Steam rockets involve a liquid to gas phase change enabling them to carry a large volume of gas compressed as a (hot) liquid. The problem not so often mentioned is it takes a lot of time (over night for instance) to heat the water up in a controlled fashion requiring typically mains electricity. Next one needs to carry it to the launch site under a thermal blanket. Should the valve fail inside a car one could risk the fate of a lobster in the kitchen. In principle a compressed liquid CO2 rocket could *not* work well because the phase change liquid to gas involves too much solid CO2 formation (see fire extinguisher) which turns to gas long after nozzle ejection and is lost for propulsion. But requiring no electricity. Other materials do not come to mind just now. Another alternative (used for safe RCS in orbit one reads) is compressed air (compressor). 70 bars of air for instance in a typical lightweight solids casing could make it to reasonable altitudes for the amateur. Even OK for the garage ;-). Similar is gas gun projectile launch where the pressure can be higher because the heavy barrel stays put. Using compressed hydrogen gas or helium one could even go for hypersonic muzzle velocities. Here burst disks can replace a valve. For all these contraptions I reckon the difficult part is buying or manufacturing the right valve. jd _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On Jul 2, 2012, at 5:01 AM, wrote: > Other materials do not come to > mind just now. Propane. But the exhaust is unburned flammable gas, unless you ignite it externally. George William Herbert Sent from my iPhone _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On Mon, 2 Jul 2012 johndom@skynet.be wrote: > ...Next one needs to carry it to the launch site under a thermal > blanket. Should the valve fail inside a car one could risk the fate of a > lobster in the kitchen. Clearly one wants to transport something like that on the *outside* of the vehicle (e.g. on an open-bed truck), not inside it. > In principle a compressed liquid CO2 rocket could *not* work well > because the phase change liquid to gas involves too much solid CO2 > formation ... Other materials do not come to mind just now. There are a number of options, although many of them are toxic or costly. (E.g., ammonia might be interesting were it not toxic -- nobody's going to breathe it voluntarily, but a suddenly-released big cloud of it can be dangerous, because someone who gets caught in it may not be able to get out quickly.) > Another alternative (used for safe RCS in orbit one reads) is compressed > air (compressor). 70 bars of air for instance in a typical lightweight > solids casing could make it to reasonable altitudes for the amateur. > Even OK for the garage ;-). Compressed *air* isn't much used for RCS, but compressed nitrogen is. Quite a variety of gases have been used in such "cold gas" rocket systems, usually for RCS but occasionally for main propulsion -- the GRAIL satellites injected themselves into lunar orbit with compressed helium! (GRAIL is doing high-precision gravity mapping, and wanted a no-liquids propulsion system to give better control of the spacecraft center of mass. Gravity measurement is so delicate that it requires careful attention to sources of error that you'd never think would matter...) Nitrogen is the usual choice when performance is not very important and the priorities are easy handling and simple hardware. One important subtlety of cold-gas systems, by the way, is that higher pressure does not improve Isp. Satellite cold-gas systems often have regulators to *reduce* the pressure of the gas before it goes to the thrusters. The energy of the exhaust jet comes from the thermal energy of the gas, not the pressure. (Higher pressure means more force per square centimeter pushing the gas out, yes, but it also means more gas mass per cubic centimeter to push, and the two cancel out.) Higher pressure mostly just gives more compact storage. (Not necessarily *lighter* storage, either, just more compact. For the same tank material at the same working stress, gas mass per unit tank mass is at best independent of pressure -- doubling the pressure doubles the gas content but also requires doubling the wall thickness. And I say "at best" because with real gases at high pressure and room temperature, non-ideal behavior starts to show up, so doubling the pressure might not double the gas density.) Henry Spencer henry@zoo.utoronto.ca (hspencer@utias-sfl.net) (regexpguy@gmail.com) _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On Mon, Jul 2, 2012 at 10:54 AM, Henry Spencer wrote: > On Mon, 2 Jul 2012 johndom@skynet.be wrote: >> ...Next one needs to carry it to the launch site under a thermal >> blanket. Should the valve fail inside a car one could risk the fate of a >> lobster in the kitchen. > > Clearly one wants to transport something like that on the *outside* of the > vehicle (e.g. on an open-bed truck), not inside it. ... and, I imagine, with standard gas bottle kinds of precautions. It's not a lot of fun if getting read-ended leads to your steam rocket flying through dudes in other cars. In the event of a more serious accident, these might also be nasty surprises for bystanders and rescue workers. In general, driving around with fire extinguishers charged with superheated steam generally sounds a bit sporty. Looking into on-site heating (as it looks like David O'Neil has done) might be prudent. -n _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket I was watching myth busters this weekend and they did more hot water heater things. They can get some altitude. Why not configure a rocket with remote pressure transucer and a heating element. Use a burst disk for launch or a valve with burst disk for precaution. A heating element could be used to bring it up to pressure and temperature. A big car battery, small generator, jumper cables? It would be much better than carrying it hot from home. Where I live it is over and hour to NEFAR and I would never make it. Thomas On Mon, Jul 2, 2012 at 12:26 PM, Nate Vack wrote: > On Mon, Jul 2, 2012 at 10:54 AM, Henry Spencer wrote: >> On Mon, 2 Jul 2012 johndom@skynet.be wrote: >>> ...Next one needs to carry it to the launch site under a thermal >>> blanket. Should the valve fail inside a car one could risk the fate of a >>> lobster in the kitchen. >> >> Clearly one wants to transport something like that on the *outside* of the >> vehicle (e.g. on an open-bed truck), not inside it. > > ... and, I imagine, with standard gas bottle kinds of precautions. > It's not a lot of fun if getting read-ended leads to your steam rocket > flying through dudes in other cars. In the event of a more serious > accident, these might also be nasty surprises for bystanders and > rescue workers. > > In general, driving around with fire extinguishers charged with > superheated steam generally sounds a bit sporty. Looking into on-site > heating (as it looks like David O'Neil has done) might be prudent. > > -n > _______________________________________________ > aRocket@exrocketry.net > http://exrocketry.net/mailman/listinfo/arocket _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On 7/2/2012 5:01 AM, johndom@skynet.be wrote: > Steam rockets involve a liquid to gas phase change enabling them to carry a > large volume of gas compressed as a (hot) liquid. > The problem not so often mentioned is it takes a lot of time (over night for > instance) to heat the water up in a controlled fashion requiring typically > mains electricity. Next one needs to carry it to the launch site under a > thermal blanket. Should the valve fail inside a car one could risk the fate > of a lobster in the kitchen. > I saw a steam rocket launch at the RRS around 2001-2002. The rocket water tank was heated on the launch rail using propane burners. It took around 30 minutes to heat if I recall. --- Carl -- Carl Tedesco Flometrics, Inc. 5900 Sea Lion Place, Suite 150 Carlsbad, CA 92010 tel: 760-476-2770 ext. 515 fax: 760-476-2763 ctedesco@flometrics.com www.flometrics.com _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket Nate Vack wrote: > In general, driving around with fire extinguishers charged with > superheated steam generally sounds a bit sporty. Looking into on-site > heating (as it looks like David O'Neil has done) might be prudent. The one actual steam rocket design I've encountered (in an article in the RRS newsletter several years ago; I forget the builder's name but the rocket was called "Scalded Cat" IIRC) used this method - the propulsion system of the rocket itself was a simple, sturdy metal tank with a nozzle; this was filled with water before launch, and the nozzle was closed by an o-ring sealed plug that was clamped in place by a latch that was mechanically released to launch. The launch stand included a set of gas burners surrounding the tank to heat it; this process took place after the operator had retreated to the remote control point. Vashon Industries once made (under the brand name "Cold Power") a toy/model rocket that worked on a similar principle, except the working fluid was Freon-12 (CCl2F2), starting at ambient temperature. (Clearly this particular material would be Not On in these environmentally-conscious times; and even the "ozone-safe" replacements such as R-134 would be undesirable in such an application, as they are considered to be strong "greenhouse gases", and hence not to be wantonly released.) -dave w _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket Next there's tetracene pellets which do not leave residues like sodium azide gas generator powder. Which is helpful if you do not want to contaminate your biprop tank contents. I could imagine tetracene could be used for gas propulsion. Or not? http://www.patentgenius.com/patent/4023352.html jd -----Oorspronkelijk bericht----- Van: arocket-bounces@exrocketry.net [mailto:arocket-bounces@exrocketry.net] Namens David Weinshenker Verzonden: maandag 2 juli 2012 19:13 Aan: Arocket List Onderwerp: Re: [AR] Steam rocket questions Nate Vack wrote: > In general, driving around with fire extinguishers charged with > superheated steam generally sounds a bit sporty. Looking into on-site > heating (as it looks like David O'Neil has done) might be prudent. The one actual steam rocket design I've encountered (in an article in the RRS newsletter several years ago; I forget the builder's name but the rocket was called "Scalded Cat" IIRC) used this method - the propulsion system of the rocket itself was a simple, sturdy metal tank with a nozzle; this was filled with water before launch, and the nozzle was closed by an o-ring sealed plug that was clamped in place by a latch that was mechanically released to launch. The launch stand included a set of gas burners surrounding the tank to heat it; this process took place after the operator had retreated to the remote control point. Vashon Industries once made (under the brand name "Cold Power") a toy/model rocket that worked on a similar principle, except the working fluid was Freon-12 (CCl2F2), starting at ambient temperature. (Clearly this particular material would be Not On in these environmentally-conscious times; and even the "ozone-safe" replacements such as R-134 would be undesirable in such an application, as they are considered to be strong "greenhouse gases", and hence not to be wantonly released.) -dave w _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket One that that has always troubled me with H2O2 and Nitric Acid is in flight tank failure or any problem that would result in nasty stuff in the air and then raining down. Thomas On Mon, Jul 2, 2012 at 3:06 PM, wrote: > Next there's tetracene pellets which do not leave residues like sodium azide > gas generator powder. Which is helpful if you do not want to contaminate > your biprop tank contents. I could imagine tetracene could be used for gas > propulsion. Or not? > > http://www.patentgenius.com/patent/4023352.html > > jd > > -----Oorspronkelijk bericht----- > Van: arocket-bounces@exrocketry.net [mailto:arocket-bounces@exrocketry.net] > Namens David Weinshenker > Verzonden: maandag 2 juli 2012 19:13 > Aan: Arocket List > Onderwerp: Re: [AR] Steam rocket questions > > Nate Vack wrote: >> In general, driving around with fire extinguishers charged with >> superheated steam generally sounds a bit sporty. Looking into on-site >> heating (as it looks like David O'Neil has done) might be prudent. > > The one actual steam rocket design I've encountered (in an article in > the RRS newsletter several years ago; I forget the builder's name but > the rocket was called "Scalded Cat" IIRC) used this method - the propulsion > system of the rocket itself was a simple, sturdy metal tank with a nozzle; > this was filled with water before launch, and the nozzle was closed by an > o-ring sealed plug that was clamped in place by a latch that was > mechanically > released to launch. The launch stand included a set of gas burners > surrounding > the tank to heat it; this process took place after the operator had > retreated > to the remote control point. > > Vashon Industries once made (under the brand name "Cold Power") a toy/model > rocket that worked on a similar principle, except the working fluid was > Freon-12 (CCl2F2), starting at ambient temperature. (Clearly this particular > material would be Not On in these environmentally-conscious times; and even > the "ozone-safe" replacements such as R-134 would be undesirable in such an > application, as they are considered to be strong "greenhouse gases", and > hence not to be wantonly released.) > > -dave w > _______________________________________________ > aRocket@exrocketry.net > http://exrocketry.net/mailman/listinfo/arocket > > _______________________________________________ > aRocket@exrocketry.net > http://exrocketry.net/mailman/listinfo/arocket _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket > >One important subtlety of cold-gas systems, by the way, is that higher >pressure does not improve Isp. Satellite cold-gas systems often have >regulators to *reduce* the pressure of the gas before it goes to the >thrusters. The energy of the exhaust jet comes from the thermal energy of >the gas, not the pressure. (Higher pressure means more force per square >centimeter pushing the gas out, yes, but it also means more gas mass per >cubic centimeter to push, and the two cancel out.) Higher pressure mostly >just gives more compact storage. That might appear evident in vacuum conditions, but it's not true (as you well know Henry) for atmospheric conditions, which (in this context) is the environment in which most amateur steam rockets operate (assuming we are ultilising gas expansion here?) Of course if we assume multi phase flow will occur in the expansion process, then yeah, there are clear limitations to any advantages you might expect from operating the nozzle at higher pressures at a given propellant temperature, but it's not useful to assume this will occur with all cold gas systems. Troy _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On Tue, 3 Jul 2012, Troy Prideaux wrote: > >One important subtlety of cold-gas systems, by the way, is that higher > >pressure does not improve Isp... > > That might appear evident in vacuum conditions, but it's not true (as > you well know Henry) for atmospheric conditions, which (in this context) > is the environment in which most amateur steam rockets operate... However, for compressed air or nitrogen, which was the specific context of this particular remark, even for amateurs "higher pressure" tends to mean the difference between, say, 1000psi and 2000psi. And that really isn't going to make much difference, even at sea level. Possibly a little, yes, if you don't lose it to issues like gas friction in the longer nozzle, but not very much. The important point is that while low chamber pressure can constrain the expansion ratio you use, it's still the thermal energy of the gas that's doing the work, *not* its pressure. Even engineering grad students have been known to assume that for really good cold-gas performance, they have to run their thrusters at 10,000psi. Not so. Henry Spencer henry@zoo.utoronto.ca (hspencer@utias-sfl.net) (regexpguy@gmail.com) _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On 2012-07-02 10:12, David Weinshenker wrote: > The one actual steam rocket design I've encountered (in an article in > the RRS newsletter several years ago; I forget the builder's name but > the rocket was called "Scalded Cat" IIRC) used this method - the > propulsion > system of the rocket itself was a simple, sturdy metal tank with a > nozzle; > this was filled with water before launch, and the nozzle was closed > by an > o-ring sealed plug that was clamped in place by a latch that was > mechanically > released to launch. The launch stand included a set of gas burners > surrounding the tank to heat it; this process took place after the > operator > had retreated to the remote control point. Dave, The owner/designer/builder/flyer of the "Scalded Cat" steam rocket(s) is Bill Inman. I'm not sure if he's a member of the ARocket list or not (I don't think he is). In any case, I'll bcc: him a copy of this message, and see if he might be interested in writing up a bit of info about the Scalded Cats to be reposted here, or if he minds if I give out his email address so that interested parties can contact him directly. Cheers, - Rick Dickinson _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket >> Evan: A friend of mine makes steam rockets - David O'Neil: >> http://www.webcomsknkwrks.com/steamrok.htm Some of his design is incredibly cleaver! Shows what can happen when one is unfettered by convention. :) -->MCS _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket The "thermal explanation of jet propulsion" is one of my favorite pet peeves, not only because it isn't really accurate, but because its also not useful as a engineering concept. Let me start by saying that this email is really written for those who are trying to learn with a "blank slate". If you already know that what I said in the first sentence is wrong, then you can stop reading this email. If you're not sure, or simply curious, read on. In a rocket engine, thrust comes from pressure. Period. Temperature doesn't generate thrust. Momentum of the jet at the nozzle exit doesn't generate thrust. Pressure acting on the walls of the chamber generates thrust. A rocket combustion chamber generates thrust because it is a pressure vessel with a hole in it. The side with a hole in it obviously has no pressure acting on the walls (because there aren't any), and therefore the net force on the chamber is positive in the direction opposite the hole. As is explained in the classic Russian texts, it is inconvenient to try to calculate thrust by integrating pressure on the walls. Instead, we use conservation of momentum as it is much more convenient. This doesn't change the fact that THRUST comes from PRESSURE. Arguments which appeal to the thermal characteristic of the process are generally attempts to draw analogies with conventional heat engines. Unfortunately, jet propulsion engines don't resemble traditional heat engines in their operation. Pressure is generated by the collision of molecules with the wall. The rate of those collisions is determined by the temperature (the speed of the molecules) and density (the frequency with which they collide with the wall). Because its hard to make a gas densometer, we instead measure pressure, which is after all only a measure of FORCE per unit area. When you have gas at a given pressure, you get the maximum amount of thrust from it in a rocket by expanding it down to the local ambient pressure. On the surface of the earth, this is approximately 14.7 psi or 1 bar. When you expand the gas down, it gets cold and eventually almost all gases will start to liquefy. Once they start to liquefy, you can't expand them anymore and get more thrust. If you're expanding nitrogen at room temperature to create thrust at sea level, the Isp will be maximized at approximately 1500 psi. Above 1500 psi the nitrogen will start to liquefy before it has expanded all the way to 14.7 psi. Below 1500 psi chamber pressure, you can increase the performance by increasing the area ratio. I apologize for the rant, but explanations of thrust which don't involve pressure are the bane of the industry. I've worked with 30 year veterans in the aerospace industry who don't understand that pressure on the walls is what makes thrust. These same people often make assumptions about how engines work which are basically wrong, because they don't understand this concept. -RM On Mon, Jul 2, 2012 at 4:24 PM, Henry Spencer wrote: On Tue, 3 Jul 2012, Troy Prideaux wrote: > >One important subtlety of cold-gas systems, by the way, is that higher > >pressure does not improve Isp... > > That might appear evident in vacuum conditions, but it's not true (as > you well know Henry) for atmospheric conditions, which (in this context) > is the environment in which most amateur steam rockets operate... However, for compressed air or nitrogen, which was the specific context of this particular remark, even for amateurs "higher pressure" tends to mean the difference between, say, 1000psi and 2000psi. And that really isn't going to make much difference, even at sea level. Possibly a little, yes, if you don't lose it to issues like gas friction in the longer nozzle, but not very much. The important point is that while low chamber pressure can constrain the expansion ratio you use, it's still the thermal energy of the gas that's doing the work, *not* its pressure. Even engineering grad students have been known to assume that for really good cold-gas performance, they have to run their thrusters at 10,000psi. Not so. Henry Spencer henry@zoo.utoronto.ca (hspencer@utias-sfl.net) (regexpguy@gmail.com) _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket -------------------------------------------------------------------------------- _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket JD, In a message dated 7/2/2012 5:03:07 A.M. Pacific Daylight Time, johndom@skynet.be writes: Steam rockets involve a liquid to gas phase change enabling them to carry a large volume of gas compressed as a (hot) liquid. The problem not so often mentioned is it takes a lot of time (over night for instance) to heat the water up in a controlled fashion requiring typically mains electricity. Next one needs to carry it to the launch site under a thermal blanket. Should the valve fail inside a car one could risk the fate of a lobster in the kitchen. Using a chemical reaction you can turn water into super saturated steam in 0.1 seconds. Working with a group in Italy, we reached >2 km on stream generated chemically. In principle a compressed liquid CO2 rocket could *not* work well because the phase change liquid to gas involves too much solid CO2 formation (see fire extinguisher) which turns to gas long after nozzle ejection and is lost for propulsion. But requiring no electricity. Other materials do not come to mind just now. CO2 presents unique engineering challenges. Knowing that CO2 has a liquid/solid/gas phase change, a nozzle can be designed to reduce the formation of solid CO2. Jim Humphreys rocket powered wheelchair was CO2 powered. It propelled Jim in his wheelchair to 40 MPH in <100 yards. jd _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket -------------------------------------------------------------------------------- _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket I remember Johnny Carson commenting the newspaper article of the rocket powered wheelchair during his monologue. http://news.google.com/newspapers?nid=1346&dat=19730422&id=I41OAAAAIBAJ&sjid=cPoDAAAAIBAJ&pg=6176,6458564 Dave CO2 presents unique engineering challenges. Knowing that CO2 has a liquid/solid/gas phase change, a nozzle can be designed to reduce the formation of solid CO2. Jim Humphreys rocket powered wheelchair was CO2 powered. It propelled Jim in his wheelchair to 40 MPH in <100 yards. -------------------------------------------------------------------------------- _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket ultimately converted as density isn’t either lost or gained. Opps… make that “mass” not density. -------------------------------------------------------------------------------- _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket I agree with everything you said, but to be fair to Henry, he was specifically referring to exhaust velocity via Isp and not thrust. Excluding specific gas properties - the exhaust velocity is significantly influenced by gas temperature and to a lesser extent influenced by pressure differential and the sensible conversion of such to mechanical work through the expansion process which is also dependent on Pa being significant to provide any significance. Fundamentally, it’s the heat component of that pressure differential that is ultimately converted as density isn’t either lost or gained. However (saying that), the optimum expansion ratio of the nozzle (that part of the rocket that accelerates the exhaust via a reactive surface) isn’t derived from the temperature or effective velocity, but by the mach number of an ideal expansion and the fluids’ ratio of specific heats. I thought Henry’s answer was a bit shifty; cherry picking operating pressures that I would refer to as “high” and “higher” to provide a comparison – which provides us with a non-trivial performance differential anyway. Not a convincing explanation of the statement “higher pressure does not improve Isp” that I was objecting to. Troy. -------------------------------------------------------------------------------- From: arocket-bounces@exrocketry.net [mailto:arocket-bounces@exrocketry.net] On Behalf Of RM Sent: Tuesday, 3 July 2012 2:26 PM To: Henry Spencer Cc: Arocket List Subject: Re: [AR] Steam rocket questions The "thermal explanation of jet propulsion" is one of my favorite pet peeves, not only because it isn't really accurate, but because its also not useful as a engineering concept. Let me start by saying that this email is really written for those who are trying to learn with a "blank slate". If you already know that what I said in the first sentence is wrong, then you can stop reading this email. If you're not sure, or simply curious, read on. In a rocket engine, thrust comes from pressure. Period. Temperature doesn't generate thrust. Momentum of the jet at the nozzle exit doesn't generate thrust. Pressure acting on the walls of the chamber generates thrust. A rocket combustion chamber generates thrust because it is a pressure vessel with a hole in it. The side with a hole in it obviously has no pressure acting on the walls (because there aren't any), and therefore the net force on the chamber is positive in the direction opposite the hole. As is explained in the classic Russian texts, it is inconvenient to try to calculate thrust by integrating pressure on the walls. Instead, we use conservation of momentum as it is much more convenient. This doesn't change the fact that THRUST comes from PRESSURE. Arguments which appeal to the thermal characteristic of the process are generally attempts to draw analogies with conventional heat engines. Unfortunately, jet propulsion engines don't resemble traditional heat engines in their operation. Pressure is generated by the collision of molecules with the wall. The rate of those collisions is determined by the temperature (the speed of the molecules) and density (the frequency with which they collide with the wall). Because its hard to make a gas densometer, we instead measure pressure, which is after all only a measure of FORCE per unit area. When you have gas at a given pressure, you get the maximum amount of thrust from it in a rocket by expanding it down to the local ambient pressure. On the surface of the earth, this is approximately 14.7 psi or 1 bar. When you expand the gas down, it gets cold and eventually almost all gases will start to liquefy. Once they start to liquefy, you can't expand them anymore and get more thrust. If you're expanding nitrogen at room temperature to create thrust at sea level, the Isp will be maximized at approximately 1500 psi. Above 1500 psi the nitrogen will start to liquefy before it has expanded all the way to 14.7 psi. Below 1500 psi chamber pressure, you can increase the performance by increasing the area ratio. I apologize for the rant, but explanations of thrust which don't involve pressure are the bane of the industry. I've worked with 30 year veterans in the aerospace industry who don't understand that pressure on the walls is what makes thrust. These same people often make assumptions about how engines work which are basically wrong, because they don't understand this concept. -RM On Mon, Jul 2, 2012 at 4:24 PM, Henry Spencer wrote: On Tue, 3 Jul 2012, Troy Prideaux wrote: > >One important subtlety of cold-gas systems, by the way, is that higher > >pressure does not improve Isp... > > That might appear evident in vacuum conditions, but it's not true (as > you well know Henry) for atmospheric conditions, which (in this context) > is the environment in which most amateur steam rockets operate... However, for compressed air or nitrogen, which was the specific context of this particular remark, even for amateurs "higher pressure" tends to mean the difference between, say, 1000psi and 2000psi. And that really isn't going to make much difference, even at sea level. Possibly a little, yes, if you don't lose it to issues like gas friction in the longer nozzle, but not very much. The important point is that while low chamber pressure can constrain the expansion ratio you use, it's still the thermal energy of the gas that's doing the work, *not* its pressure. Even engineering grad students have been known to assume that for really good cold-gas performance, they have to run their thrusters at 10,000psi. Not so. Henry Spencer henry@zoo.utoronto.ca (hspencer@utias-sfl.net) (regexpguy@gmail.com) _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket -------------------------------------------------------------------------------- _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On Jul 2, 2012, at 9:25 PM, RM wrote: > In a rocket engine, thrust comes from pressure. Period. > > Temperature doesn't generate thrust. > Momentum of the jet at the nozzle exit doesn't generate thrust. Thrust, yes. But specific impulse depends on the momentum and energy stuff. Is your rocket dominated by engine mass or fuel mass? For what you are doing, focus on the right part. Both matter at different times to different people. George William Herbert Sent from my iPhone _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket I agree that it is probably more helpful to discuss rocket operation in terms of pressure as that is more intuitive, but the temperature and the pressure are not separate at all and to say one matters and the other doesn't is misleading. Pressure itself is a non-entity. The phenomenon is just an effect of the transfer of momentum from gas particles to the walls of a container. Pressure is just one measure of the momentum contained in the particles. Temperature is another way of measuring momentum of particles. Pressure is measured in units of energy per volume after all (force per area multiplied by length over length). I am not saying that they are the same measure at all, but they are intimately related, via PV=nRT (in ideal circumstances). -Nathan Mogk On Mon, Jul 2, 2012 at 9:25 PM, RM wrote: The "thermal explanation of jet propulsion" is one of my favorite pet peeves, not only because it isn't really accurate, but because its also not useful as a engineering concept. Let me start by saying that this email is really written for those who are trying to learn with a "blank slate". If you already know that what I said in the first sentence is wrong, then you can stop reading this email. If you're not sure, or simply curious, read on. In a rocket engine, thrust comes from pressure. Period. Temperature doesn't generate thrust. Momentum of the jet at the nozzle exit doesn't generate thrust. Pressure acting on the walls of the chamber generates thrust. A rocket combustion chamber generates thrust because it is a pressure vessel with a hole in it. The side with a hole in it obviously has no pressure acting on the walls (because there aren't any), and therefore the net force on the chamber is positive in the direction opposite the hole. As is explained in the classic Russian texts, it is inconvenient to try to calculate thrust by integrating pressure on the walls. Instead, we use conservation of momentum as it is much more convenient. This doesn't change the fact that THRUST comes from PRESSURE. Arguments which appeal to the thermal characteristic of the process are generally attempts to draw analogies with conventional heat engines. Unfortunately, jet propulsion engines don't resemble traditional heat engines in their operation. Pressure is generated by the collision of molecules with the wall. The rate of those collisions is determined by the temperature (the speed of the molecules) and density (the frequency with which they collide with the wall). Because its hard to make a gas densometer, we instead measure pressure, which is after all only a measure of FORCE per unit area. When you have gas at a given pressure, you get the maximum amount of thrust from it in a rocket by expanding it down to the local ambient pressure. On the surface of the earth, this is approximately 14.7 psi or 1 bar. When you expand the gas down, it gets cold and eventually almost all gases will start to liquefy. Once they start to liquefy, you can't expand them anymore and get more thrust. If you're expanding nitrogen at room temperature to create thrust at sea level, the Isp will be maximized at approximately 1500 psi. Above 1500 psi the nitrogen will start to liquefy before it has expanded all the way to 14.7 psi. Below 1500 psi chamber pressure, you can increase the performance by increasing the area ratio. I apologize for the rant, but explanations of thrust which don't involve pressure are the bane of the industry. I've worked with 30 year veterans in the aerospace industry who don't understand that pressure on the walls is what makes thrust. These same people often make assumptions about how engines work which are basically wrong, because they don't understand this concept. -RM On Mon, Jul 2, 2012 at 4:24 PM, Henry Spencer wrote: On Tue, 3 Jul 2012, Troy Prideaux wrote: > >One important subtlety of cold-gas systems, by the way, is that higher > >pressure does not improve Isp... > > That might appear evident in vacuum conditions, but it's not true (as > you well know Henry) for atmospheric conditions, which (in this context) > is the environment in which most amateur steam rockets operate... However, for compressed air or nitrogen, which was the specific context of this particular remark, even for amateurs "higher pressure" tends to mean the difference between, say, 1000psi and 2000psi. And that really isn't going to make much difference, even at sea level. Possibly a little, yes, if you don't lose it to issues like gas friction in the longer nozzle, but not very much. The important point is that while low chamber pressure can constrain the expansion ratio you use, it's still the thermal energy of the gas that's doing the work, *not* its pressure. Even engineering grad students have been known to assume that for really good cold-gas performance, they have to run their thrusters at 10,000psi. Not so. Henry Spencer henry@zoo.utoronto.ca (hspencer@utias-sfl.net) (regexpguy@gmail.com) _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket -------------------------------------------------------------------------------- _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket JMKrell wrote on 3/7/2012: > Using a chemical reaction you can turn water into super saturated steam in 0.1 seconds. Working with a group in Italy, we reached >2 km on steam generated chemically. Amazing. We tried to think of such a reaction 12 years ago and never found one since. Are the chemicals dissolved in the water inside the tank? I cannot think of any not presenting too much dead weight and potentially nozzle clogging solid reaction products. Acid-base reaction heating is not an option; too heavy & too much salt produced. If carbide is used then I'd call it an acetylene rocket rather. If peroxide is diluted in the water to have the oxygen pressure expel the water, then I'd call it a water rocket. The diluted peroxide 'd take longer to decompose than 0.1 s. If concentrated peroxide is reacted with a liquid catalyst that 'd be an oxygen-steam rocket, not a steamrocket. Just curious how you did it. > Jim Humphreys rocket powered wheelchair was CO2 powered. It propelled Jim in his wheelchair to 40 MPH in <100 yards. Paintball guns do not produce solid CO2. This rings a bell. During expansion CO2 cools to solid form in a fire extinguisher "exhaust" where the liquid is vaporised in the atmosphere without thermal contact. So I reckon the thermal mass of cylinder and the barrel keeps it from cooling down to solid CO2 in a paintball gun. Such heavy thermal mass would not be practical in rocketry at first sight. A wheelchair does not have to fly :-). jd _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On Mon, 2 Jul 2012 21:25:42 -0700 RM wrote: > > In a rocket engine, thrust comes from pressure. Period. > > Temperature doesn't generate thrust. > Momentum of the jet at the nozzle exit doesn't generate thrust. > > Pressure acting on the walls of the chamber generates thrust. I think this is more of a philosophical question. One could argue that what really generates thrust is virtual photons exchanged between electrons in the atoms of the gas and electrons in the atoms of the chamber wall, and that pressure is just an imaginary theoretical concept. Or you could go to the other extreme and say that the ultimate cause of me gaining momentum in one direction is that something else gained momentum in the other direction, and the exact mechanism for transferring momentum is just a proximate cause, not the "real" cause. Clearly, pressure (or force in general) is a useful way of thinking about things, and so are momentum and energy. All models are wrong, some models are useful. What I find most useful is to double check my theoretical understanding by making sure something makes sense in all three frameworks. For example, I was very confused about ram jet operation until I understood a pressure based explanation. _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket Everyone, I'm forwarding comments from Bill Inman about his "Scalded Cat" rockets back to the list. He said he'd be willing to answer questions, so I'm leaving his email address in as a cc: to this post, so that others can contact him. Thanks again, Bill! (If anyone else contacts Bill for further info on the Scalded Cats, please forward the responses back to the list, as I'm sure I'm not the only one interested. Thanks!) Cheers, - Rick Dickinson -------- Original Message -------- Subject: Re: [AR] Steam rocket questions Date: 2012-07-02 22:59 From: wmjinman@aol.com To: wmjinman@aol.com, rtd@notesguy.com I warned you I could get long-winded. Below, I've added a bunch of information to the previous e-mail. You could almost go out & build a successful one with what I've got here now!!!! Bill -----Original Message----- From: william inman To: rtd ; wmjinman Sent: Mon, Jul 2, 2012 6:13 pm Subject: Re: [AR] Steam rocket questions Hi Rick, Thanks for passing this along. No, I don't believe I'm a member of that mailing list, but I have no problem with answering any questions anyone has. I would also consider writing up something explaining my "Scalded Cat" project if you'd like. I have a lot of material, and could get extremely long-winded if I don't watch out! ;-) It's been a few years, and I hate to admit, but I find myself starting to forget things. One thing I remember clearly though, is that I found that steam rockets will work under a wide variety of design variations (tank pressure and nozzle design). See, I was trying to find the "optimum" design, but was frustrated that nothing seemed to be emerging as a clear "winner". But the good news is that it doesn't much matter. - as long as you stay within certain parameters. And I can name a couple right here: 1). Be sure to take the tank pressure above 200 psi. It gets better at 400 psi, but "works ok" from 200 on. There is a BIG drop-off below 200, though. To elaborate a bit; the power (energy) is in the HEAT. Pressure is a handy byproduct of the heat. So you need to be at nearly 400 degrees F to get "steam rocket quality" power. That roughly equates to 200 psi. As you get to about 455 degrees, the pressure is around 455 psi. If your tank is strong enough and has the ability to take this temperature, it's a good operating range to shoot for. Every aluminum alloy I know of suffers serious "damage" above 300 to 350 degrees F. And that's still about 50 degrees short of what you need for a steam rocket. It may survive that temperature ONCE, but it's "temper" will be gone after that and it will be too weak to use again. 2). Likewise, it probably serves no purpose to go beyond 500 to 600 psi. Scalded Cat operated at 1500 psi, but I now think I could have got better performance at lower pressure. See, the beginning water level has to be reduced as the target pressure goes up. So at 1500 psi, I couldn't use as much water as I could have at 500. To elaborate some more; water expands as it's heated. In a sealed vessel, if there is not enough empty space above the water level to accommodate this expansion, once it expands to take up the entire space, any further heating will cause it to "go hydraulic". Before I knew better, I had this happen a couple of times - and it's SCARY!!!! The needle on the pressure gauge was moving like the "second hand" on a clock!! Fortunately, I got the fire turned off in time, and the tank didn't rupture, but it got my attention!! To make matters worse, this expansion speeds up as the temperature rises. Eventually (at around 650 to 700 degrees F.) the expansion is so extreme that to keep it from having a hydraulic rupture, you'd need to start out with the tank only about 10% full!!! However, if you keep it down to the 455 degree/455 psi range, you can fill it 75% to 80% full before you start heating. 3). Be sure to use at least a 10:1 expansion ratio (area ratio) for your nozzle. This means the cross sectional area of the nozzle exit is 10 times bigger than the cross sectional area of the narrowest part of the nozzle throat. Performance drops off badly below 10:1. It starts dropping off again if you go beyond 30:1, too. I actually found (through repeated tests that confirmed it) that there were two "humps" in the performance curve - one at 10:1 expansion, and one at 30:1 with a definite "dip" in between. 30:1 performed a little better than 10:1, but everything else performed WORSE! I don't know why, but this pattern held true all the way from 200 psi to 400 psi. Note; most of my tests were done at 200 psi, although I did a few at higher pressure. I also did a few at lower pressure, but as mentioned earlier, at 150 psi, (or below) the thrust was a fraction of what it was at 200. What came out of the nozzle more resembled the spray out of a bathroom shower head than a rocket plume. I did most of them at 200 because my ball valves and other equipment had a much higher survivability rate than at higher temp and pressure. I need to do more testing, but it appeared that at higher pressures, the performance drop-off above the 30:1 expansion ratio was less pronounced. It also seemed to be starting to rise again at 100:1 expansion on the test I did at 500 psi. 4). Nozzle exit angle doesn't seem to matter much. I tried it from narrower half-angles (like 10 degrees <20 deg included angle>) to wider ones (like 20 degrees <40 deg included>) and it didn't seem to make much difference. I even used galvanized pipe "bell reducers" as nozzle exit cones, and they performed ok!! - especially when the expansion ratio was 10:1 or 30:1!! 5). If heating the tank by an external flame, it doesn't seem to matter what you do .... it will take an hour! I've tried it with single 30,000 BTU burner and with dual 130,000 BTU burners, and it takes an hour regardless! Unless the tank isn't sufficiently shrouded. I've had people suggest I try heating the tank INTERNALLY. So I tried it!! In one set of experiments, I used a water heater electric heating element to heat with (leaks become a problem above 200 psi), and also by putting "electrodes" inside the tank and hooking it up to 120 volt AC "house current"! Both methods worked, and both took - - - - wait for it - - - about an hour!!!! A couple of notes on the electrolysis method. A) It will heat painfully slowly unless you put a bit of something like table salt in it to make the water more conductive. For a tank that holds one liter (2" nominal galvanized pipe that's 2 ft. long), one of the salt packets from a McDonald's drive-in worked well. B) A friend who's a bit of an electrician had a meter that measured amp draw, and we heated once with this meter on. We found that as the water got hotter, it drew more amperage. We believe this must mean the salty water gets more conductive as it gets hotter. A last note if heating by electric element or electrolysis. Like the shrouding when using an external flame, INSULATION becomes critical when heating internally. I had to wrap it up like a mummy in fiberglass insulation (which ended up getting all scorched-up) to heat it within an hour using these methods. 6). The tank must be shrouded or somehow otherwise protected from the wind. Trying to heat an un-shrouded tank in the wind will FAIL - guaranteed!!! I know this!! The better the shrouding (wind protection), the better your heating will go (which means you'll do it in an hour). I've run burners for 3 hours without the shrouding and not reached launch temperature. 7). Beware of the heat!! It sounds weird that heat is a bigger problem with steam rockets that are operating at 450 degrees F than with solid composite fuel that burns at several thousand. But it's true!!! The reason is that I have not found a way to isolate the heat to just the tank & nozzle. The fins will get it, the launch tower will get it, and at least the parts of the payload section nearest the tank will get it. Fiberglass & phenolic scorches, aluminum weakens, even the STEEL weakens. I was surprised that my stainless steel Scalded Cat tank was even compromised almost 10% in strength while at full temperature & pressure (according to the literature on the strengths of steel & other materials). So keep that in mind. Note; If you're saying, "but wait, "high power" rocket motors use aluminum cases and work fine", just remember that's ONLY if the phenolic liner, greased with silicone grease, and all the o-rings and phenolic bulkheads are properly installed, greased, and don't malfunction. An aluminum high power case will melt-through almost instantly if something goes wrong. Bottom line; with the ablative liners etc, most of the heat goes out the nozzle, and the aluminum case never gets to 300 degrees!! I wish I could figure out how to isolate the heat like this with a steam rocket, but so far, haven't been able to. ;-) 8) Weight is a HUGE factor in steam rocket (flight vehicle) performance. With some "high power" rockets, it's actually possible to make them too light for maximum (altitude) performance. Not so with a steam rocket - a steam rocket will NEVER be too light, and will ALWAYS be too heavy. If the tank and the rest of the rocket could be completely weightless, with ONLY the weight of the water, I once calculated (using the Rodgers Aeroscience software) that it could be too light. But that's only theoretical, since the rocket itself can't be weightless. There are a few things that contribute to this; 1) Water (the "fuel") has a low Isp potential. I could never get above about 45. Contrast that to APCP that can approach 200 in a high power motor. 2) Along with the low Isp (specific impulse), water is HEAVY. At over 8 lbs. per gallon, it's probably amongst the heaviest "rocket fuels". 3) Since it can't be made out of aluminum or carbon composite (that I know of) the tank is HEAVY. So, try to find a lightweight tank. Scalded Cat is made out of schedule-10 stainless pipe, and is thick enough (almost 1/8th inch) to handle 1500 psi. If I could have found tubing HALF that thickness and run it at 500 psi, it would have flown higher. Nevertheless, I did shave weight (sometimes a quarter ounce here & a quarter ounce there) to reduce the rocket weight by a couple of pounds, and it flew higher. (4450 ft. the 1st time and 4600 the second). So try to shave off weight wherever you can and it will make a big difference. The Scalded Cat has about a 20 lbs. tank, carries about 20 lbs. of water, and the rest of it weighs about 8 lbs. !!! Ok, there are a few tidbits of what I can remember of the old steam rocket days!!! Thanks, Bill Inman _Sent via MOTOBLUR™ on Verizon Wireless_ -----Original message----- > To: Arocket List <> > Sent: Mon, Jul 2, 2012 23:47:04 GMT+00:00 > Subject: Re: [AR] Steam rocket questions > > On 2012-07-02 10:12, David Weinshenker wrote: > > > The one actual steam rocket design I've encountered (in an article > in > > the RRS newsletter several years ago; I forget the builder's name > but > > the rocket was called "Scalded Cat" IIRC) used this method - the > > propulsion > > system of the rocket itself was a simple, sturdy metal tank with a > > nozzle; > > this was filled with water before launch, and the nozzle was closed > > > by an > > o-ring sealed plug that was clamped in place by a latch that was > > mechanically > > released to launch. The launch stand included a set of gas burners > > surrounding the tank to heat it; this process took place after the > > operator > > had retreated to the remote control point. > > Dave, > > The owner/designer/builder/flyer of the "Scalded Cat" steam rocket(s) > > is Bill Inman. I'm not sure if he's a member of the ARocket list or > not > (I don't think he is). In any case, I'll bcc: him a copy of this > message, and see if he might be interested in writing up a bit of > info > about the Scalded Cats to be reposted here, or if he minds if I give > out > his email address so that interested parties can contact him > directly. > > Cheers, > > - Rick Dickinson _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On Mon, Jul 02, 2012 at 09:25:42PM -0700, RM wrote: >In a rocket engine, thrust comes from pressure. Period. > >Temperature doesn't generate thrust. >Momentum of the jet at the nozzle exit doesn't generate thrust. > >Pressure acting on the walls of the chamber generates thrust. Ah, but Henry's point was that when you try to improve the performance of a cold-gas thruster by increasing the pressure, you also increase the density by the same factor. That gives you increased thrust, but not increased Isp, since the price of your increased thrust has been a increase by the same factor in the amount of expelled mass. If, on the other hand, you heat the gas, that increases the pressure without increasing the density, yielding improved thrust _and_ improved Isp. As with every complicated technical subject, there's more than one way to understand it, and different ways give different insights. It's quite true that thrust is an integral of pressure on the chamber walls, but it's also true that thrust needs energy that has to come from somewhere, such as from heat. -- Norman Yarvin http://yarchive.net/blog _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket johndom@skynet.be wrote: > Odd all these many words. Aren't there precise equations to settle all this. Oh, yes, and the beauty is that there are so many of them to choose from! And there isn't really anything to settle; the answer to "is it the heat or the pressure" is "yes"... -dave w _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket JD, In a message dated 7/3/2012 12:29:06 A.M. Pacific Daylight Time, johndom@skynet.be writes: JMKrell wrote on 3/7/2012: > Using a chemical reaction you can turn water into super saturated steam in 0.1 seconds. Working with a group in Italy, we reached >2 km on steam generated chemically. Amazing. We tried to think of such a reaction 12 years ago and never found one since. Are the chemicals dissolved in the water inside the tank? I cannot think of any not presenting too much dead weight and potentially nozzle clogging solid reaction products. Acid-base reaction heating is not an option; too heavy & too much salt produced. If carbide is used then I'd call it an acetylene rocket rather. If peroxide is diluted in the water to have the oxygen pressure expel the water, then I'd call it a water rocket. The diluted peroxide 'd take longer to decompose than 0.1 s. If concentrated peroxide is reacted with a liquid catalyst that 'd be an oxygen-steam rocket, not a steamrocket. Just curious how you did it. The reaction was acid-base, but not the usual chemicals. The acid was in a glass ampoule with a primacord center. They detonated the primacord and everything followed. > Jim Humphreys rocket powered wheelchair was CO2 powered. It propelled Jim in his wheelchair to 40 MPH in <100 yards. Paintball guns do not produce solid CO2. This rings a bell. During expansion CO2 cools to solid form in a fire extinguisher "exhaust" where the liquid is vaporised in the atmosphere without thermal contact. So I reckon the thermal mass of cylinder and the barrel keeps it from cooling down to solid CO2 in a paintball gun. Such heavy thermal mass would not be practical in rocketry at first sight. A wheelchair does not have to fly :-). Slow expansion and high thermal mass nozzle. Your right, we didn't want the wheelchair to fly. -------------------------------------------------------------------------------- _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket ============================================ íîâàÿ òåìà [AR] Steam rocket Isp analysis ============================================== So, since the original thread I started has produced very few useful responses, I've been working through the theory on my own, and generated a table for theoretical Isp vs starting conditions. I used the NIST webbook for all my data [1]. I assumed the water starts at saturation, and expands isentropically through the nozzle. I assumed 1 atm exit conditions. I then found the liquid fraction that produced equivalent entropy in the exhaust. I then computed exhaust enthalpy, subtracted initial enthalpy, and assumed that the excess is kinetic energy, and from there found Isp. (Worked example at 250C starting conditions and full table follow.) Does this look correct? The final number is at least in the right range. Did I get usage of enthalpy vs internal energy right? I haven't yet tried plugging in all the formulas to get C* and Ae/At yet. I'll try to add that to the table shortly. Evan Daniel Working an example at 250C: Initial entropy = 2.793 J/(g*K) Exit liquid entropy = 1.3073 J/(g*K) Exit vapor entropy = 7.354 J/(g*K) liquid fraction = (initial entropy - vapor entropy) / (liquid entropy - vapor entropy) = 0.754 Initial enthalpy = 1086 J/g Exit liquid enthalpy = 416 J/g Exit vapor enthalpy = 2675 J/g Exit total enthalpy = 973 J/g enthalpy difference = 112 J/g Ve = 474 m/s Temperature (K) Pressure (MPa) Ve (m/s) 383.16 0.143 35.1 393.16 0.199 67.9 403.16 0.270 100.4 413.16 0.362 132.9 423.16 0.476 164.8 433.16 0.618 196.3 443.16 0.792 227.6 453.16 1.003 258.9 463.16 1.256 289.7 473.16 1.555 320.5 483.16 1.908 351.1 493.16 2.320 381.6 503.16 2.798 412.2 513.16 3.348 442.8 523.16 3.977 473.5 533.16 4.693 504.4 543.16 5.504 535.4 553.16 6.418 566.8 563.16 7.443 598.7 573.16 8.589 631.1 583.16 9.866 664.3 593.16 11.286 698.5 603.16 12.860 733.9 [1] http://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185&TUnit=K&PUnit=MPa&DUnit=kg%2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm&Type=SatP&RefState=DEF&Action=Page _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket Evan Daniel wrote: > So, since the original thread I started has produced very few useful > responses, I've been working through the theory on my own, and > generated a table for theoretical Isp vs starting conditions. > > I used the NIST webbook for all my data [1]. I assumed the water > starts at saturation, and expands isentropically through the nozzle. I > assumed 1 atm exit conditions. I then found the liquid fraction that > produced equivalent entropy in the exhaust. I then computed exhaust > enthalpy, subtracted initial enthalpy, and assumed that the excess is > kinetic energy, and from there found Isp. That sounds -about- right, I think... finding conditions of matching entropy and then comparing enthalpy is the basic strategy of the standard "shifting equilibrium" calculation, which is probably a representative way to calculate performance here: if any rocket expansion process is genuinely "shifting equilibrium", it would be a steam-rocket! :) -dave w _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket Seems to be right. Our 50kN hot water rocket motor (I like this term more than steam rocket) had about 500m/s at 50 bar. Bruno SPL Am 04.07.2012 19:01, schrieb Evan Daniel: > So, since the original thread I started has produced very few useful > responses, I've been working through the theory on my own, and > generated a table for theoretical Isp vs starting conditions. > > I used the NIST webbook for all my data [1]. I assumed the water > starts at saturation, and expands isentropically through the nozzle. I > assumed 1 atm exit conditions. I then found the liquid fraction that > produced equivalent entropy in the exhaust. I then computed exhaust > enthalpy, subtracted initial enthalpy, and assumed that the excess is > kinetic energy, and from there found Isp. (Worked example at 250C > starting conditions and full table follow.) > > Does this look correct? The final number is at least in the right > range. Did I get usage of enthalpy vs internal energy right? > > I haven't yet tried plugging in all the formulas to get C* and Ae/At > yet. I'll try to add that to the table shortly. > > Evan Daniel > > > Working an example at 250C: > > Initial entropy = 2.793 J/(g*K) > Exit liquid entropy = 1.3073 J/(g*K) > Exit vapor entropy = 7.354 J/(g*K) > > liquid fraction = (initial entropy - vapor entropy) / (liquid entropy > - vapor entropy) = 0.754 > > Initial enthalpy = 1086 J/g > Exit liquid enthalpy = 416 J/g > Exit vapor enthalpy = 2675 J/g > Exit total enthalpy = 973 J/g > > enthalpy difference = 112 J/g > Ve = 474 m/s > > > Temperature (K) Pressure (MPa) Ve (m/s) > 383.16 0.143 35.1 > 393.16 0.199 67.9 > 403.16 0.270 100.4 > 413.16 0.362 132.9 > 423.16 0.476 164.8 > 433.16 0.618 196.3 > 443.16 0.792 227.6 > 453.16 1.003 258.9 > 463.16 1.256 289.7 > 473.16 1.555 320.5 > 483.16 1.908 351.1 > 493.16 2.320 381.6 > 503.16 2.798 412.2 > 513.16 3.348 442.8 > 523.16 3.977 473.5 > 533.16 4.693 504.4 > 543.16 5.504 535.4 > 553.16 6.418 566.8 > 563.16 7.443 598.7 > 573.16 8.589 631.1 > 583.16 9.866 664.3 > 593.16 11.286 698.5 > 603.16 12.860 733.9 > > [1] http://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185&TUnit=K&PUnit=MPa&DUnit=kg%2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm&Type=SatP&RefState=DEF&Action=Page > _______________________________________________ > aRocket@exrocketry.net > http://exrocketry.net/mailman/listinfo/arocket > "At that time [1909] the chief engineer was almost always the chief test pilot as well. That had the fortunate result of eliminating poor engineering early in aviation" (Igor Sikorsky) -- Bruno Berger Swiss Propulsion Laboratory E-Mail: bruno.berger@spl.ch (HTML-Mails will be dropped!) PGP: http://www.spl.ch/contact/Bruno.Berger.asc WWW: http://www.spl.ch HAM: HB9RSU _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket Jeez I'd like to help but this 'd cost me more than a working day to even explore the matter from my present knowledge. So I can imagine you do not get response. Equations scare the hell out of most people; not that they are that complicated (lying, they can really be as there are so many) but that it takes patience & time starting from the beginnings. Hey guys, why not we have an statue cast for those who guide us. I'll contribute. For AR's history 's sake. Names nor required but wellcome to mention. jd -----Oorspronkelijk bericht----- Van: arocket-bounces@exrocketry.net [mailto:arocket-bounces@exrocketry.net] Namens Evan Daniel Verzonden: woensdag 4 juli 2012 19:02 Aan: Arocket List Onderwerp: [AR] Steam rocket Isp analysis So, since the original thread I started has produced very few useful responses, I've been working through the theory on my own, and generated a table for theoretical Isp vs starting conditions. I used the NIST webbook for all my data [1]. I assumed the water starts at saturation, and expands isentropically through the nozzle. I assumed 1 atm exit conditions. I then found the liquid fraction that produced equivalent entropy in the exhaust. I then computed exhaust enthalpy, subtracted initial enthalpy, and assumed that the excess is kinetic energy, and from there found Isp. (Worked example at 250C starting conditions and full table follow.) Does this look correct? The final number is at least in the right range. Did I get usage of enthalpy vs internal energy right? I haven't yet tried plugging in all the formulas to get C* and Ae/At yet. I'll try to add that to the table shortly. Evan Daniel Working an example at 250C: Initial entropy = 2.793 J/(g*K) Exit liquid entropy = 1.3073 J/(g*K) Exit vapor entropy = 7.354 J/(g*K) liquid fraction = (initial entropy - vapor entropy) / (liquid entropy - vapor entropy) = 0.754 Initial enthalpy = 1086 J/g Exit liquid enthalpy = 416 J/g Exit vapor enthalpy = 2675 J/g Exit total enthalpy = 973 J/g enthalpy difference = 112 J/g Ve = 474 m/s Temperature (K) Pressure (MPa) Ve (m/s) 383.16 0.143 35.1 393.16 0.199 67.9 403.16 0.270 100.4 413.16 0.362 132.9 423.16 0.476 164.8 433.16 0.618 196.3 443.16 0.792 227.6 453.16 1.003 258.9 463.16 1.256 289.7 473.16 1.555 320.5 483.16 1.908 351.1 493.16 2.320 381.6 503.16 2.798 412.2 513.16 3.348 442.8 523.16 3.977 473.5 533.16 4.693 504.4 543.16 5.504 535.4 553.16 6.418 566.8 563.16 7.443 598.7 573.16 8.589 631.1 583.16 9.866 664.3 593.16 11.286 698.5 603.16 12.860 733.9 [1] http://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185&TUnit=K&PUnit=MPa&DUnit=kg %2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm&Type=SatP&RefStat e=DEF&Action=Page _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket Never did like all that fancy math, and all though I know it's needed in a world where we send people to the moon It's a lot simpler and a heck of a lot more fun to go do it by hand. Ya just got to remember to keep your head away from the front, your hands away from the back, know what the vessel is rated at, grab a camera and do a "myth buster" approach till you get all the numbers you need or want. It has worked for me for 15 years. Sure I'll never get to the moon, but then, I never wanted to either. On the other hand it's good to know that there are people that can crunch the numbers for those time that no mater how I do it I can't get it right. Robert At 08:12 PM 04/07/2012 +0200, you wrote: >Jeez I'd like to help but this 'd cost me more than a working day to even >explore the matter from my present knowledge. So I can imagine you do not >get response. >Equations scare the hell out of most people; not that they are that >complicated (lying, they can really be as there are so many) but that it >takes patience & time starting from the beginnings. > >Hey guys, why not we have an statue cast for those who guide us. I'll >contribute. For AR's history 's sake. Names nor required but wellcome to >mention. > >jd > >-----Oorspronkelijk bericht----- >Van: arocket-bounces@exrocketry.net [mailto:arocket-bounces@exrocketry.net] >Namens Evan Daniel >Verzonden: woensdag 4 juli 2012 19:02 >Aan: Arocket List >Onderwerp: [AR] Steam rocket Isp analysis > >So, since the original thread I started has produced very few useful >responses, I've been working through the theory on my own, and >generated a table for theoretical Isp vs starting conditions. > >I used the NIST webbook for all my data [1]. I assumed the water >starts at saturation, and expands isentropically through the nozzle. I >assumed 1 atm exit conditions. I then found the liquid fraction that >produced equivalent entropy in the exhaust. I then computed exhaust >enthalpy, subtracted initial enthalpy, and assumed that the excess is >kinetic energy, and from there found Isp. (Worked example at 250C >starting conditions and full table follow.) > >Does this look correct? The final number is at least in the right >range. Did I get usage of enthalpy vs internal energy right? > >I haven't yet tried plugging in all the formulas to get C* and Ae/At >yet. I'll try to add that to the table shortly. > >Evan Daniel > > >Working an example at 250C: > >Initial entropy = 2.793 J/(g*K) >Exit liquid entropy = 1.3073 J/(g*K) >Exit vapor entropy = 7.354 J/(g*K) > >liquid fraction = (initial entropy - vapor entropy) / (liquid entropy >- vapor entropy) = 0.754 > >Initial enthalpy = 1086 J/g >Exit liquid enthalpy = 416 J/g >Exit vapor enthalpy = 2675 J/g >Exit total enthalpy = 973 J/g > >enthalpy difference = 112 J/g >Ve = 474 m/s > > >Temperature (K) Pressure (MPa) Ve (m/s) >383.16 0.143 35.1 >393.16 0.199 67.9 >403.16 0.270 100.4 >413.16 0.362 132.9 >423.16 0.476 164.8 >433.16 0.618 196.3 >443.16 0.792 227.6 >453.16 1.003 258.9 >463.16 1.256 289.7 >473.16 1.555 320.5 >483.16 1.908 351.1 >493.16 2.320 381.6 >503.16 2.798 412.2 >513.16 3.348 442.8 >523.16 3.977 473.5 >533.16 4.693 504.4 >543.16 5.504 535.4 >553.16 6.418 566.8 >563.16 7.443 598.7 >573.16 8.589 631.1 >583.16 9.866 664.3 >593.16 11.286 698.5 >603.16 12.860 733.9 > >[1] >http://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185&TUnit=K&PUnit=MPa&DUnit=kg >%2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm&Type=SatP&RefStat >e=DEF&Action=Page >_______________________________________________ >aRocket@exrocketry.net >http://exrocketry.net/mailman/listinfo/arocket > >_______________________________________________ >aRocket@exrocketry.net >http://exrocketry.net/mailman/listinfo/arocket > _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket Looks correct, but FWIW you can get higher Isp by taking just the steam from the top of the tank; doing that roughly doubles the Isp. Trouble is, you end up with a tank full of hot water at the end of the burn, though ;-) but it roughly doubles the Isp. You lose overall performance (delta-v), but gain Isp. That was what I worked out eventually when I was trying to work out the best way to get water off the moon using a rotating tether; you just have the tip come down (say) a few kilometres (or less, the lower the better) above the surface and the steam vehicle jumps up to grab it. You really want maximum Isp then, not maximum delta-v, because it maximises the propellant delivery (i.e. delivery of the remaining water), but that's an unusual (and somewhat amusing) case. On 4 July 2012 18:01, Evan Daniel wrote: So, since the original thread I started has produced very few useful responses, I've been working through the theory on my own, and generated a table for theoretical Isp vs starting conditions. I used the NIST webbook for all my data [1]. I assumed the water starts at saturation, and expands isentropically through the nozzle. I assumed 1 atm exit conditions. I then found the liquid fraction that produced equivalent entropy in the exhaust. I then computed exhaust enthalpy, subtracted initial enthalpy, and assumed that the excess is kinetic energy, and from there found Isp. (Worked example at 250C starting conditions and full table follow.) Does this look correct? The final number is at least in the right range. Did I get usage of enthalpy vs internal energy right? I haven't yet tried plugging in all the formulas to get C* and Ae/At yet. I'll try to add that to the table shortly. Evan Daniel Working an example at 250C: Initial entropy = 2.793 J/(g*K) Exit liquid entropy = 1.3073 J/(g*K) Exit vapor entropy = 7.354 J/(g*K) liquid fraction = (initial entropy - vapor entropy) / (liquid entropy - vapor entropy) = 0.754 Initial enthalpy = 1086 J/g Exit liquid enthalpy = 416 J/g Exit vapor enthalpy = 2675 J/g Exit total enthalpy = 973 J/g enthalpy difference = 112 J/g Ve = 474 m/s Temperature (K) Pressure (MPa) Ve (m/s) 383.16 0.143 35.1 393.16 0.199 67.9 403.16 0.270 100.4 413.16 0.362 132.9 423.16 0.476 164.8 433.16 0.618 196.3 443.16 0.792 227.6 453.16 1.003 258.9 463.16 1.256 289.7 473.16 1.555 320.5 483.16 1.908 351.1 493.16 2.320 381.6 503.16 2.798 412.2 513.16 3.348 442.8 523.16 3.977 473.5 533.16 4.693 504.4 543.16 5.504 535.4 553.16 6.418 566.8 563.16 7.443 598.7 573.16 8.589 631.1 583.16 9.866 664.3 593.16 11.286 698.5 603.16 12.860 733.9 [1] http://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185&TUnit=K&PUnit=MPa&DUnit=kg%2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm&Type=SatP&RefState=DEF&Action=Page _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket -- -Ian Woollard -------------------------------------------------------------------------------- _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On Jul 4, 2012, at 2:41 PM, Ian Woollard wrote: > Looks correct, but FWIW you can get higher Isp by taking just the steam from the top of the tank; doing that roughly doubles the Isp. > > Trouble is, you end up with a tank full of hot water at the end of the burn, though ;-) but it roughly doubles the Isp. You lose overall performance (delta-v), but gain Isp. The tank top approach is documented somewhere. You can even hybrid and have a internal pipe to the top, with throat and nozzle at the bottom of that pipe. Thrust max while water initially in the pipe is ejected in mixed form then Isp max as steam from the rest of the water comes out. I think with enough superheat you may be able to boil everything away but not sure about the optimum point at all. This is very sensitive to initial pressure / temperature conditions. Density dropping and tanks getting heavier... Titanium tanks may be good choices. George William Herbert Sent from my iPhone _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On 4 July 2012 23:59, George Herbert wrote: I think with enough superheat you may be able to boil everything away but not sure about the optimum point at all. This is very sensitive to initial pressure / temperature conditions. Density dropping and tanks getting heavier... I believe that to boil everything away you have to have the water supercritical, but for water that's a really, really high pressure, and so I doubt it's optimal for most vehicles due to the tankage mass, although it looked good for any engines on the tether. George William Herbert Sent from my iPhone -- -Ian Woollard -------------------------------------------------------------------------------- _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket Ian Woollard wrote: > I believe that to boil everything away you have to have the water > supercritical, but for water that's a really, really high pressure, and so > I doubt it's optimal for most vehicles due to the tankage mass, although it > looked good for any engines on the tether. Well, with a controlled quantity of water, you could arrange things so that you reached your design pressure at superheated conditions (but not in the supercritical range) - the relevant quantity of water might be pretty small (like the last bit in a tank of butane or refrigerant, which is superheated gas at ambient temperature once the liquid is all evaporated) - which would reduce the total impulse, but it would at least (with enough superheat) have an all-gas exhaust stream... that's the reason that steam turbines often use superheated steam (with heat added between the boiler and the turbine nozzle): to reduce condensation to liquid as it expands in the turbine. For a rocket, it's probably a lot more effective to use more water, such that the tank is mostly full of hot water with the ullage volume containing saturated vapor - like a new, filled tank of butane or refrigerant. Historical note: Vanguard's second stage roll control used propane, electrically heated to 120 deg.F, as the working fluid for the RCS jets during powered flight. (During second stage coasting flight, the roll control system switched over to using residual helium from the fuel tank after main engine shutdown, while the pitch/yaw RCS jets - switched into the attitude control system - used residual helium from the oxidizer tank.) -dave w _______________________________________________ aRocket@exrocketry.net http://exrocketry.net/mailman/listinfo/arocket On 5 July 2012 04:28, David Weinshenker wrote: Ian Woollard wrote: > I believe that to boil everything away you have to have the water > supercritical, but for water that's a really, really high pressure, and so > I doubt it's optimal for most vehicles due to the tankage mass, although it > looked good for any engines on the tether. Well, with a controlled quantity of water, you could arrange things so that you reached your design pressure at superheated conditions (but not in the supercritical range) - the relevant quantity of water might be pretty small I thought about that, but I figured that, at minimum, there needs to be enough thermal energy in the tank initially to overcome the latent heat of vapourisation of all the water (which as it happens is very large for water). I found when I did the calculations that it looks like if you're not supercritical, then there didn't seem to be enough total thermal energy in the tank to boil off all the water. In fact thinking about it, after doing the calculation that's presumably is basically the definition of supercritical, you have enough energy in the fluid that the molecules can't stick together under any circumstances. The other way to do it would be to have your liquid already boiled, but then you're storing a gas rather than a liquid, that's not going to go well. (like the last bit in a tank of butane or refrigerant, which is superheated gas at ambient temperature once the liquid is all evaporated) I think that's slightly different. Extracting butane is done relatively slowly so that external heat has a chance to leak in and boil the butane off. Rockets don't usually have the luxury of hanging around absorbing heat like that (although for orbital use, solar thermal is a good plan, and that's basically how that works). - which would reduce the total impulse, but it would at least (with enough superheat) have an all-gas exhaust stream...that's the reason that steam turbines often use superheated steam (with heat added between the boiler and the turbine nozzle): to reduce condensation to liquid as it expands in the turbine. Yes, if you have a superheater, which again is another source of energy. But would you be better off superheating the steam or simply using the burner as a biprop? I'm pretty sure biprop. (Unless the source of superheat is nuclear I suppose). -- -Ian Woollard