Subject: Space-tech Digest #134 Contents: P2-E pressurization, misc. P2 comments (9 msgs) ------------------------------------------------------------ Date: 14 Nov 1992 03:20:28 -0500 (EST) From: "GORDON D. PUSCH" Subject: Re: Frozen Ozone (was: P2-E Pressurization) Dominic Herity writes: > What kind of specific impulse are we talking about, when used with hydrogen ? > Are there any other show stoppers, apart from detonation ? The sysop has gone home for the weekend, so I can't have that tape mounted, and the original article I got the reference from is somewhere in my hardcopy archives (i.e., packed for the move...). The reference was, unfortunately, an _Analog_ ``Fact'' article, ca. 1988, entitled ``ozone rockets,'' but I *think* it cited articles from reputable journals... My vague recollection is that the I_sp of LH2/LO3 was about 10--15% better than LH2/LF2; dunno how that compares w/ LH2/LO2... Show-stoppers might be: 1) reliably pumping ``slush qua slush'' is probably harder than pumping liquid; 2) slush might be awkward to inject into the engine (I suppose one could heat it until the LN2 boils off and the slush LO3 melts, but I bet that would be *really* touchy to get right...); 3) it might take so much LN2 to slush the stuff that the I_sp is degraded to the point it isn't worth doing... I don't know enough about *ANY* of these areas to make quantitative guesses... Regarding the detonation problem: frankly, I don't know whether anyone's ever had *enough* of the frozen stuff in one spot to find out. *Liquid*-O3 is pretty easy to make; in fact, it's what's left after LOX mostly boils away (a deep blue- purple liquid). One can make it in larger quantities by zapping LOX with UV (the kind that's got a *shorter* wavelength than UV-B, I forget whether it's called UV-A or UV-C). I guess one could freeze some w/ LN2, then hit it w/ a hammer... (don't try this at home, kids! ;-). Oh, yes: 4) even at LN2-temp, the stuff slowly reverts from O3 to O2, so it's not ``storable'' even as a cryogen. At *LHe*-temp, the decomposition mostly stops, of course, and the loss in I_sp would be less... but who wants to fool w/ LHe??? The amount of tank-insulation required to keep the stuff from boiling off instanter would cut your payload to zilch... :-/ Gordon D. Pusch ------------------------------ Date: Mon, 16 Nov 92 16:49:37 +0000 From: Dominic Herity Subject: Re: Frozen Ozone (was: P2-E Pressurization) To: space-tech@cs.cmu.edu > My vague recollection is that the I_sp of LH2/LO3 was about 10--15% better > than LH2/LF2; dunno how that compares w/ LH2/LO2... After I posted my question, I decided to work it out for myself. The CRC handbook gives heats of formation for H2O and O3. From these, I calculated that H2/O2 gives an Isp of about 5100m/s and LH2/O3 gives 5600m/s. Figures accurate to two digits - sorry, I didn't bring the envelope with me. Since a SSME actually delivers 4400m/s in vacuo, a hypothetical LH2/O3 engine might be expected to deliver about 4830m/s. The respective mass ratios to reach 9000m/s (LEO plus some allowance for aerodynamic, gravitational and other losses) would be 7.73 for LH2/O2 amd 6.44 for LH2/O3. This means that ozone can reduce fuel mass by 13% at most. Now, where did I put that hammer? :-) Best Regards Dominic Herity ------------------------------ Date: Sat, 14 Nov 92 20:58:35 -0500 From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: More on tank pressurization Bruce has the helium being heated to 300 K before injection into the propellant tanks. Is this really necessary? It would be simpler to just let the heat of the propellant warm the helium to near room temperature. Some figures: Heat of vaporization of 4He: 5.7 kcal/kg. Specific heat of gaseous 4He at constant volume: .75 kcal/kg K delta-T: about 300 K Density of helium at 273 K, 1 atm: .1784 kg/m^3 Density at 75 atm, 300 K: 12.2 kg/m^3 Heat energy needed to warm liquid helium to 300 K, 75 atm, per volume of final state: 2.82e6 cal/m^3 I don't have the figures on propane, so I'll use RP-1. Density of RP-1: 800 kg/m^3 Specific heat (at 298 K): .45 kcal/kg K delta-T of RP-1 needed to provide 2.82e6 cal/m^3: 8 K This is a rather small temperature change. This suggests that by mildly preheating the fuel, we can provide all the energy needed to warm the pressurant gas simply by bubbling it into the tank, or perhaps by using a counterflow heat exchanger on the propellant feed line. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ Date: Sun, 15 Nov 92 17:08 PST To: space-tech@cs.cmu.edu Subject: P2-E pressurization via propellant heat transfer From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) Paul Deitz suggests warming the helium needed for tank pressurization by two general methods involving the transfer of heat from propellants. I have previously considered both, and have had my misgivings about them (although I am always willing to listen to arguments). At first, it may seem that vaporizing and warming helium by transfer of heat from propane or peroxide is using "free" heat. However, the transfer cools the propellants, reducing their performance very slightly. In this respect, propellants from separate tanks used in a gas generator aren't really "wasted", as they allow us to avoid stealing energy from the main propellants. Warming propellants prior to launch is not useful, as the lower density and thus poorer mass fraction kills any increase in performance (mind you, I have not done a rigorous analysis of this). Any scheme that transfers heat from propellants to the helium suffers from the fact that as the helium temperature rises and becomes closer to the temperature of the propellant, the heat transfer rate between propellant and helium becomes smaller. It would take an infinitely large heat exchanger to get the helium to the propellant temperature. In practice, we will probably have to be satisfied with helium which is somewhat cooler than the propellant. We certainly will never be able to generate helium at a higher temperature than the propellant. 1) Method 1: Bubbling liquid helium into the oxidizer or fuel, and letting the helium warm by contact. a) The helium requires very little heat input in order to vaporize - most of the needed heat has to be added to the resultant cold gas in order to get it temperature up. This means that injected liquid helium will almost immediately vaporize, and shoot to the top of the tank in bubbles which may not be in good thermal contact with the propellant. I suspect that it will be difficult to get the helium to warm to anywhere near the propellant temperature. Warming will be a particularly acute problem in the later stages of a burn, when the propellant level is low and the acceleration is at several G, lowering contact time. b) The propellant pressurization scheme can't be tested except in large scale tests at pressure, with the actual propellants in reasonable sized tanks. Acceleration effects can't reasonably be tested at all. This is in contrast to a gas generator scheme, where the gas generator can be tested for its ability to generate high pressure room temperature helium in complete isolation from any propellant tanks. In my opinion, to lower development costs, it is critical to design the vehicle as a series of subsystems which can be tested in isolation. c) Because there may be a lag between when the liquid helium is injected and when it is warmed, pressure control may be difficult. This is accentuated by the fact that the relationship between injected helium and the resulting volume of pressurant gas is probably variable throughout the flight - at the beginning, the helium has to bubble through a lot of propellant, and acceleration is moderate leading to longer transit times and warmer helium. At the end of the flight, propellant levels and low and acceleration is high, giving low contact times and colder helium. 2) Method 2: Warming the helium in a heat exchanger, with the heat source being flowing propellant. I like this scheme much better. It is more amenable to testing in isolation, using actual propellant or a thermal equivalent. However the heat exchanger will be markedly larger than that in the gas generator using a heat exchanger, as the temperature differential is so much smaller. In the proposed generator, there will not even be a heat exchanger - energy will be transferred by mixing the hot gases from a peroxide/propane reaction with liquid helium. I think that it is not worth the effort at this stage worrying too much about the exact method of turning liquid helium into gaseous helium. Probably more than one scheme can be made to work. The only minor thing which is disagreeable about using helium for pressurization is the cost, and even that isn't so bad. I believe that someone calculated previously for me that helium was probably worth about $30,000 per ton, or about $300,000 for a medium size P2-E stage. This is insignificant to other likely costs. However, it hurts me to throw away a gas for which there is a limited supply. One method I didn't mention for saving helium (in addition to using hot helium) is to use lower tank pressures later in the flight. In particular, the P2-E booster could do with a lower terminal acceleration. This could be done by providing helium at full pressure for only half or two thirds of the burn, then shutting off the helium supply and letting the tanks blow down to a lower pressure. This is of course dependant on developing an injector and combusion chamber that are not upset by variations in chamber pressure. -- Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca ------------------------------ Date: Mon, 16 Nov 92 16:00:00 -0500 From: dietz@cs.rochester.edu Subject: Re: P2-E pressurization via propellant heat transfer My gut feeling about bubbling helium through the propellant is that the surface area would be so large that the helium would be warmed to very near the temperature of the propellant within a short distance. I would be more concerned about localized freezing of the propellants. (Didn't we talk about all this last March? Sorry for the repetition if so.) Since Bruce finds that propane is a better fuel than RP-1, due in part to higher Isp, I wonder if it would be still better to go to a fuel with a higher hydrogen/carbon ratio. Methane would have a higher H/C ratio, but does not liquify at room temperature. However, propane dissolves considerable methane. At 7.5 MPa and 298K, liquid propane saturated with methane contains .574 moles of methane per mole of propane. This gives a H/C ratio of ~2.88, vs. 2.66 for propane and ~2 for RP-1. At 9.07 MPa, .757 moles of methane dissolve per mole of propane, for an H/C ratio of ~2.93. This saturated liquid could still be pressurized with helium. Methane would want to evaporate as the tank was drawn down. The same happens with propane, I'd imagine -- at 7.5 MPa and 300 K about 4% of the propane could get left in the tank as vapor. Perhaps kinetics would prevent the vapor in the head space from coming into equilibrium with the fuel. BTW, it looks like the mole fraction of hydrogen or helium that would dissolve in propane at 298 K and 7.5 MPa are about 6% and 3%, respectively. Paul ------------------------------ Date: Tue, 17 Nov 92 08:14 PST To: space-tech@cs.cmu.edu Subject: P2-E misc. From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) Ted Anderson writes: > I want to lend whatever encouragement I can that you are doing the right > thing with your emphasis on testability in this design. An adaquate > mechanism that can be easily tested is much preferable to a "better" > mechanism that can't. This is very important for practical, affordable engineering. I keep remembering the hassles that they had testing the SSME. Everything had to be working simultaneously for any individual item to be tested. This is real hard, when everything is in a prototype stage. > I am still confused about the relationship between this proposal and the > one George Herbert was working on. Is it true that these are different > design efforts? You guys aren't working together? The proposals are completely independent, but are both addressing the same problem (low cost, dumb approaches to boosting moderate to heavy payloads). I correspond with George on specific points, but we don't work on each other's designs. I think each design should be evaluated separately - I don't want to see them put in competition with one another. > > Keep up the good work, > Ted Anderson Thanks. There is very little payoff to pursuing something like this unless there is some positive feedback. -- Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca ------------------------------ Date: Tue, 17 Nov 92 08:42 PST To: space-tech@cs.cmu.edu Subject: Alternate P2-E fuels From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) Paul Deitz writes: > My gut feeling about bubbling helium through the propellant is that > the surface area would be so large that the helium would be warmed to > very near the temperature of the propellant within a short distance. I guess we simply differ. Surface area will be large, but I worry about contact time. When thinking of helium bubbling through a liquid, the speed with which bubbles rise will be a function of the viscosity of the liquid. Liquid propane has very low viscosity in comparison with common liquids such as water. > Since Bruce finds that propane is a better fuel than RP-1, due in part > to higher Isp, I wonder if it would be still better to go to a fuel > with a higher hydrogen/carbon ratio. Not necessarily. Heading from propane towards ethane and methane raises the hydrogen/carbon ratio, and using methane rather than propane gives an Isp increase of about 2%. This rather undramatic increase is because the heat of formation per carbon atom gets worse in going towards methane (more tight bonding). Worse, the density drops very sharply requiring larger tanks. The tank issue isn't totally reversed by the increasing oxidizer to fuel mixture ratio. Very roughly, in the P2-E the effects seem to cancel out and propane is not any better performing than RP-1. I chose it because of its other virtues (cost, low viscosity, etc.) > Methane would have a higher H/C ratio, but does not liquify at room > temperature. However, propane dissolves considerable methane. At 7.5 > MPa and 298K, liquid propane saturated with methane contains .574 > moles of methane per mole of propane. This gives a H/C ratio of ~2.88, > vs. 2.66 for propane and ~2 for RP-1. At 9.07 MPa, .757 moles of methane > dissolve per mole of propane, for an H/C ratio of ~2.93. Interesting - I would like to know how you do the calculations for this. The important thing is the density - can you calculate this as well? > This saturated liquid could still be pressurized with helium. Methane > would want to evaporate as the tank was drawn down. The same happens > with propane, I'd imagine -- at 7.5 MPa and 300 K about 4% of the > propane could get left in the tank as vapor. Perhaps kinetics would > prevent the vapor in the head space from coming into equilibrium with > the fuel. The real virtue of this approach might be to eliminate the helium for the fuel tank, and have a self pressurizing fuel. The tank would be loaded with a partial load of propane, then pressurized with methane bubbled through the propane. Can you calculate whether such a mix would maintain a head space pressure of 7.5 MPa as the tank blows down? Head space would be mostly methane, with some propane. This is not as attractive a pressurizing gas as helium (considering mass), but is better than nitrogen. In any event, the fuel tank is smaller than the oxidizer tank by quite a bit and the performance hit might be bearable. > BTW, it looks like the mole fraction of hydrogen or helium that would > dissolve in propane at 298 K and 7.5 MPa are about 6% and 3%, respectively. Interesting. Again, how do you calculate this? This suggests that the amount of gas actually dissolving is negligible. -- Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca ------------------------------ Date: Tue, 17 Nov 92 19:13:46 EST From: John Roberts Disclaimer: Opinions expressed are those of the sender and do not reflect NIST policy or agreement. To: space-tech@cs.cmu.edu Subject: Re: Propane viscosity >Liquid propane has very low viscosity in comparison with common >liquids such as water. Tilting a small metal cylinder of propane, I get the impression that it's pretty "slushy". (Of course, that's at room temperature, liquified by high pressure. Liquified butane in a transparent lighter appears to be no more viscous than water.) John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ End of Space-tech Digest #134 *******************