Subject: Space-tech Digest #144 Contents: Hydrocarbon/peroxide SSTO (14 msgs) ------------------------------------------------------------ Date: Thu, 4 Feb 93 17:24 PST To: space-tech@cs.cmu.edu Subject: Hydrocarbon/peroxide SSTO From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) I enjoyed meeting Henry at the conference. He is right that I had some concerns about the design by Mitch Burnside Clapp (Burnside Clapp is a double-barreled Australian name) and Max Hunter for a hydrocarbon/peroxide SSTO. Mitch incidentally will be one of the flight controllers for the upcoming DC-X tests. Unfortunately, I had to leave the conference before the presentation by Mitch. If Henry attended the presentation (rather than one of the other simultaneous sessions), he might be able to fill in some details. I talked with Mitch, and in addition to a draft of a paper (which has little specific information about the proposed SSTO), he gave me a copy of a spreadsheet used for the calculations. The proposed vehicle, as described on the spreadsheet, uses spherical Kevlar propellant tanks, and has a gross mass of 280 metric tons, with a mass fraction of 0.967. The payload to orbit is 4.54 metric tons. Using the same assumptions as he made, I was able to put together a spreadsheet which predicted a payload of 4.85 tons (the difference in payload is probably due to some ambiguities in what mixture ratio he was assuming for the propellants). Mitch admits that the spreadsheet is only a first effort, and "needs a couple of rounds of refinement". At the conference, I pressed him on how he was planning to pressurize the propellant, as this has proven to be the single most difficult issue for my P2-E concept vehicle. He stated that the pressurizing gas would be derived by "decomposing some of the peroxide". I pointed out that this would produce superheated steam and oxygen at about 1200 K, and that this would melt his Kevlar tanks, explode his fuel and probably touch off decomposition of the bulk peroxide. He then said that the decomposition products would have to be cooled somehow. I pointed out that this would mean that all the water in the decomposition products (2/3 of the moles of material produced) would then condense. He said he guessed he would have to look more closely at the problems of pressurization. Subsequently, I looked at the spreadsheet and found no allowance for the mass of pressurization gas in the vehicle. I checked by phone with him, and he stated that he was assuming the pressurization gas mass to be in a "other mass" allowance of 4.6 metric tons in the spreadsheet, which has to account for all vehicle mass other than the tanks (2.17 tons) and the engines (2.19 tons). The 4.6 tons has to therefore allow for power, avionics, reaction control system, aerodynamic flaps (if used), landing gear, and landing propellant. However, the absolute minimum mass of gas needed for pressurization in his vehicle is approximately 2.1 tons, with probably another 0.9 tons needed for tanks, pumps, and gas generation equipment (this assumes helium pressurization at full pressure for half the flight followed by tank blowdown, storage of liquid helium in an ambient pressure tank, and a pump system and gas generator system powered by peroxide and propane). I don't think that the needed mass can be crammed within the 4.6 ton allowance. Pressurization by decomposed peroxide isn't even in the cards - to pressurize with warm (50 C) oxygen requires 16.6 tons of oxygen, which requires 45.7 tons of peroxide to be decomposed to water and oxygen, followed by cooling and condensing about 30 tons of superheated steam. I also have some other disagreements with how the vehicle was modeled. The principal disagreement I had was with the tensile strength of the Kevlar tank, and the density of Kevlar. The tank either has to be made of metal, then wound with Kevlar, or made of a Kevlar/resin composite material. To my knowledge, Kevlar is only available as fibers, so the resulting tank, however made, does not have the tensile strength of the fibers themselves (in a spherical tank, stress is in all directions necessitating fibers to run in all directions - an individual bundle of fibers has zero tensile strength at right angles to the fiber direction). Mitch and I disagree on what numbers to use: if anyone has data on the effective tensile strength and wall density of composite or fiber-wound tanks, I would appreciate it. Taking into account helium pressurization mass, my numbers rather than Mitch's for Kevlar tanks, and a couple or other items which he did not account for (propellant residuals, ullage space in tanks), I calculate a payload of - 7.3 tons (that is, minus 7.3) for the vehicle. I have written a letter to Mitch and Max outlining my problems with the design. I should be fair and point out that for some of the disagreements that I have with their design, it is possible that they are right and I am wrong. I wait with interest for an answer from them. Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca ------------------------------ Date: Fri, 5 Feb 93 15:24 PST To: space-tech@cs.cmu.edu Subject: Re: Hydrocarbon/peroxide SSTO From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) I was concerned with the assumptions made by Mitch Burnside Clapp for the composite materials used in the design of the pressurized propellant tanks for his storable propellant SSTO. I asked for numbers representing state-of-the art. Dani Eder replied: > I have some data I obtained from Aerojet on their "Advanced Liquid > Axial Stage" pressurant tanks. The ALAS is a technology development > project developing a stage suitable for SDI interceptor work. > It represents the state-of-the art in a number of areas. > > The tank is carbon fiber over a thin aluminum liner. The numbers are: > > Pressurant: He > Storage Pressure 10,000 psi > Burst Pressure 15,000 psi > Figure of Merit 1.4x10^6 in (burst pressure x volume / tank wt.) > Volume 40 cubic inches > Weight 0.45 lb > > Propellant Tank: Carbon/Epoxy overwrapped, 6061 Aluminum liner (6 mils) > > Propellants: ClF5/N2H4 > Pressure 1550 psi > Figure of Merit 1.0x10^6 in > Oxidizer Tank: > Volume 96 in^3 > Weight .22 lb > Length 9 in > Diam 4.75 in > Fuel Tank: > Volume 61 in^3 > Weight 0.17 lb > Length 9 in > Diam 3.75 in The figure of merit given (volume in cubic inches * pressure in psi / mass in pounds) is 1.0x10^6 and 1.4x10^6 for two advanced tank construction methods. A little playing with my spreadsheet indicates that the figure of merit calculated in this way is totally independent of tank size (for the same construction material and pressure, the figure of merit is the same no matter what the volume). The tanks in Mitch Burnside Clapp's design have a figure of merit of at least 6.7x10^6, and possibly higher (there is some ambiguity as to where any safety factors are applied in his design, and I don't know whether the tensile strength of the wall material he quotes is the ultimate strength, or the working strength after the application of safety factors). His tanks are spherical rather than cylindrical giving them a slight advantage, but not enough I think to account for the very high figure of merit. This tends to confirm my feeling that he is too optimistic in his figures for the properties of composite tank walls. Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca ------------------------------ To: Bruce Dunn Cc: space-tech@cs.cmu.edu, gwh@soda.berkeley.edu Subject: Re: Hydrocarbon/peroxide SSTO Date: Fri, 05 Feb 93 19:26:40 -0800 From: George William Herbert In earlier mail I sent from work regarding the tankage fraction assumptions used by Mich Burnside Clapp, I indicated that something sounded wrong about those numbers and that I'd send more data when I got home to my reference materials. I'm home, and here's what my materials have to say... Kevlar 49, 90 degree woven bidirectional layup (has max strength along x, y axies, much lower along intermediate angles) in Epoxy matrix: Ultimate Tensile Strength: 517 MPa Density: 1.33 +-45 degree woven XAS Carbon fiber, (closer to even strength in all axies) in Epoxy: UTS: 240 MPa Density: 1.53 +-90 degree woven XAS, Epoxy (again, strong only along x, y axies) UTS: 625 MPa Density: 1.53 Unidirectional XAS, Epoxy (strength only along one axis) UTS: 2040 MPa Density: 1.57 Unidirectional Kevlar UTS: 1379 MPa Density: 1.38 The fibers involved, without a matrix, have UTS of roughly 2650 MPa (Kevlar) and 3850 MPa (Carbon XAS). There are slightly stronger fibers out there, but their performance in an actual composite layup is similar in bi- and multi-directional usage. Their primary usages are in uni-directional layups. As you can see, there is a factor of 7 reduction in strength from unidirectional fiber to even bidirectional composite layup; for a real spherical tank, I would guess that a limiting stress would be on the order of 165 MPa. [For the metric-impaired (Including Me, most of the time): 1 MPa = 145 PSI in other words, Carbon XAS has a fiber strength of about 560,000 PSI but in a non-directional composite it's closer to 35,000 PSI (and you have to have a safety margin on top of that...)]. _Design with Advanced Composite Materials_, Ed. Leslie N Phillips, Springer-Verlag, New York (1989) pp. 5, 18,21, esp. 73 (table) -george william herbert gwh@soda.berkeley.edu gwh@lurnix.com ------------------------------ To: Bruce Dunn Cc: space-tech@cs.cmu.edu, gwh@lurnix.COM Subject: Re: Hydrocarbon/peroxide SSTO Date: Fri, 05 Feb 93 17:00:03 -0800 From: gwh@lurnix.COM >I was concerned with the assumptions made by Mitch Burnside Clapp for the >composite materials used in the design of the pressurized propellant tanks >for his storable propellant SSTO. I asked for numbers representing >state-of-the art. > [dani eder's data...] >The figure of merit given (volume in cubic inches * pressure in psi / mass in >pounds) is 1.0x10^6 and 1.4x10^6 for two advanced tank construction methods. >A little playing with my spreadsheet indicates that the figure of merit >calculated in this way is totally independent of tank size (for the same >construction material and pressure, the figure of merit is the same no matter >what the volume). You're right. Tankage that's strength-limited scales linearly based on the volume; volume scales with the cube of linear dimention; tank wall area with the square and thickness linearly, so volume and total tank wall volume scale equally. This doesn't hold when stiffness or secondary loads become the limit loads, but in pressurized-tank vehicles primary pressurization loads are the limiting load in all cases I've looked at. >The tanks in Mitch Burnside Clapp's design have a figure of merit of at least >6.7x10^6, and possibly higher (there is some ambiguity as to where any safety >factors are applied in his design, and I don't know whether the tensile >strength of the wall material he quotes is the ultimate strength, or the >working strength after the application of safety factors). His tanks are >spherical rather than cylindrical giving them a slight advantage, but not >enough I think to account for the very high figure of merit. This tends to >confirm my feeling that he is too optimistic in his figures for the >properties of composite tank walls. Methinks he goofed. Spherical will gain you somewhat; the tankage thickness is lower (spherical containers have half the stress as the hoop stress in cylindrical containers for the same internal pressure and radius). However, the effeciency factor, per volume of fuel, is around 1.4 to 1.6 depending on geometrical factors beyond the quick address of a netmail posting, i.e. the figure of merit should go no further than say 2.0-2.2x10^6. The factor of three discrepancy indicates to me that either a) there's a hole in Bruces' and my' quick analysies somewhere or b) Mr. Clapp is in error. I'll be posting detailed information from composites databooks later tonight to verify this. -george ------------------------------ From: henry@zoo.toronto.edu Date: Sat, 6 Feb 93 19:09:50 EST To: space-tech@cs.cmu.edu Subject: peroxide etc. >... my mother, who did physical chemistry (i.e. >blew things up at Stanford Research Institute back >in the late 60's), refused to help me work with Peroxide >if I was going to freeze or boil it or anything similar. >[Bruce, take notes: she was a pro at dealing with explosives >and thinks that Peroxide is a bad idea...]... Did she actually work with peroxide, or only hear stories? The *folklore* about peroxide in the US rocket community is uniformly pretty bad, but the actual experiences of the Brits and the NF-104 people suggest that most of it either is sheer superstition or is based on experiences involving improper techniques. It never ceases to amaze me, actually, that people who casually accept *hydrazine* think that peroxide is unsafe. I know which one I'd rather work with. At Dryden, where some of the aircraft have hydrazine-powered emergency APUs, the hangars for those aircraft are marked (roughly) "hydrazine area, essential personnel only, everyone else keep out, this means you". This, mind you, for aircraft whose hydrazine supply is in sealed tanks behind burst diaphragms; it's not as if they actually *handle* the stuff in normal operations. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ To: henry@zoo.toronto.edu Cc: space-tech@cs.cmu.edu, gwh@soda.berkeley.edu Subject: Re: peroxide etc. Date: Sat, 06 Feb 93 17:09:29 -0800 From: George William Herbert She actually worked with Peroxide, though she was primarily working on other stuff at the time. The impression she conveyed was that it was very difficult to keep it safe under production conditions. I would also like to note for the record that I don't casually accept Hydrazine. Recall that I rejected Tetroxide and Hydrazine from cons{ideration in my vehicle due to safety... -george ------------------------------ From: henry@zoo.toronto.edu Date: Sat, 6 Feb 93 21:30:55 EST To: space-tech@cs.cmu.edu Cc: gwh@soda.berkeley.edu Subject: Re: peroxide etc. >She actually worked with Peroxide, though she was primarily working >on other stuff at the time. The impression she conveyed was that >it was very difficult to keep it safe under production conditions. This may be a function of details. The British experience, written up in JBIS a while ago, was that some seemingly-plausible things (like adding stabilizers to the peroxide) were actually mistakes. My impression is that they simply did a much more systematic exploration of peroxide handling issues than any of the US teams ever attempted -- perhaps just because they were doing a lot more of it -- and hence could base their handling practices on positive knowledge of what worked and what didn't. They say that safety in production conditions was not a serious problem... and they probably have more experience with it than everyone else put together. The NF-104 people at Edwards had garden-variety USAF technicians handling it for eight years. I believe they had no serious accidents. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ From: henry@zoo.toronto.edu Date: Sat, 6 Feb 93 22:19:00 EST To: space-tech@cs.cmu.edu Subject: Re: Hydrocarbon/peroxide SSTO >... Unfortunately, I had to leave the conference before the >presentation by Mitch. If Henry attended the presentation (rather than one >of the other simultaneous sessions), he might be able to fill in some >details. I caught most of the presentation, but he actually didn't say a lot that was technical and wasn't in the paper. The NF-104 experience with peroxide handling was very positive: ordinary USAF technicians handled 90% peroxide for eight years with no real trouble. JP-5 plus 100% peroxide gives a combustion temperature of about 3000K, low enough to minimize NOx formation in the atmosphere. You can probably use reverse osmosis to purify 30% peroxide (which is a large-scale commercial chemical, used for bleaching wool). The reverse- osmosis membranes are polyethylene, which is peroxide-compatible (one of the better peroxide-compatible materials, in fact). Another interesting material, mentioned by Jordin Kare from the audience, is Spectra: a Kevlar-like material which is a variant of polyethylene. Mitch figures on a zero-length aerospike, which typically has something like 4.5% off the theoretical efficiency. This does impact throttling for limiting acceleration: you can't shut down (say) 2/3 of your engines unless you've got lots of engines, because you need a vaguely-continuous ring of exhausts to keep the aerospike's gas bubble confined. From memory of a Pratt&Whitney (?) book on aerospike designs (loaned to Mitch by Max Hunter), Mitch said ten engines is too few but fifteen might be okay. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Mon, 8 Feb 93 08:14 PST To: space-tech@cs.cmu.edu Subject: Pressurizing Hydrocarbon/Peroxide SSTO From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) > Paul Dietz writes > > About pressurizing for an SSTO... > > You should be able to tolerate more blowdown in an SSTO than > in your P2-E's first stage, because the SSTO has a higher mass > ratio. The thrust has to decline a lot at the end to avoid > excessively high acceleration. Also, since most of the flight > is in vacuum, the chamber pressure can drop without overexpansion. > > Perhaps pressurizing with helium to 1/4 or 1/5 of the flight, > then blowing down after that? One could play games with cold > helium/oxygen/hydrogen mixes, warmed by passage over a catalyst, > as I think we talked about before. > Paul - you win first prize!!!! You are completely right. For my spreadsheet model of the Burnside Clapp hydrocarbon/peroxide SSTO, assuming full pressurization (and thrust) for half the flight, followed by blowdown to half pressure (and thrust) leads to a final acceleration of 15.7 G. I had missed this. The maximum pressurization needed would seem to be full pressure for about the first 20% of the ascent. Takeoff is at 1.3 G by design. By the time that 20% of the propellant has burned off, acceleration is up to about 1.8 G, after including the effect of the increased thrust as the vehicle pulls out of the lower atmosphere. If the tank is subsequently allowed to blow down, acceleration drops off to about 1.1 G at the middle of the ascent, and then starts to build up again as the decreasing mass over-rides the effect of the gradually dropping thrust. Acceleration is up to 3.3 G by the time that 90% of the propellant has burned, and ends as 6.3 G (unless some throttling is done near the end of the flight). Assuming full pressurization for only 20% of the flight brings the required mass of helium down from 2.08 tons to 0.83 tons. My apologies for a late answer to your E-mail. I had to build a spreadsheet to model what is going on as once the blow-down phase begins, the remaining propellant mass drops linearly with time, but the pressure (and thrust) do not. The length of time that full pressurization is necessary will be determined by a trade-off. Continuing full pressurization longer than 20% of the flight will avoid some of the sag in acceleration at mid-flight (and thus lower the delta V lost to gravity losses) at the expense of requiring more pressurizing gas to be carried. A New Idea: The fact that full pressurization is not needed through the flight leads to an interesting concept. At the moment, the Burnside Clapp design has a single fuel and a single peroxide tank. Both tanks must withstand full take-off pressurization. There would be a potential saving in splitting each propellant into two tanks. The first tank would be heavy walled and would be able to withstand high pressure. Propellant from this tank would be used for the initial stage of the flight. The remaining propellant would be carried in a lighter tank, capable of withstanding only the lower pressures encountered in the later stages of the flight. When the first tank was empty, its pressurization gas would be led (through a regulator if necessary) into the second tank, which would supply lower pressure propellant to finish the flight. This saves tank mass, but adds plumbing mass and complexity. It also may present packaging difficulties - the Burnside Clapp design elegantly builds a semi-conical vehicle around a single spherical peroxide tank, and a smaller spherical hydrocarbon tank, separated by a cargo bay. Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca ------------------------------ To: uunet!zoo.toronto.edu!henry@uunet.UU.NET Cc: space-tech@cs.cmu.edu, gwh@lurnix.COM Subject: Re: Hydrocarbon/peroxide SSTO Date: Mon, 08 Feb 93 17:45:14 -0800 From: gwh@lurnix.COM >You can probably use reverse osmosis to purify 30% peroxide (which is a >large-scale commercial chemical, used for bleaching wool). The reverse- >osmosis membranes are polyethylene, which is peroxide-compatible (one >of the better peroxide-compatible materials, in fact). ..or boiling. Reverse osmosis is safer, though. Still pretty expensive, though; my figures at around a dollar a pound are holding steady after more investegation. >Another interesting material, mentioned by Jordin Kare from the audience, >is Spectra: a Kevlar-like material which is a variant of polyethylene. Spectra is primarily advantageous (at this time) for body armour, where it's "wet" performance and fatigue behaviour (multiple impacts) are superior to Kevlar. Spectra's basic strength per mass is a bit lower than Kevlar, though, so in structural applications in a composite matrix, it's not as useful. Improvements of the basic Spectra material may change that situation, but right now Kevlar is superior for composite structures. -george william herbert ------------------------------ To: uunet!zoo.toronto.edu!henry@uunet.UU.NET Cc: space-tech@cs.cmu.edu, gwh@soda.berkeley.edu, gwh@lurnix.COM Subject: Re: peroxide etc. Date: Mon, 08 Feb 93 17:39:38 -0800 From: gwh@lurnix.COM I haven't read the JBIS article (citation, anyone?)... Even if Peroxide is safe if handled correctly, I believe that Inhibited Nitric Acid 98% is a better all-around oxidizer. It's cheaper by a factor of three, not significantly more hazardous to handle (deal with spills by water washdown and application of baking soda...), roughly same energy content and density... plus, it can't explode if we accidentally contaminate it 8-) The downside is that it's more environmentally incorrect, but no worse than existing solid rockets (better, actually; only a tiny fraction of the halogen content in the rocket as compared to solids with Cl-containing oxidizers ...., and what's there is due to the "Inhibited" or hydroflouric acid content, ~1% HF) And I can get free expert chemical handling advice if I use Nitric Acid. Even if mom's wrong about peroxide, she doesn't want to help if I'm working with it. At this point, looking at my development situation, that's a problem. So I'll go with Nitric Acid. -george william herbert ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 9 Feb 93 01:25:29 EST To: space-tech@cs.cmu.edu, gwh@soda.berkeley.edu, gwh@lurnix.COM Subject: Re: peroxide etc. >I haven't read the JBIS article (citation, anyone?)... July 1990 issue, I believe. >The downside is that it's more environmentally incorrect, >but no worse than existing solid rockets (better, actually; >only a tiny fraction of the halogen content in the rocket as >compared to solids with Cl-containing oxidizers ... Actually, considerably better; as I recall, the ozone-layer problem is specifically chlorine, not halogens in general. (In particular, I believe fluorine does not behave the same way.) Of course, explaining this to Greenpeace may be difficult. >And I can get free expert chemical handling advice if I use Nitric Acid. >Even if mom's wrong about peroxide, she doesn't want to help if I'm >working with it. At this point, looking at my development situation, >that's a problem... Agreed that this is a point in HNO3's favor! Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Tue, 9 Feb 93 09:08:58 -0500 From: dietz@cs.rochester.edu To: Bruce_Dunn@mindlink.bc.ca, space-tech@cs.cmu.edu Subject: Re: Pressurizing Hydrocarbon/Peroxide SSTO Bruce Dunn wrote: > the flight. This saves tank mass, but adds plumbing mass and complexity. It > also may present packaging difficulties - the Burnside Clapp design elegantly > builds a semi-conical vehicle around a single spherical peroxide tank, and a > smaller spherical hydrocarbon tank, separated by a cargo bay. The packaging difficulties can be evaded by putting the high pressure tank *inside* the lower pressure tank. This makes the low pressure tank larger, but makes the high pressure tank lighter, so it should be a wash mass-wise. Paul ------------------------------ Date: Tue, 9 Feb 93 11:17:39 -0500 From: dietz@cs.rochester.edu To: Bruce_Dunn@mindlink.bc.ca Subject: Re: Pressurizing Hydrocarbon/Peroxide SSTO Cc: space-tech@cs.cmu.edu Bruce, How did you model the behavior of the helium in the decompressing tank? Pessimistic analysis would be to assume the gas is pure helium, expanding adiabatically. Optimistic analysis would assume the helium stays in contact with the liquid and expands isothermally. Most realistic would probably be to assume the gas expands adiabatically, but take into account the contributions of water vapor and CO2 in the gas (water vapor would release heat upon condensation, and both would tend to lower the specific heat ratio of the mixture). Would it make sense to allow the tank to blow down, but continue to add *heat* to the pressurizing gas? In the two-tank scheme this might be as simple as retaining some fluid in the high pressure tank, and blow that fluid (as a mist) into the low pressure tank's head space. Paul ------------------------------ End of Space-tech Digest #144 *******************