Subject: Space-tech Digest #136 Contents: Fuel-rich mixtures and specific impulse (7 msgs) Deep search for asteroids (2 msgs) Reactors banned in NEO? (4 msgs) ------------------------------------------------------------ Subject: Re: P2-E pressurization via propellant heat transfer Date: Mon, 16 Nov 1992 19:15:05 EST From: John F Carr > 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. What affect would dissolved He have on Isp? The molecular weight of the exhaust would be lower, but the temperature would also be lower. ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 17 Nov 92 14:10:48 EST To: space-tech@cs.cmu.edu Subject: Re: P2-E pressurization via propellant heat transfer >What affect would dissolved He have on Isp? The molecular weight of >the exhaust would be lower, but the temperature would also be lower. No general answer can be made without calculation. Such effects are often beneficial, in moderation. Almost all fuel/oxidizer combinations yield optimum exhaust velocity (i.e. Isp) when run fuel-rich, for this very reason -- a small loss in temperature is more than compensated for by the drop in exhaust molecular weight. (Note that it's usually the decomposition products of the fuel, not the fuel itself, that appear in the exhaust, so this works even with high-MW fuels like kerosene.) The effect is particularly marked when hydrogen is involved, given its very low MW. I wouldn't be at all surprised if a bit of helium helps performance noticeably. However, it's unlikely to be cost-effective, given the cost and scarcity of helium. (One reason why I would hesitate to rely much on helium is that the US government controls the major sources of it, so it implies yet another set of bureaucrats who can shut you down.) Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Tue, 17 Nov 92 20:09:45 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: Fuel-rich mixtures and specific impulse >>What affect would dissolved He have on Isp? The molecular weight of >>the exhaust would be lower, but the temperature would also be lower. >No general answer can be made without calculation. Such effects are >often beneficial, in moderation.... [Followup to a very old post to sci.space] I would very much like to see these calculations. I've tried to calculate it in several different ways for hydrogen-oxygen mixtures, and it invariably comes out that you gain in specific impulse but lose on total impulse if you replace some of your onboard oxygen with an equal mass of hydrogen. The first time I calculated this, I treated the exhaust as a hydrogen-water mixture moving at a uniform velocity. The second time, I used the specific impulse of the two gases, calculated the extent to which each would be heated, and treated the exhaust velocities of the two as independent. (I did use the simplifying assumption that all of the energy of combustion goes into velocity of the exhaust. I don't know whether the extent to which this departs from reality causes a problem, but I've seen Paul Dietz use this assumption in his calculations, so if that invalidates the results, you can blame Paul. :-) In both cases, the effective specific impulse (by which I mean impulse per unit of oxygen burned) turned out greater than that of a stoichiometric(?) mixture. However, there's also less oxygen to burn, which offsets the improved Isp to some extent. Expressing it as a formula to show total impulse as a function of the amount of oxygen replaced with hydrogen, in both cases I came up with the result that the greatest total impulse is achieved when *none* of the oxygen is replaced (in other words, the stoichiometric mix gives the greatest total impulse). As I said before, if there are more sophisticated calculations which show that this is not really the case (and ideally show what the optimum mix really is), I would really appreciate seeing them. (Books on rocket propulsion are in somewhat limited supply here, but I haven't found anything which would disprove my calculations.) If there is some reason that the oxygen tank is required to be spherical, so that (for instance) the chosen diameter of the Shuttle external tank limits the amount of oxygen that can be brought along, but the length of the hydrogen tank is not constrained, then I agree that making the hydrogen tank a little bigger and bringing along a little extra hydrogen can improve the total impulse. (And of course it prevents the engines from burning up.) But if oxygen tank shape and engine erosion are not constraints, you would do better to add hydrogen *plus* a corresponding amount of oxygen. As Clint Eastwood would say, go ahead - prove me wrong! (But please show the math. :-) John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ Date: Tue, 17 Nov 92 21:21:35 -0500 From: dietz@cs.rochester.edu To: roberts@cmr.ncsl.nist.gov Subject: Re: Fuel-rich mixtures and specific impulse Cc: space-tech@cs.cmu.edu John, To see what's going on here, let's look at the thermodynamics of the situation. Inside the rocket's combustion chamber is a higher pressure, high temperature mix of chemicals. Because it is at high temperature, the gas is not, in general, composed only of the lowest energy mix of components (water, for example, in the case of hydrogen + oxygen). Instead, it also contains H2 and O2, hydroxyl radicals, atomic hydrogen and oxygen, and so on. As the gas expands out the nozzle, it cools. Some of the energetic components react to form more stable molecules. This releases energy, which retards the cooling of the gases. The more the gas expands, the cooler it gets, and the more of this latent energy that is converted to kinetic energy. In a real rocket, the gases are still hot or have not fully reacted by the time they leave the nozzle. This reduces performance over the theoretical upper bound. Stoichiometric hydrogen/oxygen would produce a very hot gas, with a lot of energy going into dissociating molecules. If you add some extra hydrogen, this reduces the temperature so that more of the latent energy becomes available. As it turns out, in real engines the extra energy (and lower average molecular weight) improves Isp, up to a point. Just how much depends on the pressure ratio of the engine. Paul F. Dietz dietz@cs.rochester.edu ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 17 Nov 92 22:50:41 EST To: space-tech@cs.cmu.edu Subject: Re: Fuel-rich mixtures and specific impulse >... several different ways for hydrogen-oxygen mixtures, and it invariably >comes out that you gain in specific impulse but lose on total impulse if >you replace some of your onboard oxygen with an equal mass of hydrogen. I have difficulty understanding how that could happen. Specific impulse is impulse per weight of propellant. If you keep weight constant and improve specific impulse, how can you lose total impulse?!? Something is fundamentally wrong with any calculation producing that result! Sutton (Rocket Propulsion Elements) outlines the process in chapter 6 (Chemical Rocket Propellant Performance Analysis): Determine initial combustion-product composition, given known pressure, pre-combustion composition, and thermodynamic properties. That is, find the product composition that minimizes Gibbs free energy. This assumes complete combustion to equilibrium, but that's usually a good assumption; experimentally-determined combustion efficiencies are 94-99%. Gibbs free energy is G = sum for j=1 to n of (mu sub j * n sub j) where n sub j is percentage of molecules of substance j and mu sub j is free energy for substance j: mu sub j = U + pV - TS where S is entropy, U is internal energy, and typically you look this up in a table as a function of temperature for the relevant substance, and then correct for pressure. Don't forget intermediate products; Sutton's example for RP-1/LOX shows not just CO2, H2O, and H2, but also CO, OH, H, O, and O2. Then expand isentropically to the nozzle exit-plane pressure, making some kind of assumption about reaction rates during expansion. "Frozen" expansion assumes a reaction rate of zero so the initial composition is unaltered; "shifting" expansion assumes infinite reaction rates so the gas is in equilibrium at every point (note the possibility that components with high boiling points will condense into liquid or solid). Anything intermediate is inordinately hard to handle because little is known about reaction rates under such conditions. Real results usually fall between the frozen and shifting cases, which typically aren't too far apart. I seem to recall that one reasonably-good heuristic is to assume shifting flow to the nozzle throat and frozen thereafter. Conservation of entropy now gives you the exit temperature. From that and known gas properties we get enthalpy (thermal energy) change, and from that and conservation of energy we get kinetic energy content at the exit plane. Hence, finally, average exhaust velocity and specific impulse. No analytic shortcuts for all this are apparent. You have to grind through it with computerized assistance and lots of tables of thermodynamic properties of gases. It's something I have not done myself, so I can't comment knowledgeably on difficulties. The typical result is 3-12% overoptimistic, much of which is probably nozzle losses of various kinds. >If there is some reason that the oxygen tank is required to be spherical, Nope, and in fact it isn't, quite. The lower end is more or less hemispherical, but the upper end forms the nose of the External Tank. It's closer to a sphere than to a cylinder, but not very spherical. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Wed, 18 Nov 92 00:46:53 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: Specific impulse >I have difficulty understanding how that could happen. Specific impulse >is impulse per weight of propellant. If you keep weight constant and >improve specific impulse, how can you lose total impulse?!? Something >is fundamentally wrong with any calculation producing that result! Sorry - it's been a long time since I did the calculations, and I worded it badly. Does this sound any better: given the initial assumptions I used (now shown to be somewhat shaky!), and assuming the same rate of oxygen consumption as for a stoichiometric-mix rocket with equal combined mass of hydrogen and oxygen, it is indeed true that exhaust velocity and mass flow are greater in the hydrogen-rich rocket, but since the total amount of oxygen brought along is less, the rocket fires for a shorter period of time, and my calculations work out that the total impulse (thrust multiplied by time of firing) is less for the hydrogen-rich rocket. (That's still not very precise, but hopefully it's a little better.) >Sutton (Rocket Propulsion Elements) outlines the process in chapter 6 >(Chemical Rocket Propellant Performance Analysis): I'll try again to see if I can find that book. Thanks to you and Paul for your replies. If I understand them correctly, the main problem with my calculations is that the initial assumption of 100% conversion of thermal energy to useful kinetic energy within the rocket engine is not valid, due in large part to the finite length of the rocket nozzle, so other factors such as the chemical properties of the exhaust components at various temperatures become predominant. (Is that about right?) Would it be reasonably safe to assume that if for some reason a rocket engine in space were made with an extremely long nozzle, then the specific-impulse advantage of a hydrogen-rich rocket would become less pronounced? John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ From: John F Carr Subject: Re: Fuel-rich mixtures and specific impulse Date: Wed, 18 Nov 1992 11:29:31 EST > No analytic shortcuts for all this are apparent. You have to grind through > it with computerized assistance and lots of tables of thermodynamic > properties of gases. It's something I have not done myself, so I can't > comment knowledgeably on difficulties. I did some such calculations when I took MIT's rocket propulsion course. I don't remember it being very hard, but we only worked with very simple cases (hydrogen + oxygen). I wouldn't want to try computing the exhaust velocity of a hydrocarbon burning engine without a computer and a good program. I remember the instructor telling us that many of the reaction rates and thermodynamic properties of intermediate products of hydrocarbon burning were unknown. ------------------------------ To: space-tech@cs.cmu.edu Subject: Deep search From: can2can@ziggys.cts.com (Tim Edwards) Date: Sun, 15 Nov 92 08:37:10 PST A radar situated at each of the trojan points would give us a good chance of seeing around the Earth and Moon, and not missing anything in this vicinity. A chirp-radar sends a long pulse, that is frequency modulated, then uses the frequency modulation of the echo to time compress it. The result is a good 'power in the pulse to peak power handled' factor, combined with exelent resolution in range. A phased array antenna alows the signal beam to be aimed without physical movement of the array. The pahased array antennas that I am familliar with involve a _lot_ of slotted wave guides, piped into the circuitry. If the dimensions are adjusted for the dielectric constant, we should be able to have the tubes built as solid, with a core of pyroceramic so there is never a question of leakage/contamination, then a layer of whatever metal is availible, and a{glass finish to slow the evaporation of the metal. Raw materials to be lunar, we know where the lunar resources are without needing the radar first to track planetesimals. The radars will give us a lead to the resources of the planetesimals, which seem to be quite different from lunar resources. can2can@ziggys.cts.com - BBS (619)262-6384 Ziggy's Den Of Iniquity ------------------------------ Date: Mon, 16 Nov 92 15:09:06 +0000 From: Dominic Herity Subject: Re: Deep search > A radar situated at each of the trojan points would give us a good chance > of seeing around the Earth and Moon... I feel as if I walked into a room in the middle of a converstaion. Maybe I did. Apologies in advance if these points have been covered. I gather the reason for the radars is to detect near earth asteroids. What kind of range are we talking about? To get 24 hours warning of something coming at a relative speed of 50km/s, you'd need to see it at 4.3 million km. Anything less wouldn't be much use, I guess. 24 hours wouldn't be any use either unless you had a response pretty well rehearsed. :-) 200 times that range would catch almost everything inside Jupiter's orbit, which gives enough time to figure out a response after detection. Why not put the radars on Earth ? It would be much cheaper. Since the earth rotates every 24 hours and shorter notice isn't much use, nothing relevent is going to hide behind it. Since the moon travels its own diameter every 33 minutes, so nothing will hide behind that either. The only way I can imagine a dinosaur killer hiding is by coming straight out of the sun. Best Regards Dominic Herity ------------------------------ Date: 17 Nov 1992 16:27:30 -0500 (EST) From: "GORDON D. PUSCH" Subject: Reactors banned in NEO? Henry Spencer writes: > Nuclear power might be the right technical solution but it has > obvious political problems at present... I vaguely remember that at one point there was talk of getting reactors *banned* inside of Mars's orbit in general, and within the Earth's Van-Allens in particular. (Gamma-ray astronomers were particularly keen on this --- the weak-decay positrons from the unshielded reactors aboard Soviet ``Cosmos'' satelites remained trapped in the Van Allen belt, and were ``polluting'' the electron-positron annihilation line when they struck gamma-ray observatory- satelitess.) 1) Is there currently an international agreement banning reactors from near-earth orbit? (maybe I should really post this to sci.apace, sorry!) 2) Given that most reactors in orbit are unlikely to have more than "shadow- shielding," do trapped reactor-produced positrons in the Van-Allens pose a hazard to anything other than gamma-astronomer's careers? Anybody know what the typical lifetime of a Van-Allen electron is? (It should put an upper bound on the positron lifetime.) Gordon D. Pusch ------------------------------ Date: 17 Nov 1992 17:17:38 -0500 (EST) From: "GORDON D. PUSCH" Subject: P.S. to "Reactors banned in NEO?" Before anybody fires back the "Space is *BIG*. Really, *really* *BIG*" line from _Hitchhiker's_: the positrons tend to remain trapped on the flux-tube they were emitted onto. Since the magnetosphere more-or-less corotes w/ the earth at low lattitudes, that means a reactor in near-circular orbit tends to fill a thin shell w/ positrons (and electrons) --- except at GEO, where it just fills the flux-tube it's on (modulo effects due to misalignment of magnetic- and spin-poles, and deformation and convection of the magnetosphere plasma due to solar-wind pressure). Such trapped positrons really *do* exist and are observable; I read a paper about them in _Nature_ a few years ago. Gordon D. PUsch ------------------------------ Subject: Re: Reactors banned in NEO? Date: Tue, 17 Nov 92 14:50:55 -0800 From: gwh@lurnix.COM There are no agreements about reactors anywhere. For a long time, the Russians were putting up radar ocean recon sats that were nuclear reactor powered and in LEO; one fell on Canada a while back, you might recall, after it's reactor jettison system (designed to put the reactor in high, stable orbit) failed. There has been no serious talk of banning reactors from near the earth. Nuclear propulsion is universally disliked for near-earth applications, but transfer vehicles and anything going further out are fine. Nuclear reactors are not affected. Physicists may not like it, but they're not running the whole space program (thankfully 8-). Nuclear bombs are banned, in any form, in space. The test ban treaty saw to that. Which theoretically nails down any "Orion" vehicles... 8-) As if anyone would build one unless they were desperate enough to violate a treaty anyway. Which is likely scienci fiction not space-tech 8-) -george ------------------------------ Date: 18 Nov 1992 11:38:04 -0500 (EST) From: "GORDON D. PUSCH" Subject: Re: P.S. to "Reactors banned in NEO?" Samuel J. Pullara writes: > I think he meant: How long does a positron last before it is annihilated > by an electron in the Van Allen Belts? No, I meant what I said; I'd have to work it out to be sure, but my gut feel is that the Van-Allen electron density (and therefore the annihilation rate) is so small that the positron will get scattered into the loss-cone long before it annihilates --- that's why I said it would put an *UPPER* bound on the lifetime, which annihilation will reduce somewhat. Gordon D. Pusch ------------------------------ End of Space-tech Digest #136 *******************