Subject: Space-tech Digest #106 Contents: Airbreathing SSTO? (12 msgs) ------------------------------------------------------------ Date: Tue, 17 Mar 92 08:32:42 EST From: davidsen@crdos1.crd.ge.com To: space-tech@cs.cmu.edu Subject: Re: In opposition to airbreathing SSTO (longish) I'm missing what thrust to weight as a raw number has to do with the plane vs pure rocket implementation. A plane (other than VTOL) doesn't use it's thrust for lift, it uses the lift of the wings. If you look at the percentage payload of a 747 and compare to a rocket lifting the same load to the same height, you begin to see my argument. Also, planes are (mostly) fail-safe. With small exceptions at takeoff, the plane is likely to be able to softland after loss of partial power. Rockets don't seem to have that luxury. I have no axe to grind for any implementation. A really huge plane doing a drop (airbourne launch) doesn't offend me, if that drops the cost to orbit. And the maint. costs on a plane are a lot lower than on a shuttle. The combination of lift and using oxygen from the air seems to offer a reduction in fuel weight, total weight, and complexity. Liquid O2 is reasonably nasty (ie expensive) stuff to handle and store, heavy, expensive, and hard on everything it touches. If a vehicle can use less of it, that's all to the better. ------------------------------ Date: Tue, 17 Mar 92 09:39:10 -0600 From: pgf@nasa15.usl.edu (Phil G. Fraering) To: space-tech@cs.cmu.edu Subject: In opposition to airbreathing SSTO (longish) > Also, planes are (mostly) fail-safe. With small exceptions at takeoff, > the plane is likely to be able to softland after loss of partial power. Ack. These fail-safes you're talking about aren't as fail-safe as you think. NASP, _DRY_, lands in the 250-300 mph range, and needs special landing gear and a _very_ long runway to land on... ------------------------------ Date: Tue, 17 Mar 1992 11:56 EST From: "GORDON D. PUSCH" Subject: Re: In opposition to airbreathing SSTO To: space-tech@cs.cmu.edu > I'm missing what thrust to weight as a raw number has to do with the > plane vs pure rocket implementation ... It's simple: no matter how you figure, SSTO is a marginal proposition with current technology. One often finds that changes of a few percent in one's assumptions lead to *negative* payloads :-(. From that standpoint, an engine that masses 10 tonnes instead of 1 tonne means 9 tonnes of payload which will *NOT* be going to LEO ... > ... A plane (other than VTOL) doesn't > use it's thrust for lift, it uses the lift of the wings. If you look at > the percentage payload of a 747 and compare to a rocket lifting the same > load to the same height, you begin to see my argument. > True; however because of the aforementioned sensitivity of the SSTO problem to one's assumptions, one may get *radically* different answers RE: whether wings help or hurt you. Example: I understand that "Pegasus" could not have made orbit without its wings, because the best wingless launch-profile required a 45-degree angle- of-attack near max-Q; an airframe strong enough to withstand the resulting lateral aerodynamic forces would have massed far too much. Adding wings to Pegasus reduced the maximum angle-of-attack to about 15 degrees, at an earlier point in the launch-profile. By contrast, in the Shuttle, the wings are pure, dead mass on launch; at no point during launch does the Shuttle's aerodymanic lift exceed a few percent of its thrust. Again, both Hunter's SSX and the German "BETA" concept conclude that wings and/or air-breathing constitute a *net loss in payload* to LEO. On the other hand, Sanger and HOTOL both expect a net gain. In both cases, though, the conclusion is *sensitive to the assumptions made*. To my mind, the advantage of the SSX-style approach is that it uses very simple conventional, reliable engines (no "unproven technologies" or "edge of the envelope" stuff), and puts all of its "finesse" into shaving the vehicle structural-mass down. I happen to think that'll be easier, cheaper, and more "do-able" than the NASP, which uses *REALLY* exotic materials, requires the *ENTIRE UNDERBODY* as its intake/engine/exhaust/lift-surface, and can't even be *SIMULATED* accurately, so that the only assurance one can have that it'll fly is to BUILD IT and *TRY* to fly it. Like the Shuttle, I expect the NASP to be a prototype *experimental* vehicle that the NASA management will confuse with an *operational* vehicle; that's why I think it'll *INCREASE*, not reduce, cost FOB to LEO. > Also, planes are (mostly) fail-safe. With small exceptions at takeoff, > the plane is likely to be able to softland after loss of partial power. > Rockets don't seem to have that luxury ... > That's exactly what Hunter wants built into the SSX: "continuous intact abort capability." He argues that aircraft have this ability because they have *multiple engines*, not because they have wings. Ever tried to land a 747 "dead-stick?" If my fligt-simulator is at *all* accurate, it's NOT easy ... Hunter wants the SSX to be able to land safely even in the event of a double engine-failure; he estimates this requires a minimum of 8--10 engines. > ... I have no axe to grind for any > implementation. A really huge plane doing a drop (airbourne launch) > doesn't offend me, if that drops the cost to orbit. And the maint. costs > on a plane are a lot lower than on a shuttle. > It doesn't *offend* me either; I just don't think it'll *WORK*. I agree that the maintance costs of a plane are lower --- but that's because (a) transport- planes don't push the "edge of the envelope" (the Shuttle and NASP will), and (b) they don't require a maintainance-staff of 10,000 people/vehicle. I agree with Hunter that the *TRUE* problem is the "standing army," not rockets per se, and that the REAL need is to develop a rocket that has an operation-cost comparable to a transport aircraft. I have become convinced that a "plug-nozzle" rocket is *simpler* and *CHEAPER* than a jet --- no rotating turbo-machinery in the airstream, no variable inlets or exhaust nozzles, more energy-efficient, fewer moving parts, etc. > The combination of lift and using oxygen from the air seems to offer a > reduction in fuel weight, total weight, and complexity. Liquid O2 is > reasonably nasty (ie expensive) stuff to handle and store, heavy, > expensive, and hard on everything it touches. If a vehicle can use less > of it, that's all to the better. > As I said in my first posting, so it *seemed* to me also --- until I began looking into the problems in more detail. However, if a change of a few percent in one's assumptions about engine or structural mass is enough to shift the balance from wings/air-breather to VTOVL/rocket (Sanger vs. BETA), it now seems to me to be too *economically* risky a proposition; we need something that can be *GUARANTEED* to fly cheaply, not something that *MIGHT* fly cheaply, and *MIGHT* carry a useful payload. Air-breathers *MAY* be a viable option for a *FOURTH* generation launch- vehicles, once materials and combustion science have advanced enough --- and I'll even go so far as to admit that NASP *might* be a useful *EXPERIMENTAL* vehicle (if NASA weren't managing it). However I am convinced that the first *PRACTICAL* spacecraft will be something like the SSX --- *IF* it doesn't require a "standing army" to suuport it ... (Besides, LOX isn't *THAT* nasty --- and it's *very* tasty on bagels ;-) Gordon D. Pusch ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 17 Mar 92 13:17:24 EST To: space-tech@cs.cmu.edu Subject: Re: In opposition to airbreathing SSTO (longish) > I'm missing what thrust to weight as a raw number has to do with the >plane vs pure rocket implementation. A plane (other than VTOL) doesn't >use it's thrust for lift, it uses the lift of the wings... This branch of the discussion was addressing engine choice, rather than wings vs. VTOL; the two issues are indeed pretty much unrelated, except insofar as it's difficult to build a VTOL spacelauncher with airbreathing engines. As witness Pegasus, rocket-powered aircraft are easy. Lift is actually somewhat of a red herring here. Even conventional rockets use their engines for lift only fairly briefly; they do most of their accelerating nearly horizontally at very high altitude. The difference is that they get up to their operational altitude for a long enough time by a ballistic lob rather than by wings. The relevance of airbreathing engines' terribly low thrust:weight is that airbreathing launchers necessarily carry a lot of extra weight and accelerate very slowly, because their engines are heavy and not very powerful. Extra weight is just what a launcher doesn't need. And slow acceleration, while not disastrous in itself, means spending a long time accelerating in air that is thick enough to provide lift and run the engines, and that means terrible thermal problems. > Also, planes are (mostly) fail-safe. With small exceptions at takeoff, >the plane is likely to be able to softland after loss of partial power. >Rockets don't seem to have that luxury... It depends on how marginal your rocket is. New designs like ALS and SSTO tend to specify successful completion of mission with one engine failed, and SSTO specifies successful soft landing even with multiple failures. For that matter, a good many existing systems can survive partial power loss if it doesn't happen too early; the second Saturn V limped into orbit despite losing two out of five engines on the second stage, and one of the shuttle missions lost an SSME midway up. > The combination of lift and using oxygen from the air seems to offer a >reduction in fuel weight, total weight, and complexity... The price is hypersonic aerodynamics and aerothermalics (which probably isn't a word, but should be :-)). It's a steep price. Rockets avoid a lot of weight and complexity by getting up out of the atmosphere quickly. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Tue, 17 Mar 92 12:15:51 PST From: gwh@lurnix.lurnix.com (George W Herbert) To: tcsi!cs.cmu.edu!space-tech@uunet.UU.NET Subject: Re: In opposition to airbreathing SSTO We seem to be wandering a bit far here from mere airbreathing SSTO 8-) Anyway... Gordon, how much of the development of NASP and the other SSTO ideas have you closely followed? Some of yout statements don't make a whole lot of sense from what I know about the programs. NASP is not and won't ever be anything but a research air/spacecraft. It has a payload of about 500kg, which is taken up by two crewmembers and some random extra (non-essential) instrumentation. There's simply no question as to wether or not it will raise or lower costs to orbit... NASP _is not_ a launch vehicle, so it's not a valid question. The second-generation NASP-type vehicle, NDV (NASP Derived Vehicle) is intended to be. It won't get built unless the NASP meets its performance requirements. Which it is currently estimated to exceed by roughly 25%... they're being conservative. (This claim from random propulsion person in the program, though not officially announced). On a seperate subject: >As I said in my first posting, so it *seemed* to me also --- until I began >looking into the problems in more detail. However, if a change of a few >percent in one's assumptions about engine or structural mass is enough to >shift the balance from wings/air-breather to VTOVL/rocket (Sanger vs. BETA), >it now seems to me to be too *economically* risky a proposition; we need >something that can be *GUARANTEED* to fly cheaply, not something that *MIGHT* >fly cheaply, and *MIGHT* carry a useful payload. The mass estimation error bar cuts both ways, Gordon. If the operational DC design is 12% over its structural mass estimate, it's payload is _gone_. (This from memory from the AvWeek article). The estimates ought to be accurate +-3-4%, barring a misanalysis of some stresses. But if they aren't, then the design is in trouble. Also, >I have become convinced >that a "plug-nozzle" rocket is *simpler* and *CHEAPER* than a jet --- >no rotating turbo-machinery in the airstream, no variable inlets or exhaust >nozzles, more energy-efficient, fewer moving parts, etc. The RL-10 engine (well proven rocket, LH2/LOX, etc) in its highest power design is about 760 lbs (estimated mass) for 22,000 lbs thrust. The advanced GE F404 turbofan masses under 3000 lbs, produces 21-22,000 lbs thrust, and burns a whole lot less fuel than the RL10 does over an initial Mach 0-Mach 3 trajectory, when even variable inlets run out of steam. Once you've taken tankage fraction into account, the RL10 is still ahead a bit in overall mass, but not a lot. What's cheaper? A vehicle with two F404's as a booster stage will be a hell of a lot cheaper than a smaller vehicle with a RL-10. The increased mass overall is more than offset in reusability, cost (F404's run around $1.5 million each), and reliability (Yes, the RL-10 is reliable. By space standards 8-) -george william herbert gwh@lurnix.com gwh@ocf.berkeley.edu ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 17 Mar 92 18:20:26 EST To: space-tech@cs.cmu.edu Subject: Re: In opposition to airbreathing SSTO > NASP is not and won't ever be anything but >a research air/spacecraft... While this is true, one should not chastise people for having to be reminded about it, because those who've had to be reminded about it at times have included the NASP project office and the contractors. There has been considerable pressure from various quarters to make the X-30 as close to an operational vehicle as possible (at one point it was going to have a payload bay with doors, for example). Even people who should know better sometimes fail to make the distinction between the X-30 and its hypothetical operational successors, e.g. by claiming that it will demonstrate "airline-type" turnaround -- something the research planes have never been noted for. >There's simply no question as to wether or not it will >raise or lower costs to orbit... NASP _is not_ a launch >vehicle, so it's not a valid question. There is also no certainty that NASP will reach orbit (even assuming it is fully funded), although that is the official objective. > The RL-10 engine (well proven rocket, LH2/LOX, etc) in its highest >power design is about 760 lbs (estimated mass) for 22,000 lbs thrust. > The advanced GE F404 turbofan masses under 3000 lbs, produces >21-22,000 lbs thrust, and burns a whole lot less fuel... That's still a factor of 4 advantage for the rocket, and the RL-10 T:W isn't particularly impressive by rocket standards. By the way, I assume that F404 mass doesn't include the intake, which is non-trivial for high-Mach operation. >(Yes, the RL-10 is reliable. By space standards 8-) Yes, the F404 is reliable. By military-engine standards. Don't confuse them with civil-engine standards. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Tue, 17 Mar 1992 21:19 EST From: "GORDON D. PUSCH" Subject: Re: In opposition to air-breathing SSTO (longish) To: space-tech@cs.cmu.edu > From: George W Herbert > > Gordon, how much of the development of NASP and the other SSTO ideas > have you closely followed? Some of your statements don't make a whole > lot of sense ... NASP _is not_ a launch vehicle ... The second-generation > NASP-type vehicle, NDV (NASP Derived Vehicle) is intended to be. It won't > get built unless the NASP meets its performance requirements ... > Ack! It does seem I'm a bit behind the times. I can't say my knowledge on SSTO is *current*; it's mostly from _J. Prop. Power_ and _Acta Ast._ ca. 2yrs ago (VA Tech got them, but Chalk River doesn't, nor does it get _Aviation Leak_ :-(. I dropped my subscription to SPACE-DIGEST around when SPACE-TECH was formed, because I couldn't handle the noise-to-signal ratio anymore. I *do* read _Astronomy_, and occasionally pick up _Ad Astra_ ... So I stand corrected: it's the *NDV* that'll "decrease" cost to LEO from $7k/lb to $10k/lb ;-) (However, see Henry's comment that the NASP project office *also* seems to be confused on this issue ... :-( > ... Which it is currently estimated to exceed by roughly 25%... > they're being conservative. (This claim from random propulsion > person in the program, though not officially announced). > Not to impune the reputation of your source, but ... do you really *BELIEVE* him/her? Recall that such claims were also made RE: the Shuttle before it was built; the reality was (*ahem*) somewhat disappointing ... > > ... However, if a change of a few > >percent in one's assumptions about engine or structural mass is enough to > >shift the balance from wings/air-breather to VTOVL/rocket (Sanger vs. BETA), *+++++ * ... CORRECTION: I meant to write "HOTOL vs. BETA" ... *----- > >it now seems to me to be too *economically* risky a proposition; we need > >something that can be *GUARANTEED* to fly cheaply, not something that > >*MIGHT* fly cheaply, and *MIGHT* carry a useful payload. > > The mass estimation error bar cuts both ways, Gordon. > If the operational DC design is 12% over its structural mass estimate, > it's payload is _gone_. ... The estimates ought to be accurate +-3-4%, > barring a misanalysis of some stresses ... > IMHO, the NASP/NDV uncertainties are likely to be *MUCH* larger than the DC-#'s; NASP is based on "numerical wind-tunnel" (i.e., computer) calculations, and, well ... GIGO. Hypersonic aero-thermo-chemo-dynamics in the NASP flight-regime is highly non-equilibrium, and rather poorly understood, because (as of 2 years ago) the relevent reaction-rates had *NOT YET BEEN MEASURED*. I submit that noone will *REALLY* know whether the numerical model is *accurate* unless/until NASP flies ... > >I have become convinced > >that a "plug-nozzle" rocket is *simpler* and *CHEAPER* than a jet --- > >no rotating turbo-machinery in the airstream, no variable inlets or > >exhaust nozzles, more energy-efficient, fewer moving parts, etc. > > The RL-10 engine (well proven rocket, LH2/LOX, etc) in its highest > power design is about 760 lbs (estimated mass) for 22,000 lbs thrust. > The advanced GE F404 turbofan masses under 3000 lbs, produces > 21-22,000 lbs thrust, and burns a whole lot less fuel ... > [in the Mach 0--3 range] ... Once you've taken tankage fraction into > account, the RL10 is still ahead a bit in overall mass, but not a lot. > As Henry observed, that's still a factor of four advantage for the RL-10, and I, too, doubt that you've included the mass of the intakes, which probably adds another 1k--2k lbs to the jet. Also, remember that the jets and inlets are *dead mass and drag* from Mach 3 on up. If you keep them, they cut into your payload fraction. If you drop them, then they need to be safely recovered and returned. If you put them on a "C5a-class-blackbird first-stage," you have to deal with a hypersonic seperation maneuver --- such as the one believed to have cost the CIA (USAF?) an A-12 (SR-71?) while launching a GB-whatever spy-drone. I freely admit that these are all solvable problems, but it *will* increase the overall complexity and cost of the launch-system. Furthermore (as Max Hunter points out) you're going to all this expense and effort to get ``free'' oxygen from the air --- when LOX retails for $0.05/lb! You're likely to spend far more on LH2 (at $1.50/lb) lugging the excess mass and drag of your jets than you'll ``save'' !!! Finally (as Niven et. al. mention in _Fallen Angels_) LOX will be an almost *unavoidable* by-product of the hydrogen-liquifier any SSX/DC-# launch-site is sure to have (it'll be condensing on all the LH2 cryo-lines ;-) --- so *WHY BOTHER* ??? On the whole, I *still* think the SSX/DC-# looks better than an air- breather, for at least the next 1--2 decades ... (BTW, an "inlet precooler" such as Rolls-Royce proposed for HOTOL's (no longer secret!) "pegasus" engines *might* allow an H2-fueled turbojet to run up to Mach 4--6, but again, that's a massive/unproven engine technology. Should it actually work, I'll probably have to change my mind about air-breathers yet *again* --- but I'm not holding my breath; "interim HOTOL" is about as far from HOTOL as "Fred" is from what "Freedom" was planned to be ... :-( Gordon D. Pusch ------------------------------ Date: Wed, 18 Mar 92 08:13:53 EST From: davidsen@crdos1.crd.ge.com To: space-tech@cs.cmu.edu Subject: Re: In opposition to air-breathing SSTO (longish) > Furthermore (as Max Hunter points out) you're going to all this expense > and effort to get ``free'' oxygen from the air --- when LOX retails for > $0.05/lb! You're likely to spend far more on LH2 (at $1.50/lb) lugging > the excess mass and drag of your jets than you'll ``save'' !!! Well, I don't think that's a fair statement. The oxygen is 'free' in the sense that you don't have to lift it, not that you worry about the dollar cost. > (BTW, an "inlet precooler" such as Rolls-Royce proposed for HOTOL's > (no longer secret!) "pegasus" engines *might* allow an H2-fueled > turbojet to run up to Mach 4--6, but again, that's a massive/unproven > engine technology. Should it actually work, I'll probably have to > change my mind about air-breathers yet *again* --- but I'm not holding > my breath; "interim HOTOL" is about as far from HOTOL as "Fred" is > from what "Freedom" was planned to be ... :-( May it work! Obviously LH2 is a far better fuel than J2 in terms of power to weight. ------------------------------ Date: Wed, 18 Mar 92 08:06:17 EST From: davidsen@crdos1.crd.ge.com To: space-tech@cs.cmu.edu Subject: Re: In opposition to airbreathing SSTO Sender: mnr@DAISY.LEARNING.CS.CMU.EDU > > The RL-10 engine (well proven rocket, LH2/LOX, etc) in its highest > >power design is about 760 lbs (estimated mass) for 22,000 lbs thrust. > > The advanced GE F404 turbofan masses under 3000 lbs, produces > >21-22,000 lbs thrust, and burns a whole lot less fuel... > > That's still a factor of 4 advantage for the rocket, and the RL-10 T:W > isn't particularly impressive by rocket standards. By the way, I assume > that F404 mass doesn't include the intake, which is non-trivial for > high-Mach operation. Unfortunately the engines don't lift themselves... what you have to compare is the weight of engine plus fuel to produce N pound-sec of thrust net. By net I mean thrust-(Weight-engine+weight-fuel). I don't know how this comes out, I would have to spend a few hours getting figures to even get a ballpark number, I just have a suspicion that the LOX+LH2 is heavier than J2+nothing. ------------------------------ Date: Wed, 18 Mar 92 09:00:22 CST From: ssi!lfa@uunet.UU.NET (Louis F. Adornato) To: uunet!cs.cmu.edu!space-tech@uunet.UU.NET Subject: Re: In opposition to air-breathing SSTO > IMHO, the NASP/NDV uncertainties are likely to be *MUCH* larger than > the DC-#'s; NASP is based on "numerical wind-tunnel" (i.e., computer) > calculations, and, well ... GIGO. Hypersonic aero-thermo-chemo-dynamics > in the NASP flight-regime is highly non-equilibrium, and rather poorly > understood, because (as of 2 years ago) the relevent reaction-rates had > *NOT YET BEEN MEASURED*. I submit that noone will *REALLY* know whether > the numerical model is *accurate* unless/until NASP flies ... > The accuracy of numerical aerodynamic simulation (NAS) has improved greatly over the past few years. I understand that Ames ran a comparison between the same body in thier supersonic wind tunnel, the NAS system, and flight hardware, and discovered that the wind tunnel was actually _less_ accurate than the NAS due to the effects of the tunnel walls. They're now using the NAS to validate the wind tunnel results. - Disclaimer: I have all of this second hand. Lou Adornato uunet!ssi!lfa | The secretary (and the rest of the company) Supercomputer Systems, Inc | have disavowed any knowledge of my actions. Eau Claire, WI | ** Space IS our future! ** ------------------------------ Date: Wed, 18 Mar 1992 14:49 EST From: "GORDON D. PUSCH" Subject: Re: Price of LOX To: space-tech@cs.cmu.edu > From: Richard Schroeppel > > You mention that "LOX retails for $.05/lb". Is this really true? > I'm interested in the price of LN2, which seems a good deal higher. > (The "warm" superconductor discussions compared LN2 to milk in price, > which would make it $.50/lb.) I can't see any obvious reason that LOX > should be cheaper than LN2; if anything, I'd expect the opposite, > since LN2 is less hazardous. Got any data? > > Rich Schroeppel rcs@cs.arizona.edu > You've got a point; furthermore, atmospheric N2 is more abundant, and LN2 has a lower boiling point than LOX, so it can be obtained by simply letting the O2 boil out of liquid air. Hmmm ... I got my price from par.3, p.9 of Max Hunter's paper, "The SSX: SpaceShip eXperimental (draft II)." It could be a typo, or it could be that he's giving his *estimated* cost of obtaining LOX as a byproduct of his on-site H2-liquifier ... One thing I vaguely remember: isn't that "$/ltr_LN2 ~ $/ltr_milk" figure for *small lots* of LN2 in 50-ltr dewers? Maybe it's a *LOT* cheaper in bulk ... ;-) Gordon D. Pusch ------------------------------ Date: Wed, 18 Mar 92 13:14:37 -0800 From: George William Herbert To: space-tech@cs.cmu.edu Subject: Re: various things... Louis Adornato correctly pointed out that I'll never ride in a 99.99% reliable vehicle. While airliners meet this in terms of not falling from the sky and killing people, they don't even make 99% for "normal" flights w/o diversons around weather, some system failure, etc. I was describing NASA's self-determined reliability for the Shuttle, not what anything would actually achive. I know better 8-) The reason for my comment about the SSME / Challenger issue was that I wanted to communicate how little faith the NASA engineers had in the SSME. Wether or not it was the cause, if they don't trust it, why should I? On an unrelated subject, the price given for LOX in bulk of $80 and down is roughly accurate for high purity ton lots. I've heard as low as $50/ton in industrial grade. The higher prices you hear are for small lots... -george william herbert gwh@lurnix.com gwh@ocf.berkeley.edu ------------------------------ End of Space-tech Digest #106 *******************