#121
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fun with expendable SSTOs (was The 100/10/1 Rule.)
kT wrote:
Henry Spencer wrote: In article , Pat Flannery wrote: We've read up on your "Brown Bess" booster concept; if you were going to make an unmanned expendable SSTO, how would you go about it, and what propellant combo would you use? First, as Richard observed, I'd drop the "unmanned". If it's reliable enough for expensive cargo, it's reliable enough for people. Like, for example, me. :-) (There are people who suggest building relatively unreliable rockets to be used for bulk cargo -- water, fuel, etc. -- only. I don't think this actually works out well. You still need moderately good reliability, say, 80-90%, if only to avoid being fined for littering :-). I don't see a significant cost or complexity advantage to be had from the difference between that and the 98-99% of conventional expendables. If you can dependably get 80-90%, it should cost very little extra to hit 98-99%.) (Getting to 99.9% is harder, as witness the fact that no existing expendable has definitely achieved it -- there are a few uncertain cases where moderate production runs simply had no failures -- with the *possible* exception of the Soviet-era Soyuz launcher. It should be feasible, given careful design, a high flight rate, and automated production. Even 99.99% is probably not out of reach for expendables, if you sweat hard on things like systematic process improvement. Beyond that is strictly reusable territory.) The real major dividing line is reusable vs. expendable. Here, by definition, we're talking expendable. After that is the big question of whether whoever's paying for it has constraints to impose: use existing engines, no Russian subsystems, a minimum payload size, etc. They also might have opportunities to offer, e.g. use of shuttle-ET production facilities. Many of these things can severely constrain the design. Assume none of this. The major subsystem question is engines: buy or build? Buying means you don't have to get into the engine-development business, which saves a lot of trouble and may look less risky to potential investors. There are some downsides: (a) it's a lost dimension of competitive advantage, (b) the choice of existing engines is somewhat limited and can severely constrain design choices (in particular, ruling out many unconventional approaches), and (c) buying engines tends to be expensive and to involve a lot of hassles. I'd favor build, if only to relax design constraints. The major specs issue is, how much payload to what orbit? Orbital inclination affects delta-V requirement by determining how much help you get from Earth's spin. The big question for orbital altitude is whether the orbit is low enough for a direct-ascent trajectory -- continuous burn all the way up, like Gemini or Apollo -- or requires a Hohmann ascent like the shuttle, injecting into an elliptical orbit and then doing a final insertion burn at apogee. Hohmann ascent would always be more efficient if the atmosphere didn't get in the way. In real life, direct ascent usually incurs little penalty up to 300-400km, but gets rapidly worse thereafter. The nice thing about direct ascent is no engine restart. And even with Hohmann ascent, you pay a price for higher orbits. Absent outside constraints (e.g. cargo delivery to ISS), I'd favor direct ascent to 250-300km, high enough to last a little while and give the payload time to maneuver higher or be picked up by a tug. As for how much payload... depends on whether there's a specific mission constraint. If not, I would favor relatively small payloads, giving a small launcher and frequent flights, and relying on orbital infrastructure (assembly base, tug, fuel depot) to assemble larger systems. Smallness actually is not that important -- launcher cost scales much more strongly with complexity, thinness of margins, and closeness to the leading edge of technology than with sheer size -- but frequent flights are beneficial in many ways. *How* small depends on how much inconvenience you're willing to accept. There are cutoff points where inconvenience rises sharply because you can no longer launch particular objects in one piece, plus a general slow rise in inconvenience as orbital assembly operations multiply. If you want at least the option of launching people, that obviously sets a minimum size. For serious orbital operations, I see a high payoff for being able to launch a two-man ferry spacecraft, sort of a stripped-down Gemini, in one piece: it lets you have one pilot and one passenger, so the passenger doesn't need exhaustive training in emergency procedures for the ferry. Gemini weighed a bit under 4t, with early-1960s technology and greater capabilities than the ferry really needs. An aggressive modern design could come in quite a bit lighter. For serious orbital operations, the other thing that it would be nice to launch in one piece is a minimal habitation module. Perhaps inflatable... but with an expendable SSTO, a "wet workshop" approach using the spent stages is also very attractive. Say: Launch #1 carries a life-support module with consumables, integrated with the spent stage and with a docking hatch at the top. Launch #2 carries a multi-hatch docking node integrated with the spent stage, and a tug; the tug maneuvers it to mate with #1, and sticks around to supply attitude control and reboost. A ferry docks with one of the ports on the node, and you're in the space-station business. (Actually berthing would be better than docking, but that's a detail.) How much does each load have to weigh? That would need more study, but it's interesting to note that the Apollo-Soyuz Docking Module was about 2t. Could this sort of scenario be done with payloads of 2t or less? Probably, but it might get pretty tight. 5t should be lots. Let's be mildly aggressive and set the payload at 3t. I'd want to look into infrastructure issues -- size of manufacturing machinery, size of facilities, etc. -- and if it didn't look like a slightly bigger launcher would cross any boundaries that made things significantly harder or more expensive, make it bigger just on general principles. Materials etc. cost very little; the infrastructure issues are the main things that make a launcher cost more just because it's bigger, and they mostly rise in sudden jumps, not in a steady slope. And far more people have regretted making a launcher a bit too small than have ever regretted making it slightly too big. Anyway, let's cut to the chase -- this has already taken rather longer than I meant to spend on it :-) -- and look at the launcher. This is based on some past thought but without rigorous calculation for this particular design problem. Shape: a plain cylinder with a cone on top, or possibly a two-slope cone like the nosecone for Apollo 5 (which has lower drag and more usable volume) -- simple to make, simple to analyze. More the proportions of say, a Jupiter than a Delta -- the shorter, fatter shape has a bit more drag but is a lot stiffer and less prone to bending problems. Very light tanks, probably pressure-stiffened like the old Atlas. Likewise for the nose -- that was done on Atlas for SCORE and some other flights. (Here the nose stays on until reaching orbit, after which it hinges up and over to expose the payload, staying on the rocket so it goes back down when the rocket deorbits itself.) Either aluminum alloy or composite -- that would need more investigation. Composites are stronger and lighter, but more hassle to make, and there might be minimum-gauge issues with such light sheets, and composite LOX tanks are still iffy. Pressurization in the tanks is just enough for structural purposes, i.e. not very much. Boost pumps at the bottom of the tanks, or possibly the bottom of the feed lines, add enough pressure to prevent cavitation in the main pumps. (This approach is out of fashion but it has been done successfully in the past; it avoids having to make the tanks stronger and heavier to permit higher pressures.) One interesting option is to make the boost pumps jet pumps, recirculating a bit of the output from the main pumps to the jets in the boost pumps. (That too has been done.) The oxidizer is LOX -- cheap and dense. The fuel is probably propane -- slightly better performance than kerosene, less tendency to leave oily residues and otherwise misbehave, and it's still liquid and quite dense at LOX temperatures. Finally, for engines, I'm partial to the idea of an aerospike with a ring of small individual chambers. The small chambers help keep the scale of most engine-development facilities down. The aerospike provides altitude compensation and also permits a light, compact nozzle with a very high expansion ratio in vacuum. Expander or gas-generator cycle, preferably the former if enough heat can be had. (It's been done with propane.) Post-separation attitude control with propane cold-gas thrusters, and deorbit by dumping residual propellants through the engines. This is where hydrogen shines over the 'lesser fuels'. With the lesser fuels, you just barely make it to orbit, and any fuel you do have left over, you waste to deorbit the booster to then burn up in the atmosphere, which is nearly 90% of your usable payload mass, already delivered to 100 percent of orbital velocity. That's just nuts. With hydrogen, you get there, and then some, with plenty to spare. Lesser fuels make the hydrogen 100/1 rule look good. Yes, the advantages are many for rockets and Atmospheric Flight to Orbit. Double the energy in such a small package. Every performance curve in the Atmosphere times 2, rockets less because of the square. |
#122
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fun with expendable SSTOs (was The 100/10/1 Rule.)
Post-separation attitude control with propane cold-gas thrusters, and
deorbit by dumping residual propellants through the engines. This is where hydrogen shines over the 'lesser fuels'. That's the popular myth, but it doesn't hold up on closer inspection. High Isp and high vehicle performance are two different things, because Isp is not the only variable in the rocket equation. What hydrogen gains on high Isp, it loses on high dry mass, because of large heavy tanks, inferior engine T/W, and added plumbing complexity. The required mass ratio is lower, yes, but it's actually harder to achieve. Stages with SSTO-class delta-V performance using "lesser fuels" appeared several years *before* hydrogen stages with similar performance, and with fewer development difficulties too. All three stages of the Saturn V had near-SSTO performance, but the one that was closest to being a practical SSTO was the first stage -- the one that *didn't* use hydrogen. With the lesser fuels, you just barely make it to orbit... Similar story with hydrogen, if not worse, given the greater difficulty of achieving a given mass ratio with hydrogen. and any fuel you do have left over, you waste to deorbit the booster to then burn up in the atmosphere, which is nearly 90% of your usable payload mass, already delivered to 100 percent of orbital velocity. That's just nuts. Not if you have no use for the booster. If you *have* a use for it, then naturally you retain it in orbit. But if you don't, then whether it's 90% or some other number, it's just dangerous space debris and you should deorbit it. (Indeed, you may be required to do so; the regulatory agencies are getting steadily more concerned about space debris. There has already been one case of a rocket being grounded by government order because debris concerns hadn't been addressed to everyone's satisfaction.) There's nothing about this that depends on the fuel; the LOX/LH2 stages of the Skylab crew launches were deorbited in exactly the way I described, and for the same reason. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#123
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fun with expendable SSTOs (was The 100/10/1 Rule.)
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#125
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article ,
Joe Strout wrote: Not if you have no use for the booster. If you *have* a use for it, then naturally you retain it in orbit. But if you don't, then whether it's 90% or some other number, it's just dangerous space debris... Well, it's only dangerous debris if it's uncontrolled. Even if you have no immediate use for it ... it might make sense to set up an orbital scrap yard -- a crude space station that collects spent stages and basically does nothing but keep them under control until somebody wants to buy them. Depends a whole lot on whether they're all going to the same orbit. The problem is that in general, they aren't -- each customer wants a different orbit, so there is no easy way to collect the spent stages together. The major exception is if they're being used for something like space-station resupply, in which case it might make sense to collect them. Even then, it depends on whether they go all the way to the station, or only to a low parking orbit where a tug picks up the cargo. If it's the latter, spending the extra fuel to take them up to the station might not be worthwhile. And the parking orbit and the station orbit won't stay together -- their orbit planes will precess at different rates -- so even launches to the same station won't all go to the same parking orbit. There is also a general problem that *keeping* them up requires expending stationkeeping fuel to fight air drag, and the amount can be significant for big, light objects like spent stages. A collecting station can do things to minimize the problem, but it doesn't entirely go away. None of which really invalidates your point, of course. But I agree with the original poster, that wasting all that great mass already in orbit seems nuts. At a minimum, we should be saving it for future use. It's definitely an appealing idea, but it's rather harder than it sounds. Mother Nature isn't very helpful. (But if you can't save it for future use, then of course you must deorbit it.) Exactly. So the launcher *has* to have a way to do that, even if sometimes you won't use it. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#126
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fun with expendable SSTOs (was The 100/10/1 Rule.)
Hi Henry,
I believe what your doing is extrapolating trade studies beyond there range of usefulness. In some cases way beyond what the question being investigated by the trade study. To some sort of global statement about something else because certain aspects of the studies might indicate that. Extrapolating outside the useful range of data can be very dangerous and lead to false conclusion. Trade Studies if done properly, are good for comparing this or that. This and that being known quantities and done for the purposes of deciding which is best. They are also probably good for identifying area were further work, thought, consideration is required if you still want to do something where the trade study indicated a negative outcome. Not, as good as the other. With a redesign, new concept, different materials and bit of clever engineering the trade study may go the other way. This is true by orders of magnitude if a large extrapolation outside the useful range has been made. Even second order, third or fourth order effects can come into play invalidating such an extrapolation. An early example of this might be the ET to Orbit GN&C performance trade study that I did for NASA after Reagan announced his "Free ETs in Orbit" program. If I had done the obvious and simply looked at it as the deference between the two, sure it's obvious, taking up more mass cost an appropriate performance penalty. But, I liked the idea, and went about the study with a positive attitude. Taking it to Orbit wasn't just keeping it attached and doing a normal ascent profile. It also freed up some "MECO ET disposal" constraints. Removing those constraints also allowed a more optimal profile which resulted in the a performance gain at MECO. The ET could to be brought to a fairly high orbit with no performance penalty. The question of the Trade Study change from "how much" performance penalty to "how high" you can take it. Also, the OMS engines could still point through the C.G., so from a GN&C/Performance aspect there were no show stoppers. Had NASA had a more positive attitude about taking ETs to Orbit, I don't doubt that the overall outcome of the larger study would have been different. We could possibly have had a 5-6 million pound station by now. Attitude can have a lot to do with the outcome of a Trade Study. In a more recent example with john hare's light weight jet engine, wanting to build his engine, actually see it work, find a use for it. He starts out wanting to dissuade me the Atmospheric Flight to Orbit is even possible, really working against his stated goal. Then working on a Trade Study with certain assumptions to show that it's not possible. I bring up "Fluid Variable Intakes" which fits nicely with his engine and invalidating certain assumptions of his trade study. His study just swung in a different direction. If I had gotten a little more serious about entering the X-Prize, mine most certainly would have been an air breathing solution with a "Fluid Variable Intake". But, just about every turbojet in existence was developed by a government entity not commercial, and what few reasonable worn out but serviceable engine that are available are from the 50s maybe 60s. Where did all the more modern worn-out engines go? BTW john, the X-Prize which yielded a new venture by Sir Richard Branson and Virgin Galactic company still has some very big problems with his business plan/model (maybe/maybe, not, knowing what it is). At least the early part with flying suborbital zero gee tourist flight. The biggest problem I see has to do with recurring costs, turn around time, and size of his market (number of customers). Which an air breathing solution could potentially fix. Or, allow room for a competitor to undercut and take over market share if it becomes a reality. I'm sure you presentation "But the Sim Said It Would Work!" will be quite good. Later, -- Craig Fink Courtesy E-Mail Welcome @ -- Henry Spencer wrote: Post-separation attitude control with propane cold-gas thrusters, and deorbit by dumping residual propellants through the engines. This is where hydrogen shines over the 'lesser fuels'. That's the popular myth, but it doesn't hold up on closer inspection. High Isp and high vehicle performance are two different things, because Isp is not the only variable in the rocket equation. What hydrogen gains on high Isp, it loses on high dry mass, because of large heavy tanks, inferior engine T/W, and added plumbing complexity. The required mass ratio is lower, yes, but it's actually harder to achieve. Stages with SSTO-class delta-V performance using "lesser fuels" appeared several years *before* hydrogen stages with similar performance, and with fewer development difficulties too. All three stages of the Saturn V had near-SSTO performance, but the one that was closest to being a practical SSTO was the first stage -- the one that *didn't* use hydrogen. With the lesser fuels, you just barely make it to orbit... Similar story with hydrogen, if not worse, given the greater difficulty of achieving a given mass ratio with hydrogen. and any fuel you do have left over, you waste to deorbit the booster to then burn up in the atmosphere, which is nearly 90% of your usable payload mass, already delivered to 100 percent of orbital velocity. That's just nuts. Not if you have no use for the booster. If you *have* a use for it, then naturally you retain it in orbit. But if you don't, then whether it's 90% or some other number, it's just dangerous space debris and you should deorbit it. (Indeed, you may be required to do so; the regulatory agencies are getting steadily more concerned about space debris. There has already been one case of a rocket being grounded by government order because debris concerns hadn't been addressed to everyone's satisfaction.) There's nothing about this that depends on the fuel; the LOX/LH2 stages of the Skylab crew launches were deorbited in exactly the way I described, and for the same reason. |
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The 100/10/1 Rule.
"Jorge R. Frank" wrote in message ... Dense-propellant SSTOs have lower gravity losses so they need a smaller fudge factor. (They also typically have lower drag losses since the dense propellants allow smaller tanks, but that's not as significant as the lower gravity losses.) Can you explain this? I would have thought gavity loss fudge factor would be a strong function of thrust/weight. I don't understand why it would be function of density. Danny Deger |
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The 100/10/1 Rule.
Danny Deger wrote:
Can you explain this? I would have thought gavity loss fudge factor would be a strong function of thrust/weight. I don't understand why it would be function of density. It's because of the higher mass ratio, which dense propellants allow. The vehicles gets lighter faster, so acceleration increases more quickly. Paul |
#129
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fun with expendable SSTOs (was The 100/10/1 Rule.)
On 11 Mar, 06:57, (Henry Spencer) wrote:
The oxidizer is LOX -- cheap and dense. The fuel is probably propane -- slightly better performance than kerosene, less tendency to leave oily residues and otherwise misbehave, and it's still liquid and quite dense at LOX temperatures. Quick question, as an Economist might ask: If propane is better than Kerosene, why doesn't everyone use it instead of kerosene? (apart from the non budget constrained LH2 users). |
#130
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fun with expendable SSTOs (was The 100/10/1 Rule.)
Henry Spencer wrote:
Depends a whole lot on whether they're all going to the same orbit. *The problem is that in general, they aren't -- each customer wants a different orbit, so there is no easy way to collect the spent stages together. *The major exception is if they're being used for something like space-station resupply, in which case it might make sense to collect them. The other orbit is the performance optimal orbit, for KSC 28.5 degree inclination due to latitude of the site. Many customers want to just take as much as possible, this essentially includes anyone wanting to go to a lower inclination than the latitude. The number of all flights out of KSC to this inclination is most likely a much larger percentage of last stages to Orbit than the Space Stations. Having an Orbital Junk Yard with Orbital Space Tug Base here makes a lot of sense. |
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