#131
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fun with expendable SSTOs (was The 100/10/1 Rule.)
Nothing wrong with that thinking.
Things to consider when trying to use Hydrogen/Oxygen SSME type engine on your SSTO launcher: Hydrogen, all that energy in such a small package, nice. Comparatively, Oxygen much larger, allows large variations in the efficiency (ISP) vs Thrust curve with large variations in mixture ratio. All with the same engine. Expansion ratio, make it huge, so the Hydrogen/Oxygen engine can really shine where it does best. Shuttle SSMEs have a compromised expansion ratio, so that they can operate at a low altitude. At liftoff, using this engine will probably require a kick in the ass to get it going. May as well fix the backflow problem caused by the huge expansion ratio at the same time. Since it's going to be running at the extreme end of it's mixture ratio ISP/Thrust curve, may as well do something simple and relatively light weight. The lightest and simplest thing I can think of to give it a kick in the ass, is to drill a big hole right along the centerline of the engine. In the hole, run a pipe going thru the injectors, thrust chamber, throat, and into the bell. This give a simple nice clear path to inject lots of mass into the supersonic flow inside the engine bell, to eliminate any backflow problems. What should the mass be? Well, since the engine is already running on the extreme Oxygen rich side, probably any old heavy dense liquid that reacts well with Oxygen would do. Since pressures are lower in the bell, maybe even a pressure fed tank might work, even though turbines are alway nice to have. Heck, even make the pipe and Christmas tree (or, roots) in the bell out of aluminum. That way when the extra mass is gone at a high altitude, it will simply burn off clearing the thrust chamber/throat and bell of the unnecessary fluid engine plug. There are other cool things that could be done with a fluid engine plug, huge expansion ratio LOX/LH2 engine. Don't like the third propellent, ok, use the hydrogen instead of the the dense liquid. Oversize the first stage compressor of the hydrogen compressor slightly, this way some of the pressurized hydrogen can be bled off into that pipe running down the centerline. Just after the throat, split the pipe into many small pipes, branches of a Supersonic Christmas Tree. The small pipes can then begin to diverge slowly to distribute the extra mass in the bell. Crossing the flow lines in the engine at very small angles so that the supersonic flow is not disturbed too much. Maybe split the flow again a little further down. Smaller, but more total area pipes, gasifying and heating the not so high pressure Hydrogen. A exo-atmospheric SSTO with disposable Supersonic Afterburners. The Afterburners themselves little Hydrogen dispersion rockets, to distribute the not so high pressure hydrogen into the very low pressure Oxygen rich supersonic exhaust. Oh what fun, the development would be. The Trade Studies, at tweak here, a tweak there. But, magnitudes more fun to actually watch it fly and Magnitudes More to Ride. I still think Atmospheric Flight to Orbit is the way to go, I kind of like the extra payload to Orbit. -- Craig Fink Courtesy E-Mail Welcome @ -- kT wrote: Expect for the shuttle, that was pretty darn clever. I want to take everything that was good about the shuttle, and nothing that was bad about it, and apply that experience to an SSME powered SSTO launcher. At this point, a propane powered SSTO makes no sense to me at all. |
#132
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fun with expendable SSTOs (was The 100/10/1 Rule.)
alexterrell wrote:
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). Henry is somewhat isolated from reality. -- Get A Free Orbiter Space Flight Simulator : http://orbit.medphys.ucl.ac.uk/orbit.html |
#133
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article .net,
Craig Fink wrote: 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... Unfortunately, that isn't one orbit. A 28.5deg orbit with a launch at noon is in a different plane from a 28.5deg orbit with a midnight launch. The former crosses the equator roughly at sunset heading south, while the latter also has a sunset crossing, heading north -- the two directions are 57deg apart. And there's a full range in between. Yes, customers often do care about which one they get, for reasons like sun angles, which is why even 28.5deg launches usually have well-defined launch windows. Mind you, sometimes the preferences aren't that strong. You might be able to attract a modest fraction of the 28.5deg customers to a single orbit if you offered a discount. But a fair number of them wouldn't bite. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#134
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article .com,
alexterrell 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). Quick answer: it's not a lot better than kerosene; many designers are constrained to use existing engines or simple derivatives of same; and it's just unconventional enough to trip a cautious decision-maker's risk-aversion reflex. (Despite its avant-garde reputation, spaceflight these days is mostly a very conservative business.) -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#135
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The 100/10/1 Rule.
In article ,
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 not a function of density, but of Isp -- lower Isp is *better* in this respect! Consider a couple of simple cases which are easier to visualize. Vehicle X starts out being 1/4 fuel. Halfway through its burn, with half the fuel gone, it has 7/8 of its takeoff mass. If thrust is constant, that means it has 8/7 (circa 1.14) times its original acceleration. Vehicle Y starts out half fuel. Halfway through its burn it has 3/4 of its takeoff mass, so 4/3 (circa 1.33) times its original acceleration. Because its less efficient engines gobble more fuel, its mass drops off faster and its acceleration increases faster. As I mentioned in my original posting, the difference is not huge -- about 300m/s out of 9000m/s or so -- but it's significant. By itself it would not make lower Isp competitive, but it helps. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#136
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fun with expendable SSTOs (was The 100/10/1 Rule.)
On 18 Mar 2007 02:01:40 -0700, "alexterrell"
wrote: 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). Propane may be slightly better as a launcher fuel, but it's a cryogen (albiet a soft one), which makes it much worse as a missile fuel. As most everyone who has ever built a space launch vehicle has also been in the missile-building business as well, they've understandably gone and standardized on kerosene as their hydrocarbon fuel of choice. There have been a few launcher-only design groups, but none of them so lavishly funded that they could afford to develop *everything* from scratch, and so far they've all found it expedient to borrow bits of kerosene-rocket technology from the missile-based side of the industry than build their own propane rockets from scratch. The more ambitious your goals, though - and this is a fairly ambitious hypothetical project - the more you are driven towards building stuff from scratch rather than borrowing extant but inadequate technology, and this opens the door to propane. Doesn't make it a no-brainer, though. SpaceX, which did design its engines and propellant tanks from scratch, still uses kerosene. -- *John Schilling * "Anything worth doing, * *Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" * *Chief Scientist & General Partner * -13th Rule of Acquisition * *White Elephant Research, LLC * "There is no substitute * * for success" * *661-718-0955 or 661-275-6795 * -58th Rule of Acquisition * |
#137
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article nk.net,
Craig Fink wrote: What should the mass be? Well, since the engine is already running on the extreme Oxygen rich side, probably any old heavy dense liquid that reacts well with Oxygen would do. Since pressures are lower in the bell, maybe even a pressure fed tank might work... If you're injecting into the supersonic flow, you might as well just use water. There won't be *time* for any significant reaction; the exhaust is moving *fast* at that point, and accelerating fiercely, and it just doesn't stay in the nozzle for long. The usual rule of thumb is that reaction rates even in the already-mixed combustion gases are effectively zero after the throat. Even just reasonable mixing -- which is what's needed if you want to bulk out the exhaust this way -- is doubtful with just a straight pipe. You need some kind of atomization, even if it's only plugging the end of the pipe and drilling a bunch of holes in its side just above the end. Some kind of non-trivial injector discharging into the chamber will work a lot better. At which point, of course, you can start thinking about how much hydrogen and how much dense fuel is appropriate. Interestingly enough, you might find that the percentage of hydrogen keeps going down as you analyze more carefully. Surprise, you've just discovered that dense fuels are better for SSTOs than hydrogen... I still think Atmospheric Flight to Orbit is the way to go, I kind of like the extra payload to Orbit. Odd that people who actually study this in detail quite consistently find that it gives less payload to orbit. (E.g., the HOTOL team found that going all-rocket, with other assumptions left unchanged, would more than double their payload. Same story as dense fuels vs. hydrogen: lower Isp, but also much lighter hardware bringing the mass ratio way up.) -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#138
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article . net,
Craig Fink wrote: I believe what your doing is extrapolating trade studies beyond there range of usefulness. No, actually, I'm looking at real rocket stages that were actually built and flown. Achieving high single-stage delta-Vs is consistently harder with hydrogen than with dense fuels; that is actual experience, not trade studies. Dense-fuel rocket stages with SSTO delta-V numbers were in production for operational service in 1962. The first hydrogen stage that came anywhere close was the S-II, whose development difficulties (notably in dry-mass control) were severe. Extrapolating outside the useful range of data can be very dangerous and lead to false conclusion. Quite so. For example, much of the belief in *hydrogen* comes from the implicit belief that you can replace (say) kerosene with hydrogen without making a large difference to the stage's dry mass. That was a common assumption in early studies, but is verifiably false in actual experience. Other propellant substitutions had made little difference in dry mass, but the range of propellant temperatures and densities involved was relatively modest, and so the impact of hydrogen's extremely low temperature and very low density was grossly underestimated. 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. Was that for the original two-OMS-burn ascent, or the later "direct" ascent? My understanding was that taking the ET to orbit originally showed a net gain, but that turned into a loss when the more efficient "direct ascent" trajectory became usual. ...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. I don't believe you explained what "Fluid Variable Intakes" are. It's not a term I'm familiar with either, although I don't keep up with the current fads :-) on the airbreathing side. I'm sure you presentation "But the Sim Said It Would Work!" will be quite good. Unfortunately, I'm not going to make Space Access this year, to my great annoyance, so the talk won't appear. Deadline problems. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#139
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fun with expendable SSTOs (was The 100/10/1 Rule.)
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#140
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fun with expendable SSTOs (was The 100/10/1 Rule.)
"Henry Spencer" wrote in message ... In article . net, Craig Fink wrote: ...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. I don't believe you explained what "Fluid Variable Intakes" are. It's not a term I'm familiar with either, although I don't keep up with the current fads :-) on the airbreathing side. It would have needed negative mass to counteract the engine weight to a degree necessary to match pure rocket stage performance. I lost interest somewhat in supersonic intakes when it became clear that there could not be a cost advantage to them. If I ever get started again, I have business reasons for a continuing interest in some airbreathing cruise. If my business model matches reality closely enough, The mass penalties will be acceptable. If I get money, it will most likely come from pilot/businessmen. If we ever fly, it will probably have to be from airports that have not done the paperwork to become spaceports. Fad maybe, but I hope to compete in a slightly different field of my own choosing. This does not require that I be deluded in thinking it performs better for real space missions. I'm sure you presentation "But the Sim Said It Would Work!" will be quite good. Unfortunately, I'm not going to make Space Access this year, to my great annoyance, so the talk won't appear. Deadline problems. -- This eases the sting of missing two in a row for me. It is very annoying to miss my mini vacation repeatedly. spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
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