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#21
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Multiple Engines???
In article ,
David Shannon wrote: Alas, SSTO fuel fraction is prohibitive. Not necessarily. When people have been pushed hard to try to build expendable stages with that sort of fuel fraction, they have generally succeeded. And with 1960s technology, too, in some cases. Yup, true. I meant (thinking context evident) "RLV" SSTO. Apologies. Wings, heatshield, deorbit propellant - all dip hard into payload. Leaving off the wings helps considerably with that. :-) Deorbit fuel is not a big deal, but heatshield and landing systems are certainly an issue. On the other hand, we *do* have better technology now than the guys who built (e.g.) the Titan II first stage. To me, it seems challenging, but far from hopeless, especially if you are willing to innovate rather than just believing parametric models -- based on orthodox past practice! -- for everything. (How much does a horizontal lander's landing gear weigh? Orthodox parametric guesswork is 4%. NASA RLV parametric guesswork is 3%. The B-58 landing gear, in 1957, was 1.5%... and the Voyager gear was 0.9%.) Assume the VentureStar was built and worked as advertised. 257 klb inert, 50 klb payload, 2313 klb LHOx, 8 Aerospikes, fuel fraction .883 2 smaller editions, 3 Aerospikes on Booster, and 1 on Orbiter, have *together* 198 klb inert, 50 klb payload, 927 klb LHOx, 4 Aerospikes, fuel fraction .824 The only added complexity is crossfeed, already proven on the STS. No, the added complexity is that now you have to develop three different configurations -- two different vehicles plus the stack. That has a tendency to cost 2-3x as much as a single vehicle. It also adds a bunch more failure modes. The SRB separation may look simple but it isn't; NASA spent a lot of time and money making sure it would work. But work it does, yes? So far, yes. :-) That doesn't mean it's a good idea, especially for a new design that wants reliability and low development cost. -- MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | |
#22
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Multiple Engines???
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#24
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Multiple Engines???
(Greg) wrote in message m...
(David Shannon) wrote in message 2STO typically uses 1/3 the propellant for a given payload, although vehicle empty weights are higher. I don't have a problem with 2STO for a RLV. But how do you get the first stage back? If you have good mass ratio between the 1st and 2nd stage, its likley to be going fast at the seperation point making it quite hard to do. I suspect the TSTO can glide back. Even at mach 8 separations, is the first stage all that far down range, more than 100-200 miles? It'll have a lot of energy from altitude and speed that can be spent turning around and gliding back. However, despite my ill-edumucated suspicions, I do note many early US shuttle concepts seemed to favor lots of jet engines. The "winged Saturn", the Saturn V flyback first stage, had 10 jet engines. Mike Miller, Materials Engineer |
#25
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Multiple Engines???
Leaving off the wings helps considerably with that.
D'accord. I would be happy with a steerable parachute on a capsule - as long as I can land back at the takeoff point ( R! L! V!, R! L! V!) 2 smaller editions, 3 Aerospikes on Booster, and 1 on Orbiter, have *together* 198 klb inert, 50 klb payload, 927 klb LHOx, 4 Aerospikes, fuel fraction .824 The only added complexity is crossfeed, already proven on the STS. No, the added complexity ... three different configurations ... It also adds a bunch more failure modes. Yes indeed. So, don't use differing configurations in the stages. Consider the design path where the stages are externaly identical (eg General Dynamics "Triamese" http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld020.htm ) This approach has theoretical limitations that are outweighed by practical advantages. You only have to spend skull-sweat on the orbiter - the boosters are simplified variants (no OMS, less TPS, etc, etc) I can live with the risks in stage separation. |
#26
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Multiple Engines???
Leaving off the wings helps considerably with that.
D'accord. I would be happy with a steerable parachute on a capsule - as long as I can land back at the takeoff point ( R! L! V!, R! L! V!) 2 smaller editions, 3 Aerospikes on Booster, and 1 on Orbiter, have *together* 198 klb inert, 50 klb payload, 927 klb LHOx, 4 Aerospikes, fuel fraction .824 The only added complexity is crossfeed, already proven on the STS. No, the added complexity ... three different configurations ... It also adds a bunch more failure modes. Yes indeed. So, don't use differing configurations in the stages. Consider the design path where the stages are externaly identical (eg General Dynamics "Triamese" http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld020.htm ) This approach has theoretical limitations that are outweighed by practical advantages. You only have to spend skull-sweat on the orbiter - the boosters are simplified variants (no OMS, less TPS, etc, etc) I can live with the risks in stage separation. |
#27
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Multiple Engines???
But how do you get the first stage back?
A 1993 NASA study (TP-3335) considered the 2STO case where 47% of the ascent propellant was carried by the booster, which had a fuel fraction of .874. It staged at T+105 seconds at 83,175' and 2,900 ft/sec. Even so, it was then only 10.5 nautical miles laterally from the pad. A glide return was perfectly feasible. |
#28
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Multiple Engines???
But how do you get the first stage back?
A 1993 NASA study (TP-3335) considered the 2STO case where 47% of the ascent propellant was carried by the booster, which had a fuel fraction of .874. It staged at T+105 seconds at 83,175' and 2,900 ft/sec. Even so, it was then only 10.5 nautical miles laterally from the pad. A glide return was perfectly feasible. |
#29
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Multiple Engines???
In article ,
David Shannon wrote: Consider the design path where the stages are externaly identical (eg General Dynamics "Triamese"... This approach has theoretical limitations that are outweighed by practical advantages. You only have to spend skull-sweat on the orbiter - the boosters are simplified variants (no OMS, less TPS, etc, etc) The practical problem with biamese and triamese is that almost anything you do to simplify the boosters starts you off down the slippery slope of building two different vehicles. It's very hard to stop that. Just leaving systems out looks easy, but often it means a lot of extra engineering to assess what *happens* when you leave those systems out, and what drives development cost is not materials but engineering effort. Later on, when weight is excessive or there's a bit of a performance shortfall, well, we're already building two different configurations, so we'll just make them a little *more* different... Biamese or triamese is a win only if the boosters are the *same* as the orbiter. Same TPS; if it doesn't get as hot, that's nice. Same OMS; okay, we can leave its tanks empty on the boosters. Same systems, all of them. Maybe we fill the boosters' cargo bays with tanks, but if so, any permanent fittings we need to add go in the orbiters too. It takes very strong engineering leadership to make this work. -- MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | |
#30
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Multiple Engines???
In article ,
David Shannon wrote: Consider the design path where the stages are externaly identical (eg General Dynamics "Triamese"... This approach has theoretical limitations that are outweighed by practical advantages. You only have to spend skull-sweat on the orbiter - the boosters are simplified variants (no OMS, less TPS, etc, etc) The practical problem with biamese and triamese is that almost anything you do to simplify the boosters starts you off down the slippery slope of building two different vehicles. It's very hard to stop that. Just leaving systems out looks easy, but often it means a lot of extra engineering to assess what *happens* when you leave those systems out, and what drives development cost is not materials but engineering effort. Later on, when weight is excessive or there's a bit of a performance shortfall, well, we're already building two different configurations, so we'll just make them a little *more* different... Biamese or triamese is a win only if the boosters are the *same* as the orbiter. Same TPS; if it doesn't get as hot, that's nice. Same OMS; okay, we can leave its tanks empty on the boosters. Same systems, all of them. Maybe we fill the boosters' cargo bays with tanks, but if so, any permanent fittings we need to add go in the orbiters too. It takes very strong engineering leadership to make this work. -- MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer pointing, 10 Sept; first science, early Oct; all well. | |
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