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SSTO propulsion overview
I agree that hypersonic SCRAMJET propulsion research is completely
pointless. But there is a region between standstill and about mach 6 where air breathing propulsion is very attractive. Simple calculations show that you can achieve a factor of 10 savings in fuel consumption if you use ramjet like engines. If your design is correct you get a 15 to 1 air fuel mixture ratio which results in an Isp of about 4000 at standstill. As you start to move the mass flow increases and the engine leans out, just getting better and better with speed, up to about 2km/s at which point it is just easier to close the air intakes and use on-board oxidizer. There have been numerous real hardware experiments and proofs of concept that show the performance of ramjets, ejector ramjets, strut jets etc. An example of an air augmented rocket is the russian GNOM missile see: http://www.astronautix.com/lvs/gnom.htm This missile has less than half the mass of a rocket. Basically my calculations show that you can save the entire first stage if you build an air breathing booster. I have actually invented and built an engine, see at vtol.net that gets an Isp of 4000 at standstill. An air breathing rocket can be built that uses air first and then switches to on board oxidizer and gets into orbit with an overall mass ratio of 6. See also vtol.net/air.htm I keep repeating myself, nobody ever reads the archives. It is very easy to search for stuff and read it. Zoltan |
#12
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SSTO propulsion overview
"Zoltan Szakaly" wrote in message om... I agree that hypersonic SCRAMJET propulsion research is completely pointless. Agree. But there is a region between standstill and about mach 6 where air breathing propulsion is very attractive. Simple calculations show that you can achieve a factor of 10 savings in fuel consumption if you use ramjet like engines. Your simple calculations are a bit off. A dense fuel SSTO would have a mass ratio of 16. Even from mach 6, the upper stage would have a mass ratio of about 6. Since you mention this below, you must know better. This would be a factor of 2 &2/3 if the ramjet used no fuel at all. It also does not include getting the ramjet up to supersonic speed where it becomes operational. Also, fuel is much cheaper than the hardware it burns in. If your design is correct you get a 15 to 1 air fuel mixture ratio which results in an Isp of about 4000 at standstill. As you start to move the mass flow increases and the engine leans out, just getting better and better with speed, up to about 2km/s at which point it is just easier to close the air intakes and use on-board oxidizer. Your exhaust velocity will be on the order of 1,000 m/s, which gives a real Isp in the 1,500 range. This is at standstill if you compress the air sufficiently. As you move faster, inlet drag becomes a factor which heavily penalizes lean operation, which reduces effective Isp from your engines. There have been numerous real hardware experiments and proofs of concept that show the performance of ramjets, ejector ramjets, strut jets etc. An example of an air augmented rocket is the russian GNOM missile see: http://www.astronautix.com/lvs/gnom.htm This missile has less than half the mass of a rocket. All of your examples seem to be about fuel consumption, ignoring the increased mass, cost, and complexity of the hardware and flight profile. A Saturn V class rocket might have $1M in fuel. This is not a consideration in the current market. Basically my calculations show that you can save the entire first stage if you build an air breathing booster. I have actually invented and built an engine, see at vtol.net that gets an Isp of 4000 at standstill. An air breathing rocket can be built that uses air first and then switches to on board oxidizer and gets into orbit with an overall mass ratio of 6. See also vtol.net/air.htm What is the T/W of this 4,000 second engine? Have you calculated inlet mass at all? Are you aware of the mass and complexity of effective supersonic inlets? How are you compressing the air? I keep repeating myself, nobody ever reads the archives. It is very easy to search for stuff and read it. Quit repeating yourself and say something new and convincing. Some of us have read on the subject a bit. Zoltan |
#13
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SSTO propulsion overview
In article ,
Azt28 wrote: And are hypersonics a good idea for anything at all? For high-speed cruise within the atmosphere -- assuming you have some urgent reason to want to do that -- they look promising... I think the sonic boom forbids any hypersonics aircraft service. Not if it's military. And there are some theoretical possibilities for low-boom or no-boom flight, although whether they will work at hypersonic speeds is unclear. -- MOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | |
#14
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SSTO propulsion overview
In article ,
Zoltan Szakaly wrote: But there is a region between standstill and about mach 6 where air breathing propulsion is very attractive. Simple calculations show that you can achieve a factor of 10 savings in fuel consumption if you use ramjet like engines. Unfortunately, that's much more attractive for cruising missions than for accelerating ones, because the price is much heavier engines. Even at modest altitudes, the oxygen content of air is *four orders of magnitude* less, per unit volume, than that of LOX. So you inevitably need big heavy machinery to handle air. (If you thought liquid hydrogen was a "fluffy" propellant, awkward to handle because of its bulk, atmospheric air is enormously worse.) There have been numerous real hardware experiments and proofs of concept that show the performance of ramjets, ejector ramjets, strut jets etc. An example of an air augmented rocket is the russian GNOM missile... Yeah, they're fairly interesting for *cruise* missions. But that's a very different class of problem. This missile has less than half the mass of a rocket. Which is of almost no importance, for launchers. The added mass of the rocket is almost all LOX. Liquid oxygen is one of the cheapest substances on Earth. In particular, it's much cheaper than airbreathing engines. Basically my calculations show that you can save the entire first stage if you build an air breathing booster. That's plausible. But so what? You've turned a rocket first stage into a jet first stage. In the process, you've made it harder to build and more difficult to develop. For what? To save *LOX*? WHY??? -- MOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | |
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SSTO propulsion overview
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#16
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SSTO propulsion overview
"Henry Spencer" wrote in message ... In article , Zoltan Szakaly wrote: But there is a region between standstill and about mach 6 where air breathing propulsion is very attractive. Simple calculations show that you can achieve a factor of 10 savings in fuel consumption if you use ramjet like engines. Unfortunately, that's much more attractive for cruising missions than for accelerating ones, because the price is much heavier engines. Even at modest altitudes, the oxygen content of air is *four orders of magnitude* less, per unit volume, than that of LOX. So you inevitably need big heavy machinery to handle air. What is the maximum possible T/W you see from a turbine based air breathing engine. How inevitable is the question. Do you see a fundamental T/W limit at 100, 40, 15, or some other number? At what T/W do air breathing engines become performance competative with the lower stage rocket thrust they replace? Competative does not necessarily mean desirable in this case, just not a penalty. During a previous discussion I accepted that 120/M seemed to be a reasonable break even for an air breather that supplies all the acceleration from the ground. I suggested a few weeks ago that for a VTVL SSTO, 28 to 43 might be a reasonable requirement for units designed for the landing mass only, not operating supersonic at all during launch phase. Would you agree with these requirements for break even performance? How much better would they have to get to be desirable as opposed to break even? What is the performance requirement for an ICH tee shirt? :-) |
#17
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SSTO propulsion overview
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#18
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SSTO propulsion overview
Ian Woollard wrote:
(Henry Spencer) wrote in message ... Basically my calculations show that you can save the entire first stage if you build an air breathing booster. That's plausible. But so what? You've turned a rocket first stage into a jet first stage. In the process, you've made it harder to build and more difficult to develop. For what? To save *LOX*? WHY??? A reusable flyback booster? Which still need not be hypersonic (or supersonic at all) to be effective. Super/hypersonic seperation of stages is not a trivial issue, and a subsonic air-breathing carrier that gets you into less dense air, and a rocket powered second stage whose exhaust expansion is less of the inevitable compromise between sea-level and vacuum is a desirable thing.... -- You know what to remove, to reply.... |
#19
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SSTO propulsion overview
Ian Woollard wrote: (Henry Spencer) wrote in message ... Basically my calculations show that you can save the entire first stage if you build an air breathing booster. That's plausible. But so what? You've turned a rocket first stage into a jet first stage. In the process, you've made it harder to build and more difficult to develop. For what? To save *LOX*? WHY??? A reusable flyback booster? You can have that without using jets on the ascent, only during the return flight. That is a solved problem. Heck, they can even use the same fuel as the ascent rocket engines, for sane choices of fuel :_ |
#20
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SSTO propulsion overview
In article ,
johnhare wrote: ...the oxygen content of air is *four orders of magnitude* less, per unit volume, than that of LOX. So you inevitably need big heavy machinery to handle air. What is the maximum possible T/W you see from a turbine based air breathing engine. How inevitable is the question. Do you see a fundamental T/W limit at 100, 40, 15, or some other number? I'm not a turbine-engine guy, so it's a little hard for me to call. My understanding is that the fighter-engine guys are now in the 10-11 range, and it's taken them thirty years to get there from the 7-8 range. The air temperature at the turbine inlet is now well above the melting point of the turbine blades (!). (The blades are single crystals of very stubborn alloys, with cooling vents blowing [relatively] cool air out onto their surfaces to keep the hot stuff at a distance.) That technology isn't too far from its limits. 15, maybe? Radical design changes might perhaps take it farther. But that's harder to predict. I'd be surprised to see 25. (I do get surprised sometimes.) Systems which don't use turbomachinery can do better on mass, but they have a hard time doing as well on air handling, and they generally don't work at low speeds. (Mind you, the turbomachinery tends not to work very well beyond about Mach 3.) Hybrid systems, rocket/airbreather combinations, can do still better. The question there is whether there's enough Isp gain to be worth it. At what T/W do air breathing engines become performance competative with the lower stage rocket thrust they replace? Competative does not necessarily mean desirable in this case, just not a penalty. Given the other constraints they impose -- for example, they tend to need reasonably clean airflow, which is not easy to come by on the surface of a lower stage -- I think I'd call for at least 40, and that's not going to be easy, especially as speed builds up. (Good LOX/kerosene rocket engines with sea-level nozzles are up around 125.) During a previous discussion I accepted that 120/M seemed to be a reasonable break even for an air breather that supplies all the acceleration from the ground. I suggested a few weeks ago that for a VTVL SSTO, 28 to 43 might be a reasonable requirement for units designed for the landing mass only, not operating supersonic at all during launch phase. Would you agree with these requirements for break even performance? I wouldn't strongly *disagree*, but that reflects limited feel for the problem rather than deep conviction that those are good numbers. :-) -- MOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | |
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