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Were liquid boosters on Shuttle ever realistic?
Alain Fournier wrote:
On Oct/19/2017 at 9:48 PM, JF Mezei wrote : On 2017-10-19 20:15, Alain Fournier wrote: It was my claim from the very start that LH2/LOX does NOT offer better performance for the first stage. If discussing engine performance only, would it be correct to state that SSMEs with higher ISP would offer better performance then RP1 engines or SRBs since it has better Isp? Performance is a somewhat vague term. Higher ISP shouldn't be your measure of performance. If you measure performance by ISP, SSME is better, but it will cost you dearly if you measure performance by cost in dollars. That applies to all stages, though, so if LH2/LOX has poor performance (in dollars) on the first stage, it will have equally poor performance on all other stages. Better ISP means more thrust per pound of fuel, does it not? Yes. Or is there something magical which makes LH2 engines perform not as well at sea level? Isn't the difference in tuning engines for sea level vs vacuum more the engine bell shape/size? It is very difficult to pump enough of LH2 to the combustion chamber in a very short time. You end up needing more engines or bigger engines to have enough thrust. RS-68A seems sufficiently 'high thrust' at over 700,000 lbs of thrust. Using two of them gives you F-1 levels of thrust with almost half again the Isp. Two of them will weigh (dry weight) about half again what an F-1 weighs, but that difference is lost in the noise when you look at the difference in fuel weight you get from the Isp advantage. Does 14.7 difference in PSI outdoors make a difference when it comes to the turbines and combustion chambers which are assume are a tad more pressuzised ? is the only drawback of LH2 engines (from engineering) the aerodynamics of the bigger tanks, which for the first stage, must endure max Q ? The bigger tanks giving greater aerodynamic pressure is a problem. But it is not as important as the problem of getting enough thrust. You can get enough thrust, but it will cost you dearly. Or not so much. RS-68A cost is significantly less than SSME and it's designed to be thrown away after one use. It gives up some performance over the SSME but the high thrust and lower cost make that more than worthwhile. An RS-68A costs about 20% of what an RS-25 costs. As for aerodynamics, R.H. Coates, lead propulsion engineer for SLS, seems to disagree with you. When asked why RP-1/LOX is better for first stage he said, "Refined petroleum is not the most efficient thrust-producing fuel for rockets, but what it lacks in thrust production it makes up for in density. It takes less volume of RP-1 to impart the same thrust force on a vehicle, and less volume equates to reduced stage size. A smaller booster stage means much less aerodynamic drag as the vehicle lifts off from near sea-level and accelerates up through the more dense (thicker) part of the atmosphere near the earth. The result of a smaller booster stage is it allows a more efficient ascent through the thickest part of the atmosphere which helps improve the net mass lifted to orbit." Given a choice between him as an authority and you, well, I'm going to go with him. From a cost point of view, would RP1 engines be much easier and simpler to make or do they represent same level of complexity and thus cost of manufacture ? Bingo! Pumping a tonne of RP1 per second is much easier than pumping a tonne of LH2 per second. Both because the LH2 will have a much bigger volume and because it is very cold. To get the same thrust from a LH2 engine will cost you much more than from an RP1 engine. Both F-1 and RS-68 are gas generator fed engines. RS-68A reportedly costs on the order of $10-$12 million each. I don't find a cost for what an F-1B (the current version) would cost, but I'd bet they're not cheap. SpaceX Merlin, also a gas generator fed engine, but with much less thrust than an F-1, costs around 20% of what an RS-68 costs. Yes, an LH2/LOX engine is going to cost more than an RP1/LOX engine of similar thrust, but don't let RS-25 cost lead you to exaggerate the difference. -- "The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man." --George Bernard Shaw |
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