Your Questions About Using Eft
Charles asks…
Design question. What if we had SRBs and detachable EFTs in 1969?
I’ve tried to picture what a moon rocket would look like if we had those instead of having to rely on the propulsion system used with the Saturn V. What do you think it would have looked like? What would the key differences be? Do you think it might employ a second SRB stage?
I figure it would be smaller, for sure. Or maybe as large but they could send more stuff.
Just thought I’d throw that out there for you aeronautics fans to have fun pondering.
You know, guano, I have always marveled at the Saturn V. I couldn’t see how the thing could hold its own weight up, much less accelerate from zero to 60 in ten seconds — STRAIGHT UP!!
Have you ever seen that? I read once that by the time the bottom of the rocket reaches the top of the gantry, it’s going 60 mph. The reason it looks so much slower is because it’s so frikkin’ BIG.
MyDigiResults answers:
Actually, solid rockets are less efficient than liquid fueled ones. The specific impulse of the Shuttle SRB is 242 seconds at sea level; while the first stage of the Saturn V rocket had a specific impulse of 265 seconds (the specific impulse roughly tells you how long you can get 1 lb of thrust from 1 lb of fuel; evidently scaled to the massive thrust those rockets have).
Further, the second stage of the Saturn V had an even better specific impulse, partly because its engines were designed to operate in near vacuum, and because they used liquid hydrogen (instead of the jet fuel like kerosene used in the first stage). The specific impulse of the second and 3rd stage was 421 seconds. Which is almost as good as the Shuttle main engines (also using liquid hydrogen) at 453 seconds (but only 363 seconds at launch, due to the air pressure)
So, using solid rockets would have meant those would have had to be larger and use more fuel than the liquid fuel engines used to acheive the same payload and performance.
On the other hand, if you think about keeping the Saturn V as it was and ADD solid rocket boosters, or external fuel tank, well, it may not have worked as well. The extra thrust that the rocket would have had to sustain would have required structural reinforcement, and I am not sure the additional weight would have not completly balanced the extra thrust. Same thing for having external tankage. The Saturn V actually jettisioned several tanks on the way up (one for each stage) while the shuttle brings along a rather large one nearly all the way to orbit; at the end, it is empty, but still quite massive. And an external tank on the side adds to the aerodynamic drag, while it is not as bad when it is stacked vertically.
A lunar rocket using solid fuel would have had to been larger, and not as performing as the Saturn V.
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