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For whatever reasons, it seems like the standard upper stage engine in the U.S. inventory is the RL-10. A high performance engine with nice specific impulse (Isp = 450-465 s), but very low thrust (25 Klbs).

For Ares-1, the initial notion was to try a Space Shuttle Main Engine (SSME) on the second stage, and air start it, which ended up being a non-starter.

SSME is a great performer in Isp (452 seconds in vacuum and 366 seconds at sea-level) and much higher thrust (512 KLbs thrust), but that is literally 20 times the thrust.

Due to the difficulty in air starting (really starting period) an SSME, they changed to the notion of using a J-2X with good Isp (448 s in vacuum) and about half the thrust (294 Klbs).

Ares-1's payload to LEO was not huge (56 KLbs), compared to, say, a Delta-4 Heavy that uses the RL10 as an upper stage.

So a 20 fold increase in thrust seems like it would be more of a hindrance than help. J-2X with only a 10-fold increase in thrust seems like it would be problematic as well.

You can throttle all you want, but a 10-fold or a 20-fold thrust increase seems like a big deal to accommodate.

I understand that Isp is king in upper stages, which is why the fairly low thrust RL10 is so effective (similar to the discussion in the question How is an RD-180 engine a reasonable replacement for 2 NK-33/AJ-26 engines on Antares?).

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SSME wouldn't have been insane. RL-10 is used on relatively lightweight upper stages: Centaur and DCSS plus their payloads are in the 20-40 ton range, and the thrust-to-weight ratio winds up being in the ballpark of 0.3:1 -- not enough to lift off, but reasonable for circularizing your orbit once the first stage gets you most of the way to altitude.

If you've got 10 times the thrust, you can push a much larger payload at the same acceleration, so Ares-I second stage would be a much larger beast.

I don't know exactly how big the conceptual SSME-driven Ares-I second stage would have been, but you know how I love back-of-the-envelope math:

With a shuttle SRB at 12000kN thrust, a total weight of 800 tons could have started off the pad at 1.5:1 thrust-to-weight, which is respectable. SRB is 600t, so second stage plus Orion would have come to 200t. SSME at 1860kN pushes that at just about 1:1 TWR when it lights up, increasing as fuel burns off, which is indeed much higher than most RL-10 stages, but not unacceptably high.

According to Wikipedia, it seems like Orion was going to eat too much of that 200 tons mass budget:

Shortly after the initial design was approved, additional tests revealed that the Orion spacecraft would be too heavy for the four-segment booster to lift

So the design evolved from from a 4-segment SRB + SSME stage to a 5-segment SRB + J-2. The five-segment booster increases thrust to 16000kN (and presumably weight to about 750t), giving you 1088t off the pad at 1.5:1. 338t upper stage, J-2X at 1310kN, yields a much lower TWR at separation, about 0.4:1 -- very similar to what you see for RL-10-driven upper stages.

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Rumor had it that the air-start of the SSME at stage separation was doable, but restarting it for any kind of orbital burn was not. I have never seen this openly discussed, and can hardly believe it wasn't addressed in the conceptual design, but something made them switch to J-2X with the subsequent dire effects on the program.

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    $\begingroup$ You think the J-2X was a bigger project than air starting an SSME? I am not so sure. :) $\endgroup$ – geoffc Dec 18 '14 at 19:40
  • $\begingroup$ The impacts were more the increase in upper stage size and weight due to the J-2X's lower ISP. $\endgroup$ – Organic Marble Dec 18 '14 at 19:47
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    $\begingroup$ You would really prefer to not throw away uber expensive SSMEs also. $\endgroup$ – Erik Dec 18 '14 at 20:50

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