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What is the specific impulse of the RCS thrusters on the Apollo Service Module? This document cites 281 seconds for the LM's RCS thrusters, but I cannot find similar specs for the SM.

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Some resources (Wikipedia, Astronautix) give 312 seconds for the R-4D, but I think that's for the large-nozzle modern version.

Marquardt's "Apollo SM-LM RCS Engine Development Program Summary Report" gives the nominal specific impulse as 280 seconds, and gives figures for flown engines on Lunar Orbiter I-V as 276.1 to 279.5 seconds. The Lunar Orbiters, the Apollo LM, and the Apollo SM all used the R-4D. The exact specific impulse will vary with mixture ratio and tank pressurization, so I don't think the 276-281 second range is particularly significant.

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