# Once in Earth orbit, what is the strongest propulsive acceleration an astronaut has experienced?

Early crewed launches subjected astronauts to large propulsive acceleration, usually right at the end of a stage before shut down and separation when the mass for that stage is minimum. (see acceleration plots in What is BECO? (Gemini) Same as MECO? that go to 7.5+ g!)

Once in orbit, maneuvers are usually quite gentle. The next very strong g forces astronauts experience are during aerodynamic deceleration during reentry.

But once in Earth orbit, what is the strongest propulsive acceleration an astronaut has experienced?

For the purposes of this question trajectories on the way to/from the Moon can probably count as being in Earth orbit, but I think that once in a lunar orbit or on the Moon, those wouldn't count.

• Do you care whether it is inside a vehicle, outside a vehicle, or either? – DrSheldon May 30 '20 at 6:54
• @DrSheldon either is fine – uhoh May 30 '20 at 7:02
• Over what period are you measuring the acceleration? I would assume that at the moment of ignition, the acceleration would be infinite for an infinitesimal short time. (Assuming spherical cows.) More realistically, the "jolt" of ignition will have very high acceleration for a very short time. – Jörg W Mittag May 30 '20 at 7:41
• @JörgWMittag I'm not sure that's actually true for real engines and realistic scenarios, but if you have an answer of "infinity" and you can support it then go ahead and post it. For answers in general though, let's assume this is sustained propulsive acceleration for at least something like 0.5 seconds. – uhoh May 30 '20 at 7:56

Most orbital maneuvers are pretty gentle, because if your engines can accelerate you at a significant fraction of a gee, you probably brought too much engine mass.

Early space capsules had very small maneuvering engines, but somewhat more powerful retrorockets used to lower their orbit promptly for reentry. In Mercury's case there were three retrorockets which normally fired in overlapping sequence; the acceleration would be about 0.4g per rocket or less than 1.2g total. I didn't do the math on it, but at a glance I see that Gemini's retrorocket thrust is higher by roughly the same proportion that the spacecraft is heavier, so it should have similar acceleration. Vostok used a liquid bipropellant retrorocket yielding about 0.35g.

Voskhod had a big solid backup retrorocket in addition that would have produced about 2g for 3 seconds or so but I believe it was not used on the two Voskhod.

Soyuz uses its maneuvering thrusters for retroburn, producing a small fraction of a gee.

If we're counting translunar burns, but not atmospheric reentry (because that would be non-propulsive), then I believe the highest g-force would be Apollo 8's TLI. Apollo 8 lacked a lunar module (though it did have a fairly massive ballast in place of it), so it was the lightest translunar Apollo. At the end of its translunar burn, it would have been accelerating at about 1.5g.

The other likely suspect was the Apollo CSM, which has a rather oversized engine, but the maximum acceleration it could achieve is under 0.75g when completely dry.

• I believe aerobraking that doesn't result in reentry should still count. – SF. May 30 '20 at 23:56
• Are there any examples of crewed aerobraking not resulting in prompt reentry? – Russell Borogove May 31 '20 at 17:47
• Might Apollo 11's trans-Earth injection burn have been higher? Apollo 11 had the lightest CSM stack of any of the Moon-orbital missions, and at the end of the TEI burn, would have been carrying the least fuel. – Mark May 31 '20 at 21:09
• TEI seems to be excluded by the wording of the question, and as I note in my answer, an empty CSM could only get up to ~3/4 g in expending the last of its propellant. – Russell Borogove May 31 '20 at 21:52