How big was the acceleration during the different phases after the stage separation? Would it be safe to land a human being that way? (at least taking only the g-forces into account, as I suppose there might be too much heat anyway by design).

I can think of 4 different phases:

  • boostback burn
  • re-entry aerobraking (maybe this one is not significant enough on its own if it "happens" at the same time as the burn)
  • re-entry burn
  • hover-slam
  • $\begingroup$ It never de orbited since it never went to orbit. $\endgroup$
    – Antzi
    Dec 22, 2015 at 20:08
  • $\begingroup$ @Antzi true. Tried to fix that but I could only come up with "redirecting". Maybe you can suggest a better term. $\endgroup$
    – jkavalik
    Dec 22, 2015 at 20:16

1 Answer 1


The whole process is probably very human-survivable.

The very end of the hover-slam is about 2g acceleration (dry mass of stage ~23 tons, single Merlin 1D rated at 723kN, throttled down to 70%, so 723 x 0.7 / 23 = 22m/s2 assuming it lands bone-dry).

The boost-back is done on three engines but with some fuel remaining in the tanks. Our user Hobbes computes that given the speed at separation and the duration of the boostback burn that it exceeds 4.6g acceleration; it's potentially as much as 8g. That would make it the roughest part, but comparable to the ascent forces of Mercury or Gemini manned flights.

Aerodynamic braking force will be much less than it would for an orbital reentry (because of the lower speed); at a guess it would contribute less than 1g of force to the experience.

I would also guess that the jolt at touchdown is substantially less than that of an Apollo-style space capsule splashdown, which could be pretty rough, particularly if the capsule hits a rising wave.

  • 3
    $\begingroup$ the speed of the stage at separation is 5000 km/h. The boostback burn reduces that to 0 and provides some speed for the trip back, with a burn time of 30 seconds. For a speed of 0, acceleration is $46 m/s^2$ or 4.6 G. $\endgroup$
    – Hobbes
    Dec 23, 2015 at 14:22
  • $\begingroup$ Ah, good thinking! Incorporated. $\endgroup$ Dec 23, 2015 at 15:23

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