Apologies for asking a dumb question, but I'm going to ask it anyway... I was watching a music video where the singer duct-tapes her ex to the side of a rocket to exile him into space, and it made me wonder -- if someone actually attached a person to the outside of the rocket (or, if you prefer less cruelty in your hypotheticals, a roughly person-shaped and person-sized mass), would the rocket's control systems be able to compensate and still reach (roughly) its intended orbit? Or would that be enough imbalance to cause the rocket to lose control of its trajectory? For my hypothetical, assume a Falcon-9 or comparable rocket, and that the duct tape is sufficiently strong to hold :)

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    $\begingroup$ the falcon 9 user guide spacex.com/media/falcon_users_guide_042020.pdf gives allowable height for the C of G but no allowable C of G offset sideways. $\endgroup$ Jul 15, 2021 at 0:04
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    $\begingroup$ Neither the video nor the question mentions the fairing, and I am 99.44% certain the engines have the control authority to handle the CG offset. Let me do the math... $\endgroup$ Jul 15, 2021 at 0:27
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    $\begingroup$ Hey, duct tape works for everything else, why not this? $\endgroup$
    – GdD
    Jul 15, 2021 at 7:12
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    $\begingroup$ My main concern would be aerodynamic buffeting during max-q, resulting from the very un-aerodynamic boyfriend impinging on the supersonic airflow. The sidewalls of a rocket are rather thin, and an asymmetrical supersonic impingement could be a bad day. $\endgroup$ Jul 15, 2021 at 13:07
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    $\begingroup$ As for center of gravity: no issue at all. the Falcon9 first stage can handle a sudden weight (loss of thrust-same thing!) of 60 tonnes, out on a 1.4 meter off center arm, and continue on with a smile. On the second stage, the engine gimbal of 8.5 degrees could handle an elephant hanging off the side. Worry about added mass, not CoG disruption. $\endgroup$ Jul 15, 2021 at 13:13

1 Answer 1


For the Falcon 9, the worst-case point should be near the end of the second-stage burn, when the mass of Alison Goldfrapp's ex is largest relative to the remaining mass of the rocket, and only a single engine is available to steer, with a relatively short moment arm between the gimbal and the center of gravity.

I assume that we are not launching any non-ex-boyfriend payload. The dry mass of the stage is not precisely known, but is close to 4000kg; the payload fairing is about 1700 kg. Stage is 12.6m long; let's assume for simplicity that the ex-boyfriend is mounted at the same height as the normal center of gravity and masses 100kg, contributing 0.0172 of the total mass stage. This moves the CG proportionately; assuming the CG is normally centered, the CG will offset by about 31 millimeters toward our special guest.

I don't know precisely how high up from the engine gimbal the normal CG is, however. The engine contributes somewhere around 10-15% of the mass (the Merlin 1D masses ~470kg, but the vacuum version on the second stage adds a big (but thin-walled) nozzle extension that I don't know the mass of) and the rest of the mass should be fairly symmetrically distributed in the portion above the engine, so the same proportional distribution logic suggests the CG would be something like 4 meters above the gimbal plane.

The gimbal angle needed to reach a 31mm-off-center CG at a distance of 4 meters is about 0.43º. As long as the engine can gimbal this far, it can keep the thrust line pointing through the offset center of gravity, and as long as it can gimbal a few more degrees than that, it can steer its way efficiently onto any sensible trajectory.

I haven't ever seen a source for the gimbal range of the Merlin engines, but it's certainly multiple degrees, probably more than 5º in any direction. This worst-case point, when the second stage is burning the last of its propellant, also happens to be when the least steering authority is needed -- the stage is light, and atmospheric pressure is near zero. Throughout the rest of the flight, the CG will be closer to the centerline, requiring less gimbal deflection, so we should have scads of control authority throughout the flight.

Alison's former paramour will interfere with the aerodynamics a bit, producing some additional drag and creating some supersonic shockwave effects that might cause structural failure, but I think this can be mitigated by using additional duct tape to create a smooth fairing.

Thus, I believe the dead weight that Ms. Goldfrapp is trying to jettison will go to space today.

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    $\begingroup$ Additional payload would reduce the CG offset, yes. I think the drag issue is a bigger concern than the unbalanced mass, so I’m inclined to leave the payload out — though, of course, F9 can easily send multi ton payloads to GTO, and OP didn’t specify a destination orbit. $\endgroup$ Jul 15, 2021 at 12:49
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    $\begingroup$ OP stipulated that we assume the duct tape holds. $\endgroup$ Jul 15, 2021 at 19:21
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    $\begingroup$ I would be happy to present a list of candidates for the position of counterbalance. $\endgroup$ Jul 15, 2021 at 21:57
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    $\begingroup$ One should program an ex-bf add-on, so to speak, for Kerbal Space Program that can simulate what happens to the rocket -- and the bf! $\endgroup$ Jul 15, 2021 at 23:42
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    $\begingroup$ I'm sooo going to use "mitigated by using additional duct tape" in a technical report someday! :-) $\endgroup$
    – TooTea
    Jul 16, 2021 at 9:14

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