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In a gravity assist does the incoming velocity of a space craft effect how much speed it gains. For example say a rocket was approaching Jupiter for a gravity assist with speed v and another rocket was approaching with speed 2v. Would the incoming speeds (v and 2v) effect how much speed they are able to gain from Jupiter (for example, would the rocket at speed v gain more than the one at 2v because it spends more time around the planet and therefore has more time to gain more speed)?

I am looking for a proper quantitive relationship or proof of the effect of the velocity of a spacecraft going into an assist on the ability of the spacecraft to gain or lose speed. I am also curious what the effect on its ability to change direction (deflection angle) would be as well. If anyone can provide the proof or direct me somewhere where it is done it would be greatly appreciated.

Alexander Ivanov
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  • The change in velocity depends on both incoming velocity and "impact parameter" (how close it would pass if it continued along a straight line). You can read about all of this by searching this site for "gravity assist" and "impact parameter" or just clicking on the gravity-assist tag. – uhoh Jul 09 '20 at 02:54
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    @uhoh I found a formula for the delta-v and in it is only the gravitational parameter, hyperbolic excess velocity (related to the incoming velocity) and height of periapsis. I was wondering how the impact parameter effects the delta-v - maybe the height of periapsis takes it into account? They are different values however. – Alexander Ivanov Jul 09 '20 at 05:52
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    @uhoh I found the answer from answers in these two questions: https://space.stackexchange.com/questions/6582/how-much-delta-v-can-we-squeeze-out-of-a-gravitational-slingshot-and-what-factor?noredirect=1#comment146796_6582 and https://physics.stackexchange.com/questions/128356/gravitational-slingshot-maximum but I am still trying to understand the derivation for the delta-v formula (if you can help me with that I would greatly appreciate it). Should I answer my question by putting in the links? – Alexander Ivanov Jul 10 '20 at 05:28

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When the bat strikes the ball, the ball will gain velocity equal to twice the bat's velocity times the cosine of strike angle. The gain is independent of the ball's initial velocity. It's only the velocity of the bat and deflection angle that matter.

The same physics applies to gravity assists. However, a faster spacecraft must fly closer to the planet to achieve the same deflection angle and velocity gain. It spends less time in the gravity field and therefore must be deflected with higher acceleration. Flying closer is sometimes impossible without crashing into the planet or its atmosphere. This limits the range of possible deflection angles for fast spacecraft.

Rainer P.
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