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When returning the side boosters of the latest Falcon Heavy launch to their landing site, their trajectories keep them right next to each other, and I noticed that a "wall" of vapor is visible midway between them. This made me wonder whether flying two boosters near each other has any performance effects in terms of fuel savings or waste. Does anyone know?

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  • $\begingroup$ I'm thinking that landing near launch site may make the booster fly into the turbulence the rocket produces at take off. This may also affect descent performances. $\endgroup$ – Manu H Jun 26 at 13:20
  • $\begingroup$ Do we have any evidence for the actual distance between the two during the reentry phase? $\endgroup$ – asdfex Jul 1 at 9:29
  • $\begingroup$ @asdfex, I don't, but I would be interested to know. $\endgroup$ – foobarbecue Jul 2 at 1:21
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There is likely minimal effect.

At rocket speeds, there is very little effect of shear stress, the only significant effect is particles hitting the leading surface of the rocket.

Also due to how fast hey are going, the effect of the rocket of "pushing air out of the way" does not have time to get far ahead of the rocket, and this drops further behind as you move radially away from the rocket.

In fact the 'wall' in your question is the boundary of this region of influence.

As the leading bit of neither rocket is inside the volume effected by the other, neither is significantly effected (at least in terms of net force, vibration/temperature etc is a different game as these don't just effect the leading bits).

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    $\begingroup$ I like this answer but I wonder if it's all still true even when the rockets are descending / travelling in the direction their exhaust is going? $\endgroup$ – foobarbecue Jun 26 at 1:46
  • $\begingroup$ @foobarbecue interesting question. The same observation that if it was outside of the cone around the end of the exhaust plume then little effect would occur for the same reason. It didn't look to me like this was happening, but its hard to be sure. If it did happen, it would hinder the performance. It would cause less drag and therefore need more active deceleration. As there is no reason for them to be so close on decent, i doubt it would come up. However, if they started trying to land two cores on one drone ship, this could well be an issue. $\endgroup$ – ANone Jun 27 at 16:13
  • $\begingroup$ @ANone This answer lacks sources, can you provide some evidence for your claims? $\endgroup$ – ReactingToAngularVues Jul 1 at 9:00
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If the boosters were closer together, like at separation, there is a complex interplay of shockwaves that produces lots of turbulence. If the vehicles stay in each other’s turbulence, it will need to be corrected for by the guidance system, probably using up more RCS fuel. These boosters are far enough apart during descent that they are outside of each other’s shockwave.

The Falcon Heavy boosters are detached from the core with a pneumatic ejection system in order to have them out of the way quickly.

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  • $\begingroup$ This answer lacks sources, can you provide some evidence for your claims? $\endgroup$ – ReactingToAngularVues Jul 1 at 9:00

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