# Second stage thrust used to slow first stage

Is it possible to use a large lightweight sail deployed on the first stage shortly after separation from the second stage to receive exhaust thrust from the second stage engine, so as to slow the first stage down?

• You would still need something else to separate them fast enough before you are able to deploy your sail.
– user10509
Commented Apr 21, 2018 at 12:47
• probably not. What's your use case? The sail would have to be robust to withstand being torn apart aerodynamically, and may require significant control to keep the first stage stable. The effect would be short lived as the first stage decelerated and the second stage accelerated the distance would grow, and the effect would diminish with the square of the distance.
– user20636
Commented Apr 21, 2018 at 13:44
• A first stage is not able to entry an orbit on its own. Gravity and the atmosphere would slow down the first stage anyway. Just separate the stages shortly before or after the first stage burns out.
– Uwe
Commented Apr 21, 2018 at 20:20
• @JCRM a use case is not necessary to ask a question.
– uhoh
Commented Apr 22, 2018 at 2:02
• +1 There is a close vote for "unclear what you're asking" which is strange. This one sentence question is absolutely clear, and certainly on-topic.
– uhoh
Commented Apr 22, 2018 at 2:04

tl;dr: Comparing 6 to 16 m/s delta-v to the 2nd stage velocity at separation of about 2,000 m/s it seems there's just not much help.

alt-tl;dr: To quote @LorenPechtel's insightful comment:

Well, if you're using a photon drive. Otherwise it disperses so fast you gain almost nothing.

The question is clear. While I'm not a rocket scientist I'll venture an answer. I'll use a Falcon 9 1st and 2nd stage ballpark numbers as nominals for an envelope-back calculation.

Let's assume the stage separation has no large "kick" and that there's a cone at the top of the 1st stage that deflects the thrust impinging from the 2nd stage sideways. This cone magically expands to a diameter of 100 meters (50 meter radius) and manages to convert half of the force from the 2nd stage thrust that hits it into a decelerating force of the 1st stage until they are separated by 100 meters, at which point much of the 2nd stage exhaust expands so much that it starts missing the cone.

t = metric ton = 1,000 kg

2nd stage thrust:                      105 kN
2nd stage mass :                        97  t

remaining propellant
0%       10%       20%
-----------------------------
1st stage mass:                     26        65       105        t
1st stage acceleration:            2.0       0.8       0.5      m/s^2
2nd stage acceleration:            1.1       1.1       1.1      m/s^2
time to 100 m separation:          8.8      10.3      11.3        s
1st stage delta-v                 16.2       8.3       5.6       m/s


Comparing 6 to 16 m/s delta-v to the total velocity at separation of about 2,000 m/s it seems there's just not much help.

Consider that the last ten seconds of the 1st stage flight had nine equivalent engines accelerating it, ten seconds of half the thrust of one engine won't do much to reverse that.

• Did you get disrupted in the middle of writing this? You reference a couple of papers about the expansion of the exhaust plume but then provide no information obtained from them. Commented Apr 22, 2018 at 4:01
• at first stage separation there will be air pressure, depending on the use case this will vary, and not all upper stages have t/w > 1 -- which is why I asked for a use case -- but it's hard to envisage a situation where it would be useful any way due to the square of the distance thing. However with a low thrust upper stage and residual thrust 1st stage it might be enough to prevent recontact
– user20636
Commented Apr 22, 2018 at 4:04
• @LorenPechtel you are right, I didn't include enough information to explain the image or refer to the links. I think I'll use those as a basis for a new question instead. Thanks!
– uhoh
Commented Apr 22, 2018 at 4:13