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SpaceX's attempts to land involve using the first stage engines to slow the booster down, and use grid fins to perform trajectory adjustments.

Parachutes generally aren't rigid and can throw the rocket off course when landing on something as small as a barge, but is it an option to use airbrakes - possibly resembling the grid fins, but with a solid surface - to slow the rocket down during descent, saving fuel, and helping prevent hard landings?

If not, what are the drawbacks preventing SpaceX from using rigid airbrakes?

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    $\begingroup$ To slow down the stage enough, the airbrakes would need to be as large as a parachute. The arguments against parachutes also apply to airbrakes. See space.stackexchange.com/questions/7718/… $\endgroup$
    – Hobbes
    Jan 17, 2016 at 19:59
  • $\begingroup$ @Hobbes: There's little doubt the first and third burn need to be performed as they are - first initiating return from space, and last - performing the landing. But there are many ideas about the second burn, shortly after the atmospheric reentry, where passive methods could appear superior. $\endgroup$
    – SF.
    Jan 18, 2016 at 2:47

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It seems that they initially planned to use the landing legs as air brakes.

Elon Musk stated that:

If you then do some interesting things, like look at our landing gear, they're essentially like giant body flaps, so the drag - when we deploy the landing gear, the drag massively increases, so we have dual use of the landing gear as giant body flaps and as landing gear. That actually cuts the terminal velocity in half and therefore the fuel - the propellant we need to stop the vehicle in half, and actually it's quite an efficient method of landing precisely.

This maybe just an earlier idea which didn't materialized. Looking at the earlier reusability video, you see that legs are partially deployed as air brakes. But with the latest video, legs are no longer used in that manner so it is likely they gave up on the idea. My speculation is that they indeed intended to use landing legs for attitude control during the unpowered decent, but didn't work well during the initial test landings out in the ocean and switched to the idea of grid fin higher up on the body.

Also came a cross a picture that they were wind tunnel testing the deployed legs at one point.

To give a little more context, I like to add that the above comment was made on 10/24/2014, after two landing test flight had been conducted with the landing legs, flight 9 and 10 on 2/18/2014 and 7/14/2014. But also, on 6/17/2014, they already tested the steering fins for the first time on F9R test flight and only 3 months after the comment, flight 14 flew with grid fins on 1/10/15. So it is likely that by the time he made the comment, it was already decided that the air braking with legs is no go.

There was also a twitter exchange on this subject on 11/22/14 and he stated:

Using legs as air brakes to drop terminal velocity in half requires slight redesign & more data. Maybe flight 21.

SpaceX news article on 12/21/15 before the Orbcomm-OG2 launch has a conflicting information regarding grid fins.

The first attempt to touch down softly on water failed, as we tried to control the rocket with small attitude thrusters alone. While it works well for a smooth, blunt body shape like Dragon, that turns out to be a hopeless proposition for something the shape of a rocket booster. Falcon spun out of control and smashed into the water at high speed.

We then added four grid fins in an X-wing configuration to give us the necessary three axis control under high dynamic atmospheric pressure, which peaks at 1.5 tons per square foot.

This solved the control problem and we were able to do two successful soft landings in the water. Max altitude of the rocket stage was 210 km, which doesn't matter a lot, and max transfer kinetic energy was 200 GJ.

The first attempt must be referring to flight 6, and two successful soft landings in the water can only be referring to flight 9 and 10. But the grid fin were not flown until flight 14, so the success of flight 9 and 10 cannot be attributed to the grid fin as mentioned in this article. Well, obviously, that article was written in a hurry as stated at the bottom:

T-zero in 15 minutes, so have to sign off. Apologies for any typos in the above.

-- Elon

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  • $\begingroup$ Thanks I had been looking for that kind of quote, I was sure I had heard that, and when landing video showing the legs coming out in the last 10 seconds, I was bummed. I had hoped the legs would have been used aerodynamically. $\endgroup$
    – geoffc
    Apr 5, 2016 at 16:48
  • $\begingroup$ It was interesting for me that I also remembered the quote, but I have been misunderstanding that just having legs added to the body adds enough drag (without deployment) to reduce the terminal velocity by half. Now that I reread it, he was obviously talking about the drag generated when they were partially deployed as shown in the original video. I would love to hear the story behind this design change and what was the ruling factor for that. $\endgroup$
    – hshib
    Apr 5, 2016 at 16:59
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    $\begingroup$ Unfortunately using the legs as airbrakes puts the drag at the wrong end of the craft; it would have a strong tendency to flip upside down at high speed. $\endgroup$
    – Russell Borogove
    Apr 5, 2016 at 17:12
  • $\begingroup$ To expand on russels comment, the center of lift would very likely in front of the center of mass (rel to moving air). This makes the craft aerodynamically unstable as this is the point where the net drag is applied. (works just like a wind vane) But I do not think this was the reason for the change of plan, as this is a fairly standard consideration. $\endgroup$
    – Arthur D
    Jun 14, 2016 at 23:11
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SpaceX needs a solution that can work at hypersonic speeds, and then down as slow as possible. They have enough fuel allocated to use the engine to kill velocity, and they need steering fins anyway.

Thus grid fins do the trick for steering across the performance domains with the engine providing sufficient deceleration as needed.

Air brakes would likely only work at the very end of the flight, where the engine is mostly controlling the speed of descent.

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    $\begingroup$ "They have enough fuel allocated to use the engine to kill velocity ..." -- In principle, if rigid air brakes were practical, they could allocate less fuel for landing and therefore more to boost cargo. $\endgroup$
    – Keith Thompson
    Apr 5, 2016 at 16:25
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Air brakes would have to withstand very high temperatures, since they basically convert the kinetic energy (speed) into friction (heat). So they would need to be big and expensive.

The rocket already has a side which is very heat-resistant (the engine side, with the exhaust nozzles). So they can use that as a kind of very small air brake. The grid fins are tiny air brakes as well.

Since these don't provide enough braking power, they simply light the engines one more time to cancel out the biggest part of the kinetic energy.

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