Pictures of SpaceX's "Big Falcon Spaceship" (BFS) delta wing don't seem to show presence of any aerodynamic control surfaces. How will it's attitude be controlled when encountering the atmosphere?

To go a bit further, BFS V1 looked like center of lift was very close to center of gravity, where BFS V2 looks like center of lift is very aft from center of gravity.

One thing to notice on both versions is the use of blunt wedge at trailing edge, like seen on X-15 vertical stabilizer.

Edit: @PearsonArtPhoto As far as I try to understand, movable ballast sled won't act on the same axis on both BFS and Dragon. I think either vernier nor reaction wheels can match aerodynamic forces on delta wings, and grid fins should be as fat as delta wing to turn it 180° Anyway you're right we'll see the solution soon.

movable ballast sled

  • $\begingroup$ As far as I'm aware, SpaceX hasn't given us concrete details on this. Best we can do is speculate probably $\endgroup$
    – Dragongeek
    Commented Jan 19, 2018 at 10:41

2 Answers 2


According to Elon, mainly with aerodynamic control surfaces.

Compared to the design I showed last time, you can see that there’s a small delta wing at the back of the rocket. The reason for that is in order to expand the mission envelope of the BFR Spaceship. Depending on whether you’re [landing on] a planet or a moon that has no atmosphere, a thin atmosphere, or a dense atmosphere, and depending on whether you’re reentering with no payload in the front, a small payload, or a heavy payload, you have to balance the rocket out as it’s coming in. So the delta wing at the back, which also includes a split flap for pitch and roll control allows us to control the pitch angle despite having a wide range of payloads in the nose and a wide range of atmospheric densities. We were trying to avoid having to have the delta wing, but it was necessary in order to generalize the capability of the Spaceship such that it could land anywhere in the Solar System.

(Bold mine)

There are also meant to be attitude control thrusters operating on gaseous methane and oxygen that would make them much more powerful than the cold gas nitrogen thrusters used on current vehicles, but he didn't emphasize them much in terms of entry control.

  • $\begingroup$ Great input thanks! Waiting to see this split flap. $\endgroup$
    – user19132
    Commented Jan 19, 2018 at 16:05

It isn't known exactly, there are a couple of possibilities.

Dragon uses "Movable ballast sled" in order to control it's attitude when reentering. Basically it moves a weight around to control the angle in which it is reentering.

The second possibility would be using control jets. This seems unlikely. It might also use the main rocket engine thrust gimbaling to control the attitude, or even a grid fin design similar to Falcon 9.

Bottom line, I suspect it will use the movable ballast sled design, or else add control surfaces to the wings. We will probably know more in July or August, when another update will be made in regard to the BFR.

  • 2
    $\begingroup$ A ballast sled is good for setting the "neutral trim" attitude, but because it acts near center-of-mass (thus has a short moment arm), and is heavy by design (thus is slow to shift), it's not optimal for active, dynamic adjustments. $\endgroup$ Commented Jan 19, 2018 at 17:38
  • $\begingroup$ @RussellBorogove in case of a capsule, I'd say most effective way to use some ballast sled would be to tell the crew to unfasten their seat belts and move around according to where they should weigh. $\endgroup$
    – user19132
    Commented Jan 19, 2018 at 18:18
  • $\begingroup$ @qqjkztd Consider that angle of attack adjustments must be made at the correct times during entry to reach the landing target while staying within the vehicle's performance envelop for G loads, aerodynamic loads, re-entry heating, etc. An automatic, movable weight sled seems vastly preferable to the re-entry equivalent of 'Chinese firedrills.' $\endgroup$
    – Saiboogu
    Commented Aug 3, 2018 at 18:30

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