BFS (SpaceX's Big Falcon Spaceship) is in development and has seen between 2016 and 2018 three major design modifications.

screenshot from Everyday Astronaut Tim Dodd video

January 2018 I asked here how BFS planned to manoever during aerobraking.

Latest SpaceX presentation answered this question with this simulation:

enter image description here (source)

Movable control surfaces have been added, canard and fins, which will steer the vehicule during aerobraking, in roughly the same way one skydiver can change its attitude by moving its limbs, providing differential drag around center of gravity. And this looks like a great idea.

Anyway it feels like (unavoidable opinion-biased statement when considering something which doesn't even exist except in simulations) this design could provide one complicated heat shielding of the whole ship (mostly on hinges) and complicated double purpose of aft fins also acting as main landing legs.

Question is: How viable would it be to have (see animation below) one monolithic heat shield protecting hinges, limited amount of movable control surfaces (2 instead of 4) and conventional landing legs (retracted in animation below)

Note that there's one complexity increase in designing rotatable and slidable surfaces, and that those control surfaces could be lighter since they don't need to whistand the weight of the whole ship on the ground)

Also note that yaw control may be assimilated to paper helicopters.

google image "paper helicopter" (qq jkztd)

views (qq jkztd)

landing sequence (qq jkztd)

heat shielding concerns :

heat shielding (qq jkztd)

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    $\begingroup$ What's your actual question here? $\endgroup$ Oct 22, 2018 at 23:25
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    $\begingroup$ Is what concept viable? You need to describe the concept here. Don't assume we've all seen or read whatever this is you've found. Provide links if you can. $\endgroup$
    – Ingolifs
    Oct 23, 2018 at 6:56
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    $\begingroup$ Aerodynamic consideration may be best suited in aviation.SE $\endgroup$
    – Manu H
    Oct 23, 2018 at 7:11
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    $\begingroup$ This should be a textbook example of how to improve a question. Good job. $\endgroup$
    – Ingolifs
    Oct 23, 2018 at 8:46
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    $\begingroup$ The sliding mechanism is going to be problematic from a "sealing the slot" perspective and from the sheer amount of volume it's going to take up inside the ship. $\endgroup$ Oct 23, 2018 at 11:43

2 Answers 2


Partial answer, focusing on the landing legs only:

The SpaceX solution where the fins contain the landing leg mechanism has two advantages over your solution:

  1. the landing legs are placed far apart and provide better stability,
  2. the landing legs are farther away from the engines, so they are heated less.

The drawback is a heavy structure for the fins.

In some earlier versions of the BFR, the landing legs were integrated in the base (as in your solution):

enter image description here

The current evolution (2018) has the landing legs at the tips of the fins, and they're much farther apart than in the previous image:

enter image description here

so SpaceX had the opportunity to evaluate both choices. The fact they moved the landing legs into the fins tells me that this is the better solution.

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    $\begingroup$ "the optimal solution" -> until the next iteration $\endgroup$ Oct 23, 2018 at 11:42
  • $\begingroup$ 1. how are they farther apart compared to classical landing legs? 2. why are they farther away from the engines? $\endgroup$ Oct 23, 2018 at 11:54
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    $\begingroup$ I've added images to illustrate the difference. $\endgroup$
    – Hobbes
    Oct 23, 2018 at 12:05
  • $\begingroup$ @Hobbes Nice input thanks! (with added swept angle) those sliding surfaces could slide all the way down in "landing mode" and provide two contact points far away from longitudinal axis, while third contact point could be integrated in the 9m diameter body. $\endgroup$
    – user19132
    Oct 23, 2018 at 14:30
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    $\begingroup$ Making the sliding mechanism able to slide all the way down and to hold up 1/3 of the weight of the ship is going to make it even heavier, volume-expensive, and hard to seal. $\endgroup$ Oct 23, 2018 at 16:00

This question is opinion-based and answers have to be speculative, but I hope this remains open since it is great fun :-)

Concerning the heat shield, I don't see any concern with the current BFS design compared to your proposal. The main part of the fuselage has to be covered anyway. In the current design, four surfaces with hinges need to be protected, in your design it is two surfaces, two hinges, and two linear bearings.

I expect advantages of the official design over your proposal in two respects

  1. Complexity: four hinges vs. two hinges + two linear bearings translating said hinges. The latter method introduces a whole new kind of engineering problem and makes the hinges even more complicated. Your visualizations are awesome, but a detailed sketch of the moving parts would make the problems more obvious. The linear movement would need some sort of very stiff rails or something like that. As a rule of thumb, interference of complexities is not additive but multiplicative.
  2. Bending momentum: the hinges will carry large bending loads. Now I don't know how they intend to bear them, but since the fins are placed symmetrically, one could think of spars transmiting the loads between the static hinges. In your design the whole center structure of BFS's body needs to be capable of taking the loads.
  • $\begingroup$ nice input thanks! I agree it cannot be something else than opinion-based, but I mostly agree it's a great fun thought experiment. Regarding heat shielding, covering one smooth uniform surface (see edited question) with ablative material is easier than covering complex geometry details. Torsion is already present in current spaceX concept when yawing, between canard and aft fins. $\endgroup$
    – user19132
    Oct 23, 2018 at 14:09
  • $\begingroup$ @qqjkztd right, there's no way to avoid torsion. $\endgroup$ Nov 12, 2018 at 14:50

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