The Dragon that Musk presented\ I I don't think anyone has the seen the actual dimensions of dragon version 2, but assuming it is what SpaceX will be using to take people to space, I think it will have issues regarding stability during descent. At-least some amount of the cross section of the spacecraft appears to be squarish (shown in pink and blue) . This is probably a unintended consequence of having descent engines to the side ( which is now redundant since they aren't doing propulsive landing). This will affect both lift features, and also gyroscopic stabilization during ballistic descent, if at all possible, will be a lot harder because it would be harder to rotate around its principal axis, since the frame is not symmetric. Is this correct? Also the frame features intended for propulsive landing, like the big channels that leads to the nozzle (circled in green) will seriously affect the overall dynamics, especially during descent, especially since they don't seem to be symmetric.

  • $\begingroup$ What makes you think symmetry is critical @user2277550? $\endgroup$
    – GdD
    Nov 5 '18 at 13:34
  • $\begingroup$ @GdD For soyuz for exaple, lift is critical during re-entry, and it has to be somewhat actively controlled (I am not sure about the control part). But anyway, if you don't have a somewhat stable shape, there is no way you can control lift right? For ballistic descent, drag about the principal axis would be higher without symmetry (not sure how serious this is) but it seems an un-necessary complication. Of course there are a lot of approximations that go on here, but SpaceX's design seems to be somewhat too problematic. $\endgroup$ Nov 5 '18 at 13:36
  • $\begingroup$ I'm not 100% sure what you mean when you talk about asymmetry here @user2277550, maybe a picture or two would help. If you mean the shape of the TPS itself then the asymmetry is an intended part of the design to give life and control to re-entry rather than a pure ballistic descent. $\endgroup$
    – GdD
    Nov 5 '18 at 13:45
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    $\begingroup$ "the whole craft is" NOT "spun up for gyroscopic stabilization during ballistic descent" $\endgroup$ Nov 5 '18 at 17:16
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    $\begingroup$ Your video describes a ballistic descent under no active control. The spacecraft spins randomly due to buffeting, canceling out the offset CoM that lets it provide lift when actively steered. A typical descent is actively steered with RCS to hold a desired angle of attack. See this video - youtube.com/watch?v=HgTNzDCc0gk -- This is not 'spin stabilized,' just a natural roll that occurs without guidance. $\endgroup$
    – Saiboogu
    Nov 6 '18 at 15:06
  1. The pink and blue sections are not square, they are cone sections.
  2. There are 4 thruster units on the capsule, and they are arranged in a regular pattern. The capsule is symmetric.
  3. I have seen no statement by SpaceX that they will use gyroscopic stabilization.

Here's a diagram of the capsule showing that the capsule is symmetric:

Crew Dragon

During a ballistic descent, the capsule will fall heatshield-first, and the rest of the capsule including the thruster fairings is in the lee of the heatshield. Most of the ballistic descent will be at supersonic speeds, where the capsule is in the lee of a shockwave.

Without doing an aerodynamics analysis, I suspect it wouldn't be very difficult to spin the capsule.

The thrusters still serve a function: they are the abort system.

It seems to me that the channels leading to the thrusters would have a stabilizing effect: when the capsule goes off-axis it exposes one of the channels more to the airstream, creating more drag which pushes the capsule back, until all 4 channels have minimal exposure and the capsule is in equilibrium.

You want to avoid spinning the capsule, because it uses parachutes during the final stages of the descent. If the capsule would spin at that point, you'd tangle the parachutes.

  • $\begingroup$ Gyroscopic stabilization, at least with Soyuz, is done as a last ditch option to flatten the trajectory during ballistic descent. Ballistic descent itself is done when the spacecraft lift can't be actively controlled, i.e. as a extra safety measure. I am not sure why my question got so many negative votes. $\endgroup$ Nov 6 '18 at 11:11
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    $\begingroup$ @user2277550 I don't think it was clear to most responders that you are talking about emergency and last ditch measures, rather than a routine descent. Without that it could be seen as a "based on this one photo I'm goign to claim that a major company is being incredibly stupid" question which doesn't usually go down well. $\endgroup$ Nov 6 '18 at 11:30
  • $\begingroup$ @Hobbes The front view, IMHO, seems to be problematic. The black parts, when spun around the axis will make it difficult to control the lift. And the black part itself was mainly intended for propulsive landing which they are not using (I read). Of course they could have put it into a wind tunnel, tested it a lot, and put those values into the control system. But there seems to be too many superfluous things in the design itself. $\endgroup$ Nov 6 '18 at 11:36
  • $\begingroup$ @SteveLinton I would think that such emergency measures have a significant bearing on basic design. $\endgroup$ Nov 6 '18 at 11:37
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    $\begingroup$ @user2277550 Capsules roll naturally when not guided (ballistic descent). Stability is natural due to the shape, not gyroscopic. $\endgroup$
    – Saiboogu
    Nov 6 '18 at 15:09

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