\ 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.
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3$\begingroup$ "the whole craft is" NOT "spun up for gyroscopic stabilization during ballistic descent" $\endgroup$– Organic MarbleCommented Nov 5, 2018 at 17:16
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1$\begingroup$ I have not been able to find a reference to Soyuz using spin stabilization during any reentry. $\endgroup$– HobbesCommented Nov 6, 2018 at 12:46
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1$\begingroup$ Downvoted till a reference is provided showing the spin stabilized Soyuz. $\endgroup$– Organic MarbleCommented Nov 6, 2018 at 13:23
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1$\begingroup$ @OrganicMarble It is an ESA official video. Not a random one. $\endgroup$– user2277550Commented Nov 6, 2018 at 13:40
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3$\begingroup$ 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. $\endgroup$– SaibooguCommented Nov 6, 2018 at 15:06
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- The pink and blue sections are not square, they are cone sections.
- There are 4 thruster units on the capsule, and they are arranged in a regular pattern. The capsule is symmetric.
- 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:
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.
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$\begingroup$ 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. $\endgroup$ Commented Nov 6, 2018 at 11:11
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1$\begingroup$ @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. $\endgroup$ Commented Nov 6, 2018 at 11:30
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$\begingroup$ @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. $\endgroup$ Commented Nov 6, 2018 at 11:36
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$\begingroup$ @SteveLinton I would think that such emergency measures have a significant bearing on basic design. $\endgroup$ Commented Nov 6, 2018 at 11:37
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3$\begingroup$ @user2277550 Capsules roll naturally when not guided (ballistic descent). Stability is natural due to the shape, not gyroscopic. $\endgroup$– SaibooguCommented Nov 6, 2018 at 15:09