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I just watched the NASA JPL video of the 2014 test of the Low-Density Supersonic Decelerator (LDSD).

Tangential thrusters are used to spin up the craft for stability before the main rocket ignites. The first screenshot shows a pair; the second one fires a few seconds after the first.

Question(s): The spin-up looks so nice. Is this pair balanced by at least one more on the opposite side? After engine cut-off, a pair of tangential thrusters facing the opposite direction fires sequentially and it looks like the craft is almost perfectly de-spun with almost no residual rotation. Are these precision calibrated or is there some kind of control to stop them at the right instant?

I have two small side-questions. I can split them off if someone would like to answer them separately.

  1. In the "previously unreleased high definition, high resolution, high speed video" (2nd screen shot, starting at about 01:10) what is holding the camera?
  2. Each step seems to be accompanied by sound. Is there enough air at that altitude to carry sound to a microphone, or is it Foley?


OK here is the actual video:

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There was no active control for spin/de-spin. We did a very good job (if we do say so ourselves) controlling the mass properties of the vehicle with spin testing and balance masses. Yes, there were balanced pairs of spin motors on each side. They were all made in a single lot for consistency in performance.

In the screen shot you can see the boom holding the camera in the bottom of the picture.

All of the sound you hear is real, picked up by the GoPro microphone. You might notice the sound go down in intensity through the main motor burn. That is due to the vehicle increasing in altitude, with the lower densities reducing the sound transmission.

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    $\begingroup$ I was hoping you would see this question and answer it:-) $\endgroup$ – PearsonArtPhoto Dec 15 '16 at 19:11
  • $\begingroup$ Are the despin motors smaller to compensate for the mass spent by the spin motors, or is that a negligible consideration? $\endgroup$ – Russell Borogove Dec 15 '16 at 21:28
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    $\begingroup$ No, the spin-down motors were the same size, and from the same lot as the spin-up motors. We simply angled the spin-down motors out more, to increase the cosine loss by the amount needed to account for the decrease in the moment of inertia due to the spent main motor propellant. $\endgroup$ – Mark Adler Dec 16 '16 at 8:01
  • $\begingroup$ I've just linked this question about the careful balancing of spin-stabilized spacecraft to here. $\endgroup$ – uhoh Apr 15 '17 at 15:16
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I would guess that the total impulse of the tangential rockets is calibrated closely enough that no active control is needed. They aren't imparting a huge amount of spin, and a small residual wouldn't be a problem. (They look like solids, and while it is possible to design solids for early cutoff, it's unusual.)

Foley is extremely unlikely on a video like this; the primary goal isn't entertainment and no one wants to give crackpots ammunition for claims of fakery. I'd guess that some of the sound is conducted to the microphone through the body of the vehicle, but also a substantial amount via air. If air pressure is ~1% of sea level, if I've done my math right, you should expect about a 40 dB reduction in sound volume, which is substantial but not extreme - like turning a volume knob about halfway down.

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    $\begingroup$ Ah, I assumed the pressure would have been much lower to simulate the upper atmosphere of Mars, but maybe this is near the upper practical limit of balloons for such a heavy payload. These are pretty noisy things to begin with, and the lens cover popping off being so much louder is consistent with the attenuation. Any idea what's holding the camera for the high-speed video segment? It's not still hanging from a balloon, is it? $\endgroup$ – uhoh Dec 15 '16 at 18:32

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