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Parachute opening

Open parachute

MARS2020/Perseverance footage of parachute opening shows that parachute cover ejected by the lander (together with other "things") followed same trajectory of parachute box, until parachute itself opened and started slowing down the lander.

But the parachute cover was not on parachute, so probably it was not slowing down as much as the lander.

What does prevent the cover from falling right into the parachute and possibly the lander itself in this EDL design?

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  • $\begingroup$ I wish I could give you complete answer, but I suppose it's going to be simple - the odds are immensely against it happening. Thing if you had a box falling at 600+ miles high and you ejected the lid off of it while traveling 950mph, the odds of the lid falling back on would be astronomical, no? $\endgroup$ – Mikey Feb 24 at 21:49
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    $\begingroup$ @Mikey it would seem not to be a problem in real life, but if you're falling through fairly thin air on a ballistic trajectory and you threw the lid off and then immediately slowed down quite a lot, the lid seems like it can't be that far away when it passes you. $\endgroup$ – Starfish Prime Feb 25 at 10:35
  • $\begingroup$ I read somewhere that there is a video of Viking parachute testing showing debris creating a hole in parachute... $\endgroup$ – jumpjack Feb 25 at 12:39
  • $\begingroup$ @jumpjack do feel free to share the video with us, or the paper talking about it, that might mention the nature of the debris and the damage to the parachute and so on. $\endgroup$ – Starfish Prime Feb 26 at 10:46
  • $\begingroup$ @StarfishPrime I was told about it in a forum, I looked for it with no luck. $\endgroup$ – jumpjack Feb 26 at 12:26
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This perhaps isn't the most useful answer, but from what research I've been able to do it seems that nobody cares. This seems weird, given the risk to the parachute, if nothing else. I can only imagine that the cover's very light weight and high drag ensure that it doesn't come anywhere near the rest of the lander or the parachute, and this is presumably so obvious and uninteresting to all the scientists and engineers involved that they don't think to mention it even in an offhand way.


There have been various things written about the design and use of supersonic parachutes with mortar launchers over the past 50 years or so... Viking 1 used them, as have the various Mars landers that have been sent there since.

There are various things people say about the size of the charge to eject the parachute, and the metal sabot that the gas generator pushes against that in turn shoves the parachute out is a thing that has been highlighted as a potentially risky bit of gear that could punch a hole through the canopy or hit the lander if not dealt with properly.

From Overview of the Mars Science Laboratory Parachute Decelerator Subsystem I see things like

The sabot capture bag assembly is similar to that developed during the MER and Phoenix programs. Its primary function is to capture the sabot after mortar fire, to prevent it from damaging the canopy.

but not one word about the cover of the parachute deployment system, except to note that it exists!

I found a Viking lander-era paper, "Parachute Mortar Design" which I read for free on a website of possibly dubious provenance (I can't read Chinese to say either way, but I won't link it here just in case). It can be bought for something like 30 dollars which no-one should be prepared to pay, quite frankly.

Anyway, the author states:

It is usually aluminum and can be attached to and remains with the apex of the parachute.

and aside from a discussion of shear forces to ensure that the cover is blown off and the mortar stroke isn't limited, that's it. I strongly suspect that supersonic parachutes won't keep the cover tethered to them, however... I don't see it in any of the relevant images.

Finally, I found this: Development and Qualification of the Mars Science Laboratory Mortar Deployment System which says little more than

The pack proceeds to displace the cover that is mounted on the top of the mortar tube.

So there you have it. The cover just gets blasted out of the way, and no-one gives it a second thought. I'll leave you with a nice gif of it happening (and the sabot being caught, by way of a bonus).

Perseverance parachute mortar test

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  • $\begingroup$ do you really see a cover "blasted out of the way" in that video/gif?!? It remains exactly on the parachute trajectory; then the parachute slows down, while the cover does not. (not visible in video, but it's elementary physics) $\endgroup$ – jumpjack Feb 25 at 12:42
  • $\begingroup$ @jumpjack and yet it can be inferred that it doesn't hit the parachute on the way back down, because doing that with a supersonic parachute under load would be Bad. It does, therefore, get blasted out of the way. $\endgroup$ – Starfish Prime Feb 25 at 13:17
  • $\begingroup$ Just by chance. 2,5B$ save, by chance. $\endgroup$ – jumpjack Feb 26 at 10:38
  • $\begingroup$ @jumpjack oh, don't be ridiculous. Why do you think these people are idiots? Do you honestly think that they didn't consider this problem at any point in any of the supersonic parachutes they've made and deployed, successfully, in the last fifty years? They talk about other risks, after all, ones they've successfully identified and mitigated. Just not this one thing you're interested in. So either we have fifty years of people with this huge blind spot and massive luck, or... they've thought about it and realised that it isn't a problem. And that's my whole point. $\endgroup$ – Starfish Prime Feb 26 at 10:44
  • $\begingroup$ Are you aware of the design change which some years ago suddenly led to modify the attitude before atmosphere entry inerface of ALL future missions, as to prevent cruise stage from impacting the lander after being left behind? Additionally, in the press conference about the Perseverance EDL, one of the scientists just said "sky is big, it's difficult that debris hit the parachute, which anyway is quite strong". This is not a nice thing to hear for an engineer like me. $\endgroup$ – jumpjack Feb 26 at 13:41

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