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I was going to put a whole back-story for this question here, but thought it'd be better as a Google+ post instead, for reasons that should be obvious to anyone who reads it.

So, during high velocity returns, is there a chance that a parachute will fail if the capsule is travelling too fast when it's deployed? If so, what's the optimal speed/altitude range for deployment?

If speed is not the cause of these failures, what is, and how can I mitigate it?

I should also say that I'm using Mechanical Jeb. For reasons explained in this question, I now always bring the MechJeb unit back with me. In the case of the most recent failure, the MechJeb unit was positioned between the capsule and the parachute, something that had worked for me before. But for my rendezvous mission, I used one of the radial mount options, keeping the (proper sized) parachute connected to the top of the capsule.

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Possibly related: kerbalspaceprogram.com/forum/showthread.php/… –  kalina Sep 22 '12 at 16:43
    
@MBraedley Have you looked at the flight log after impact? It won't (usually) tell you what caused a failure, but it should tell you where the failure is occurring, which might give some insight into your issue? –  DwarfSlice Sep 22 '12 at 16:58
    
indeed, you should get something along the lines of "Structural failure on linkage between Mk16 Parachute and Mk1-2 Command Pod" –  kalina Sep 22 '12 at 17:01
    
@pixel: Hmm, there is some useful information there that makes me think of another possibility. I've updated the question. –  MBraedley Sep 22 '12 at 17:38
    
in one of my ships everything below the command pod breaks away.. which is fine by me as I don't need a separator. –  reto Jan 7 '13 at 12:49

2 Answers 2

KSP has an advanced physics engine and while the planets of the Kerbol system are different to the Sol system, the mechanics are similar:

In real life a heavier vessel requires more parachutes to slow to a safe speed. The chutes undergo huge stresses when they fully deploy. If these stresses are too great, the parachutes could simply tear off.

All chutes partially deploy at around 30km (in Kerbin, it varies on different planets). While partially deployed they have around 1% effectiveness - so still useful.

Regular chutes fully deploy at 500m and drouge(Mk25) chutes at 2500m.

It sounds like aerobraking was working for you, but you didn't have enough chutes. A single standard chute(eg. mark16) is only good for around 1t of vessel - that's a little more than a mk 1 command pod. I recommend using Mark2-radial chutes and putting on a few of them. How many? That's a question for KSP parachute calculator

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The rule of meassure I use is to only deploy the parachute when below the speed of sound (330 m/sec). I tend to wait until 250 m/sec, but I've never had a parachtute ripped off when under 330 m/sec.

The parachute doesn't do much anyway until you hit 500-1500m (Kerbin) altitude (most videos on Youtube show 500m, I'm playing 0.19.1 and it's usually roughly 1350m for me.). If at that point you're still going faster than 330 m/sec; may Kerbal God have mercy on your poor Kerbals' souls...

Edit: I do not use MechJeb, so I have no idea of its weight. The figures above are for a pod (doesn't matter how many Kerbals) and a parachute. If it's heavier, I'd scale those numbers accordingly.

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parachutes will start stabilising the flight path as soon as deployed AFAIK as drogue chutes, but won't fully open until 500m above ground level (which can be a ways up if you're landing in a mountain region). –  jwenting Apr 16 '13 at 9:22
    
Yep, but if you're still going too fast when deploying, it'll be ripped off. I haven't landed on a celestial body with an atmosphere other than Kerbin, so I'm not sure if variations in atmosphere (pressure, density, ...) will have an effect on those numbers. –  Flater Apr 16 '13 at 12:45
    
+1 To jwenting regarding altitude. KSP's built-in altimeter shows ASL (above sea level - or, more precisely, above mean sea level) only. Parachute deployment happens based on AGL (above ground level) altitude except when over water (when "ground level" is effectively below sea level). I highly recommend getting MechJeb, which will help to show you the difference and makes a whole lot of other useful information and tools available. –  Iszi Jul 25 at 18:39

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