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enter image description here

Image shows a myriad of lines holding down the parachute. I can't even count them all. Is this really necessary?

All those strings must surely add a lot of weight to the Soyuz TMA. I would think that only 12, 6, or maybe even just 3 lines are all that's necessary. Surely we have strong enough strings whose tensile strength is enough to hold the weight of the capsule?

Also, don't all those lines pose a greater risk of tangling up the chute? The fewer lines, the more safety. Or is this incorrect somehow?

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    $\begingroup$ I count 64 lines in the photo. Due to ambiguities I think the accuracy is ±5. $\endgroup$ – wallyk May 29 '15 at 5:23
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All round parachutes have many suspension lines, and they're needed to distribute the load on the canopy, keep its shape, and their hooking (or even re-hooking) geometry during deployment can help with reefing and keeping its shape. It's not because we can't make suspension lines strong enough. Fewer suspension lines would require stronger canopy that would then likely result in even heavier parachute, and fewer sections would result in less controlled reefing and it would be more prone to collapsing and canopy tearing.

Soyuz TMA main parachute is a rather large annular pull down apex type that deploys after the spacecraft has already decelerated to subsonic speed with the help of two pilot and four drogue parachutes 15 minutes before landing[1] that reduce the rate of descent to ~ 80 m/s (288 km/h)[2] before the main parachute deploys, and it has a small apex hole to increase stability and reduce oscillation. From that linked Wikipedia page:

A variation on the round parachute is the pull down apex parachute. Invented by a Frenchman named Pierre-Marcel Lemoigne it is called a Para-Commander canopy in some circles, after the first model of the type. It is a round parachute, but with suspension lines to the canopy apex that apply load there and pull the apex closer to the load, distorting the round shape into a somewhat flattened or lenticular shape.

Some designs have the fabric removed from the apex to open a hole through which air can exit, giving the canopy an annular geometry. They also have decreased horizontal drag due to their flatter shape and, when combined with rear-facing vents, can have considerable forward speed.

So, while all those suspension lines do add to parachute's weight, and there's another point you missed in your question that a large parachute is even more difficult to fold properly (terribly important with parachutes!), if it used fewer suspension lines, it would have fewer sections that would somehow have to be even stronger then. And it would have problems keeping its shape once deployed because of larger sections, and due to this more prone to instabilities or tearing of the canopy.

So even if we could construct a chute with stronger suspension lines and canopy fabric, we likely wouldn't use any fewer suspension lines. As for entanglement of suspension lines, well there's always a chance you'll be having a really bad day, but that risk is reduced by using a single main parachute and pilot chutes that pull it out and help it reef. If lines entangle, there's still a good chance that the canopy will be able to keep its shape, lines will only stretch out (hook) from a bit higher up. Compare that with usually three main parachutes that most other spacecraft use, and you start appreciating that it's a tradeoff, and no solution is bulletproof.

Using many main parachutes means they can get entangled, canopies can bump with each other and partially or fully collapse, and there's even more suspension lines, but you can still get home for dinner if only one of them fails. With a single one, if that one fails, you have to deploy the spare one[3] and there might not be any time for that. It's a calculated risk and a design choice. But there isn't much point in reducing the number of suspension lines and parachute sections.


References:

  1. NASA - Soyuz Landing
  2. RuSpace - Soyuz landing profile
  3. ESA - Soyuz TMA (PDF)
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  • $\begingroup$ Well for entangle and misdeploy problems, another idea i had was to use a few compressed air cartridges to pull out the chute in three directions 120 degrees apart. But your point about canopy fabric strength is well taken. That's really too bad because, if I'm interpreting your answer right, we really could use a lot less strings if we had a very strong fabric. Do you happen to know the tensile strengths of the typical parachute fabric? I want to compare them to things like kevlar. $\endgroup$ – DrZ214 May 29 '15 at 2:53
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    $\begingroup$ Then why not less stings that divides towards the top $\endgroup$ – Antzi May 29 '15 at 3:02
  • $\begingroup$ @DrZ214 No, even if we had very strong canopy fabric and suspension lines we still wouldn't use fewer of them, not for this purpose at least. Otherwise yes, there are kevlar parachutes, see that Wikipedia link under Ribbon and Ring section mentioning parachutes used for nuclear bombs. But those are deploying at supersonic speeds and they don't reduce rate of descent as well. Point of Soyuz TMA parachute is to reduce descent rate as well as keep horizontal velocity small. $\endgroup$ – TildalWave May 29 '15 at 3:04
  • $\begingroup$ @Antzi Less strings dividing towards the top is exactly what hooking means. Suspension lines can be grouped and hooked at different distance to the canopy. But it will affect deployed chute's geometry as their angle with respect to it then changes. Refer to the PDF in references (page 4), you'll notice that Soyuz main chute is re-hooked so that suspension cables branch out at two different distances to the canopy: "15. Re-hook the main canopy for symmetrical suspension,...". $\endgroup$ – TildalWave May 29 '15 at 14:44
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    $\begingroup$ @DrZ214: If we had a stronger fabric for the canopy, we'd just make it thinner in order to reduce weight. $\endgroup$ – jamesqf May 29 '15 at 18:24
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The number of ropes (suspension lines) is not arbitrary. Between every two gores (sectors) there is a load tape which transfers load from fabric canopy to lines. The width of lobed sectors (gores) is limited by strength of the fabric. Typically, the fabric is at least two times weaker than the fiber it is made of. The rope (unidirectional fiber structure) is the optimal way to transfer loads. Multiple ropes with small diameter ropes are stronger than an equivalent-section (and -weight) cable.

The parachute of Soyuz has a single canopy with larger diameter than the three parachutes of Dragon for example. Dragon parachutes also have 'myriad' ropes.

The spacecraft parachutes are optimized by trade-off between number of gores, thickness of fabric etc.

parachute terminology

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