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The Orion reentry vehicle will have a parachute system. Like Apollo, they'll have 3:

   Orion Parachute Drop Test

Orion Parachute Drop Test on May 1, 2013

A model of NASA's Orion spacecraft glides to a successful touchdown during a test of its parachute system on Wednesday, May 1. Orion's three main parachutes, which slow it gradually down for landing, weigh 300 pounds each and can cover almost an entire football field.

Photo and caption source: NASA

They seem so nicely separated in pictures. But from a physics standpoint, the aerodynamic force seems like it should be upward, and that would tend to mush all the chutes together. That would seem quite dangerous, by decreasing the area or making them crumple.

Is there anything about the design of the parachutes and tethers that prevents this? Is there something about the aerodynamics that keeps this from happening? Or is some bumping just not a big deal?

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up vote 20 down vote accepted

It's just the aerodynamics. There is high pressure where the air spills out the side that tends to push them apart more than the forces that you mention that pulls them together.

Good thing too. A giant parachute with the same drag would take too long to open.

Clustering is very commonly used for cargo.

cluster another cluster a third cluster

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Much more air flows out the top of a round parachute than out the sides - round canopies without vents oscillate rather violently, notice all the openings in that picture. If you see a canopy with no vents it's probably made of porous fabric, nylon woven for the purpose leaks a LOT of air - you could tape it over your mouth and still breathe comfortably.

Round canopies with drive vents ( search for "Para-Commander", a last-generation steerable round parachute) do not cluster as their forward speed will cause collisions, and modern sport-skydiving canopies are very unfriendly to their peers. Any jumper with both canopies out has to land very carefully as the system can collapse with little provocation, and that's usually fatal for the person under it.

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Yes, stability is critical, which requires porosity. Either geometric porosity, material porosity, or both. –  Mark Adler Jan 25 at 1:19
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