How do they get parachutes to deploy at certain altitudes?

[Awass]

What makes parachutes go from semi-deployed to deployed at a set altitude, not just in KSP but in real life too?

[TheCanadianVendingMachine]

Air drag and resistance.

[NASI Director]

Air drag and resistance.

What he said, but I'll explain it in a more in-depth approach:

On the Soyuz for example, there are two parachutes: the first one (I'll reference it as A), that is attached to the second one (. So anyway, the Soyuz is on its merry little way, and deploys A-chute. A-chute is really small, no where near enough to slow down the Soyuz. However, it still creates drag and slows itself down, and since it's attached to the B-chute, it pulls it out in a straight line so the lines don't get tangled and so the parachute doesn't get all flipped over or ripped. So the B-chute deploys, along with A-chute, and the Soyuz gets to fall down safely. But because the Russians really care for their people, especially on submarines, they added some solid rocket motors to the bottom of the Soyuz to slow it down for a softer landing.

TL;DR: Air resistance is a magnificent thing.

EDIT: In KSP, from what I can tell, the parachutes deploy once you hit a certain amount of atmosphere, and even then only slow you down the further you go, hence why parachutes don't work well on Duna, but are OP on Eve.

Edited October 13, 2013 by NASI Director

[zekew11]

in KSP, there is a certain air pressure required for semi-deployment. If a chute is semi-deployed (and therefore the air pressure requirement met), then, at a certain radar altimeter reading (500m for standard chutes, 1500 for high altitude chutes.), it will go full-deployed.

[Awass]

What he said, but I'll explain it in a more in-depth approach:

On the Soyuz for example, there are two parachutes: the first one (I'll reference it as A), that is attached to the second one (. So anyway, the Soyuz is on its merry little way, and deploys A-chute. A-chute is really small, no where near enough to slow down the Soyuz. However, it still creates drag and slows itself down, and since it's attached to the B-chute, it pulls it out in a straight line so the lines don't get tangled and so the parachute doesn't get all flipped over or ripped. So the B-chute deploys, along with A-chute, and the Soyuz gets to fall down safely. But because the Russians really care for their people, especially on submarines, they added some solid rocket motors to the bottom of the Soyuz to slow it down for a softer landing.

TL;DR: Air resistance is a magnificent thing.

EDIT: In KSP, from what I can tell, the parachutes deploy once you hit a certain amount of atmosphere, and even then only slow you down the further you go, hence why parachutes don't work well on Duna, but are OP on Eve.

I mean within the use of a single parachute, it usually does not fully deploy immediately. It will remain somewhat folded and semi-deployed until a certain altitude, and I'm wondering how that works.

Watch how the final parachutes fully deploy at the last second. That's what I mean.

[TheCanadianVendingMachine]

It is exactly the same. The chute deploys, but the air drag isn't high enough so it "idles"

[Awass]

It is exactly the same. The chute deploys, but the air drag isn't high enough so it "idles"

So how do you design a parachute to "idle"?

[TheCanadianVendingMachine]

Deploy it at a higher altitude. That way, the terminal velocity of the chute is lesser than the cargo. The parachute fly's out and idles

[razark]

It's called "reefing". Run a line around the base of the chute and it will keep the canopy from fully opening. Once the line is cut, the chute is free to open. This allows the reefed chute to slow the vehicle somewhat. Allowing the chute to fully open a too high a speed would provide a sudden deceleration, resulting in damage to the chute, the vehicle, or more likely both.

[Frederf]

In the software parachute deployment is a two-step process. The valid states are not-deployed (normal), armed (blue), partial (yellow), and full (green). The user decides to initiate the sequence which is otherwise automated. When armed the code checks if the next stage of deployment is satisfied. For partial it's an aerodynamic density. Full is I believe always a height above surface. If the user initiates automated deployment within one or both parameters for partial or full deployment then no delay occurs to that step.

In reality the process is computer controlled via various automatic steps based on sensor inputs. The physical canopy can be restrained as razark says to temporarily limit full deployment.

[Sandworm]

Another trick that is to install a donut-shaped piece of fabric over the chute's lines. I forget the exact name of the device, but it works to slow the deployment of the full chute by holding the lines together in a tight ring. As the main chute gains air it expands and pushes this donut down the lines, eventually allowing the chute to fully deploy. See the following vid. Look for the ring of fabric traveling down from chute to aircraft.

Edited October 13, 2013 by Sandworm

[Specialist290]

For the record, if you're wondering the maximum altitude at which your parachutes will first partially deploy, this atmospheric density table lists and highlights them.