Parachute proper usage

[seaces]

Ok I have some question regarding the parachute usage.

how do you know to which altitude the "Min. pressure" setting corresponds to? For me it would be much more simple to set the altitude at which it would pop out. But in this format I have no idea how to calculate for each planet, where parachutes are possibility, how to set proper pressure to deploy them at desired altitude automatically. They had to go for more complicated format.....

To be honest I have not never used the automatic deployment via these setting because I do not know when they will pop up and I do not know either will be my craft by that time going enough slow and not to break chutes. I would always do it manually. Wait for safe conditions and just pop them up. However I am into RT2 mod and maybe this auto deployment could sometimes be of use.

[Mr. Speed]

You can use the Presmat Barometer to check the pressure per altitude. you can check it with a simple test flight. 4KM is for example a usually safe parachute deployment altitude and pressure at this altitude would be 0.54atm. Here are some common deployment altitudes:

 

[nobodyhasthis2]

Also see pressure tables from wiki. Here is one for Kerbin

Kerbin pressure

Altitude (m) Pressure (Pa) Pressure (atm) 0 101 325 1.000 2 500 69 015 0.681 5 000 45 625 0.450 7 500 29 126 0.287 10 000 17 933 0.177 15 000 6 722 0.066 20 000 2 546 0.025 25 000 989.0 0.010 30 000 401.3 0.004 40 000 78.99 0.001 50 000 15.56 0.000 60 000 2.382 0.000 70 000 0 0.000

 

[GoSlash27]

seaces,

 deployment pressure can be estimated by desired altitude, scale height and surface pressure IAW the following formula:

2.718^(-A_d/H_s) * P_s = P_d

Where:

Ad= deployment altitude in meters

Hs= Scale height in meters (check the wiki)

Ps= Surface pressure in atmospheres

Pd= Deployment pressure in atmospheres.

 

 This is not exact because Ksp doesn't use a single pressure gradient and pressure also varies with temperature. Also, this gives no guarantee that your parachute will survive even though you've deployed it at the correct altitude.

But as an example, to deploy a parachute at 6km at Duna:

6km deployment altitude/ 5.7km scale height = 1.053

2.718^ (-1.053)= .349

.349* .0667 atm surface pressure = .0233 atm deployment pressure.

I have had instances where I had a capsule in a degrading orbit with failing batteries and I wanted to pre- deploy the chute so it would still work safely even with no control.

a 4km deployment on Kerbin would work out to e^(-4k/5.8k), or about .50 atm.

HTHs,
-Slashy

 

Edited February 22, 2016 by GoSlash27

[Plusck]

Earlier, I started to reply to this topic using the data available on the wiki, but I wasn't entirely sure about what changed with 1.0.5, and I wasn't sure how up-to-date the wiki really is (certainly post 1.0 for the atmophere, but does it include the 1.0.5 tweaks?).

So assuming the data is correct, I would guesstimate that a setting of about 0.35 would work well for Kerbin and Laythe. That corresponds to a fairly normal re-entry profile for me on Kerbin in any event. For Eve you could go much higher, say 1.0. For Duna though, you'd really have to be very sure of your entry profile: 0.05 would give you an absolute maximum of 2500m of room to land in, and even then you'd have to be pretty sure that your ship has enough drag to be slow enough at that stage.

[Snark]

It's also important to bear in mind that much of this discussion doesn't directly pertain to what the OP really cares about, which is not figuring out altitudes per se, but rather "how do I know ahead of time at what point my parachutes will be safe to deploy."

All of the above advice is excellent.  However, even if you have perfect knowledge and understanding of "what's the pressure at a given altitude for planet ___", that's not enough to answer the OP's real concern.

The fact is, the thing that determines "is it safe to open my parachutes" is determined by how fast you're going, in addition to pressure.  And there's no simple formula for that, because it depends on a lot of things-- not just the nature of the planet's atmosphere, but also the planet's radius, gravity and (crucially, for this question) some variables that have nothing to do with the planet, but rather with your piloting and your ship design.

"How fast will my ship be going when it reaches altitude X on planet Y" depends on how fast you're going when you hit atmosphere; how steep your entry angle is; your ship's ballistic coefficient (i.e. how draggy it is, for its mass); and how much lift your ship generates as it enters (which is a function both of your ship design and of your AoA while entering).

It's possible to come up with a general rule of thumb, for planets with very thick atmospheres-- e.g. "pop open at 6 km on Kerbin" usually works, for example-- but (except for Eve) even those aren't guaranteed safe:  there could be an inconveniently tall mountain poking its nose up, or you could have a reentering ship that lethally combines a high ballistic coefficient with too much aerodynamic stability, so that it can faceplant while still going well in excess of the safe parachute speed.

So if you can manage it, the safest bet is always to pop your chutes when the indicators go white.  If you have a situation where you can't do that (e.g. you want unattended activation in RemoteTech), then you'll have to combine some "rules of thumb" with careful ship design, but it's hard to generalize the rules of thumb because they'll depend on the design.

[seaces]

Thnx you for explaining people.