Parachute tweaks

[egoego]

After the 1.1.2 update I made a new clean install (still without mods) and started a new career game. I just made it into orbit and had build a rocket that could transport 2 kerbals into orbit and back which I used for a couple of tourist contracts. The parachute of this rocket is tweaked to Min Pressure 0.75 (all the way to the right), as I was told in the new in game tutorials, and Altitude 1200 (default is 1000 but I like to have a little extra buffer).

Today I was flying a tourist contract with that rocket again and I came down over the Kerbin Highlands (before it was water or Grasslands). The parachute opened too late and didn't deploy fully before the pod crashed into the ground and killed the tourist and the pilot.
EDIT: I had staged the parachute way before that, at about 25km.

My Question:

Why did the parachute deploy too late?

Is it because the airpressure in the Highlands is too low for the Min Pressure setting?
Or is it because the tweaked Altitude is measured above sea level instead of ground level?

Edited May 19, 2016 by egoego
spelling

[Pecan]

Max pressure above highlands happens at exactly the same sea-level altitude as over the, er, sea.  When the parachutes open at that pressure above the sea they still have lots of altitude in which to slow you down.  Over the mountains West of KSC, for instance, which reach to 4km high in parts, they might not even semi-deploy before you smash into the ground.

Pressure/altitude settings are nice but it's probably best/safest to stage them manually as soon as their icons have gone grey again after re-entry - once speed falls below ~250m/s

[Plusck]

33 minutes ago, Pecan said:

~250m/s

Well, it's a bit more than that now

45 minutes ago, egoego said:

Altitude 1200 (default is 1000 but I like to have a little extra buffer).

...

Or is it because the tweaked Altitude is measured above see level instead of ground level?

You really don't need to add more altitude, unless you're trying to slow a very fragile big craft and want several parachutes to open progressively at different heights. You can generally reduce that 1000m to 800m or less and still have virtually no chance of encountering a problem.

The full deploy "altitude" is your distance from the ground, not sea level. Air pressure, however, is completely dependent on height above sea level as Pecan says.

[egoego]

Ok, thanks to Pecan and Plusck I now know that my Min Pressure tweak setting was my problem.

In this thread about the highest montain on Kerbin (6767m) I found the air pressure on that peak to be 0.258, which is way lower than I expected. The advice from the tutorial mission to set the Min Pressure to 0.75 is dangerously missleading!

Also, given that I am not sure that my rocket is slow enough to open parachutes at 7000m, I can only hope never to hit that mountain. 

[Plusck]

37 minutes ago, egoego said:

Ok, thanks to Pecan and Plusck I now know that my Min Pressure tweak setting was my problem.

In this thread about the highest montain on Kerbin (6767m) I found the air pressure on that peak to be 0.258, which is way lower than I expected. The advice from the tutorial mission to set the Min Pressure to 0.75 is dangerously missleading!

Also, given that I am not sure that my rocket is slow enough to open parachutes at 7000m, I can only hope never to hit that mountain. 

The tutorial advice to set semi-deploy pressure to 0.75 is new to 1.1.

It's always been an option to tweak parachutes - and is absolutely necessary if for some reason you don't have control over the craft (e.g. you have a probe with hardly any electricity left, and you aren't sure it'll still have power low in the atmosphere) - but default player behaviour has tended to be to stage parachutes manually all the time.

So if you can guess where you're going to hit the ground, and there are no mountains or highlands there, then go ahead and set to 0.75 or 0.6 (≈3000m). If not, leave semi-deploy pressure low and stage the parachute manually when you're sure to be going slowly enough.

And yes, if you do end up in the mountains, pray that your flightpath puts you in a valley. On a typical re-entry profile, safe deploy speed might not be reached until about 4000-4500m altitude, leaving no time to be braked by chutes.

(edit: physics actually lends a helping hand here: if you are in low orbit and set Pe to about 20km, you'll end up picking up a lot of vertical velocity and be able to deploy parachutes safely at around 4000m. Being in low orbit though, it is relatively easy to guess your landing zone. If you are in a much higher orbit (like the 800km orbit in the Orbiting 101 tutorial) and set Pe to 20km, that additional re-entry speed will give you a much flatter trajectory lower down, and you actually slow to safe parachute speeds at around 7500m. So if you are too high to judge your landing zone easily, physics has the effect of making it matter less!)

Edited May 19, 2016 by Plusck

[bewing]

First: I have never had exactly the failure that you describe in 1.1.2, so take this with a grain of salt -- a few more experiments are necessary to know exactly what failed.

In previous versions, I can say for a fact that parachutes had to deploy in a particular order. First you stage them, then they have to partially deploy, and then they can fully deploy. The two deployment steps took time.

Supposedly, this got fixed with 1.1.0. Now a parachute is supposed to be able to go from staged to fully deployed in one step. But the deployment still takes time. Radial chutes deploy much faster than an MK16.

So, the partial deployment pressure that you mentioned (.75) occurs at an absolute altitude of about 2200m above sea level. The 1200m full-deployment altitude is supposed to be measured straight down to the surface of the ground (not sea level, as said above). But if your ship is coming in with some horizontal velocity toward a sheer cliff, then you may not be too many seconds away from hitting a hillside when the chute starts to open.

Now, I recently saw in the bugtracker a report that the parachute deployment altitudes were not acting right. I just did a bunch of searches and scanning by eye, and I cannot find that report again. IIRC, they said that the full deployment altitude was not being measured right. So you seem to be confirming that report, if I could just find it again.

 

[Warzouz]

On 18/5/2016 at 8:01 PM, Pecan said:

Max pressure above highlands happens at exactly the same sea-level altitude as over the, er, sea.  When the parachutes open at that pressure above the sea they still have lots of altitude in which to slow you down.  Over the mountains West of KSC, for instance, which reach to 4km high in parts, they might not even semi-deploy before you smash into the ground.

Pressure/altitude settings are nice but it's probably best/safest to stage them manually as soon as their icons have gone grey again after re-entry - once speed falls below ~250m/s

I totally agree with that. As I said in another topic. you could auto-deploy chute in 1.0.4, but atmo and/or chutes where changes since 1.0.5. It's more dangerous now. I use to auto-deloy chutes. Now I deploy them manually.

As for the 1000m limit, it's fine for kerbin. Beware on Duna though.