When is a parachute safe to open?

[RizzoTheRat]

Does anyone know how to work out the safe opening conditions for parachutes?

I found this interesting post from @Gaarston calculating terminal velocity with chutes but can't find anything on the safe limits.

 

Presumably the safe limit is based on a maximum force, which will come from drag, so as the Cd and Area will be fixed for a given parachute type it'll be directly proportional to Dynamic pressure, Q.  However I can't find any info on it.

Using kOS to track Q of a fairly speedy capsule (3 mk1 capsules and a Stayputnik) as it re-entered, I found the Q peaked at about 36kPa when I stopped getting flame effects, it then dropped to about 14kPa, with the radial chutes turning white at around 18kPa, then Q started to rise again with the chutes turning orange at 31kPA and red at 35kPa.  I've not entirely got my head around Q having 2 peaks though, I was expecting it to build to a peak and then drop off as it's directly proportional to drag, I guess there's some mach effects there increasing the effective area.

 

The info on the radial chutes in the VAB says they have a tolerance of 4000kPa, which presumably isn't dynamic pressure as that would equate to about 2500m/s at sea level.  I'm not even sure that could be a maximum dynamic pressure they can withstand when stowed as they'd presumably overheat and explode well before that.

I did find an entry of "Breaking Force = 100" in the radial parachutes cfg file, but no idea what that could be as they're taking way more force than 100N, and I'd need to have a bit of a wade through the topic linked above to try and work out what the Cd and Area are to be able to calculate an actual force at my experimentally derived dynamic pressure to see if it's linked to that, but 100kN sounds too low to me. I might try sticking an accelerometer on a capsule to measure the force when the chute deploys to see if it relates to 100 anything.

 

Anyone got any ideas? 

Edited April 18, 2018 by RizzoTheRat

[Tyko]

In the current version of the game (I think it was added in 1.3.x) there's an option in the right-click menu of the chute that lets you set when it will open. The option "Open when safe" does a pretty good job of delaying deployment until you're craft is slow enough for the chute to survive. for Main Chutes that's around ~350m/s.

Using this Open when safe setting I'll usually trigger my parachutes to stage in space when i'm about to enter the atmosphere - I usually set it to trigger in the same stage where I detach my anything below the heat shield. 

With this setup the chutes are activated, but the Open when safe instructions delay deployment until your craft is slow enough.

[blakemw]

I've also observed the two peaks, I think the first peak is from slowing down to subsonic velocity (relative to the atmospheric density), and the second peak is from hitting denser atmosphere.

[RizzoTheRat]

16 hours ago, Tyko said:

In the current version of the game (I think it was added in 1.3.x) there's an option in the right-click menu of the chute that lets you set when it will open. The option "Open when safe" does a pretty good job of delaying deployment until you're craft is slow enough for the chute to survive. for Main Chutes that's around ~350m/s.

I hadn't noticed that, I've used the deployment pressure option before but that must be new functionality since I last looked. 

What I'd like to do is use kOS to trigger the chutes at a time to help control the range, ie if I'm overshooting the target they fire as soon as possible, and if I'm undershooting they fire as late as possible.  At the moment I'm using 320m/s and 30kPa as the early and late limits but it would be nice to try and calculate them more accurately, but I could use the "when safe" option for the early trigger.

[Snark]

On 4/18/2018 at 8:19 AM, Tyko said:

Using this Open when safe setting I'll usually trigger my parachutes to stage in space when i'm about to enter the atmosphere - I usually set it to trigger in the same stage where I detach my anything below the heat shield. 

^ This.  This is what I do for my own ships, all the time.   Works like a charm.

Max safe speed does depend somewhat on pressure, so the actual speed you're going when the "when safe" chutes open can vary a bit depending on your craft's ballistic coefficient and how steeply it's reentering.  For example, a craft like a lawn dart that has a very high ballistic coefficient may penetrate considerably deeper into the atmosphere before it slows down enough for chutes to open... and in that deeper, thicker atmosphere, the safe speed may be lower.

But for fairly typical plain-vanilla reentries  on Kerbin of fairly-typical ship designs, it usually works out to be around 300-350 m/s for the regular chutes, and roughly double that for drogues.

[pandaman]

'Before impact' is generally considered to be the safest option. 

[LordFerret]

It's not very scientific, but I usually wait until the velocity drops to about 250m/s or 1000m* (safety margin) for small stuff ... and for 'large' modules, I use multiple drogues deployed in stages of 1000m decreasing altitude starting around 5000m* before opening main chutes at 1000m*.

(* above ground level)

 

When all else fails, I watch the when it's safe indicators.

[RizzoTheRat]

On ‎20‎/‎04‎/‎2018 at 11:48 PM, Snark said:

Max safe speed does depend somewhat on pressure, so the actual speed you're going when the "when safe" chutes open can vary a bit depending on your craft's ballistic coefficient and how steeply it's reentering.  For example, a craft like a lawn dart that has a very high ballistic coefficient may penetrate considerably deeper into the atmosphere before it slows down enough for chutes to open... and in that deeper, thicker atmosphere, the safe speed may be lower.

Yeah, that's why I'm assuming it's based on Q (1/2 * Rho * V^2), but the fact they're not safe at higher speeds and attitudes where Q is much lower suggests another constraint.  Possibly a limit on Mach number might make sense.

I'd have thought there'd be some data somewhere on it but can't find anything.

I can't even do accurate drop experiment in kOS as I can't find any parameter than I can log that tells me if the chute is safe or not, so I could log data on when the chute first becomes safe to open (by setting them to deploy when safe), but can't figure out when they become unsafe again which is what I'm after.

Q < 30 kPa works for my current 1.25m craft but I'll have to experiment when I've researched bigger/heavier/draggier stuff.

 

Edited April 23, 2018 by RizzoTheRat

[Not Sure]

2 hours ago, RizzoTheRat said:

Possibly a limit on Mach number might make sense.

I’m like the rest who just have a little rule of thumb, and this is mine. Below Mach 1 seems to work every time. I appreciate the thoughts being put into this, will be used by many in the future.

[Corona688]

I believe "safe" means "unlikely to rip off'" - so the velocity where a partially-deployed parachute will not rip off, even if it can't slow down.

"Unsafe" would be the point when it would rip right off a heavy ship and give a light one a heck of a whack as it rapidly slows down.

When I attach multiple chutes I like to have some 'deploy when safe' and 'deploy when unsafe', in the hope that the unsafe ones might sacrifice themselves to turn an unsafe descent into a safer one.

Edited April 23, 2018 by Corona688

[RizzoTheRat]

I'm not convinced the weight of the ship should matter, as the force is drag, where the ships mass comes in to it is in how quickly the chute decelerates it, and what terminal velocity it teaches.

If you deploy one when it's unsafe does it slow the ship at all or just rip straight off?

 

Good point on Mach 1 generally being a safe speed.  The speed of sound is sqrt(Constant * Pressure/Density), so I can track that on a drop test and see where the chutes open if they're set to open as soon as safe.

[pincushionman]

There’s also the color of the staging icon, which can be useful in determining the appropriate speed. If it’s green, it’s probably safe. Yellow, maybe. Red, it will always break.

This, of course, assumes you have enough parachute for your capsule. I don’t think it accounts for that. So it’s not very useful for planning.

[Archgeek]

What I'm led to wonder is what happens when you hit the chutes on that first dip -- do they pop in drogue mode and slow the craft enough to not rip off when you hit that denser atmosphere?  Can we even get the min pressure low enough to get them pop out at that height?  How silly must it look for the chute to be dancing around the plasma streamers?

[Corona688]

I've discovered that parachutes open **MUCH** later on heavier craft.  With two drogues and two radials on one airplane tube, drogues open well upwards of 600m/s quite high in the atmosphere.  With three chutes and two drogues on two airplane tubes plus a one-man cockpit, I'm screaming "oooooopeeeeeeen" at it everywhere from 10km on down until it snaps open somewhere 300-400m/s if I'm lucky, or lithobrakes when I'm not.

So I bet it's doing the full-out calculation of how much force will be exerted on that chute given that velocity, pressure, and payload.