Learning about the freakin' parachute

[MissileMonkey]

From the bottom up, here is the spacecraft I've been experimenting with in the sandbox mode. Took a couple of cues from Scott Manley's tutorials.

LV-T30 engine

FL-T100 fuel tank

Second FL-T100 fuel tank

Third FL-T100 fuel tank

MK1 command pod

Mk16 parachute

(appropriately staged engine and parachute, using SAS)

With this configuration and some experimentation with throttle and angle of ascent, I can achieve altitudes of up to 70 km straight up, but at those altitudes I'm still losing the spacecraft due to "chute destroyed by aero forces and heat" on descent.  Even angling the ascent direction thirty or so degrees off of vertical did not help.

So on a hunch, I added a component to the stack.

LV-T30 engine

FL-T100 fuel tank

Second FL-T100 fuel tank

Third FL-T100 fuel tank

TD-12 decoupler

MK1 command pod

Mk16 parachute

(also appropriately staged, engine, decoupler, parachute, using SAS)

The parachute is now more than capable of surviving even vertical descents from 70 km, since I dump all that excess weight before deploying it.

In real life, a parachute can only function properly if it is not overloaded.  The information in the program itself and the KSP wiki are a bit lacking on the technical specifications of the chute and the mass it is rated to carry. For that matter, there's no mass figures on any of the other components. A google search didn't turn up much more.

I guess my question is: In KSP, are things like trying to figure the load on the chute just guesswork, or can we really apply rocket science to it?

Edited June 29, 2021 by MissileMonkey

[Snark]

Moving to Gameplay Questions, since that's a better venue to get answers to these kinds of questions. 

7 hours ago, MissileMonkey said:

From the bottom up, here is the spacecraft I've been experimenting with in the sandbox mode. Took a couple of cues from Scott Manley's tutorials.

<simple rocket design>

With this configuration and some experimentation with throttle and angle of ascent, I can achieve altitudes of up to 70 km straight up, but at those altitudes I'm still losing the spacecraft due to "chute destroyed by aero forces and heat" on descent.  Even angling the ascent direction thirty or so degrees off of vertical did not help.

Yes, it's not surprising that this runs into problems.  The issue, though, is actually not what you think it is-- more on this below.

7 hours ago, MissileMonkey said:

In real life, a parachute can only function properly if it is not overloaded.  The information in the program itself and the KSP wiki are a bit lacking on the technical specifications of the chute and the mass it is rated to carry. For that matter, there's no mass figures on any of the other components. A google search didn't turn up much more.

I guess my question is: In KSP, are things like trying to figure the load on the chute just guesswork, or can we really apply rocket science to it?

It's understandable that you might think that the problem that destroys the parachute is "load", such as how much mass it's supporting... but that's actually not the issue, at all.  I mean, at all.  You could literally have a 1000-ton spaceship with a single Mk16 parachute on it, and that wouldn't destroy the parachute from "overloading", because that's simply not a thing in this game.  (It wouldn't slow the 1000-ton ship down very much, so your "landing" would be at pretty high speed and would likely destroy the ship from the crash, but it wouldn't rip the parachute off while in midair.)

What kills the parachute isn't load.  It's airspeed.  The problem you're facing in situations like this-- "my stupid parachute keeps breaking!"-- isn't that your parachute is overloading, it's that your vehicle's terminal velocity while falling is too high.

 

The technical details of what's happening

Here's how it works:

1. You can stage the parachute at any time, including in space in a vacuum.  Once you've done that, it will semi-deploy as soon as it's able.
2. The parachute won't deploy at all if the atmospheric pressure is below 0.04 atmospheres (by default, you can tweak that).
3. The parachute has a certain "critical speed", such that if it is deployed when above that speed, it gets destroyed.  (This is the problem you're running into.)
   • This critical speed varies with pressure.  Near sea level, it's around 250 m/s.  Higher up near the pressure limit for semi-deployment, it's around 400 m/s.
4. If it's not deployed at all yet, while you're falling, then by default the parachute is set to "deploy when safe", so if you're initially falling faster than the safe speed, it won't deploy.  This doesn't slow you down... but at least it doesn't rip the parachute off. 
5. A typical scenario is that your craft will slow down somewhat due to atmospheric drag as it falls, so eventually its speed will fall below that critical speed, at which time the parachute will deploy.
6. In the happy case, it then stays deployed and un-destroyed all the way down.  For one thing, even in its semi-deployed state, it adds some drag, which helps to slow you down a bit more.
7. When you fall below the "full deployment" altitude above the surface-- which is 1000 m by default, though you can tweak it-- then it opens all the way and slows you down by a lot.

The problem that you're running into is in step 6 above.  Your falling craft has a terminal velocity that's too high.  It's built like a javelin:  dense and slender.  In other words, its ballistic coefficient is too high.

 

What's happening to your ship

So what's happening to you is, at some point you're falling and you're slow enough for the chute to semi-deploy... but the craft is still too heavy and aerodynamic to be slowed down, so it keeps its speed (and maybe even continues to accelerate from gravity) as it falls down into denser air.  The critical speed gets lower as the air gets thicker, but you're still going way too fast, so eventually your speed gets above the critical speed and it rips the chute off.

Since the problem is that your ship's terminal velocity is too high... then the way to fix this is to reduce your terminal velocity.

 

Okay, so how do I reduce my ship's terminal velocity?

As with so many things in KSP, there are multiple possible ways of solving the problem. 

Solution #1:  Alter the ship by staging

You've already discovered this method on your own, so congratulations!

7 hours ago, MissileMonkey said:

TD-12 decoupler

...By throwing away those fuel tanks and engine, you change your craft from a sleek, slender, aerodynamic, dense javelin with a high terminal velocity, to a craft that's much less aerodynamic (with the blunt end of the capsule leading the way), and which is also much more "draggy" relative to its mass.  So air drag slows it down by a lot more, and therefore it can safely fall with no problems.

Personally, this is usually my own go-to solution.  It's simple, it works, and (in career mode) is available very low on the tech tree.

Solution #2:  Alter your trajectory

The shallower the angle that you launch at, the more time you have to lose speed due to drag and the less punishment your chutes take.  You mentioned a 30 degree angle from vertical-- that's still pretty steep.  Aiming for a more shallow trajectory than that can help, depending on ship design.

Solution #3:  Use drogue chutes

A drogue chute, such as the Mk12-R, is one that's specialized for slowing down a craft from very high speeds, down to a safe speed for "standard" chutes (like the Mk16) to operate.  Things to know about drogue chutes:

• They can handle much higher speeds.  The "critical speed" (that it can tolerate before being destroyed) is roughly double that of a standard chute.
• They have a much higher drag while semi-deployed than a standard chute does.  So they help slow things down quite a bit even before they're fully deployed.
• They have a much lower drag while fully deployed than a standard chute does, so they're not very useful for landing (if you tried to land on drogue chutes alone, you'd probably hit the ground at 30-40 m/s, ouch).
• Their minimum pressure to semi-deploy is substantially lower than a standard chute, meaning that they can open (and start slowing you down) at a significantly higher altitude.
• Their default altitude to fully deploy is 2500 m, which is a lot higher than a standard chute's 1000 m.

So, what you can do for your ship, for example, is to add a couple of Mk12-R chutes to it, in addition to the Mk16, and put them all in the same stage for convenience.  As the ship falls, the drogues will deploy first, and will substantially slow the ship so that when the Mk16 deploys, it should stay safely under its critical speed.

Solution #4:  Increase drag via orientation

Another way  you could potentially mitigate this is to alter the orientation of your craft.  For example, you could use drag, and make your craft as draggy as possible by falling sideways.

Solution #5:  Use "body lift" to descend at a shallower angle

Another option would be to use body lift to try to descend at a shallower slope.  Body lift is what you get when the fuselage of your cylindrical ship is angled to the oncoming airstream, generating some lift.  (Kinda like when you hold your hand out the window of a moving car, and angle it so that it deflects the airstream downwards-- your hand gets pushed up).  About the optimum angle for maximum body lift is around 25-30 degrees.  If you're falling nose first, you'd want to keep your nose pointed about 30 degrees higher than .  If you're falling tail first, then you'd want to keep your nose pointed about 30 degrees lower than .

Some limitations of the body-lift technique:

• It works better when you're falling at a not-so-steep angle.  You mentioned angling away from the vertical by 30 degrees-- that's still pretty darn steep.  You may have better luck with a shallower angle than that.
• If your craft is too aerodynamically stable-- i.e. if it acts like a badminton birdie with all the mass in the front and all the drag in the back-- then the limited control authority from your reaction wheel may not be enough to give you that 30 degree angle.  Your craft may end up getting held fairly rigidly in the worst (i.e. most aerodynamic) position, and you can't generate the angle needed to get lift.  How well this works for you will depend on the design of your craft, which depends on the shape and the mass distribution.

Using body lift becomes much more powerful and practical if you've got some steerable fins on the back of your ship, such as the AV-R8 Winglet.  These allow you to steer with the power of the airstream, so you're pretty much guaranteed to be able to get some reasonable angle on your orientation.

Solution #6:  Put wings on it

With wings on the craft-- even stubby ones, like the AV-T1 Winglet-- you can glide instead of plummeting, which lets you descend at a much shallower angle and therefore slower, which is safe for the parachutes.