Perfectly symmetrical craft flips out on reentry

[Kobymaru]

Hi! So I have been designing my Lander for eve, and run into problems on testing.

I wanted to use the 10m heat shield to slow down, but it kept flipping out on me. I thought it was just my craft being asymmetrical, but after some testing, turns out that it's not. 

Check out the following craft: 

 

It only consists of parts on the stack, and the RCS blisters in a 4-way symmetry. Now when I use this craft to reenter in a steep angle, with RCS enabled and set to hold Retrograde, the craft keeps flipping out!!

It always ends up in this position:

Here is the craft and quicksave file for reproduction: 
https://www.dropbox.com/s/somq58cybk5aohw/flipper.craft
https://www.dropbox.com/s/p2p1j25ybptcv3o/heatshield-flipout.sfs

How can this even happen? Is there any way to fix this? Or is the 10m heat shield ultimately useless for reentry? Cuz that would be a real bummer.

Edited April 30, 2017 by Kobymaru

[Numerlor]

Try moving com closer to heatshield

[Kobymaru]

11 minutes ago, Numerlor said:

Try moving com closer to heatshield

I'd love to, but sometimes the design just doesn't allow that. Pancakes might be nice for going down, but especially on Eve they're a problem when going up.

I already pushed my CoM as far down as I could. The craft above is just for illustration.

Edited April 30, 2017 by Kobymaru

[Numerlor]

Don't know about anything else, guess you'll have to wait for people that know what they're talking about

Edited April 30, 2017 by Numerlor

[Cpt Kerbalkrunch]

The common explanation I've seen for this problem is to think of the arrow. It has it's weight at the front end (the arrowhead, which is aerodynamically shaped), and it's drag at the back end in the form of feathers.

You have a short, stubby arrow with a large bulbous head. So it flips. I know of two ways to solve it. Either make your crafter even shorter (effectively lowering CoM even further) so that the shield can't flip you, or make it longer and put a ton of drag on the other end. It will have to be quite a bit to counteract the drag of the shield.

And if you're going to Eve, it's the atmospheric layer between 50,000m and 40,000m (I don't know what it's called) that you need to survive. That's where all the action is. If you survive that, you're good. Just make sure your draggy bits have a high heat tolerance or it will end badly.

[bewing]

There is passive and active stability. Passive stability comes from having the Center of Pressure behind the Center of Mass. The Center of Pressure comes from the drag of each part. The part at the very front of your craft has the most drag -- so the Center of Pressure is right up at the very front, and not behind the CoM.

So how do you fix it? Active stability control, mainly. More Vernor thrusters and tailfins are my choice. But they need a proper lever arm distance from CoM. They can't be near the CoM. So you need to stick them out on a girder way behind the rest of the ship. You can put some medium-sized horizontal winglets on that girder too, to add drag to the back end.

 

[Kobymaru]

1 hour ago, Cpt Kerbalkrunch said:

The common explanation I've seen for this problem is to think of the arrow. It has it's weight at the front end (the arrowhead, which is aerodynamically shaped), and it's drag at the back end in the form of feathers.

You have a short, stubby arrow with a large bulbous head. So it flips.

Here is my understanding: Both situations, arrowhead front and arrowhead back are stable. One is obviously the lower-energy state, so one is "really" stable and the other is metastable.

If there are any small perturbations to the metastable state (arrowhead backwards), then the craft starts flipping. It never starts flipping just by itself. Now where are those small perturbations if I have an active steering system that is set to cancel out any small perturbations?

Think of it as balancing a broom on your nose. Sure, it's hard, because the broom wants to flip around, but with enough practice you can "steer" the broom so that it always points upwards. What I thought is that the Retrograde SAS setting would just do the balancing for me.

 

1 hour ago, bewing said:

So how do you fix it? Active stability control, mainly. More Vernor thrusters and tailfins are my choice. But they need a proper lever arm distance from CoM. They can't be near the CoM. So you need to stick them out on a girder way behind the rest of the ship. You can put some medium-sized horizontal winglets on that girder too, to add drag to the back end.

This is what I don't get: why do I need a lever arm, and why do I need any more than minimal torque?

If I align the CoP and the CoM perfectly along the direction of motion all the way from reentry, theoretically I would never need any steering at all. Practically, I would think a small reaction wheel would suffice to keep the two aligned. But even with a reasonably strong RCS system, they still slide away?

I can't help but think that there is more going on. Especially since the flipout doesn't seem to be quite random. 

[Cpt Kerbalkrunch]

3 minutes ago, Kobymaru said:

I can't help but think that there is more going on. Especially since the flipout doesn't seem to be quite random. 

I thought the same until I figured it out through trial and error. It sounds like you want a larger ship, so you'll need to elongate it, and put lots of drag at the top end. What @bewing said about fins is exactly what I mean. They're a good idea, unless you're going to Eve. I have a two-Kerbal ship heading for Eve right now. I first used Big S wings as giant fins to stabilize the ship. And they worked great. Right up until they melted.

[Harry Rhodan]

26 minutes ago, Kobymaru said:

This is what I don't get: why do I need a lever arm, and why do I need any more than minimal torque?

Your craft basically rotates around your CoM, so small forces acting on the long lever arm are extremely strong while minimal forces on the short end are pitifully weak. Or to use your broom analogy: try to balance the broom on the heavier brush end, it will be much harder to control.

[Nicias]

You can also put a couple of girders pointing out the back at an angle with inflatable heat shields. They will give you enough drag to make sure you go nose first.

[Cpt Kerbalkrunch]

13 minutes ago, Nicias said:

You can also put a couple of girders pointing out the back at an angle with inflatable heat shields. They will give you enough drag to make sure you go nose first.

Exactly. I used decouplers instead of girders but, same concept. I have a 200 ton Eve lander, and they work perfectly.

[Kobymaru]

1 hour ago, Nicias said:

You can also put a couple of girders pointing out the back at an angle with inflatable heat shields. They will give you enough drag to make sure you go nose first.

1 hour ago, Cpt Kerbalkrunch said:

Exactly. I used decouplers instead of girders but, same concept. I have a 200 ton Eve lander, and they work perfectly.

Could you guys post a screenshot of that? That would be much appreciated.

[Starman4308]

3 hours ago, Kobymaru said:

Here is my understanding: Both situations, arrowhead front and arrowhead back are stable. One is obviously the lower-energy state, so one is "really" stable and the other is metastable.

If there are any small perturbations to the metastable state (arrowhead backwards), then the craft starts flipping. It never starts flipping just by itself. Now where are those small perturbations if I have an active steering system that is set to cancel out any small perturbations?

Think of it as balancing a broom on your nose. Sure, it's hard, because the broom wants to flip around, but with enough practice you can "steer" the broom so that it always points upwards. What I thought is that the Retrograde SAS setting would just do the balancing for me.

Thing is, you're always getting a little bit of wobbling, and while SAS will try to compensate, you will* eventually reach a point where it wobbles just a little bit too far for SAS to correct, and it slips out of control.

*Yes, you will. No matter how hard you try. Murphy's Law is stronger than the laws of physics.

[Cpt Kerbalkrunch]

1 hour ago, Kobymaru said:

Could you guys post a screenshot of that? That would be much appreciated.

You can see it's a large lander/ascent vehicle for Eve, with shields fore and aft.

 

 

The key is that the top shields are slanted. Each time the craft tries to flip, they act as rudders and straighten it back out. I got the idea when I saw an airbrake panel swing out to correct course. I wondered if they would help on an Eve descent, but they kept burning up. Next I tried Big S wings with elevons, but they burned up too. This finally did the trick. After you pass 50,000m (when things get interesting) it gets bent in all kinds of different ways, but it holds together and works perfectly. I still engage the Vernors after I pass 60,000m, but I'm not sure if I need them anymore.

I know people like to go small for Eve, but this thing will put 2 Kerbals on the surface and back into orbit. Trying to get them home without refueling, but that might be pushing it. (Forgive the one-Kerbal portrait: I threw this up for a quick screenshot). I'm sidetracked right now by the Retro Rescue Challenge, but I'll get back to Eve in a minute.

[Vanamonde]

How about some airbrakes at the upper end? 

[Nicias]

Here is mine:

I don't know why, but that crazy fairing totally works. It looks like it should have a ton of drag but it doesn't.

I have drogue chutes on the small hardpoints, in addition to the on the main vehicle. I inflate all HS's before reentry. SAS to retrograde (surface). Once the drogues go off. I stage away the upper HS's, deflate the lower one, and then jetison it. If you don't have a patch to deflate your HS's, you can weight a little later and then chuck the bottom one once the main chutes fully deploy.

[Cpt Kerbalkrunch]

22 minutes ago, Nicias said:

Here is mine:

Nice. I want to do it without drilling this time, so I had to go bigger. Do you have any issues dumping your bottom shield? For my driller I sent last time, I used just the one shield on the bottom as well, but I couldn't get rid of it without losing part of my ship. Ended up landing on it and, luckily, was actually able to drill through it.

[swjr-swis]

7 hours ago, Kobymaru said:

Now where are those small perturbations if I have an active steering system that is set to cancel out any small perturbations?

Even discounting pilot error or tiny deviations of the perfect heading, SAS itself adds perturbations, unfortunately. Especially when trying to keep pointed prograde or retrograde, it has a tendency of starting a self-amplifying swing about the marker, which at high speeds can very quickly swing it beyond control.

[Kobymaru]

Thank you guys, very good info in here! Turns out Eve is *even harder* than i thought.

[Nicias]

11 hours ago, Cpt Kerbalkrunch said:

Nice. I want to do it without drilling this time, so I had to go bigger. Do you have any issues dumping your bottom shield? For my driller I sent last time, I used just the one shield on the bottom as well, but I couldn't get rid of it without losing part of my ship. Ended up landing on it and, luckily, was actually able to drill through it.

No, I deflated it and jetisoned it after my main chutes caught, I had enough horizontal velocity still that it didn't fall below me.

[Cpt Kerbalkrunch]

6 hours ago, Kobymaru said:

Thank you guys, very good info in here! Turns out Eve is *even harder* than i thought.

It's extremely tough, but probably the most rewarding when you finally make it.

One more quick tip:

If you have your inflatable on the bottom of your ship, you're gonna need a way to get rid of it. Even with chutes deployed, it'll fly right into your ship when you jettison it. There are cheats you can use to deflate it, like Nicias mentioned, but if you wanna play it straight you're gonna need to get creative.

[bewing]

Not all that creative. Your ship will naturally tend to wobble with the heatshield attached at the front, when you are in thick air. With some minor steering adjustments, you can enhance the wobble. Then time doing the jettison of the heatshield so that your ship is sideways when you hit the button.

 

[Cpt Kerbalkrunch]

1 hour ago, bewing said:

Not all that creative. Your ship will naturally tend to wobble with the heatshield attached at the front, when you are in thick air. With some minor steering adjustments, you can enhance the wobble. Then time doing the jettison of the heatshield so that your ship is sideways when you hit the button.

 

I guess maybe I just wasn't experienced enough that first time. I didn't just have it wobbling, I had it rocking from one side to the other. But even when it was completely horizontal (no matter which side), it would still fly into the ship the moment it was decoupled. Finally just landed on it and drilled right through it. This time I've solved the problem, but I remember it being really tough.