Spaceplane turns on reentry

[Mat2ch]

Hi,

I'm trying to build a huge spaceplane that is capable of delivering a Rockomax Jumbo-64 fuel tank into LKO. I've made several attempts just to build plane huge enough to carry one someday. My latest attempt is this:

It is capable of going into LKO and returning with enough fuel to at least try a landing. (Actually you see it here returning from Orbit, heading home. Didn't make it though, not enough liquid fuel left)

But on reentry it flips to it's back (actually to its tail). I observed this behavior now several times with different spaceplanes, but on the first ones I didn't take too much care about CoM or CoL, they should fly.

But on this one I had the CoM right in the middle of the cargo bay and CoL was almost next to it. As you can see there's a fuel tank in front of the cargo bay and one in the back and they're getting emptied evenly. The liquid fuel tanks are also balanced out around the CoM.

It only happens on reentry when hitting the thicker atmosphere. But it is stable again, if I manage to point it prograde (which is hard, because it's wildly spinning) and apply some thrust. (There's even a half full liquid fuel tank in the front just for balance, I do not feed from it and I checked that the fuel is still there)

So what happens here? I'm out of ideas.

Edit: I managed to fix it thanks to your answers. The problem were the air intakes at the front creating too much drag. I moved them to the back, rebalanced the plane and now it glides during reentry like a charm. As if it was made for it. Oh wait.

Thank you all very much, I never had though that this would cause my problems.

Edited March 22, 2015 by Mat2ch

[Empiro]

Your pictures don't work quite right, but I'm assuming you're playing under stock aerodynamics (in FAR you probably wouldn't make it into orbit without a tailfin). Most likely, if your CoM is very close to the CoL, then the issue is with placement of things that have higher than normal drag in stock, like air intakes and such. Either try moving them back or you can also try pumping fuel manually forwards as your descending. If you have trouble nosing up as you get close to the ground, you can then pump the fuel back.

Also, if you plan your de-orbit burn appropriately, you don't need fuel at all to make the landing -- you can glide in like what the space shuttle does.

[Starhawk]

Where are your intakes? Intakes have an enormous amount of drag and can cause craft to flip around.

Just a wild guess.

Happy landings!

[Deddly]

Mat2ch; first of all, welcome to the forum! Please do a little experiment for me and tell me what you see (screenshots would be perfect):

In the Space Plane Hangar, activate the CoM and CoL indicators and then empty out the fuel in the main tanks. Does the CoM move back at all?

Next - still with the fuel removed - grab the whole plane and tilt it upwards 5 degrees at a time, taking screenshots at each step. At some point, the CoL will come ahead of the CoM, and that will show the point at which the angle of attack is too great for your plane, and it becomes unstable. Please post the screenshots so we can see the results.

Edited March 20, 2015 by Deddly

[InterCity]

The only thing I can think of is that the center of lift is ahead or not far enough behind the center of mass. Your engines weight quite a bit, and if you don't have that orange tank in the cargo bay, your CoM may shift considerably. Try either moving the engines a bit ahead, or the wings backwards. That might help.

Also, when you design the plane. leave the cargo bay empty. If your CoM goes ahead, it's generally far easier to deal with than when your CoM goes backward in flight.

You should as well add a tail fin or two, just for a good measure. They'll help you fly the plane if nothing more.

[jarmund]

I'm thinking that with most of the fuel used up, your plane becomes back heavy - Loads of engines in the back, with no fuel forward to balance it out anymore.

[Mat2ch]

Hello, thank you for all your answers. You are a very helpful and nice community, I think I'll like it here. I'll try to answer as many questions in the following posts and hope I don't "spam" to much.

[Mat2ch]

Hello Empiro,

Your pictures don't work quite right, but I'm assuming you're playing under stock aerodynamics (in FAR you probably wouldn't make it into orbit without a tailfin).

you are absolutely right I'm playing without FAR and I'm using only stock parts. The only mod I've got installed is MechJeb, because I got lazy to always watch my speed and creating an fine tuning nodes. Also there are a few bugs in the Linux version which make it really hard to turn the camera and/or use the manouver nodes.

The tail fin is missing due to my lazyness. Since I don't need it most of the time I just don't place it. But I've changed this now.

Most likely, if your CoM is very close to the CoL, then the issue is with placement of things that have higher than normal drag in stock, like air intakes and such. Either try moving them back or you can also try pumping fuel manually forwards as your descending. If you have trouble nosing up as you get close to the ground, you can then pump the fuel back.

I think you and Starhawk are right. My air intakes are in the front as counter balance to the heavy engines. Looks like I need something different.

Also, if you plan your de-orbit burn appropriately, you don't need fuel at all to make the landing -- you can glide in like what the space shuttle does.

Most of the time I fear to overshoot and make it too short. But with time comes practice.

[Mat2ch]

Mat2ch; first of all, welcome to the forum! Please do a little experiment for me and tell me what you see (screenshots would be perfect):

Hello Deddly and thank you for the warm welcome!

In the Space Plane Hangar, activate the CoM and CoL indicators and then empty out the fuel in the main tanks. Does the CoM move back at all?

CoM and CoL on the full plane:

And here on the empty plane:

Next - still with the fuel removed - grab the whole plane and tilt it upwards 5 degrees at a time, taking screenshots at each step. At some point, the CoL will come ahead of the CoM, and that will show the point at which the angle of attack is too great for your plane, and it becomes unstable. Please post the screenshots so we can see the results.

Ange of attack screenshots:

This looks pretty stable to me. But I've got to change the position of the air intakes.

[Mat2ch]

The only thing I can think of is that the center of lift is ahead or not far enough behind the center of mass. Your engines weight quite a bit, and if you don't have that orange tank in the cargo bay, your CoM may shift considerably. Try either moving the engines a bit ahead, or the wings backwards. That might help.

Also, when you design the plane. leave the cargo bay empty. If your CoM goes ahead, it's generally far easier to deal with than when your CoM goes backward in flight.

You should as well add a tail fin or two, just for a good measure. They'll help you fly the plane if nothing more.

Hello JebKerboom,

yes the tail fin is a good idea. I've added it now. I've designed the plane around the empty cargo bay. I'm trying to create a design that will get me into orbit with an empty cargo bay. I will have to redesign it if I want to transport fuel tanks with it, because the engines have not enough power.

[Mat2ch]

I'm thinking that with most of the fuel used up, your plane becomes back heavy - Loads of engines in the back, with no fuel forward to balance it out anymore.

I tried to design it the way that the fuel gets consumed evenly. I guess Empiro and Starhawk are right with the air intakes. Will resdesign and just try it.

[Deddly]

Ange of attack screenshots:

Well done, Mat2ch. Looks like your plane will be nice to control on the way up. Your fuel balancing is great, assuming the tanks empty evenly, but you seem to have checked that in flight. At re-entry speeds, though, I would suggest the problem must be your CoM. Try having your CoM slightly in front of your CoL by adding a little more wing at the back. This should make it very stable because it will automatically want to point forwards. The way you have it right now, it looks like it will have a slight tendency to flip, which gets greater and greater as the angle of attack increases.

Edited March 22, 2015 by Deddly

[Mat2ch]

Well done, Mat2ch. Looks like your plane will be nice to control on the way up. Your fuel balancing is great, assuming the tanks empty evenly, but you seem to have checked that in flight. At re-entry speeds, though, I would suggest the problem must be your CoM. Try having your CoM slightly in front of your CoL by adding a little more wing at the back. This should make it very stable because it will automatically want to point forwards. The way you have it right now, it looks like it will have a slight tendency to flip, which gets greater and greater as the angle of attack increases.

Thank you, Deddly, for your suggestions. I would've never though of just tilting the plane in the hangar to see how it will behave at different flight angles. I've redesigned it a bit and now it flies really great.

Edited March 22, 2015 by Mat2ch
Typo

[Deddly]

Glad to hear that! Happy flying!