Space Plane Reentry issue

[Redwizard000]

My Space plane is going bonkers!

The plane takes off and flies just fine. I can get it to orbit Kerbin with no issues. When I try to land I set the plane in a nice nose down position going through the atmosphere. While it is descending through the fire it is fine. When it hits about 20km I open the air vents and turn the jets back on. The plane still flies fine. If I touch any control the plane goes nuts. It just starts spinning around in every direction. Even if I just tap the control the plane spins out of control.

It only happens on reentry. If I don't go into orbit and just fly around on Kerbin I can do flips, barrel rolls and loops and the plane flies fine.

The center of mass and lift are in the right places and all the fuel is in the center of the plane, and the fuel is almost all used up on landing anyways.

I am not sure if I am doing something wrong or if it is a bug or something.

I looked through the space plane tutorials on The Drawing Board and I wasn't able to find anything about it.

I am thinking the problem may be that the fuel is being used up in orbit and that is affecting the balance, but I am not sure why the plane works fine until you tap a control.

Anyone have any ideas? (The most recent crash cost Jeb his life... But I guess he knew the risks when he was volunteered.)

Thanks

-RW

[Moar Boosters]

It sounds like the center of mass might be moving due to the fuel tanks emptying.

Go into the VAB and right click on all the fuel tanks to empty them, and then see what happens to the center of mass/lift.

The reason it's working until you touch the controls is probably down to the SAS. SAS is pretty good at stabilizing things that are already going in a straight line, but struggles to help an unstable craft if you're performing maneuvers.

[Kasuha]

Go into the VAB and right click on all the fuel tanks to empty them, and then see what happens to the center of mass/lift.

Notice how full each fuel tank is when it goes out of control.

Replicate that level of fuel in each tank in VAB and check what it did to center of mass vs center of lift.

[TheD007]

How much SAS torque do you have? At 20km you do not have enough atmosphere to use control surfaces effectively. I have found that that is normally a center of drag issue caused by high drag parts (open intakes and docking ports) in the front. The first option is to redesign to have docking ports in the back of the plane to try to stabilize the ship. The second is to keep the intakes closed until you are below 10km. A third option is to turn on a gimbaled engine before you try to turn as that will provide some more control but you probably don’t have enough fuel or air for that to work.

[allmhuran]

During re-entry drag is a big factor. Intakes at the back really help. The surface mounted "scoop" intakes have pretty high drag and are easy to mount at the rear.

High mass parts also have high drag, so if you have designed an aircraft with the engines near the middle of the fuselage, perhaps on the wings (like a commercial airliner), then this will create a lot of drag forces in a very unstable position.

Of course, the problem with engines at the back is that they want to draw fuel from the front to the back, which seriously messes with your centre of mass, which is the main reason why many of my planes do indeed have engines slung under the wings (so they draw evenly from the front and rear of the fuselage).

If this is how your plane is designed, a solution is to still have the engines under the wings but set the wings further back, and then set the engines further back again by putting them behind additional fuel tanks (if you can make use of them) or structural fuselage pieces. Don't forget to tweak your lift surfaces after doing this because your CoM will have shifted.

[Redwizard000]

Thanks for the suggestions. When I get home from work I will give them a try.

[Pecan]

...If I touch any control the plane goes nuts...

Then don't touch any control!

Seriously - I had the same thing and, since you don't get to try re-entry until you've practiced getting into orbit in the first place it's almost the final lesson you learn.

Drag, as has been mentioned, is the issue. As you're re-entering you have almost orbital velocity and, as you're coming down, the air and drag is increasing, all of which is slamming the brakes on for your plane. Don't even try to 'fly' it until you're below 10km and ~300m/s horizontal speed. Until then just keep it close to prograde so it doesn't flip/spin on its own. Once it's back in thick air add a little throttle and slowly bring the nose up, again not moving too far from prograde. You should be able to level-out above 5-6km so you've still got lots of height and are now back flying normally.

[Vanamonde]

This is a very common problem. In addition to the things mentioned above, you can enlarge the control surfaces at the back of the plane, and/or move them farther back so that they're applying more leverage.

[Claw]

At the risk of tooting my own horn, here is a tutorial I wrote. Section 4 talks about Center of Mass design. There is also Center of Lift considerations and it explains how to tweak fuel levels.

http://forum.kerbalspaceprogram.com/threads/65638-Basic-Airplane-Space-Plane-Aero-Tutorial?p=905336#post905336

One more thing to keep in mind. When you fine tune the CoM/CoL relationship, remove the landing gear. Because landing gear is massless in flight, it causes the CoM marker in the SPH to be incorrect. The CoM usually shows too far forward for most designs, which causes the craft to be unstable.

[Redwizard000]

yeah it looks like when the plane runs out of fuel the center of mass moves way to the back of the plane.

That is kind of an annoying problem

[Claw]

yeah it looks like when the plane runs out of fuel the center of mass moves way to the back of the plane.

That is kind of an annoying problem

Welcome to the world of space planes.

[Zourin]

Mass is likely the main culprit. It's too far back. Before reentry, push fuel as far forward as possible so that you're more of a lawn dart with a weighted nose, and pre-open rear air intakes to help keep your initial alignment along your velocity until you hit thick atmo where your wings should be able to take over properly.

The moment you take a weighted tail out of alignment with motion is when you start doing a 'bullet tumble', especially if you're 'falling' and not 'gliding' at that point.

[Redwizard000]

I am just going to put like 50 parachutes on it and land it in zeppelin mode...

[Zephram Kerman]

This is why I try to put all my fuel tanks exactly at the CoM. That way, full or empty, they don't affect it at all. If a lot of fuel is needed, the tanks could be in a row, left to right.

[Tank Buddy]

If you use the RCS Build Aid: http://kerbalspaceprogram.com/rcs-build-aid-v0-2/

It will show you the wet and dry mass of your craft. This way you can see how where the CoL will be when empty, and build accordingly.

[Claw]

But don't forget that completely full or completely empty may not be your worst case (most aft) CoM location.

You can also get creative with fuel lines, or lock out certain tanks from feeding during flight to control the CoM. There's also a fuel balance mod call TAC Fuel Balancer. I've not used it, but I know several people have. I don't know if it's 0.23.5 compatible.

Parachutes and Zeppelins are also an approved Kerbal solution.

[Redwizard000]

Parachutes and Zeppelins are also an approved Kerbal solution.

Hmm... What would Jeb do?

[Jean Deaux]

I have FINALLY made a successful space plane, I've known about the C.o.M. from the many threads I've read and have loaded the TAC fuel balancer to keep the weight distribution even throughout the plane at all times. Soon as I hit atmosphere I lose all control of the plane. Even after slowing down to as slow as take off speeds I'm unable to regain any control (to include giving Mechjeb flight control). As such, I'm not entirely convinced center of mass is the entire culprit.

[LethalDose]

I have FINALLY made a successful space plane, I've known about the C.o.M. from the many threads I've read and have loaded the TAC fuel balancer to keep the weight distribution even throughout the plane at all times. Soon as I hit atmosphere I lose all control of the plane. Even after slowing down to as slow as take off speeds I'm unable to regain any control (to include giving Mechjeb flight control). As such, I'm not entirely convinced center of mass is the entire culprit.

Few questions so we can help:

• Are you using FAR?
  
• What altitude is "Soon as I hit atmosphere"? 70 km where the atmo starts, or like 35 km where the atmo is noticeable.
  
• How slow is "slow as take off speeds"? And at what altitude are you going this slowly?
  
• What is your re-entry profile (what height are you deorbiting from? How low are you setting your periapsis on your deorbit burn? etc)?
  
• Did you check the CoM at different fuel levels as previous posters have noted?
  
• and finally, got any pics?
  

[Hodo]

I have FINALLY made a successful space plane, I've known about the C.o.M. from the many threads I've read and have loaded the TAC fuel balancer to keep the weight distribution even throughout the plane at all times. Soon as I hit atmosphere I lose all control of the plane. Even after slowing down to as slow as take off speeds I'm unable to regain any control (to include giving Mechjeb flight control). As such, I'm not entirely convinced center of mass is the entire culprit.

Ok, you need to post a picture of your craft, preferably with the CoM and CoL indicators on from a decent angle.

I think I know what the problem is, but need to see the craft and know if you are using FAR.

[Jean Deaux]

I was actually starting to make a video of the whole thing and believe I found the missing piece of the puzzle, the angle of re-entry. I had been coming in nose slightly elevated figuring the angle of attack would help reduce speed. Looks like that was the cause of the problem. I came this time nose on the horizontal plane and this time I came in without a hitch. Want to see the video? Here it is!

And to answer your questions, no FAR, all stock ship short of Mechjeb and TAC Fuel Balancer.

[LethalDose]

Well, congratulations on making it back to the KSC in one piece (mostly). I'm glad you figured out what the issue was. I use FAR and DER, so my advice for re-entry probably wouldn't have helped much anyway. With my SSTOs, I keep my nose just below where FAR gives me the "Large AoA/Sideslip" alert on re-entry for the exact reasons you mentioned: reducing airspeed. So, I think you're thought process is correct, the stock drag model is just goofy.