Shuttle Spins out in Atmosphere

[markymarkandthefunkybunch]

Hi, so since 1.1 came out I decided to build a space shuttle mostly based on NASA's space shuttle. The whole thing launches great and it can get into orbit just fine, however problems arise when I go to reenter the atmosphere. Reentry goes smoothly til im about 20-15km in the air. At that point my shuttle spins out and begins to point engine first at the ground. Any ideas on why this might be happening? I've attached a picture of my shuttle's COM and COL with the ammount of fuel it has during reentry:

 

 

I'd greatly appreciate any help, thank you!

 

[FullMetalMachinist]

23 minutes ago, markymarkandthefunkybunch said:

Any ideas on why this might be happening?

Long story short, the CoL marker in the SPH is lying to you. It's behind and slightly below your CoM, which might make you think that it's stable. (ok, not really lying, just not telling you the whole story) 

The CoL marker doesn't show the drag that will be produced by that big, long fuselage that's way out in front of CoM. This gives the drag forces a long lever arm to wrench the nose around backwards. 

Edited April 1, 2016 by FullMetalMachinist

[markymarkandthefunkybunch]

18 minutes ago, FullMetalMachinist said:

This gives the drag forces a long lever arm to wrench the nose around backwards.

So should I try to eliminate as much drag on it as possible?

[Plusck]

Also, before finalising any design in the SPH, rotate the root part with markers showing so that you can see what the game thinks of your centre of lift at different angles of attack. This doesn't show drag as such but body lift is somewhat related...

[FullMetalMachinist]

1 hour ago, markymarkandthefunkybunch said:

So should I try to eliminate as much drag on it as possible?

Well, the only way to do that is to get rid of those parts . 

The other thing you can do is fly with very little angle of attack. The problem with that is that on reentry you need a high AoA in order to bleed off speed. 

Are you starting to see now why building and flying a successful shuttle replica is considered one of the hardest things to do in KSP? 

The answer to your current problem is a redesign that puts as much weight as possible up front, moving the CoM forward and giving your wings and control surfaces a similarly long lever arm to counter the drag forces. 

The question on exactly how you do that is a little tricky, primarily because you have those three big, heavy engines stuck to the back. I would start by putting any monoprop fuel tanks and anything else you can think of in between the cockpit and the cargo bay. Also you can add canards to the front, though that destroys any thoughts of "purity" with regards to the replica status of your shuttle. 

Also, try to move the main wings and control surfaces as far back as you can, and make sure you have a set of body flaps. (if you don't know what that is, do a Google Image search for "sts body flap") 

Edited April 1, 2016 by FullMetalMachinist

[Reactordrone]

you could also try putting airbrakes on the back to simulate the drag of the split rudder on the real space shuttle. That should reduce its tendency to want to fly backwards.

[markymarkandthefunkybunch]

40 minutes ago, Reactordrone said:

you could also try putting airbrakes on the back to simulate the drag of the split rudder on the real space shuttle. That should reduce its tendency to want to fly backwards.

 

1 hour ago, FullMetalMachinist said:

Well, the only way to do that is to get rid of those parts . 

The other thing you can do is fly with very little angle or attack. The problem with that is that on reentry you need a high AoA in order to bleed off speed. 

Are you starting to see now why building and flying a successful shuttle replica is considered one of the hardest things to do in KSP? 

The answer to your current problem is a redesign that puts as much weight as possible up front, moving the CoM forward and giving your wings and control surfaces a similarly long lever arm to counter the drag forces. 

The question on exactly how you do that is a little tricky, primarily because you have those three big, heavy engines stuck to the back. I would start by putting any monoprop fuel tanks and anything else you can think of in between the cockpit and the cargo bay. Also you can add canards to the front, though that destroys any thoughts of "purity" with regards to the replica status of your shuttle. 

Also, try to move the main wings and control surfaces as far back as you can, and make sure you have a set of body flaps. (if you don't know what that is, do a Google Image search for "sts body flap") 

 

1 hour ago, Plusck said:

Also, before finalising any design in the SPH, rotate the root part with markers showing so that you can see what the game thinks of your centre of lift at different angles of attack. This doesn't show drag as such but body lift is somewhat related...

 

Thank you all for your help, i'll take all your advice, thanks again! I'll post a picture if I get it working.

[GoSlash27]

MarkyMark,

 That's a fine looking shuttle!

 I'm sure you'll get it all sorted out with the above advice. Adding on to Reactordrone's comment, you can actually mimic the split rudder on the real shuttle by using 2 rudders and reversing the deployment direction of one of them.

A couple pics I hope you'll find helpful:

Good luck!
-Slashy

[markymarkandthefunkybunch]

2 hours ago, GoSlash27 said:

MarkyMark,

 That's a fine looking shuttle!

 I'm sure you'll get it all sorted out with the above advice. Adding on to Reactordrone's comment, you can actually mimic the split rudder on the real shuttle by using 2 rudders and reversing the deployment direction of one of them.

A couple pics I hope you'll find helpful:

Good luck!
-Slashy

Thank you! Thats a great idea, the split rudder. I did manage to land my shuttle, after moving the COM forward a bit using a fuel tank. Here it is landed, a little short from the runway lol.

[FullMetalMachinist]

4 minutes ago, markymarkandthefunkybunch said:

Thank you! Thats a great idea, the split rudder. I did manage to land my shuttle, after moving the COM forward a bit using a fuel tank. Here it is landed, a little short from the runway lol.

Great! I'm glad you got it working well.