What makes spaceplanes flip backwards?

[Captain Sierra]

I had this happen on one of my latest SSTO cargo plane builds so I'd like to know what it is that causes a spaceplane to flip backwards during reentry. The thing insists on coming back down tail first. What are the reasons it could be doing this?

I'm not asking craft-specific, just in general so I know what I'm looking for when I troubleshoot it.

[bdito]

It generally happens when the center of mass ends up behind the center of lift as the fuel tanks empty.

It acts like a dart, where the heaviest part of the plane wants to lead the vessel, so it flips around.

When designing your plane have the CoL and CoM visible and then empty your tanks. If the CoM goes behind the CoL you'll have to readjust your design. Or you can install a fuel balancing mod to ensure your CoM doesn't change much as the fuel drains.

Edited January 27, 2015 by bdito

[Alshain]

Two possibilities, you didn't say if you were using FAR or not, #2 is mostly for FAR.

1. Center of Mass moved as fuel ran out. The plane will want center of lift behind center of mass and if it isn't it will try to get it that way.

2. You came in with the plane too far off the prograde and went into a spin when it hit the atmosphere. Make sure the plane is "generally" pointed in the direction of travel. You have some leeway of course, but if you come in broadside against the wind, that will be a problem.

[Captain Sierra]

Center of mass does move quite close to the CoL, but it does not pass it.

I am not using FAR, and I am keeping it at a 15 degree AoA relative prograde on entry.

[Pecan]

Lots of people say CoM but I've found Centre of Drag has a much greater effect, especially during re-entry when drag is slowing the vehicle so sharply.

Intakes are high-drag. They want to be behind the CoM and if you put them in front of it ... they don't want to stay there.

[bdito]

Lots of people say CoM but I've found Centre of Drag has a much greater effect, especially during re-entry when drag is slowing the vehicle so sharply.

Intakes are high-drag. They want to be behind the CoM and if you put them in front of it ... they don't want to stay there.

Is there any way to see what the center of drag is?

[Alshain]

Is there any way to see what the center of drag is?

No, but you can kind of figure it out. Air intakes create a lot of drag, while a couple of them in front of the CoM may be tolerated, if you are air hogging and they are all in front it will cause the craft to pull around. I didn't mention this earlier because you got it into space and that problem would exist going up as well as down.

You really need to put up some pictures, we are stabbing in the dark.

[Frank_G]

Well usually this occurs, because it lacks some weight at the front, which was there when the fuel tanks were full. Try to fill the most forward tank with the remaining fuel and oxidizer before re-entry. That should solve your problem, but is just a workaround.

Better to keep an eye on how fuel is used from the tanks during construction and make sure the center of mass does not shift too far away from its origin. Try emptying the fuel tanks in the SPH with the CoM and CoL Marker on. That will give you the idea of what to do, to create a solid and stable spaceplane. The task gets harder with spaceplanes that transport payload of different weight and size... its always a walk on a thin line.

[Pecan]

...that problem would exist going up as well as down...

It certainly can. We all know a lot of designs that are fine until they get high and fast and then flip-out.

On the other hand, before I learnt this, I had a few planes that were fine on the way up but impossible during re-entry. I'm assuming this was simply because the main speed-run during ascent comes above the thick atmosphere, whereas the aerobraking spike on re-entry is once you hit it.

(usually I'm air-hogging 8 intakes per engine, in Stock + DRE)

- - - Updated - - -

Well usually this occurs, because it lacks some weight at the front, which was there when the fuel tanks were full.

We're talking about when it isn't Com moving behind CoL.

[panzer1b]

A few tips (some of this may or may not have already been covered):

Always have CoM in front of CoL.

Ideally the CoM will not shift much, but if it shifts, make sure to either manually balance tanks, use mods, or design craft such that thin does not happen (say place tanks around plane's natural CoM when it has no fuel in it).

Keep intakes behind CoL, this is not a big deal with low amount of intakes, but if like me you use roughly 1 Ram/ton of mass, all that drag counts alot, and it will want to end up as far behind you as possible, so place it there in there in the 1st place). While this isnt really ideal, if you cant fit intakes behind the CoL, close then during reentry. During ascent you are usually too slow or too high for it to matter much (if ascent profile is your typical gun vertically to 30km, level, max speed, then punch out once ur max velocity). For shallower ascents it could be an issue either direction. Reentry is just too fast decent rate that will flip you over, unless you do it extrmeley carefully, and aerobraking is guaranteed to do it when you come in at 10Km/s to an altitude of 2km.

[Captain Sierra]

To reply to EVERYONE, I have some big intakes on there about parallel the CoM. CoD is gonna be pretty close to the CoL. CoM shifts around a good bit and I need to rearrange some fuel so the CoM doesn't migrate to right on top of the CoL during the climb. All the params look good bone dry (how it descends). On my phone until ~7pm. Hold out for images till then.

[GoSlash27]

Lots of people say CoM but I've found Centre of Drag has a much greater effect, especially during re-entry when drag is slowing the vehicle so sharply.

Intakes are high-drag. They want to be behind the CoM and if you put them in front of it ... they don't want to stay there.

^ This.

Intakes generate more drag than stock parts and wings at very high velocities, and can cause a spaceplane to fly backwards at very high velocity/ low pressure.

Active control surfaces can also have this effect if not balanced longitudinally, such as elevators with no canards to balance them. Since control surfaces can generate *negative* drag when deflected (infiniglide), they can easily overpower the balance and force the aircraft to want to fly backwards in reentry.

Best,

-Slashy

[Tank Buddy]

I recommend using RCS Build Aid. It shows a Dry CoM icon along with the CoM one, so you can make sure that your plane is balanced when full and empty.

http://forum.kerbalspaceprogram.com/threads/35996-0-90-RCS-Build-Aid-v0-5-5

[Laie]

It certainly can. We all know a lot of designs that are fine until they get high and fast and then flip-out.

On the other hand, before I learnt this, I had a few planes that were fine on the way up but impossible during re-entry.

Also, during ascent you have the engine gimbals to help you keep things under control. Plus, ascents tend to be much more controlled -- some of my flippy designs behaved quite well as long as I was just as attentive on the way down as I was on the way up. And whether you have FAR or not, you shouldn't have your plane pointed at the horizon when your actual trajectory is a steep descent.

[KerikBalm]

Ascent typically has the CoM farther forward, it typically moves back as fuel is consumed (given that engines are pretty much always going to be behind the CoM, its hard to avoid this completely- especially since engine gimbal doesn't work right if they are in front of it), and of course gimbal... and you'll want to be pointing prograde to minimize drag when you are high and fast/going orbital

Also, lift and drag are not proportional (drag is to V^2, but lift in this game - until the next release- is proportional to V, not V^2) - so if you have lifting surfaces for control, at high speed they lose to drag, but at lower speeds they may compensate just fine.

What I highly recommend, is that you keep your intakes close on re-entry -> at least any intakes that are in front of the CoM