Stabilizing SSTO spaceplanes during reentry.

[Specula]

I've made several basic and a single complex SSTO spaceplane design, all of which have been quite effective for their purpose except for one simple and common problem they all share: They all spin wildly out of control during atmospheric decent from space or from very high altitude flying (above 30,000m). I've only successfully pulled a single one out of it's deathspin to land it, and it's frankly INFURIATING me. I simply don't understand why spaceplane SSTOs do this. Are there any SSTO designers out there that have had this problem with earlier designs and worked out a solution for yourself? Considering this seems to happen to all my planes no matter the size or shape, I don't think my craft themselves are the problem, I'm guessing either it's the overall 'wide wing/delta wing' design shape I stick to or there's some other unknown mechanic or problem I haven't addressed.

Any help from the better/most knowledgeable SSTO designers on what causes this and how they fix it would be really appreciated.

Addendum: I'm a stock player. Please don't suggest using modded parts/flight control mods to fix it.

Atmosphere skipper:

Orbital/Interplanetary SSTO:

Edited February 22, 2014 by Specula

[MaverickSawyer]

Well, it all boils down to the relationship between the center of lift and the center of mass. Sounds like your CoL is forward of your CoM, which makes your spaceplanes flip-happy. Try pumping fuel forward to help stabilize it. Also, set up the craft in the hangar with a typical fuel load during the reentry phase by using tweakables, and see what's going on.

[bsalis]

Some pictures would help.

Sounds like flameouts... or the CoM moving behind the CoL as the fuel drains.

[Tex_NL]

Unless you're using aerodynamics mods like FAR (and you've already stated you're not) wing shape does not matter. Wing placement however does.

Without pictures I can only guess where your problem originates but I do have an idea. It has probably to with CoL and CoG relations. At take off I assume your plane flies stable, CoL just behind CoG. But as you expend fuel your CoG will shift. Possibly to behind your CoL. While in space this is totally irrelevant but as soon as you hit atmo again your CoL comes back into play. If it is indeed in front, your plane will almost certainly flip and loose control.

To check this drain all fuel tanks in the SPH to the level they are when you reenter the atmosphere and pull up the CoG and CoL markers. With a little luck you can solve your problem simply by moving fuel back to the front of your plane before reentry.

[Specula]

Huh, after a quick bit of testing, I think you're both right. The CoM and CoL being out of line seems to be causing it. Thanks folks. Managed to follow Maverick's advice and while not perfect, showed vast improvements.

[DocMoriarty]

My CoL is usually (there can be excpetions if CoThrust is not exactly behind CoG) a tiny tad behind and above the CoG and my tank configuration and the order in which fuel is flown out of the tanks is always made in a way that CoG never changes during the whole operation time of the aircraft.

Example:

This is a long range space explorer. I could have used a bigger tank in the middle but then i wouldn't have been able to fly out fuel balanced. Note that the structure at the rear end with the 3 fuel lines is actually attached to the middle tank (kinda tricky). Due to the fuel pipes fuel is first flown out of all 4 aircraft fuel tanks simultanously, then the small ones between the aircraft tanks, then the front and end tank in the middle and then the center tank. And the CoG is precisely in the center of the center tank. This ensures, that the CoG never moves, no matter how much fuel is already flown out. This is also important since there are 3 more engines below the craft for VTOL operations in vacuum (visiting moons, Moho etc.).

[1greywind]

If you use more than one fuel tank, make sure that your engines drain fuel evenly from each one. Otherwise Center of Mass of your space plane shifts from it's original position and if shift is large anough to put CoM in front of Center of Lift and your plane becomes instable.

Also you can simple add many reaction wheels or control surfaces to overwhelm instability.

[Specula]

Sorry, should have included some pics. Anyway, here's a few pics of my two current SSTOs/High altitude circumnavigation planes:

Atmosphere skipper:

Orbital/Interplanetary SSTO:

Ah, so it's true you can add more than one control wheel. Thanks, I was under the impression that any more than one did nothing and was just extra, wasted weight.

[Geschosskopf]

Addendum: I'm a stock player. Please don't suggest using modded parts/flight control mods to fix it.

What about a mod that adds no flight controls, doesn't add or change parts, and only functions in the VAB/SPH? I'm thinking RCS Build Aid. This mod will show you 2 CoM markers, one with full tanks and one showing the average CoM position as fuel burns. You can thus move parts forwards and backwards until both markers are in the same place, which means the CoM won't move regardless of fuel consumption. Then you don't have to pump fuel around to fix balance problems during flight. You still have to fly the thing yourself and you still build it entirely out of stock parts. It just saves you the frustration of having to discover balance problems after spending all the time needed to fly the mission to the point where you lose control.

[Hodo]

Geschosskopf is right, the RCS build aid is one of those great tools like Kerbal Engineering Redux. It doesn't change the game any, just gives you much needed information that should be in the stock game anyway.

[Xavven]

I'm admittedly terrible at designing aircraft, and one of my high altitude Kerbin explorers entered spins constantly when coming down from 24,000m. Let's just say I got a lot of opportunities to practice spin recovery. Wikipedia has a great article on spins, and there's a paragraph about spin recovery too (http://en.wikipedia.org/wiki/Spin_%28flight%29#Entry_and_recovery). The TLDR version is to throttle back and apply opposite rudder. Sometimes you may have to pitch forward. This video helps:

http://www.fightercombat.com/upright-flat-spin/

[GluttonyReaper]

Geschosskopf is right, the RCS build aid is one of those great tools like Kerbal Engineering Redux. It doesn't change the game any, just gives you much needed information that should be in the stock game anyway.

To be fair, you can simulate this in stock by using tweakables to empty all the fuel tanks.

[MaverickSawyer]

I'm admittedly terrible at designing aircraft, and one of my high altitude Kerbin explorers entered spins constantly when coming down from 24,000m. Let's just say I got a lot of opportunities to practice spin recovery. Wikipedia has a great article on spins, and there's a paragraph about spin recovery too (http://en.wikipedia.org/wiki/Spin_%28flight%29#Entry_and_recovery). The TLDR version is to throttle back and apply opposite rudder. Sometimes you may have to pitch forward. This video helps:

http://www.fightercombat.com/upright-flat-spin/

That's why my spaceplanes have a lot of airbrakes at the back as well. If I tumble, I toggle them open, and I suddenly have a HUGE jump in drag at the back of the plane, causing it to both stabilize in a nose-first attitude AND shed a ton of speed, allowing me to regain control.

[nhnifong]

While in the SPH, you can temporarily drain all the fuel in your tanks to check that the COL will still be behind the COM even when empty. Jus right click the tanks and drag the fuel level down. Don't forget to drag it back up before final flight!

[Hodo]

To be fair, you can simulate this in stock by using tweakables to empty all the fuel tanks.

You can, sort of, but you can't have both the dry and loaded CoM icons at the sametime like you can in the RCS build aid.

[Geschosskopf]

You can, sort of, but you can't have both the dry and loaded CoM icons at the sametime like you can in the RCS build aid.

Furthermore, RCS Build Aid does NOT show the empty fuel tanks CoM, figuring you can see that yourself. What it shows is the AVERAGE CoM, which you can only guesstimate without it. Further, without the mod you can't see the average CoM move as you move parts around.

[Hodo]

Furthermore, RCS Build Aid does NOT show the empty fuel tanks CoM, figuring you can see that yourself. What it shows is the AVERAGE CoM, which you can only guesstimate without it. Further, without the mod you can't see the average CoM move as you move parts around.

You can check the boxes which will set the CoM for empty instead of average. Which will show you the empty tanks, you can even set it so only certain types of fuel are empty and others are still full. Like RCS vs liq/ox tanks.