Spaceplane flips up during reentry

[onlinegamesz]

Hi guys, I build this SSTO space plane, it works really good when taking off and reaching space but the moment I reenter the atmosphere it starts flipping up and after that it flips in all directions and becomes completely uncontrollable, I placed my spaceplane in the screenshots below, suggestions are appreciated. Thanks

 

 

[Ohm is Futile]

I'm thinking there's a lot more drag on the bottom than the top. Also, is that the dry CoM or wet one? Does it move a lot? Also, it looks like your CoL is actually slightly ahead of your CoM. If burning fuel makes it the CoM move further backwards, then it's no wonder that it gets unstable, especially with the engines off.

[AeroGav]

Problem 1 - CG moves aft as fuel burns away

Let's briefly add up the major dry mass items of your plane -

Ahead of CG  (by approx 2 long mk2 fuselage sections distance)  = cockpit , 2 tons

Behind CG (by approx 2 long mk2 fuselage sections' worth of distance) = NERV,  3 tons

Behind CG (by approx 0.5 long mk2 fuselage sections' worth of distance) = 2 x 1.8 ton whiplashes

Behind CG (by approx 075 long mk2 fuselage section's worth of distance) = 2 x 2ton rapiers

You can see that you have far more dry mass behind CG than ahead of it.

On takeoff, this is countered by having most of your fuel stowage in front of CG.    But once that burns off, your CG will move aft.

The easiest way to avoid falling into this trap again is to get RCS build aid, it shows a red dot which is where your CoM will move to when the tanks are empty.

 

Problem 2 -  Blue indicator not allowing for aero forces on non-wing parts

Even with your CG/CoM at its furthest forward position, before the fuel burns off,   you have 66% more fuselage length in front of CG than behind it.   The stock blue indicator only takes account of wing and control surface parts. 

I strongly recommend you get CorrectCoL mod installed so it takes account of aero forces acting on every part, including the fuselage.  Makes airplane design much less prone to trial and error.   I suspect your CG is way further forward than you think.

 

RCS build aid and CorrectCoL are very small mods with a tiny memory footprint that won't affect loading speed, because they don't add any new parts and therefore have no extra textures to load on startup.

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[AeroGav]

OK,  I had a go at recreating the craft from the picture. 

I didn't realise you had a cargo bay, instead of a fuel tank immediately behind the  cockpit.   This means you have a similar amount of fuel ahead and behind CoM, it doesn't move all that much and actually goes forward as it burns off (red dot)

But,  my CoM ends up much further aft than yours, which is halfway between the two short rocket fuel tanks.   You must have a heavy payload in the cargo bay, and once you unload it, the ship becomes tail heavy.  My "copy" had no cargo.

 

[bewing]

Also note that all MK2 fuselage parts are wing parts, so the stock indiators work fine with them.

 

[Spricigo]

9 minutes ago, bewing said:

Also note that all MK2 fuselage parts are wing parts, so the stock indiators work fine with them.

 

Well,  kinda.  But there is a lot of drag the stock CoL is clueless about. 

[bewing]

True. Drag is not included.

 

[juanml82]

Two things. As Aerograv said, you should try to put the dry CoM at the center of the plane. Since the engines are heavy and are at the back of the plane, that's hard to achieve. See if you can redisign with some of the engines around the middle of the plane, or if you can put the fuel tanks further ahead. Also, the Big-S wings can store fuel. Canards at the nose might be helpful in general, but I'm not sure if they'll help with reentry.

 

Once you have that accomplished, you should reenter more smoothly. Also keep an eye on your horizontal speed. By the time the flaming reentry effects are over, your velocity will approach subsonic speeds. If you don't put your nose prograde by then, you may slow down too much (horizontally) and stall