[SOLVED] help needed, SSTO unstable on reentry

[bloodbird]

Hello, 

I need help figuring out what is wrong with my SSTO, the first 2 versions didn't go in orbit, they would yaw sideways, then loose roll control as I would fight the yaw then total flatspin and the aerodynamic forces would tore it to shred (V1 did it at 300 m alt, fixed it by moving the COM further forward, V2 still did it in high alt, fixed by adding 3 pair of hidden main wings inside the back for extra lift) then v3 did well, easily went in orbit, but on the return trip, the deadly flatspin of doom came back, i tried even more weight in front but i become a flatsining lawndart...

so now i do the next logical step, ask better player for some tips

 

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Edited March 15, 2018 by bloodbird
solved

[Jens Lyn IV]

Based on your description, the craft has negative static stability in the yaw axis. The SPH shows centre of lift in the pitch axis, so you can't rely on it for yaw stability.

I don't use mods, so I'm not familiar with many of these parts, but:

• I see a vertical surface clipped inside the nose; remove it. The horizontal canards are fine, but vertical fins should be at the rear only.
• Your tail fins are at a severe angle; this makes them contribute lift, but also lessens their stabilising effect. Try making them vertical. Alternatively, since you clearly don't mind clipping, you could hide vertical tail surfaces inside the rear fuselage, just ahead of the engine nozzles.
• The cupola appears to be attached radially; if so, you have a source of severe drag far in front of the centre of mass, possibly enough to destabilise the vehicle. If the above suggestions aren't enough, try removing the cupola.

Good luck.

Edited March 15, 2018 by Jens Lyn IV

[Jakalth]

Weight balance will be an issue.  I see a huge problem when the craft is light on fuel.  As you burn fuel, the COM will creep forwards.  As it creeps forwards, your ability to maintain control(Pitch mostly) will be greatly decreased.  The hidden canards in the front of the craft gives it more tolerance to this, but it might still be too much.  The weight issue would not effect its ascent due to the craft being out of most of the aerodynamic effects before it shifts forwards too much.

The front vertical stabilizer might help a bit on the ascent, but as Jens Lyn IV said, get rid of it if you can.  The vertical stabilizer up front just increases aerodynamic torque.  as the craft starts to turn to the right, the front vertical stabilizer will act like a vain and force the nose to turn even more.  This can easily turn a slight wobble into a violent spin.  Vertical stabilizers should not be any farther forward on an aircraft then it's center of mass.  For example,  even the winglets of modern commercial and civilian aircraft are slightly behind the center of mass of the craft to prevent inducing aerodynamic rotation, even though they are only for smoothing the airflow over the wing.

Drag...  The front of the craft does seem to have a slightly higher amount of drag, for its profile, then the tail.  Mostly due to that cupola.  But, you have the countermeasures to compensate for this.  The rear airbrakes.  Add an action group that lets you deploy a few of the rear most airbrakes to increase drag on the tail once your inside the atmosphere.  This should help counter some of the front end drag.  You should be able to retract the brakes once the craft has slowed down a fair amount and the rest of the control surfaces can keep it under control.

 

My suggestion is to try tweeking it in the order above; Weight, front stabilizer, then airbrakes/drag adjustments.

[bloodbird]

wow!  i mean, i still crashed, but only cuz i touched down to fast XP

I rocked the Dunkird soundtrack all the way down and kept it undercontrol!

Thanks Guys! You helped me tame it and i even got to keep my tourist cupola ;P