Ssto

[cheeseit]

Having a bit of trouble lately with my SSTO's.

Take off is fine and it handles well at low speeds but once I start to reach high speeds (1000m/s+ in the upper atmosphere around 20,000m) when I attempt to tweak the pitch of the craft to lower/stop the vertical climb it begins to overpower the auto pilot and cause the craft to flip over backwards. Even at full lock trying to keep the nose down it just ever so slowly tilts up and then eventually enters an uncontrollable spin.

It only does this after I adjust the pitch at high speed. If I just leave it at the initial angle it will continue until the craft runs out of intake air and an engine burns out.

Edited April 28, 2014 by cheeseit
hit enter by mistake >.>

[cantab]

Is the Centre of Mass behind the Centre of Lift, either initially or perhaps once some fuel has burnt?

[LethalDose]

Pics will really help trouble shoot this. Are you using multiple engines? RAPIERs? etc.

[The Silent Majority]

Pics, but, based on the description it sounds like the COL is moving in front of the COM.

Either that or the COT is below the COM.

Then again, i've only made a few SSTO's.

[federally]

It sounds like the Center of Thrust is below the Center of Mass. Once the ship lightens and aerodynamic forces lessen having the CoT lower then CoM causes unending back flips.

But you mention that sometimes it makes it to space fine, so maybe not because this CoT below CoM problem would screw you up a ton in space as well.

So maybe as fuel burns your CoM is moving back behind your Center of Lift.

[SpaceK531]

In flight, try moving some of your fuel forward by alt+clicking two tanks and pressing the correct "in" or "out" button. It saved my plane the first time I did an SSTO, I wasn't expecting it to happen but I'm glad I thought of the solution.

[cheeseit]

I don't think it's purely a CoM vs CoL issue because if I sit and wrestle with the plane after it spins out I can regain control and then it will fly normally again.

I understand that as the fuel burns the CoM shifts but I can't understand why it goes from Stable to unstable after I lightly touch the pitch. Even if I turn auto pilot on and off it does nothing. It's a pain to sit there tapping D to keep the nose down.

Sorry for the lack of pics wasn't posting from my home PC.

Edited April 28, 2014 by cheeseit

[FREEFALL1984]

As I learned last week the centre of drag could also be too far in front of the centre of thrust, try strictly putting intakes at the back of your craft this essentially gives your craft a flight, much like a dart, which keeps it stable at really high speeds, Also have the bulk of the fuel tanks right over the centre of mass, this way as your fuel runs out your centre of mass wont shift dramatically.

Another interesting concept I've been playing with is having detachable control surfaces, so here's what you do.

1 - build a normal craft with the centre of lift behind the centre of mass as normal,

2 - Take all the fuel out of the front tanks, and watch how the centre of mass shifts towards the rear,

3 - Now place a set of winglets on the front of the craft, and then tweak the main wings to shift the COL into optimal position.

4 - Now this is how the crafts COM and COL will look after expending the first fuel tank.

5 - Refill the front fuel tank and place some decouplers on the front of the craft,

6 - Add some winglets onto the decouplers.

7 - move the decoupler and winglets into a position which shifts the COL into optimal position.

8 - Stage the whole thing, and when in flight, after the front tank becomes empty you should start having a tough time controlling its now that you should stage the decouplers.

This can be done multiple times for as many fuel tanks as you have on board, allowing you to maintain perfect control during the full flight, only problem is it kinda ruins the idea of a reusable space plane for those purists who like to keep their SSTO vessels fully intact,.

[Monger]

The faster you get, the more drag you got. Also: the higher you fly, the more your rudders need to work in order to stabilize an unstable plane. I got the same phenomenon: everything seems to work until you reach a certain height and speed, and then the plane starts to flip. When it whirls down into the lower atmosphere and looses speed, it slowly stabilizes again, and you can start to ascend again. At some point it will flip again, etc. pp.

Take a look at your rudder control in the bottom left corner. Usually all arrows should be centered, but with an imbalanced plane the auto pilot slowly needs to tilt the rudders more and more - up to the point where it hits the limit. If you would switch off the autopilot just for a fraction of a second there, your ship will immediately turn over. And this is exactly what happens once you are trying to manually alter your course.

Long story short: take a look at the rudders. If they have trouble either keeping the plane nose down or up, you have a balancing problem. Either add more rudders/flaps, or try to balance your CoL/CoM more carefully.

[O-Doc]

Your canard is on backwards which is shifting your "real" CoL during a pitching moment.