Spaceplane having problems with sonic barrier... or something...

[Leeksoup]

Hi there!

So I\'ve managed to build a stable spaceplane, the Xing Long III (Mandarin for Star Dragon III- not original, I know) out of almost entirely stock parts- the only non-stock on it are the fuel ducts, since I like how Kosmos\' look and they don\'t matter, and a couple radial-mount parachutes for those 'oh crap' moments. It\'ll take off, barely, on its two standard airbreathing engines, then once I\'m over water I\'ll kick on my four ramjets. At 10km up, I\'ll shut off my jets, running only on rams. Using this configuration I can fly for... a long time, probably even do a circumnavigation if I\'m careful, at heights of up to 16km.

However, there\'s my problem. See, this is supposed to be a *space*plane, so I want to go to space. I ripped out a couple of my jet fuel modules, replaced them with stock rocket fuel and the gimballing engine, and hooked it up. 16km up, doing about 400 m/s... good. Rocket ignition!... which leads to horrible over-lift, or something, and me spinning wildly out of control after I hit about 500 m/s. Usually, this ends with me desperately trying to pull out of an uncontrolled spin as I plummet back to Kerbin, half the time succeeding, half the time dying horribly.

What can I do to make my craft more stable at.. what is it, exactly? High elevations? I did put RCS on it... High speeds? Is it just because it\'s so freaking heavy? As a side note, is there any way to manually turn off rocket engines the way you can airbreathing ones? Thanks guys!

[G Addict]

For shutting off rocket engines, try disabling flow of fuel through the tank the rocket is mounted on. I think that should work, though by the looks of it shutting off the rockets in this manner will also kill the jets. The reason you\'re getting stability issues is because rocket engines are very heavy compared to jets and your control surfaces aren\'t enough in the extremely thin atmosphere to steer the plane, which is made worse by the rockets burning far back from the centre of gravity. You should be good if you put stupid numbers of RCS thrusters on the plane but I don\'t like that solution. Try going easy with the throttle on the rockets until you\'re out of the atmosphere - IIRC it\'s more efficient to go relatively slowly in the atmosphere, then punch it once you\'re out anyway, plus the lower speeds will stop air resistance messing with your trajectory.

[Mr_Orion]

Try using a lower throttle when you ignite the rockets, maybe 1/2 throttle or so.

[Ydoow]

Meh, I don\'t like spaceplanes.

My problem is that my payload, a 2 liquid tank rocket with gimballing can\'t pull it\'s heading above the horizon, and slowly slips back down to the sea...

Over-lift is interesting though.

The air is really thin, so definitely depend on gimballing rocketry.

If it is the wings, you could try shedding them, or put the rocket engines above the center-of-gravity on the plane, so it will push the nose down on ignition.

[E. F. Kranz]

The rockets aren\'t providing thrust to the center of the craft. ASAS and control surfaces will fight this to some extent, but ultimately, it is a killer. All thrust must be on the center plane of the craft.

[boolybooly]

When you light a spaceplane rocket you usually have to zero trim as the very next action ( alt + X ) because the sudden increase in speed makes the trim up much more potent and the plane should now be powerful enough to just go in a straight line. Steering with delicate dabs on fine setting (capslock toggles this and the pitch and yaw markers turn blue to indicate it is active) can help avoid oversteer.

If trim is zero and you are still pitching up then you either have excess drag, maybe on top or possibly a bit on top and more at the front causing any deviation to be magnified, (or you could have wings too far forwards or a wing part at an angle or have the rockets below the center of mass.) In your case imho the canards are quite far forwards and you need to try to bring them back as far as you can. If the plane wont lift off without them forwards then you can try bringing the main wings forwards a bit and move the canards back as far as you can.

There is a time between ignition and 30,000m altitude where the ships can be very tricky to steer, but after 40km it is all rocket and you can begin to ignore aerodynamics entirely. However I think the wings on yours are above the line of rocket thrust and in rocket mode their mass and that of the fins will tend to tip your ship upwards very slightly in rocket mode but you should be able to counter that with manual control. If you can fit an ASAS on there it will help a lot. I find its very tricky to fly in KSP without an ASAS.

imho