Spaceplane Help

[Comrade_]

Hi, I have made myself a spaceplane that gets to about 17,000m then it flips backwards. The thing is, if I turn off the 2 turbojets I have, it doesn't flip, forcing me to use my rocket engine earlier and as a result I run out of fuel. Here's a pic of it.

Someone said add more wings to it, but it doesn't seem to help, thats the only reason there are a bunch in the back.

[Jod]

Try moving the aerospike to the front of the plane and air intakes to the back(do you close the intakes when you turn off the jet engines?), that helped me get my first stable SSTO, might work in your case too.

[Cal'Mihe]

Your two extra outboard tanks (and engines ? Don't recognize them) is set higher than the rest of the fuselage. The trust from your jet engines is therefore below the center of mass. The problem becomes apparent at 17k meters because by then you have burnt a bit of fuel, the center fuel tanks therefore weight less in relation to the engines, your center of mass moves upwards, while your trust stays in place, causing your plane to flip nose up.

Imagine pushing against the center of a big box, that causes the box to move straight forward.

Now imagine that you are pushing against the lower edge of the box. Still pushing in the same direction. But now your box will start to rotate. Same thing is happening to your plane.

[Comrade_]

Those 2 extra tanks have a S.A.S on them and an avionics package

[Comrade_]

Now it flips even earlier, I have all the intakes open, is that bad?

[Markus Reese]

Alright, I cannot see center of mass on the plane in relation to thrust so I will go back to the first one to start. First plane looks aero flight stable, but lack of wing surface means that when it gets high, the off centre of gravity thrust causes a rotation. When you hit 17000, you simply lose the wing surface to counter this rotation also combined with the loss of fuel mass.

The second one could be a result of your center of lift being too forward of your center of gravity? Possibly too much upper drag?

[Nao]

There are several factors that could make the plane flip.

One is aerospike engine behind center of mass (CoM) which naturally makes the craft more unstable due to drag.

Another is having a control surface at the front. If you go higher than 30deg attitude i would expect the plane to flip (from what i can see on the pictures) just due to center of lift (CoL) moving forward. You can go to SPH and rotate whole craft (with [shift]) with CoL and CoM visible to see how does it change with attitude. If CoL goes too far forward, add more control surfaces to the rear or remove the front ones.

Placing air intakes in front of CoM have destabilizing effect aswell.

And there is of course more silly mistake of fuel change moving CoM behind CoL.

[Sephta]

the common reason a plane flips at this altitude is the drag induced from the needed pitch: all parts in the front will push back above the CoL, your engines will push forward below the CoL: there you have a nice torque for lifting your nose up.

you can counter this by: adjusting your weight at this point (pump fuel forwards), reducing your need of pitch (pre-pitch the wings instead) , adjust the direction of thurst (you have to judge the way of your CoM moving, as you burn your fuel.)

The easiest way is to make your plane a bit nose heavy and give it enough lift (and pitch) with the wings alone (give them an angle up or down, depending where they are)

Edited June 10, 2013 by Sephta