Loss of control of an SSTO

[700NitroXpress]

So I have an SSTO that achieved orbit, then I did a re-entry, and I was flying it at 22,000 meters give or take a couple hundred at about 1650 meters per second and I was descending. The craft felt unstable so I was being careful with the controls and then it nosed up hard and spun out of control. I regained control for a brief period at 8,000 meters and now going about 600 meter per second, but then the same thing happened again. Is this because the SSTO afterburner tanks were empty and throwing of the center of mass, or is something else to blame here?

The turbo jet engines all have equal air intake and were running at about 0.4 and the flame-out threshold is 0.1 so they were still running.

Here's the craft on the runway and in max velocity sustained flight.

Update

I have stability for sustained atmospheric flight by locking the forward fuel tank until the others are depleted. I have action groups for all engines. I'm now testing parachutes to see if they can pull me out of a chaotic spin and then cut the chutes using an action group for those.

Edited October 30, 2013 by 700NitroXpress

[sal_vager]

As you burn fuel your center of mass gets closer to your center of lift, making you less stable.

When your center of mass moves behind your center of lift, your craft tries to fly backwards.

[700NitroXpress]

Alright... any ideas on how to counter a chaotic roll and spin once this has happened to regain control of the craft?

[sal_vager]

If you have any fuel left, shift right-right the tanks and pump it forwards, or fit drogue chutes, or your intakes at the back so drag helps keep you pointing the right way.

[Thogapotomus]

Try to transfer your remaining fuel forward if possible to keep your center of mass where it needs to be. Getting out of a spin can be tough or almost impossible if your craft is too unbalanced. Best advice I can give is to lock in your SAS and wait untill the atmosphere gets thicker. Your spinning will slow and when you're getting closer to your prograde vector, throttle up and try to stay prograde till your stability comes back. Then it's just an issue of avoiding the ground while not spinning out again.

[700NitroXpress]

If you have any fuel left, shift right-right the tanks and pump it forwards, or fit drogue chutes, or your intakes at the back so drag helps keep you pointing the right way.

Ok, so this helped me identify the problem. It looks like the forward MK2 adapter with the liquid fuel was throwing off the center of mass. So pumping from the rear to the front fixes this for a short period and I can recover it from a flame-out. However, this is when I haven't fired the afterburners and just kept it within the atmosphere, so they're still at full fuel.

I'm going to test some parachute configurations.

[700NitroXpress]

Try to transfer your remaining fuel forward if possible to keep your center of mass where it needs to be. Getting out of a spin can be tough or almost impossible if your craft is too unbalanced. Best advice I can give is to lock in your SAS and wait untill the atmosphere gets thicker. Your spinning will slow and when you're getting closer to your prograde vector, throttle up and try to stay prograde till your stability comes back. Then it's just an issue of avoiding the ground while not spinning out again.

Yeah, my main problem was that when I went back into the thick atmosphere, it was still spinning out of control so getting it to prograde or retrograde was impossible because the spinning was causing loss of air intake, resulting in engine misfires.

[Thogapotomus]

Set up an action group to shut down your engines.

Also note, even after your jet engines flame out, they'll still be pouring out fuel unless you shut them off.

[700NitroXpress]

Set up an action group to shut down your engines.

Also note, even after your jet engines flame out, they'll still be pouring out fuel unless you shut them off.

Already have action groups for both engines and I was trying to shut the down and then reactivate, but to no avail.

[700NitroXpress]

The spin out seems to happen whenever I push the craft over 1400 meters per second at high altitude.

[arq]

As much as people like to make wide, stocky (not to be confused with stock) craft, this is a common problem with them. One thing that helps immensely is to add a conventional tail with horizontal and vertical stabilizers. The vertical stabilizer will make it very difficult to go into a spin sideways and having the horizontal so far back will give your elevators much greater pitch authority, helping them to keep you under control. Also, it can be helpful if you have stacks of fuel tanks to make them drain from the back instead of the front, which means you don't have to do it manually.

[DaveofDefeat]

Make sure your center of lift is behind and above the center of mass in the Hanger.

[700NitroXpress]

As much as people like to make wide, stocky (not to be confused with stock) craft, this is a common problem with them. One thing that helps immensely is to add a conventional tail with horizontal and vertical stabilizers. The vertical stabilizer will make it very difficult to go into a spin sideways and having the horizontal so far back will give your elevators much greater pitch authority, helping them to keep you under control. Also, it can be helpful if you have stacks of fuel tanks to make them drain from the back instead of the front, which means you don't have to do it manually.

Good stuff, I'll try this next.

[Thogapotomus]

As much as people like to make wide, stocky (not to be confused with stock) craft, this is a common problem with them. One thing that helps immensely is to add a conventional tail with horizontal and vertical stabilizers. The vertical stabilizer will make it very difficult to go into a spin sideways and having the horizontal so far back will give your elevators much greater pitch authority, helping them to keep you under control. Also, it can be helpful if you have stacks of fuel tanks to make them drain from the back instead of the front, which means you don't have to do it manually.

Agreed. Tails seem to be severely under rated on space planes, but they help maintain control immensely. Even if you just stick an I-beam off the rear of the plane with just stabilizing wings, (no control surfaces) it'll help.

[700NitroXpress]

Ok, here is my new design with tail fins and an additional SAS system on the rear. I've also added 4 parachutes with action group cut control and I'm testing it now.

[Thogapotomus]

Though you may not need it, it might help to put a vertical stabilizer at the rear as well. Parachutes are actually a good idea too. I might put some on my planes in case of emergencies.

[700NitroXpress]

Ok, so with the center tail fin, it decides to turn either left or right off the runway.

[700NitroXpress]

OK, so adding a full tail fin caused horrible yaw on the runway followed by explosions. However, adding a top tail fin and two additional airfoils to the afterburners is working really well. It's still not going down the runway 100% straight tho, I suspect that the landing gear is to blame.

I'm conducting flight testing with the plane shown here:

Edited October 27, 2013 by 700NitroXpress

[Starglider]

I had this problem once. I think the back end of your plane is trying to take off before the front end does, due to all that lift on the rear. You want the front end to take off first. More your aft landing gear forward, or add lift to the front. If you move the aft landing gear forward, you may need to lower them further on cubic octagonal struts in order to get the takeoff and landing clearance for the back end of your plane. If you add lift to the front you may encounter issues with the center of mass. Its all a balancing act between sometimes conflicting requirements. I found adjusting the gear to be the simpler solution.

[SpaceSphereOfDeath]

Try making your wings dyhedral (if that's how you spell it), that means you should tilt them down, this will make it more stabile

[DaveofDefeat]

Try making your wings dyhedral (if that's how you spell it), that means you should tilt them down, this will make it more stabile

Dihedral means you angle the wings upwards, Raising the center of lift above the center of mass and making it more stable.

[700NitroXpress]

Here is my final version. I have parachutes for emergencies that managed to save me from a plunge to the surface without ripping the plane apart. Landing gear has been adjusted, plane goes straight down the runway, but has to go all the way to the drop off before it leaves the ground.

Here is max speed achieved without afterburners: 1716.0 m/s