Space plane help

[Redshift OTF]

I am flying your plane now and will give a full report soon. The plane has multiple problems that are not immediately obvious! Interesting though.

[Redshift OTF]

I can confirm it is not a problem with fuel use causing the Centre of Mass to Move behind the Centre of Lift.

Part of the problem is the high tail. If you go in the SPH and set the thrust of the rocket engine to zero you will see the Centre of Thrust of the jets is very close to the Centre of Mass and the Centre of Lift. It is also at a 45 degree angle to the Centre of Lift. This is causing the plane to want to pitch up a lot and if you do go out of control you can't use the thrust of the jets or their gimballing to correct yourself until the air gets thick enough to allow the wings to correct your flight direction. You have also disabled some of the control surface functions. Turning them all on makes things a little better as well. Your plane also has a tendency to want to fly sideways!

The real problem is the air intakes, (well maybe, see later). You are getting more air coming into the right intakes than the left. I have no idea why. At first it is not a problem but when you are really going fast at 20km with both engines on full the radial engines become in-effectual, (which is normal), but the right Ram intake has 0.2 air coming while the left has none. I would imagine the extra drag on the right hand side is pulling the plane to the right. This also means you don't have enough air coming in to sustain both engines at full speed and to top it off the causes only the right engine to flicker flame out which pulls you to the right even more and you've lost control. Perhaps the behaviour of the air intakes is a bug? I do run MechJeb but I toggled some of the settings and managed to get it to stop doing what it was doing and then flew up to 35k before the jets properly started flaming out.

I went back into the SPH and removed and put back the jets and air intakes back on in symmetry mode and that seemed to stop the un-even intake of air once but I still got low altitude flame outs. I took the plane out again and the problem was back! I did manage to nurse it through the early flame out stage by lowering my angle of attack so maybe that was also affecting the air flowing into the intakes.

TL : DR - I don't know what's wrong with it. I would recommend rebuilding it by moving the jet engines and wings back so it is more stable, try a different wing and add more intakes.

Edited October 5, 2014 by Redshift OTF

[RizzoTheRat]

Never considered the CoT being an issue, interesting. Weird that the intakes could be different though. I'm certainly learning a fair bit playing with this, thanks.

When you say add more intakes, in an earlier version I had 3 or 4 radial intakes on each engine too, but took them off as they just seemed to cause drag and not give me any more airflow at high altitude/speed where I needed it. Presumably I could add a third Ram intake on the nose and bin the docking port for now.

[Redshift OTF]

So I moved the whole engine wing assembly back, put some extra winglets on the front to balance the CoL with the CoM and replaced the radial engine intakes with 2 more pairs of ram intakes. Oh and I ditched the tail winglets. Works perfectly and no flame outs until 35km. Weird. I think the radial engines might be bugged or something.

Edit: I also fully reactivated the control surfaces on the back of the wings so they pitch now.

Edited October 5, 2014 by Redshift OTF

[O-Doc]

So I moved the whole engine wing assembly back, put some extra winglets on the front to balance the CoL with the CoM and replaced the radial engine intakes with 2 more pairs of ram intakes. Oh and I ditched the tail winglets. Works perfectly and no flame outs until 35km. Weird. I think the radial engines might be bugged or something.

The CoL in the OP is well in front of the CoM. The best way to design a plane is to get your CoL/CoM relationship right before adding control surfaces. Rizzo was using control surfaces as a crutch to fix the Col/CoM. Without some solid experience and the explicit goal of making your craft aerodynamically unstable, you will get into trouble when the plane is not flying dead straight. Mostly due to a very dramatic shift in CoL when servos are in a non-neutral position. With SAS this is "most of the time" during ascent.

[RizzoTheRat]

Made some mods based on Redshift's assessment last night, moved the wings and engines back, ditched the winglets and radial intakes, added another 4 ram intakes...and took it to about 35km and 1800m/s before switching engines. Successfully put it in a 80km orbit with enough fuel left to refuel a craft that's just returned to Kerbin orbit ready to go on another mission.

Still not got the CoM/CoL relationship quite right as with not much fuel left it was a bit unstable in pitch, if I let the nose get too high or low it was very difficult to recover, and resulted in a hairy moment about 2km out from the runway meaning I ended up landing it on the grass not the runway. Bit it achieved its mission and Bill got it back on the ground in piece.

Still needs some work but I think I'm getting the hang of it, thanks for all the input.

[StainX]

Jup, every plane flies a bit differently,... i still try to build a spaceplane that can hold high AoA during reentry, for the some info on almost everything look at this awesome tutorial by keptin.

[Redshift OTF]

Woohoo! Glad it is working out. You could try draining all the fuel out of the plane in the SPH and seeing if it affects the CoM/CoL relationship much. The CoM usually moves back because engines are the heaviest part of your ship, not a lot you can do about that I don't think. It's even worse when you start using LV-N's on spaceplanes. :/

[RizzoTheRat]

Yeah, I check everything with the fuel drained out and the CoM was close but still ahead of CoL.

Started a new game in 0.25 last night and got a contract to test a BACC booster in flight. Decided to use the BACC as the body of a plane and managed to produce a completely stable and easy to fly plane. Its follow up variant to test a skipper and a radial engine didn't work quite as well though and became unstable in pitch beyond about 20-25 degrees AoA on the return flight with less fuel, which certainly made landing interesting. CoM was still ahead of CoL but I think I had the canards too far forward and the off centre drag of the Mk55 radial sat on it's back probably didn't help. Now I know I've got a design that is stable and works well I can make modifications to the BACC plane to explore CoM/CoL variations and I will get the hang of it.

Then it'll be time to have a go with NEAR/FAR...