Space Plane troubleshooting: I NEED HELP

[Lobo1544]

I'm working on my first SSTO. I started and was able to get it int orbit, but wasn't left with any fuel. After making modifications, at 14000m or so, the nose starts to drop. I mash the S key, but it stays low and slowly starts dropping. At this point only my jet engines are going. I have enough air intakes to use the jets until about 20000m, so those are not the problem. The CoL is just barely behind the CoG and I haven't had any problems with stability. Also, I noticed, even when only the jets are going, my rocket fuel slowly depletes.

Edited October 27, 2013 by Lobo1544

[Sirrobert]

When the fuel tank drains, the CoM will move back. If it's so close, it might even get behind the CoL.

Maybe that's it?

[Scarecrow88]

And your jet engines will also use your rocket fuel. The rocket engines need the additional oxidiser to run - the jet engines rely on the intakes to supply the oxygen they need.

[Johnno]

It's hard to tell, is the tank between the rocket engine and the FL-T800 tank a liquid fuel tank?

Either way, liquid fuel to the jet engines will be taken from the tank furthest away, which likely will be the FL-T800 tank. Which would show that your rocket engine is losing fuel, except that it hasn't lost oxidizer and it would take the liquid fuel from the regular liquid fuel tank if it has any when needed. So as such you haven't lost rocket fuel.

And yeah, if you're taking liquid fuel out of the FL-T800 your CoM will move toward the rear, upsetting the balance which might be fine at liftoff.

[Lobo1544]

When the fuel tank drains, the CoM will move back. If it's so close, it might even get behind the CoL.

Maybe that's it?

I tried moving the CoL back, but that caused it to start dropping even earlier.

[LeFudge]

By rocket fuel do you mean Oxidizer? Or is it just that the green bar is draining? Liquid fuel is interchangeable so if you do have a jet fuel tank behind that FTL800 it will drain out of the front tank first

Also your "nose down" problem, make sure you're using angle snap when you attach your engines radially like they are. It looks really close but I can't tell from just this image, it sounds like your jet engines might be a little too high up, if your center of thrust is slightly above your COM it will push the nose down.

[Sirrobert]

I can't tell exactly from the picture, but are the fuel tanks on the jet engines to the side rocket tanks or plane tanks ? The difference is that rocket tanks have liquid fuel AND oxidizer, while the plane tanks have only liquid fuel.

If they are rocket tanks, than replacing them with plane tanks would lighten the back of the plane, moving the CoM forward

[Lobo1544]

By rocket fuel do you mean Oxidizer? Or is it just that the green bar is draining? Liquid fuel is interchangeable so if you do have a jet fuel tank behind that FTL800 it will drain out of the front tank first

Also your "nose down" problem, make sure you're using angle snap when you attach your engines radially like they are. It looks really close but I can't tell from just this image, it sounds like your jet engines might be a little too high up, if your center of thrust is slightly above your COM it will push the nose down.

The I re-did the engines with angle snap... still drops at about 12000m. I didn't know how fuel flows, but know that I do I understand why the foremost tank was draining.

[JoCRaM]

Select the craft in the SPH, and press Shift-W a couple of times, to see how the lift/CoM changes when you're in a nose depressed situation.

As you get to higher alitidudes your ram air intakes will be grabbing less air - this means they will be lighter, which will move your CoM forward, leading to the drop you observe.

My advice would be to move the scoops to the CoM (perhaps by swapping half of them with the goo pods), and give the rear winglets a 5 degree twist so the front is slightly lower than the back.

[JoCRaM]

It's hard to tell, is the tank between the rocket engine and the FL-T800 tank a liquid fuel tank?

it would take the liquid fuel from the regular liquid fuel tank if it has any when needed. So as such you haven't lost rocket fuel.

I think the liquid fuel tanks are radially attached, and so their fuel would not be available to the rocket.

the "stack demand" fuel system sucks for space planes.

The way I'd fix that in sandbox is to yellow piple the jet fuel to the rocket fuel, (or attach the fuel/engine pods via I-beams (no cross feed) to stop them stealing rocket fuel.

[700NitroXpress]

You should replace the jet engines from the basic jet engine to the turbo jet engines. They'll take you to a higher altitude and achieve higher air speed, then do engine shutdown and switch to rockets. Also, you can use the rockets and turbo jets together because the additional thrust of the rocket will provide more air intake for the turbo jets and allow you to achieve max speed and altitude before you run out of air.

Also, you don't need RCS on anything unless you're docking.

This plane work as an SSTO.

All turbo jet engines and 215 power engines to achieve orbit.

This version also works with the nuclear engines.

Edited October 27, 2013 by 700NitroXpress

[Lobo1544]

Well, a friend recommended that I add flaps to the front of the capsule, but still make sure the CoL is behind the CoG. This worked great! Although I do have to keep the nose up manually around 20000m-40000m, its been working great! I wouldv'e used turbo but Im building this is career mode. And the RCS is to dock to my refueling station.

Edited October 27, 2013 by Lobo1544

[700NitroXpress]

Well, a friend recommended that I add flaps to the front of the capsule, but still make sure the CoL is behind the CoG. This worked great! Although I do have to keep the nose up manually around 20000m-40000m, its been working great! I wouldv'e used turbo but Im building this is career mode. And the RCS is to dock to my refueling station.

Good deal, I usually build planes with the CoM and CoL equal, but having it slightly behind is good for lightweight planes and slightly in front is good for heavyweight planes.

[Lobo1544]

This is the design I eventually came up with. Works great.

Not sure if it has enough fuel to rendezvous and dock with my station, but either way I just made my first SSTO!

[LeFudge]

What I usually do now to keep the plane balanced is build the main fuselage out of the jet fuel tanks, put a couple of I-beams or girders on the back to mount the engines to and then the LAST thing I put on is the rocket fuel tanks, just put them right on the CoM so that way when they drain your center of mass wont move too much (Oxidizer is much heavier than Liquid Fuel)

I cruised around KSC on the nuke engines alone just to run it out of fuel and it still lands beautifully with all the rocket fuel gone and about a tank and a half of jet fuel.

Edited October 27, 2013 by LeFudge