Spaceplane Follows Parabola (can't break 20k)

[robotmonkeys]

I'm having problems getting m SSTO plane to maintain altitude. Usually I can get it up to at least 20,000 m, but that's about it. I've noticed craft seems to start slow down at high altitudes and follow a parabolic arc. Even I have to point the craft at about 45 degrees up to just to maintain a level trajectory, but with that attitude, I slow down even faster. Switching off the turbojets and switching to the rocket doesn't help. The damage is done.

I've tried shifting my fuel from the front to the back of the jet, but that doesn't help.

Any thoughts?

[Laie]

Don't go quite as high / don't milk them for as long. If you have any data display mods, you will notice that thrust decreases with altitude. Even though the jets may keep running well above 20km, most of the work happens below.

Part of what makes spaceplanes more difficult since 1.0 is that the jets cut out at an altitude where the air is still comparatively dense. You not only want to build horizontal speed (though that's still the main point), but also have some upward momentum that helps you into thinner air.

[MockKnizzle]

My guess is that you don't have enough lifting surface for the weight of your craft. In the thick lower atmosphere your wings are enough, but once you get into thinner air you're having to use engine thrust to maintain vertical speed like a rocket.

Care to post a picture of the craft so we can see if this is the case?

[capi3101]

I would say that you've got your CoM too far forward of your CoL, but you also say that shifting fuel aft doesn't help (which kinda rules that out). Could be insufficient pitch authority; I know I've had that issue my own self on several occasions.

I'm with MockKnizzle - let's take a look at your craft; it'll help us diagnose potential problems far more easily.

[Fearless Son]

What Laie said, all jet engines start to substantially lose thrust above 20,000 meters, regardless of how many air intakes you cram on there.

A big part of it is that you cannot simply milk the jets until they cut out, then turn on the rockets. In most designs, there will be an intermediate period where the rockets will be active at the same time as the jets. Right click on one of our jets and watch the window monitoring its thrust output. Watch the thrust rise as your altitude goes up. Once your thrust starts to level out, and then dip, that should be when you fire the rockets. Let them carry you up. Leave the jets on to add their thrust to your horizontal speed for some time, as they can continue accelerating you on top of what the rockets themselves are doing, but cut the jets (and the drag from the intakes) once their thrust starts to get too low and leave the rockets to do the rest.

[ajburges]

Actually, you want to light rockets when you lose speed.

Aerobic engines have thrust multipliers in two curves: velocity and altitude. There is a speed were jets hit peak thrust (1000 m/s for turbos, 1300 m/s) the longer you linger there on jet power, the more thrust you get from your hyper efficient engines, and the less dV you expend to orbit. Going faster than this while your engines run wastes rocket fuel.

As far as parabolic arc, good space plane ascent transfers from aerodynamic flight to ballistic trajectory as max speed is reached (and the lift surfaces can efficiently alter trajectory). For a mass optimal craft, you actually want to enter a gravity turn where you circularize between 50-60 km and the rest of orbital insertion is a Hohmann transfer. This minimizes the thrust of your orbital insertion burn and this OMS/ascent rocket mass. My goal is to have prograde at least 20° above horizon when I light my rockets.