What's the ideal ascent profile for a spaceplane?

[Trekkin]

With rockets, I get it; you go straight up or nearly so until you\'re out of really thick air, pitch over gradually, and burn till your apoapsis is where you want it; the ascent profile looks like an exponential curve, if you will.

Planes, though, are still a complete mystery to me. How do you fly them into space most efficiently? What factors, if changed on a plane, change how it\'s flown into space?

This is probably not answerable in complete depth, but anything you can tell me would be most appreciated.

[Oggula]

Very interesting question, I wouldn\'t mind knowing

I\'d guess it looks kind of like a tan curve on its side. You go flat to get up to speed, pitch up to 45 degrees at the center of the atmosphere so you climb rapidly, then level out near the top so you an use the remaining air to gain horizontal speed before climbing to orbit.

[theflyingfish]

Oggula has it right. I remember ascending using that profile on the XR-2 in Orbiter, with the scrams and everything.

However, I found that with current KSP engines, this doesn\'t really work. The 'High Altitude' engines are most efficient at sea-level, and as you gain speed they don\'t really increase thrust enough to continue accelerating, and in order to really go into space with auxiliary rocket engines you need to get to a speed far higher and faster than what the engines can currently do. I think (and I am not sure about this) that the drag v. speed curves on KSP are all out of whack. There is far too little drag at low speeds and far too much at high speed. IN order for the space planes to be effective, they must be able to reach extremely high speeds at extremely high altitudes, which is impossible with the current engine efficiency curves and drag v. speed curves.

It doesn\'t really matter, anyway, because the rumor is that KSP is due for a aerodynamics engine upgrade in the near future that will render anything I say here completely moot.

PS: I\'ve come close to achieving orbit with a spaceplane turbine/rocket hybrid by modifying some cfg. values, but am currently plagued by staging problems.

[hebdomad]

Hopefully your right on that one. I\'ve also heard that aerodynamics are in for an overhaul, so for now I\'m going to stay clear of the Space Plane Hanger until it\'s fixed. This also includes putting space planes on rockets in the VAB... Because that\'s even more messed up.

[Zephram Kerman]

It\'s not as bad as you might think. It seems to me that the drag model is not the issue (other than the way it\'s always been). The problem is the engine parts are still basically placeholders. The ramjet should be utterly helpless while sitting still on the runway, but it gets full efficiency.

Regarding trajectories: it really depends on engine performance, so everything will change as the new features in version 15 get improved. My strategy for now is to use engines in stages according to their 'sweet spot' or performance envelope. A good example is Oggula\'s and theflyingfish\'s experience with Orbiter\'s Delta Glider XR-2. (In that ship, I would take off horizontally with the rockets. Once the ship reached the scramjet\'s performance envelope, it would be used for much better efficiency. Then it shuts down and back to the rockets to finish the ascent.)

Another example: just today, I visited Minmus using four ramjet engines as vertical first stage boosters. (That probably won\'t work for long.) I launched straight up, then gradually tipped over until nearing the ceiling for those engines. At the end of the performance envelope, I was flying horizontally as high and fast as those engines allow. Ran them out of fuel, then jettisoned and fired the rocket stage to finish the ascent.

[Stevenator1]

Indeed the RAM jet physics are nowhere near complete. For now, I\'ve completely gotten rid of my first stage solid boosters on the bottom, favoring usually about 8 jets. The minimal fuel use is easily outperforming a solid booster, and I can get a good horizontal velocity before running out of fuel and firing up the main liquids.

[antbin]

My hunch at an ideal profile for my KPS Delivery IV craft, as far as I can tell in 0.15:

[list type=decimal]

[li]If using jet engines, take off at 100% thrust, climb ASAP as steeply as possible[/li]

[li]As turbojets approach 100/200 intake boost, let nose fall until plane is barely climbing at just 10m/s or so (which depending on weight / lift will be with nose up ~30 degrees). Try to keep airflow at 60/60[/li]

[li]Keep climbing until craft isn\'t accelerating much more (or not gaining altitude)[/li]

[li]Switch over to rockets, pitch up to 90 degrees, let the wings transfer horizontal velocity to altitude[/li]

[li]As periapsis approaches 50-60km, drop the nose down towards the horizon and burn to orbital velocity[/li]

This is sort of like the S-curve profile, or as close to it as I\'ve gotten in KSP so far.

[marko dash]

the current engines arnt really ramjets, they are turbofans (basic jet engine) and turbojets (duh).

[NovaSilisko]

An incredibly basic chart for what I do with spaceplanes and rockets. Simple, but it gets the general idea across.

With both, you want to maximize your horizontal velocity as much as possible. Any time spent going straight up is wasted fuel, as you\'re just fighting against gravity until you pitch over.

Once your apoapsis is out of the atmosphere, either continue burning slowly until you reach it and then throttle up, or cut engines and drift to apo, then reignite them and finish your orbital burn.

[antbin]

Androsynth tells me that my profile isn\'t the best - he\'s found that a steady 50 degree climb is more fuel-efficient.

Useful tip: Atmospheric engines are more efficient than rockets down to 8%.