Some orbital mechanics questions

[Netris]

Even if I play KSP since .13, there's some question that came into my mind a few days ago. Mostly about rendez-vous as I'm finishing to build Mir and I'll begin building the ISS. So I thought it would be good to ask the community because some people are nearly astrophysicians

So let's begin.

• What is the best way to reach a really high orbit ? Direct ascent by brute force or intermediate orbite then transfer to the higher orbit ?

• For rendez-vous, is it more efficient to direct ascent at the launch phase angle or go for a lower orbit and then transfer ?

• Is it more efficient to begin the gravity turn at low altitude as you have to add less horizontal velocity when circularize ?

I guess it's everything for the moment ^^

[TheRabbit]

I use an orbit transfer for my large orbit sats. For rendezvous it depends on what orbit it's at, if it's at 100,000 direct ascent, if it's at 500,000, then yeah, go into a lower orbit. And for the gravity turn, just start at 10,000, it saves fuel.

[georgTF]

1. I think if you punch it for a really high Ap, you'll have less gravity to deal with when raising Pe. That said, the orbit would have to be really high, i don't think it's any good for a station.

2. It should be more efficient to do a direct ascent, but if you miss you'll waste more fuel correcting. Using maneuver nodes is also easier if you already have an established orbit. But if you know the angle then go direct and complete the rendezvous normally once you close in.

3. Hmm, tough. Usually it depends on the weight and thrust of the thing. Slow rockets are better off doing it after 10km as the air thins considerably and velocity picks up more easily. You really don't need to lose fuel to gravity. Another factor is staging. It may be better to delay the turn if you'll be dropping tanks very soon. Generally grabbing horizontal velocity between 5-10km is not worth it as the gain is too small.

[Godot]

I haven´t made any practical experiments with it, but normally (at least for the standard 100km orbit) I prefer a rather steep ascent ...

with a gravity turn to ~20° at 10km and another to ~40-45° at 20-25km (and additional turns at higher altitudes as i see fit).

My reasoning is that a high vertical speed component ensures, that I quickly pass the lower atmospheric regions and therefore minimize losses due to drag