Polar Landing: Inclination changer Vs Stop-and-hop

[Doozler]

When doing an inclination change from an equatorial to a polar orbit, it is obviously more efficient to burn direct to your new orbit (i.e. at 135 degrees) than to stop completely, then burn south-north (by a factor of root 2). (and I understand it is even more efficient to raise Apoapsis and then change inclination there).

But if you need to land at the poles, is it more efficient to stop all equatorial orbital velocity, then do a sub-orbital hop? It seems that this changes the situation as your polar "sub-orbit" does not need the full measure of dV.

I have been doing the full inclination change and then a standard landing so far, but it occurs to me that this might not be the most efficient.

Any thoughts?

Edited May 1, 2014 by Doozler

[Jimbimbibble]

Which planet/moon is this on?

[EdFred]

If you know you are going to the poles, why not change your inclination (at the latest) when you enter the SOI. If you are intercepting on a prograde orbit, just make a burn at 225° or 315° even with the horizon on the nav ball until your orbit is polar. That's the cheapest dV you'll get aside from trying to change it well before SOI intercept.

[FREEFALL1984]

For a full 90 degree orbit change assuming a landing on an atmospheric body, I personally would be tempted to raise my apoapsis to a much higher altitude and then change my inclination at apoapsis, then burn retrograde and kill my periapsis and target a landing area on the pole in question. This is assuming you're not using DRE or want to keep it realistic since you'll be hitting the atmosphere at a terribly dangerous angle which IRL would be out of the question. But as Edfred said, burning before entering SOI is always the best option (assuming the planet isn't kerbin) and if it is kerbin then why not just take off to the north anyway

Edited April 30, 2014 by FREEFALL1984

[Claw]

I keep rereading and thinking about your question, but this part keeps throwing me off.

But if you need to land at the poles, is it more efficient to stop all equatorial orbital velocity, then do a sub-orbital hop? It seems that this changes the situation as your polar "sub-orbit" does not need the full measure of dV.

It's unclear to me exactly what you mean by stopping all equatorial orbital velocity, then do a sub-orbital hop. If you were to stop all equatorial orbital velocity without doing anything else, then you would fall out of orbit. Now your dV expenditure would include fighting gravity while setting up your sub-orbital.

I would say generally you're better off getting the inclination change out of the way as early as possible (and when you are traveling as slow as possible). What does that mean?

- If you know before establishing an orbit that you're going to the pole (and have no reason to be equatorial) then go straight into a polar orbit. This can be done straight from launch (off of any body), or well before you get to the SOI change. (As others have said.)

- If you're already in orbit, do the inclination change at as high of an AP as you can afford. If you're just barely capturing into a highly elliptical orbit, then keep the orbit elliptical. Do the plane change out at AP, then circularize.

If you're already equatorial, then it's going to be expensive to go polar. The most efficient is like you said, to raise the AP, change inclination, then lower the AP.

You can burn to a sub-orbital polar landing by burning mostly retrograde, but also slightly north (or south). This effectively kills your equatorial velocity while establishing a polar sub-orbit. That would be more efficient than simply doing the entire plane change 90 degrees, then dropping your PE. But it's also probably a bit more tricky to manage.

[Kasuha]

Best way to enter polar orbit is to insert into one. It is much easier to enter polar orbit than equatorial orbit because you can do so when coming from any direction - just make sure your trajectory goes above north or south pole.

When launching to polar orbit, switch to orbital speeds before you start gravity turn, then perform the turn so your velocity goes exactly towards noth or south at the end of the turn. This means you need to compensate for your initial perpendicular speed caused by planet rotation.

When you're already in equatorial orbit, bi-elliptic transfer is the best way to change it to polar. Landing requires you to clear all your speed, while bi-elliptic plane change can be done for about 0.8 of your orbital speed.

[Doozler]

Cheers everyone!

You can burn to a sub-orbital polar landing by burning mostly retrograde, but also slightly north (or south). This effectively kills your equatorial velocity while establishing a polar sub-orbit. That would be more efficient than simply doing the entire plane change 90 degrees, then dropping your PE. But it's also probably a bit more tricky to manage.

Ah yes, this makes perfect sense. I think this didn't occur to me as I sttarting thinking about actualy landing first and then going sub-orbital. After that I realised I didn't have to actually actually land before going polar. I didn't then make the final step of burning direct from equitorial to polar sub orbital.

The reason for the slightly odd change is that I was on extended science mission at the Mun, with a staion with MPL/ fuel in an equitorial orbit. I just needed the last couple of polar biomes and was on the margins for dV.