reducing orbit - burn retrograde at apoapsis or periapsis for efficiency?

[mrklaw]

Have a contract to get a basic satellite into orbit around the sun (regretting it now). Seems to be taking huge amounts of dv to get anywhere near (also blooming slow with a nuclear engine).

Just wondering if it makes any meaningful difference where I burn retrograde? that is - do I do one burn at apoapsis and burn until my periapsis is where I want, and then circularise; or multiple smaller burns to lower the orbit overall?

And are there any more direct routes to get in close (need to be about 2,500,000,000ish)? I was just launching to the east at sunrise, and continuing to burn until escaping Kerbin's SOI and entering a sun orbit at about 13,000,000,000.

[Xannari Ferrows]

Apoapsis. That is where your overall kenetic energy is lowest. Though it also depends on your current orbital parameters.

[Red Iron Crown]

A bielliptic transfer is likely cheapest delta-V wise, but takes a considerable amount of time.

[n0xiety]

I had the same problem and had to carry a mamooth engine to test with me even closer than that to sun. Well according to mechjeb the manuver needed more than 12k dV so i found the solution of using gravity assist from planets. It's complicated tho and changes from situation to situation so i would recommend checking out a guide if it seems hard to you. Other than that fastest way is to burn at apoapsis if you have the dV for the job.

Edited May 25, 2015 by n0xiety

[justidutch]

A bielliptic transfer is likely cheapest delta-V wise, but takes a considerable amount of time.

Given that the examples quoted in the Wiki article would give you only 0.2 - 1% savings of Delta-V, is that really worth it? Sounds like a straight-up Hohmann is the only option, barring any fortunate gravity assists as mentioned by n0xiety

[Red Iron Crown]

I haven't run the numbers, it depends on the ratio of the SMAs. Larger ratios equal more savings, moreso if it can be combined with a needed inclination change.

[Snark]

(The following assumes that what you want is to be in a specific orbit that's low around the sun, e.g. approximately circular at low radius. This wasn't actually explicitly stated in the thread. If what you want is just, for example, to "lower a periapsis so it's close to the sun, but high apoapsis is OK", the strategy would be different.)

Anything that involves getting into a low orbit around the Sun is going to be just brutal in terms of dV. The lion's share of the work is going to be lowering your apoapsis once you're down close to the sun, and other than using a planetary gravity assist, there's not a lot that can help you with that.

That said, you can at least try to minimize the amount of dV you use on your initial burn that drops your periapsis down to the sun (while apoapsis is still up around Kerbin): you want to make maximum use of the Oberth effect, which means you want to be in as low an orbit as possible around Kerbin, and then accelerate like blue blazes to do a single high-impulse burn at low altitude.

You mentioned that you're using nuke engines, which means you can't just go WHAM and do it in one go, but you can at least help yourself out by making a few passes. That is, start out in low circular Kerbin orbit. Figure out where you're going to make your eventual Kerbin-escape burn. Make a burn at that spot, without enough to actually escape-- all you'll do is burn for a minute or so right at that location, which will raise your apoapsis and put you in elliptical orbit around Kerbin with your periapsis still really low. Coast around your orbit, until you come to periapsis again, do another burn. Continue this as many times as necessary to get it so that you're in a highly elliptical orbit around Kerbin with apoapsis really high and periapsis really low. This will have you doing in the neighborhood of 3000 m/s at periapsis. That's when you do your big burn to escape Kerbin.

As for lowering your apoapsis when you're around the Sun: wait until you're at periapsis and do all your burn then, again to maximize Oberth effect. You do not want to do a bunch of little burns. Wait until you're lowest and fastest, and then one big huge brutal burn.

It might be possible to shave some dV with bielliptic transfers, as folks have suggested, but would need to know the exact orbital parameters to work out the numbers.