Multiple gravity assists on the same celestial object?

[Urben]

Hi there,

I am playing career at the moment and I try to get to the low-Kerbol (Sun) orbit. I dont have yet unlocked the atomic engine or the ion engine. Not even the mainsail engine, so getting much fuel out of Kerbin is quite hard.

I tried some ways to solve this quest. A gravity assist on eve saved som dV but its not enough. My idea now is to do multiple break maneuvers on Kerbin to lower my speed while orbiting the sun. I am about to try that, but maybe some guys here have some tips to share? Or is it even impossible?

[Streetwind]

It's possible - ESA's Rosetta mission did it (four assists total, going Earth -> Mars -> Earth -> Earth). However it takes a lot of time and is really difficult to pull off in KSP because of the rounding errors in orbit calculation.

[Urben]

Well I am not trying to precalculate the whole mission, I will see how it will do. Once Im entering the Kerbin SOI Ill do some burns to get as close as possible. Im sure this will cost only about 100dV each time.

[Kasuha]

I was able to crash into the Sun without using nuclear engines or mainsails (or slingshots) when I was playing KSP demo, so you don't need these, you only need to set up your stages right. Also if you set up your course right, you have free return because your apoapsis stays in place where Kerbin will be one its orbit around Sun later.

If you want to get near Sun using slingshots, your best bet is slingshot off Jool. Return to Kerbin might be more complicated in that case.

Multiple slingshots off the same body are possible, but there is a limit. With each slingshot, the angle between your trajectory and the body's trajectory changes and when these trajectories are parallel, you cannot gain anything by slingshot anymore.

[Taki117]

The most efficient way to get to Low Kerbol Orbit is via something called a Bi-Elliptic transfer.

The parts are a tad old, but the concept remains the same.

[MarvinKitFox]

To go straight to Kerbol, you need about 8500m/s.

But first you need to escape Kerbin's SOI from low orbit, about 1000m/s

But first you need to achieve LKO, about 4500 m/s

total = 8500+1000+4500=plenty

(14000m/s)

You can save some by going OUT to jool (~900 m/s), then using its grabbity to sling you counter-orbit, to fall straight in to Kerbol. Jool has that much grabbity slingshot and more to spare.

this could bring your total delta-v budget down to some 4500+1000+900=6400, a *much* more manageable sum.

All this, however, will only give you a single parabolic flyby of Kerbol.

If you want low Kerbol *orbit*, you need to dish out another 66500m/s, with no recourse to aerobraking, nothing to slingshot off, *nothing*. Good luck with that!

[Kasuha]

The most efficient way to get to Low Kerbol Orbit is via something called a Bi-Elliptic transfer.

Bi-elliptic transfer is only most (fuel-) effective if you cannot use gravity slingshots (and if you don't mind the mission takes ages).

[Urben]

Multiple slingshots off the same body are possible, but there is a limit. With each slingshot, the angle between your trajectory and the body's trajectory changes and when these trajectories are parallel, you cannot gain anything by slingshot anymore.

That makes sense... What if I put the apoapsis at the outside of Kerbins orbit? So the rendevouz at the crossing cant be parallel?

[Armchair Rocket Scientist]

As a matter of fact, there's a real mission being planned which is extremely similar to yours:

http://en.wikipedia.org/wiki/Solar_Probe_Plus

It plans to use multiple gravity assists from Venus, although an assist from Jupiter was also planned.

In KSP, Eve flybies may be even more effective than Venus flybies in real life; Venus is slightly smaller than Earth, but Eve is more than twice as massive as Kerbin!

[Kasuha]

That makes sense... What if I put the apoapsis at the outside of Kerbins orbit? So the rendevouz at the crossing cant be parallel?

You start with your apoapsis above Kerbin and periapsis below Kerbin. There's no crossing without that. Assuming you'll be slingshotting to raise your apoapsis, you'll raise your periapsis with each slingshot as well. And when your periapsis will be on Kerbin level, you're done. And your trajectory is parallel.

You can see an example of such double slingshot off Kerbin here. Notice the angle reduces after first slingshot and trajectories are parallel after second.

[Jouni]

A 4200 m/s escape burn from LKO against the direction of Kerbin's orbit around the Sun will get you close enough to the Sun. After leaving Kerbin's SoI, you should fine tune the orbital period to be 30 minutes less than 40% of Kerbin's orbital period. Then, after five orbits around the Sun, you will have a very close encounter with Kerbin, making it possible to return almost for free.

That 4200 m/s from a single burn is quite a lot, so you'll need powerful engines. On the other hand, even a simple Mun landing requires at least 3000 m/s of delta-v from LKO, so the total delta-v budget of the mission is not that much. If you are willing to let a kerbal stay in a command pod for 7 months, you can achieve it with 10-15 tonnes at LKO, depending on the scientific payload. The basic idea is to use a few drop tanks and a single LV-T30 as the engine.

[arq]

I've been toying with the idea of a sending a probe to low Kerbol orbit via repeated Moho assists. You can repeatedly swing around Moho until your periapsis is as low as you want and your apoapsis is at Moho, and this can be done almost for free. Of course, getting your apo down to circularize will still take fuel.