Interplanetary launch

[Augustulus91]

I am making my first steps towards interplanetary travel. I am launching a probe, that i want to place in an orbit around Duna.

I've read somewhere that if you're planning to leave Kerbin's SoI, there is no need to do a gravity turn.

Most tutorials start with a ship in orbit around kerbin, and start burning from that point.

What is the most fuel-friendly? Placing the probe in orbit (using a gravity turn), or just keep on burning straight, until i leave Kerbin's SoI?

Edited August 11, 2013 by Augustulus91

[jebster]

I think it would work like this:

If you perform a gravity turn and use Kerbin as a slingshot towards Duna, you will save fuel getting there, but you will have to spend more fuel slowing yourself down. This is because you will gain velocity as you orbit around the sun and by the time you enter Duna's SoI you will be moving very quickly.

If you burn directly out of Kerbin's SoI and into Duna's, you will use a large amount of fuel leaving, but your velocity upon reaching Duna will be much less.

I'd think that using Kerbin as a gravity slingshot would be more fuel conserving as you can just aerobrake into Duna and lose the majority of the velocity, conserving much fuel.

[SRV Ron]

I'm not sure which is more efficient, an escape from orbit, direct burn at dawn, a direct burn after midnight, or a direct burn that is timed to use Mun for a slingshot to escape velocity. I may test that againin the near future with several probes to find out which is most efficient.

To burn directly to Duna, launch at dawn and shut down as soon as you hit Kerban escape. To do a more efficient burn from orbit, get a prograde Mun encounter where the encounter with Mun is near the dawn side of Kerbal and set up a slingshot boost to get escape velocity.

For heading to Eve, launch straight up at dusk to escape velocity.

If using the Mun slingshot, be mindful of the plane angle you leave Mun from and adjust the mid course correction maneuver accordingly or you will have to make that planer change during a later mid course correction that will cost you more fuel.

If you can encounter Duna retrograde, you can use its SOI to slow you down even more and save fuel or save even more fuel using aerobraking. (Very difficult to set up but worth it if short on fuel.)

Edited August 11, 2013 by SRV Ron

[Frederf]

Direct ascent is what you're describing. You absolutely want to do a gravity turn or similarly optimized ascent as if you were aiming for a parking orbit. The directness of the direct ascent profile comes from the zero time delay from reaching a stable orbit until burning for escape. If you take a typical ascent-park-escape mission profile and reduce the time spent in park you approach the direct profile. The nice thing about eliminating the parking orbit is you can choose a lower point at which to go hyperbolic since the time spent in the thin upper atmosphere is so small that the drag losses are overshadowed by the deltaV gains from being lower. The downside is the absurd levels of planning and precision required.

[Armchair Rocket Scientist]

Thanks to the Oberth Effect, the lower you burn off your fuel, the more you get out of your dV. Even for an escape trajectory, a gravity turn will give you better efficiency, as seen

. In addition, getting into a parking orbit gives you much more flexibility on when you start your transfer burn; if you miss your burn window in a 30-something minute orbit, you can try again on the next orbit. If you miss your burn window on the launch pad, you'll have to wait six hours for Kerbin to rotate back into position.

For optimal efficiency, you should use a prograde parking orbit, so you can take advantage of the extra velocity from Kerbin's rotation. Then you should perform your transfer burn such that you escape Kerbin travelling in the same direction as Kerbin orbits, giving you a hohmann transfer out to Duna. This will give you the lowest possible velocity relative to Duna when you encounter it. Finally, tweak your encounter to fly as close to Duna as possible, taking advantage of the Oberth Effect again to make your capture burn easier. Then you can perform any aerobraking necessary to get your final orbit.

The same principle applies to superior planets (Dres, Jool, and Eeloo). For Eve, the principle is the same, but you want to time your burn to escape Kerbin travelling opposite Kerbin's orbital movement, cancelling out some of your orbital speed relative to Kerbol and fall inward to Eve. Moho is more complicated, because it has such an inclined and eccentric orbit.

[Augustulus91]

Thank you for the response, i will go through all the different answers, but i assume that doing a gravity turn is clearly the better option (as i am not using any tools, so it's hard to time certain orbits (for Mün slingshots)

[Motokid600]

Treat yourself to Engineering Redux. It's something no one should go with out.