Landing effciency - Prograde or Retrograde?

[Xurkitree]

We all know that it is more efficient to launch with the planet's rotation, in order to add extra speed to your craft.

But my question is, is it more efficient to land on a body (assuming there is no atmosphere), from a prograde orbit or a retrogade orbit?

I haven't been able to make too much sense out of this question, and looking around doesn't give many satisfactory answers.

For the sake of arguement, let's just say that the ship fires retrogade until it has an impact course, then does a suicide burn.

Any thoughts?

[The_Rocketeer]

Landing from a prograde orbit is more efficient, for the same reason that taking off into a prograde orbit is more efficient - there is less difference between your starting energy state and your goal energy state, i.e. you need less delta-V to match actual velocity and altitude with desired velocity and altitude (or in layman's talk, your starting motion is more similar to your finishing motion if you're flying round the planet the same way it's turning than if you're going the other way).

For a frame of reference, let's take a planet that isn't rotating noticeably at all. To land you need to match speed with the surface, or in other words, by the time you've descended to the ground, you need to have come to a dead horizontal stop.

If the planet is spinning in the same direction as you are orbiting, you don't need to slow down that much - you still need some horizontal motion, enough to equal the speed of the surface's rotation.

If the planet is spinning in the opposite direction to your orbit, you need to slow down more - in fact, you need to pass a dead stop and gain motion in the opposite direction.


The only time it ever really makes sense to go retrograde is when you want to cash in your orbital speed for a big payout in altitude, and even then only if altitude is the only desired criteria.

Edited June 6, 2018 by The_Rocketeer

[Raptor9]

2 hours ago, Xurkitree said:

But my question is, is it more efficient to land on a body (assuming there is no atmosphere), from a prograde orbit or a retrogade orbit?

The easy answer is yes, it is always more efficient to land on an atmo-less body by going in the direction of rotation.  However, sometimes it may not make that much of a difference, depending on the orbital/rotation characteristics of the body itself.  For an example that a lot of players have discussed prior, is whether or not to enter into a prograde orbit around the Mun, or approach the Mun on a free-return trajectory, but requires entry into a retrograde orbit.  In the specific case of the Mun, which has a rotational period the same as it's orbital period around Kerbin, the delta-V savings of landing prograde is quite small.

You can determine this by comparing your Orbit speed readout to your Surface speed readout.  A 20km equatorial orbit around the Mun requires 544 m/s, but is 534 m/s across the surface of the Mun, meaning that the "speed of the Mun's surface" in the direction of rotation is about 10 m/s.  Now if you flip it around and compare the two speeds during a 20km retrograde orbit, the craft's orbital speed is the same, but the surface speed is 554 m/s.  So in reality, choosing to land on the Mun from a prograde orbit versus a retrograde orbit only equates to 20 m/s difference of horizontal velocity that you need to cancel out from orbit to touchdown.  If you have a ship or a probe that has such tight propellant margins that 20 m/s of delta-V is the difference between success or failure, I doubt many players could perform such a landing with consistent results given such a margin.

Other planets/moons that have insignificant (less than 40m/s) prograde/retrograde "speed differences" are Minmus, Moho, Gilly, Ike, Tylo, Bop, and Pol.  Due to either really low gravity, like Gilly or Minmus, or being tidally-locked with their parent planet, like Tylo.  Even Dres is hardly worth mentioning with a difference of 55 m/s.  These comparisons were done from low, circular equatorial orbits, but I doubt they would be much different from higher altitudes.

[Snark]

On 6/6/2018 at 9:51 AM, Xurkitree said:

But my question is, is it more efficient to land on a body (assuming there is no atmosphere), from a prograde orbit or a retrogade orbit?

As the previous posters have mentioned, it's more efficient to land from a prograde orbit, for the same reason it's more efficient to take off from one.

Another bonus:  if you're going through reentry on a world with an atmosphere... it'll be a gentler reentry (i.e. lower surface-relative velocity) if you reenter from a prograde orbit than from a retrograde one.

Moral of the story:  generally, you always want to be prograde, for everything, unless you happen to have some specific reason why you need to be retrograde (e.g. matching a retrograde target orbit or something).

[Tyko]

35 minutes ago, Snark said:

As the previous posters have mentioned, it's more efficient to land from a prograde orbit, for the same reason it's more efficient to take off from one.

Another bonus:  if you're going through reentry on a world with an atmosphere... it'll be a gentler reentry (i.e. lower surface-relative velocity) if you reenter from a prograde orbit than from a retrograde one.

Moral of the story:  generally, you always want to be prograde, for everything, unless you happen to have some specific reason why you need to be retrograde (e.g. matching a retrograde target orbit or something).

Yea, I was always a bit surprised that the Apollo missions used retrograde lunar orbits. They gained free return trajectories, but lost efficiencies in the LEM. When you consider Apollo 13 it was clearly the right choice though.

[Snark]

25 minutes ago, Tyko said:

Yea, I was always a bit surprised that the Apollo missions used retrograde lunar orbits.

At the Moon, it doesn't matter whether you're prograde or retrograde (to any significant degree), simply because the rotation is so slow.

The Moon is about 10,900 km in circumference, and takes over 27 Earth days to rotate once.  That means that its speed of rotation at the equator is under 5 m/s... compared with nearly 1700 m/s for low circular orbit around it.  So for all intents and purposes, you can treat the Moon as not rotating at all-- the dV difference between prograde and retrograde would be absolutely negligible.

And free return trajectories are pretty important, even if you're planning on braking to orbit and not using it-- because what if something breaks.  I expect that somebody was thinking of Apollo-13-like scenarios before Apollo 13.

The same principle applies to KSP.  It's always advantageous to be prograde rather than retrograde... but how much benefit you get totally depends on the rotation speed of the body.  If it's really low (e.g. Moho), then the difference is small enough that it just doesn't matter.

[Xurkitree]

Thanks a lot for clearing that up guys!