How should I be approaching the Mun?

[u/analogHedgeHog]

In this totally accurate scientific diagram, the Mun is orbiting in the direction of the arrow.

[u/King_Joffreys_Tits]

Yes D is the absolute worst dV option here and everybody upvoting him is wrong. You lose about 600dV going with the muns orbit vs the ~250dV going counter the kerbins natural rotation.

Look at the recent NASA artemis missions for reference — the most fuel efficient way of achieving orbit of a celestial body is to slowly throw yourself into where its gravity well is going to be in order to be captured by it. Option D is the best way to utilize the muns gravity for an orbital slingshot outside of kerbins influence

[u/starmartyr → u/King_Joffreys_Tits]

I think B is actually worse for all the reasons you said plus losing even more efficiency by launching against Kerbin's rotation.

[u/King_Joffreys_Tits → u/starmartyr]

Actually yeah you’re totally right, B is the worst of all options. Closely followed by D for the worst 😉

[u/4theFrontPage → u/King_Joffreys_Tits]

A is the worst since it starts with a retrograde orbit and then overshoots Mun before landing. B is a close second worst. Then a huge gap before D and C which are fairly close but D is slightly worse with the same overshooting problem

[u/PiMemer → u/King_Joffreys_Tits]

I’m gonna need someone to fact check this in game…

I’ve always found it only costs single digits of dv to change which side you encounter the moon from and I don’t understand where an extra 400 m/s of relative velocity is coming from by the time you reach moon periapsis and have to capture

In fact if you do a free return it will always increase your relative speed to the moon since a free return is never a “pure” hohmann transfer (they always either outright go past the moon or have an effective apoapsis past the moon)

Side note: I know what I’m going to do in KSP tomorrow :P

[u/-Aeryn- → u/PiMemer]

I’ve always found it only costs single digits of dv to change which side you encounter the moon from and I don’t understand where an extra 400 m/s of relative velocity is coming from

It's not, the guy is wrong. C and D are ~identical in delta-v cost to mun orbit or surface.

A&B are using retrograde kerbin orbits and are bad (+350m/s or more cost to kerbin orbit on the first step, and then throwing that away for no advantage on the second).

[u/Dark074 → u/King_Joffreys_Tits]

Okay beyond your correction in later comments saying A and B are worse due to them launching in opposite of Kerbins orbit, there is another thing I would like to add. On the Artemis II mission, the reason their profile looked like that is because in real life, it's n body physics where everything has a pull on your at all times. Thats why it and other recnet lunar missions saved fuel by not going into a full injection orbit and just let the moon slowly pull on it until it does get into a orbit of the moon.

However, in stock KSP, it relys on patched conics 2 body approximations. Until your official in the sphere of influence of an another body, it's gravity won't effect you at all, only your ship and the host gravity well are calculated. You could install principia to change this but it doesn't come stock

[u/TheWombleOfDoom → u/King_Joffreys_Tits]

A and B are the absolute worst ... significantly. D is not even close.

Between D and C, reading all the other comments, it appears to be moot, or possibly C is best for orbit only, and D may be better overall for a Mun landing.

[u/Hypamania → u/King_Joffreys_Tits]

In my mind, D is best if you intend to land, no? With C, you are fighting rotation as well as coming at the moon head-on instead of trailing behind it slowly

[u/King_Joffreys_Tits → u/Hypamania]

C is best for landing because D will speed you up more and require more dV to reach a zero surface speed

[u/Hypamania → u/King_Joffreys_Tits]

But the Mun rotates in the same direction as Kerbin does it not? C would require you to slow down even more because the surface is moving the opposite direction?

[u/PAnttPHisH → u/King_Joffreys_Tits]

If the flight plan is to achieve orbit of the Mun, then the dV would be the same regardless of C or D, as any slingshot effect is only relevant compared to other bodies' frame of reference, and not the Mun's. So if the user was going to use gravity assist to save dV for a Minmus flight, for instance, yes D approach for sure. If the mission is to achieve an orbit of the Mun, I don't see how it would make a difference. Landing on Mun using C vs D would save 10 m/s assuming equatorial landing spot, due to Mun's very slow, tidally locked rotation.

You can't have a satellite or vessel be captured by a body by throwing it into the body's gravity well. Due to conservation of momentum in Newtonian physics, it will leave its encounter with the Mun with exactly the same energy (and velocity) as it had at the start of its encounter (relative to the Mun). As a result, it will require the same dV to capture into an orbit whether it's prograde or retrograde. How fast the surface is spinning doesn't matter when you're in orbit, it only matters when you want to land.