So why do people go to the Mun when it is right over the horizon?

[RainDreamer]

I have seen a lot of Mun landing guides that says this, and I wonder why. Is it for "straight up" launch that doesn't try to go to kerbin orbit first and instead launch all the way to the Mun?

Edited January 11, 2015 by RainDreamer

[Reactordrone]

It depends which horizon you're talking about. If you launch with the Mun to the east then you'll do a normal gravity turn and keep burning to get a Mun intercept. If the Mun is on the western horizon you'd have to burn straight up to get an intercept.

[Zourin]

Munrise is the earliest point for a "direct intercept". Munset is 'last call' before you will likely need to spend the fuel for a full kerbin orbit, plus the fuel for the transfer burn.

I'm pretty sure I don't have that backwards. Now I can't remember if Kerbin rotates faster than the Mun's orbit.

Edited January 11, 2015 by Zourin

[eddiew]

By and large, they probably mean 'from orbit' in the first place - this is a fair assumption, since this is where you can be setting manoeuvre nodes with time to think. However, if you want to do the shortest time in the pod possible (maybe you have TAC life support installed) then you can do it as a direct injection burn, meaning you arrive at orbital altitude and velocity at the right time to be burning to your target. Ultimately this will result in circularising around Kerbin anyway, as part of the transition to the more elliptical orbit, but means you don't have to physically loop round Kerbin first.

As for the vertical burn all the way, that's a discussion comes up quite often; the short version is that it's easier to orbit first, and doesn't cost significantly more delta-v, so most people prefer to stabilise LKO first.

One way to think about it (it's not the right model, but it fits with common sense)... If you go straight up, the distance is less, but you always have Kerbin underneath you all the way until Mun's SOI, dragging you straight backwards. That is, you're fighting 100% of gravity to keep going outwards. If you orbit first, then you go out in an ellipse, the pull of gravity behind you is off-centre and some of it goes into making your ship turn sideways, leaving you only fighting maybe 70% along your retrograde, letting you make the same AP for less delta-v from the same starting altitude.

[RainDreamer]

Ah, then my assumption is right. It is for a direct burn to mun, whether it is from orbit or from surface, huh.

[Vectura]

Though when in LKO, burning prograde when the mun is just rising guarantees an encounter. Assuming you are orbiting the right direction (to the east)

[Wanderfound]

It's not for a direct burn, it's for a Hohmann transfer.

For any Hohmann transfer, there is a phase angle between the location of your exit burn and the location of the target at the time of the burn that you have to get right for the transfer to succeed. Basically, you need to raise your apoapsis to the height of the target orbit, but you need to do it in the right place so that the ship and the target arrive there at the same time in order for the target's gravity to capture the ship.

For the Mun, that angle just happens to be roughly what you get when you burn at Munrise. It's an easy visual cue that allows Mun transfers without navigational aids; you can do it with a Tier 1 Tracking Station fairly simply.

Although you burn straight towards the Mun, that location isn't where you go: instead, you swing into an extreme elliptical orbit, with the apoapsis 90° off to the left of the direction of the burn. It's at this apoapsis that you eventually encounter the Mun (which has itself orbited around to that location by the time you get there).

Edited January 11, 2015 by Wanderfound

[OhioBob]

I have seen a lot of Mun landing guides that says this, and I wonder why.

It's really just a coincidence that it works out that way. If you are in a minimum orbit around Kerbin, say about 70-75 km, then Mun will rise when the phase angle is about 114 degrees. This is the angle between Mun and your spacecraft as measured from the center of Kerbin. If you burn at that instant to raise your spacecraft's apoapsis to the height of Mun's orbit, it will take about 7.4 hours for the spacecraft to travel halfway around Kerbin from its current periapsis to its apoapsis. The spacecraft will have traveled 180 degrees from its starting point, but since it started 114 degrees behind Mun, it will have traveled 180-114 = 66 degrees relative to Mun's starting position. In that same 7.4 hours that is takes the spacecraft to reach apoapsis, Mun will have traveled 69 degrees around Kerbin. Therefore the spacecraft and Mun will arrive at approximately the same point in space at the same time. With just a little bit of tweaking you can have a very nice encounter.