Trans-Lunar Injection

[Entroper]

So it seems like, over the course of the next few updates, we'll have a new goal: Landing on the moon (Mun, Kluna, whatever it's called -- I'll use the non-proper 'moon' to refer to it). I thought I'd get the ball rolling (so to speak) and start figuring out how to get there.

I'm going to assume a starting point of a low circular orbit around Kerbin, with the orbital plane synchronzied to that of the moon. We already know how to do those steps (even if the instrumentation for the latter part is a bit lacking at the moment), it's the next step that's the interesting one.

At some altitude above Kerbin, between the planet and the moon, will be a point at which the pull of Kerbin's gravity is equal to the pull of the moon's gravity. It seems reasonable to me to start a Hohmann transfer orbit and aim for this altitude at apoapsis. Since the transfer orbit calculation doesn't take the moon's pull into account, you should end up overshooting this altitude by some margin, meaning you will end up on a path where the moon's gravity has more influence.

At this point, depending on the moon's size and mass, you will likely be in an escape trajectory heading around the moon, and using standard orbital maneuvers, you can make a retrograde burn at periapsis to place your craft into a lunar orbit, and you'll be all set. Alternatively, if you overshoot your trans-lunar injection too far, you may be on a collision course with the moon, and may need to make a prograde or even a radial burn to increase your apoapsis to an altitude above the moon's surface before circularizing. Either way, you will have achieved lunar orbit.

Now, backtracking a bit, we have a few more things to determine about the trans-lunar injection burn. Following normal orbital mechanics, in order to place the apoapsis of the transfer orbit closest to the moon, you would want to start your burn 180 degrees around Kerbin from the moon's location. However, the moon will be revolving around Kerbin, so rather than placing your apoapsis near where the moon is, you need to place it near where the moon will be after you make your ascent. This requires finding the period of your transfer orbit, and determining how far the moon will travel during this time. Then you must travel the same number of degrees past 180 before making your injection burn.

One final note is that your orbit around Kerbin should be in the same direction as the moon's orbit. This sounds obvious, but the reason this is important is because it results in a lower velocity relative to the moon when you complete your transfer, since you and the moon are going in the same direction around Kerbin. It is certainly possible to do it the opposite way, but it will require more delta-V.

Any comments? Anything I'm leaving out / getting wrong?

[DoctorEvo]

*sigh* I don't understand why this is such a high priority for the userbase. You'd think having multiple orbital objects to rendezvous with would be the logical next step...

Anyways, the standard way to get there is to boost into a low holding orbit and then perform a normal Hohmann Transfer to get there. That's not all, though... since you want to get there in one hop (rather than just get to the same altitude and then have to wait several agonizingly-long orbits to sync up), you have to time it right so that your apokee will be approximately where the moon will be when you get there. The first time I did a lunar transfer in Orbiter, I just sorta wung it in terms of timing, and surprisingly enough I found myself on a collision course with the moon. I still don't know how much of it I can attribute to skill or just dumb luck. 8)

Anyways, if you manage to perform your burn at a reasonable moment, you will find yourself approaching the moon on a (relative) hyperbolic escape trajectory. Odds are, you'll need to make a mid-course correction so that your trajectory comes close to the moon without hitting it; this can be done by simply turning perpendicular to your (moon-relative) flight path and burning in that direction for the proper length of time. Orbiter has instrumentation that makes this easy; KSP does not at the moment. Thus, most of you will swing wide or crash until you get the hang of it (best to err on the side of swinging wide, unless you're just going for a direct descent). From there, you simply wait till perilune, then burn retrograde until you're in a nice, low lunar orbit.

From there, I'll let you figure out the rest. If the moon lacks an atmosphere, expect landing to take a LOT of fuel (probably more than you brought with you).

[Entroper]

Is there an echo in here?

[DoctorEvo]

-in here? -in here? -in here?

[Sundowner]

*sigh* I don't understand why this is such a high priority for the userbase. You'd think having multiple orbital objects to rendezvous with would be the logical next step...

Actually, it would be mandatory if we want Jeb to come back from the 'Kluna' (would personally prefer the Kearth also to be a moon of another planet ). It would be possible to get enough fuel to achieve lunar orbit, and come back, but landing... bringing all the fuel needed for the trip back to the moon surface, and back up to its orbit - some heavy cheating will have to be involved. Much easier to first have possibility to dock and undock from objects, put fuel storage on lunar orbit, and leave it there while we play with the lunar crash... err... lander. You know, like it was done 42 years ago

Some game saving system would also be nice... no to redo the whole mission, when we crash to the fuel tank coming back from Kluna surface

Sending Kerbals to their moon is a fun project, but for it to happen we need a lot of other content in this game.

[tziz]

It is possible to use cheat engine to enable time accelleration. Using it, I used a hohmann transfer orbit from a circular orbit of 51km to a circular orbit of 80,000km (I actually realise now that this is twice as far out as any appropriately scaled moon would be, however it did have an orbital time of 28 days, so it's somewhat close in atleast one way).

Once I achieved orbit, there was pretty much no way I'd actually let it run for the full 28 days (or rather 1.4 days using a 20x speedhack.

I was able to complete an exit burn that brought me down onto the planet, with 2 further navigational burns at the 40,000 and 5000km mark to bring my vector in line with the planet. I entered the atmosphere at an alarmingly steep angle at 4km/s, before coming down to a quiet landing...

(and then blowing up on touchdown, ah, ksp....)

[HarvesteR]

Most of my earth-moon flights in orbiter were done like this:

* get yourself to low-earth orbit (aligned to the moon's plane)

* wait until you're opposite the moon's future position and burn prograde a lot. (I use transfer MFD for that bit)

* then coast until you're in the moon's sphere of influence

* once there, burn sideways until you get a non-crashing, non-getting-lost-in-space trajectory

* At moon periapsis, burn retro until the orbit is circular enough for your taste.

* For extra credit, before reaching periapsis, burn up and down to get your inclination the way you want it.

* then deorbit and land

All of this should be possible with the new orbiting system, once we rig up an interface to display it, that is... that's a big task in itself, because this interface has to be very well thought out, so as to not be too complex for non-orbiter-nuts like ourselves, but still useful.

About the ton of fuel you'll need for a moon landing, it's not as much fuel as one would expect. You'll be landing at a world with only 1/6th the gravity, so a small engine with good fuel efficiency could do the job.

Cheers

[Winston]

I'll attempt to do a free return trajectory like this, purely sightseeing. Perhaps shit out some pointless satellites while I'm there.

[LukeTim]

*sigh* I don't understand why this is such a high priority for the userbase. You'd think having multiple orbital objects to rendezvous with would be the logical next step...

But rendezvous - to a relative layman like myself - seems so much harder than a lunar injection.

I just about understand Hohmann and Bi-elliptic transfers, but I haven't got a clue how to change the plane of orbit... it sounds really hard... and AFAIK it is a necessity for orbital rendezvous.

[DoctorEvo]

But rendezvous - to a relative layman like myself - seems so much harder than a lunar injection.

But, you see, that's the thing - lunar transfer IS a rendezvous. You're rendezvousing with the moon. It's a rendezvous, with a catch - pretty much every step of rendezvous must be done during or before the transfer, which is only half-an-orbit long. Also, there's the whole issue of your target having its own gravity well...

I just about understand Hohmann and Bi-elliptic transfers, but I haven't got a clue how to change the plane of orbit... it sounds really hard... and AFAIK it is a necessity for orbital rendezvous.

Plane changes are - in one form or another - a necessary part of a lunar transfer as well. There are ways you can perform a transfer that don't necessarily require you to align planes with the moon until you get there, but these are actually even HARDER to set up than a normal plane alignment.

But that's not even the hard part. The hard part is syncing your transfer with the moon, so that you don't end up at the right altitude but the opposite side of the planet from the moon.

Of course, all of this suddenly gets easier when you have an obscenely excessive amount of delta-V to work with. The Buck Rogers method - simply pointing towards your target and blasting there as fast as you can - becomes a viable option. In that particular case, I could see lunar transfer becoming easier than rendezvous - but only because your target is so much bigger.

[GroundHOG2010]

The best way for me to do a luna landing is with earth orbit rendezvous. This allows me to use current rockets to build a rocket in space to send to the moon (if it works).

[guitarist123]

Has there been any idea on about how far the moon would be? If I had to make a estimate, I would use the Kearth to Earth ratio. (1/0.0940719~). Using this I found the moon should be about 34,107.649km at Perigee, and 38,137.689km at Apogee.? (This is all thinking the kerbal universe is proportion to the Kerath Earth size ratio.)

Perigee. (362,570km * 0.0940719 = 34,107.649km)

Apogee. (405,410km * 0.0940719 = 38,137.689km)