Landing on the Mun

[closette]

But getting there is half the fun! Thanks for taking a look. Yes, I just focused on the descent phase because it\'s easier to analyze, and for most people does not involve staging at this time so the mass stays ~ constant.

Approaching directly or from an elliptical orbit - not covered, and if you\'re after a particular landing spot you\'ll need a gravity turn and some optimal control theory to get the most fuel-efficient approach. Speaking of which, here\'s an interesting demonstration of that (in 1-dimension, but still a tough problem) on the Mathematica site: http://demonstrations.wolfram.com/MoonLandingSimulation/

At least the equation above gives one a ballpark delta-v to land as a function of altitude, for those who choose to enter orbit first with their landing stage and look for a nice spot. Something which I rarely do myself, but now I know that lower is better if I can get there with my trans-munar stage first.

Oh, and playing around briefly with the KSP Orbit Mechanic Java tool (settings: Mun, Hohmann transfer) I don\'t see a big difference in delta-v from a large initial orbit (300km altitude) to say a 5km vs a 10km altitude final orbit. Both add an additional ~ 200 m/s delta-v. Of course almost no-one enters the Mun\'s Sphere of Influence with a circular orbit but these figures are a rough guide.

[Neomex]

What do those two yellow and pink markers mean?

I never know which button to use when to turn properly.

[Katalliaan]

What do those two yellow and pink markers mean?

I never know which button to use when to turn properly.

The yellow/green markers represent your movement (the circle is the direction you\'re heading in, the other is the opposite), and the pink ones point you to and away from the space center.

[KhorneFlakes]

This is off-topic, but something I just noticed from the OP: Holy carp, a fellow FreeSpacer!

[Ydoow]

I\'m usually just happy to land safely and not too fussy about picking a spot, although Kosmo-not\'s video was very instructive. It looks like a ~10 km orbit is the lowest you can safely go for a 'drop down from orbit' technique, but I am curious myself if it would be more, or less, fuel-efficient to drop down from a higher orbit? (You\'ll have less orbital velocity to cancel out, but farther to fall vertically afterwards).

That ~10km part will be helpful.

I\'m also interested to find the sweet spot for fuel efficiency.

I bet if you did a high orbit slow down, then use eject-able/disposable RCS units to make a controlled fall to the Mun it would be pretty efficient, no? The gravity should be pretty low up high, so I\'d think an RCS unit could do a controlled fall. When empty, dump them, leaving you more rocket fuel for a landing and take off.

[closette]

@Ydoow,

Interesting question. Take a look at the delta-v curve for 'fall' as a function of altitude in reply #23. Now look at the parts specifications on the Wiki (I just did):

http://kerbalspaceprogram.com/~kerbalsp/wiki/index.php?title=Parts

If you are going to use and dump RCS, the appropriate mass to use for each segment is (#RCS thruster blocks + #RCS tank + stack decoupler). If you use 1 thruster block + tank per segment, that\'s 1.75 mass units, for a Munar weight at 200km altitude of 0.792 force units. The total thrust of that segment=1 unit, so you\'re barely slowing down. And as you get closer to the Mun you\'ll need more than 1 thruster block firing just to counteract the weight (=2.9 force units at the surface).

It might be possible to use RCS for the Munar descent, just not very efficient IMHO. By all means worth a try all the same! You can do the reverse though, i.e. take off from the Mun with ejectable RCS, and even make it back to Kerbin.

Interestingly, you inspired me to calculate that the delta-v for {pod+RCS tank+thruster block+stack decoupler} = 530 m/s (someone please check my Tsiolkovsky math!). This is just a bit less than the orbital speed near the Mun\'s surface, AND the orbital speed of the Mun around Kerbin. Those stock parts are well designed for a challenging game! (I\'m probably not the first to notice this).

You do better without a stack decoupler. I don\'t use one between the pod and RCS tanks on my Munar lander because of the cost of dragging it all the way to the Mun and back, but it means I have to land Soyuz style (with an RCS retro-burst) when I get back to Kerbin.

[E. F. Kranz]

I know for a fact you can leave the moon with only RCS; it stands to reason that you could also land.

[closette]

I know for a fact you can leave the moon with only RCS; it stands to reason that you could also land.

Well the situations aren\'t quite reversible. You lose mass as you burn up fuel either way. In fact one NASA program simulated descent by running the ascent simulation with fuel tanks artificially filling up, then reversing time. A neat trick.

[VincentMcConnell]

Easy to land. Simply keep your engine aligned retrograde the entire time. If you velocity starts heading lateral, point your engine in the direction that you\'re moving and burn until your retrograde indicator dances back towards the center. As long as you align your spacecraft retro during the entire powered descent, you should touch down with little to no lateral velocity. You\'ll get the hang of it. Practice is all you need.

[AlexanderB]

easier said than done.. in the last moments you might need to correct out the small difference in horizontal speed, but the result was that I pancaked my last attempt flat into the ground while almost at touchdown, because the rocket responded really abruptly and sensitively to the controls..

[closette]

Keep at it AlexanderB, and you\'ll get the hang of it. The lander I use has an ASAS module (which I leave behind on the Mun) to make controlling it less 'twitchy', but I found that trying to be a prefectionist when nulling out lateral speed did me more harm than good. I\'ve skidded into a safe landing with 4-5 m/s lateral motion, but again the lander I use is short, not tall, so less likely to topple.

Doesn\'t mean I get it right every time though, which is part of the fun of the game!

[Kosmo-not]

I\'ve recently converted to using RCS landers. With those, it\'s simple to zero out your horizontal speed.

[Xivios]

I prefer a combo. My lander does have a gimballing LFE, but it\'s also got an RCS array for horizontal velocity nulling and an ASAS to help the whole mess along. Namely, the ASAS can keep the thing damn close to perfectly vertical, without worry of poorly-placed RCS pushing it over when trying to translate. End result? Translational movement is probably under 1m/s at touchdown.

[Braun]

You have to realize that it\'s not you who produces lateral motion. It\'s the body you\'re descending to that\'s drifting away.

Synchronizing that is a pain...

Just something I wanted to say since I never found it anywhere - this is my perspective on that little... problem.

[Xivios]

Um, no, it\'s not. The navball switches to 'Surface' from 'Orbit' automatically when you descend to a certain altitude, I can\'t remember what the switch is on the Mun, but on Kerbin it happens around 30,000 to 32,000 meters (which is almost exactly when my mun rocket looses it\'s first stage, hence my memory of it), and you can switch it back and forth by clicking the display.

When the navball says 'surface' and you\'ve nulled your lateral movement, you are synced - you are descending/ascending/hovering in a vertical line relative to the surface. Easy as it was before planetary rotation was added.

[Kosmo-not]

Um, no, it\'s not. The navball switches to 'Surface' from 'Orbit' automatically when you descend to a certain altitude, I can\'t remember what the switch is on the Mun, but on Kerbin it happens around 30,000 to 32,000 meters (which is almost exactly when my mun rocket looses it\'s first stage, hence my memory of it), and you can switch it back and forth by clicking the display.

When the navball says 'surface' and you\'ve nulled your lateral movement, you are synced - you are descending/ascending/hovering in a vertical line relative to the surface. Easy as it was before planetary rotation was added.

36km for Kerbin

12km for Mun

[AlexanderB]

Keep at it AlexanderB, and you\'ll get the hang of it. The lander I use has an ASAS module (which I leave behind on the Mun) to make controlling it less 'twitchy', but I found that trying to be a prefectionist when nulling out lateral speed did me more harm than good. I\'ve skidded into a safe landing with 4-5 m/s lateral motion, but again the lander I use is short, not tall, so less likely to topple.

Doesn\'t mean I get it right every time though, which is part of the fun of the game!

I know landing a lander with wings and everything is pretty easy, I was testing someone elses design.. (shute, pod, decoupler, asas + 4 rcs thrusters, rcs tank, 2 fueltanks, gymbal engine, no landing legs) and its really too tall and fragile to land properly made it on the first try but on takeoff I went the wrong way and ended circling the sun, subsequent tries after that all resulted in missing the moon, or pancaking because of the tall skinny lander with no legs. not such a great design (the whole rocket is really tall and skinny, in my (short) experience stubby rockets work better with the weird drag model anyway), but, to be fair, still capable of going to the mun and back, wich was the question.

next run might very well be a RCS only lander, see how that goes.. rcs from the mun back to kerbin works great indeed, so why not

[Braun]

When the navball says 'surface' and you\'ve nulled your lateral movement, you are synced - you are descending/ascending/hovering in a vertical line relative to the surface. Easy as it was before planetary rotation was added.

That is until you start to power up again because your descend is too fast and overshoot. The upwards acceleration decreases the snycronized drift which causes your craft to move opposite to the body\'s movement.

[VincentMcConnell]

I modded my RCS tank and used the nova punch pack and was able to take off from Earth using nothing but RCS, wrap into lunar orbit, initiate a deorbit burn and then land, take off and return.

My landing gear was just winglets and I did it all with RCS. Not that hard if you\'ve modded the tanks.