Most fuel efficient way to get to the moon and back?

[Malekh]

I have a craft that\'s very near the fuel limit for getting to the mun and back, I just need to eek out a little more efficiency, so what\'s the optimal route?

As it is, I head towards 90 when I get over 12kish m and I am at full 90 at around 50k. I set up for the minimal possible periapsis at 70k, and then burn for the mun when I see it rise.

I end up on a direct landing path to the mun, I don\'t go into orbit around it first, and then I do a full burn around 20-25k up from the surface of the mun (which bring me VERY near to the surface), and then do a slow decent.

Now is there a more fuel effecient way to do it? Is a longer, slower burn to slow down on the moon landing better than a full burn? Anything I should be doing differently?

Is there a way of taking off from the moon that guarantees that I don\'t have to retroburn to hit Kerbin?

[Huojin]

You could be yawing over too early. I tend to wait until around 30km or 500-550 m/s before beginning to yaw towards a heading of 090. At 12km, you\'re ploughing through pretty thick atmosphere.

I don\'t think my braking burns are much more efficient than yours, considering I do two, about 40km apart, with your burn height somewhere in the middle, but yeah... I\'d go with the yawing too early thing. And if you need a little extra push to return because you\'ve run out of fuel, you could always stick an RCS tank and thrusters and use that to eke out a return.

[Hypocee]

The only thing I have to add is that slow versus fast doesn\'t affect the total delta-V per unit of fuel, but it does affect the total fuel efficiency of your trajectory. The ideally efficient pilot is as close to a gun as possible, and always burns at the maximum available power; the concept is called 'gravity drag', and can be demonstrated by extending it to an extreme: Imagine throttling down until you had .0001 m/s acceleration upwards. You\'d burn all your fuel hovering a foot above the pad, for effectively zero result. Similarly, slow-hover down from a klick up and you\'ll burn all your fuel then fall. Real landings have to make allowance for things like human reflexes, telemetry and control authority.

[closette]

Regarding landing on the Mun from a height, it reminds me of (apparently) one of the very first video arcade games by Atari:

http://lander.dunnbypaul.net/ - an online example based on the original.

Apparently the most efficient descent profile is called an 'optimal control problem' and not easy to solve, and very dependent on payload, fuel, starting height and velocity, and max. thrust and fuel rate - in other words it will be different every time!!!

Since gravity provides a constant impulse per unit time, i believe that the most efficient descent is also the shortest-time descent, but a free-fall most of the way down followed by a 'white knuckle' full burn takes a lot of finesse - and luck!

I too like to hear others\' strategies for an efficient Mun landing.

[closette]

Is there a way of taking off from the moon that guarantees that I don\'t have to retroburn to hit Kerbin?

Yes there is. What you need to do is counteract the ~540 m/s orbital velocity of the Mun around Kerbin so that you will fall almost straight back to Kerbin. There are a few ways to do this starting from Mun orbit, but if you launch from somewhere near the left edge of the Mun\'s near side (as seen from Kerbin), here\'s a more direct method which I adapted from a Youtube video:

1. Taking off from the Mun\'s surface, at 1-2km altitude, point at the center of Kerbin.

2. Keep burning and adjusting the spacecraft\'s nose to point in the direction of velocity (or a wee bit above it if you\'re not climbing fast enough), UNTIL you achieve a hyperbolic escape trajectory with an opening angle of about 90 degrees (i.e. a 'right angle bend').

This should send you away from the Mun in a direction opposite to its orbital velocity- initially at about 900 m/s, but this will slow (ideally to about 540 m/s) by the time you escape the Mun\'s sphere of influence.

When Kerbin\'s gravity grabs you, you\'ll be in a nearly radial orbit which will send you screaming back to Kerbin, as you requested. If you have any fuel left you may even want to raise perikerb a little bit so that you will aero-brake into the atmosphere a little more gently.

I\'ll come back with a screenshot if the above is not clear, but it takes a while (and some luck) for me to be able to get to the position where I can launch from the Mun\'s left near side in the first place! (This area is usually where I end up landing, by the way).

You can adapt this method to work from Mun orbit (pro or retrograde), as long as you end up shooting the guys backwards along the Mun\'s orbit at ~540 m/s. Good luck!

[Malekh]

You could be yawing over too early. I tend to wait until around 30km or 500-550 m/s before beginning to yaw towards a heading of 090. At 12km, you\'re ploughing through pretty thick atmosphere.

I don\'t think my braking burns are much more efficient than yours, considering I do two, about 40km apart, with your burn height somewhere in the middle, but yeah... I\'d go with the yawing too early thing. And if you need a little extra push to return because you\'ve run out of fuel, you could always stick an RCS tank and thrusters and use that to eke out a return.

Well after some testing, waiting with yawing over at all, until after 30k is definitely a bad idea. It uses way more fuel, I simply don\'t build up enough speed. Starting to turn at around 12,5 seems about right (mind you that\'s not a full yaw to 90, just a gradual slow yaw), and I think I\'ll actually have enough fuel now. I\'ll post my rocketship in the showcase if I\'m successful soon *fingers crossed*

Any other ideas and tips are much appreciated!

[chimpychimp]

Do you guys not have trouble yawing at the correct angle whilst still in lower atmosphere? My best ship design so far (pony mk 6) is really really stable, and barely uses any SAS autocontrol whilst ascending, and Ive not really played around with it much yet. But for every single other ship I have built, yawing whilst in atmosphere is too darn difficult. Taking off SAS autocontrol usually means the ship begins to drift on all 3 axis on its own, meaning steering it requires superhuman piloting.

[Ferrit]

Do you guys not have trouble yawing at the correct angle whilst still in lower atmosphere? My best ship design so far (pony mk 6) is really really stable, and barely uses any SAS autocontrol whilst ascending, and Ive not really played around with it much yet. But for every single other ship I have built, yawing whilst in atmosphere is too darn difficult. Taking off SAS autocontrol usually means the ship begins to drift on all 3 axis on its own, meaning steering it requires superhuman piloting.

Without seeing the desing in question I\'m going to have to provide some general advice instead of anything specific.

I generally avoid much in the way of maneuvering in the lower atmosphere (below 10\'000m to 12\'000m), only a rotation so the rocket is heading 90 East and maybe up to 10 degrees of lean to the East. After that slowly ease the nose down to 45 degrees at about 50\'000m and then all the way down to 0 degrees by 70\'000m (or intended altitude).

Fins, the non moving ones, can help aerodynamicaly stabilise a rocket and for one particular design I have to use the C7 Delta wings as fins! Best of luck to you!

Back to the main thread:

Closette: Lunar Lander was the first proper game I ever programmed a version of. Fond memories

For general Mun landings I enter orbit at 10\'000m, kill horizontal speed and then drop to the surface, eliminating any dregs of horizontal speed with RCS and braking with main engines at the last moment I dare.

To save fuel on return to Kerbin I\'ve started to preform aerobraking orbits, allowing a highly eccentric orbit return to Kerbin (ignoring direct return) over a half dozen or so orbits. In fact about half the duration of my Mun missions are the aerobraking orbits, though I want to decrease that a lot (hence this thread is of much interest!).

[chimpychimp]

Thanks Ferrit, as I say on my most recent design its actually very stable, and as you say I leave the turning to horizontal till quite late (does this waste more fuel?).

Ive actually made it to the moon 4 times now, with 2 landings! My main problem now is landing straight, I cant seem to eliminate all of my orbital velocity in order to just drop straight down. If I understand correctly, you retroburn at the green cross on your orbital tracjetory, and you also have to retroburn the greencross for your lunar velocity (caused by you being pulled down by the moons gravity). Problem is I can never seem to get the two crosses matched up, and most my landings involve skidding along the ground at about 10m/s - im amazed ive landed successfully at all.

Any tips?

[closette]

Chimpychimp,

Congratulations on your Mun landings. It is tricky to land vertically and I usually have a few m/s of un-cancelled drift over the ground when I land.

I do a bit of coarse transverse velocity cancellation at high altitude, when the spacecraft\'s vertical velocity is still rather low, just by pointing the spacecraft in the main view.

After that, as the spacecraft descends, its velocity vector naturally becomes more vertical anyway so I don\'t try to fight it. Instead I always thrust opposite to velocity (centering the yellow 'X' on the Nav ball), and if you do that down to about 6000m, you should naturally be close to vertical by then with this procedure.

In the last few thousand meters, I also use WASD keys to gently 'push' the yellow even cross closer towards the zenith spot on the Nav Ball (which is just marked by a dot with lines radiating out of it). If I get the cross close to the zenith and downward speed is around 20 m/s, I tend to leave it alone, engage SAS, and concentrate on fuel and landing speed right. Having my 'Buzz Aldrin' at my side to call out fuel status helps a lot!

Hope the above makes sense. Of course, doing all this while trying to pick a good landing spot makes for a busy few minutes, and it\'s pretty exciting. For some reason I always seem to get phone calls during this time!

[chimpychimp]

Thanks for your tips closette, ive just this second returned my kerbals home - my first fully sucessfull munar mission!

Ive found that instead of switching between the surface and orbit velocity marker and trying to center on each green X, I just switched to the surface marker. My hohmann transfer was perfectly timed to create a near vertical orbit straight to the muna surface without the need for any retroburns, so I think thats what helped.

The last thing im still struggling with is getting all of the various bearings straight. I know that the flat plane between 90 degrees and 270 degrees is the orbital path of the mun, but actually figuring out when to burn to initiate the orbit for a hohmann transfer is really difficult. Is there a marker I am supposed to use on the glob ball thingy?

[Hypocee]

Orbital transfers are quite difficult There is not currently a marker for an ideal Hohmann transfer, in part because it relies on either a big fat UI or the assumption of a mun landing, and in part because there\'s debate over whether it would hurt the fun to have the machine tell you GO HERE FOR SUCCESS HUMAN PILOT without having 'bought' the machine in the metagame. Many people use the rule of thumb 'burn when it rises above Kerbin horizon in front of you'.

You\'ve succeeded multiple times already, but you might also find the RCS translation controls useful/fun. Tune the main engine thrust for near-cancellation of gravity, then adjust your velocities directly rather than relying on tilt.

[chimpychimp]

Mmmhm I think I need more practice with the RCS controls before I try that, Ive started to use them but im still far to slow and durrr which way is up to make any use of them during a hasty descent.

I modified my best shuttle, now pony mk8, to have the crew pod attached to a RCS fuel block then a stage ejector thing, rather than the other way around. That way its like a mini shuttle in case things go tits up with my main engine. Just now I tried someones instruction of how to do a free return orbit, and although I was thrown back into kerbal orbit I would have been stranded, but luckily I had the RCS for a messy uncontrolled retroburn. Great fun 8)

[closette]

@chimpychimp, glad to be of help - the only way I got to the Mun and back was to learn from others: on this forum, on other fansites, and on Youtube.

The 'marker' I use for initiating Trans Munar Injection from an eastward orbit around Kerbin is very simple - when I see the Mun rise just over the horizon, I give it full thrust along the velocity vector, then switch to Map view, and only cut the engines when the Apopsis is equal to the Mun\'s orbital radius. Seems to work every time.

I\'m still struggling with finding a good 'marker' to initiate the escape burn from low Mun orbit that will send me back along Mun\'s orbit at 540 m/s, for a near radial plunge back to Kerbin. Any advice appreciated/

[SteevyT]

For getting back, I\'ve found that burning right about at the Mun\'s orbit line in front of it to escape works fairly well.

[Raziel]

I\'ve managed to get a small spaceship to the mun by following this method: [ship: 1 command pod: decoupler: 4 LFEs : 1 LF engine (no gimbaling): 6 SRBs ]

At 20k from launchpad tilt 10degrees, at 35k : 45 degrees, at 50K turn to 75 degrees and continue burning this way. You will manage to get an orbit crossing the mun\'s trajectory if you time things right, otherwise you will fall back to kerbin and enter atmosphere as the periapsis will still be inside the atmsophere. You shoud also have enough fuel to get into orbit with the mun (but not land on it). [PS You can always crash on it the kerbal way ]

[Rickenbacker]

I\'ve been thinking about how to achieve orbit as efficiently as possible - that is after all where you burn most of your fuel. Is the most efficient strategy to just burn at max throttle and pitch over gradually, or is there a way to manage throttle to use less fuel on the way up?

[Rickenbacker]

Interesting... I tried to launch with full throttle all the way to orbit, and a gradual pitch over. This let me get to the Mun with about half my second stage full of fuel, where previously I only had a tiny fraction of fuel left. So I guess it IS more efficient to get the hell out of the thick air as fast as possible.

[Hypocee]

Possibly, in a given craft, in KSP, currently But exploring and working out the calculus of less gravity drag vs. more kinetic energy imparted to thick air is among the things real aerospace engineers are for. KSP\'s drag model is also currently incomplete in one way and broken in another - though if I understand correctly both fixes will push it in the 'power hard' direction.

[dashcunning]

I don\'t know if it\'s the most efficient return possible, but the following method seems to work well and is based a specific, very repeatable timing.

Get into a 5-6 km eastwards orbit around the Mun and burn at 40° past the the Mun\'s forward orbital line. An easy way to set this up is to go to the orbital view and when you are crossing that line aim 40° below the horizon on the navball and set ASAS. Wait for that aimpoint to become the horizon and then start your burn. Get up to 830 m/s and then throttle down. Your trajectory will be about as good as you can get and by burning at a low altitude (and higher speed), you get the most out of the oberth effect. I think, I\'m only an amateur rocket scientist

Your orbit around Kerbin will have a Pe in the atmosphere (depending on the various specifics during the burn), easy to fine tune for aerobraking. Below ~30,000 for an immediate descent with no further orbiting. Above ~40,000 to remain in an orbit, from which you can make further adjustments. Say for landing back at KSC!

[Kosmo-not]

My ascent plan into Kerbin orbit goes like this:

Burn straight up until 13k, then nose over to 55 degrees.

Once navball goes into orbit mode, point nose prograde.

Continue burning until apokee is close to desired altitude and cut of thrust.

Before reaching apokee, thrust again to get apokee to desired altitude. (should be pretty close to orbital velocity after this)

Final gentle burn for orbit at apokee.

My thought process was to be able to put almost all my thrust into the prograde vector.

I used to burn straight up until out of the atmosphere, but then I took an arr....

Seriously, though. It does save a good amount of fuel going up at an angle.

As for drag, I hear optimizing your throttle setting has to due with your TWR (thrust to weight ratio). I cannot remember the optimal TWR, but I did see it in this forum somewhere.

[Hypocee]

As for drag, I hear optimizing your throttle setting has to do with your TWR (thrust to weight ratio). I cannot remember the optimal TWR, but I did see it in this forum somewhere.

My guess is you\'re referring to a few of Alchemist\'s posts, most clearly stated in this one. Strictly speaking it\'s about adjusting your instantaneous speed which has implications for your design TWR.

[closette]

@Hypocee,

Thanks for the reference to Alchemist\'s post about thrust/weight ratio, but I\'m wondering did you find an answer to the question you posted below it:

I\'d seen you run through that before. Is it always the case that 50% power devoted to drag is optimal? Precisely or rule-of-thumb? Is it Some-Russian-Or-German-Guy\'s Formula so I could look up the derivation?

I am curious about this 50% 'rule' too.

[Hypocee]

Nope! Still vaguely wondering.

[PD]

I\'m fairly sure that efficiency basically requires:

Efficient burns (as hard and as short as possible, to limit 'gravitational drag')

As few burns as possible.

If you don\'t need to land on the Mun, then the most efficient way of doing things is to burn for a 'Free Return Trajectory'.

How to achieve Free Return

Burn into 120,000 parking orbit 090 degrees.

Wait for Mun-rise.

Burn for AP to reach about 14.5 Million m.

Shut off engines and wait.

You should head out to the mun, come fairly close to it between 500,000 and 1,500,000m of the surface, before being slingshotted retrograde to it\'s orbit.

From there, just wait, and you will be re-captured by Kerbin, with a Periapsis either in the atmosphere or on the ground.

You just got home without a single burn. Efficient? Yes!

PD