Return Mun Lander? (Answered, but further suggestions most welcome)

[Mister Spock]

I've long had trouble making a Munar lander that has enough delta-V to make it back to Kerbin. I land fine, and usually I have enough fuel to make it back to Munar orbit, but it seems I'm always looking at about 300-350 m/s delta-V left once I'm in Munar orbit, which is cutting it close for making it back to Kerbin. (I suppose I shouldn't be orbiting Mun at all, but even jetting right toward Kerbin to escape Mun, I still cut it close.)

I've tried one 400-size fuel tank with legs; one 200-size tank with four 100-size radially attached with fuel lines; and other configurations. Always I seem to be just short of what I need to get home. Yes, I can slap on a docking port and refuel in orbit, but I'd prefer just to go home directly.

Any advice on configuration for return landers? I did search on landers here, but much of the advice isn't focused on *return* landers. Thanks in advance.

Edited May 11, 2015 by Mister Spock

[TheXRuler]

hmmmm you could either add in some staging (drop the empty t100's).

Also launch not directly toward kerbin but toward the retro direction of your kerbin relative orbit. lift off and turn so that you are going in the opposite direction to which the mun is orbiting.

For an optimal gravity turn launch when you are either directly in the center of the kerbin side or directly on the other side. Well not optimal but close enough

Edited May 11, 2015 by TheXRuler

[Randazzo]

If I absolutely need to retrieve the lander itself, I have two methods:

1. Make it a two stage lander (landing/return) with enough fuel in the return stage to get back to LKO. This has the downside of being a touch larger than a usual lander.

Example:

2. Instead of making a heavy lander, make a larger transfer stage and leave it in orbit. Once you bring the lander back up, dock with the transfer stag and let it carry you back home. It is possible to do this with a dummy stage that is just a fuel tank and engine, but it is far easier if you put a probe core and reaction wheel on the transfer stage so you can align it easily. It will also require solar panels and a battery. You can also build your lander so that it jettisons the ascent stage after docking to increase the dV budget of the transfer stage.

Example:

Sorry that last image is dark, the transfer stage is performing it's escape burn back into Kerbin orbit. These are just my preferences, there are undoubtedly other ways to get this done. I also use lots of mods, as you may have noticed, but you can get the same functionality out of stock parts. It's just less pretty.

[Mister Spock]

XRuler: Thanks for the advice. I'll try that trajectory.

Randazzo: Thanks for your reply. I do rather like the idea of an Apollo-style mission, so I may try that. Rendez-vous and docking takes me a while, but I do enjoy it.

Anyway, for now I'm trying a combo of your suggestions: a two-stage lander with a docking port, and a separate refueling vessel. I'm playing Kerbal Construction Time, so I'll build the refueling vessel first, to ensure that it's in place before I launch the Mun shot.

Incidentally, it's not that I want the lander back; it's that I want my Kerbal and the science results back. Kerbals are precious: I have only five right now. I'm playing a moderate-to-hard career mode, never reverting (relying solely on KCT sims to test), accepting my losses. Plus I'm using USI Life Support. Plus I get attached to my Kerbals. So I don't want to do what I used to do -- just stuff some random Kerbal into a capsule and let him sit on the Mun forever, planting flags occasionally. I want to bring the Kerbal home!

[TheXRuler]

Oh, a fellow Kerbal lover, nice I thought I was starting to think I was alone in this.

I myself use TAC-LS and it has made me do crazy things like fly a 19000 m/s dV mission to duna to save my kerbals with barely 80 days of life support left xD

[Mister Spock]

Yep, in my last save, I used TAC-LS, and I did similar stuff. USI Life Support has a less-lethal penalty by default (Kerbals just go on strike if they're hungry), but I still can't stand to see it happen. I like the gameplay challenges that life support mods bring.

[Fishslap]

You could always build a big enough ship that it will have fuel when it gets there. It is possible to soft land even rockets on Mun if it`s flat enough. If it`s about lander construction I doubt that`s the problem. You can give it a good go almost right at the start of career mode if you do trajectories and orbits right. For the escape you barely need any fuel to get back at all. Remember to escape backwards from Mun when you leave, and not directly at Kerbal. We`re not moving in lines here but in orbits. A line is always slower and more fuel inefficient. Mun moves at 512 m/s along its orbit. The faster you can escape the gravity the faster you`ll be picked up by Kerbal and the less fuel you`ll need. It`s the same with every planet and moon. Naturally the fastest way back therefore is directly back along Mun`s orbit. Just a short burst in the right direction and you`ll go right down your new orbit to Kerbal`s atmosphere. Minmus is the same and it`s almost as if you fall down to Kerbal if you leave it the right way even though it`s very far away. Gotta love science right!

To explain why a bit, if you leave Mun along its forward path you`ll be moving at 500-600 m/s but, like I said above, Mun will be moving at 512 m/s along its orbit (Therefore effective escape velocity= 600m/s - 512m/s = 88m/s) so it`ll take a long time to outrun it basically. Go the other way and you`ll be moving as fast while Mun will be moving away from you (Effective escape velocity 600m/s+512m/s = 1112 m/s). Huge difference as you can see. If you go in a straight line toward Kerbal you`ll be somewhere in between. Let`s average out 88 and 1112 and say you`ll have an effective escape velocity of 600m/s. But you will also have less control over the peri down in Kerbal`s gravity well and will need more fuel to get down. You will also stay longer in Mun`s gravity and you will lose orbital speed while waiting to get picked up. Always leave backward:) That`s what we`re doing when we launch every rocket to the east really. West (From the Space Center) is the direction of Kerbal`s orbital movement around the sun and it therefore takes longer to escape Kerbal`s gravity if you go out the wrong way. And because of the equatorial spin it is also harder to get out of the atmosphere that way so a double whammy of physics right in your face. You should try leaving on a reverse orbit and compare how much fuel it takes to the normal way. I haven`t tried it in KSP yet but I wouldn`t be surprised if it took twice as much burn to get to Mun going west. Maybe more. Simple concept really, once you stop and think about it.

Edited May 12, 2015 by Fishslap

[FancyMouse]

Isn't 300m/s enough for returning from Mun orbit? You just need to get a correct escape angle (read: play with maneuver node), then burn to make a Kerbin Pe at around 30km, then let atmosphere brake you down - the burn should usually be just 280m/s or so.

And of course, landing skill is something that needs practice and you could save a lot of dV from that as well.

[Mister Spock]

@Fishslap: Thanks for your detailed reply. I thought I read somewhere that the optimal angle was about 30 degrees off the Mun's retrograde path, but in any case, your explanation helped me understand much better.

Interesting you ask about taking off to the west from Kerbin: I just did this to achieve a 180 degree inclination to match the orbit of a stranded astronaut (for a rescue mission). The westerly launch cost me an extra 400 m/s delta-V or so. Perhaps more interesting, I returned to Kerbin using the same clockwise inclination, and my re-entry was by far the hottest, scariest I've had in 1.0x. I suppose that's because I was entering "against the grain," as it were? I was moving west while Kerbin was rotating east, causing more speed and heat friction?

@FancyMouse: Yes, 300 m/s is about enough; it's just that I am always right around that 300 m/s mark, and a Kerbil's future is at stake. Yep, my landing skills are not all they could be. I gather it's more efficient to avoid over-burning for a long time during descent, but the "safe" approach is to over-do rather than to gamble on a sudden burst of power 1000 meters above the surface. As I mentioned, I'm playing a custom-almost-hard-no-revert game, with Kerbil Construction Time, so I only get one chance to get it right -- although for tricky maneuvers, I do "sim" using KCT a half-dozen times before doing the actual mission!

[FancyMouse]

@FancyMouse: Yes, 300 m/s is about enough; it's just that I am always right around that 300 m/s mark, and a Kerbil's future is at stake. Yep, my landing skills are not all they could be. I gather it's more efficient to avoid over-burning for a long time during descent, but the "safe" approach is to over-do rather than to gamble on a sudden burst of power 1000 meters above the surface. As I mentioned, I'm playing a custom-almost-hard-no-revert game, with Kerbil Construction Time, so I only get one chance to get it right -- although for tricky maneuvers, I do "sim" using KCT a half-dozen times before doing the actual mission!

As long as it's not below 280m/s it should be fine. If no revert, then try manually reduce the engine thrust when getting close to the completion of maneuver action, and then do fine adjustment (with 0.1 or even less thrust and throttling you can get really high precision - that's how I get my comm satellite network of 400km height with <10m precision) - and you can achieve high precision orbit without wasting .

[Mister Spock]

Update: Success! This is my first KSP career in which I've landed a Kerbil on the Mun and returned her to Kerbin safely on the first* try! (*Well, I did "rehearse" bits of the journey using the "sim" feature of KCT, but that's still not the same as the Main Event, especially in a no-revert game.) The multi-stage lander worked great: I landed with a bit of LFO in four radial tanks, which were feeding a fifth central tank, keeping it full. After EVA, flag-planting, and general tomfoolery, Staemma Kerman climbed back in and just shot straight up, as I was on the "retrograde" side of the Mun. You guys were right: I simply skipped Mun orbit and went directly to Mun escape, and the Mun disappeared ahead of me more quickly than I'm used to, which was great. I dropped down into Kerbin's atmosphere with 100 m/s delta-V to spare. Re-entry always scares me, but it went smoothly. I brought back 300 science, which will jump-start my research. Yay!

I did have one slightly off-topic question. When landed on the Mun, I had a heck of a time trying to figure out which way was "counterclockwise" -- i.e., whether I was looking above or below the ecliptic plane. I finally settled it by targeting the transfer stage debris I'd dropped in Mun orbit; that debris was orbiting counter-clockwise, and that confirmed to me which way the Mun was orbiting. Is there a simpler way? Is there some way to enable "orbit is going this way" arrows?

[FlipNascar]

There aren't those arrows as such, but it's worth noting that all of the celestial bodies, in stock KSP, orbit and rotate in the same direction. So this means that if you look East when you are on the body, you are looking in the direction of rotation.

[Mister Spock]

That's helpful, FlipNascar. So if I'm on the surface of the Mun, and if the sun is up, I can tell which way the Mun is rotating (and therefore orbiting) relative to the sun. That should be good enough.

Little directional arrows on the system display would still be nice. I've noticed that the sample orbits for contracts do include directional arrows.

[Fishslap]

"As long as it's not below 280m/s it should be fine. If no revert, then try manually reduce the engine thrust when getting close to the completion of maneuver action, and then do fine adjustment (with 0.1 or even less thrust and throttling you can get really high precision - that's how I get my comm satellite network of 400km height with <10m precision) - and you can achieve high precision orbit without wasting ."

Yes of course. But the idea is to escape its gravity as quickly as possible, which you do by moving back along the orbit because then Mun is obviously moving away from you. If you go straight up you`ll stay in the gravity well longer and therefore you have to create more m/s yourself = you need more fuel. I just picked a number. You are using the map and stopping the burn when the periapsis hits Kerbin`s atmosphere anyway right? I was just talking about escape vectors and not really the velocity of the lander.

Edit: Just thought I`d post this to demonstrate the concept. A short burst back along Mun`s orbit and you make an apo, and naturally the peri is at the bottom of the well. Going in any other direction should require more fuel before the apo gets made, and they might not be this convenient if the burn starts circularizing the Kerbin orbit while you get away from Mun.

Edited May 13, 2015 by Fishslap

[Fishslap]

@Fishslap: Thanks for your detailed reply. I thought I read somewhere that the optimal angle was about 30 degrees off the Mun's retrograde path, but in any case, your explanation helped me understand much better.

Interesting you ask about taking off to the west from Kerbin: I just did this to achieve a 180 degree inclination to match the orbit of a stranded astronaut (for a rescue mission). The westerly launch cost me an extra 400 m/s delta-V or so. Perhaps more interesting, I returned to Kerbin using the same clockwise inclination, and my re-entry was by far the hottest, scariest I've had in 1.0x. I suppose that's because I was entering "against the grain," as it were? I was moving west while Kerbin was rotating east, causing more speed and heat friction?

@FancyMouse: Yes, 300 m/s is about enough; it's just that I am always right around that 300 m/s mark, and a Kerbil's future is at stake. Yep, my landing skills are not all they could be. I gather it's more efficient to avoid over-burning for a long time during descent, but the "safe" approach is to over-do rather than to gamble on a sudden burst of power 1000 meters above the surface. As I mentioned, I'm playing a custom-almost-hard-no-revert game, with Kerbil Construction Time, so I only get one chance to get it right -- although for tricky maneuvers, I do "sim" using KCT a half-dozen times before doing the actual mission!

Good to hear. And you`re right about the velocity to escape Mun. But I was talking about the speed you need in order to find a good peri on Kerbin. And again, you need to develop much less speed using your fuel if you go backward rather than forward because Mun just disappears along its orbit as soon as you throttle up. It is actually a very good idea to stay in orbits for a long time at first to experiment a little with the various angles you can use to leave. If you grab the central wheel you can move your maneuver node around the orbit to see where you get picked up. And a short burst from Mun`s orbit will intersect with Kerbin only if you go backward. Straight down it`s a medium burst, and you usually also have to build speed first to get picked up and then brake again after to shorten the peri. So you might actually end up with two medium bursts just to get home. And of course if you go the wrong way you`re not very likely to make it back at all, at least not early on. You`ll be tailed by Mun for ages before you can even find a peri on Kerbin. And then it might not even be deep. You can sometimes even find yourself in a circular orbit when you get free and have to again brake to cut an apsis down to reentry level. I think 30 degrees retrograde sounds right. But it depends on what you`ve been doing on Mun and whether you have a circular orbit when leaving or not. You certainly don`t want to waste fuel circularizing your Mun orbit before leaving. In the picture I posted above I came out at a somewhat inclined orbit and ended up tangential to Mun`s orbit going outward, so it was certainly not perfect. But it was still only a 12 second burst with this lander:

Anyway, I guess you`ve proven that taking off to the west is a bad idea then. Those numbers sound about as I expected, and it is certainly not for people running out of fuel at Mun... I`m not aware of any reentry difference depending on which direction you`re coming from though. Apparently there is more air resistance at the poles I am told - at least in real life - because cold air is more compressed than warm air (heat causes everything to expand/vaporize). What you say about the direction sort of makes sense though. I guess we`ll have to study up and find out about it:)

Edited May 13, 2015 by Fishslap