Returning from the Mun with 1000 DeltaV

[Dragonchampion]

So I have landed on the surface of the Mun and now need to get home. But after about 10-20 quicksaves and reverts, I can't seem to get a return trajectory... just a permanent orbit around Kerbin where I end up stuck with no fuel. How do I return from the Mun with 1000 DV?

Edited July 22, 2014 by Dragonchampion

[Talavar]

Launch your ship into 90 degree orbit. then burn prograde when you start getting to the side of the moon that faces kerbin. Conserves the most fuel. Check out this graph it may help. http://forum.kerbalspaceprogram.com/threads/25360-Delta-V-map according to this graph, You are short on fuel. this is of-course showing teh DV required to get there.. not sure if it will work well in reverse if you add the nodes getting to LKO. Assuming it takes as much to get into orbit as it does to break orbit, run it backwards and add the DV needed - 4500 to escape kerbins atmoshere.

Edited July 22, 2014 by Talavar
Changed graph

[palioxis1248]

It's important that your escape trajectory is aligned opposite the direction that the Mun is travelling. This gives you the biggest decrease in Kerbin periapsis for a given amount of fuel. As to where you should start burning around the Mun to achieve that, barring calculation it's mainly a matter of trial-and-error.

[Pecan]

This deltaV map (http://i.imgur.com/NKZhU57.png), which is the one I use, shows you need 580m/s to get into low orbit around Mun and another 230 + 80 to return - for a total of 890m/s. As palioxis says you need to be leaving Mun opposite to its direction of travel; you are slowing your Kerbin orbital speed and thus fall to a lower altitude around it. Concentrate on getting the periapsis into Kerbin's atmosphere (69km or lower, preferably about 20-40km). When you fall into the atmosphere 'aerobraking' will degrade your orbit even more, lowering the apoapsis a lot and the periapsis a bit. It might take several orbits but as long as you're entering the atmosphere each time you will, eventually, hit the ground.

[Dave Kerbin]

How far is your landing site from the Mun's equator? Are long as you are close you should be able to do it (if you want to post your persistent file I could even test it for you).

If I was doing it I would fly it like this:

-While on the ground look at your navball and find the orange 'north' line that runs though part of it. Go counter clockwise from that line 90 degrees to find 'east'. It's just like when you launch from Kerbin and turn right, only on the Mun you might be twisted around so you need to figure out the right direction.

-Take off at full thrust and unless you need to clear a crater wall immediately turn 45 degrees east.

-Quickly switch to map view and hover your mouse over the AP marker. When it hits about 10km (or even 8) press X to cut engines.

-Drop a maneuver node on your AP point and pull the prograde pointer until it shows a roughly circular orbit.

-The maneuver should give you a direction and burn length. Point in that direction and start burning at about the half way point (so if it is a 20 second burn, starting your engines at T-10 seconds from the maneuver node)

-You should be in a low, fast orbit around the Mun. You may want to calculate your total delta-v at this point.

-Zoom out a bit so you can see the orbit line that the Mun follows. That line crosses your own ships orbit twice. Start by placing a maneuver node on one that is in the direction the Mun is traveling.

-Again drag the prograde marker out until the total m/s is close to but not quite equal to the amount of remaining delta-v.

-Now start moving the node around, you'll want to move it closer to the side of the Mun facing Kerbin. This is much easier with PreciseNode since you can use keyboard shortcuts but it can be done with just the mouse. Turn the camera a bit so you don't accidentally pull the normal/anti-normal part of the maneuver node while you are moving it.

-You should be able to find the point where it will take you out of the Mun's orbit and swinging low enough to Kerbin to aerobrake

-Use the same technique to burn for this manuever node as you did for the first one

-You should hopefully have a few drops of fuel left (and maybe RCS too). If you find that you didn't quite get low enough to aerobrake in Kerbin's atmosphere then wait until you reach the AP in your new orbit around Kerbin and use any remaining fuel and RCS to brake.

[Taki117]

not sure if this has been mentioned already, but if you are not worried about getting into a stable Kerbin orbit (i.e you are going straight to landing) set your Periapsis at about 30km and just let the Atmosphere, and gravity, do the work for you.

[DeepOdyssey]

Wait till you have earth on the horizon on the east. Then ascend a little and burn 90* degree hdg.

[Vanamonde]

Have a look at my moon mission walkthrough. The return leg of the trip is about halfway down.

[Dragonchampion]

Thanks to landing accidentally in the polar crater, I can't seem to do it even with all these tips. I finally got sick of it and just turned on cheats. Bullshit, but so is this situation.

Oh, and I have Mechjeb too, and I know exactly how much DeltaV I have, and I can eaisly hit orbit, and when I use the "return from a moon" it gives me some bullshit 2 million m height orbit.

Edited July 22, 2014 by Dragonchampion

[Dave Kerbin]

Thanks to landing accidentally in the polar crater, I can't seem to do it even with all these tips. I finally got sick of it and just turned on cheats. Bullshit, but so is this situation.

Oh, and I have Mechjeb too, and I know exactly how much DeltaV I have, and I can eaisly hit orbit, and when I use the "return from a moon" it gives me some bullshit 2 million m height orbit.

Ok, I think I understand the situation now.

[EdFred]

If departing the Polar Crater, lift off and immediately head north, and burn until you get a Kerbin PE of 30km.

[Dragonchampion]

Ok, I think I understand the situation now.

Oh, I forget. Everyone dispises Mechjeb, thinking that people using it are disgusting casuals that shouldn't be playing in the first place. Take your prejudice somewhere else; I came here for help, not to be judged.

[Taki117]

I point you here

[Dragonchampion]

Alright thanks Taki.

[Dave Kerbin]

Oh, I forget. Everyone dispises Mechjeb, thinking that people using it are disgusting casuals that shouldn't be playing in the first place. Take your prejudice somewhere else; I came here for help, not to be judged.

If you are going above the atmosphere you aren't playing 'casual' (if KSP can even be mentioned in the same sentence given that it's a complex physics simulator that requires you to do literal rocket science). MechJeb isn't a cheat but it can easily mislead players into think that MechJeb's solution to a given problem is the right one. When you ask MechJeb to return you to Kerbin or dock with another ship it is not giving you the best solution it is giving you a solution and more often then not it is the brute force solution which is good for a computer but bad for a human (not only is MechJeb wasteful of monopropellent when docking, trying to copy its method as a human pilot will result in docking being about 10 times harder then it should be).

Give me a few minutes and I'll show you, in pictures, exactly how to return from the Mun Polar Crater using less then 1000 m/s.

[Dave Kerbin]

Ok, somehow you've gotten to the Mun's polar crater. I'm used a 2 stage lander because I wanted to match your delta-v as closely as possible. The extra solar panels and communication dishes are to adjust the mass of the ascent ship slightly so that it has a total delta-v of 1000.7 m/s. The big engine on the descent stage has been shutdown and so it will only serve as a launchpad for our return.

Now in my original set of comments I suggested that if you where close to the equator you should find east and take off in that direction. Since we are much closer to the pole we'll take a different tactic - take off and fly directly south at the same 45 degree angle.

In map view we wait until the AP is around 8 or 10km and shutoff the engine, we are aiming for a low and fast orbit. If you have a low thrust to weight ratio (small engine, heavy ship) you'll want to go to the high end, maybe even go to 12km because you'll need some vertical clearance for the burn after this one. Using a maneuver node we plot out how long and what direction we need to burn to get into this low orbit.

My maneuver node told me that I needed to burn for 16 seconds so I pointed my ship at the blue indicator and when it read T-8s I throttled up the engine. This put me into a low orbit around the Mun with around 371 m/s of delta-v remaining to get home. This flight was done in a fresh save so I actually had the Mun lined up nicely. If it hadn't been lined up I could have waited a while in orbit until the orbital paths crossed. I'm placing a maneuver node close to where my orbit meets the Mun's orbit on the 'front' side of the Mun (if you considered the Mun a vehicle moving forward in orbit). Next we drag out the pro-grade vector to break Mun orbit. Like I said before an easy way to do this is to take the available delta-v and create a maneuver with slightly less then that (so maybe 360 m/s).

Once the maneuver is in place we start moving it forward until we find the 'sweet spot' where it dips low into Kerbin's atmosphere or even disappears into the planet. In this case using 360 m/s is more then enough to go into Kerbin, even when I then reduced it to 276 m/s it was still more then enough to easily reach Kerbin and I had to nudge it a bit around the sweet spot to get a 30 km entry instead of plowing straight into the ground (I'll admit this is stock so that actually works but I've play a lot with Deadly Reentry so I've picked up the habit).

The burn is done the same way as the orbital insertion burn and we are safely on our way back to Kerbin.

Edited July 22, 2014 by Dave Kerbin

[Pecan]

*Applause* a masterly demonstration there.

[You might want to check MJ again though. While your main point is true, it's much better at docking now, doesn't blast RCS all the time].

[AeroAviation]

I would just get into LKO then use my kerbal to push the ship into the atmosphere.

[Dragonchampion]

Now THAT, Dave Kerbin, should be posted on the wiki a a guide. Incredible work, thanks!

EDIT: THought I did forget to mention I use TAC Life support so I can't just wait around for the paths to cross. Also, I want to ask if it is economical to ditch that much of your ship like that? I mean, that much money wasted from not being able to reclaim it must be a lot.

Edited July 22, 2014 by Dragonchampion

[Rhomphaia]

Now THAT, Dave Kerbin, should be posted on the wiki a a guide. Incredible work, thanks!

EDIT: THought I did forget to mention I use TAC Life support so I can't just wait around for the paths to cross. Also, I want to ask if it is economical to ditch that much of your ship like that? I mean, that much money wasted from not being able to reclaim it must be a lot.

Luckily the Mun is tidally locked to Kerbin, get the launch right and you won't have to wait.

[Dave Kerbin]

Also, I want to ask if it is economical to ditch that much of your ship like that? I mean, that much money wasted from not being able to reclaim it must be a lot.

I did it in a sandbox file so there was no cost. It was a two stage lander because I wanted the ascent part to match your reported delta-v. If I had used a normal Mun lander then some of the fuel would be burned during landing, make it impossible to plan precisely how much fuel (and delta-v) would be available for ascent. I considered having drop tanks or shifting fuel around after landing but it was just much more simple to spec out an ascent ship that matched your delta-v and then throw it on top of a transit/landing stage and add a launch stage that was equally simple.

Luckily the Mun is tidally locked to Kerbin, get the launch right and you won't have to wait.

I usually overlook that. Of course if I had landed somewhere else near the poles, maybe 90 degrees east or west, then I would need to wait several days while my orbit shifted in relation to the Mun to get the cheapest return, though I am somewhat convinced now that with 1000 m/s available you wouldn't need to wait, just find the best fit for that orbit and go.

TAC life support makes things more complicated, since a low delta-v return can take more then 24 hours. I'm not sure how tight it was on your ship, but playing BTSM getting to and returning from the Mun in a short amount of time is very important since every minute you are consuming precious power and batteries add a lot of weight at that stage in the game (I've got it under 10 hours for the entire round trip in order to get it under the mass limit for certain tech levels).

[Dangerous_Beans]

@Dave Kerman: i have a quick question about your explanation on the previous page. since you are on the prograde side of the Mun, why not: launch north @45 degrees until your apoapsis is ~8km, then burn at the horizon until you get the desired Kerbin periapsis?

intuitively that would seem to me to be the more efficient method (and also a bit faster), so i assume that i am missing something.

[Dave Kerbin]

@Dave Kerman: i have a quick question about your explanation on the previous page. since you are on the prograde side of the Mun, why not: launch north @45 degrees until your apoapsis is ~8km, then burn at the horizon until you get the desired Kerbin periapsis?

intuitively that would seem to me to be the more efficient method (and also a bit faster), so i assume that i am missing something.

Done correctly it could be faster, I'm not entirely sure it would be more efficient if we are thinking about the same thing. I went south partially because I felt it was more intuitive but also because it provided the space to break things up into nice clear pieces. Without mods (like PreciseNode) getting around in KSP can be less then accurate, sometimes even frustrating. Using the stock maneuver node interface is far from accurate and while a player who already knows what they are doing can compensate and even make manual corrections mid burn it doesn't help someone who has not done that kind of maneuver before and is trying to learn from an example. For that reason I did it without combining steps or making intuitive adjustments (that could only be made by someone who already knew how it worked) to get things perfect. Stock KSP control is imperfect so any instructions or plans need to accept imperfection.

[Dangerous_Beans]

Done correctly it could be faster, I'm not entirely sure it would be more efficient if we are thinking about the same thing.

yeah, thinking about it further i don't think it would be any more efficient. it was just one of those thoughts where i didn't entirely realise that we are dealing with orbital velocities in a vacuum. like getting out of orbit by burning towards the planet.