Getting a low Kerbin periapsis while leaving the Mun?

[UndyingWar]

So I've landed on the Mun, I'm low on fuel, and the only way I can get back to Kerbin is to get a very low periapsis. It will probably have to be 500km or less. Not eight million like I've been getting. How can I escape the Muns gravity, and get a low Kerbin Periapsis at the same time?

 

Is 142 liquid fuel even enough to make it home?

Edited March 13, 2016 by UndyingWar

[basic.syntax]

You may be stuck, the safest thing would be to bring a rescue craft with extra crew capacity, that you are more confident in, attempt to land near the same spot, and transfer the crew over. Or rendezvous in Mun orbit if the Mun craft has enough to at least get to orbit.  (trying to land near the same spot can be extra challenging.) 

[UndyingWar]

12 minutes ago, basic.syntax said:

You may be stuck, the safest thing would be to bring a rescue craft with extra crew capacity, that you are more confident in, attempt to land near the same spot, and transfer the crew over. Or rendezvous in Mun orbit if the Mun craft has enough to at least get to orbit.  (trying to land near the same spot can be extra challenging.) 

I actually just figured it out. Is there a way for me to delete this post? Or should I post the solution?

[Warzouz]

If you have MechJeb (node editor tool) or Precise Node mod, you can try to get the most precise ejection angle. Get the max burn you can get from your engines, then tweak the node by sliding it. A node editor is best because you can do that very precisely.

While you're doing that, check the Kerbin periapsis. At one point it should get to a minimum.

Note that you can slide the node using the stock node handle, but that's much much less precise and very frustrating.

If you can't lower your Pe in the atmo, then you're stuck. In that situation, the best thing to do is to stay in Mun orbit and do a rescue mission.

[basic.syntax]

18 minutes ago, UndyingWar said:

I actually just figured it out. Is there a way for me to delete this post? Or should I post the solution?

Congrats! I'm just curious if you "tried stuff" until it worked and got the craft back to Kerbin on your own.  Its not necessary to delete the post or go into detailed detail, unless someone asks and you're feeling like writing it up  

[UndyingWar]

1 minute ago, basic.syntax said:

Congrats! I'm just curious if you "tried stuff" until it worked and got the craft back to Kerbin on your own.  Its not necessary to delete the post or go into detailed detail, unless someone asks and you're feeling like writing it up  

That's pretty much what I did. I tried stuff until it worked. Instead of going 2,200,000m away from the Mun to get into Kerbins gravity like I normally do, I got the apoapsis up to something like 200,000,000 and that gave me a periapsis of 20,000.

[DancesWithSquirrels]

Post what you did, it will be helpful for others who run into this.

As I understand it, the best use of your fuel would be (assuming you landed near the equator of the Mun) to wait until your landing site is on the trailing edge of the Mun, then burning straight up opposite the Muns orbital velocity as hard as you can. That way you burn as low in the gravitational field as possible and maximize the Oberth effect, which is surprisingly helpful.

But before that you should calculate your delta V and check it against one of the various Kerbal dV cheat sheets to see if you're even close. Or in a more Kerbal approach use F5/F9 to save before launch from the Mun, and just try it out...

 

Edit: Do just _one_ burn for escape and into a low Kerbin periapsis, not two - control from the map mode. I do these burns in map mode after tabbing to focus on Kerbin, and just watch my periapsis drop until I'm at the right altitude.

Edited March 13, 2016 by DancesWithSquirrels

[GoSlash27]

I have a pic of what to do Lemme see if I can find it...

Ahh. Here we are...

Best,
-Slashy

PS For future reference: In order to tell how much DV you have left, we'd need to know fuel, oxidizer, current ship mass, and engine type.

 

Edited March 13, 2016 by GoSlash27

[Nich]

1 hour ago, GoSlash27 said:

I have a pic of what to do Lemme see if I can find it...

Ahh. Here we are...

Best,
-Slashy

PS For future reference: In order to tell how much DV you have left, we'd need to know fuel, oxidizer, current ship mass, and engine type.

 

Although technically you want to do that from the ground without going into a parking orbit to save a little more dv but your more likely to waste dv if you don't set a maneuver node.

Edited March 13, 2016 by Nich

[Wemb]

22 hours ago, basic.syntax said:

You may be stuck, the safest thing would be to bring a rescue craft with extra crew capacity, that you are more confident in, attempt to land near the same spot, and transfer the crew over. Or rendezvous in Mun orbit if the Mun craft has enough to at least get to orbit.  (trying to land near the same spot can be extra challenging.) 

I don't think anyone else said (apologies if they did), you can, at a pinch, always get out and push. (really, you have a jetpack that magically refills itself whenever you reboard the ship).

Wemb

[wumpus]

22 hours ago, DancesWithSquirrels said:

Post what you did, it will be helpful for others who run into this.

As I understand it, the best use of your fuel would be (assuming you landed near the equator of the Mun) to wait until your landing site is on the trailing edge of the Mun, then burning straight up opposite the Muns orbital velocity as hard as you can. That way you burn as low in the gravitational field as possible and maximize the Oberth effect, which is surprisingly helpful.

But before that you should calculate your delta V and check it against one of the various Kerbal dV cheat sheets to see if you're even close. Or in a more Kerbal approach use F5/F9 to save before launch from the Mun, and just try it out...

 

Edit: Do just _one_ burn for escape and into a low Kerbin periapsis, not two - control from the map mode. I do these burns in map mode after tabbing to focus on Kerbin, and just watch my periapsis drop until I'm at the right altitude.

Except that burning straight up is going to be inefficient (you might get away with it on Minmus, but Mun is a harsh mistress).  Basically you are throwing away 1/TWR (for munar weight) of fuel.  What you really need is to wait until Kerbal is overhead and firing horizontally away from the Mun's rotation (of Kerbal.  Mun is tidally locked and doesn't rotate so which side shouldn't matter).  You still want to do a single burn, and keep burning until your perigee stops decreasing.  Hopefully, you will still have enough fuel to set up a maneuver node that will get you into the atmosphere.  If it is still close enough, you could get out and push, but that is *slow* and only works if you are really close (I'm hoping a single burn will get you there).

If not, your best bet is to make a circular orbit as high above the Mun as possible and as circular as possible.  If you care about calendar time, you might want to install kerbal alarm clock and send a Duna/Eve probe during this time, as docking in high Munar orbit is kerbal-time slow (faster and easier in human-time, but expect to be abusing time-acceleration.  So slow in kerbal-time).

[suicidejunkie]

1 hour ago, wumpus said:

Except that burning straight up is going to be inefficient (you might get away with it on Minmus, but Mun is a harsh mistress).  Basically you are throwing away 1/TWR (for munar weight) of fuel.  What you really need is to wait until Kerbal is overhead and firing horizontally away from the Mun's rotation (of Kerbal.  Mun is tidally locked and doesn't rotate so which side shouldn't matter). 

Mün being tidally locked means you can't wait for Kerbin to be overhead, because if it isn't overhead now, it never will be.

Additionally, you don't want Kerbin to be overhead, but on the western Horizon.

If you've landed on the eastern side of Mün, then Kerbin will be to the west, and straight up will be directly against the Mün's orbital velocity.  In this case, burning straight up in one blast is best, and you can do the entire return burn very close to the surface, for a discount of 10-20% over going to orbit first.

 

Anywhere else, you need to go to orbit first so that you can boost in the right direction without impacting the surface.

[wumpus]

3 minutes ago, suicidejunkie said:

Mün being tidally locked means you can't wait for Kerbin to be overhead, because if it isn't overhead now, it never will be.

...

Anywhere else, you need to go to orbit first so that you can boost in the right direction without impacting the surface.

I knew it was tidally locked, but forgot all the implications.  Also while I've always found the "western horizon" to be when to burn, I'm not sure why.  I feel you should be killing as much of your orbital velocity wrt to Kerbin, but I've done much better with a burn away from Kerbin (from orbit).

You also don't need full orbit, suborbital will likely do (circularization burns don't help you get back to Kerbin at all) assuming you are on the "right half" (halfway around would justify orbit.  I'd want less than 1/3 and ideally 1/4 of the way around to try a suborbital) of the Mun to get away with this.  You just need to be traveling over the horizon of the Mun so you can set up a maneuver node home.  Note that the ideal place for you maneuver node is as low as possible (for Obereth effect), presumably roughly near apogee (this isn't all that important, but don't be coming down fast) and most importantly have time to set it up and execute the burn.

Best guess is that if you need to go fully into orbit, you already know you aren't going to make it home (how low were the orbits you were trying?  You want them as low as possible, and try to put the maneuver node home where the orbit of your ship intersects the orbit of the Mun (see above) where the vessel is heading toward Kerbin.  You might have to adjust the maneuver node a little, but it is close to there).  Just get into a high orbit and wait for a rescue ship.