Get My Kerbals Home

[Mr Shifty]

This is more of an interesting orbital mechanics problem to try to solve than a gameplay question:

Problem:

• Munar command module (Mk1-2) in a 30km x 30km polar orbit (inclination 95 degrees) around the Mun with 3 kerbals on board.
  
• 600 m/s worth of fuel in a decouplable service module attached to the CM. (There're probably a few 10s of m/s of RCS available too.)
  
• 10 hours of battery power available for life support. (I'm playing the BTSM mod.)
  
• My current Munar orbit is perpendicular to the Mun's orbit around Kerbin. (i.e. I just arrived at the Mun from Kerbin and have the polar orbit you might expect from that transfer.)
  

Can I get my kerbals back to Kerbin safely without launching a rescue? If so, how?

Edited July 5, 2014 by Mr Shifty
Added info about perpendicular orbit.

[Taki117]

Sure you can, what you have to do is ensure that your orbit around the poles of the Mun lines up with the Muns orbit (You may need to time warp a bit) once that happens simply burn on the "front" of the Mun. (Or "back" depending on your direction, maneuver nodes help here) and once you've done that make sure your kerbin periapsis is inside the atmosphere (For a Mun return 20km is about the lower limit) then just drift on back.

I direct you to this post for a visual representation of what I'm talking about.

Edited July 5, 2014 by Taki117

[Mr Shifty]

Sure you can, what you have to do is ensure that your orbit around the poles of the Mun lines up with the Muns orbit (You may need to time warp a bit) once that happens simply burn on the "front" of the Mun. (Or "back" depending on your direction, maneuver nodes help here) and once you've done that make sure your kerbin periapsis is inside the atmosphere (For a Mun return 20km is about the lower limit) then just drift on back.

I'd never thought about it, but that would work, except that I'm starting at perpendicular (vessel orbit is rotated 90 degrees relative to Mun's orbit) and it would take nearly 10 hours just to get the orbits parallel. But it might be enough to just go halfway...

[Taki117]

I'd never thought about it, but that would work, except that I'm starting at perpendicular (vessel orbit is rotated 90 degrees relative to Mun's orbit) and it would take nearly 10 hours just to get the orbits parallel. But it might be enough to just go halfway...

I have no experience with BTSM but if you can get close enough a simple retroburn once you leave the Muns SoI to drop your periapsis (Maybe with a radial component) to hit the atmosphere should do the trick.

[Master Tao]

You also seem to have fair amount of extra delta-v.

I suggest playing around with maneuver nodes near the poles pulling to one side or the other until you get an Kerbin periaspsis low enough to at least aerobrake within your time limit (probably < 40 km) using no more than about 620 m/s dV. A couple of minutes before the maneuver node, burn off all the RCS, since it's less efficient than than the liquid fuel. Then just burn to finish the maneuver node. If it's really, really tight, you can also use the ejection force from the decoupler while you're still close to the Mun for a few m/s.

Edited July 5, 2014 by Master Tao

[Mr Shifty]

I have no experience with BTSM but if you can get close enough a simple retroburn once you leave the Muns SoI to drop your periapsis (Maybe with a radial component) to hit the atmosphere should do the trick.

I'll have to try that and see if it works.

[GoSlash27]

10 hours is the sticking point here. Do I understand correctly that you have no solar panels or RTGs aboard? And your engine doesn't have an alternator?

If that's the case, you don't have time to wait for your burn, let alone sending up a rescue.

Where is your orbital plane in comparison to Kerbin? parallel to Kerbin's surface being 0* and perpendicular being 90*?

[Mr Shifty]

10 hours is the sticking point here. Do I understand correctly that you have no solar panels or RTGs aboard? And your engine doesn't have an alternator?

If that's the case, you don't have time to wait for your burn, let alone sending up a rescue.

Where is your orbital plane in comparison to Kerbin? parallel to Kerbin's surface being 0* and perpendicular being 90*?

I added that info in the OP: perpendicular, i.e. I just arrived at the Mun (30 minutes ago.)

No solar panels or RTGs as those are unlocked further down the tech tree in BTSM. The engine (Poodle) has an alternator, but it's not going to help much. The CM burns 750 units of electricity per hour. I have just over 7500 units on board.

[GoSlash27]

Okay, so your orbit is more or less perpendicular to Kerbin's surface?

I would say set a node over whichever pole has you going away from Kerbin and burn prograde to escape Munar SOI. After that, burn retrograde for the atmosphere. I'm just not sure if you can do all that and get out of the can within 10 hours. Hopefully the burn will generate enough electricity to give you some breathing room! [edit] <-- pun definitely *not* intended!

Any nonessential systems you can shut down to conserve electricity?

Edited July 5, 2014 by GoSlash27

[Mr Shifty]

Okay, so your orbit is more or less perpendicular to Kerbin's surface?

I would say set a node over whichever pole has you going away from Kerbin and burn prograde to escape Munar SOI. After that, burn retrograde for the atmosphere. I'm just not sure if you can do all that and get out of the can within 10 hours. Hopefully the burn will generate enough electricity to give you some breathing room! [edit] <-- pun definitely *not* intended!

Any nonessential systems you can shut down to conserve electricity?

SUCCESS! This is pretty much what I did. I waited about an hour to give me a little parallel velocity, then was able to set up a maneuver node that got me back in just under 9 hours. The engine burn gave me just enough extra power to get everyone home safely. My kerbals are very appreciative. Thanks to all!

[GoSlash27]

Glad it worked out. That was a squeaker!

Congrats,

-Slashy

[cantab]

Congrats.

With 600 m/s in the tanks, the first thing I might have tried is setting up a normal/antinormal burn from my orbit round the Mun, so that I'd eject much like from a usual departure and have my Kerbin periapsis dropped right down for landing. Whether that would have been possible I don't know.