Using the Mun for interplanetary missions?

[Travisfv]

Now this may seem like a dumb idea, but I'd really like to know from the people who truly understand this game.

I want to build a space station as a launch platform for interplanetary missions, but I want it to orbit the Mun.

What I'm looking for is if there are any advantages or disadvantages as compared to using Kerbin as the launching place.

I know the Mun's SOI is smaller, and it takes much less Delta V to break orbit, but could I use that combined with a slingshot around Kerbin as a fuel efficient way of traveling to another planet?

Also, what is a good amount of Delta V to carry with me to another planet? And the right TWR?

I've been very much inspired by another post to build a ship that will make multiple trips, and I'm really looking to make it as easy as possible on myself. Up until now my interplanetary missions have been....non existent? I've put probes on other planets, but I'm still waiting to achieve my biggest goal. Put a Kerbal on another planet, then bring him home.

[Scotius]

http://forum.kerbalspaceprogram.com/showthread.php/25360-Delta-V-map

I'm sure it will be useful.

As for Mun station, it will be more useful as logistics hub (if you use Kethane mod). Deliver your fuel to the station in Mun's orbit, then use dedicated tanker to haul it to space station in Kerbin's orbit. Remember, to get to Mun station you have to use fuel to brake into orbit. You won't save any fuel this way. If you refuel in Kerbin orbit it's just one straight burn into interplanetary transfer.

[Travisfv]

Well, what I want to do is for the ship to operate solely from Mun orbit. When it returns it will aerobrake around Kerbin and return to the Mun station. This ship is never meant to land ever again, and it will be reused for multiple missions. That way I never have to launch it from the ground again.

And if what you say is true for using it at as logistics hub with harvesting Kethane from the Mun, then it seems like it would be even better for this task because it will be easier to refuel the station and the ship without travelling to Kerbin's orbit and docking with that station. The only launch from Kerbin will be for crew changes and for a payload changed, which could be done in one launch.

[donlokiman]

Well, what I want to do is for the ship to operate solely from Mun orbit. When it returns it will aerobrake around Kerbin and return to the Mun station. This ship is never meant to land ever again, and it will be reused for multiple missions. That way I never have to launch it from the ground again.

And if what you say is true for using it at as logistics hub with harvesting Kethane from the Mun, then it seems like it would be even better for this task because it will be easier to refuel the station and the ship without travelling to Kerbin's orbit and docking with that station. The only launch from Kerbin will be for crew changes and for a payload changed, which could be done in one launch.

With Kethane I can see the Mun or Minmus being a very nice place to stage missions from. I think for it to work well though we would need ways to launch with empty fuel tanks. Weight of fuel is a big issue when launching large interplanetary ships.

[Sirrobert]

Well, what I want to do is for the ship to operate solely from Mun orbit. When it returns it will aerobrake around Kerbin and return to the Mun station. This ship is never meant to land ever again, and it will be reused for multiple missions. That way I never have to launch it from the ground again.

And if what you say is true for using it at as logistics hub with harvesting Kethane from the Mun, then it seems like it would be even better for this task because it will be easier to refuel the station and the ship without travelling to Kerbin's orbit and docking with that station. The only launch from Kerbin will be for crew changes and for a payload changed, which could be done in one launch.

You need to break when at the Mun to get back into Mun orbit.

It'll probably be cheaper to ferry fuel from the Mun (kethane) to the ship parked in Kerbin orbit

[Travisfv]

This is going to take a long time to set-up all the operations, but once I get going I will let you guys know if it works. This is will be my first interplanetary mission, so I'm hoping that it goes right.

[Kerbolar]

Donlokiman: I've almost completed the first of two methods that I plan for eliminating excess fuel. Simple parts first, a more interesting plugin later.

OP: I like your idea, particularly the Kerbin slingshot. Reversing into Mun orbit will be a fuel drain though, as mentioned. But I think you should do it because it will rock.

[Ralathon]

It is useful as a last stop before hitting interplanetary space. You only need very little dV to get from the mun to any other place (provided you burn when the mun is at the right position) so you can escape the Kerbin system with nearly full fuel tanks. Saves you from hauling half empty tanks all over the system.

It isn't useful as a permanent hub though. The mun moves around and depending on the position it is either very hard or very easy to get into orbit around it. A mission plan that would be more consistent:

Low Kerbal Orbit for assembly and crew transfer -> Munar orbit for fuel topping -> interplanetary missions -> Back to low kerbal orbit.

[Kimberly]

If you use orbital construction, though, does that make the moon more appealing? Or would orbit around Kerbin still be better?

[Travisfv]

It is useful as a last stop before hitting interplanetary space. You only need very little dV to get from the mun to any other place (provided you burn when the mun is at the right position) so you can escape the Kerbin system with nearly full fuel tanks. Saves you from hauling half empty tanks all over the system.

It isn't useful as a permanent hub though. The mun moves around and depending on the position it is either very hard or very easy to get into orbit around it. A mission plan that would be more consistent:

Low Kerbal Orbit for assembly and crew transfer -> Munar orbit for fuel topping -> interplanetary missions -> Back to low kerbal orbit.

So if I include what you're saying, here is a new outline.

1. Launch to Orbit.

2.Transfer Crew and move to Mun Orbit.

3. Refuel at Mun Station.

4. Move to Interplanetary.

5. Return, circularize around Kerbin.

6. Refuel and change out crews.

7. Move to Mun orbit.

8. Top off at Mun with Kethane.

Repeat 4-8.

So how is this for a general setup?

[Khrissetti]

if you're using it for interplanetary missions then you'll waste dV by not taking advantage of the Oberth effect.

http://en.wikipedia.org/wiki/Oberth_effect

Plus as others have said you'll waste fuel circularising to rendezvous with your Mun station when you return to the Kerbin system. Basically, a Mun station on its own is cool, but if you want it to be useful, you'd be better off building low around Kerbin

[Maxwell Fern]

The Oberth effect stop being relevant when you refuel your ships before leaving.

An ejection burn to Duna takes 1061m/s from a 70km LKO, 606m/s from a 12 000km orbit (Mun's orbit around Kerbin). So take your ship on a (very) high Mun orbit, refuel it with the help of and Kethane ground base, leave Mun's SOI.

[Djhcc26]

I did an Eeloo transfer starting from minmus orbit and saved 650 D/V. Jool transfers save about 750 d/v, and I'm sure its a just as large of a savings for the other planets as well. In order to calculate what your gonna save, you can take how much d/v it took you to get to the mun and subtract the escape velocity from the mun to get your savings. IIRC it's about a 450 savings to transfer from the mun to interplanetary, and when going to Duna that's only 550 d/v spent(less than a munar transfer) and if you time it right return trips cost less than 300 d/v. I've launched from minmus just about every time I've launched to another planet.

Edited July 2, 2013 by Djhcc26

[Djhcc26]

if you're using it for interplanetary missions then you'll waste dV by not taking advantage of the Oberth effect.

http://en.wikipedia.org/wiki/Oberth_effect

Plus as others have said you'll waste fuel circularising to rendezvous with your Mun station when you return to the Kerbin system. Basically, a Mun station on its own is cool, but if you want it to be useful, you'd be better off building low around Kerbin

He is using the oberth effect if he slingshots arond Kerbin on his way out. He's actully using it more efficiently that way, and if he aerobrakes on the return trip he isnt wasting any d/v, hes actually saving it.

[Mr Shifty]

The Oberth effect stop being relevant when you refuel your ships before leaving.

An ejection burn to Duna takes 1061m/s from a 70km LKO, 606m/s from a 12 000km orbit (Mun's orbit around Kerbin). So take your ship on a (very) high Mun orbit, refuel it with the help of and Kethane ground base, leave Mun's SOI.

This. The Mun's altitude happens to be just about the optimal altitude from which to leave Kerbin for minimum delta-V transfers to Eve and near optimal for Duna.

He is using the oberth effect if he slingshots arond Kerbin on his way out. He's actully using it more efficiently that way, and if he aerobrakes on the return trip he isnt wasting any d/v, hes actually saving it.

Lowering your periapsis to Kerbin's orbit uses much more fuel than you'd save with the Oberth effect. As a general rule of thumb, it's almost never efficient to lower your orbit to take advantage of Oberth. Oberth tells you where to burn on your current orbit (periapsis), not what orbit to shoot for.

[Horn Brain]

It only makes sense to do this if you're mining kethane from the Mun and storing it in munar orbit. If you're getting your fuel from Kerbin, then you're just wasting time and fuel moving it up to the Mun. I did some analysis on this, but the forum monster ate it. :-(

[Mr Shifty]

I did an Eeloo transfer starting from minmus orbit and saved 650 D/V. Jool transfers save about 750 d/v, and I'm sure its a just as large of a savings for the other planets as well. In order to calculate what your gonna save, you can take how much d/v it took you to get to the mun and subtract the escape velocity from the mun to get your savings. IIRC it's about a 450 savings to transfer from the mun to interplanetary, and when going to Duna that's only 550 d/v spent(less than a munar transfer) and if you time it right return trips cost less than 300 d/v. I've launched from minmus just about every time I've launched to another planet.

This is not quite the case. It is true that higher orbits have less orbital energy, and therefore are easier to escape from, but lower orbits can take advantage of the Oberth effect, i.e. their energy can be used more efficiently to increase velocity. It takes about 2500 m/s to do a transfer to Jool from Minmas's orbit (46,400km). But it only takes about 2000 m/s to do it from 100km. Eeloo is even worse; it costs nearly 700 m/s more to transfer from Minmas's orbit vice 100 km.

[ComradeGoat]

This is not quite the case. It is true that higher orbits have less orbital energy, and therefore are easier to escape from, but lower orbits can take advantage of the Oberth effect, i.e. their energy can be used more efficiently to increase velocity. It takes about 2500 m/s to do a transfer to Jool from Minmas's orbit (46,400km). But it only takes about 2000 m/s to do it from 100km. Eeloo is even worse; it costs nearly 700 m/s more to transfer from Minmas's orbit vice 100 km.

What about using the Oberth effect when burning from Münar orbit? The nice thing about the Mün is that you can get within 3km of the surface and be generally confident of not crashing into it.

ETA: This is why I'm concentrating on Tylo for my Joolian outposts now rather than Laythe: a long way up the Joolian gravity well, and an Oberth effect to die for.

Edited July 2, 2013 by ComradeGoat