Good Idea? Minimus refueling base Super Heavy Lifter

[Sublight]

First:  IM ON A XBOX.  I HAVE NO MODS.
I recently went to Duna and noticed that it took somewhere around 90% of my fuel to get past Minimus.  I am thinking about making a fueling station there, so by the time I get done there, I can refuel, and have enough fuel to go anywhere that I want to go.  I will basically stage all of my missions FROM Minimus.

Im thinking about making a base around minimus to be used to refuel.

Im going to build a enormous drilling rig, that can carry a massive amount of ore to the Space Station.  Because the gravity is so low, this should be easy.  The docking will be a pain, but getting into orbit will be easy.

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Either that or make one in orbit of the moon.  I cant make a drilling rig quite as big there, but that's a big portion of my fuel to get there also.  Anyone else do this?  And is this a bad idea?

 

[bewing]

There are a lot of players who completely avoid mining/converting/refueling. But for those of us who do mine, that's what we all do, yes. Because the travel times to the Mun are shorter, it's a bit faster to set up a refueling base at the Mun -- but you have to do a lot more upsy-downsy stuff to get the same amount of fuel into orbit. I put my "base" on the surface and then shuttle the refined fuel up to orbit with a reusable lander. I do the same on Minmus too -- but it takes longer to get it all started. I play in hard mode, and for some reason I never have any ore on the flats, which is (of course) the easiest place to land.

[Streetwind]

Minmus is a popular destination for refueling stations, for all the reasons you mentioned. But at the same time, it's not something that everyone does, because there are downsides too.

 

For starters, you save a bit less fuel than you think you do. Minmus' tiny gravity well doesn't give you much in the way of Oberth Effect. If you don't know what that is, the super-simplified explanation is: you use less fuel to gain a certain amount of energy when you do it deep inside a gravity well. KSP models this, meaning that a four digit dV interplanetary transfer starting from Minmus is a less efficient burn than the same burn performed in low Kerbin orbit. Ultimately you still come out on top, because you save dV for no longer having to lift your orbit to the altitude of Minmus first, but it's less than you think.

Second, it puts you at an inclination. Minmus is tilted six degrees with respect to Kerbin's equator, so for all flights to destinations within Kerbin's SoI, you have to deal with that inclination to various degrees. For all flights into interplanetary space, the effects are lessened, but also less predictable. You may luck out and reduce your required plane change, you may end up starting your burn at Minmus' AN/DN and not be off-inclination at all, or you might end up in a situation where it makes your required plane change noticably worse.

Third - and this is the big one, much bigger than the two really minor downsides above - it restricts your departure windows. For interplanetary trips, you want to start your burn at a certain "ejection angle" relative to prograde of Kerbin's orbit. That angle is different depending on which planet you want to go to, and sometimes (when inclined and eccentric orbits are involved, like with Moho and Eeloo) even on each specific transfer window. Now, in low Kerbin orbit, you loop around once per 30 minutes, so you can just go whenever you want. But Minmus takes a long time to go around Kerbin. You might find yourself in a situation where you have your whole mission set up, the giant star cruiser all fueled up at Minmus, the departure time is two days from now and... woops, Minmus is going to take another 38 days to arrive at the spot where you need to begin your burn!

The first and second downside can be somewhat mitigated by what I like to call an 'Oberth Dive'. Instead of burning from Minmus to interplanetary space, you instead burn the other direction, towards Kerbin. Drop your periapsis nice and low, and make your maneuver node just above the atmosphere. You can then do a combined escape burn and inclination correction deep in the gravity well while your vessel is going really fast, which gives you lots of Oberth oomph, more than enough to offset the fuel you spent going back towards Kerbin. But that maneuver is more difficult to set up and execute than a normal escape burn. And it doesn't mitigate the third downside at all, because you still need to launch at the correct position. It'll just be a different position. And in fact, because of the precision required, it actually makes the departure timing problem worse.

That timing problem is so big and so prohibitive that pretty much nobody ever launches missions from Minmus at all. Instead, they launch their spacecraft into low Kerbin orbit, then fly a dedicated tanker in from Minmus, which meets up with the waiting spacecraft, fuels it, and then returns to Minmus. Depending how big the tanker is, how much fuel the spacecraft needs, and how much fuel you need to spend to rendezvous with the spacecraft and return to the fuel station (you're coming in from and returning to up to six degrees of inclination, remember), you might need multiple tanker trips.

 

At that point, you need to ask yourself: do I want to invest the IRL playing time to fly these tanker missions for each interplanetary launch I do, or do I just want to get on with it and build a bigger lifter? Because that's what you're ultimately trading: your IRL time for ingame funds. And perhaps you could be doing a contract mission in the same time you'd normally fly refueling trips, and earn the same or more...

Edited December 11, 2016 by Streetwind

[radonek]

Yeah Minmus is a good place for refueling station, but it requires some additional planning to make best use of. You have to account for both your orbit and Minmus own position in orbit when departing, which limits launch window a lot compared to LKO. But leaving Kerbin with almost full tanks instead half empty is appealing.

As for mining rig, IMO best way to would be to have complete setup on surface and tanker/lander to haul processed fuel up. Much easier to dock simple tanker with departing ship then docking whole stack with a station.  Moving fuel from mining base to tanker is tricky without mods though.

[Waxing_Kibbous]

Here is a great YT channel to watch- he plays *mostly* stock (trajectories mod which is not critical, no part mods that I am aware of). A two video suggestion that may help:

 

[bewing]

I want to disagree somewhat with Streetwind. Proper "Oberth Dives" may be difficult to calculate, but they are trivial to set up by eye. And the 3200 m/s of Oberth from a Minmus dive is well worth the cost compared to the measley 2300 m/s you get at Kerbin. You have to plan ahead at least 30 days to set one up, and it is true that it restricts your launch window to a 20 day cycle. The window problem is indeed an issue for inner planets, because they move so fast.

However, for any jaunt to an outer planet, the 1km/s Oberth bonus is large enough to be very useful. The launch windows are not that tight that being off by 10 days is a problem. The inclination problem can always be fixed with just a few m/s of burn at the proper node.