Mun or Minmus for resource bases?

[peachoftree]

At first, minmus seems the obvious choice, but the inclination and longer distance seem to overweigh the low gravity benefits. My plan if I set up a moon base is to do all my refueling at the moon, since I don't want to do a ton of aerobraking. My question is if it's worth it to aerobrake every time you have a fuel delivery from minmus or to refuel at the moon. (refueling at minmus is not an option due to its very long orbital period and inclination.)

Edited June 3, 2015 by peachoftree

[TheXRuler]

I think minmus is waaaaay better. Think of it this way, orbital velocity around the mun at low alt is ~570 m/s if I remember correctly and around minmus it's only ~160 m/s or so. That means more fuel into orbit. Also lower gravity means less gravity losses, and you don't need as much engine mass on minmus due to the low gravity. Insertion into munar orbit can be rather expensive in terms of dV due to the high orbital velocity. Also, when departing from minmus you don't need much dV to leave kerbins SOI as you are already close to the edge.

However inclination is a problem as you say. To circumvent that I would reccommend a reasonably large (huge if you dare landing it) fuel lifter and a LV-N powered fuel tug in low minmus orbit. Refill the tug and leave minmus, refuel a massive fuel depot placed at the same orbital altitude as minmus with out the inclination. That way you can have a lot of fuel in high kerbin orbit without the added inclination.

I had a discussion about something similar a while back on these forums and while departing from LKO is cheaper in terms of dV (due to the oberth effect) you will actually have more fuel once you have reached escape velocity if you refuel very high up in kerbins SOI since it costs almost nothing to reach escape velocity from there

Edited June 2, 2015 by TheXRuler

[Laie]

Neither is really truly worthwhile: launching extra fuel from Kerbin is easy and totally affordable (unless you're playing on special difficulty).

But if you want to do it, Minmus is preferable. Most launch windows are weeks or months apart; just get your tanker to Minmus ASAP and return it to LKO at the first opportunity, then let it sit there until it's needed. Every 30 days or so, there will be a launch window where you can return without plane change. Or just suck it up; we're only talking about something like 50m/s anyway.

Alternatively, you can launch your interplanetary mission right from the Mun. In that case, fuel from the Mun may be cheaper overall. Requires some maneuver-fu and a mod like precise node for sanity's sake. And of course, you have to launch with enough fuel to reach the Mun in the first place.

[TheXRuler]

Have they really made carrer so easy that launching some additional 40 tons has no real funds impact?

I mean like half the mission cost is the booster, and if you can save 50% payload mass then that should make it more than worthwile.

[CapnSlappy]

I started a normal career, and the first sounding rocket netted me somewhere around half a million roots. Funding seems to be of little consequence.

[peachoftree]

but what about the oberth effect, isn't it hard to get a proper ejection angle when you have a days long orbital period?

[TheXRuler]

Those are two completely different things That is what I was referring to. I took part in quite a lengthy discussion on the topic of oberth effect and the most efficient way to leave kerbin and the consensus was, if you are not refuelling try avoid circularizing and instead go straight for an ejection. That is the most dV efficient way. However the way to leave with absolute maximum dV left would be to have a fuel depot right on the edge of kerbins SOI. Then fly up, refuel and do maybe a 15m/s burn to eject yourself into solar SOI. That way you have mor dV in solar orbit. Ejection angles aren't hard if the orbital period is longer. you just have to plan things a little more carefully since the most important factor for an efficient transfer is not hte time of departure but the angle. At least that's what I remember somebody smarter than me saying.

[jarmund]

I've always used Minimus for this since I started with Kethane eons ago, and here's why:

- Low gravity makes for easy landing of heavy miners (my miner doubles as a fuel barge)

- Low orbital velocity for fuel-efficient orbits from takeoff

- Many flat areas to mine from

- Somewhat easy to get a gravity assist from Mun after visiting.

- It has a nice color, and it might be made out of mint pudding

The only downsides that I can think of:

- More fuel needed to reach from LKO

- inclined orbit making it a somewhat tricky last stop for an interplanetary expedition

- You can't take off your EVA gear to taste the pudding

[peachoftree]

What about this video by scott manley where he uses the oberth effect to get a 50% efficiency gain from just escaping from a high orbit? (skip to around 9:30 for the quote)

Edited June 2, 2015 by peachoftree

[merendel]

Minmus is generaly better than Mun for a refueling base. the extra 300-400dv to reach orbit for the tanker is rather significant when luging a full load of fuel. However unless your really pinching pennies or really enjoy lots of ferry trips I wouldnt bother carting fuel down to LKO from minmus. Its far simpler and more time efficent to build a fuel launcher with a recoverable booster if you want a LKO depot.

As for minmus itself I often have a fuel base on the planet and a depot in orbit. I send interplanetary ships up to minmus to refuel befor being sent out. it only takes a couple hundred m/s to escape minmus for a low flyby of kerbin to get an oberth assisted boost out to your destination. most launch windows have enough leway in them that you can still hit the target easy enough a week on either side of them. If you leave minmus when its in the right position a week or two either before or after the launch window you've already got the 900 dv or so worth of orbital energy already that you dont have to spend agian. minmus's inclination has a relitively minor effect on your transfer orbit to another planet. I just wouldnt recomend leaving for moho from minmus just cause the timeing to hit that little speedster is soo tight and minmus wont line up with it very often.

[Snark]

Jarmund sums it up pretty well. Minmus is the clear winner; it's simply no contest.

- There's very little difference in the dV needed to transfer from Kerbin-to-Mun versus Kerbin-to-Minmus. It's like 100 m/s of difference.

- It takes far more dV to land or take off on the Mun, versus Minmus. 807 m/s versus 274 m/s.

[Orbital Vagabond]

Jarmund sums it up pretty well. Minmus is the clear winner; it's simply no contest.

- There's very little difference in the dV needed to transfer from Kerbin-to-Mun versus Kerbin-to-Minmus. It's like 100 m/s of difference.

- It takes far more dV to land or take off on the Mun, versus Minmus. 807 m/s versus 274 m/s.

escape velocity is also much lower from Minmus, and it may actually take less dV to transfer from cis-Minmus orbit to LKO than from a cis-Munar orbit to LKO. if it's more, it's not by much.

[Laie]

but what about the oberth effect, isn't it hard to get a proper ejection angle when you have a days long orbital period?

The orbital period may be as long as you like. What matters is that you have the periapsis at about the right place and reach it at about the right time. I like to have a probe in LKO that serves no other purpose than to plot transfer burns. It's maneuver node then marks the place and time where my interplanetary vessel ought to be.

Give or take five degrees doesn't matter very much. The importance of timing depends, but for anything but Moho you can well afford to be off by a whole day.

It helps a lot if you have a tool like precise node, or MJ's node editor.

Generally: when you begin your interplanetary trip at either moon, the cheapest method is by means of a low Kerbin periapsis and the oberth effect there. The Mun has a three-day period; Minmus' is more like sixty days, and the orbit has an inclination to boot. Technically speaking, Minmus would still be more efficient, but there's also the hassle to be considered. Which brings me back to my original suggestion of just launching with enough fuel and performing ISRU only at the destination, if at all.

Have they really made carrer so easy that launching some additional 40 tons has no real funds impact?

Not on flag planting alone. But almost every mission I do also fulfills at least a "build station around $destination" contract. Often also station around Kerbin and the Sun. There's a lot of money lying by the wayside.

Edited June 2, 2015 by Laie

[Requia]

Have they really made carrer so easy that launching some additional 40 tons has no real funds impact?

I mean like half the mission cost is the booster, and if you can save 50% payload mass then that should make it more than worthwile.

You can do an orange tank to LKO for ~25k after recovery if you can hit KSP on reentry. The mammoth engine is really really good and doesn't even need boosters.

Edited June 3, 2015 by Requia

[peachoftree]

Thank you everyone for the responses, I'll definitely be setting up my resource base on minmus:)

[Snark]

...uses the oberth effect to get a 50% efficiency gain from just escaping from a high orbit?

Oberth effect = good, yes.

You'll get a better Oberth effect boost from Minmus than from the Mun, for two reasons:

1. Minmus has lower orbital velocity around Kerbin than the Mun's; it requires less dV to drop your initial periapsis down to LKO.

2. You pick up more speed falling from Minmus than from the Mun, meaning a higher speed at Kerbin periapsis, so better Oberth effect.

...That said, Minmus would be the best choice anyway just for the reduced dV needed to land and take off from it, so Oberth shenanigans are just icing on the cake.

[Requia]

Minimus isn't a great choice for oberth effect burns, the time betweens windows is crazy long. You can be a little off both on the angle and the timing, but Minimus is excessive. You could maybe set it up set it up ahead of time, but it'd be really hard to figure. Delivering the fuel to KSO is only a little more dv needed for the transfer craft (about 350 m/s by my math, though you also need to build a minimus<-->KSO capable tanker).

Interestingly most of that loss comes from it being ~200m/s cheaper to drop from minimus to a peri of 70 than KSO to 70.

Hmm, the Mun is only 6 days between windows, though a bit harder to plan it up to 3 days off the transfer window is viable, a quick search says Mun return for 276, so that's around ~150m/s less than dropping to 70 at KSO, for a bit more speed at peri, might make up for the harder to plan ejection burn.

[TheXRuler]

@Requia I don't quite get what you are saying, the time between windows to where and why would it be harder to figure it out? As long as you place the manouvre node at the right angle it should be just as hard or easy to find the right angle. Like a week off is fine as long as you adjust the ejection burn to interplanetary space accordingly. Also, if you first place one node to lower your PE to ~80km (should cost no more than ~200 m/s) and then do your ejection burn at the lowest point you will get a massive boost from the oberth effect because you have a really damn high AP and the lowest possible PE meaning you should be going some 3000m/s at PE. That would indeed be a little harder to figure out but then again you just need to set up you retrograde burn for the low PE, then the ejection burn at PE and then just drag the first node around to get the timing right... trial and error and a bit fiddly yes, but not that hard as long as you aren't trying to do the actual math.

[KerbMav]

Mun has no transfer windows, a transfer burn can be initiated every 30-40 minutes in LKO; a direct ascent to Mun does not safe dV, the often repeated phrase "deep in the gravity well" is somewhat misleading. A vessel is fastest at its lowest point in orbit, during ascent it is not in an orbit and at a PE of 75km above Kerbin it can have different orbital velocities depending on the height of the AP.

What does seem to help is during shorter purely retro-/pro-grade burns to not aim at the node but at the retro-/pro-grade marker - atleast the maneuver node has still some dV left when I reach my target orbit.

[Sharpy]

What about this? Minmus as the refinery (production). A big depot on a narrow, elongated Kerbin orbit reaching Minmus ascending/descending node and with Periapsis low over Kerbin.

- you don't need to match the orbital plane for refuelling; the depot is on the ecliptic plane.

- You have used up fuel taken from Kerbin to match the depot's orbit; that's a well-invested dV, which you can use for Oberth with Kerbin and Mun, departing Kerbin on Hohmann to other planets etc - unlike dV wasted on matching the plane. More room in the tanks

- Changing such elongated orbit by a lot is very cheap when done in Periapsis so matching phase with the depot should be easy. (especially that it will be moving at snail's pace around Minmus' ascending node 90% of the time; the dives into Kerbin gravity well relatively short.

- The depot should be reachable by a shuttle with fuel from Minmus for resupply. Again, at the distant orbit speeds, you need very little dV to meet up, and the shuttle has all the fuel it wants readily at hand.

The only problem with this is Mun possibly spoiling the depot's orbit.

[Skorpychan]

Minimus for mining and refining. Mun is easier to access, so maybe stick a secondary fuel station there. Since it's not actually refining anything, all you need is a big fuel tank with a rocket to get it there, a probe core, panels, and some batteries. Don't even need RCS on it, unless you're using it as a monoprop depot or you're reusing part of the Minimus design.

[Mister Dilsby]

I have my major mining operation on the Minmus flats and shuttle ore to conversion stations in both Minmus orbit and LKO. That way if it's the right day of the 'minth' for an interplanetary transfer from Minmus orbit the ship can refuel and depart from the "Minmus Outbound" station, and if not it can leave from LKO.

As to inclination issues, remember that there are two days in each minth when you can make your trans-minmus injection burn at an ascending or descending node, making the intercept trivial. If you are doing long term station filling operations then timing is not so critical and you can wait a few days for those windows.

[Tex_NL]

If you want REAL efficiency you should consider the following:

Bring your interplanetary mothership to Minmus and start fuelling it there. It does take some careful planning but at the appropriate moment make a burn and drop down to Kerbin keeping your periapsis just above Kerbin's atmosphere. Do not aerobrake or circularize. At periapsis you go for your interplanetary burn getting the maximum out of the Oberth effect.

[Requia]

@TheXRuler Because there's no easy way to get the maneuver nodes right when you're in orbit around a moon instead of the planet, you actually *have* to do the math, the farther off angle your peri is the less efficiency you actually get from this method, and it can very easily go negative (especially compared to the high orbit park, which as I pointed out is only a few hundred dv more). And for minimus while being a week off might be perfectly acceptable, minimus might be weeks plural from being in the right position, so you're gonna have to do the peri drop before the transfer window even comes up then wait.

Comparatively if you're in high orbit around kerbin instead of around a moon, you still end up over 3000m/s after dropping peri to 70 (really no reason to cushion this, if it says 70 you won't get drag) at peri, but its dead simple to move the manuever node till your peri is the right angle. The actual cost is that the maneuver node drop is ~400 instead of ~200-

[Laie]

@TheXRuler Because there's no easy way to get the maneuver nodes right when you're in orbit around a moon instead of the planet, you actually *have* to do the math,

No you don't. As I already wrote earlier in the thread, it helps a bundle if you have some probe to plan an ordinary maneuver in LKO. Then switch to the interplanetary vessel that's currently in orbit around a moon, select the probe as target, and -voila- you see when and where you should be at periapsis around Kerbin.

From this point, it takes me about five minutes of trial and error until I have a maneuver that returns me from the moon in just the right fashion.

Getting from a munar orbit directly to the ejection burn is easy; from Minmus, not so much, because of it's 60-day orbital period. In the worst case, you have to leave Minmus 50 days early, and also lower your apoapsis as to not encounter the Mun in the mean time. Probably need a plane change as well. For all that, Minmus is still more fuel-efficient.... but from a efficiency-vs-hasse point of view, I much prefer the Mun.