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Broke dead dogg

What's more efficient to get into orbit, a tank full of fuel or ore?

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I'm trying to design a refuel station for minor orbit and I'm trying to decide where to have the refinery, on the station or on the lander? and of course it comes down to mass. So I'm trying to figure out which is more efficient to get from the munar surface, into orbit. Ore or liquid fuel

I haven't been able to find any details about the weight of a unit of ore/fuel so any help would be appreciated

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7 hours ago, Broke dead dogg said:

So I'm trying to figure out which is more efficient to get from the munar surface, into orbit. Ore or liquid fuel

Part of it depends on the ship design.  It boils down to this:

  1. If you're using the small ISRU, it's overwhelmingly better to bring up fuel instead of ore.  The small ISRU is hideously inefficient and throws away like 90% of the ore it mines.  (Which is why, personally, I never ever use the small ISRU, ever.)
  2. If you're using the big ISRU and you want your miner ship to be small (including cargo load), it may be more efficient to bring your ore to orbit, since the ISRU is dead weight that you're schlepping to and from the surface each time.
  3. If you're using the big ISRU and the miner ship is big (i.e. so the ISRU itself is a relatively small percentage of the total ship mass when loaded), then it's likely more efficient to do your refining down on the surface.

So it comes down to play style.  For myself, I like big heavy miners with massive cargo capacity (to minimize the number of round trips), so I always go for #3:  that is, my miner ships have great big fuel tanks and tiny ore tanks, and do all their refining on the surface.

Here's why it can be considerably more efficient to do your refining on the surface, depending on ship design:

Let's say you're mining on the Mun.  That's a lot of dV involved.  You need to spend 600ish m/s of dV to get from the surface to orbit, or vice versa.  So you're spending quite a lot of fuel just to do that, on each round trip.

If you do your refining in orbit, that means that when you land your Mun miner, it has to carry a big load of fuel down to the surface.  Why?  Because you have to send its get-back-to-orbit-with-a-full-load fuel along with it, down FROM orbit.  Which means you're schlepping a massive load of fuel (much more than the mass of an ISRU) on a round trip you don't need to.

Whereas if you can refine on the surface, you can design your miner so that when it touches down, it's running on empty.  That is, when it undocks from your orbital fuel depot, it only has to carry enough fuel to land, empty, which means a lot less mass.

Just as an example, let's run some numbers.  I'll assume you have a miner with a cargo (i.e. ore plus LFO) capacity of, say, 32 tons, and a dry (empty) mass of, say, 18 tons.  (That allows 4 tons of empty tankage, 2.5 tons of drills, 4.25 tons of ISRU, 1 ton for a command pod of some sort, 2.5 tons of engine, and another 3.25 tons for landing legs, solar panels, fuel cells, struts, or whatever else.)  YMMV, but that's a pretty reasonable ballpark number.  We'll assume your engine's efficient, like a Poodle at Isp 350s.  (If it's less efficient than that, the numbers get worse for doing orbital refining.)

Math in spoiler section.

Spoiler

What does the budget look like on each round trip for this surface refiner?   We'll assume you need 650 m/s of dV for each trip to and from the surface.

  1. Take off with 32 tons of cargo (it's all fuel, in this case, plus whatever smidgeon of ore it fits in its tiny ore tanks).
  2. Use 8.63 tons to get to orbital fuel depot.  (23.37 tons remaining.)
  3. Off-load 19 tons at orbital depot.  (4.37 tons remaining.)
  4. Use 3.86 tons to land on Mun again.  (0.51 tons remaining.)
  5. Touch down with 0.51 tons of fuel to spare as a safety margin.

That's when you do your refining on the surface.  It allows delivering 19 tons of cargo to orbit for each round trip.

Now let's see what happens if you do your refining in orbit.  Same ship, except it doesn't carry an ISRU, so its dry mass goes down to 13.75 tons.

  1. Take off with 32 tons of cargo (fuel plus a lot of ore).
  2. Use 7.90 tons to get to orbital fuel depot.  (24.1 tons remaining.)
  3. Off-load 11 tons at orbital depot.  (13.1 tons remaining.)
  4. Use 4.64 tons to land on Mun again .  (8.46 tons remaining.)
  5. Of the remaining 8.46 tons that you touch down with, 7.90 of that will be the fuel you'll need to get back to orbit again.
  6. So your remaining safety margin at touchdown is, again, about half a ton of fuel.

 

So there you have it.  For the above scenario, on-surface refining will net you 19 tons of cargo to orbit per round trip, whereas orbital refining will net you 11 tons per round trip.  Considerably more efficient, in this case, to do on-surface refining.

Note, however, that the case for on-surface refining becomes weaker for higher Isp engines, and for lower dV needed per round trip.  Thus, for example, if you were doing this on Minmus rather than the Mun, it probably wouldn't make much difference which strategy you use-- both because Minmus has such a tiny amount of dV needed to orbit, and also because its gravity is so low that you could probably use high-Isp nuclear engines on your miner.

Which means it depends on the situation.  :)

 

Incidentally, there's yet a third way you could set this up, which is even more efficient:  A surface refinery that stays on the surface.  In this scenario, you have a miner/refiner that lands once and never takes off again.  It just sits and sucks, so to speak.  You have your orbital fuel depot.  And then you have your fuel lifter, which is basically just a big flying fuel tank and nothing else.  The lifter lands next to the miner/refiner, connects to it, loads up on fuel, then flies to orbit.  This way, you're not schlepping the dead weight of the refinery or the drills between surface and orbit, and it's essentially the ideal scenario.  The downside, of course, is that now you have to master precision landing so that your lifter can land right next to the refiner... without actually crashing into it... and in such a way that you can hook it up to the refiner in order to take on a load of fuel.

It's a neat idea, and I've tried it, and it works.  But I ended up not doing it much-- mainly because the hassle of having to nail the landing precisely every single time outweighed (for me) the relatively small efficiency gain.  If I've got a 50-ton ship with 32 tons of fuel, that's not all that much worse than a ship that can save 7 tons of ISRU-plus-drills of dead weight, so I usually just go for the miner/refiner that lands and takes off, myself.

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Posted (edited)

18 minutes ago, Broke dead dogg said:

So I'm trying to figure out which is more efficient to get from the munar surface, into orbit. Ore or liquid fuel

Depends wich Converter you're using.

18 minutes ago, Broke dead dogg said:

 

I haven't been able to find any details about the weight of a unit of ore/fuel so any help would be appreciated

http://wiki.kerbalspaceprogram.com/wiki/Ore

http://wiki.kerbalspaceprogram.com/wiki/Liquid_fuel

http://wiki.kerbalspaceprogram.com/wiki/Oxidizer

 

So... Ore is twice as dense as fuel, but the big converter converts 1 unit of ore into 2 units of fuel, meaning your vessel's mass stays the same when converting ore to fuel. 

the small converter inputs more ore and outputs less fuel, so you should use this one on the surface.

Edited by Physics Student

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1ton of Ore becomes 1ton of fuel(with the big ISRU). So if you know which type of fuel you want while still on the surface, fuel tanks are the way to go. A lander with a built-in ISRU(drill and all mining stuff), tiny Ore tank and oversized fuel tanks. The bigger this lander is, the less appreciable it's mass becomes. A fueled 50ton lander is very useful, since the small ISRU doesn't decrease it's dv anymore.

If you go big enough, you can make the orbital fuel depot be the lander...it would occasionally go down to mine,refuel and back to orbit.

Mun has some gravity, meaning you get less fuel to orbit. Minmus is much easier in this regard.

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An additional, comparatively minor argument for refining on the ground is if you are doing any orbital manoeuvring and need monopropellant (which is likely since you're running a lander back and forth), then although the large ISRU will still take one unit of ore to produce two units of monopropellant, the monopropellant itself is less dense at 4 kg/L rather than liquid fuel and oxidiser's 5 kg/L.  This makes the refined product substantially less massive than the raw ore:  twenty percent mass savings is plenty of reason to refine on the ground.

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3 hours ago, Snark said:

...So it comes down to play style.  For myself, I like big heavy miners with massive cargo capacity (to minimize the number of round trips), so I always go for #3:  that is, my miner ships have great big fuel tanks and tiny ore tanks, and do all their refining on the surface....

Could we also point out that big miners (or landers in general) are paradoxically easier to land.  Making them low and wide means they can handle just about any slope short of an actual cliff.

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I almost always do a multipurpose mining vessel with ISRU included,  so I bring up fuel rather than ore.  By the numbers, this is slightly less efficient since the ISRU has to be hauled up and down.   It also potentially adds tankage weight,  since my ships carry both LF and LFO tanks, and sometimes I only want one or the other.   

But this saves some in-game time (since the drills and converter can work at the same time).  It also avoids the need to dock with the refining station every trip (I can send my miner straight to the ship I'm refueling).  

Finally, having the ship carry fuel gives it potentially huge range when repositioning.   My mining ships  can launch,  top off on one of the moons,  there go pretty much anywhere.   Whereas a mining ship that only carries ore is limited to its dedicated fuel supply.   

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In my opinion, you should have a mining lander bring up a ton of ore up to a station, which has a big ISRU unit.

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