Optimal Refuelling Tactic

[Whyx]

Okie dokie,

I have a main equatorial Space Station at 100km above Kerbin and a refuelling station 50km above Minmus. I have a number of large orange fuel containers on my Kerbin SS that I wish to fill up. Is it more viable to place a mining vessel on Minmus and travel from Minmus to Kerbin orbit to refuel the Kerbin SS or do lots of little SSTO trips, the problem with SSTO is that I can't figure out how to get enough lifting power to bring a decent amount of fuel to orbit from Kerbin's surface.

Any extra tips on refuelling would be greatly appreciated.

[Plusck]

Maximum possible mass ratio of stock LFO parts is 9:1 with zero acceleration. Add a Terrier, probe core, docking port and solar panels to an orange tank and this drops to about 7.4:1, meaning you should be able to get about 7000 m/s out of it (but weak acceleration), with about the minimum sort of acceleration that you need to drop from Minmus to LKO and back again with reasonable cosine losses.

Deduct 1000 m/s for the transfer from Minmus, and you lose 10 tons of LFO.

To get back up to Minmus, you need to keep 2 tons of LFO onboard.

So each orange tank coming from Minmus will be able to disgorge 20 tons of LFO, or 62.5% of an orange tank, or 1800 LF and 2200 Ox.

What you definitely don't want to do is have anything else on your transport tankers (like mining gear). If you make them much bigger and use a Poodle instead, you'll get a slightly better percentage profit on each run.

 

At the end of the day, the question is: can you be bothered spending all the time and effort doing this? Sending huge fuel-laden rockets up from Kerbin will cost funds but will cost a fraction of the time and effort.

Extra tip: rather than either of those options, take a contract for bringing a class D asteroid into orbit around Kerbin, and bring that down to a reasonable level to mine and fill your Kerbin station. Class E would be better but they are a pain to move. Without a contract it will cost you funds to do it, but you might find a good candidate coming in when no contracts are up, and it's always rewarding (I find)...

Edited November 27, 2017 by Plusck

[Aegolius13]

Both are viable options,  so I think out mostly comes down to what sounds more fun. Do you like flying big spaceplanes,or landing on Minmus?  Both options are a bit fussy and will require some repetition. 

The SSTO option will require you to pay for fuel,  but not for the hardware itself (assuming you can land back at the runway).  The mining rig is the opposite - they are pretty expensive up front but give you free fuel forever.   So cost may come down to how long you expect to use it - longer service life favors mining. 

The mining option is also probably more versatile,  since you can also refuel stuff at Minus.

If you go with mining,  I would try to make a vessel that can aerobrake on return to Kerbin.  This can save a bunch of fuel. 

[Geschosskopf]

3 hours ago, Whyx said:

Any extra tips on refuelling would be greatly appreciated.

Well, the 1st tip is, unless your game goals are pretty much all within Kerbin's SOI, it's generally not worth the time and trouble to set up a fuel infrastructure in Kerbin's SOI.  Sure, it will save you some fuel but not that much, and it takes a long time to pay off the sunk costs of building the infrastructure to begin with.  Plus all the time spent humping fuel around takes time away from doing other, more interesting things.  At other planets, sure, it's very much worth the trouble (besides giving you something to do there besides plant a flag).  But at Kerbin?  Not so much.

That said, the general rule for a fuel infrastructure is as follows:

• Mine and refine on the surface of the low-gravity moon.
• Have specialized fuel lifters to take the fuel to low orbit of the low-gravity moon.  These dock with a collection of tanks in low orbit at the low-gravity moon, pump the fuel into those tanks, and return to the surface.  These lifters tend to be the biggest ships in your entire fleet because they need to haul enough fuel at once to avoid having to make an excruciating number of trips, plus have enough of their own to take off and land again.
• Have specialized fuel carriers that pick up fuel at this collection of tanks and move it to the collection of tanks in orbit of the main planet in the system.  These ships should be as bare-bones as possible, yet still big enough to keep the number of trips to a minimum.

Alternatively, you can use the Pathfinder mod, which now has cargo mass-drivers and mass-catchers.  Fire the fuel to orbit with this, then fire it between stations.  Then you don't need any ships or as much time, but you do need lots of electricity.  MKS also has some sort of orbital resource transfer system, too.  So with these, you probably spend as much if not more money, but at least you don't have to spend the time flying fuel around.

[bewing]

Additionally to what is said above, you always have to engineer a fuel margin into every craft that comes back to Kerbin.

Which means that every craft that's about to reenter at Kerbin has extra fuel aboard. So dock them all with your station and siphon the extra fuel off before reentry. It does nobody any good to bring coal to Newcastle, or excess fuel to KSC.

 

[Aegolius13]

5 hours ago, Geschosskopf said:

• Have specialized fuel lifters to take the fuel to low orbit of the low-gravity moon.  These dock with a collection of tanks in low orbit at the low-gravity moon, pump the fuel into those tanks, and return to the surface.  These lifters tend to be the biggest ships in your entire fleet because they need to haul enough fuel at once to avoid having to make an excruciating number of trips, plus have enough of their own to take off and land again.
• Have specialized fuel carriers that pick up fuel at this collection of tanks and move it to the collection of tanks in orbit of the main planet in the system.  These ships should be as bare-bones as possible, yet still big enough to keep the number of trips to a minimum.

I would think that in many cases, you can use the same ship for both roles.  I.e., both should be large volume, low thrust, high efficiency.  You could potentially save some time and a little fuel by consolidating so you don't have to rendezvous for the switchoff.   A lifter might have a few things the lander did not need, like landing legs, but if the differences are small it might still be easier to go with one ship.

Of course, if you're mining somewhere with high gravity, or an atmosphere or whatnot, the two craft could end up pretty different.  

[Spricigo]

9 hours ago, Plusck said:

At the end of the day, the question is: can you be bothered spending all the time and effort doing this? Sending huge fuel-laden rockets up from Kerbin will cost funds but will cost a fraction of the time and effort.

^this. 

IOW, what I said early in another thread:

For me, the real question is: How much of my time (in real life) it will take to have the fuel I need where and when I need?

Some people will bring fuel from the surface of [celestial body] with an efficient nuclear tanker. I'd rather use the effective TwinBoar powered supertanker.

 

[Geschosskopf]

1 hour ago, Aegolius13 said:

I would think that in many cases, you can use the same ship for both roles.  I.e., both should be large volume, low thrust, high efficiency.  You could potentially save some time and a little fuel by consolidating so you don't have to rendezvous for the switchoff.   A lifter might have a few things the lander did not need, like landing legs, but if the differences are small it might still be easier to go with one ship.

Of course, if you're mining somewhere with high gravity, or an atmosphere or whatnot, the two craft could end up pretty different.  

Well, it does depend on the gravity of the fuel source, but in general I'd use separate ships everywhere but Gilly (I don't consider Bop and Pol as viable fuel sources).  Definitely Vall to Laythe, and even Ike to Duna.  Minmus is kinda on the border line but even there (if I actually bothered with a Kerbin-SOI system), I'd probably use 2 ships.

The reason is that the fuel lifters have to be ginormous, able to fill up anything in no more than 2 trips, maybe 3 at most in exceptional cases.  Any more than that and refueling takes way too much time and repetition.  The time isn't just yours but also gametime, when each trip is like a week 1-way (Minmus-Kerbin) or transfer windows between Joolian moons are several days apart.  So, their cargo tanks are 1/2 as big as whatever they're filling, plus then the fuel for their own consumption.  All that fuel is very heavy, which means you need LOTS of lander legs so the thing won't collapse under its own weight.  And you need relatively large, heavy engines, even in relatively low gravity, to get to orbit carrying all that.  Lugging all that from the source to the customer eats into cargo capacity, which means you need to make more trips.

Doing 2 separate ships also allows some parallel processing.  While the space-only tanker is schlepping along for a week or 2, the lifter can be back at the mine reloading and hopefully will be ready to go again when the tanker gets back.  If you just have a lander/tanker combo, then you need to have large storage tanks at the mine so it can be making fuel while the lander/tanker is making a run.  Which means a bigger, more complex, more Kraken-enticing base.  I find it better to refine directly into the big lifter than storage tanks, and then have a dedicated space-only tanker to keep the base limited to just the drills and ISRU.

[Aegolius13]

11 hours ago, Geschosskopf said:

Well, it does depend on the gravity of the fuel source, but in general I'd use separate ships everywhere but Gilly (I don't consider Bop and Pol as viable fuel sources).  Definitely Vall to Laythe, and even Ike to Duna.  Minmus is kinda on the border line but even there (if I actually bothered with a Kerbin-SOI system), I'd probably use 2 ships.

The reason is that the fuel lifters have to be ginormous, able to fill up anything in no more than 2 trips, maybe 3 at most in exceptional cases.  Any more than that and refueling takes way too much time and repetition.  The time isn't just yours but also gametime, when each trip is like a week 1-way (Minmus-Kerbin) or transfer windows between Joolian moons are several days apart.  So, their cargo tanks are 1/2 as big as whatever they're filling, plus then the fuel for their own consumption.  All that fuel is very heavy, which means you need LOTS of lander legs so the thing won't collapse under its own weight.  And you need relatively large, heavy engines, even in relatively low gravity, to get to orbit carrying all that.  Lugging all that from the source to the customer eats into cargo capacity, which means you need to make more trips.

Doing 2 separate ships also allows some parallel processing.  While the space-only tanker is schlepping along for a week or 2, the lifter can be back at the mine reloading and hopefully will be ready to go again when the tanker gets back.  If you just have a lander/tanker combo, then you need to have large storage tanks at the mine so it can be making fuel while the lander/tanker is making a run.  Which means a bigger, more complex, more Kraken-enticing base.  I find it better to refine directly into the big lifter than storage tanks, and then have a dedicated space-only tanker to keep the base limited to just the drills and ISRU.

Good points; I guess you can emulate staging by moving the fuel to a ship with smaller tanks and engines after you've burned off what's needed to get back to orbit.  

Personally, I am just so lazy  that I usually use a single ship to mine, lift off, and send fuel to stations, so I don't need to do any precision landings or extra rendezvous.  I know I'm not getting great fuel efficiency, but I try to make up for that by using stupidly large mining rigs, so that even a fraction of the full fuel load ends up being a lot. 

[Wanderfound]

On 28/11/2017 at 4:35 AM, Whyx said:

the problem with SSTO is that I can't figure out how to get enough lifting power to bring a decent amount of fuel to orbit from Kerbin's surface.

How much spaceplane tech do you have available?

 

https://forum.kerbalspaceprogram.com/index.php?/topic/30639-sstos-post-your-pictures-here~/&do=findComment&comment=2979331

Edited December 2, 2017 by Wanderfound

[Draalo]

I agree that taking efford of a Mun/Minmus base takes long.

But there may be other reasons to build one:

Practicing your skills to build a base. Is there enough power supply and radiation? Are all Tanks well sized? Experience the difference between uncrewed and having 2 Engineers abord. How long does it take to produce a certain amount?

Simulate Duna conditions by retracting a quarter of your paneels.

Practice surface docking und using subassemblys for rovers with dockingports.

Practice precision landings meanwhile...

 

Finally: build the Mother of all Ships. Fly it into Minmus orbit and refuel. Make a grand tour

That ship can be really big but just needs to have enough fuel to reach Minmus.

Picture MK III cockpit, cabin, payload, lab, Lf fuselage (long) surounded by 8x NERV radial.

With a front dockingport to carry a lander.

Visit Jool and his moons, head back to Kerbin ... and its still reusable

 

Edited December 2, 2017 by Draalo
typo

[Whyx]

Thanks for all the fantastic replies guys, I’ve built my surface station as an exercise in surface building because it’s something I’ve not done before. I’ve found it challenging to say the least. Based on the advice given I’m going to try and set up a network of refuelling around eve and Duna and eventually make a base on one - probably duna because that seems like a lot of fun