Refueling vs docking an entire tank

[Technical Ben]

I do both. I put takes up for use later. If the ship already has a tank, it refuels. I never "swap" as such, but do ditch empties in planets/moons to save on return weight.

[Red Iron Crown]

There's a lot of overlap between "fuel station" and "tanker", too. My largest tanker is 5 full orange tanks and a large RCS tank, I think some would consider it a fueling station.

[einsteiner]

When I launch a transfer stage (three orange tanks and a NERVA) I use the orange tanks as fuel for Mainsails, which I drop once in orbit. Then I send a tanker to refill the transfer stage. This way I can reuse the tankage that got me into orbit to carry fuel for interplanetary transfers too.

[RexKramer]

Decouplers and maybe sepratrons as well. Throw them out to the side.

Or, alternatively, put the engines at the front of the ship and drop tanks off the back.

That's exactly what I do. I mount two (sometimes 3 or 4) LVT-N's radially to a large tank. (Use fuel lines to keep the radial tanks/engines fed)

The extra tanks are added to the back of the ship, I like to use large decouplers. I just add extra tanks to the back of the ship as needed, and turn them loose as they are emptied.

One problem is there is a practical limit to how many tanks you can add to the back of the ship, and have it remain controllable. Extremely long ships joined by large decouplers get wobbly, and difficult to aim accurately. Extremely heavy payloads require much more fuel, so the 'add extra tanks to the back' method has a practical limit of how much mass you can get to, say, Jool. I keep my landers quite light, so it works well for me.

Also, the added weight of extra tanks, combined with an accompanying addition of extra engines, can increase your burn times. I'm content to deal with longer burn times, knowing that adding engines also adds weight and decreases efficiency.

[RexKramer]

That's exactly what I do. I mount two (sometimes 3 or 4) LVT-N's radially to a large tank. (Use fuel lines to keep the radial tanks/engines fed)

The extra tanks are added to the back of the ship, I like to use large decouplers. I just add extra tanks to the back of the ship as needed, and turn them loose as they are emptied.

One problem is there is a practical limit to how many tanks you can add to the back of the ship, and have it remain controllable. Extremely long ships joined by large decouplers get wobbly, and difficult to aim accurately. Extremely heavy payloads require much more fuel, so the 'add extra tanks to the back' method has a practical limit of how much mass you can get to, say, Jool. I keep my landers quite light, so it works well for me.

Also, the added weight of extra tanks, combined with an accompanying addition of extra engines, can increase your burn times. I'm content to deal with longer burn times, knowing that adding engines also adds weight and decreases efficiency.

Not sure what the etiquette is for responding to your own post, but...

I wanted to add that if you mount LVT-Ns radially to a vessel, the thrust will not be impeded by adding extra tanks to the back of the vessel.

HOWEVER, you do want to be cautious about components mounted radially to the rear tanks. I usually mount some 'stuff' to drop tanks, including rear-mounted drop tanks. This could include radially mounted small control module, RCS tanks and thrusters, batteries, solar panels, etc. I have very rarely encountered a situation where the extra stuff mounted radially to aft-drop-tanks has interfered with the forward radial-mount LVT-Ns, but is HAS happened. So now I make an effort to mount auxiliary equipment on aft drop tanks such that with a little careful alignment I can keep it clear of the thrust of the forward LVT-Ns.

[magnemoe]

one problem with having drop tanks is placement. you have to design the ship for them from the start so that they can be symmetrically placed and have balanced fuel draw. one question is how do you drop a tank in a burn, every time i have tried to drop a tank in a burn i have ether hit it and broke something important off my ship or it just stayed in place in front of my ship.

I have used docked tanks I drops sometimes. Vall is an typical excample, you need more fuel to land on Vall than you need to go to the minor moons so I used an drop tank who was dropped before landing.

Downside is as you say that the ship has to be designed around it, the lander also required refueling after Vall, the tank just took it from 2km/s to 3km/s dV.

For large ships the dock tanks is an structural weakness. Landers get balance problems, the Val lander had an bottom docking port and side tanks and engines who drew from center.

An mun lander on the same setup who has and dockable goo and material lab together with an fuel tank might be an good idea as you collect the data and drop the lab anyway.

However you must also refuel as its give more flexibility.

[Seret]

Isn't a lot easier to put your ship in orbit, without big tanks, and then launch and dock the tanks themselves to it?

What if your ship is already in orbit?

Much of my work in space is done with reusable tugs with LV-Ns. These are quite frugal with fuel, and it would be impractical and annoying to launch another rocket each time one of them got low on fuel. The largest type has a max fuel capacity of about 2/3rds of an orange tank, the smaller ones are about half that. It's much easier to put a fuel depot in orbit around one of the bodies they're operating around, and top up whenever they need it. If they were topping up straight from the tankers then I'd end up with tankers hanging around in orbit anyway, effectively just acting as fuel depots.

It's also convenient if you're running an exploration mission at the Mun or Minmus where you're shuttling back and forth to the surface. The lander(s) can refuel and offload their science each time they dock.

[Alymon]

... These days I need few dedicated supply missions to them, mostly to replenish jet fuel in my LKO station but even that's getting rarer ....

I completely forgot about jet fuel. I've been building stations with RCS and Liquid Fuel, but never thought about putting jet fuel on board.

[costermonger]

I used to have a lot of issues with performance (then I found Active Texture Management and I got better) so my fuel depots were pretty minimalistic; a control unit with probe core, SAS, battery and some solar panels, then a tank body with (usually) a jumbo 64 and RCS. 4 docking ports total. That kept the partcount down a bit so my 200-part monstrosity ship or whatever didn't turn my computer into a pile of goo as I get within physics range.

[LethalDose]

IMO, the stations are convenient and useful because you can dock multiple vessels at one at once, and they act as a relatively static rendezvous points for my tugs, interplanetary vessels, and resupply missions.

[Zylark]

I find Orbiting Stations to be quite useful. They act as a hub for smaller ships, crew transfers and of-course fuel storage.

I don't tend to do refueling missions by themselves. For one thing, my standard heavy-lifter usually leave me with anything from 20% to near 100% of fuel left over in the central stage after lifting something to orbit. So rather than let any go to waste, I dock at my Kerbin Station, store the fuel there, dump the central stage and let the payload carry on whatever mission it was on. As in this example of a habitat module going for the Mun (though with this particular launch the central stage left over fuel was transferred to the Mun Station before landing the payload on Mun):

(Standard Asparagus design, outer shell of solid boosters, then a circle of 6 Jumbos with Skipper engines where two are dropped at a time and a central stage of another Jumbo with a Skipper. For more oomph, I could use the new NASA parts that came with 0.23.5, but so far there have been no need.)

Here is an earlier version of my Mun Station when in use as I was doing my massive "squeeze all the science I can out of the Mun" mission.

(Mun-Lander with 4 attachment points for science pods in the process of changing out used pods for new ones and refueling at the Mun Station)

If using the Kethane mod, filling up stations and fuel-depots in far away places is a breeze. Strictly speaking not all that necessary for Mun, as it is so close by, but it is a great place to test out designs for more compact stations and not least the Kethane extraction and transport.

(In background the Kethane Truck which brings the green-gold to my orbiting refinery. Then from left to right the Habitat, the Miner and the Power-Supply with a small converter to provide rocket fuel and mono-prop on site to the Kethane Truck and Crew-change shuttle. I try to keep my Kerbals on a 90 day rotation. The PSU module was very hard to land, so for my future expansion to Duna/Ike and later the Jool system, a more compact one will have to be designed). All connected together using KAS fuel lines. Though obscured in this picture, between the Truck and Base is a ground-pylon. Fuel line goes from Base to pylon and then from pylon to Truck - extending the range of the KAS fuel line, allowing me both to not be that accurate when landing and dispensing of the need for a cumbersome Fuel Rover)

And borrowing much from the Kerbin Station, my "New" Mun Station/Refinery with two large Kethane converters and still room to expand by another 3 huge fuel tanks. Same as the old station really, only the middle section was dumped. Didn't need the 4 extra docking ports on the jumbo tank any more, and the solar-power array was a bit too elaborate and complicated, sucking up frames in the process

(left to right: big-ship docking port and large kethane converter module with solar-panels, fuel module with radial connectors for more fuel-tanks, another large kethane converter module, mono-prop, crew and small/medium ships docking module. Docked already is a Tug for transporting stuff within the Kerbin system and a tiny-tug for moving parts around - or provide emergency mono-prop for ships that by some blunder have gone dry and thus unable to dock safely)

The only downside to space-stations is that they are usually made of a lot of parts, which if you try to dock really big ships there to refuel or whatnot, your frame-rate will soon plummet down into the low single digits. The solution is as simple as it is obvious. Have a fuel truck. As with my upcoming Duna mission.

Pick up plenty of fuel at Kerbin Station (same design as the Mun station, only with some extra stuff in the middle still attached).

(Fuel truck on the right)

Then get the fuel truck to your massive interplanetary ship, dock and transfer fuel, like with this one going to Duna quite soon.

(left to right: Lander, 4xHub with two docks and two Kethane finding probes, mother-ship with 6x Science-Pods for high above, low orbit and surface of both Duna and Ike. And a Crew and Command section in the pointy end of the ship. not quite done yet, will add a Jumbo drop-tank at the very rear - just to be sure as I will not carry any Kethane gear on this trip. Notice the uncanny resemblance to the Fuel Truck. The Duna Discovery was pretty much based around the Fuel Truck, as I found out fully fueled the Fuel Truck got a stunning 9000 and change dV...)

A fuel truck is also great for bringing out fuel to ships that by some freakish accident have suffered under the catastrophic failure of Jeb to read the fuel-gauge properly...

Of-course, all this gear do require a bit of science gathered. But even early in the career, some forward planning - sometimes as simple as putting a couple of docking ports on your main lifter - can give you an excellent ad-hoc orbital fuel dump. Like this one, still orbiting Minmus after I did my science gathering missions there quite early in the campaign. Still have plenty of fuel in it. Might just have to send my Fuel Truck over to scoop it up...

Edited April 11, 2014 by Zylark
minor additions

[cantab]

I completely forgot about jet fuel. I've been building stations with RCS and Liquid Fuel, but never thought about putting jet fuel on board.

Jets use the same liquid fuel that rockets do, so you may as well stick with the regular tanks, especially considering one orange tank carries over ten times the fuel of any spaceplane fuselage, and because the fuselages are heavy a regular tank drained of oxidiser still offers better tankage than the Mk 1 fuselage and not far off the others.

[-Velocity-]

I've personally never seen much of a need for refueling stations, not around Kerbin, at least. It's so easy just to launch up a tanker when you need more fuel.

However, around other planets, the primary concern is minimizing the number of times you have to do an interplanetary trip. So it makes sense to send a big spacecraft to serve as an exploration hub/refueling base.

[Seret]

I've personally never seen much of a need for refueling stations, not around Kerbin, at least. It's so easy just to launch up a tanker when you need more fuel.

Depends how much fuel you need though. If you're using really massive ships and thirsty engines then you can probably gobble all the fuel in a reasonable sized tanker. But my standard tanker only lifts about an orange worth to LKO (I don't even use all the fuel in its core stage, let alone the long-range transfer stage) and that's enough to refuel multiple ships for me, so a fuel depot in LKO makes a lot of sense.

[FREEFALL1984]

I find them incredibly useful, for a big interplanetary mission here is my normal course of actions, first I launch an empty orange tank with RCS, and a command pod on the top, and a few docking ports. I get this thing into a stable and predictable orbit, and I launch my first mission ship, which has only empty fuel tanks (dropping the weight and reducing lifter requirements) I then dock this (and the core of the lifter with about half an orange of fuel remaining) and empty all the fuel into the "fuel station" then I deorbit the spare orange tanks reducing part count by a large margin. Then I launch the next ship, also empty, and repeat the process, then when the mission window comes near I simply transfer all the spare fuel which I have accumulated into the fuel station into the ships and assemble the ship ready for launch. This means I'm only lifting empty ships into orbit. So for instance if I need to launch a nuclear tug, which would normally weigh about 40t, I only have to lift 15tons and the fuel is transported up over 3 or 4 launches. which happens naturally due to my unnecessarily large margin of error lol

[theend3r]

Refuelling stations are certainly useful in many cases. Their use for SSTO refuelling is obvious but even with simple interplanetary ships you want to launch them in one piece since they can be strutted to hold together that way and refuell them in orbit. I have a launcher that can launch about 100 000 units of fuel into 80-130km orbit which is usually sufficient for really long periods of extensive use. All interplanetary missions are refuelled there before continuing on their way.

This is my largest refuelling station and largest man-made object in my universe, weighting about 3000t ~ class E asteroid:

Edited April 14, 2014 by theend3r