Orbiting refuelling stations? Best way to do them?

[SSgt Baloo]

I've been playing around with putting a big fuel tank into orbit for use as an orbiting gas station. Right now, I'm just trying to come up with something that I can put into orbit.
 

The current design consists of a 2.5m fuel tank with a 2.5 meter clamp-o-tron docking port on one end and a 1.25 meter clamp-o-tron docking port at the other. There are four RCS thrusters at each end for attitude control. They use liquid fuel and oxidiser, since I figured station-keeping wouldn't be much of a drain, plus there are four small LF-O motors at one end, in case it needs to change orbit or de-orbit completely. There are four large solar arrays for power, a 2.5m probe core, and a 2.5m battery (probably overkill, but... *shrugs*).

I'm in the process of adding lights to make it more visible. Is this design even remotely adequate?

The fuel tank is one from a parts pack that is as big as the orange 2.5m tank, but considerably lighter.

Edited February 13, 2016 by SSgt Baloo

[Evanitis]

21 minutes ago, SSgt Baloo said:

Is this design even remotely adequate?

I think it's more than adequate. I kinda' obsessively try to save system resources by having as few vehicles and parts as possible in constant use. For me, an empty fuel-pod (or a half-used lifter-stage) is almost a refuelling station. If it has a docking port it's good, but I usually have a klawed adapter on orbit, so it's absence isn't a too big of a problem either.

Power generation and reaction wheels are comfortable, but unless you wanna' fill something really low on fuel, it's no big deal either. I think I never ever used a docking light - the silhouettes and the stock instruments felt totally enough (though I must admit, as an RPM user I have very delicate docking indicators in IVA).

I also realized lately that I have like 3-4 barebone pods on LKO that were half-used stages or test-payloads. I thought about building a base from them, but than I realized that it's handy to have them apart from each other. This way fuel is always close when I feel the need for it.

Edited February 13, 2016 by Evanitis

[Sir_Robert]

As a functional station, it's more than enough. Nice touch with the engines on the side, good thinking.

Those fancy stations you see in screenshots serve no purpose other than looking good (and melting CPUs). The rule with everything in space travel is: keep it simple

[Jakalth]

I've found that a fuel pod of that size, having 16 of the small rcs blocks is enough to not only change alignment, but also shift orbits and perform docking maneuvers.  But, using the small rcs blocks does mean having to have seperate fuel for them(monopropellant) which is a downfall.  Also, having a single 2.5m sas module on each end can perform most of the turns the fuel pod will need, other then for sliding sideways, or performing big orbit changes.

 

That design is probably more then you need for fuel transfer, especially if the other craft does the docking.  In fact, if the other craft does the docking, all the tank needs to do is turn to face the docking craft.  So in this case, you could even drop the RCS and just use SAS for turning, saving on part counts.

 

As for the lights, I'd stick with only 2 at most.  Otherwise they can produce to much light and cause the other craft to get whited out and make visibility worse instead of better.  And if your using them to light up the docking port, turn down their intensity to 0.3 or lower for the same reason.  It takes very little light to make the ports show up in the dark of space.

[Snark]

Yep, nice start you got there!

Definitely overkill on the solar panels, you don't even vaguely need four Gigantors.  Yes, they look cool.    But they're also very fragile and can get in the way if you're docking things to the station.  You could replace them with a simple ring of a dozen or so of the OX-STAT static panels, you'd do fine.

It's often a good idea to spam docking ports, so that you have lots of options for expanding later.  For example, you could put a 2.5m docking port at each end, so you can daisy-chain big craft.  Then put a quartet of the 1.25m ports around the "equator", perhaps projecting out a ways on some girders to give a bit of "clearance".  Bonus points if you squeeze in a couple of 0.625m ports somewhere.

Reason why it's handy to have excess docking ports:  sometimes it's a handy place not just to go to refuel, but to "park" docked ships until you need them.  Once you start attaching more ships, your "parking lot" can get crowded...

I'd also suggest giving it some monopropellant storage, even if it doesn't actually need monopropellant itself.  Once you start doing a lot of docking operations, your orbital ships will need to top off from time to time.

Edited February 13, 2016 by Snark

[RexKramer]

7 hours ago, Evanitis said:

I kinda' obsessively try to save system resources by having as few vehicles and parts as possible...

This.

I suggest using the largest fuel tank you can, not multiple smaller ones.  As Snark said, your fuel depot may end up becoming a parking spot for several ships.  It's handy to just leave them tethered in one location rather than randomly drifting in space.

So, with several vessels docked to your depot the part count can get high quickly, which leads to fps issues, or lag.  Keeping the part count as low as possible on your fuel depot helps, do this by using the largest tank available, instead of tacking on multiple smaller tanks.  Other than docking ports (can't ever have too many of those..), try not to put a bunch of extra parts on it unless they are absolutely necessary.

If one Orange tank isn't enough, I go to the bigger size tanks instead of adding more orange tanks.

[SSgt Baloo]

Made another one, incorporating most of your suggestions*. I still have more than the desirable number of solar panels, but I really don't like running short of power. For the time being, I've left the enormous lifter attached, since its fuel will be available at any docking port, right? Now I know I can get it here, I know can rebuild the lifter to be smaller.

* Except I forgot about the 0.65m docking ports until just now. 



Dept. of "...And Furthermore...": http://s28.photobucket.com/user/SSgtBaloo/slideshow/Orbiting%20Gas%20Station-2a

ETA: Forgot to mention that, in addition to the 2.5m probe core, there is also a 2.5m battery and SAS unit, as well. The first version had those, as well. The RCS units are mainly because with such a massive booster,  getting pointed towards the maneuver node was taking too long. For orbital attitude adjustment, I can turn off the RCS and use SAS. Mostly I just point it towards "normal", so some of its solar cells will always be facing the light, when it's available.

Edited February 14, 2016 by SSgt Baloo

[Snark]

21 minutes ago, SSgt Baloo said:

I still have more than the desirable number of solar panels, but I really don't like running short of power.

Understandable.    Just bear in mind that except for a few things that your fuel depot is unlikely to be doing (science lab research; ISRU; mining; operating an ion drive), KSP ships really need only tiny amounts of electricity.  Just a few panels go a long way.

24 minutes ago, SSgt Baloo said:

For the time being, I've left the enormous lifter attached, since its fuel will be available at any docking port, right?

That's correct. As long as things are docked together, the game considers them all to be one ship.  So you can have any number of things all docked together in a big hodge-podge, and all of their fuel is available to each other, and to all their docking ports.

25 minutes ago, SSgt Baloo said:

Except I forgot about the 0.65m docking ports until just now. 

Meh, don't worry about it too much.  I find that most of the time, I only use the 0.625m ports for a window in mid-career before I've researched the 1.25m ones.  Once I have those, I use those.  In principle I suppose the .625m ports might be useful for resupplying really tiny unmanned probes, but I find that in practice, that never happens with me-- I don't bother resupplying them, I just launch new ones if I need 'em.

[Temstar]

Even if you do use tiny probes, it's easier to just have a claw on the depot to handle dodgy crafts. The claw has the added bonus that you can even dock things without docking port.

[SSgt Baloo]

Oh, yeah. One more thing...

 

I wanted a nose cone to protect the docking port on the way up, but did not want space junk floating around later, so I equipped it with a decoupler and nosecone. The decoupler was turned so it would take the nose cone with it. The nose cone had eight seperatrons on it (possibly overkill - it worked a treat!). After leaving the atmosphere, but before circularizing the orbit, I pointed the ship retrograde and triggered the nosecone's decoupler and seperatrons (I had them in an action group, for convenience). It decelerated very well, and very likely exploded on the way down or, possibly, when it landed on some poor Kerbal's head.

[Snark]

30 minutes ago, SSgt Baloo said:

I wanted a nose cone to protect the docking port on the way up, but did not want space junk floating around later, so I equipped it with a decoupler and nosecone. The decoupler was turned so it would take the nose cone with it.

Yup, that's a handy trick. 

30 minutes ago, SSgt Baloo said:

The decoupler was turned so it would take the nose cone with it. The nose cone had eight seperatrons on it (possibly overkill - it worked a treat!). After leaving the atmosphere, but before circularizing the orbit, I pointed the ship retrograde and triggered the nosecone's decoupler and seperatrons (I had them in an action group, for convenience)

If you enjoyed the fireworks show, then great.    However, it's worth noting that you really don't need the sepratrons.  (In fact, the drag and mass from the sepratrons almost certainly outweighed the aero benefit you got from the nosecone.)

 Nor do you need to point retrograde when you blow the nosecone.

As you say, you pop it when you get out of the atmosphere before circularizing, so anything at all that you don't have with you when you do the circularization burn is going to fall back into the atmosphere.  It would take major rocket power for it not to fall back to ground.

So you can just pop the nosecone with nothing more than a decoupler.  Nosecones are lightweight, so the decoupler will easily have enough oomph to blow the cone clear of the ship.

And you can be pointed any which way... just make sure you don't do it pointed perfectly prograde shortly before it's time to do your circularization burn, or you could end up slamming into it from behind and getting the kind of fireworks you don't like.    But that's easy, just point your nose slightly away from prograde when you pop the cone, and that will divert enough that you'll easily miss it when you go roaring past.

 

Edited February 15, 2016 by Snark

[SSgt Baloo]

The first time I set off the nose cone it went on a hugely elliptical orbit that would have taken forever to decay enough for it to fall back to Kerbin. Maybe I'll just use two seperatrons next time, or just turn away from prograde slightly to put it on a more divergent trajectory than dead ahead.

 

[Temstar]

Just decouple your nose cone once you've turned off your engine and is on your way coasting to AP. Unless you're THAT good at getting a nearly circular orbit with one continuous engine burn your PE will usually be quite low (read - below the ground) while you're coasting.

And if you're really that good at getting a circular orbit with one burn then you'll just have to cut the engines while PE is still below ground, jettison the nose cone off to the side then start the engine back up.

[Jakalth]

What I find works best in this case, is to tuck the sepratrons under the nosecone in such a way that they go most of the way through the decoupler, but not enough to touch the part the decoupler attaches to.  This way the sepratrons are out of the aero model equation, but still can kick the cone the extra distance.  Also, turning down their thrust to 50% helps prevent them from burning the front of the rocket on activation.

[Sir_Robert]

9 hours ago, SSgt Baloo said:

The first time I set off the nose cone it went on a hugely elliptical orbit that would have taken forever to decay enough for it to fall back to Kerbin. Maybe I'll just use two seperatrons next time, or just turn away from prograde slightly to put it on a more divergent trajectory than dead ahead.

 

Actually you don't even need a decoupler. Just attach the nose cone directly to the docking port. You can right click the port and decouple the nose cone when in space

[SSgt Baloo]

 

3 hours ago, Sir_Robert said:

Actually you don't even need a decoupler. Just attach the nose cone directly to the docking port. You can right click the port and decouple the nose cone when in space

No kiddin'? I thought a decoupler could only decouple from another decoupler.

 

[Sir_Robert]

49 minutes ago, SSgt Baloo said:

 

No kiddin'? I thought a decoupler could only decouple from another decoupler.

 

Yup, ports can decouple from other things. They just can't dock to anything other than other ports

It just pops right off

Edited February 15, 2016 by Sir_Robert

[Edax]

Also, sticking docking ports on struts helps keep distance between the docked ship and the refueling station.  Sometimes the shape of the plane just won't fit on a docking port attached directly to the body of the station.  Plus, the extra distance means an accidental collision won't damage anything important, and your free to use the front and back ends of your station for modular expansion.

Edited February 15, 2016 by Edax

[SSgt Baloo]

See? When you start answering questions I didn't even know to ask, that's when I get to learn something I didn't know I needed to know.

[StormKat]

I really like your design.  Personally, I prefer the RTGs to power my tugs.  Those solar panels can get hung up on things.

My fuel tank pods are really simple.  The tug uses beams to position the RCS for easy maneuvering.

[Snark]

On 2/15/2016 at 8:38 PM, Jakalth said:

What I find works best in this case, is to tuck the sepratrons under the nosecone in such a way that they go most of the way through the decoupler, but not enough to touch the part the decoupler attaches to.  This way the sepratrons are out of the aero model equation, but still can kick the cone the extra distance.

Actually, the above statement (which I've bolded here) is incorrect.  Stock KSP aero is fairly simplistic and does not take part occlusion into account.  If part A is radially attached to part B, then part A will generate drag.  Doesn't matter if you clip it inside the part, or use the editor translate tool to hide it inside something; it'll generate the same drag.  The only way for a radially attached part not to generate drag would be if you have it inside a closed fairing, cargo bay, or service bay; you can't eliminate the drag by hiding it behind something.

So basically, short of using fairings or something, it's impossible to put a sepratron somewhere that it's usable but isn't giving drag.

(Adding to the confusion is that stock reentry heating does take occlusion into account, which is why a heat shield can protect radially-attached things behind it as long as they fit inside the heat shield's "footprint".)

Note that the above statement applies to stock aero.  If you're running FAR, I believe it applies a more sophisticated model, and the technique you describe would work there.  But for stock, not so much.

Edited February 18, 2016 by Snark

[David104]

On 15.2.2016 at 1:03 AM, Snark said:

In principle I suppose the .625m ports might be useful for resupplying really tiny unmanned probes, but I find that in practice, that never happens with me-- I don't bother resupplying them, I just launch new ones if I need 'em.

I never used the small docking ports neither untill I figuered out they have a big advantage (if you use KIS)! They fit into a backpack of a kerbal and can be attached by them. The big ones are too heavy..saved me a mission once.

[KerbonautInTraining]

Something I didn't know until many months of playing is to have a dedicated backup battery.

Right click on the probe core (or any battery of your choice) and click the icon next to the charge meter. This will lock the battery from giving or receiving charge. If the other batteries run out you can use the backup (by re-enabling the battery) to get your solar panels some light. 

A ship of that size could easily run on a couple 1x6 panels.