Refueling Stations, do they make sense in terms of D/V and/or ship complexity?

[WhiteKnuckle]

Sup everybody, this is my first post here and I hope I'm not asking a question that's been answered eleventy billion times in the last week.

My question pertains to the Kethane mod pack, although I guess it would also apply to simple fuel tankers as well.

I've only been playing for a few weeks, but I'm getting ready to start launching missions into the outer system, and I'm wondering if having a refueling operation in orbit of say Duna makes any sense at all?

Example: Sending a ship to Jool.

A: Go from Kerbin to Duna, refuel the ship there, then burn for Jool.

vs.

B. Direct burn from Kerbin to Jool.

Obviously (A) ads a lot of time waiting for launch windows, but that's what timewarp is for.

I'm assuming that having to get into a Duna orbit, and then meeting up with a refueling tug in orbit costs a lot of D/V in the long run, but would be offset by the ability to use smaller/lighter/simpler ships that don't want to either rip apart, or require 10 minute long burns (ran into that with my first manned mission to Duna, no fun)

I guess I'm just wondering if some of you more experienced players bother with refueling at all, or just build bigger better ships and deal with the associated problems?

Sorry for the rambling post, thanks for reading.

[iBeej]

I guess I'm just wondering if some of you more experienced players bother with refueling at all, or just build bigger better ships and deal with the associated problems?

You are going to find a mix of both. There technically isn't a "wrong" way to do it. Of course there are more efficient ways, but it really depends on your launch vehicle, how much ÃŽâ€v you have, etc. For example, I tend to build larger ships with a lot of ÃŽâ€v and don't bother with refueling. With exception to one planet. EVE. I found it's a lot easier to do a return mission from EVE when parking a command module in orbit and launching a smaller lander that will later redock and return home. Because lifting a heavy lander from EVE to orbit becomes exponentially harder with the more parts you add.

So the times I use refueling stations would be:

1.) Around EVE

2.) Around Jool (To visit the moons and return home)

3.) Around KERBIN when attempting to make SSTO interstellar trips

If you are doing ANY kind of serious true SSTO interstellar trips, you are going to be using refueling stations.

[Tank Buddy]

I can agree with iBeej. It really depends. Some times if you are going to have ludicrous fuel consumption you'd better plan a refuel, but most of the time you should build your ships to have enough DV to get where they're going and back.

Although having refueling modules in far away places (pretty much anywhere outside of Kerbin) will allow you to easily repeat missions with the same ship.

I did something like that on Ike, I used the extra fuel from the too large transfer stage to allow more than 5 extra landings!

[WhiteKnuckle]

Yeah I kinda figured that there are too many variables for there to be one "correct" way to do it.

I think I'm going to go ahead and launch a refueling station to Duna, A. Because I've already built most of it, and B. I figure even if I don't use it for planed missions it's still nice to have as a contingency plan in case things go all pear-shaped.

[EtherDragon]

Consider the Dv budget required to enter Duna circular orbit, and leaving that orbit again. I doubt that a refueling station there would help you save anything (would actually cost more to utilize) than going for direct ascent. If you want Duna's help, the best option is through gravity assist.

Basically, a fuel depot around the departing and arrival planets should be plenty. Stick a depot around Kerbin - suddenly your modular SSTO can un-mount its RAPIERS, and connect a pair of Nukes - refuel, and be ready to head to Jool. Stick a depot around Jool - and that same SSTO can easily use the various moons to gravity sling back to Kerbin for direct reentry.

[SuperBigD60]

They probably really start to make sense when you're using kethane.

Then you don't have to take all the weight of all fuel when you fly the station components out.

You can mine it and produce it on site. (Probably makes more sense for Ike than Duna.)

Also, Extraplanetary launchpads would be another way to increase efficiency. Putting a launchpad on a tiny moon llike Ike would decrease the size of the launcher required. Probably even still more efficient if you factor in the mining equipment you'd need to bring.

Anyway, that's my two cents.

[MarvinKitFox]

Going from planet A to planet B, witha detour to C, even if you can refuel at C, never makes sense. Entering, orbiting then leaving a planet's gravity will virtually always cost more than the refuel advantage.

If you insist on having it orbit Duna, make it a *very* high orbit, to minimise penalty.

Putting a fuel tank in a Duna-type orbit does makes sense, as splitting the transfer orbit from Kerbal to Jool into two parts, without adding a gravitational point inbetween, makes no delta-v difference. Picking up fuel in this scenario would make a definite but small advantage.

Of course there is the tiny matter of waiting another couple days/months/lifetimes for the orbits to match up.

Deeper into the inner system, where delta-v gets ludicrous and orbital periods decrease a lot, a mid-space fueller is a great idea.

But realistically, a refueller only makes sense when it is orbiting just above the place you launched from, and you wish to not throw away pieces of your ship via staging.

i.e. the fuel must be in a place where you won't spend fuel, just to detour to the fueller.

Now if you are using Kethane, you have in effect a refuelling station on every solid body out there. Definitely worth it!

I currently have a science vessel sucking every point of science I can from Mun. It's now on its 7th landing, and I plan to do 8 more stops before heading home. Every 2-3 stop is a Kethane refuel!

[SSSPutnik]

Putting a fuel tank in a Duna-type orbit does makes sense, as splitting the transfer orbit from Kerbal to Jool into two parts, without adding a gravitational point inbetween, makes no delta-v difference. Picking up fuel in this scenario would make a definite but small advantage.

You still need to get the fuel out of X gravity well to the pickup point. I can't see how you would get a positive gain. ( And launch it on a trajectory where your mother ship doesn't have to slow or change course much).

Taking a Kethane plant with you can make sense.

[Streetwind]

Tangentially related - I've seen some people suggest that a refueling station at Minmus would be a good idea. After all, if you are at Minmus then the dV required to leave Kerbin's SoI is miniscule because you already expended most of it getting to Minmus. And the low gravity means that adjusting your heading, inclination and other such things are extremely cheap. Finally, you can also take advantage of Minmus' own speed relative to Kerbin (either on the outer pass for accelerating towards the outer planets, or on the inner pass for decelerating towards the inner ones). Getting the fuel to the Minmus depot would be trivial in the presence of Kethane since mining it directly on Minmus and lifting it to orbit there is effortless in the low gravity.

On the other hand, I see lots of advice to burn directly from Kerbin orbit into an interplanetary transfer, because of the Oberth effect. Since that effect is directly related to gravity, Minmus would have very little of it. The question is then, does that make Minmus a better or worse departure point? Oberth effect versus intrinsic orbital energy and refueling at the edge of the SoI... what wins?

[mhoram]

On the other hand, I see lots of advice to burn directly from Kerbin orbit into an interplanetary transfer, because of the Oberth effect. Since that effect is directly related to gravity, Minmus would have very little of it. The question is then, does that make Minmus a better or worse departure point? Oberth effect versus intrinsic orbital energy and refueling at the edge of the SoI... what wins?

Minmus has an orbital inclination of 6°, which also makes it unattractive as starting point for interplanetary travel.

[ComradeGoat]

Going from planet A to planet B, witha detour to C, even if you can refuel at C, never makes sense. Entering, orbiting then leaving a planet's gravity will virtually always cost more than the refuel advantage.

If you insist on having it orbit Duna, make it a *very* high orbit, to minimise penalty.

This neglects two things: aerocapture and the Oberth Effect. Duna is so small that the latter makes using it as a staging post to Jool impractical, however, despite entering orbit being free, due to aerocapture.

Putting a fuel tank in a Duna-type orbit does makes sense, as splitting the transfer orbit from Kerbal to Jool into two parts, without adding a gravitational point inbetween, makes no delta-v difference. Picking up fuel in this scenario would make a definite but small advantage.

Not really. The most efficient burn to Jool is done as close to LKO as possible, to utilise the Oberth Effect. In your scenario, not only do you lose the advantage of Oberth for the latter part of the trip, you also have to circularise your solar orbit to rendezvous with the fuelling station by burning to raise your perihelion. Once you've done that, you need to raise your aphelion to Jool without the benefit of the Oberth Effect.

Doing a direct burn to Jool gets you the Oberth Effect at LKO departure all the way to your aphelion, and raising your perihelion is free because that's basically what aerocapture at Jool is doing.

Frankly, you'll do far better with a fuelling station at Laythe, perhaps topped up by a Kethane mining operation on Vall. Vall is easy to land on and take off from, Laythe is easy to get to from Kerbin because you can do pretty much the entire "stopping at the other end" thing through aerocapture (once at Jool, again at Laythe).

All you have to do is make sure your vessel has enough delta V to burn from LKO to Jool, but that's probably less than the delta V you'd need to burn from LKO to your fuelling station and stop when you get there.

You also have the problem of where to top your fuelling station up from. In a solar orbit, you basically have to ship it from Kerbin. If it's at Laythe, you can mine it on Vall if Kethane is your thing.

[Streetwind]

Minmus has an orbital inclination of 6°, which also makes it unattractive as starting point for interplanetary travel.

Who says you have to match Minmus' inclination? You can make your orbit level with the plane of the solar system. Then at worst, Minmus' own orbital motion gives you a small nudge upwards or downwards at the ascending or descending node, respectively. That can probably be corrected with RCS alone in a midflight course correction. soon after leaving Kerbin's SoI. And if you happen to start at the highest or lowest point above/below the plane, then you fly completely level by default - just a little bit above or below the plane.

I don't see that as a bigger issue than trying to hit a planet in an inclined orbit from Kerbin's level one. It's just the reverse process (and maybe starting from Minmus can even let you hit that planet more easily).

[mhoram]

Who says you have to match Minmus' inclination?

There is a difference between "make your orbit level with the plane of the solar system" while in the SOI of Kerbin and

"make your orbit level with the plane of the solar system" while in the SOI of Minmus.

After refueling at Minmus (in an Minmus-Equatorial-Orbit) and leaving the SOI of Minmus, you have a 6° inclination relative to Kerbin and to the solar system, because Minus has this inclination.

Or in other words: try to reach the refueling station at Minmus from Kerbin, while your inclination to Kerbin is 0°.

Edited January 24, 2014 by mhoram

[WhiteKnuckle]

Thanks everybody, glad this sparked a larger discussion.

In regard to the Minimus tangent, would it make sense to refuel in Minimus orbit, then burn back towards Kerbin, and then make your interplanetary burn at Kerbin Ap? Seems like this would be the best of both worlds since you'd already be mostly out of Kerbin's gravity well with full fuel, but you'd also get the Oberth effect bost as you passed by Kerbin.

Now I'm wondering if I should just take my Duna refueling tanker that I spent all last night assembling in LKO and send it off to Vall instead... it's probably got enough D/V... probably.

Maybe.

[Skorpychan]

I'm planning to use them extensively. It seems a great way for landers to be re-used.

For Kerbin, it's mostly used to top up my cruise stages after use; they get left in Kerbin orbit, so they can just hook up to a station and refill their tanks.

I also have a set of tankers for refueling things in Kerbin's SOI, in case I screw up.

I'm sticking one around Duna as preparation for biomes being added there; my landers carry enough fuel to get down to Duna and back up again, but not enough to do much afterwards, so a station makes sense for them to use as a staging point. And with a lab hooked up to it, I can transmit data easily as well.

Eve missions will require one as well; Gilly is a pain in the ass to get to, and Eve's gravity well is ridiculous. Anything coming out of there will be empty on fuel once it gets an orbit, and I'm planning on using dedicated ascent systems to get kerbals and their experiments out the atmosphere. Also, it'll be a stopover for Moho missions.

Jool has lots of muns, and it's far away from Kerbin. It needs a fuel station and a set of tankers. This will have to be flown out and assembled with multiple missions.

[AlexisBV]

I also thought of the minmus station, then passing by Kerbin for Mr. Oberth. I still need to run the numbers on that though, because decreasing your Pe from Minmus to Kerbin is not free. Is it offset by Oberth effect gains? Something to ask the Brain.

Oh, and quite offtopic:

I do not fear failure. Failure is data. I fear not learning from failure. Failure to learn from failure is stupidity.

Failure to learn from failure is not stupidity. It's metadata.

[WafflesToo]

There is a difference between "make your orbit level with the plane of the solar system" while in the SOI of Kerbin and

"make your orbit level with the plane of the solar system" while in the SOI of Minmus.

After refueling at Minmus (in an Minmus-Equatorial-Orbit) and leaving the SOI of Minmus, you have a 6° inclination relative to Kerbin and to the solar system, because Minus has this inclination.

Or in other words: try to reach the refueling station at Minmus from Kerbin, while your inclination to Kerbin is 0°.

You do realize that 6dg inclination energetically translates to fiddly-squat once you reach interplanetary space, right? You can translate from a polar orbit around Kerbin and still not have a significant solar inclination.

[ComradeGoat]

I also thought of the minmus station, then passing by Kerbin for Mr. Oberth. I still need to run the numbers on that though, because decreasing your Pe from Minmus to Kerbin is not free. Is it offset by Oberth effect gains? Something to ask the Brain.

Yes, increasing your eccentricity to get a low Kerbin Pe for the Oberth Effect is more than worth it.

The difficulty is that Minmus' orbit is so long, you may miss your launch window while waiting for the right point to do this.

[TimePeriod]

I use refuelling stations on the Mun rather then on Kerbin as the Mun has much lower gravity to deal with.

[WafflesToo]

I use refuelling stations on the Mun rather then on Kerbin as the Mun has much lower gravity to deal with.

Minmus is even more attractive for the same reason

[TimePeriod]

Minmus is even more attractive for the same reason

True thou the mun is easier to hit

[mhoram]

You do realize that 6dg inclination energetically translates to fiddly-squat once you reach interplanetary space, right? You can translate from a polar orbit around Kerbin and still not have a significant solar inclination.

Valid argument. I can see now the point of my misjudgement.

[Skorpychan]

True thou the mun is easier to hit

Yes, but that's what makes Minimus so attractive.

[TimePeriod]

Yes, but that's what makes Minimus so attractive.

That you have to spend more fuel, time and effort to hit?

[I_Killed_Jeb]

The concept of a minimus refueling station is bizarre to me. Just put the station in LKO, you dock to it for the same dV +/- a few dozen m/s as getting into orbit, and then you do your transfer with maximum Oberth effect. What benefit is there to a minimus station?