Advantages of high kerbin orbit refueling

[Alistone]

So... I know everybody loves talking about how great the Oberth effect is for preparing for interplanetary burns.

But this wonderful tool seems to be confirming something I suspected.

http://ksp.olex.biz/

That the delta V of your ejection burn is less if you leave from a higher parking orbit.

For example, from Kerbin to Duna:

100km parking orbit: delta V of ejection burn 1043.06 m/s

500km parking orbit: delta V of ejection burn 882.08 m/s

2,000km parking orbit: delta V of ejection burn 690.65 m/s

Admittedly my stations tend to be in orbit of muns or low kerbin orbit; I expect the longer period of the more distant orbits makes rendevous a bit more time consuming. But it may also have the benefit of slowing down the constant turning caused as the target progresses through its orbit. This would make the docking easier.

Yes, it takes more fuel to get your station way up there, but if you are docking to refuel anyways you seem to be better off from a higher orbit.

Additionally, since ejection burns take quite a while, by not being perfectly horizontal at the start/end often requires a correction burn. By having a much longer orbital period (and a smaller ejection burn) this variance and correction burn should be reduced.

Last night was busy putting all the HOME modules on minmus. Tonight I'll make a 8,000,000 m orbit station and see how easy/difficult it is to make it work docking work.

The real future use of a "tug" may be to have efficient engines to pull fuel tanks from LKO out to a 8,000,000 orbit station.

I'll try it out later tonight. Thoughts?

[maltesh]

If you're going to completely fill up, and depart straight from a circular orbit (instead of, say, plunging to an atmosphere-scraping orbit that ejects you in the correct direction) then yes, the closer your destination is to Kerbin, the higher the optimum departure altitude winds up being. When I ran the numbers a few weeks back, the results ranged from about 11,000 km for Eve down to about 200 km for Eeloo.

There used to be a post that went a lot deeper into the specifics of this sort of thing, by someone else, but the forum implosion ate it.

[RawChicken]

Well, I was expecting that, but never sis the maths.

I dont usually go to a refueling station, I just go Kerbin->wherever Makes the game moar awesome!

[Alistone]

Well, I was expecting that, but never sis the maths.

I dont usually go to a refueling station, I just go Kerbin->wherever Makes the game moar awesome!

Yeah, my first duna landing and return was when I had an "accident" that damaged one of the the docking ports when lifting up 2 large orange tanks and a tug for a station. My 3 man tug/lander was just sitting on top of nearly 2 full orange tanks of fuel... No parachutes, but managed a powered soft landing on Duna and had enough fuel to make it back to orbit to redock with the orange fuel tanks which still had plenty of fuel for the trip home to kerbin.

This was stock... So redocking for fuel really doesn't seem to be essential for kerbin departure... but still seems essential if you're planning a return trip from an atmospheric surface.

Besides, space stations are cool and why have them if not for some functional purpose.

[maltesh]

Looking at my numbers, for the most part, you do get some saving over departing refueled from standard 100km orbit, based on the standard equatorial/circular assumptions.

[TABLE=width: 500]

[TR]

[TD]Destination

From Kerbin[/TD]

[TD]Optimum Refueling Altitude (km)[/TD]

[TD]Ejection Delta-V from Optimum(km/s)[/TD]

[TD]Ejection Delta-V from 100 km orbit (km/s)[/TD]

[/TR]

[TR]

[TD]Moho[/TD]

[TD]680[/TD]

[TD]1.661[/TD]

[TD]1.693[/TD]

[/TR]

[TR]

[TD]Eve[/TD]

[TD]11309[/TD]

[TD]0.551[/TD]

[TD]1.012[/TD]

[/TR]

[TR]

[TD]Duna[/TD]

[TD]7775[/TD]

[TD]0.649[/TD]

[TD]1.047[/TD]

[/TR]

[TR]

[TD]Dres[/TD]

[TD]1020[/TD]

[TD]1.476[/TD]

[TD]1.554[/TD]

[/TR]

[TR]

[TD]Jool[/TD]

[TD]360[/TD]

[TD]1.918[/TD]

[TD]1.921[/TD]

[/TR]

[TR]

[TD]Eeloo[/TD]

[TD]209[/TD]

[TD]2.089[/TD]

[TD]2.082[/TD]

[/TR]

[/TABLE]

So the savings are maybe useful when departing for Duna or Eve, however, there are obvious problems with putting a refueling station in a circular orbit over Kerbin at 11,309 km. It's probably not really worth it for the other destinations: the meager savings are chump change in terms of the other burns you're going to have to do.

And of course, I've not run the nubers based on going elliptical to a low Kerbin Periapsis from your refueling orbit, then burning at periapsis to eject to your destination. I suspect if I did that, very high Kerbin orbits would be the optimum position for the refueling.

[Volodyuka]

hmm, for me, it's easier to assemble interplanetary mission on orbit, then refuel it by launching refueling ships to space (+ bringing additional crew to mission) and after that going to final destination, also refueling station in my opinion needed for return mission, orbiting another celestial body, jool for example.

[RoboRay]

You burn more fuel in total stopping at a higher station to refuel, but your craft can be smaller and less massive because it doesn't need to hold as much fuel at one time.

I have a fuel depot out beyond Minmus, barely still in Kerbin's SOI, but it's really only useful if my departing craft needs another 1km/sec of delta-V on its mission.

[N_las]

If money would be relevant in ksp, you would see that refueling the station itself in a high orbit is so much more costly than a station in low orbit, that it isn't a good idea.

[Kenken244]

It does make sense, though, if you are assembling a large interplanetary ship to eve or duna to asseemble it in a higher orbit

[drakesdoom]

The idea of a high orbit refueling station becomes much more economical if you are refining your fuel on minimus or the mun instead of kerbin.

[Jason Patterson]

Whether it's efficient or not depends strongly on how you are counting delta-v used. If you only consider the cost from the ejection burn onward, a higher altitude winds up being cheaper. If you instead consider the total cost of a mission, including getting it and its fuel up to your refueling station, lower altitude absolutely crushes everything else. There's just no comparison.

The problem lies in considering the fuel that is waiting at high altitude to be a free resource. It's not just the station that has to be boosted to a high orbit - you had to spend an additional several hundred m/s getting it from LKO to a high altitude orbit. If you're trying to do something very delta-v intensive, like an Eve return mission, then you may not care about launchpad to landing delta-v, and only the portion of the delta-v spent getting out of the system is worth considering to you. For routine interplanetary missions though, the additional time and effort spent getting fuel all the way to high orbit is just not worth it.

If you are doing something like an Eve return, there are better options as well. You can start your interplanetary vehicle off burning in the appropriate direction and burn into a highly elliptical orbit around Kerbin, with a periapsis that scrapes the clouds and an apoapsis that almost hits the Mun (or goes higher, if you've got it timed right.) Once you've spent the ~750 m/s you need to make that happen, you refuel in that highly elliptical orbit and finish your burn at periapsis. You get all the benefits of Oberth and need very little delta-v to finish getting out of the system. Note that this still only "saves" you ~750 m/s, so if you're doing something that is emptying your fuel tanks it might be worth it, but it's just about as easy to simply add another booster stage at launch.

[tavert]

^ what Jason said.

Fuel doesn't magically materialize into a high-altitude orbit around Kerbin. If you need a refueling stop, you should do it at a low orbit around wherever the fuel happens to come from, to minimize the cost of getting the fuel there. If you are getting fuel from the Mun's surface or Minmus' surface, then you can bring your interplanetary ship to Mun or Minmus orbit for refueling. Your transfer windows will be further apart since you'll need whichever moon you're starting from to be in the right place relative to Kerbin when you do your ejection, which might be a bit of a pain. Due to aerobraking, it would also be quite cheap to bring fuel from the surface of a moon back to a low orbit around its parent planet.

[Eric S]

I'll kind of third that. The only time I've considered refueling in the Kerbal system anywhere other than LKO was when the fuel was coming from Kethane processing, and then I tend to do it in low orbit of whatever body the kethane is being mined on.

[KerbActual]

Quick question: How does one transfer fuel from one tank to another? Is a mod required or can it be done in stock?

[tavert]

alt-right click both tanks

[Ringer]

If money would be relevant in ksp, you would see that refueling the station itself in a high orbit is so much more costly than a station in low orbit, that it isn't a good idea.

Not to mention you can't time warp appreciably fast below 600km. I used to put another vessel in a high orbit just so I could switch to it and warp to wait for an optimal transfer with the lower-down ship in question. These days I launch everything interplanetary to 600-650; it's a good enough place for a station as well.