Convenient orbit for a fuel station?

[Jart]

I am putting a new fuel station up and was wondering what folks found for a convenient orbit. I assume I will be using it to fuel up missions to the moons and even other planets. Just stick to 100k or go up... or even down..

[Tex_NL]

For Mun and Minmus you definitely don't need to refuel. Putting the fuel station there just for this is, for the lack of a better word, a waste of fuel. For interplanetary missions, stay low. The lower you start your interplanetary burn the more efficient it will be. 100 km is a nice altitude, easy to reach and leaves plenty of room to rendezvous.

On the other hand, if you're planning to ignore the benefits from the Oberth effect and choose to leave Kerbins SOI before making your interplanetary burn it might be wise to refuel as high as possible, way past Minmus but still in Kerbin orbit.

[sojourner]

I would go at least 200km for 2 reasons. First, it gets you far enough from Kerbin to reduce framerate drop from rendering the planet. Two, you can use a higher time warp above 160km.

[G'th]

There's a couple convenient spots.

LKO (~75-80km) is a good place to put a fuel station you intend to use to your designs that have an upper stage with no fuel loaded at launch. This lets you take larger payloads into Low Kerbin Orbit with lifters that would otherwise be unable to lift whatever your payload is, the reason being because you take out a lot of the weight (in fuel) from the upper stages, with the intention of loading that fuel while in LKO. The downside is that you have to be good at rendezvous as the low orbit isn't very forgiving.

Anywhere from 150km to 300km is a good spot for stations you intend to use with part heavy crafts as well as just crafts in general, as at this altitudes you wont' be rendering Kerbin's ground details anymore and will instead just see the texture of it, so you'll gain a better amount of FPS. Due to the medium orbit you'll have plenty of room for rendezvous and it'll be a good staging point for most transfers. Little downsides.

Orbit around either Mun or Minmus or a distance farther out than either will be a good place to receive returning interplanetary crafts that you intend to send back out, as it won't take much to enter Kerbin's SOI at an orbit closer to one of the moons or the station itself compared to trying to reach a station at LKO for instance. Downside is that crafts coming up from Kerbin will require a good amount of delta v to get to the station.

[Kasuha]

Orbit around either Mun or Minmus or a distance farther out than either will be a good place to receive returning interplanetary crafts that you intend to send back out' date=' as it won't take much to enter Kerbin's SOI at an orbit closer to one of the moons or the station itself compared to trying to reach a station at LKO for instance. Downside is that crafts coming up from Kerbin will require a good amount of delta v to get to the station.[/quote']

Most effective (dv-wise) parking orbit for passing ships is a highly elliptic one, with periapsis near 70 km and apoapsis around Minmus or Mun level. That's the orbit that costs least dv to establish as you use Oberth effect when braking at your low Pe. It also costs least dv to leave the orbit again although it may require some waiting till the orbit is aligned.

The downside of such orbit is, each ship will have its own, based on the direction from which it came. For such parking it is impossible to build an orbital refuelling station (or they'll have to spend significant dv on aligning) so it's better to send up individual refuelling missions. Also not everybody handles elliptic parking orbits at funny inclinations well.

My personal favorite orbit for an orbital station is 650 km above surface. It's high enough that Kerbin doesn't take up half the sky (and doesn't render in much detail), it's possible to time warp all levels at that height, and departure from that height costs way less dv for near targets and almost the same dv for distant targets (Eeloo, Jool, Moho) compared with LKO.

[Jart]

G'th, we are thinking along the same line of thought. I am keeping my science at a slow pace and plan on running a Kerbal/Nasa style game. So a lot of smaller craft in relation to the game with a lot of rendezvous, command modules, landers etc..

Sojourner - excellent points.

Tex - Thanks for a clue-in on the Oberth effect. Slingshotting seems to be a challenge I want to accept in the game.

So now I have a lot of choices! hmmm I will probably try them all...

No problem with FPS that I can perceive in the near future. I guess any amount can take down any computer but I am well set in that regard.

[Kerba Fett]

I put all my stations at either 200 or 250 km in a circular orbit.

I'll assume you know how to use maneuver nodes to rendezvous with the station.

When you launch a ship that's going to dock with the station you just raise it's AP till it intersects with the station orbit. Once you get up to AP you raise your PE till it's out of the atmosphere. 70 to 80km. Since your orbit is faster than the station all you need to do is sit in the map view and wait till your intersect distance starts getting low. On what you think will be the final orbit you add a maneuver node to fine tune your PE to minimize the intersect distance. I can usually get mine down to 2 or 3km. Finally you make another maneuver node to match orbits with your target. This goes right at your intersect point and is usually just burning prograde to circularize your orbit.

While you could also put your ship into an eliptical orbit above the station and get the same effect, you'd need to burn fuel to get above the orbit then burn more to drop down into it again later. If you put your station too low then the smaller difference between the ships and the stations orbital periods will mean it will take more orbits to line up.

This image is a bit cluttered, but the blue line is the ship and the station is on the yellow line near the orbital inclination marker. You can see my position at intersect lined up with the AP marker and the stations position at the next orbit just below it.

[Pecan]

My Kerbin station is at 250km, allowing lower and higher phasing-orbits at 100km and 400km. For vehicles that can only just make it to LKO I use a never-landing tractor to pull them to the station (or wherever), or to refuel them. Maximising Oberth for outgoing vehicles just means dropping periapsis to LKO; you're dropping from the station so when you burn at periapsis you're travelling faster than from a circular LKO.

[Jouni]

I have a minor refueling station for spaceplanes at 120 km. The altitude is a compromise between the amount of fuel required to reach the station and the waiting time for a transfer window after a badly timed launch. It takes quite a bit of bad luck to be able to come up with a design that can reach a 70 km orbit, but not a 120 km orbit.

My main refueling station is at a 60 km orbit around Minmus. Because I'm using the Kethane Pack, it's faster and easier to transfer more fuel there than to LKO. Most ships never go to the station itself, but wait for a tanker at a higher orbit. After refueling, ships preparing for an interplanetary launch move to a high orbit around Kerbin (usually around 30 Mm). While launches from LKO would be a bit more efficient, it's much easier to do accurate 15-minute burns at a high orbit.

[fireflower]

I've used two kinds of fuel depots: Minmus orbit and at-target rendezvous. The reason you don't want to have a naked space station in some arbitrary orbit above Kerbin is because it's not just about the fuel you get there but the ease of getting there. Minmus has enough gravity to make it a lot easier to rendezvous with than a station orbiting Kerbin.

The idea with using Minmus is that you're less than 50 m/s ÃŽâ€v from an interplanetary trajectory, so if you refuel at a station there, you save almost 1 km/s ÃŽâ€v. The problem is Minmus might not be at a convenient place in its long orbit for your interplanetary burn. Also, once you develop heavy-lifters, it's just not worth it. An extra 1 km/s ÃŽâ€v is no longer a big deal when you can throw 400,000 kg into LKO, especially considering the extra work you have to do. Making a stop at Minmus is just the final step; first you have to build the damn station, move it to Minmus, and keep it fueled. Even if you're fueling it with kethane from Minmus itself, it's a lot of effort for 1 km/s ÃŽâ€v. Kethane requires you build yet another spacecraft, a lander/ascender/tanker, and land, mine, process, and ascend every time you want to fly an interplanetary mission. No thanks.

The second kind of fuel depot is just a fuel tank with an engine and a probe core you throw at your destination. When your crew gets there, lands, and ascends back to orbit, they rendezvous with it to siphon the gas to fuel their way home. This is what I did to get the crew back from Moho, except I staged a depot at both Moho and Eve, because the first depot didn't have enough gas to get the crew all the way back home. Again, this is superseded by heavy-lifters. You can go anywhere in the Kerbol system if you have a 23 km/s ÃŽâ€v asparagus-staged NTR, and answer to no one.

Ultimately my advice is very stereotypical of KSP: Build a bigger rocket. Devise a heavy lifter so outrageously overpowered you aren't scrimping and saving for every m/s you can find. There is no honor in ÃŽâ€v poverty. When you roll that beast out of the VAB I want the heavens to tremble.

[Jouni]

The idea with using Minmus is that you're less than 50 m/s ÃŽâ€v from an interplanetary trajectory, so if you refuel at a station there, you save almost 1 km/s ÃŽâ€v. The problem is Minmus might not be at a convenient place in its long orbit for your interplanetary burn. Also, once you develop heavy-lifters, it's just not worth it. An extra 1 km/s ÃŽâ€v is no longer a big deal when you can throw 400,000 kg into LKO, especially considering the extra work you have to do. Making a stop at Minmus is just the final step; first you have to build the damn station, move it to Minmus, and keep it fueled. Even if you're fueling it with kethane from Minmus itself, it's a lot of effort for 1 km/s ÃŽâ€v. Kethane requires you build yet another spacecraft, a lander/ascender/tanker, and land, mine, process, and ascend every time you want to fly an interplanetary mission. No thanks.

You don't really save that much by launching at Minmus. Some would say "Oberth effect", but that's not really the point. When escaping a planetary body, you lose a certain amount of energy (for a fixed mass). But when you are moving faster, that same loss of energy translates into a smaller loss of speed. Because the same burn gives the same delta-v at high and low speeds, it's better to do transfer burns when you are moving as fast as possible.

Additionally, you need a certain velocity relative to the Sun to be able to use a certain transfer window. At a low orbit, you may already be close to (or even past) that velocity when traveling to the right direction, so you don't need to accelerate that much to get to the right velocity and to compensate for gravity losses. At a high orbit, your velocity relative to the Sun is pretty much the same as the velocity of Kerbin, so you may actually be farther from the destination.

Let's use the Launch Window Planner for some concrete numbers. Starting from day 1, the best transfer window to Duna requires 1034 m/s from a 100 km orbit, and 596 m/s from a 30000 km orbit. In this case, you are saving something by refueling at a high orbit, but the savings are less than half of what you need to get there in the first place. If you are going to Jool instead, you need 1986 m/s from a 100 km orbit, and 2490 m/s from a 30000 km orbit, so you are actually losing by getting to a high orbit to refuel.

Whether refueling stations around Kerbin are useful depends on the kind of ships you are using. With disposable ships, there is not that much point in refueling before launch, because you can just build a bigger lifter and launch the ship with more fuel. On the other hand, if you are using modular reusable ships, you usually need different modules for different missions. In that case, refueling is just a small part of the refitting between missions.

Edited February 2, 2014 by Jouni

[Alchemist]

With starting interplanetary missions from Minmus (or just edge of Kerbin's SOI), the best approach would be:

1) Escape Minmus against its direction of propagation

2) Get highly elliptic Kerbin orbit with periapsis just outside the atmosphere

3) Burn at the periapsis to get your tranfer trajectory.

The total cost wouldn't differ much from low orbit assembly, but the delta-v requirements for the final craft will be about 1 km/s lower (you are getting back what you spent by delivering all this to Minmus). But you have to plan for proper ejection angle.

[ComradeGoat]

Yeah, burning direct from Minmus to, e.g. Jool actually uses more fuel than burning from LKO, unless you can do the periapsis dropping thing. The problem with doing that is that Minmus takes so long to orbit Kerbin, while you wait for the ejection angle, you'll miss your launch window.

If you're mining fuel with Kethane, a station at Minmus may make overall sense, but be aware your ships will still need more delta V to go places than they will if starting from LKO.

[Prince of Rockets]

Why not geosynchronous orbit? Seems like a convenient spot. I personally try to build everything into single launches and thus don't use refuelling stations... But if I used them, I'd want them above geosync, because the remaining delta V requirement to exit the Kerbin system is minimal at that point. It also seems like the best spot for a interplanetary SSTO refuelling station.

[m4rt14n]

200km is ideal for me. because if you fail at 1st rendezvous from launching from KSC, you can safely drop periapsis down and catch-up. 100km is of course much cheaper for spaceplanes leaving kerbin, but adjusting rendezvous is annoying because there is only so much you can drop down into before hitting the atmosphere.

[fireflower]

I should add, with the high part count typical of massive spacecraft, lag becomes an issue. Particularly if I join two of those spacecraft together, I start my burn at an orbit of 166 km. This is because the rendering distance for the surface of Kerbin is 160 km. At and above that altitude, the high part-count is much less of a problem. The extra 6 kilometres gives some buffer distance for rendezvous and altitude loss due to long burns. I suppose if there's a place to put a gas station, it's there, but I haven't done it.

[Monger]

I'd vote for 200 km as well. It is close enough to Kerbin so that you can get there within one or two stages, and far enough so you can navigate there easily. The less distance you have between atmosphere and your target, the more tedious it becomes to intercept your station from "behind".

[technion]

There should be room for discussing the efficiency of the TWR here too.

With a heavy nuclear rocket, a burn to Jool from 100km might take you 2/3 of the way around Kerbin. That's an added inefficiency. From 300km, the problem would be minimised.

[fireflower]

With a heavy nuclear rocket, a burn to Jool from 100km might take you 2/3 of the way around Kerbin. That's an added inefficiency. From 300km, the problem would be minimised.

It's not really a problem. You can limit yourself to the ÃŽâ€v for escape velocity. And you can do what the Indian Mars probe did: Make multiple burns.