Should I build a refueling station at the edge of Kerbins SOI?

[Aedile]

It's cheaper from LKO.

Refueling station at the moons makes sense in 3 cases -

Exploration of said moon. This is very effective. Best placed on low polar orbit around said moon.

Refueling on return, though you are probably better off with aero capture.

Going to a different planet. In this case, the trick is not to escape directly from there, but rather, escape to kirbin, in such a way that you are at periaps exactly at the location and time you need to start burning for your transfer orbit. This could be useful , depends where you are going. You will need to do a mid course plane change as well to match inclinations. With the help of a mod such as precise node, you can do this from high Mun orbit 'relatively easy', as Mun's orbit is not inclined, and has relatively short period, and the trip takes just a few hours. Still it's one of those things which does not make much sense, unless you use a mod and produce fuel there and have a reusable program going on. Building bigger rocket and refueling it in LKO directly by a fuel ship, is much, much simpler. Mun slingshot is also simpler.

Refueling station on circular orbit at the edge of Kirbin SOI does not have much practical value if any, and efficient rendezvous take long time. Also they are difficult to refuel.

[Skorpychan]

What do you know, you were right: high parking orbits really are bad for your fuel economy.

Since I couldn't believe my intuition was wrong, I did some math. The image I'm linking is the burn delta-V (shaded contours) needed as a function of the height of a circular starting orbit around Kerbin (horizontal axis) and the desired delta-V relative to Kerbin's orbit around the sun (vertical axis). The red curve along the lower left is the height at which you burn the least fuel for a given interplanetary delta-V.

Bottom line: if you're headed for Eve or Duna, you want to burn from just inside the Mun's orbit. For any other destination, you want to be a lot lower. There go my station plans...

*snip*

(Pardon the messy plot. I'm better at math than I am at pretty graphics.)

Forgive me for having to ask, but how am I meant to read that?

I mean, I understand what the lines are, but what do the axes and the colours represent?

(I failed physics and mathematics, y'see)

[John FX]

Myself I have a small refuel station for aerobraking craft in LKO which gives enough fuel to get to the main station at Minmus and after refuelling my craft fully there they exit Minmus using minimum fuel and adopt a parking orbit just inside kerbin SOI and exit Kerbin SOI so that on the next orbit I am catching up to or just ahead of Kerbin and re-enter the SOI to get a slingshot. You can get pretty much anywhere with not much fuel although there is a year added onto every journey...

Obviously for direct burns everything said so far is great, good work reporting all this. Its good to have stuff like this to refer to when planning a mission.

[mhoram]

I played around with different transfer orbit periapsis altitudes and the differences for the graph are not large in the range 70-100km.

One additional information is quite relevant: when to use exactly a single and when to use a double burn via a 80km x current altitude transfer orbit.

This graph shows it.

(GPL V3 licensed source code)

It uses basically the same setup as Starstrider42:

Take the altitude of your circular starting orbit and locate it on the horizontal axis.

Take the desired excape velocity at SOI and locate it on the vertical axis. Be aware that this is not the dV value you see on the dV-charts. The markers on Starstrider42s graph should help you get the magniture for different planet targets.

On Starstrider42s chart you can see by the color of the resulting x-y-point how much dV you have to spend to achieve this transition.

On my chart you can see, when to use a direct burn and when to use a double burn via a 80km x current altitude transfer orbit.

Forgive me for having to ask, but how am I meant to read that?

Say you are on a circular orbit near Minmus and want to go to Jool.

Starstrider42s graph tells you that you need an escape velocity of ~2750 m/s for Jool.

According to my graph, a double burn is more efficient.

The total dV you need in order to reach Jool is then about 1250 m/s.

Edit: And in order to get back to the OP's question:

If you want efficient single-burn post-refueling transfers, then a refuelingstation near SOI is not recommendable.

If you want a refueling station from where to start most fuel efficiently, then placement near SOI is recommendable.

Edited April 8, 2014 by mhoram

[Starstrider42]

Forgive me for having to ask, but how am I meant to read that?

I mean, I understand what the lines are, but what do the axes and the colours represent?

(I failed physics and mathematics, y'see)

Horizontal axis = height of your starting orbit around Kerbin (assumed circular). Since this thread is about refueling stations, I assume that actually getting up to that orbit is free -- either you refuel from a station there, or you use a mod like Extraplanetary Launchpads to build the ship there. The vertical lines are reference values for kerbostationary equatorial orbit (KEO), the Mun's orbit, or Minmus's orbit. These calculations do NOT account for the gravity of the Mun or Minmus, so launching from a station orbiting either moon will cost a little extra.

Vertical axis = how fast you want to be flying away from Kerbin AFTER you've left Kerbin's sphere of influence. The horizontal bands are the speed needed to intercept a given planet. The width is because the planets' orbits are eccentric, and you need a lower speed to intercept Eve and Moho at their apoapsis (or periapsis for the other planets). This assumes you've already taken care of orbital alignments ("phase angles"), and ignores inclination changes. So, to be safe, add a bit to the numbers on the chart, especially for Moho and Eeloo.

The colors don't mean anything except making the lines/bands distinct.

So to read the chart, pick the altitude of your refueling station along the horizontal axis, then move up to the band of the planet you want to fly to. The shaded contours tell you the delta-V you need to go from your space station's orbit to an intercept trajectory for the planet, assuming perfect timing.

Or do it backwards: if you're planning a space station to support lots of flights to Duna, find Duna's band on the vertical axis, then find the smallest delta-V within that band and read off the best station height from the horizontal axis. This puts your ideal station at about 8000 km (if you're always using single burns, first chart) or beyond the orbit of Minmus (if you're using Eric S's more difficult method, second chart).

I hope I answered your question somewhere in that Wall of Text.

[Skorpychan]

Horizontal axis = height of your starting orbit around Kerbin (assumed circular). Since this thread is about refueling stations, I assume that actually getting up to that orbit is free -- either you refuel from a station there, or you use a mod like Extraplanetary Launchpads to build the ship there. The vertical lines are reference values for kerbostationary equatorial orbit (KEO), the Mun's orbit, or Minmus's orbit. These calculations do NOT account for the gravity of the Mun or Minmus, so launching from a station orbiting either moon will cost a little extra.

Vertical axis = how fast you want to be flying away from Kerbin AFTER you've left Kerbin's sphere of influence. The horizontal bands are the speed needed to intercept a given planet. The width is because the planets' orbits are eccentric, and you need a lower speed to intercept Eve and Moho at their apoapsis (or periapsis for the other planets). This assumes you've already taken care of orbital alignments ("phase angles"), and ignores inclination changes. So, to be safe, add a bit to the numbers on the chart, especially for Moho and Eeloo.

The colors don't mean anything except making the lines/bands distinct.

So to read the chart, pick the altitude of your refueling station along the horizontal axis, then move up to the band of the planet you want to fly to. The shaded contours tell you the delta-V you need to go from your space station's orbit to an intercept trajectory for the planet, assuming perfect timing.

Or do it backwards: if you're planning a space station to support lots of flights to Duna, find Duna's band on the vertical axis, then find the smallest delta-V within that band and read off the best station height from the horizontal axis. This puts your ideal station at about 8000 km (if you're always using single burns, first chart) or beyond the orbit of Minmus (if you're using Eric S's more difficult method, second chart).

I hope I answered your question somewhere in that Wall of Text.

Oooh, I get it now! Thank you!

[Northstar1989]

First of all, your graph is inaccurate because you're completely neglecting the orbital velocity you have around the Mun or Minmus. While the orbital velocity you would have around Minmus is nothing to write home about, with proper ejection from a low Munar orbit (and making a much higher Delta-V burn than you need simply to escape Munar obit), you can easily keep a significant amount of velocity from a Munar orbit.

Second, Minmus has the MAJOR advantage of being able to Kerbin-dive. Because the orbital velocity relative to Kerbin of Minmus itself is very low, all you have to do is escape from Minmus' SOI in the retrograde direction, and you'll basically dive right straight in towards Kerbin. It's very easy to get a periapsis of around 75-90 km with VERY low fuel expenditures from Minmus. At that point, if you timed it correctly, you'll have the benefits of BOTH a super-charged Oberth Effect (since your starting velocity is much higher than you would normally have at that altitude) AND a high-orbit refueling. This allows for unquestionably the lowest Delta-V transfers from Kerbin SOI of virtually any starting location.

Regards,

Northstar