Tankers and Rendezvous in High Altitude Orbit

[LittleItaly]

I've been developing my Lotus-class orbital tankers in the hopes of eventually reaching Duna, Eve, and beyond. I've been using the Oberth Effect to maximize my efficiency in getting the tanker up to altitude. Currently it is sitting at around 34,000,000 in Kerbin orbit (that's around midway between the Mun and Minmas). Is this a good altitude for interplanetary journeys to rendezvous with for liquid and RCS fuel?

Thank you!

[Uberick]

If you're planning on having outgoing ships rendezvous with the tanker on the way OUT of Kerbin's SOI, you might want to put the tanker as close as possible to Kerbin itself(70-80km orbit). That will allow the outgoing ship to launch, refuel, and make a very efficient burn in low Kerbin orbit. If you make an ejection burn while very far out from Kerbin, you will not be able to utilize the gravity well(Oberth Effect) to make your ejection burn as efficient as possible. I don't know for sure if that's correct(haven't done maths), but that's what I'd guesstimate.

maybe someone more qualified can give a concrete answer

[tavert]

I agree with Uberick. The only reason you might have for putting a tanker at high altitude is to get slightly further after refueling with the same interplanetary ship. But if you consider overall fuel consumption, it takes a lot of fuel to get the tanker to high altitude in the first place. You're better off having a slightly larger interplanetary ship (if needed), and doing all refueling and ejection burns at low altitudes, escaping your way out of Kerbin's SoI from there.

[purpletarget]

If you make an ejection burn while very far out from Kerbin, you will not be able to utilize the gravity well(Oberth Effect) to make your ejection burn as efficient as possible. I don't know for sure if that's correct(haven't done maths), but that's what I'd guesstimate.

The flip side to this is that refueling that far out in the SOI, you shouldn't need much Delta-V to complete an escape burn anyways, and you start it with a full tank of gas. So you're craft should leave the SOI with about an extra 1km/s delta-V.

However, that also has to be measured off against the risk that your orbital period that far out is about 31 kerbin days...so what is the risk that your interplanetary transfer window can open and close in that amount of time if your tanker is in the wrong position for the transfer burn?

[Alistone]

I had a similar thought to you; and I thought that the wider orbit would make docking maneuvers easier; and you already have more potential energy when you're that high up.

But to be efficient you need your transition burn to start at as high a velocity as possible. At 31,000,000 you are orbiting at less than 100 m/s. By starting with such a low velocity you can't take advantage of the Oberth effect during your ejection burn.

Higher orbits tend to be better for a Duna trip because it requires a relatively slow ejection velocity. But for trips that require higher ejection velocities they are far more efficient to start at a higher velocity in low kerbin orbit to take advantage of the Oberth effect.

[Mr Shifty]

It is often more efficient to launch interplanetary transfers from high altitude for precisely the reason other commentors have said: it takes less energy to get to escape velocity from a high orbit. This is counterbalanced by the Oberth effect, which means that there is actually an optimal altitude for each transfer, which I helpfully tabulated here:

http://forum.kerbalspaceprogram.com/showthread.php/36615-Efficient-Hohmann-Transfer-Altitudes

[macmodder]

I have recently taken to getting in a 1,000,000m orbit of the mun, refuelling there. It only takes ~40m/s extra speed to escape kerbin. I do have a refuel twice though, but I'm getting better with practice

[tavert]

It is often more efficient to launch interplanetary transfers from high altitude for precisely the reason other commentors have said: it takes less energy to get to escape velocity from a high orbit.

That's not efficiency. That's energy you already have by putting it into potential energy, and you're pretending you never had to spend it because of refueling. Spending fuel to increase your potential energy is always wasteful.

[Mr Shifty]

That's not efficiency. That's energy you already have by putting it into potential energy, and you're pretending you never had to spend it because of refueling. Spending fuel to increase your potential energy is always wasteful.

I understand, but I don't. If I live on the 46th floor of the building, it takes less effort for me to get to the swimming pool on the 50th than it does for a guy coming from the lobby. I mean sure, I had to get myself there in the first place, and I had to lug all my food up there, but those are sunk costs by the time I start my climb from my front door. I don't start more tired than the guy in the lobby.

Would it make more sense if I said "It takes less fuel to launch interplanetary transfers from a high altitude..." rather than talk about efficiency?

[tavert]

Stretching the analogy a little too far here, but those don't have to be sunk costs since you had a choice of where to live - 46th floor was a bad choice, you had to bring all your furniture etc way higher than if you had picked closer to the ground. (And that's not really fair to call "home" since you don't actually start up there.)

Only if you are very clear that you're only considering the fuel cost starting from a given altitude to the interplanetary destination, and neglecting the (non-negligible) cost of getting to that altitude in the first place.

Edited June 20, 2013 by tavert

[Eric S]

Would it make more sense if I said "It takes less fuel to launch interplanetary transfers from a high altitude..." rather than talk about efficiency?

I think that's a little more accurate. I'd probably use the term depart rather than launch just to try to be more clear about what fuel you're interested in tracking.

What it comes down to is that if you're shipping all your fuel up from Kerbin, it will take more fuel total (including the fuel for getting the fuel into HKO) to depart from HKO than LKO, but the ship leaves the Kerbin SoI with more delta-v on board. If, on the other hand, you're mining kethane on Minmus, then LKO has less of an advantage.

Several months ago, before the forum rollback, we did a pretty thorough writeup on LKO refueling versus refueling in Minmus orbit. The conclusions were basically:

1) If you're not mining kethane, the only times refueling anywhere but LKO made sense was if your delta-v budget was insanely tight or if the ship that needed to be refueled was somewhere other than LKO and couldn't easily return to LKO, and neither of those cases were common enough to really worry about.

2) Departing from Minmus made sense if you had refueling facilities on Minmus and either were on a tight delta-v budget or you'd need to refuel anyway.

3) Departing directly from Minmus saved some post-refueling delta-v over departing from LKO, but there was an alternate strategy that did much better. Basically, burn away from Minmus like you're returning to Kerbin, keeping your periapsis at about 80km, then burning at periapsis to reach the necessary velocity for the transfer. The timing on this is tricky, but for a Duna transfer, it would mean using less than half as much post-refueling delta-v compared to refueling in LKO. Since that's a mostly linear savings, it's not going to make a radical difference if you're going to Moho, but again, if your delta-v budget is tight, it might be essential. If the delta-v budget isn't tight, it isn't worth doing it unless you're confident you can manage the transfer timing accurately, as course corrections can eat through your savings and then some if the timing is off.