Ideal orbit for refueling station

[michal.don]

Hi guys,

I would appreciate your opinions on my problem.

I'm currently building a refueling system for my ships and SSTOs on Kerbin orbit. The refueling ship is quite huge - about 100k capacity of LF + Ox. The fuel itself will be delivered to the ship from my mining and refining operation on Minimus, that is already operating. I have also already figured out the way to deliver the fuel to my "gas station" from Minimus.

The thing that remains uncetain to me is the ideal location of the refueling ship. How high/low should the orbit be? I've figured out some of the pros and cons of hing and low orbit:

LKO - pros: simplier rockets and SSTOs can reach the refueling station from surface of Kerbin, lower wastes of fuel when heading out of Kerbin SOI

cons: grater distance and fuel waste when transporting the fuel from Minimus to refueling ship.

I can't really decide which of the reasons above is the most important, so I have no idea whether to place the refueling ship at 75x75 orbit, or somewhere far out, for example higher than Mun.

What do you guys suggest?

Thanks,

michal.don

Edited September 30, 2015 by michal.don

[SpeedDaemon]

I prefer about 130km for my Kerbin stations.

- It allows 100x time warp to cut down on rendezvous time (cut-off is 120km)

- Low enough to reach with spaceplanes

- High enough that you can put your PE below it without hitting atmosphere to make rendezvous easier

[michal.don]

I prefer about 130km for my Kerbin stations.

- It allows 100x time warp to cut down on rendezvous time (cut-off is 120km)

- Low enough to reach with spaceplanes

- High enough that you can put your PE below it without hitting atmosphere to make rendezvous easier

That's the altitude that I prefer for my space stations as well - mine are at 125 and 150 km orbits. And I'm aware of the advantages of these orbits when it comes to rendezvous and docking. The thing I'm concerned about is the fuel transfering and transfer burns (when leaving Kerbin SOI) efficiency, that aren't involved whed building a space station.

[mattssheep4]

I personally prefer somewhere between an 80x80 and 100x100 km orbit, because there is room above and below its orbit for easier rendezvous. You can use aerobraking to save dV for deceleration from Minmus or other destinations. Usually SSTO's and other vehicles requiring refueling need fuel sooner after achieving orbit rather than later, so a lower orbit is better for that in my opinion. It also takes less time to rendezvous in a lower orbit because the vessels are moving faster closer to the planet.

[capi3101]

Folks will generally tell you that the lower your refueling stations are, the more the departing craft will be able to benefit from the Oberth Effect after the refueling operation is complete (TL;DR of the Oberth Effect - it saves you some delta-V in the long run, making your burn inherently more efficient).

Myself, I use a 100 km orbit; it's still low enough that it's easy to reach but not so low that you risk rendezvous maneuvers putting you in the atmosphere. I'd say for Kerbin 85-100 kilometers is ideal. By the same token, refueling stations in orbit of Mun and Minmus should be relatively close to their surfaces; I usually put them around 20-25 kilometers if I can at all help it (and I'm not yet convinced whether or not that's too high for Minmus).

[mattssheep4]

I usually put them around 20-25 kilometers if I can at all help it (and I'm not yet convinced whether or not that's too high for Minmus).

I never have Minmus stations any higher than 15 km.

[michal.don]

I've heard about the Oberth effect, but I still don't know how important that is - does it save 1, 10, 100 or 500 delta-V on a journey to Duna? But a good point with the aerobraking - I didn't think of that, and I believe that it helps save lots of fuel.

So it looks like my refueling station will float in a LKO,

thanks guys, you've been really helpful,

michal.don

[capi3101]

I've heard about the Oberth effect, but I still don't know how important that is - does it save 1, 10, 100 or 500 delta-V on a journey to Duna? But a good point with the aerobraking - I didn't think of that, and I believe that it helps save lots of fuel.

So it looks like my refueling station will float in a LKO,

thanks guys, you've been really helpful,

michal.don

Honestly, how much delta-V it saves is solely dependent upon how low the craft is at the time it makes the transfer burn. There was a site out there that explained the Oberth effect in layman's terms and did the math simply; I'll have to find the link again, and when I do I'll edit this post with the link. For now, I'll point you to the Wikipedia page on the subject, suck as that does. I do recall that the savings aren't terribly much - no more than a few hundred m/s, though so often a mission succeeds or fails for want of a few hundred more m/s of delta-V in this game...

I never have Minmus stations any higher than 15 km.

See, I was thinking 15 was plenty high my own self. I'll have to try that next time I put up a refueling depot up that way.

[OhioBob]

I've heard about the Oberth effect, but I still don't know how important that is - does it save 1, 10, 100 or 500 delta-V on a journey to Duna?

You can always use the Launch Window Planner to see what the ÃŽâ€v difference is between orbits of different altitudes. However, that's only half the story. You also have to consider that it takes more ÃŽâ€v to get into a higher parking orbit in the first place. For low Kerbin orbits (up to ~200 km), a ballpark approximation is that is takes about 1 m/s more ÃŽâ€v to get to Duna for every 1 km increase in the parking orbit altitude. For Jool it takes about 1.5 m/s per 1 km.

[FlyingPete]

A lower orbit will be more efficient for transferring fuel from Minmus to LKO, since you can aerobrake to shed most of the excess velocity. A lower orbit means less delta-V to bring the periapsis up out of the atmosphere at rendezvous. The lower altitude makes it simpler to build and operate a SSTO crew shuttle too. For Kerbin system operations, this is what I'd do.

However, if you're going interplanetary, it's worth considering that with a higher orbit, you can first make a retrograde burn with your spacecraft to lower your periapsis, and thus make use of the oberth effect as well as the higher initial energy you get from a higher orbit. Pros and cons to both approaches.

[michal.don]

So, I decided to make a little update for those curious about this, or even those thinking about building something similar.

I placed my refueling station in 85x83 km orbit, built a nuclear-powered tug for moving the refined fuel from low minimus orbit to the station and sent the first round of fuel from surface of minimus to orbit to meet the tug. The ships docked and I managed to do the transfer burn from Minimus to Kerbin orbit with periapsis about 60 km for aerobraking just for about 250 m/s of delta-V. So far, everything went great.

What I didn't expect was the ineffectiveness of aerobraking. I couldn't go too low with the periapsis so my ship didn't burn, so it took tremendous amount of time to slow down to a reasonable speed. As a matter of fact, I'm still not done - after about 15 orbits through Kerbin atmosphere my apoapsis is still above 2000 kms, and I lost one of my solar panels to heat because I got a little unpatient

I know that this is probably the most efficient way of getting fuel to Kerbin orbit, but it's just too time-consuming. Considering the fact that I would have to do four trips like this one to fill my refueling station, I give up on this approach and will probably do it the expensive way and build a monstrous rocket and get the refueling station to orbit already filled.

Anyway, thanks again for your advice,

michal.don

[FlyingPete]

One alternative is to have a refuelling depot in orbit of the Mun. You can launch a craft with just enough fuel to get there, then refill the tanks at the Mun and go interplanetary from there. Do a two-stage transfer burn that first ejects from Munar orbit and drops your Kerbin periapsis to a low altitude, then at that periapsis burn to eject from Kerbin. The interplanetary ship can be slightly smaller as you effectively start with an apoapsis at the Mun's orbit. You could work around Minmus directly, but the longer orbital period and inclination makes it a bit trickier to get a transfer set up.

[capi3101]

I wouldn't suggest a nuclear-propelled tug if you're staying within Kerbin's local system. I'd also suggest you use the protected solar panels - the kind that you can deploy and retract - and make sure they're retracted while you're in atmo (especially much below 45k; higher than that and you're generally okay to have them out, in my experience anyway). Make sure you go in butt-first - your rocket engine can generally take a great deal of heat before it tries to 'splode, and in my experience it's almost as effective as ablator at protecting the rest of your craft. Finally, how low is your aerobraking periapsis? Coming from either of Kerbin's moons, a Pe of between 32.5-35k is what you want; much lower than that and you're probably heading in, higher than that and it'll take an insane number of tries before you get low enough.

60k for aerobraking? Yeah - that's way too high.

[michal.don]

I wouldn't suggest a nuclear-propelled tug if you're staying within Kerbin's local system. I'd also suggest you use the protected solar panels - the kind that you can deploy and retract - and make sure they're retracted while you're in atmo (especially much below 45k; higher than that and you're generally okay to have them out, in my experience anyway). Make sure you go in butt-first - your rocket engine can generally take a great deal of heat before it tries to 'splode, and in my experience it's almost as effective as ablator at protecting the rest of your craft. Finally, how low is your aerobraking periapsis? Coming from either of Kerbin's moons, a Pe of between 32.5-35k is what you want; much lower than that and you're probably heading in, higher than that and it'll take an insane number of tries before you get low enough.

60k for aerobraking? Yeah - that's way too high.

Yes, I should have thought of the aerobraking before putting the Gigantor panels on my vessel - my bad Ok, thanks, I'll give it another try with another tranport vessel better suited for the job.

michal.don

[Goomblah]

I personally took a weird hybrid approach - it's not Mun orbit, it's not Minmus orbit, and it's not LKO, but it's close (as in 250-300m/s away) to all three.

My Kerbin Gateway Station is in an approximately 24x24Mm (24,000km) orbit - about twice the Mun's orbital altitude. This allows for exceptionally cheap transfers to and from Minmus every 10 days or so, as well as easier course plotting (no SoI change to deal with, no orbital inclination relative to Kerbin/Mun either)

Getting out there from LKO is a bit expensive - about as much as achieving orbit around the Mun or Minmus, but it's generally worth the extra design hassle for most craft. Getting fuel from Minmus to the station is super easy, and although the two-step interplanetary transfer (drop Pe to 80km or so, then do the burn) is a little trickier than just going from LKO, you actually save a significant amount of dV due to Oberth and all the potential energy you are storing in the higher orbit. This also makes things easier on low-TWR spacecraft, since the escape burn is smaller.

[magico13]

Yes, I should have thought of the aerobraking before putting the Gigantor panels on my vessel - my bad

Gigantors are retractable...

I'm doing something similar to you, but stupidly forgot about aerobraking and used the wrong solar panels (might swap them out with KIS). I can drop down to about 50km for aerobraking and supplement that with burns at periapsis to drop the speed even further. Once I swap out the panels I can drop that lower probably.

My station is around 85km. I've been using 80-100km for nearly every launch this play through.

[michal.don]

Gigantors are retractable...

Yes, I'm aware of that But even when retracted, they tend to burn rather quickly.....

Anyway, I just tested another design for the transporting purpose, and it behaves much better - I'm capable of aerobraking enough in just three orbits, which is much, MUCH better, so I might give this a go...

Thank you guys very much, you helped a lot.

michal.don