Refueling Stations, do they make sense in terms of D/V and/or ship complexity?

[WafflesToo]

The concept of a minimus refueling station is bizarre to me. Just put the station in LKO, you dock to it for the same dV +/- a few dozen m/s as getting into orbit, and then you do your transfer with maximum Oberth effect. What benefit is there to a minimus station?

Kethane mod lets you mine fuel from off Kerbin. Dragging it down to LKO costs more than transferring your mission material up to the mining site.

[ComradeGoat]

Kethane mod lets you mine fuel from off Kerbin. Dragging it down to LKO costs more than transferring your mission material up to the mining site.

But if you fly to Jool from Minmus, you need more delta Vs to get there than if you do a burn from LKO.

[Skorpychan]

That you have to spend more fuel, time and effort to hit?

Yes, but you don't hit it as hard as the Mun, due to lower gravity.

[BubbaWilkins]

But if you fly to Jool from Minmus, you need more delta Vs to get there than if you do a burn from LKO.

It requires just 360 DeltaV to launch from Minmus and Escape Kerbin's SOI.

It requires 930 to do the same from LKO (70km).

Ideally, you wouldn't even set a fuel station on Minmus, but rather put in orbit around it. Then you would use a fuel extractor to pick fuel from the surface and place it at the station for your crafts to top off.

Minmus actually requires less Delta V to land on than the Mun does.

KSP to Mun Surface requires minimum of 6,250 DeltaV

KSP to Minmus requires a minimum of only 5,770 DeltaV

This chart is awesome: http://forum.kerbalspaceprogram.com/threads/41652-A-more-accurate-delta-v-map

Edited January 24, 2014 by BubbaWilkins

[ComradeGoat]

It requires just 360 DeltaV to launch from Minmus and Escape Kerbin's SOI.

It requires 930 to do the same from LKO (70km).

Most of the delta V to Jool comes *after* you reach Kerbin escape velocity.

Seriously, try it: put a ship in a solar orbit, just beyond Kerbin's SOI and an identical one at LKO, both fully fuelled. Do a direct burn to Jool with both; the one starting in LKO will arrive with more fuel left.

The closer to Kerbin periapsis you are, and the lower that periapsis is, the more each unit of fuel burned will raise your aphelion. You could try to use the Oberth Effect at Minmus, but it's tiny in comparison. Starting from there is sabotaging you, not helping you.

Edit:

I've just done it myself. Two identical ships, one in LKO (just above 100km), one in orbit around Minmus (20km, ish), Hohmann transfer to escape Kerbin's SOI and raise aphelion to Jool's orbit.

From Minmus orbit: 2365 m/s

From LKO: 1980 m/s

Conclusion: Refuelling at Minmus for a Jool transfer is not merely wasted effort; it's actually counterproductive.

Edited January 24, 2014 by ComradeGoat

[SuperBigD60]

(10 characters-ish)

Edited January 24, 2014 by SuperBigD60
redundant

[ComradeGoat]

I think he's saying it takes more dV to burn from LKO to Minmus, then to Jool, than just burning from LKO to Jool

makes sense to me, though I imagine the difference is minimal

No I'm not. See my post above. It takes more delta V to burn from Minmus to Jool than it does from LKO to Jool, even before you take into account the fuel burned to get from LKO to Minmus.

Starting from both, with a fully fuelled ship, the difference is about 400-500m/s (in LKO's favour). If you count the fuel taken to get to Minmus as well, then LKO wins by about 1,500 m/s.

Hardly minimal! This is counterintuitive, I grant but it is correct. You can easily check it yourself using manoeuvre nodes.

And it's "she", BTW.

[SuperBigD60]

No I'm not. See my post above. It takes more delta V to burn from Minmus to Jool than it does from LKO to Jool, even before you take into account the fuel burned to get from LKO to Minmus.

Starting from both, with a fully fuelled ship, the difference is about 400-500m/s (in LKO's favour). If you count the fuel taken to get to Minmus as well, then LKO wins by about 1,500 m/s.

Hardly minimal! This is counterintuitive, I grant but it is correct. You can easily check it yourself using manoeuvre nodes.

And it's "she", BTW.

My apologies, milady.

You apparently type faster than me and I hadn't noticed your comment.

The effect you meantioned was on my mind, and I did agree with you before you proved it, for what its worth.

[ComradeGoat]

My apologies, milady.

You apparently type faster than me and I hadn't noticed your comment.

The effect you meantioned was on my mind, and I did agree with you before you proved it, for what its worth.

No problem, I edited it in afterwards :-)

[Mobjack]

Refueling stations are always a waste unless they are at your destination planet to begin with.

If you make the effort to go to another planet on route to your destination, you should get a gravity assists which is free dV.

Refueling at the planet requires slowing down to be captured, then speeding up again to your original speed again which is a huge waste.

[WafflesToo]

It's hard to wrap your head around isn't it? XD But Minmus' local orbital velocity of 274m/s gets totally dominated by Kerbin's SOI velocity moving around Korbol at 9, 284 m/s.

[WafflesToo]

But if you fly to Jool from Minmus, you need more delta Vs to get there than if you do a burn from LKO.

Very true and you are all very correct that for a one-shot mission it's wasteful... However, if you are operating a small fleet of persistent ships that are being reused from one mission to the next then where you put your stations matters. Yes, it costs make dV to depart from Minmus,. But it costs fewer tons than having to haul fuel down from Minmus to LKO.

[magnemoe]

Refueling stations are always a waste unless they are at your destination planet to begin with.

If you make the effort to go to another planet on route to your destination, you should get a gravity assists which is free dV.

Refueling at the planet requires slowing down to be captured, then speeding up again to your original speed again which is a huge waste.

My 100 ton from orbit grand tour was based on this, I went from Eve to Duna, here I split the ship, the unmanned lower stage went directly to Jool, only the manned lower stage entered low orbit sent down an lander, continued to Ike then Dress and finally Jool.

I managed to get down to 100 ton by letting most of the fuel skip a lot of expensive burns.Yes I could saved more by sending fuel directly to Jool and Duna but that would not make it an grand tour

Now sending spare fuel to Jool if you want to explore all the moons make a lot of sense. Taking the large mothership into low orbit around all the moons does not.

[ComradeGoat]

Very true and you are all very correct that for a one-shot mission it's wasteful... However, if you are operating a small fleet of persistent ships that are being reused from one mission to the next then where you put your stations matters. Yes, it costs make dV to depart from Minmus,. But it costs fewer tons than having to haul fuel down from Minmus to LKO.

Sounds like a potential use case for a fuel station at Minmus, but bear in mind in making the logistics easier by not hauling fuel around kerbin's SOI, your persistent interplanetary ships are going to have to be larger because getting most places from Minmus requires more fuel than getting there from LKO, and getting back to Minmus to refuel is a bit more expensive too (not much though because the heavy lifting is mostly done by aerobraking there).

Still seems like a no-brainer to me. If you want to mine fuel at Minmus, haul it to a station in LKO and use that as your interplanetary refuelling depot, then you only need the capacity in one ship: your fuel tanker, rather than all of them. It's not going to hurt much either as you can aerobrake to LKO, and you'll be returning to Minmus almost empty, so one nuke will give you decent TWR and burn not much fuel.

Now what about Jool? I think for much the same reasons, you want your fuelling station at Laythe, and your mining operation at Vall (although IME Laythe aerobrakes are always a bit fraught). Must get round to giving Kethane a try sometime. It sounds like a lot of fun.

[WafflesToo]

Sounds like a potential use case for a fuel station at Minmus, but bear in mind in making the logistics easier by not hauling fuel around kerbin's SOI, your persistent interplanetary ships are going to have to be larger because getting most places from Minmus requires more fuel than getting there from LKO, and getting back to Minmus to refuel is a bit more expensive too (not much though because the heavy lifting is mostly done by aerobraking there).

Still seems like a no-brainer to me. If you want to mine fuel at Minmus, haul it to a station in LKO and use that as your interplanetary refuelling depot, then you only need the capacity in one ship: your fuel tanker, rather than all of them. It's not going to hurt much either as you can aerobrake to LKO, and you'll be returning to Minmus almost empty, so one nuke will give you decent TWR and burn not much fuel.

Now what about Jool? I think for much the same reasons, you want your fuelling station at Laythe, and your mining operation at Vall (although IME Laythe aerobrakes are always a bit fraught). Must get round to giving Kethane a try sometime. It sounds like a lot of fun.

Actually I did get to rethinking that a bit after posting. I'll have to run the math to see which way burns fewer tons, hauling the kethane down to my LKO station first, or having the mission leave from the Minmus station.

[magnemoe]

Note that if you plan an extended Mun mission, with multiple landings it make a lot of sense to put fuel and extra science equipment in low Mun orbit, dock and grab more goo containers and labs before landing again. Here you put the fuel at the target.

Kethane breaks this too, with Kethane it makes far more sense to reuse stuff.

[palioxis1248]

One use for a refuelling station is exploiting the Oberth effect

1. Have a station scraping the edge of Kerbin's SOI in a circular orbit and keep it filled.

2. Have your vessel rendezvous with said station instead of burning to leave Kerbin.

3. Top up tanks, then burn to reduce periapsis to typical LKO altitudes. <-- This step works because orbital velocities at the edge of the Kerbin system are insanely low.

4. Exploit the Oberth effect at periapsis to go further than you could go just by boosting from LKO with a full ship.

I haven't tried this out yet, but all theories indicate it should work.

Possible delta-v equivalent variants of this are left as an exercise to the reader.

[Mitchz95]

One use for a refuelling station is exploiting the Oberth effect

1. Have a station scraping the edge of Kerbin's SOI in a circular orbit and keep it filled.

2. Have your vessel rendezvous with said station instead of burning to leave Kerbin.

3. Top up tanks, then burn to reduce periapsis to typical LKO altitudes. <-- This step works because orbital velocities at the edge of the Kerbin system are insanely low.

4. Exploit the Oberth effect at periapsis to go further than you could go just by boosting from LKO with a full ship.

I haven't tried this out yet, but all theories indicate it should work.

Possible delta-v equivalent variants of this are left as an exercise to the reader.

Wouldn't that be kind of imprecise, though?

[palioxis1248]

I don't quite follow. Imprecise?

[ComradeGoat]

I don't quite follow. Imprecise?

The main problem with this is that your orbit round Kerbin takes so long that you will not likely line up your launch window with the burn to lower periapsis.

[Mobjack]

One use for a refuelling station is exploiting the Oberth effect

1. Have a station scraping the edge of Kerbin's SOI in a circular orbit and keep it filled.

2. Have your vessel rendezvous with said station instead of burning to leave Kerbin.

3. Top up tanks, then burn to reduce periapsis to typical LKO altitudes. <-- This step works because orbital velocities at the edge of the Kerbin system are insanely low.

4. Exploit the Oberth effect at periapsis to go further than you could go just by boosting from LKO with a full ship.

I haven't tried this out yet, but all theories indicate it should work.

Possible delta-v equivalent variants of this are left as an exercise to the reader.

Why not just have a fuel station at LKO obit?

It is going to costs fuel getting a fuel station to the edge of Kerbin's SOI. The orbital velocity at the edge of the Kerbin's SOI is low but it is no trivial either. It would be more efficient and practical having a fuel station at LKO instead.

[WafflesToo]

Wouldn't that be kind of imprecise, though?

I see what Palioxis is trying to do. It would just be a matter of planning a little more in advance. If I remember right it will take the ship about six days to fall from SOI limit back to 75km above Kerbin so you would want to time your departure to be six days early... of course there could be a big problem getting your departure angles to line up. I suppose a solution to the problem is six-or-so stations in a ring and just pick the one you need in advance... which would involve (you guessed it) even more advanced planning.

Some people really enjoy the minutia of the game and if that's their thing then who am I to tell them it's wrong? Personally, that's getting a bit too involved for my tastes

[WafflesToo]

Why not just have a fuel station at LKO obit?

It is going to costs fuel getting a fuel station to the edge of Kerbin's SOI. The orbital velocity at the edge of the Kerbin's SOI is low but it is no trivial either. It would be more efficient and practical having a fuel station at LKO instead.

Because palioxis is trying to maximize the Oeberth effect by burning at absolute maximum velocity. I don't think you can get much more maximized than falling from the edge of Kerbin's SOI down to atmospheric scraping altitudes

Getting the ejection angle to line up would be difficult, but it would be a heck of a stunt if you could pull it off

[WafflesToo]

Note that if you plan an extended Mun mission, with multiple landings it make a lot of sense to put fuel and extra science equipment in low Mun orbit, dock and grab more goo containers and labs before landing again. Here you put the fuel at the target.

Kethane breaks this too, with Kethane it makes far more sense to reuse stuff.

Kind of what I've been trying to do with my current playthrough of the game. I haven't started exploiting Kethane yet (it's coming) but I have a fuel tanker that can get just over 5-1/2 tons of fuel into LKO for very little cost (less than half what a single one-shot Mun mission would cost). It will become even more attractive once I get a Kethane operation up-and-running.

[WafflesToo]

Actually I did get to rethinking that a bit after posting. I'll have to run the math to see which way burns fewer tons, hauling the kethane down to my LKO station first, or having the mission leave from the Minmus station.

Just did the math and departing to Moho (which I think is the second- or third-most distant point from Kerbin... energetically speaking) from Minmus's orbit (didn't include Minmus's Oeberth effect to simplify the calculation) costs 2090 m/s of dV.

Departing directly from LKO on the other hand costs 1670 m/s of dV; saving the mission 320 m/s of dV.

But hauling fuel down to LKO from Minmus' orbit costs 230m/s to drop down to a 32km aerobrake and another 160 m/s to get the orbit circularized at my space-station's altitude. Already up to almost 400m/s without taking any orbital maneuvering into account.

Without looking too much farther into it I'm thinking that it still makes sense to just go ahead and leave from Minmus, accepting the additional mission dV since the extra fuel will be burnt on your exit burn anyway and its mass will (largely) be gone from the mission; troubling it little more for the remaining burns it would have to make.

Of course this assumption breaks a little as mission mass goes up... And this only works for interplanetary missions. For local missions (such as Munar exploration) it makes far more sense to have a local fuel source and not leave its SOI at all.

Edited January 25, 2014 by WafflesToo