Orbital mechanics - Launching interplanetary missions from Minmus

[Traches]

Minmus is a great place to mine kethane, what with its super low surface gravity and all, so that's where my mining base is set up. Now, I could move that fuel to low kerbin orbit and launch interplanetary missions from there, but it seems like a waste to haul it so far down kerbin's gravity well just to haul it all back out again.

If the timing is right, minmus is a great place from which to leave the Kerbin system, seeing as it's so close to the edge of the SOI anyway. What I'm not sure on is what to do when the timing is wrong-- say the window for a given transfer puts my trajectory straight through the middle of the system, and by the time minmus comes around in its orbit the window is long gone.

I see a couple solutions, but I'm not using any math-- just intuition. I could see burning just outside of kerbin's SOI, with enough relative velocity to be clear of the planet before the transfer window opens. If you've got the time, you can do this when minmus is moving in the planetary prograde or retrograde direction, and save yourself some delta-v. I could also see setting up a slightly different orbit from minmus- one that stays within kerbin's SOI but will put you on the correct side of the planet when the window arrives. Finally, I could see heading down kerbin's gravity well, then doing a burn at periapse to slingshot yourself out of the system in the direction you need. This one seems like it would be the most difficult to plan and execute.

I have no idea which would be the most efficient Delta-V wise. Thoughts from those more informed than I are welcome!

Edited September 9, 2013 by Traches

[Captain Sierra]

What I would do if I were you is run the kethane op from minmus with a regular supply line from a kerbin refueling station at mid alt (250-300km thereabouts). Orbit the interplanetary ship empty and then fuel it from the station. Then you can refuel all your kerbin missions with supply runs from Minmus.

My kethane operations are like this. Based on the Mun, but with the massive supply runs coming from Minmus infrequently and much smaller, more often runs from the Munar surface, with all conversion done on the surface right after drilling. Then I ship the fuel back down to one of my kerbin refueling stations.

[Decent Weasel]

Being bonkers and a bit amateurish I just designed all of my interplanetary ships to dispatch their own kethane mining systems, whether through a lander or through putting down the entire ship. This did have the advantage of some weight savings, letting me carry just enough fuel to get to my destination, then refine fuel on-site for the trip home. I'm sure there's a perfectly good reason not to do this...

[Zeroignite]

I use extraplanetary launchpads for my big ships, launching everything from Minmus. What I've found to be the best strategy is to do a minimum delta-v Minmus/Kerbin escape about a week before the launch window, then hohman to wherever I want to go from solar orbit. It's easy and saves a ton of fuel relative to dropping back down to LKO.

[Eric S]

I've played around with this a lot, and have generally found the most fuel efficient way to do an interplanetary transfer from Minmus is to depart Minmus in the retrograde direction, burning your periapsis down to about 80-100km, and then doing your transfer planet at your periapsis. The thing is, the timing of this can be a bit annoying. You've got a wide margin of error on your planetary phase angle, but your ejection angle is more critical, especially when most of your ejection velocity is in place before your ejection burn. Because of the tight timing margins on this, I strongly recommend quicksaving before you start your burn to depart Minmus, at least the first time or two.

Boosting straight out from Minmus either directly into a transfer or out into solar SoI and then doing a hohman are both less fuel efficient, but easier to do.

[Traches]

What I would do if I were you is run the kethane op from minmus with a regular supply line from a kerbin refueling station at mid alt (250-300km thereabouts). Orbit the interplanetary ship empty and then fuel it from the station. Then you can refuel all your kerbin missions with supply runs from Minmus.

My kethane operations are like this. Based on the Mun, but with the massive supply runs coming from Minmus infrequently and much smaller, more often runs from the Munar surface, with all conversion done on the surface right after drilling. Then I ship the fuel back down to one of my kerbin refueling stations.

I thought about that, but I'd like to find the smart answer rather than the hard answer!

Being bonkers and a bit amateurish I just designed all of my interplanetary ships to dispatch their own kethane mining systems, whether through a lander or through putting down the entire ship. This did have the advantage of some weight savings, letting me carry just enough fuel to get to my destination, then refine fuel on-site for the trip home. I'm sure there's a perfectly good reason not to do this...

Might do that too!

I use extraplanetary launchpads for my big ships, launching everything from Minmus. What I've found to be the best strategy is to do a minimum delta-v Minmus/Kerbin escape about a week before the launch window, then hohman to wherever I want to go from solar orbit. It's easy and saves a ton of fuel relative to dropping back down to LKO.

I've been looking at that mod, but I'm afraid it'll make things too easy. As far as the escape a week ahead, that's definitely one way I was looking at doing it. If the timing works out I'll probably just do that.

I've played around with this a lot, and have generally found the most fuel efficient way to do an interplanetary transfer from Minmus is to depart Minmus in the retrograde direction, burning your periapsis down to about 80-100km, and then doing your transfer planet at your periapsis. The thing is, the timing of this can be a bit annoying. You've got a wide margin of error on your planetary phase angle, but your ejection angle is more critical, especially when most of your ejection velocity is in place before your ejection burn. Because of the tight timing margins on this, I strongly recommend quicksaving before you start your burn to depart Minmus, at least the first time or two.

Boosting straight out from Minmus either directly into a transfer or out into solar SoI and then doing a hohman are both less fuel efficient, but easier to do.

I had a hunch the burn at periapse would be most efficient. I could see making small prograde/retrograde adjustments while you're inbound to LKO to fine tune the timing of your arrival, but that would involve a fair amount of setting and resetting maneuver nodes! The challenge will be fun though, I'm gonna try it a few times.

[magnemoe]

I've played around with this a lot, and have generally found the most fuel efficient way to do an interplanetary transfer from Minmus is to depart Minmus in the retrograde direction, burning your periapsis down to about 80-100km, and then doing your transfer planet at your periapsis. The thing is, the timing of this can be a bit annoying. You've got a wide margin of error on your planetary phase angle, but your ejection angle is more critical, especially when most of your ejection velocity is in place before your ejection burn. Because of the tight timing margins on this, I strongly recommend quicksaving before you start your burn to depart Minmus, at least the first time or two.

Boosting straight out from Minmus either directly into a transfer or out into solar SoI and then doing a hohman are both less fuel efficient, but easier to do.

I did some testing during my grand tour, this was from Ike to dress but the setting is pretty similar.

Found that the cost was 2500 m/s with sol escape first however if I entered Duna orbit with Pe at 80 and Ap just inside Ike possible SOI the cost was reduced to 1500 m/s, both to low Dress orbit.

I let mechjeb find the launch time and burn angle of burn and that worked well. Note that you get problems if if your orbit is wrong aligned, so as Eric says do an quick save.

[Van Disaster]

I've been looking at that mod, but I'm afraid it'll make things too easy. As far as the escape a week ahead, that's definitely one way I was looking at doing it. If the timing works out I'll probably just do that.

You have to mine and move all your own raw materials, process those into other materials and move that to the pad, fuel the ship on the pad, and ship your crew up there - I'm not exactly sure how that's easier than the infinite stockpile at KSC, even if you're avoiding most of the gravity well.

I haven't actually tried an interplanetary launch from Minmus, but I'd have thought dropping into Kerbin SoI & using Kerbin itself to point you in the right direction would be simplest ( and not too complicated if you play with maneuver nodes enough ); changing your orbit when you've just left Minmus SoI is really cheap, even if you want to leave the system the opposite side of Kerbin. In the latter case I think I'd do my transit burn in the Sun's SoI though.

[Dangerous_Beans]

you don't have to drop down into Kerbin SOI, if you raise your orbit you can escape Minimus SOI while still staying in Kerbin SOI (burning prograde from the far side of minimus should do this). going down the Kerbin gravity well just seems like a waste of fuel in this scenario.

the orbital periods are starting to get long out there, but the furthest off you can be from a launch window is about 7 days. i'm not sure if that is enough to cancel out the fuel savings from launching from so high.

[Eric S]

you don't have to drop down into Kerbin SOI, if you raise your orbit you can escape Minimus SOI while still staying in Kerbin SOI (burning prograde from the far side of minimus should do this). going down the Kerbin gravity well just seems like a waste of fuel in this scenario.

It's not a waste, the Oberth effect means massive differences between a Minmus burn and a barely-above-kerbin's-atmosphere burn. I'll run a few missions to measure the difference when I get the chance.

[futrtrubl]

Mining on Minmus and shipping back to LKO is not all that fuel inefficient. dV wise the biggest burn is up from LKO to Minmus, but considering you are only sending your empty tanker ship that's not whole lot of fuel. On the return when your tanker is full and heavy you are able to aerobrake back into LKO and save a ton of fuel there.

According to the deltaV map:

Empty:

LKO -> Minmus -> Capture -> Landing

920 + 80 + 240 = 1240 so only enough fuel for about 1400-1500dV while empty needed (margin for screw ups/inclination changes etc)

Full:

Launch -> Escape -> Circularize in LKO

240 + 180 + 100 = 520 so only enough fuel for about 700dV while full needed.

Also you only need to size you fuel tanks to carry 1500dV while empty or 240dV when full, which ever is the greater amount of fuel and that depends on your ship and how much Kethane it can carry, if you carry a converter with you (since you can convert and refuel between burns on the return leg).

I recommend shuttling Kethane and doing the major conversion at your fuel base since, if you use the heavy converter there, you are able to transport less mass as currently the LFO created is heavier than the Kethane it is made from and you are also able to convert it to other things as the need arises.

[Zeroignite]

I recommend shuttling Kethane and doing the major conversion at your fuel base since, if you use the heavy converter there, you are able to transport less mass as currently the LFO created is heavier than the Kethane it is made from and you are also able to convert it to other things as the need arises.

Bad advice. Kethane is less dense then LFO, so you waste extra mass in fuel tanks. Additionally, you lose some mass during the conversion process.

[Captain Sierra]

Mining on Minmus and shipping back to LKO is not all that fuel inefficient. dV wise the biggest burn is up from LKO to Minmus, but considering you are only sending your empty tanker ship that's not whole lot of fuel. On the return when your tanker is full and heavy you are able to aerobrake back into LKO and save a ton of fuel there.

According to the deltaV map:

Empty:

LKO -> Minmus -> Capture -> Landing

920 + 80 + 240 = 1240 so only enough fuel for about 1400-1500dV while empty needed (margin for screw ups/inclination changes etc)

Full:

Launch -> Escape -> Circularize in LKO

240 + 180 + 100 = 520 so only enough fuel for about 700dV while full needed.

Also you only need to size you fuel tanks to carry 1500dV while empty or 240dV when full, which ever is the greater amount of fuel and that depends on your ship and how much Kethane it can carry, if you carry a converter with you (since you can convert and refuel between burns on the return leg).

I recommend shuttling Kethane and doing the major conversion at your fuel base since, if you use the heavy converter there, you are able to transport less mass as currently the LFO created is heavier than the Kethane it is made from and you are also able to convert it to other things as the need arises.

For me, this is in fact the most efficient way because I service my entire kerbin operations, not just interplanetary missions. So most of the time, it has to get dragged down to mid-ish kerbin orbit anyways.

[futrtrubl]

Bad advice. Kethane is less dense then LFO, so you waste extra mass in fuel tanks. Additionally, you lose some mass during the conversion process.

While Kethane is less dense (per unit, not per volume) everything else you said is incorrect.

You actually SAVE mass in tanks. One KL-TL80 (the largest kethane tank, holds 16000 units or 32T) which has a dry mass of 3.25T fills one Jumbo 64 LFO tank ( the large orange tank) and one FLT-100 ( the small striped tank) which together have a dry mass of 4.0625. So you save 0.8125 in dry mass alone by hauling kethane instead of LFO.

Secondly, like I said in my first post, you GAIN mass in the conversion process, the 32T of kethane becomes 32.5T of LFO saving you 0.5T of mass you need to haul.

So in total for each large Kethane tank you haul instead of the LFO tanks to carry it's equivalent you save 1.3125T, thereby decreasing the amount of fuel used to haul it and therefor delivering an even greater percentage of your cargo.

Edward

Edited September 12, 2013 by futrtrubl

[Geschosskopf]

It's not a waste, the Oberth effect means massive differences between a Minmus burn and a barely-above-kerbin's-atmosphere burn. I'll run a few missions to measure the difference when I get the chance.

I'm missing something here.....

I have always been under the impression that the purpose of the interplanetary transfer burn was to get your speed up high enough to escape the SOI you're currently in. Such being the case, it seems to me the best place to do the transfer burn is where you already have the highest possible orbital velocity within the current SOI, so the burn has to add the least amount of speed. To me, that means being as close to the central body of the SOI as possible. Thus, I've always assumed that leaving direct from Minmus would required more delta-V than leaving from LKO, because your orbital velocity is lower out at Minmus' distance. Is this not the case?

Now, diving down from Minmus to a low Kerbin Pe would give you higher starting speed. However, you still have to account for the delta-V needed to get out to Minmus in the 1st place, and that makes it a wash. For example, suppose you ignore the position of Minmus and do a 2-step transfer burn starting at LKO. In the 1st step, you go into an elliptical orbit out as far as Minmus, except Minmus isn't there. Then you coast around this orbit and when you get back to Kerbin, you do step 2 of the burn, finishing it off. This will require the same amount of delta-V as doing the burn all in 1 go from LKO, because Oberth does not give you any free speed increase if you're already in a closed orbit around the body in question.

All Oberth is doing here is keeping your orbit elliptical. If you don't do step 2 of the burn, you just go back around the same elliptical orbit you established in step 1. If Oberth was actually giving you free speed, you wouldn't have to do step 2 at all. Instead, each time you passed your Pe, it would be like burning prograde there, so your Ap would get higher and higher each orbit until eventually you escaped. But this doesn't happen, as seen by the fact that comets have constant, instead of increasing, orbital periods .

So, the only way to pick up Oberth speed from Kerbin towards your eventual escape is to already be on an escape trajectory (hyperbolic, open orbit) when you pass Kerbin. That pretty much rules out doing this from a starting point anywhere within the Kerbin system. You'd have to be coming into the system from Eve or Moho, like how the Voyagers were doing at each planet they flew by.

[Eric S]

I'm missing something here.....

I think it's more that we're not measuring the same thing.

Thus, I've always assumed that leaving direct from Minmus would required more delta-V than leaving from LKO, because your orbital velocity is lower out at Minmus' distance. Is this not the case?

This is in fact the case.

Now, diving down from Minmus to a low Kerbin Pe would give you higher starting speed. However, you still have to account for the delta-V needed to get out to Minmus in the 1st place, and that makes it a wash.

Worse than a wash, actually, if you spend delta-v to circularize at Minmus. However, here's where we got to the part where we're not measuring the same thing. The OP is talking about running a kethane-based refueling operation on Minmus, so I don't think he's interested in total delta-v expenditures, but rather the maximum delta-v expenditure between the pre-refueling phase and post-refueling phase. In that case, there's less delta-v expended departing Minmus with a low-kerbin periapsis where you do the rest of your transfer burn.

This usually beats departing directly from LKO by at least 400 delta-v, if you're not counting the delta-v to get to the refueling station against that maneuver, which means that as long as your post-refueling-phase delta-v budget is higher than your pre-refueling-phase delta-v budget, you will be able to get by with a planetary transfer stage that has that much less delta-v (or the same stage, but with a higher safety margin). I think that for most planetary transfers, the delta-v savings are closer to 600 delta-v, but it's been long enough since I've done this that I'm not certain I'm remembering it accurately. The last time I did this was back in 0.18.2.

If you're looking at the total delta-v expenditure, then yes, this is more costly than refueling at LKO or departing directly from there, depending on whether you needed to use your transfer stage to circularize. If you hadn't spent the delta-v to capture into a Minmus orbit then leave it, then the delta-v expenditure would look like a regular two-kick departure, so whatever delta-v you used on those two Minmus-related maneuvers would be an extra cost.

[Geschosskopf]

Worse than a wash, actually, if you spend delta-v to circularize at Minmus. However, here's where we got to the part where we're not measuring the same thing. The OP is talking about running a kethane-based refueling operation on Minmus, so I don't think he's interested in total delta-v expenditures, but rather the maximum delta-v expenditure between the pre-refueling phase and post-refueling phase. In that case, there's less delta-v expended departing Minmus with a low-kerbin periapsis where you do the rest of your transfer burn.

OK, I get what you're trying to do but it seems to me that getting such a savings would be a rare occurrence. To start with, unless your Kerbin Pe is exactly on the ejection angle to the target, you have 3 choices. Either:

1. you make a burn en route between Minmus and Kerbin to move the Pe to the correct place, or
   
2. you circularize at Kerbin so you can ride the low orbit around to the correct ejection angle (which defeats the whole purpose of using Minmus), or
   
3. you settle for doing the ejection burn before or after Pe (probably after if Pe is on the edge of the air to begin with).
   

All of these add delta-V costs. And the only way to avoid them is if at T-3 days (or whatever the travel time from Minmus to Kerbin is), Minmus happens to be on the direct line between where Kerbin will be on ejection day and the interception point at the target's orbit.

On top of this, Minmus' orbit is inclined enough to be a bother and it's unlikely that it'll be on an ascending or descending node when you leave it. The further off plane it is, the more up or down you'll need to add to your burn (or save for a midcourse correction) to actually hit the target. It seems to me that these 2 factors would eat up most of any savings in radial delta-V you get from going to such trouble.

[Eric S]

OK, I get what you're trying to do but it seems to me that getting such a savings would be a rare occurrence. To start with, unless your Kerbin Pe is exactly on the ejection angle to the target, you have 3 choices.

It takes some trial and error to get the periapsis in the right place, but it's not as hard as you make it sound. Your orbit around Minmus is fast enough that you have a good bit of flexibility, waiting an extra orbit changes where the periapsis would be just over a degree, which is well within the margin of error if you're trying to eyeball the ejection angle, and not too far off even if you're using protractor or the like. The big exception is Moho, which has a very narrow transfer window.

I've done this more than once, and while there are losses due to having a less than perfect periapsis and inclination, I have yet to have a planetary transfer that doesn't come in at least 300 delta-v below what it would take for a transfer from LKO, even after course corrections, assuming I get everything close to right. If I blow the periapsis timing, I can easily spend twice what I "saved" with midcourse corrections, which is why I always suggest quicksaving before attempting this.

[Dangerous_Beans]

Now, diving down from Minmus to a low Kerbin Pe would give you higher starting speed. However, you still have to account for the delta-V needed to get out to Minmus in the 1st place, and that makes it a wash. For example, suppose you ignore the position of Minmus and do a 2-step transfer burn starting at LKO. In the 1st step, you go into an elliptical orbit out as far as Minmus, except Minmus isn't there. Then you coast around this orbit and when you get back to Kerbin, you do step 2 of the burn, finishing it off. This will require the same amount of delta-V as doing the burn all in 1 go from LKO, because Oberth does not give you any free speed increase if you're already in a closed orbit around the body in question.

i thought the Oberth effect was due to the preservation of momentum; if your rocket is moving quickly then the propellent has more kinetic energy, cause it's also moving, and that energy has to go somewhere. it's just relative to the speed of the rocket, regardless of the system the rocket is part of.

put another way, i can't see why this would occur in a parabolic escape orbit, but not in a highly eccentric orbit. either way, your velocity is still highest at the periapsis so the fuel will do extra work because it has more kinetic energy.

I'm probably missing something, i haven't studied physics in a decade.

[Talonsin]

While it may not be the most efficient route, having my interstellar ships stop at "MoonBase-Minmus-1" before heading out into the deep black is awfully cool.

[Garek]

because Oberth does not give you any free speed increase if you're already in a closed orbit around the body in question.

Oberth never gives you free speed, the Oberth Effect just means it's more efficient to burn fuel at the periapsis of an eccentric orbit (or a flyby) than at it's apoapsis or in a circular orbit. The "free-speed-effect" of a flyby is called gravity slingshot, I think.

[Traches]

While it may not be the most efficient route, having my interstellar ships stop at "MoonBase-Minmus-1" before heading out into the deep black is awfully cool.

Fact. After all, way cool is at least 3/4 of the whole point!

[JiWint]

if you have to travel to moho,or eve,wait until minmus is closest to the sun,and burn minmus prograde.