Most efficient way from Minmus-refueling-station to interplanetary?

[Magzimum]

I have a mining station on Minmus. And I have the ambition to do a bunch of interplanetary missions in the near future. What's the recommended way to use the Minmus fuel, without making Mr. Oberth upset too much (see fig 1)?

I see a bunch of options, but none seem easy or efficient. If I have a fully fueled mothership at Minmus, I could:

• (1) Fly back to LKO. Advantage is that launch is trivial. Disadvantage is that I lose a LOT of fuel to get down from Minmus, and lower both Ap and Pe.
• (2) Adjust my plane to Kerbin's equator, then lower only my Pe to 80-100 km, and launch for interplanetary at Pe. A lot more efficient than option 1, but much more difficult to plan, especially when I need to get an intercept with one of the smaller planets (I didn't forget you, Dres).
• (3) Just fly off from Minmus, straight out of the Kerbin SoI. By far the easiest to plan, because the whole interplanetary intercept is planned when you're on a solar orbit. This seems to make Mr. Oberth really upset though. Major disadvantage is that the extra fuel needed has to be taken outside of the Kerbin SoI, so if you want to drop those extra tanks, they are lost and cannot be recycled/refueled. This in turn means that the refueling business now also includes an extra launch from Kerbin for new tanks.

If I have a tanker, I could:

• (4) Fly the fuel to LKO. I could repeat this as often as necessary, since the fuel is essentially for free. It would either mean lots of flights (my current tanker can deliver "only" about 20000 units of liquid fuel to LKO in 1 flight, but I have some ambitious plans). This is boring and repetitive, and downscaling the payload is not an option I want to consider in this thread.
• (5) Launch a new tanker, which is even larger, possibly including an upgraded mining station. Fly the huge tanker to LKO, and refuel whatever needs refueling. Costly (but possibly the most Kerbal way, and already my preferred option).

Of course, a combination of the options above is possible too. For those using Minmus (or Mun) bases for refueling, how do you approach this issue?

Figure 1, Mr. Oberth, already slightly upset with all of us getting fuel so cheaply from Minmus (source)

 

[Superfluous J]

I have always done (2) above. It took 3-4 times to really get the process down because things (generally, Mun-shaped things) tripped me up a lot. (3) though isn't as inefficient as you think it is if you do all the burning to your destination from Minmus, instead of burning out to Sun orbit and then burning from there to your destination.

(1) is terrible. Never ever even consider it as an option.

Edited September 14, 2016 by 5thHorseman

[Magzimum]

Regarding (3), I tried to burn to Jool from Minmus directly, but the burn time was in the same order as the time of 1 entire orbit... so I was just really confused where to start the burn.

And I could not really do the burn over multiple orbits, because I was out of Minmus' SoI after just a few hundred m/s acceleration... so there was no 2nd orbit. Burning from Minmus' orbit seems to work only with LFO, so that you can keep the burn time short. Right now, that's not much of an option, since I just got myself a few nerv-based ships that I am quite fond of.

[Superfluous J]

10 minutes ago, Magzimum said:

Regarding (3), I tried to burn to Jool from Minmus directly, but the burn time was in the same order as the time of 1 entire orbit... so I was just really confused where to start the burn.

And I could not really do the burn over multiple orbits, because I was out of Minmus' SoI after just a few hundred m/s acceleration... so there was no 2nd orbit. Burning from Minmus' orbit seems to work only with LFO, so that you can keep the burn time short. Right now, that's not much of an option, since I just got myself a few nerv-based ships that I am quite fond of.

Well there are trade offs to the NERVs, that are generally worth it. You will have to accept that Oberth won't be giving you much of a hand no matter if you're in Minmus' SOI, Kerbin's, or Sun's. But you get that sweet sweet Isp efficiency to make up for it, and that works everywhere.

Not sure the best way to go to Jool from Minmus with such low TWR. I'd probably try to exit Minmus as it was swinging around the back of Kerbin, and just keep burning. Basically your trajectory out will look a lot like a LKO trajectory to Jool with decent TWR, only it'll be way out at Minmus' orbit.

[Diche Bach]

Hmmm, it is an interesting question and I haven't got this far yet.

What about:

1. Put the fuel into a fuel depot in Minmus orbit. Me, I'd use those "inflatable" fuel tanks in that one mod (cannot recall the name now). = -150dV? (if that?)

2. For Interplanetary missions:

 A. Launch your interplanetary ship into Kerbin orbit (or skip it and go straight for Minmus) = - 750 to 1200 dV (you can get to Minmus cheap if you get the right Mun gravity assist and the stars are aligned)

B. Rendezvous with the fuel depot in Minmus orbit = - 150 dV??

C. Escape Minmus orbit and return to "very high" Kerbin orbit = - 150 dV (probably a bit less)

D. Execute your burn for interplanetary mission from there: in very high Kerbin orbit (between 45 and 47 million or whatever it is)

So, just totally ballparking here . . . that would mean that for each interplanetary mission, you are expending between 1200 and 1650 dV, considerably less than the bare mininum to get into LKO, which (I think) means that (given any interplanetary ship that was not constructed in orbit has to effectively 'get to LKO' anyway) for missions launched from Kerbin, this is still more efficient.

If you always insure that the fuel "payloads" you transfer are "large" (say 8 or 9K dV without payload) then even better.

The other thing you could do to up the efficiency: Also have a "tank farm" depot in LKO (or even at ~240km). Design a craft with the sole purpose to deliver empty "Interplanetary Stages" to this Tank Farm. They can be mostly empty (include enough to get to Minmus perhaps). 

If you put a docking port on the retrograde side of your interplanetaries and configure your first couple stages to have just enough to get to the Tank Farm. Once it is at the tank farm, dock up with an "Interplanetary Stage" with lots of fuel storage (but is mostly empty) and head for Minmus.

Because I use Stage Recovery, this would be particularly appealing as I'd be recovering more or less everything.

Of course, the even MORE efficient method is to skip Kerbin altogether. Build orbital construction facilities around either Mun or Minmus and build the spaceship in orbit! There is at least one mod that allows you to do this.

When/if humanity actually starts to get serious about exploring the solar system, it is my understanding that this is how it is likely to be done: getting into orbit and to the moon will only involve transporting personnel and materials/finished products that cannot be fabricated in space. With a few asteroids captured into orbit around the Moon, and Lunar bases that extract what there is to be had there (as well as self-sustaining orbital/lunar food and air production facilities) much of the currently prohibitive costs of space flight will be defrayed.

Of course, this requires a level of infrastructure that we are not likely to see anyone willing to make the initial investments on for quite some time.

[Plusck]

1 hour ago, 5thHorseman said:

I have always done (2) above. It took 3-4 times to really get the process down because things (generally, Mun-shaped things) tripped me up a lot. (3) though isn't as inefficient as you think it is if you do all the burning to your destination from Minmus, instead of burning out to Sun orbit and then burning from there to your destination.

(1) is terrible. Never ever even consider it as an option.

Definitely agree with this.

Again, though, gate orbits. These are the altitudes at which you need the minimum burn to get a given V_inf on leaving Kerbin's SOI. Since that part of the ejection burn that corresponds to raising Ap to Minmus is very cheap (it costs you what you need to burn from Minmus to drop Pe, so therefore 50-150m/s depending how far down Kerbin's gravity well you go), it's therefore the difference between raising Ap to Minmus and raising it beyond to escape the SOI with the right V_inf that matters.

Hmmm. Scratch that last part - they're pointless musings since in all cases, your most efficient strategy (with the possible exception of a burn to the closest planets - Duna or Eve) is certainly to lower Pe as far as possible.

However, for option (2), while dropping Pe down to 70km is certainly the most efficient, you don't need to drop all the way to make significant gains and if you have weak engines, you really don't want to have so little time at Pe that you can't benefit from Oberth and end up doing most of your burn while climbing away from Kerbin.

So I'd compromise - drop Pe to well within Mun's orbit, but leave it high enough that your ejection burn time covers only a small segment of your trajectory as you pass Pe.

[Randazzo]

Whenever I've bothered to set up fuel operations, I've always ended up delivering the fuel to LKO with a tanker of sorts. My first foray had a tanker similar to the one you describe (in terms of volume of fuel it carried) and it did indeed take ages to fuel up my Sarnus-bound monstrosity. In my later attempts, I had a fuel tug that hauled much larger amounts of fuel from Minmus to LKO, but I didn't design it to land so I still needed a surface-to-orbit tanker. I also put up an enormous fuel station in LKO so I could deliver fuel before having a ship up there to receive it. With such massive vessels involved though, docking got to be a bit of a challenge.

Ultimately, all fuel infrastructure attempts have ended in terrible boredom. It's always much faster to just launch the fuel from the surface of Kerbin, and at that point in my career games, funds aren't that much of an issue.

[Red Iron Crown]

I settled on this method:

1. Launch the interplanetary ship into LKO, using the mothership's fuel to help.
2. Bring full tanker tug down to LKO and rendezvous/dock.
3. Start the ejection burn, bringing Ap to Minmus altitude.
4. Top off interplanetary ship and detach tanker tug.
5. Interplanetary ship completes the ejection alone.
6. Tug waits for a Minmus encounter (or burns to get one) and returns for refueling.

This has a few advantages:

• The amount of fuel supplied from Kerbin is minimized, as much as possible is "free" from ISRU.
• Easier to hit a launch window in comparison to a departure from Minmus.
• The interplanetary ship can be smaller/lighter than one that leaves from LKO without fueling, because it doesn't need to hold fuel for the first 950m/s or so of the ejection burn. Smaller ship = cheaper to launch to LKO in the first place.
• The tanker tug can use expensive high efficiency engines with little penalty as they are reused repeatedly. The tanks get reused too, it's essentially a 100% reusable stage for the first part of the ejection.

[Aegolius13]

There are few threads around here on "gate orbits" you might want to check out.  Basically if you could leave from anywhere fully fueled, a gate orbit is the ideal orbit to get to another body with the least fuel consumption.  There's a different gate orbit for each destination.  In general, they're higher than LKO - the savings from already being higher in the gravity well outweighs the Oberth loss, up to a point.  Since Minmus is pretty far out near the edge of the SOI, it's probably further out than most gate orbits.  But being able to leave fully fueled is a big advantage.  

As you say, the main issue may be leaving at the right time.  Since Minmus' orbit is relatively slow, it might be hard to hit a launch window perfectly.

You could also consider refueling at the Mun, which might be closer to the gate orbits, has more Oberth, and revolves around Kerbin more quickly for those launch windows.  Obviously it takes more delta-v to lift fuel off the surface of teh Mun, but if you refuel your planetary ship in orbit, that doesn't hurt your maximum range.  

All that said, I have just done direct burns from LKO on my interplanetary missions, as the savings was not worth the extra work (or I was deliberately avoiding refueling, so I had to design around the tyranny of the rocket equation).  

 

One thread on gate orbits:

 

[Mattasmack]

I'm in the process of setting up a construction / fueling station around Minmus and I've been pondering the same thing.  I haven't flown any missions from the station yet in-game, but I've been using KSPTOT (Trajectory Optimization Tool) to explore options for making interplanetary transfers.  A two-burn departure (similar to your option 2) is definitely the most efficient way to go, but probably requires using something like KSPTOT to find the burns.

What I've been simulating is a craft starting from Minmus's position in orbit of Kerbin. (... with Minmus deleted from KSPTOT's database; otherwise the craft starts in the center of Minmus and everything falls apart!)  So the effects of Minmus itself are neglected.  An optimized two-burn departure for Duna takes only 370 m/s, while a single-burn departure is more like 630 m/s.  In the two-burn scenario, the first burn drops the craft to an orbit with low periapsis around Kerbin, and also includes a small normal component for plane change and a small radial component to adjust the time when the craft hits its Kerbin periapsis.  The second burn, at Kerbin periapsis, is almost purely prograde.  The single-burn departure turns out to have a lot of radial component as well as prograde, because Minmus' orbital period is long and isn't in the best position when the transfer window comes around.  (This is probably close to as good as it gets for a single-burn departure; if Minmus happens to be on the wrong side of Kerbin when the window comes around you could end up fighting Minmus' orbital velocity.)

[Wcmille]

I went to Eve recently doing something similar, but from Mun (which I think is better for this). Here's how I did it.

1. Set Up A Template Ejection

At location A, I found the ejection orbit I wanted using a space station in a nice, circular LKO, that wasn't going anywhere.

2. Match Your Mothership Kerbin Pe to that Ejection Point

My mothership, in orbit around Mun at B, then planned a burn that would put my Pe at the exact ejection point where A is located. If you do not put your Pe there, you will need a radial burn component, which is bad.

To find your launch orbit, you have to realize that your Kerbin-frame Ap is going to be somewhere at C, so you will need to have a date where your moon (Mun or Minmus) is appropriately placed. I just advance the orbit several times. For Mun, its not as tedious as you might think.

3. Create a Parallel Ejection for Mothership

Once I've got my Pe matched (you don't have to actually intercept the space station, you just need to see the shape of the orbit), I add a maneuver node for the mothership at A. Your mothership will need to target the space station to see its ejection orbit.

Now you are just adjusting the prograde and normal vectors until your mothership ejection is parallel to the station's ejection. Where you exit the SOI (if it's close) isn't nearly as important as your vector being parallel.

Comments

This maneuver probably saved me about 600 dV since it takes about 830 dV to get to Mun, and I'm spending only 235 dV to get that far.

This technique also gives you as much Oberth as you want, just set your station closer to Kerbin.

I like using Mun because it's coplanar with Kerbin and it's orbital period is 6 days, so you can always get close to the ideal launch window.

[Aegolius13]

Interesting... I was just kind of playing around on my Jool 5 and I ended up doing a similar maneuver, dipping from Pol  orbit to near Jool.  Just happened to get a really easy intercept with Kerbin.  Not sure what "market price" is to go Hohmann straight from Pol to Kerbin, but my thing took about 1500 m/s.  

 

[Wcmille]

I believe that if the transfer dV is low, 3 is better. If the transfer dV is high, 2 is better, so you can get the Oberth benefit. I don't have a guess what the threshold dV is.

[Magzimum]

Thanks for all the feedback!

@Wcmille, I like your suggestion to use a dummy drone (or in your case a space station) to find the ejection orbit out of Kerbin's SoI. I would imagine that this is useful for almost everybody, since everybody has a station or some satellite in LKO. Thanks for the tip. It makes my "option 2" a little easier.

5 hours ago, Aegolius13 said:

Interesting... I was just kind of playing around on my Jool 5 and I ended up doing a similar maneuver, dipping from Pol  orbit to near Jool.  Just happened to get a really easy intercept with Kerbin.  Not sure what "market price" is to go Hohmann straight from Pol to Kerbin, but my thing took about 1500 m/s.  

From Bop back to a Kerbin intercept took me 1150 m/s last night, so that is a bit curious. I did notice that it mattered enormously if you intercept Kerbin at your solar Pe, or somewhere else (meaning you need to use more fuel at Jool, and a lot more to stay in Kerbin's SoI once you get there). Did you 'rendezvous' with Kerbin at your solar Pe?

How low to Jool did you go? Close to its atmosphere (well below Laythe)? Just asking because I struggled with the Jool return last night, and because I intend to do a few more Jool visits in the near future.

 

 

[Sharpy]

The best approach delta-V-wise, would be to launch from Minmus, get a pretty low periapsis by Kerbin, and perform Oberth maneuver during Kerbin fly-by.

But getting the encounter that way is a total b*tch. Very small inaccuracies result in very large errors, as you're operating at high speed, tight maneuvers and very little wiggle room, as both the departure from Minmus and the flyby burn need to be timed perfectly, or you'll end up wasting more fuel on corrections than you're saving on the maneuver.

I did it once for Duna. The craft was floppy and I had to throttle down during the burn or it would break. What should have saved me about 800m/s of delta-V, cost me half a year extra and a capture burn of good 1500m/s more than it should be.

Edited September 15, 2016 by Sharpy

[Plusck]

7 hours ago, Wcmille said:

I believe that if the transfer dV is low, 3 is better. If the transfer dV is high, 2 is better, so you can get the Oberth benefit. I don't have a guess what the threshold dV is.

Interesting. I am sure there is a single equation that would give this.

However, since I already have a spreadsheet with all the various planets and C3s worked out for each planet, I decided to do it the simple, reiterative way. The result is this excel monstrosity. Which also completely ignores any possible Oberth benefit from Minmus itself...

Still, what it does show is that if you want to make any dv saving at all, then no matter what your destination you will always be better off dropping Pe to the Mun's orbit or lower.

If, however, you insist on making a bigger dv saving (say 30% of the cost from leaving directly from Minmus) then you have to drop your Pe to something lower than 2500km (for Eve, Duna or Dres), and lower than 1500km that if you're going to Jool or Eeloo (or Moho, assuming you go the intelligent way with a higher escape burn from Kerbin and lower capture burn at Moho).

Edited September 15, 2016 by Plusck

[Aegolius13]

Looks like my ejection burn was at about 3,970 km over Jool, so well within Laythe orbit.  Buy the ensuing transfer orbit was a little crooked.    Would normally try to get it a bit cleaner, but I had a ton of fuel left do it didn't matter.  It was also a pretty slow transfer, probably because of the part going out beyond Jool orbit.

Here is a pic of the maneuver.

http://i.imgur.com/gsm3h5j.webp

 

Edit:  should also note that I captured at Kerbin with a heat shield, so I have no clue what the injection burn would have been.

Edited September 15, 2016 by Aegolius13

[foamyesque]

Personally I launch out into a circular, non-inclined orbit at Minmus's altitude (i.e., dust off from Minmus at the AN/DN relative to Kerbin) and, once there, plan out a periapsis lower + ejection kick. It lets you fix your ejection angle pretty precisely since half (or in some cases, much more!) of the ejection burn is already done.

[Laie]

On 9/15/2016 at 11:23 AM, Magzimum said:

I like your suggestion to use a dummy drone (or in your case a space station) to find the ejection orbit out of Kerbin's SoI. I would imagine that this is useful for almost everybody, since everybody has a station or some satellite in LKO. Thanks for the tip.

Markup probes are indeed priceless. Upvoted @Wcmille's suggestion.

I'd like to add that the choice between 1) going back to LKO and 2) dropping your PE from Minmus, does not need to be a strict either/or decision. There is (literally!) a lot of room to play with. Dropping your AP just enough to avoid brushing up against the Mun, for example, isn't very costly and avoids headaches.