Interplanetary travel - LKO vs Minimus as the starting point

[JoeSchmuckatelli]

I don't have the maths to figure this out myself - but is there a significant difference in efficiency determined by where you start?  Obviously, a LKO burn gives you higher initial speed - but then you're starting out having to fight all of Kerbin's gravity to escape - whereas with a Minimus burn you're starting from a lower gravity body...

 

I presume if you start from Minimus you want to wait until the moon is going retrograde to Kerbin's orbit to go to the inner planets - but prograde to the outer - or does that matter?

 

... Where I am going with this is - trying to decide whether it's worth my time to set up a mining operation on Minimus to assist in exploring the rest of the system.

Edited January 22, 2016 by JoeSchmuckatelli

[Gaarst]

If you start from Minmus you spare the ~850dV to get there from LKO, but you have the orbital inclination to account for and you will not have Kerbin's Oberth Effect (Minmus' is weaker, because orbital speed is smaller).

Overall, I don't really know how much dV you spare by starting from Minmus, but if your goal is to do a mining operation, then Minmus is the best choice IMO. If you just want to send small crafts to other planets, you don't need to do a mining operation.

[Evanitis]

Just check the the Transfer Window Planner, and it will show that with the ideal angle, launching from Minmus orbit needs -a lot- less dV. The difference is almost 1000 m/s for a Duna transfer.

However the windows are smaller, you have to do the hassle of refueling and you can't slingshot (which is fun). Though mining is fun too, so use the methot that pleases you more at any given time.

I am silly.

Edited January 22, 2016 by Evanitis

[Aethon]

LKO is more efficient.  

I used to drop down from Minmus orbit and swing by Kerbin for the Oberth effect.  While there is a significant dv savings (as long as you refuel at Minmus), it's a pain to get Minmus and your ejection angle to line up, and still be inside your transfer window.

Edited January 22, 2016 by Aethon

[Plusck]

Yes, someone has worked all this out. There are particular "most efficient" orbits for burns to escape the SOI, depending on how fast you have to go to escape. The faster your Kerbin SOI escape velocity is, the lower down into its gravity well you can be while remaining at that most efficient orbit.

http://forum.kerbalspaceprogram.com/index.php?/topic/33699-efficient-hohmann-transfer-altitudes/

Interestingly, for Eve and Duna, the optimal orbits straddle the Mun's orbit. Therefore if you are going to refuel somewhere before escaping Kerbin on your way to Eve and Duna, then the best place to do it is in orbit around the Mun.

Minmus is not the best place to do this because you either lose the Oberth effect (which is an extemely expensive choice, once you're out by Minmus), or you have to burn back to a low Kerbin Pe before burning back out (losing 160 m/s or whatever).

And from Minmus, you have a serious problem with timing and difficulty in plotting your escape burn, and difficulty getting the inclination right.

[Tex_NL]

15 minutes ago, Aethon said:

I used to drop down from Minmus orbit and swing by Kerbin For the Oberth effect.  While there is a significant dv savings (as long as you refuel at Minmus), it's a pain to get Minmus and your ejection angle to line up, and still be inside your transfer window.

That is indeed THE most efficient way to set up and interplanetary journey.
Park and refuel in low Minmus orbit. It will be very cheap to drop down to low Kerbin and at periapsis go full throttle prograde to escape Kerbins SOI. You'll get 850dV from Minmus for free.

As you said very tricky to do right, it will take some careful planning and a bit of luck to align everything right. If you're REALLY lucky you might be able to get an additional gravity assist from Mun or Minmus. Doing this same thing from Mun would be less efficient but easier as Mun has a shorter orbital period and 0^o inclination.

Edited January 22, 2016 by Tex_NL

[JoeSchmuckatelli]

16 minutes ago, Plusck said:

Yes, someone has worked all this out. There are particular "most efficient" orbits for burns to escape the SOI, depending on how fast you have to go to escape. The faster your Kerbin SOI escape velocity is, the lower down into its gravity well you can be while remaining at that most efficient orbit.

http://forum.kerbalspaceprogram.com/index.php?/topic/33699-efficient-hohmann-transfer-altitudes/

Interestingly, for Eve and Duna, the optimal orbits straddle the Mun's orbit. Therefore if you are going to refuel somewhere before escaping Kerbin on your way to Eve and Duna, then the best place to do it is in orbit around the Mun.

Minmus is not the best place to do this because you either lose the Oberth effect (which is an extemely expensive choice, once you're out by Minmus), or you have to burn back to a low Kerbin Pe before burning back out (losing 160 m/s or whatever).

And from Minmus, you have a serious problem with timing and difficulty in plotting your escape burn, and difficulty getting the inclination right.

I looked at this - but want to be sure I get it.  Does this mean that for a Kerbin - Moho run, I should circularize a 700km orbit until the transfer window opens, then decelerate until I get a low Kerbin Pe, and then accelerate at Pe out of the system?

[Evanitis]

21 minutes ago, Tex_NL said:

You'll get 850dV from Minmus for free.

I was waiting for that value (expecting it to be a bit higher). Let me pull some numbers from the planner... A Duna transfer from LKO (100km) needs 1680 dV at the window. Subtracting the dV saved by by dropping the PE to Kerbin orbit (850), it still needs a 830 m/s burn. While a direct Duna transfer from a 20km Minmus orbit takes only 260 m/s.

Am I missing or misinterpreting something?

I am stupid, and I only leave that here to make it obvious for those who read it. You are right.

Edited January 22, 2016 by Evanitis

[Evanitis]

11 minutes ago, JoeSchmuckatelli said:

Does this mean that for a Kerbin - Moho run, I should circularize a 700km orbit until the transfer window opens, then decelerate until I get a low Kerbin Pe, and then accelerate at Pe out of the system?

Read the fine print too:

"- It is typically less efficient overall to transit to a higher altitude in order to begin a transfer because the fuel expended getting to a higher altitude outweighs the gain you get from launching your transfer at the optimal altitude. Nevertheless, this might be useful information for where to put your refueling station."

(from the same post)

Read: it's a waste of fuel unless you plan to refuel on 700 km.

Edited January 22, 2016 by Evanitis

[Yasmy]

Here's an old table of approximate delta-v costs from various moons to interplanetary destinations, showing the direct transfer cost vs. the cost if you dip down to low orbit around the parent body.  Also shown for comparison is the rough cost starting in low orbit around the moon's parent.

Edited January 22, 2016 by Yasmy
Got rid of giant clunky forum link in favor of traditional url.

[Yasmy]

1 hour ago, Tex_NL said:

<snip>Doing this same thing from Mun would be less efficient but easier as Mun has a shorter orbital period and 0^o inclination.

I agree with everything Tex_NL said, but additionally want to point out that if you are starting from low munar orbit, and want to go to Duna or Eve, it is cheaper to go directly from the Mun, than it is to dip down to LKO and burn there.  That's only for Duna and Eve though.  For other destinations from the Mun, it is significantly cheaper to drop down to Kerbin and perform the interplanetary burn there. (You can see this in the table in the linked thread one post up.)

Edited January 22, 2016 by Yasmy

[Plusck]

2 hours ago, JoeSchmuckatelli said:

I looked at this - but want to be sure I get it.  Does this mean that for a Kerbin - Moho run, I should circularize a 700km orbit until the transfer window opens, then decelerate until I get a low Kerbin Pe, and then accelerate at Pe out of the system?

Not quite.

This is not at all a question of decelerating from that orbit. In fact, that would be exactly like making your burn from whatever Pe you bring it down to, to start with. Or the same as splitting your burn over two orbits (which in itself is fine, and sometimes needed for very low TWR craft).

As Evanitis pointed out, the fine print is quite clear about it probably being a waste to raise your orbit that high. For Kerbin, I would go further and say it is most definitely and absolutely a waste to do that.

Where it would not be a waste would be, for example, within the Jool system. With all the moons there, it is relatively easy to get gravity assists to raise your orbit around Jool. Therefore the different numbers for "most efficient orbit" could be a great help in leaving the Jool system. Although again, you really want a gravity assist from a big moon to leave Jool, so it's more a question of combining tehniques to get the most efficient outcome.

And of course, if you want to put a refuelling station somewhere, then for missions to Eve or Duna, that somewhere is in orbit around the Mun. For Moho or Jool, you would ideally want to refuel in a lower orbit. However, if you have time to spare and are immunised against jittery nodes and planning headaches, then refuelling at Minmus then dropping Pe to LKO works equally well for all destinations.

The principle behind the table is simply that the lower you are in the gravity well, the bigger the Oberth effect, but the more energy you need to escape the gravity well. There is a point where increasing Oberth effect is not going to make up for the lower energy of your orbit.

 

And one final point: those figures are for circular orbits. If you are on a highly eliptical orbit, the point at which it starts being wasteful to continue a burn, rather than just orbit again and burn a second time at Pe, is far lower than those high "most efficient" altitudes.

Edited January 22, 2016 by Plusck

[Plusck]

1 hour ago, Yasmy said:

Here's an old table of approximate delta-v costs from various moons to interplanetary destinations, showing the direct transfer cost vs. the cost if you dip down to low orbit around the parent body.  Also shown for comparison is the rough cost starting in low orbit around the moon's parent.

Interesting data!

I was seriously wondering about the "Ike to Kerbin" vs "Ike to Duna to Kerbin" question the other day. I suspected it would be cheaper to do it directly, so I'm quite pleased to see the data. : D

[FullMetalMachinist]

Another option that avoids the hardship of getting the Mun or Minmus' orbits to match with the transfer window is to refuel in LKO. I have a mining operation on Minmus and a refueling tanker that bounces between Minmus orbit and LKO. 

Basically, lifting refined fuel to Minmus orbit is cheap, and getting that fuel to LKO is cheap with proper use of aerobraking. Then you launch your interplanetary ship, dock with the tanker, refuel, undock, and make your ejection burn. Just remember to leave enough fuel in the tanker to get back to Minmus orbit. 

[JoeSchmuckatelli]

7 minutes ago, FullMetalMachinist said:

Another option that avoids the hardship of getting the Mun or Minmus' orbits to match with the transfer window is to refuel in LKO. I have a mining operation on Minmus and a refueling tanker that bounces between Minmus orbit and LKO. 

Basically, lifting refined fuel to Minmus orbit is cheap, and getting that fuel to LKO is cheap with proper use of aerobraking. Then you launch your interplanetary ship, dock with the tanker, refuel, undock, and make your ejection burn. Just remember to leave enough fuel in the tanker to get back to Minmus orbit. 

That's starting to sound like the way to do it... If I do it.  As of right now it just seems easier to launch a fuel tank - then launch my mission package, and refuel from that tank.

 

Thus far I haven't run into prohibitive costs in my Career mode game - but might just do a small mining operation at Minimus just to complete a contract (Initially I was thinking some over engineered huge mining ship).  Unless someone can tell me it turns out to be cheaper to refine fuel at Minimus and ship it to Kerbin for example, I'm starting to wonder if there's any reason to do anything other than launch fuel from Kerbin.

[Snark]

16 minutes ago, FullMetalMachinist said:

Another option that avoids the hardship of getting the Mun or Minmus' orbits to match with the transfer window is to refuel in LKO. I have a mining operation on Minmus and a refueling tanker that bounces between Minmus orbit and LKO. 

Basically, lifting refined fuel to Minmus orbit is cheap, and getting that fuel to LKO is cheap with proper use of aerobraking. Then you launch your interplanetary ship, dock with the tanker, refuel, undock, and make your ejection burn. Just remember to leave enough fuel in the tanker to get back to Minmus orbit. 

+1 to this.

You can even take it a step further, if what you're trying to do is maximize the amount of dV your interplanetary ship has remaining when it leaves LKO:  Get the ship into LKO, then do a partial ejection burn that leaves it in an elliptic orbit with Pe very low, and Ap fairly high (e.g. most of the way to the Mun), oriented such that the direction you're traveling at Pe is the direction you want for your ejection burn.  Then park it there, in that elliptical orbit.

You can then refuel it (by taking your tanker to match the elliptical orbit), so that when you finally eject, you have a smaller burn to do, since you've already done a lot of the work by raising your Ap before refueling-- and you're doing your ejection burn at low orbit, to maximize Oberth.

This doesn't necessarily maximize your overall fuel expenditure (since your refueling tanker needs to do more dV worth of maneuvers)... but it packs the most dV into your interplanetary ship without requiring additional fuel tankage.

[FullMetalMachinist]

6 minutes ago, Snark said:

You can then refuel it (by taking your tanker to match the elliptical orbit), so that when you finally eject, you have a smaller burn to do, since you've already done a lot of the work by raising your Ap before refueling-- and you're doing your ejection burn at low orbit, to maximize Oberth.

This. Is. GENIUS! 

I would have never thought of doing it, but that makes perfect sense. The only hard part is being able to line up your partial-ejection-eliptical orbit to the correct ejection angle days ahead of time to give you time to do the refueling. 

[Snark]

1 hour ago, FullMetalMachinist said:

The only hard part is being able to line up your partial-ejection-eliptical orbit to the correct ejection angle days ahead of time to give you time to do the refueling. 

Heck, that's not even hard.  You don't have to go into a multiple-day orbit, here; the lion's share of the dV is in raising Ap the first few thousand km.  (Witness the fact that boosting to Minmus' orbit from LKO only costs about 50 m/s more than boosting to the Mun's.)  The orbital period is short enough that you don't have to worry much about missing your launch window.

So to set it up, all you do is:

1. Get to circular LKO.
2. Go ahead and set up a maneuver node to go to your destination planet, as if you were going to make direct burn.
3. Start the burn, but only do the first 700 m/s (that's enough to keep your Ap comfortably below the Mun).  Then delete the maneuver node.
4. Do your matching thing with the fuel tanker, and refuel your ship.
5. Now set up a new maneuver node at Pe to send you to the destination.  It'll be essentially identical to the last one, just 700 m/s smaller because you've already done part of the burn.  The fact that it's a day or so later should make very little difference to your launch window.

In practice, I don't often end up doing this, myself-- it's basically equivalent to just including an extra (full) drop-tank on the interplanetary ship.  Having the tanker refuel the ship in elliptical orbit is essentially just recovering/reusing the drop tank.    I do occasionally do this if the ship design is one where bolting an extra tank on is awkward, or I have a refueler that's fortuitously placed.

Edited January 22, 2016 by Snark

[maceemiller]

Me, id go from LKO just as Kerbin is my home.....

[FancyMouse]

After trying out different options, what I usually do now is to have my launch stages capable of delivering LKO+700~900m/s, and the last launch stage reserves a tiny bit of fuel to deorbit itself at its apoapsis. Least hassle while saves transfer stage fuel.

I only refuel ships that don't return (i.e. hoppers/planes staying on a moon/planet forever for science/contracts, also tugs). 

Edited January 23, 2016 by FancyMouse