How does this happen?

[NewtSoup]

I'm still building my Minmus base, Rura Penthe.  I decided to run two missions at once.  One to launch a triplet of ore tank modules and one to launch an orange fuel tank storage module.

As you can see I launched the two missions pretty much two days apart with the idea of giving me two days grace to orbital maneuver and landing the Ore Tanks on Minmus etc.

Both had the same dV requirements naturally and I set up maneuvers to capture into Minmus Orbit.  However:

Instead of two days grace I've ended up with two hours and indeed the second mission is arriving before the first ( taking two days and two hours less to arrive than the first mission!)
I'm sure there's a logical explanation for this.  Can someone tell me what's happened?

Edited August 14, 2017 by NewtSoup

[Cpt Kerbalkrunch]

If I had to guess (and I do ), I'd say your first rocket had a more efficient maneuver that allowed for a good encounter with Minmus. The second rocket was a bit off. To compensate and still get a good encounter, you had to burn longer. The second ship was then travelling faster and actually passed the first. I see this quite a bit when I launch multi-vessel missions to the outer planets.

Accept as it applies to the future efficiency of your maneuvers, I wouldn't worry too much about it. Just burn to capture with your first (second?) ship and then leave it in orbit 'til you capture with the other. No need to rush your maneuvers and maybe miss a node.

[NewtSoup]

The first one is moving at 280 m/s but is 2 days out so it will have slowed somewhat.  The second one is just a few hours out from Kerbin and moving at over 900 m/s which is expected for a Minmus transfer ( relative velocities of course).  But as you say, I shall put it out of my head.  Dump the pair in orbit around Minmus, out of trouble, and deal with them in the order I need to once they're both there.  Sometimes I get an idea of how something should work in my head and then worry when it doesn't even when it doesn't matter.

[Cpt Kerbalkrunch]

It's something to think about for the future, though. You'll notice the second ship (arriving first) will need to burn more to capture. Of course, more is a relative term at Minmus since it's so easy to capture. When traveling to other planets though, the discrepancy can be much greater. Sometimes enough to ruin an otherwise good mission.

[Snark]

11 hours ago, NewtSoup said:

Instead of two days grace I've ended up with two hours and indeed the second mission is arriving before the first ( taking two days and two hours less to arrive than the first mission!)
I'm sure there's a logical explanation for this.  Can someone tell me what's happened?

The transfer time to Minmus is overwhelmingly dominated by the "slow end" of the transfer ellipse-- that last little bit out there close to Minmus.  A relatively small difference in your ejection from LKO-- either in direction, velocity, or both-- translates to a big difference in travel time.  An "ideal" (minimum dV) transfer to Minmus takes ~9 days... but it requires only a small amount of extra dV to shave days off of that.

So, small fluctuations in your trajectory there lead to big fluctuations in travel time.

[NewtSoup]

1 minute ago, Snark said:

The transfer time to Minmus is overwhelmingly dominated by the "slow end" of the transfer ellipse-- that last little bit out there close to Minmus.  A relatively small difference in your ejection from LKO-- either in direction, velocity, or both-- translates to a big difference in travel time.  An "ideal" (minimum dV transfer) to Minmus takes ~9 days... but it requires only a small amount of extra dV to shave days off of that.

So, small fluctuations in your trajectory there lead to big fluctuations in travel time.

So basically I travel by butterfly.  Oddly enough some of the best navigators in the world.

[Snark]

16 minutes ago, NewtSoup said:

So basically I travel by butterfly.  Oddly enough some of the best navigators in the world.

...see, they don't "flutter".  They just have a very very large number of maneuver nodes. 

 

Actually, a better analogy would probably be golf.  A ball may spend more time dribbling those last few yards along the green than it takes to fly hundreds of yards down the fairway.  And in the initial, powerful drive off the tee, there's only a very small difference between overshooting and undershooting the green.  (Which is one of the reasons why golf is hard.)

[WanderingKid]

So, the question is answered, but I'd like to add a note.  Traveling to Minmus and the *ahem* Butterfly dribble effect is something that will heavily influence any interplanetary transfers you'll do.

Mun's great as a first target.  Minmus actually teaches the little bits and pieces that end up getting exponentially multiplied for interplanetary transfers.  That last 10 dV is murder!

[Snark]

9 hours ago, WanderingKid said:

the *ahem* Butterfly dribble effect is something that will heavily influence any interplanetary transfers you'll do.

Actually, not so much.   Some of them, yes, but not all of them.

Where it becomes really blatant is when the transfer ellipse is extremely eccentric.  That's when you get a really super-slow apoapsis speed, and small changes in the initial ejection make for big changes in travel time.

The effect doesn't show up much for the Mun, because a Mun transfer ellipse isn't all that eccentric (the Mun's not orbiting all that high, relative to LKO).  Minmus has a much more pronounced effect because of the eccentricity of the transfer.

The same principle applies to interplanetary transfers.  Kerbin to Duna, or Kerbin to Eve?  Not much effect-- it takes a lot of dV to make a big difference in travel time.  But going to Eeloo?  Yah, it'll make a difference.

[Pand5461]

7 hours ago, Snark said:

The effect doesn't show up much for the Mun, because a Mun transfer ellipse isn't all that eccentric (the Mun's not orbiting all that high, relative to LKO).  Minmus has a much more pronounced effect because of the eccentricity of the transfer.

Never thought about it before, but... If we scale real-life Moon's orbit by the ratio of Kerbin/Earth radii, the SMA would be 36 Mm, much closer to Minmus's than Mun's. And adding just a bit extra dV easily shaves off a few days from IRL Moon transfer of course.