Hohmann transfer screwiness around the moons of Jool

[WafflesToo]

Having some trouble doing Hohmann transfer calculations. I was working out transfer maneuvers between Vall and Laythe and I'm getting an ejection velocity of 2451.91 m/s, and exit velocity of 347.21 m/s and an error for the ejection angle.

The problem is, escape velocity from Laythe is 2635.36 m/s

I ran this through that rather excellent Hohmann calculator and came up with identical results...

Am I to take this to mean that there is no Hohmann transfer window between Laythe and Vall, making transfers more of a brute force affair? I do find that hard to beleive... but it's not outside the relm of possiblity I suppose. :/

Anyway, here's my data and my math for a Laythe -> Vall transfer. Maybe somebody else can spot my mistake.

r1 (Parking orbit) = 565,000 meters

r2 (SOI radius) = 3723646 meters

v2 (SOI exit velocity) = 347.21 m/s

u (Laythe gravitational parameter) ~ 1.96E+12 m^3/s^2

v1 (Ejection velocity) = SQRT((r1*(r2*v2^2-2u)+2*r2*u)/(r1*r2)) = 2451.91 m/s

ve (Escape velocity) = SQRT(2*u/r1) = 2635.36 m/s

[Geschosskopf]

Am I to take this to mean that there is no Hohmann transfer window between Laythe and Vall, making transfers more of a brute force affair? I do find that hard to beleive... but it's not outside the relm of possiblity I suppose. :/

I don't know what calculator you're using but it doesn't surprise me you're getting strange results from it. Flying around between the moons of Jool is strange just in the stock game, especially between the big 3 of Laythe, Vall, and Tylo. Out by Bop and Pol, it's no big deal.

As I understand things, it's sort of an artifact of KSP's small scale compared to reality. You have the planet-sized SOIs of these moons all close together and changing position to each other rapidly as they orbit within the monumental SOI of Jool. There's nothing in the real world like this, even at Jupiter, so strange things happen.

It's really not so much brute force as it is illogical. For instance, you'd think that for going from Laythe to Vall, you'd need phase and ejection angles similar to going from Kerbin to Duna. While the phase angle between the moons is similar, the ejection angle is bizarre. Sometimes it'll be on the retrograde side of Laythe instead of the prograde side. Sometimes you'll get a Vall encounter by leaving Laythe pointed straight at Jool. It may depend on the positions of the other moons, I don't know, but it does seem to vary each time I do it. No matter what it looks like within Laythe's SOI, however, if you drag and tweak the node enough to get a Vall encounter, that's what you'll get. Eventually. I say "eventually" because you'll often get 3 or 4 unexpected encounters back with Laythe in the early part of the trip, that come out of nowhere and then disappear for no reason.

As to the delta-V required, it usually seems to take about 800m/s for the transfer burn between Laythe and Vall, IIRC. Maybe 1000m/s but no more than that. Capture at Vall is additional.