Non-Hohmann Transfer Orbits

[MilkToast]

I've got a ship I would love to get to Duna soon.

The nearest Hohmann transfer window is in 209 days (thank you Kerbal Alarm Clock) - with travel time of a month or so, I'm looking at 250 days to get there.

What I'd like to do is get there sooner - leave BEFORE the window. I do not want to timewarp - rather, I've got a ship already enroute from the last transfer window, and I'd love to get there shortly after it does.

I'm looking for tips and thoughts on how to do it.

Edited January 29, 2014 by MilkToast

[User Unrelated]

You don't HAVE to leave during departure windows.

check out the launch planner...

http://alexmoon.github.io/ksp/

the shortest time that it says you would need to get there (from LKO, to LDO) is 38 days (with the optimal Dv minimization taking between 50 and 60 days)

You would need a mega heap of Dv, but you should be able to get there sooner... I'm not quite sure what method to use, though.

Probably just pack way more Dv than you need, and ham-fist your way over there.

[Guest Brody_Peffley]

You don't HAVE to leave during departure windows.

check out the launch planner...

http://alexmoon.github.io/ksp/

the shortest time that it says you would need to get there (from LKO, to LDO) is 38 days (with the optimal Dv minimization taking between 50 and 60 days)

You would need a mega heap of Dv, but you should be able to get there sooner... I'm not quite sure what method to use, though.

Probably just pack way more Dv than you need, and ham-fist your way over there.

The shortest I've ever seen someone get to duna is around 4 days or 6. forgot

[Tank Buddy]

As long as you can put your trajectory through Duna's orbit while it is there, you can transfer. If Duna is pretty far ahead, you can only wait, or you will have to throw your orbit far out to wait for an intercept. I would just time warp until the launch window.

Also, you can launch 2 ships in the same window, and the second one to depart can get there first, it just depends on the trajectory. If you missed the launch window by too much, there's not much you can do.

[Zuni]

The best way is to do a hohmann. The easiest way is to strap a mainsail to a capsule, turn infinite fuel on and burn towards Duna until you get there.

[ComradeGoat]

If you're nearer the last launch window than the next one, it's easy: just burn past it until you get an intercept, keeping your perihelion at Kerbin.

If you're nearer the next launch window than the last one, it's harder, because you'll have to lower your perihelion to speed your orbit up and catch it up. You might even have to resort to a manoeuvre node to get an intercept, or something.

Anyway, you can always get there, more or less. As someone said above, you just have to arrange to be in the same bit of space at the same time it is. Treat it like orbital rendezvous, only bigger, because that's what it is.

[ComradeGoat]

As long as you can put your trajectory through Duna's orbit while it is there, you can transfer. If Duna is pretty far ahead, you can only wait, or you will have to throw your orbit far out to wait for an intercept. I would just time warp until the launch window.

Not true. You just need to exit Kerbin such that your perihelion is lower than Kerbin's orbit by a suitable amount so you can catch Duna up, and your aphelion gets the intercept. It's still faster than waiting for the window.

[alexmun]

The Hohmann transfer from Kerbin to Duna comes around about every 227 days. If you're 209 days from the next transfer, that means you're only about 18 days past the last transfer. The Duna launch window is pretty big, so you should still be able to launch right now without a big delta-v penalty. For example, if I use the calculator in my sig to find a launch window on day 84 (18 days after the first optimal launch window, which occurs on day 66) I get an ejection delta-v of 1162 m/s vs the optimal ejection delta-v of 1046 m/s, so you're only getting a penalty of about 120 m/s for launching late. Now, if you aren't doing an aerocapture, you may have a greater penalty on the insertion side because you will end up catching up to Duna with a greater relative velocity (in my example it ended up being 1189 m/s vs 647 m/s). I should also note the time of flight for the later launch is much longer (120 days vs 66 days), but you'll still get there earlier than if you waited for the next window to come around.

The bottom line is (1) launch windows are bigger than you think, and (2) you can always get from point a to point b, it's just a matter of how much delta-v you have to spend.

[MilkToast]

@Alexmun - Very helpful. That calculator is a piece of work.

I went ahead and did the burn two or three days later, at slightly greater expense.

Thanks!