Interplanetary Transfers

[Branjoman]

Just a quick question on transferring to other planets...

My current method is simply get into orbit around Kerbol and twiddle around with a maneuverer until I get an encounter with said planet. Is it fine to pack an adequate amount of fuel and do this, or should I wait for orbital transfers?

[Jimbimbibble]

Sure, it works, but you're probably wasting 1000 m/s of deltaV. It is significantly more efficient to do transfer burns in low orbit.

[Anquietas314]

If that's fun for you, then sure I guess you can stick to your current method. If on the other hand you care at all about efficiency, I would highly recommend waiting for the appropriate transfer window and doing the burn from low Kerbin orbit

This calculator will let you see the relative costs of choosing to do a transfer at different times (and help you pick a good one - the blue bits are the cheap areas ):

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

[Astroman29]

This might help, too:

http://ksp.olex.biz/

[EndOfTheEarth]

I used to use the exact same method you're describing, but after an exceptionally nasty rescue mission to Bop, I realized that waiting for the optimal transfer windows uses less fuel and gives me a greater safety margin for my return.

If you are doing one-way flights, or just starting out, then continue doing what you're doing. If you're doing a round-trip mission and don't want the headache of planning a rescue fuel tug, it helps to wait.

[Greep]

Well you don't necessarily have to use the transfer window, here's an easy fuel efficient way of getting to another planet:

1)Just barely escape kerbin

2) circularize and get inclination with planet to 0ish by either burning anti-normal on AN or normal on DN.

3) Make a maneuver node anywhere and burn prograde or retrograde until you intersect with the planet's orbit, and then burn a few more dV

4) Pick the Maneuver node by the circle (not prograde, the center of the maneuver node) and slide it along your orbit until you get an encounter. Tada! When you think about it, this is exactly the same as the transfer window method, you are just letting the maneuver node show you where the transfer is!

Technically, I think this can be inefficient if you're aiming for an ellitpical orbit... but not by much.

Keep in mind, if you use no method or a bad method, being off by 1000 m/s is not as bad as it can get, you can be several thousand dV off, and if there's no aerobraking option, you will end up paying that offset DOUBLE.

Edited December 29, 2014 by Greep

[Anquietas314]

Greep: while that is an easy method, it's certainly not fuel efficient. Doing your transfer while deep in Kerbin's gravity well saves you a fair bit of delta-V, then there's the part about plane change (there are better ways to do that - you ideally want to do most of that in your transfer burn). Plus remember every bit of delta-V in upper stages means a lot of extra fuel earlier on (ion drive aside... that's practically free )

That said, it's nowhere near as bad as the "straight line" method

[Greep]

Ah yeah, that is true. Not efficient, just not horrible. It's pretty annoying that it's so hard to slide the Maneuver node while in kerbin to get the perfect ejection angle, though :/

While we're on the topic, is there a way to fix inclination for interplanetary while in kerbin? Or any situation where you're in an orbit trying to transfer to an orbit within the same system. I've been trying to figure out how to get from, say, pol to bop, and it's a pain.

[Anquietas314]

While we're on the topic, is there a way to fix inclination for interplanetary while in kerbin? Or any situation where you're in an orbit trying to transfer to an orbit within the same system. I've been trying to figure out how to get from, say, pol to bop, and it's a pain.

Not necessarily fix it, but you can offset your inclination in such a way that you arrive at your destination It's expensive, but sometimes cheaper than doing a mid-course plane change.

[Greep]

Ah yeah.. I've tried it... and I always end up spending more fuel than I would've saved. Except in that one case, changing inclination close to jool is almost a non-option, but the method is just "set a maneuver node and wing it until I don't spend much fuel". I was just wondering if there was a more.. scientific way of doing it.

[Anquietas314]

I was just wondering if there was a more.. scientific way of doing it.

Well, there is if you like really complicated maths. Otherwise, alexmoon's calculator

[LethalDose]

While we're on the topic, is there a way to fix inclination for interplanetary while in kerbin? Or any situation where you're in an orbit trying to transfer to an orbit within the same system.

To fix the inclination on the exit burn, you must perform the burn at the AN or DN between the two planets orbits, which may not coincide with the launch window, It's also a bit of a nightmare figuring out how much inclination you need in the current SoI to change the inclinication by the correct amount in the parent's SoI. I think it is universal, though, that the inclination change around the original body will always be greater than the the resultant inclination around the parent's SoI (e.g. a escaping from a 30 degree inclined orbit around Kerbin will result in a 10 degree inclined orbit around the sun).

Based on some math I did for another thread, I don't think bi-elliptical transfers are really dV savers until you're doing very large inclination changes (> 60 degrees), already needing highly elliptic transfer orbits.

[Red Iron Crown]

Based on some math I did for another thread, I don't think bi-elliptical transfers are really dV savers until you're doing very large inclination changes (> 60 degrees), already needing highly elliptic transfer orbits.

48.9 degrees is the break-even point for bielliptic inclination changes, according to people smarter than me. Interestingly, initial orbit size is not a factor.

[LethalDose]

48.9 degrees is the break-even point for bielliptic inclination changes, according to people smarter than me. Interestingly, initial orbit size is not a factor.

I think this is one of those situations that I think may vary by starting radius, target radius and mu

There's some more messages further down there that give other thresholds, e.g. > 38.9 degrees you should adjust AP to certain value and above 60 you should go as high as you can.

[Red Iron Crown]

I was misremembering and linked to the wrong post in the right thread, thanks for the correction. Though if I'm reading it correctly it's about an inclination change only, where you wish to return to the same initial orbit, in which case starting orbit doesn't matter. The math is more complex (and admittedly a bit beyond me) if changing orbit size as well.

Edited December 29, 2014 by Red Iron Crown
Punctuation!

[mpk10]

I use what I consider to be a compromise method between the "just escape Kerbin and fiddle with Maneuver nodes" and "use a calculator and wait for appropriate transfer windows".

1. Regardless of where I am in any transfer window scenario, I do my first burn from low Kerbin orbit using the Oberth effect. (I'm sure you can read about it, but roughly speaking, from a standard prograde orbit, burn on the sunny side for the inner planets, on the dark side for the outer planets.) I put my Apoapsis roughly around the same distance (by eye) as the orbit of the planet Im headed for.

2. Once i get to an acending or decending node, i burn a second time to match inclinations and adjust my apoapsis to be right on the orbit line of my target.

3. JUST after i pass my "closest approach" indicator, i do a third burn (prograde, or retrograde) to push or pull the "target position" indicator to within an encounter. (Sometimes, if my "target position" is crazy far, like on the opposite side of Kerbol, i'll wait another year so i can make a smaller adjustment, which i realize defeats the purpose of avoiding the transfer window in the first place, but whatever, i'd rather avoid the calculators. )

I know it's not the most efficient, but i think it's a good balance between the fun inherent in the game and the fact that if we just wanted to do things perfectly, we'd put it all in the hands of computers. The last thing we need is a Skynet on Kerbin! i just can't see Jeb as a John Connor type Kerbal.

[mhoram]

1)Just barely escape kerbin

2) circularize and get inclination with planet to 0ish by either burning anti-normal on AN or normal on DN.

3) Make a maneuver node anywhere and burn prograde or retrograde until you intersect with the planet's orbit, and then burn a few more dV

4) Pick the Maneuver node by the circle (not prograde, the center of the maneuver node) and slide it along your orbit until you get an encounter. Tada! When you think about it, this is exactly the same as the transfer window method, you are just letting the maneuver node show you where the transfer is!

If you want numbers on the Delta-V needed for this kind of transfer to the other planets, I made some tests. Hava a look in this thread: A Delta-V Map for simple flightpaths.

[Foxster]

There is a complication with KSP and transfers: warp.

If you are waiting anything up to 2 years for an efficient transfer window then you want to be above 600km so that you can get max warp. Otherwise you'll wait all day in real-time for that window.

[Greep]

Well the bright side of it is at least you get really experienced at aerobraking altitudes, as that cuts off a lot of the circularization inefficiency.

But yeah... that looks pretty horrid for eeloo/moho/dres

There is a complication with KSP and transfers: warp.

If you are waiting anything up to 2 years for an efficient transfer window then you want to be above 600km so that you can get max warp. Otherwise you'll wait all day in real-time for that window.

Nah, just go to ksp and warp ALthough if you do that, it's probably a good idea to get 80k altitude at least so you don't run into krakens.

Edit: or warp before launch of course. Not like Duna's going anywhere in 10 minutes.

Edited December 30, 2014 by Greep