Faster (ballistic?) planetary transfers

[The Pink Ranger]

On a typical Hohmann transfer you intercept the target 180 degrees around the parent body from where you burned but I noticed that if you expend more dV you can intercept the target sooner on its orbit. Are there specific angles to shoot for which always hold true? For example if I expend a little more dV during the transfer burn I can intercept the target at about 90 degrees from the burn point. If I burn some more I can intercept the target at an even more acute angle. Hope you understand what I'm getting at.

[Red Iron Crown]

You should play around with Alexmoon's transfer calculator. In the advanced settings you can restrict the transfer time to help calculate a higher energy, lower transfer time transfer. You can also restrict the launch window so you're not waiting around for months for your transfer.

[The Pink Ranger]

I appreciate the info but KSP's main draw for me is that it's such a good conceptual tool and I'd rather be able to intuit and eye-ball things than rely on a calculator.

Anyway look at page two of this PDF. There's a diagram that illustrates how the transfer orbit from Earth to Mars changes with greater expenditures of dV. I noticed the intercept points look pretty evenly spaced at about 45 degrees from each other and I was wondering if this is something that applies ALL of the time.

[Red Iron Crown]

Those are just examples, you can choose any point in between the points shown and have an energy cost in between the two adjacent examples.

[The Pink Ranger]

Ah gotcha. So as long as you burn at the transfer window and the transfer orbit intersects the target's orbit you will always get an intercept.

[Kasuha]

Hohmann transfers are optimal transfers (in terms of dv spent) between two circular orbits if their diameters are close enough (otherwise bi-elliptic transfer is better at a cost of more time spent) and in absence of other massive bodies (otherwise a gravity slingshot can be used). Faster transfers are not called hohmann transfers but they are of course possible.

Alexmoon's calculator will not give you hohmann transfers in general (because it optimizes for more parameters), although in most cases the difference is negligible if you let it to find local minimum in the porkchop plot.

Below are two examples of non-hohmann transfers which spent more dv to save some transfer time. More can be found on corresponding Wikipedia page.

[Kasuha]

Ah gotcha. So as long as you burn at the transfer window and the transfer orbit intersects the target's orbit you will always get an intercept.

That is not true, but you'll likely find out yourself if you try it. Non-hohmann transfers don't really have transfer windows, you can launch anytime and there is always certain set of parameters (ejection angle, spent dv, transfer time) that will get you to the target, but you need to use specific set of values to get an intercept, not just launch anwhere or at any speed.

[The Pink Ranger]

That is not true, but you'll likely find out yourself if you try it. Non-hohmann transfers don't really have transfer windows, you can launch anytime and there is always certain set of parameters (ejection angle, spent dv, transfer time) that will get you to the target, but you need to use specific set of values to get an intercept, not just launch anwhere or at any speed.

Yea I thought about it a little and realized that you have a very small margin to get an encounter when you're doing a transfer. However when I play with the stock Kerbol system and I'm perfectly lined up with the ecliptic during my burn I notice that if I continue to burn after the initial "Hohmann encounter" I will get a couple of more encounters (at what I think are specific angles) sooner on the target's orbit.

While I realize that you can transfer from any phase angle you want I think my original assumption that there are intercept points at specific angles from the initial burn point might still hold weight. So I guess the only way to confirm this is to take advantage of this wonderful tool squad has provided for me.

Anyway the only reason I'm asking instead of just testing it out is because I play with RSS where all of the planets are slightly off the ecliptic so you almost never get encounters and you almost always have to do mid course correction burns to get on the target's plane. I guess I'll report back when I've figured it out.

[JDCollie]

I appreciate the info but KSP's main draw for me is that it's such a good conceptual tool and I'd rather be able to intuit and eye-ball things than rely on a calculator.

Anyway look at page two of this PDF. There's a diagram that illustrates how the transfer orbit from Earth to Mars changes with greater expenditures of dV. I noticed the intercept points look pretty evenly spaced at about 45 degrees from each other and I was wondering if this is something that applies ALL of the time.

Wow, looking onward in that document, the way that transfer burns are achieved with VASMIR drives is vastly different than how I currently do transfer burns in KSP, since if I'm understanding the document correctly, the VASMIR allows the spacecraft to apply thrust for the duration of the voyage for optimal (in terms of speed + fuel efficiency) transfer. (Relevant since I use life support mods) In comparison, my own transfer times seen very slow.

[Kasuha]

Anyway the only reason I'm asking instead of just testing it out is because I play with RSS where all of the planets are slightly off the ecliptic so you almost never get encounters and you almost always have to do mid course correction burns to get on the target's plane. I guess I'll report back when I've figured it out.

I'm not sure if there is an equivalent of Alexmoon's planner for RSS (I wouldn't be surprised if there was one) but I am almost certain that even in RSS, ballistic transfers can be used efficiently to get to the destination. As a simulation, you can check the difference in optimal transfer to Eeloo in Alexmoon's calc. Ballistic transfer comes at 3559 m/s, while transfer with mid-course plane change needs 3556 m/s dv. That difference is completely negligible and you'll lose more on inefficiences in your burn.

[Nori]

Wow, looking onward in that document, the way that transfer burns are achieved with VASMIR drives is vastly different than how I currently do transfer burns in KSP, since if I'm understanding the document correctly, the VASMIR allows the spacecraft to apply thrust for the duration of the voyage for optimal (in terms of speed + fuel efficiency) transfer. (Relevant since I use life support mods) In comparison, my own transfer times seen very slow.

Agreed. Wonder if the time control mod really works for the long Ion burns... Or with the NFT VASMIR drives.

In any case the document is super interesting and rather exciting.

[cantab]

In practical terms, if you're in your parking orbit and know you want to leave now, and you have a certain amount of delta-V to use, just set a manouevre with that much delta-V prograde, then move it around the orbit until you get an encounter or close approach with the target, then refine as needed.

This will work for standard Hohmann transfers if you know how much dV you need and that you're in a transfer window.