Oberth Effect and Interplanetary Burns?

[Vedexent]

I'm wondering if anyone knows of any mods, or online web applications, that help one to set up using the Oberth effect on an interplanetary ejection burn.

I'm thinking that ideally, one would want to set up a highly elliptical orbit around Kerbin, with a periapsis of - say - 100km, and the apoapsis as close out of the edge of the Kerbin SOI as possible.

Provided one doesn't have problems with accidental slingshot maneuvers around Minmus of the Mun, one would be going max possible orbital speed at the periapsis; maximum Oberth effect on one's burn to Duna, Jool, or wherever.

The problems I see with this are:

• Timing: One would have to "tune" the orbital period so that the craft arrives at periapsis just as one needed to be burning for ejection
  
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• Angle: You would have to set up the initial elliptical orbit up so that the hyperbolic ejection path after the periapsis burn would be either in prograde or retrograde to Kerbin's orbit. I'm thinking the direction of the "long axis" of the ellipse would be related to the Kerbin phase angle around the Sun.

Has anyone had any successful experience setting this all up?

Does anyone know of any tools / mods that could help set this up?

Or - alternatively - have I missed something and it's not as complicated as it seems?

Thanks,

[Kosmo-not]

I start with a circular orbit of around 70km. I don't usually have to perform multiple burns.

[Vedexent]

I start with a circular orbit of around 70km. I don't usually have to perform multiple burns.

Agreed - that's the optimum simple case: lowest orbit, highest "natural" velocity, best efficiency.

Doing a deep dive from a highly elliptical orbit would give you a much higher velocity, and thus more of an effect - but it seems way more complicated to set up.

[stupid_chris]

Actually, that technique won't affect the amount of delta V at all. See, Oberth effect is just about speed, and the distance you can get your apoapsis at is too about speed. Now, if you want to set your apoapsis to the edge of the SOI, you'll have to burn nearly to the escape velocity. The thing is, you will have exactly the same velocity the moment after you complete that maneuver to push your apoapsis out than when you will come back, falling from your apoapsis. So Oberth effect will affect you the same way if you keep burning than if you were to fall back down, if you were thrusting moments before doesn't matter, it'S just about speed. You'll use up the exact same amount of fuel if you just keep pushing your apoapsis out to get that encounter with Duna than if you just go out to the Edge of the SOI and come back down to do finish your transfer.

So yeah, just do it normally, don't bother with that kind of complicated math.

[Vedexent]

You'll use up the exact same amount of fuel if you just keep pushing your apoapsis out to get that encounter with Duna than if you just go out to the Edge of the SOI and come back down to do finish your transfer.

So yeah, just do it normally, don't bother with that kind of complicated math.

Ah, Got it

I could see that the return velocity coming back from apoapsis would be way higher and increase the effect, but I didn't take into consideration the energy used to set up the orbit - and yeah, I can see that there just isn't a net gain here.

Thank you!

edit: I do have a Kethane refinery / re-fueling station in orbit of Minmus though, so perhaps the idea isn't completely dead if I can replace the fuel needed to get out to Minmus. Still - the original question was a "no starter".

Edited July 24, 2013 by Vedexent

[ramnrmeul]

Well, if you're dealing with Ion drives, what you're proposing is actually valid. The ion drives have such a low acceleration, that you would use a lot more Delta-V than is necessary than if you burn multiple times at periapsis.

[Alistone]

Ah, Got it

I could see that the return velocity coming back from apoapsis would be way higher and increase the effect, but I didn't take into consideration the energy used to set up the orbit - and yeah, I can see that there just isn't a net gain here.

Thank you!

edit: I do have a Kethane refinery / re-fueling station in orbit of Minmus though, so perhaps the idea isn't completely dead if I can replace the fuel needed to get out to Minmus. Still - the original question was a "no starter".

It's not really a "no starter". It is actually an intermediate step for people with low T/W ratios. They will burn at periapsis and create an eliptical orbit and they will continue to do this each pass of their periapsis orbit to do as much burning near periapsis as possible. BUT as soon as their apoapsis is no longer in the Kerbin SOI, they cannot do this again because they won't be coming back to Kerbin's SOI any time soon... so at that point they just do whatever is left for their interplanetary burn.

The main point is that there is no reason to try to setup your elliptical orbit first. You may as well just let it fall where is falls as you do your interplanetary burn.