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To Oberth, or not to Oberth?


DrunkenKerbalnaut
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Let's keep it simple:

 Arrive at Minmus SOI from Kerbin. Hyperbolic orbit with perigee at 150km, generally equatorial. Desired orbit is 15km x 15km, equatorial. I can play this two ways:

 

1) Immediately burn retro, dropping Pe to 15km. Then burn retro at Pe to lower Ap to 15km. 

2) immediately burn nadir to tighten Pe down to 15km. Then burn retro at Pe to lower Ap to 15km. 

What at is the difference? What is the trade off between the two methods? Would (1) actually effect  entry speed in a meaningful way, if it were Duna and not Minmus? Would (2) provide more of a boost if I was only using the encounter for a slingshot?

These are my suspicions, but I find it hard to quantify any F5/F9 results. When should I be employing which method?

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If you are after efficiency, then you should aim to get your intercept Pe to be as close as possible to your final orbit, so an early en route correction burn is optimal.

If you already are at 150 km Pe then Mr. Hohman is your friend.

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1 hour ago, Shpaget said:

If you are after efficiency, then you should aim to get your intercept Pe to be as close as possible to your final orbit, so an early en route correction burn is optimal.

If you already are at 150 km Pe then Mr. Hohman is your friend.

i usually throw on a few Pe tweaks when im aligning my orbital plane (and this is after my injection burn, because plane changes use less fuel when your orbit is eccentric). im usually in the within a few meters of my target when i get to Pe, and i just need to lower Ap where i want it.

Edited by Nuke
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easiest to test it. Set up an radial burn to lower Pe, then an circulation burn, note sum of the two burns.
Then an braking burn who lower your Pe then an circulate burn. 

Gut feeling is that radial first is cheapest, on the other hand you don't gain much Oberth from Minmus so testing it would be interesting. 

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Energy isn't the only game in town. You also have to kill angular momentum. This is why Hohmann isn't always your best option for transfer. You want to be as low as possible to bleed energy, but you want to be high while bleeding angular momentum. The maneuver that maximizes angular momentum loss without giving you even more energy to bleed at periapsis is a retrograde burn.

In general, almost all of your burns should be prograde or retrograde. There are very few exceptions. Notably, inclination change, intercept adjustments when you just have to be at a certain place at certain time, and the last bit of landing on an airless body if you are going for optimal efficiency.

 

Edit: Scratch that. What I wrote above is true for 1-body approach from infinity. You brake retrograde early, dropping perigee. If you are in KSP physics, highly hyperbolic, and already entered SOI, this changes things. First, you made a nav error with such approach, but there is no use crying over it. Energy is your main enemy now. You need to do a minimal burn that redirects you towards a lower periapsis at first opportunity. In general, this won't be exactly radial, unless you already reached your initial periapsis. But given size of SOI and high entry, it's going to be mostly radial. Basically, the earlier you make this adjustment, the more retrograde and more efficient the burn. But if you wait until the last moment, you'll have to burn radial and waste fuel. Still not as bad as burning retro at the last moment, though.

Edited by K^2
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