Oberth Effect

[ohlookabirdie]

So i've done a few interplanetary trips before career mode was active and was flicking through a few threads on here looking for new more efficient ways to do voyages to the planets now that it is active (.22). one term that pops up quite regularly is "the oberth effect" and i have no idea what it is... or how it works. could someone please explain it to me in terms of the ksp universe and get me on the right steps to my first zero-mods interplanetary flight

playing .22 career mode with no mods, used to using mechjeb in sandbox and its great fun trying to relearn how to fly a rocket.

[TheReaper]

in the most basic form, burning at periapsis facing prograde is efficent, since planets gravity is bosting your speed a little bit more, it is most usefull when you need to burn multiple time, multiple burns ussualy happens when you need a very low thrust engine and need a very high delta V, such as burning several km delta V with a single nuclear engine, burning whenever you get near periapsis, when a little more when you get away a little more from periapsis, and that is all it, remember only prograde burns get more efficent at periapsis, inclination changes and retro burns are better at apoaps

Edited October 20, 2013 by TheReaper

[Specialist290]

Essentially, the basic principle of the Oberth Effect is that the closer you are to the center of gravity of the body you're orbiting when you execute a burn, the more "bang for your buck" you'll get out of your fuel.

This page on MyKSPCareer does a good job of going into more detail about it. For that matter, I'd highly recommend reading the rest of the site as well.

[ohlookabirdie]

thanks guys... this seems pretty intuitive anyway... i seem to have been doing it without knowing

[blizzy78]

http://en.wikipedia.org/wiki/Oberth_effect

[BlazeFallow]

Quick question to piggy back off this. I don't know anything about the physics engine in the game, but it feels pretty robust (with some obvious flaws) when I play. Is the Oberth effect actually calculated in this game? Or is it something so inherent in the physics calculations that it just happens and there doesn't need to be additional calculations on the part of the game to factor it in? I read the wikipedia article on the Oberth effect and couldn't surmise my own answer.

[madbadger2742]

Oberth is a natural effect of orbital mechanics, so I highly doubt there are any special calculations made for it.

[allmappedout]

The Oberth effect is simply the result that as velocity increases linearly, kinetic energy increases at its square.

Calculation of Work Done is Force applied over a distance Work (Wiki) (ie: if you push a spaceship in a vacuum with the same force for 1metre, it doesn't go as far as if you push it with the same force for 2 metres). Since you're travelling faster, for the same given burn time, you're travelling through more of your orbit (since you're travelling faster and distance is speed*time), so you do more work for the same amount of applied force.

It's like when you push a swing - you don't have to apply as much force at the middle of the swing (when you're moving fastest) to increase the height the person gets, than if you applied it at the top of the swing. Conversely, if you apply the same force at the bottom of your swing, you go higher (which is the same as burning at Periapsis, when you think about it).

The Work-Energy principle kind of sums it up nicely in conjunction with the first statement:

The work done by all forces acting on a particle (the work of the resultant force) equals the change in the kinetic energy of the particle.

As kinetic energy increases, you get more work done.

Edited October 24, 2013 by allmappedout