Why does the oberth effect work?

[Red Iron Crown]

Gravity doesn't cause the Oberth effect per se, but it can create conditions in which you can maximize it. It is entirely possible to get more Oberth effect when being affected less by gravity. E.g. a 1000x100km orbit will gain more from Oberth at periapsis than a 70x70km orbit, even though gravity is greater at 70km than at 100km. Speed is the critical factor, not gravity.

Adding velocity to a higher velocity imparts more kinetic energy than adding velocity to a lower velocity.

This is one of the most concise ways to put the Oberth effect into laymans terms I've ever read. +1

So it is, though it should be mentioned that subtracting velocity from a higher velocity removes more kinetic energy than subtracting velocity from a lower velocity. I.e. the Oberth effect works for retrograde burns, too.

Edited February 20, 2015 by Red Iron Crown

[RainDreamer]

So it is, though it should be mentioned that subtracting velocity from a higher velocity removes more kinetic energy than subtracting velocity from a lower velocity. I.e. the Oberth effect works for retrograde burns, too.

So that is why bi-elliptic transfer works! Still doesn't understand why retrograde burns work that way too though.

[LostOblivion]

Thanks for the clarification. I guess moving faster out of the gravitational pull of the primary body only means getting a higher apogee.

I guess the Oberth effect, like relativity, is hard or nearly impossible to understand intuitively.

[Red Iron Crown]

So that is why bi-elliptic transfer works! Still doesn't understand why retrograde burns work that way too though.

It's conservation of energy. A repost from a previous thread about Oberth:

As a thought experiment, imagine the following:

Two identical ships are docked together, with one facing prograde and the other facing retrograde. An explosive charge is placed between them and detonated at periapsis thrusting the ships apart with equal force in the prograde and retrograde directions, with each ship's kinetic energy changing by the same amount. Each ship perceives the other as fuel it has expended to change its velocity. The ship headed in the prograde direction, you will admit, gains extra kinetic energy due to the Oberth effect. Due to conservation of energy, the ship headed in the retrograde direction must have lost exactly the same amount of energy as the ship going prograde gained, so it must have experienced the Oberth effect as well.

The Oberth effect is symmetrical. The exhaust of the ship gains or loses exactly the same amount of "extra" energy as the ship itself.

[Bryce Ring]

Why is Oberth effect now the relationship between V and KE? I thought that relationship was explored long before Mr. Oberth?

Why is Oberth effect now not how to and why you get a Higher AP? Isnt that where it was coined.

The Oberth effect AFAIKnew was the exploitation of How KE can be most effectively gained such as the way LostOblivion described, and with the how and why it is effective relative to orbital maneuvers.

Without a gravitational influence the oberth effect is useless as an exploit for orbital mechanics.

What is eK without an Opposing force (A Relative force) such as Gravity air or clay for that matter. ??

These are honest questions.

Please don't think I am digging or defending a thought, I just want to know.

To understand Oberth effect. Would I need to first understand eK and then apply the principals to orbital maneuvers ?

Thanks;)

Bryce.

If I had to sum it up, I'd just say, let gravity do all it can to increase your Velocity, then, once Gravity is no longer Increasing the Speed part of your Velocity, IE Gaining Altitude now, Use your deltaV so as to reduce time in local Gravity as you climb, allowing more Momentum to be converted to Potential energy.

Edited February 21, 2015 by Bryce Ring

[Red Iron Crown]

Why is Oberth effect now the relationship between V and KE? I thought that relationship was explored long before Mr. Oberth?

The Oberth Effect is not really a separate physical law, it is an observation of how objects which can accelerate independent of speed change kinetic energy. The reason it was a revelation is not because it changed the way we calculate kinetic energy (that's still Newton's KE=0.5mv^2), it's that the observation wasn't made before because so few surface-bound or atmosphere-bound objects can accelerate independent of speed.

The Oberth Effect, simply, is this: It is most efficient to change kinetic energy and orbit size by changing speed when moving fastest.

The kinetic energy change is what changes the size of the orbit, increasing when burning prograde and decreasing when burning retrograde. To maximize the amount of energy change and orbit size change, perform those types at the fastest point in the trajectory (at low periapsis).

Why is Oberth effect now not how to and why you get a Higher AP? Isnt that where it was coined.

The Oberth effect AFAIKnew was the exploitation of How KE can be most effectively gained such as the way LostOblivion described, and with the how and why it is effective relative to orbital maneuvers.

That's a pretty good description of how the Oberth effect benefits spacecraft (with the addition that you get a lower Ap when burning retrograde, like in a capture burn). It's not why it works, but it is how to take advantage of it.

Without a gravitational influence the oberth effect is useless as an exploit for orbital mechanics.

What is eK without an Opposing force (A Relative force) such as Gravity air or clay for that matter. ??

Gravity doesn't cause the Oberth effect, but we use gravity to create situations where we can harness it effectively. In a gravity system, speed, altitude and energy are closely related, the Oberth effect trades altitude for speed so it is cheaper to change energy.

If I had to sum it up, I'd just say, let gravity do all it can to increase your Velocity, then, once Gravity is no longer Increasing the Speed part of your Velocity, IE Gaining Altitude now, Use your deltaV so as to reduce time in local Gravity as you climb, allowing more Momentum to be converted to Potential energy.

That's good advice, but it's not the time reduction in gravity that matters so much as getting your speed up where it is easier to change its energy. For a retrograde burn, the Oberth Effect actually increases your time close to the gravity well.

Edited February 21, 2015 by Red Iron Crown