oberth effect

[JtPB]

could you explain to me what exactly happens in oberth effect, & why it happens? (graphic illustration may very help)

i tried to read about it on wikipedia but i understood nothing

EDIT: ok, got it, stop the spam

Edited March 4, 2014 by JtPB

[J.Random]

As I understand it:

The more massive planetary body is, and the closer you are to it, the higher your orbital velocity is. When you burn fuel, momentum conservation works, and it's linear. You always get the same dV for the same amount of fuel spent (not counting the loss of mass). But kinetic energy is proportional to velocity squared, so, the higher your velocity is already, the more kinetic energy you get for the same increment in velocity.

[FanaticalFighter]

J.Random is correct. The delta-v is the same, no matter where you burn, but when the burn is deeper in a gravity well, the change in kinetic energy is higher.

[blizzy78]

but when the burn is deeper in a gravity well

It has nothing to do with gravity, but with your current velocity. Which of course is higher deeper down a gravity well.

[SSSPutnik]

Here is a summary of what this great thread decided.

Edited February 28, 2014 by SSSPutnik
This edit sums up discussion from the whole thread.

[Pecan]

To put it so simply that even I can understand it (although probably incorrectly ^^)...

Your ship and its fuel-load fall from apoapsis to periapsis, gaining speed all the time. At periapsis they are at their fastest = (Fastest + Burn) m/s.

Your ship and its fuel-load climb from periapsis to apoapsis, losing speed all the time. At apoapsis they are at their slowest = (Slowest + Burn) m/s.

(Fastest + Burn) is more than (Slowest + Burn).

Edited February 24, 2014 by Pecan

[SSSPutnik]

The burn has to be made prograde to get this effect, the less prograde the lesser the boost. Burning retrograde will negate the effect I believe.

You're basically trying to "drop" your exhaust with as little speed as possible. Ideally your exhaust speed = ship speed. (Eg.. If exhaust speed is 5000ms and your ships speed is 5000ms then the exhaust leaves your ship with 0 velocity). 5000-5000=0

[peadar1987]

The burn has to be made prograde to get this effect, the less prograde the lesser the boost. Burning retrograde will negate the effect I believe.

You're basically trying to "drop" your exhaust with as little speed as possible. Ideally your exhaust speed = ship speed. (Eg.. If exhaust speed is 5000ms and your ships speed is 5000ms then the exhaust leaves your ship with 0 velocity). 5000-5000=0

Nope, the Oberth effect still applies to retrograde burns (although I'm not 100% sure how or why!)

[Red Iron Crown]

The simplest explanation of the Oberth effect I've come across is this:

The energy state of a given orbit is fixed. The balance between potential energy and kinetic energy may change, but the total of the two remains the same.

To change that energy state, work must be done.

Recall that Work = Force * Distance

For a given burn time, the force is constant. But if the burn is done at a higher speed, the distance covered during that burn time is greater, so the work completed is greater. More work completed means greater amount of energy added to the orbit. This applies whether burning prograde or retrograde.

[SSSPutnik]

That doesn't gel with the wiki description of it... Well it does, but then exhaust velocity seems to play a part... Geez looks like even real rocket scientists couldn't get this one for a while...

If you burn retrograde your ship is slowing, so you are reducing the effect, since you move less distance over time. Not only that but burning in the direction of movement means the exhaust leaves your ship at exhaust velocity PLUS the speed of your ship.

The wiki also says its used to increase speeds during flybys...

Edited February 24, 2014 by SSSPutnik

[Red Iron Crown]

If you burn retrograde your ship is slowing, so you are reducing the effect, since you move less distance over time. Not only that but burning in the direction of movement means the exhaust leaves your ship at exhaust velocity PLUS the speed of your ship.

It applies to retrograde burns as well. If a spacecraft burns for 10 seconds retrograde at a starting velocity of 1000m/s and for 10 seconds retrograde at a starting velocity of 100m/s, which burn covers more distance?

It gels with the Wiki description perfectly, take a look at the very first formula given in the article.

[Carter]

It seems intuitive to me that the Oberth effect would apply at both periapsis and apoapsis, but simply reversed. The most efficient way to add energy to your orbit is to burn directly prograde at point of highest velocity (periapsis). Therefore the most efficient way to subtract energy from your orbit would be to burn directly retrograde at point of lowest velocity (apoapsis). You'll also get more bang for your buck by performing radial and normal maneuvers at apoapsis, but maybe I'm stretching to say Oberth applies to those, I'm really not familiar with the specific definition.

After playing this game for a while without really reading anything about spaceflight you start to just "get" stuff that when you attempt to explain to someone else, they just don't understand unless you showed it to them in-game. Like teaching an old person how to use an iPhone without actually putting one in their hands.

[Red Iron Crown]

It seems intuitive to me that the Oberth effect would apply at both periapsis and apoapsis, but simply reversed. The most efficient way to add energy to your orbit is to burn directly prograde at point of highest velocity (periapsis). Therefore the most efficient way to subtract energy from your orbit would be to burn directly retrograde at point of lowest velocity (apoapsis).

This may seem to be correct intuitively, but it isn't. Oberth effect applies to both prograde and retrograde burns. The most efficient way to change the energy of an orbit is to burn at periapsis.

[SSSPutnik]

It applies to retrograde burns as well. If a spacecraft burns for 10 seconds retrograde at a starting velocity of 1000m/s and for 10 seconds retrograde at a starting velocity of 100m/s, which burn covers more distance?

It gels with the Wiki description perfectly, take a look at the very first formula given in the article.

Well the answer depends on the amount of thrust.

I presume the 1000ms one would, under typical thrusts.

The very first equation only describes rocket speed vs thrust, it is not describing the Oberth effect.

Check this description, if you burn retrograde you are adding to the exhaust masses orbit so it won't/can't work as I understand it.

"

But that total energy is split between the rocket ship and the exhaust. The Oberth effect is an observation that your rocket ship ends up with more energy if the exhaust ends up with less energy. By "dumping" the exhaust when you're lower in the gravity well, it ends up in a lower orbit with less energy. Therefore, your rocket ship ends up with more energy."

I'm getting a spreadsheet out and calculating... I am prepared to eat humble pie.

Edited February 24, 2014 by SSSPutnik

[Rhomphaia]

Well the answer depends on the amount of thrust.

I presume the 1000ms one would, under typical thrusts.

The very first equation only describes rocket speed vs thrust, it is not describing the Oberth effect.

Check this description, if you burn retrograde you are adding to the exhaust masses orbit so it won't/can't work as I understand it.

"

But that total energy is split between the rocket ship and the exhaust. The Oberth effect is an observation that your rocket ship ends up with more energy if the exhaust ends up with less energy. By "dumping" the exhaust when you're lower in the gravity well, it ends up in a lower orbit with less energy. Therefore, your rocket ship ends up with more energy."

I'm getting a spreadsheet out and calculating... I am prepared to eat humble pie.

When you are burning retrograde, you are not adding to the energy of the rocket, you are subtracting from it, so you want the energy in the exhaust. the faster you are going, the more energy you add to the exhaust, the more the rocket looses.

[Red Iron Crown]

The very first equation only describes rocket speed vs thrust, it is not describing the Oberth effect.

Are we looking at the same thing? Distance is a component of the first equation:

delta Ek = F * s

where F is thrust and s is distance

Whether F is positive or negative, the magnitude of delta Ek increases as s increases.

[SSSPutnik]

OK, plugging numbers into the equation, if you want less speed, burning retrograde, your velocity is less. Using the example given here

http://www.projectrho.com/public_html/rocket/mission.php

If I have initial velocity of 3200ms and want to come to a stop its telling me I need -25ms dV burn, it clearly doesn't work slowing down...

And it makes no sense technically as you would be getting something for nothing then, your exhaust is being boosted ahead of you into a higher energy orbit rather than dumped at a lower energy orbit.

[Red Iron Crown]

As Rhomphaia notes, when burning retrograde you are trying to subtract energy from your craft, so you want more of the energy to end up in the exhaust. It is the same "something for nothing" you get when burning prograde.

[KerbMav]

Could we get one of the guys from NASA over here please?!

[SSSPutnik]

Emailed Project Rho guy, hoping for answer....

[Red Iron Crown]

He posts on here:

http://forum.kerbalspaceprogram.com/threads/28428-Orion-aka-Ol-Boom-boom

[Red Iron Crown]

Interesting example here of Oberth effect reducing the delta-V needed for an orbital capture: http://www.clowder.net/hop/railroad/Oberth.html

It compares a rendezvous with Phobos using Oberth versus a rendezvous with a Martian Trojan asteroid.

[Ventis]

I think this graph explains it best and should settle the argument:

It would seem that Red Iron Crown is right.

[Tokay Gris]

The Oberth-effect ist really quite simple.

And it is not some esoteric effect... it is just maths.

It all depends on how a rocket engine works. You throw exhaust out and actio vs. reactio, the rocket moves forward.

Lets make it simple. Lets say, the exhaust fumes masses as much as the rocket and the rocket is standing still.

If the exhaust leaves the rocket with a certain force, half of that force is in the exhaust and half is in the rocket. Both move away from each other with the same speed. Both have then the same kinetic energy.

If the rocket is moving, then still exhaust and rocket move away from each other and the same speed (if both have the same weight).

BUT since the rocket was moving, the exhaust has less kinetic energy, because it is has the force minus the speed the rocket had before burn.

And the rocket has the burn force PLUS the speed. So the rocket has more kinetic energy than the exhaust.

This goes so far that the exhaust can be moving forward AFTER the burn, if the exhaust velocity is smaller than the velocity of the rocket.

Lets say, exhaust velocity is 5000 m/s. But the rocket before the burn is moving at 10.000 m/s.

Then, after the burn, the exhaust is STILL moving at 5000 m/s forward. While the (hypothetical) rocket (of same mass) is moving forward with 15.000 m/s.

(At higher velocities, this gets funky, because of Einstein, but these speeds we are not talking about and relativity effects are not computed in KSP anyway.)

[Jouni]

That doesn't gel with the wiki description of it... Well it does, but then exhaust velocity seems to play a part... Geez looks like even real rocket scientists couldn't get this one for a while...

Early physicists were confused, because the conservation of momentum seemed to be in conflict with the conservation of energy. Then Oberth showed that you don't get energy for free with the conservation of momentum, and everyone was happy again.

Except that they decided to give a fancy name to the thing, which gave people the impression that there is something complicated or confusing about the Oberth effect. If you just accept that there is such thing as the conservation of momentum, then the Oberth effect becomes a trivial consequence of it. You gain delta-v from a rocket burn, and you lose kinetic energy to gravity. Because kinetic energy is proportional to the square of the velocity, you have more velocity remaining at a certain altitude, if you did the burn while moving at a high velocity than at a low velocity.