Oberth effect and nodes.

[SSSPutnik]

Question, do maneuver nodes take the Oberth effect into account?

Edited February 18, 2014 by SSSPutnik

[adinfinitum]

Yes, they do take the Oberth effect into account.

[SSSPutnik]

Thanks, all I needed.

[blizzy78]

Actually, they don't, because there is nothing to account for. A maneuver node is nothing more than a direction and a change in velocity in that direction. The Oberth effect comes naturally because of your current velocity right before executing the node.

[SSSPutnik]

Err so when burning at the node, I just end up using less fuel for the same dv change?

[technion]

Err so when burning at the node, I just end up using less fuel for the same dv change?

I don't believe fuel consumption can change without an ISP change (or the rocket algorithm would be broken).

A few tests demonstrate that a manoeuvre node to Jool from low down shows a lower dv than one in a high orbit. In other words, the node accurately reflects what is required to burn, it's the dv needed to get where you want, and therefore the size of the node that changes.

[blizzy78]

Err so when burning at the node, I just end up using less fuel for the same dv change?

It does not matter if you're near to a planet (going fast) or far from it (going slow.) It does not affect your vessel itself, especially not your engine's Isp. Therefore, the fuel requirement is the same for the same change in velocity.

[maccollo]

This got confusing pretty fast.

Simply put, the oberth effect does manifest itself when you plot maneuvers at different altitudes above the planet.

If you come in for a capture burn around Eve it will require much less delta V if you do it right above the atmosphere than at the edge of the SOI.

[SSSPutnik]

Yup, now I have zero understanding of the Oberth effect as enacted in KSP...

What if I do a burn at periapsis for a gravity slingshot?

If I place maneuver node, it says X ms change required.

For there to be any effect I should be using less fuel for the same dV change!?

[KerbMav]

The change in dV always uses the same amount of fuel (per dV for a given craft with a given TWR), but the needed change in dV is lower, because you are already faster than higher up.

[SSSPutnik]

OK got it again, so the effect is totally dependant on altitude and speed.

[maccollo]

Got two images that sums up the whole thing better than I could do with words.

https://dl.dropboxusercontent.com/u/22015656/oberthEffect2.png

https://dl.dropboxusercontent.com/u/22015656/oberthEffect1.png

The resulting orbits are pretty much identical, but the second one took less deltaV to achieve.

Edited February 17, 2014 by maccollo

[blizzy78]

Essentially, the Oberth effect and fuel requirements for specific delta-v requirements have nothing to do with each other. The Oberth effect is simply that you will need less delta-v (and hence, less fuel) to perform your maneuver, but for a given delta-v requirement the fuel requirement will always stay the same, regardless of current velocity. (As I explained earlier, the vessel's current velocity does not affect the vessel in any way, so as a consequence, the fuel requirement must stay the same.)

[SSSPutnik]

Blizzy, I think you're wrong on second half. Reading the wiki on Oberth it has everything to do with velocity at burn time, and the higher your TWR the better. It sounds like the best is when your fuels kinetic energy has its highest value.

Briefly burning the engine (an "impulsive burn") prograde at periapsis increases the velocity by the same increment as at any other time (\Delta v). However, since the vehicle's kinetic energy is related to the square of its velocity, this increase in velocity has a disproportionate effect on the vehicle's kinetic energy; leaving it with higher energy than if the burn were achieved at any other time."

You're basically "stealing" kinetic energy from your fuel.

[John FX]

Question, do maneuver nodes take the Oberth effect into account?

Simple answer.

Yes. You do not need to do anything to account for the Oberth effect, it is included in the Node.

nice example images Maccollo

[wmheric]

Blizzy, I think you're wrong on second half. Reading the wiki on Oberth it has everything to do with velocity at burn time, and the higher your TWR the better. It sounds like the best is when your fuels kinetic energy has its highest value.

Briefly burning the engine (an "impulsive burn") prograde at periapsis increases the velocity by the same increment as at any other time (\Delta v). However, since the vehicle's kinetic energy is related to the square of its velocity, this increase in velocity has a disproportionate effect on the vehicle's kinetic energy; leaving it with higher energy than if the burn were achieved at any other time."

You're basically "stealing" kinetic energy from your fuel.

I think what Blizzy was trying to say is that fuel requirement and a physics "effect" have no relation to each other. You do not "end up using less fuel for the same dv change". Given the same rocket design, the fuel requirement is always the same for a certain delta-v requirement. You save fuel because the delta-v requirement is less, the phenomenon that delta-v requirement is less at a higher speed is termed the Oberth effect; it isn't really something that pops up when you thrust -- it is just there.

The answer to the question "do maneuver nodes take the Oberth effect into account?" is no, because it doesn't take into account some physics. It is like asking "do engine burns take the Newton's Second Law into account?". KSP implements classical mechanics, so any physics consequence is just there in the game.

I'm not trying to be picky here. I want to fight physics misconceptions

[blizzy78]

I think what Blizzy was trying to say is that fuel requirement and a physics "effect" have no relation to each other. You do not "end up using less fuel for the same dv change". Given the same rocket design, the fuel requirement is always the same for a certain delta-v requirement. You save fuel because the delta-v requirement is less, the phenomenon that delta-v requirement is less at a higher speed is termed the Oberth effect

That's basically what I was trying to say, yes. Moving faster does not magically make your use of fuel more effective; the fuel requirement stays the same for a given amount of delta-v. Instead, the delta-v required to achieve a final velocity magically goes down when moving faster because of the Oberth effect.

[Poynting]

The oberth effect does not affect your ship in any way. If you make the same length burn with the same ship you will get the same DeltaV (obviously the amount of fuel you have remaining makes a difference as once your ship has less mass you will get more acceleration for the same amount of fuel). The oberth effect occurs dependant on WHERE you make your burn. Burning low down in a gravity well (say to escape the planets SOI) not only accelerates you ship away from the planet, it also reduces the time for which gravity has to slow you down as you escape. It also works the opposite way round. If you come shooting into an SOI towards a planet, you travel quickly towards it not giving the planets gravity a long time to accelerate you (Therefore you dont need to use fuel removing this velocity when you want to slow down relative to the planet). Also once you burn, the closer to the planet you are the more time gravity has to slow you down and therefore the less velocity you need to remove to prevent yourself flying back out into interplanetary space. Your rocket isnt influenced by where it is (Constant ISP, Constant fuel consumption for a given throttle setting, DeltaV produced only depends on the properties of your rocket), but the resulting change in orbit for a given amount of DeltaV is VERY dependant on the previous orbit and WHERE the burn is made.

(An illustration of the oberth effect is the act of hovering. Using 400m/s I can hover for x number of seconds if I burn continuously for x seconds. However if I expend all 400m/s is a small fraction of x I will fly up and away from the gravitating body.)

Oberth effect rules of thumb:

When going to another planet make your transfer burn whilst still in low kerbin orbit (Its more difficult than going into the suns SOI but you save a lot of fuel).

When arriving at another planet make your deceleration burn as close to the planet as possible.

[Poynting]

Got two images that sums up the whole thing better than I could do with words.

https://dl.dropboxusercontent.com/u/22015656/oberthEffect2.png

https://dl.dropboxusercontent.com/u/22015656/oberthEffect1.png

The resulting orbits are pretty much identical, but the second one took less deltaV to achieve.

Yes this is a perfect example of a capture burn taking advantage of the oberth effect. Nice pictures Macollo

[SSSPutnik]

Yes, got it, but I think there is some misconception here, its not where, its the speed that matters. Its more efficient close to the periapsis as that's when you are travelling fastest.

[Poynting]

Theres no misconception, but your right my explination isn't perfect (I'd be amazed if it was!). Your right the pure oberth effect is definitely about speed. The faster you are going the more kinetic energy is imparted to the ship due to the fuel you are burning also having kinetic energy on top of the chemical stored energy. (Wikipedia does a decent job of explaining it http://en.wikipedia.org/wiki/Oberth_effect)

I just wanted to impress the fact that its a connected system and that the oberth effect is used in connection with gravitational effect to produce the fuel savings. So it the positioning of your ship that will allow you to find the highest velocities nearest a gravitating body. Its then the interaction with gravity that produces the benefit. You can't use the oberth effect in free space to make a more efficent change to your velocity.

[SSSPutnik]

No... Well in the real world I guess you could but it would be pretty weak.

By misconception I didn't mean you, but some of the other posters seem a little confused on the subject.

For instance the hovering example. That's not Oberth, that's pure f=ma really.

Edited February 17, 2014 by SSSPutnik

[adinfinitum]

You can't use the oberth effect in free space to make a more efficent change to your velocity.

It's not so much a more efficient change to your velocity as it is a more efficient change to your specific orbital energy, which is the sum of the potential and kinetic energy of your craft. Even in free space your kinetic energy would increase proportional to the square of your velocity, so a change from 2000m/s to 2500m/s would get you more kinetic energy than 500m/s to 1000m/s. But since there you aren't orbiting a body, there is no orbit or specific orbital energy to calculate anyway, and the oberth effect would be irrelevant, although it still technically occurs. However, since you are always in the SOI of some body in KSP, you can always use the oberth effect to your advantage.

It's true that the Oberth effect isn't some calculation the game would have to take into account after you place a maneuver node, it's simply a handy side effect of the vis-viva equation, which is how your specific orbital energy is calculated.

Because the semi-major axis of your orbit increases proportionally to your orbital energy, and the orbital energy changes proportionally to the square of your velocity, you get a much larger change in orbit for the change in velocity when you are traveling fast as opposed to slow. An example is comparing a trip to the Mun vs Minmus. It takes about 850m/s dV to get your orbit up to the Mun, at 12,000 km, while for about 920m/s dV, only an additional 70m/s, you can get your orbit up to Minmus, at 47,000 km.