Oberth effect - intuitive explanation(s)

[Cheaterman]

I found an intuitive explanation of the Oberth effect in a way anyone can experiment at home - especially these sunny days.

Imagine someone in a hammock, slowly oscillating: if this person wants to keep oscillating, there are two ways he could give energy to his hammock.

The first one is pushing his finger towards the ground when he's at the highest point of his trajectory. That makes him apply a prograde force at his lowest speed and is very inefficient - I tried it so I suggest you do as well.

The second (and most efficient thanks to the Oberth effect) way is to push with his finger in a tengential direction when he's at the lowest point of his trajectory - and therefore at the highest speed. Again, I suggest you try this at home, but you're not fighting gravity in any way while doing that which makes your energy transfer more efficient.

In orbit it's the same thing, except the performance gain is less impressive than with the hammock due to the fact you always end up fighting gravity at some point - as long as you're not doing a timed launch with a direct ejection.

Please feel free to add your own intuitive experiments of the Oberth effect in everyday life!

[spikeyhat09]

if i understand the metaphor correctly, to exploit the oberth effect wouldnt one need a smaller hammock

[SleepyWacKo]

great, now explain what is a hammock?

[Juanfro]

It't like a spaceship made completely with struts. But they don't have SRB for some reason, I must research why.

[Arran]

great, now explain what is a hammock?

One picture tells more as a thousant words

[Guest]

One picture tells more as a thousant words

It's "more than a thousand words", but I like yours too .

[Frederf]

I struggle to find a common household way to describe that kinetic energy rises as the square of speed. Very little in our daily lives operates on a "fixed delta-v" basis like rockets do. Because friction is so prevalent often high speed is the worst time to increase speed since the friction increases at a rate exceeding the square. Low speeds minimize many kinds of friction but downplay the exponential relationship between speed and energy.

Pendulums like a hammock or playground swing are good simple models for the interplay of motion and potential motion like what happens in spacecraft orbits.

[numerobis]

My intuition about the Oberth effect is just that the faster I go, the less time gravity has to affect me over any given distance, so the less gravity drags me down.

Conversely, the faster I'm falling, the less extra speed I will have built up when I get down to periapsis.

[Kosmo-not]

The Oberth effect simply states that the power a rocket produces is greater when the rocket is travelling faster.

[KerbMav]

My intuition about the Oberth effect is just that the faster I go, the less time gravity has to affect me over any given distance, so the less gravity drags me down.

Conversely, the faster I'm falling, the less extra speed I will have built up when I get down to periapsis.

Gravity doesnt need any time to affect you.

Burning retrograde also doesnt mean "pushing" the ship further away, it accelerates the ship so its orbit changes accordingly.

I read the wiki article three times now - but Im still like ... uhu ... sounds nice ... what??

[Flowz0r]

Very intuitive, problem is that your example has nothing to do with the oberth effect.

[KerbMav]

OK, this site takes a different approach.

To summarize what I understood:

A rocket is like a gun, although we have to switch the model around, as in the rocket being the barrel and the fuel/exhaust being the projectile.

The rocket shoots out fuel/exhaust and its speed increase is equivelant to the recoil of the gun.

Now would the gun travel at the muzzle velocity of the projectile, the bullet would just be released hanging dead in the air (and dropping down if inside a gravitational field/fired on e.g. earth).

Now it says that because the rocket put energy into the fuel by expelling it yet the exhaust's "backwards" speed is lower by the amount the rocket is going forward, their is a difference in energy that has to be left somewhere because energy cannot be destroyed. So the Oberth effect is the transfer of the kinetic energy of the exhaust onto the rocket.

Still puzzles me - the fuel had the same kinetic energy as the rocket while still in the tanks - wasnt the energy used do stop that motion? How can it be transfered onto the rocket?

[Mephane]

Still puzzles me - the fuel had the same kinetic energy as the rocket while still in the tanks - wasnt the energy used do stop that motion? How can it be transfered onto the rocket?

Well energy can not be "used up", of course! It has to go somewhere, and the only place it has to, is the spaceship.

[Somerled]

Oberth effect (from wikipedia): the use of a rocket engine when travelling at high speed generates much more useful energy than one at low speed.

For us, the operative word is "useful." The most useful energy you want to change in an orbit is your specific orbital energy. Like kinetic energy, this changes as a square of velocity. If you start your burn while moving faster, your SOE increases much faster.

Mathematically, this is due to an extra term appearing as a result of the square of velocity (find an equation for SOE and do the math, V+dV, it won't take long). Physically, you must consider your reference frames: your orbits change in very different ways at periapsis and apoapsis. One change is more efficient than another in imparting orbital energy to the rocket. Intuitively, we think of gaining delta-v, but that's a term relating directly to burn times; our true goal of using delta-v is to change our energy, which changes as a square of velocity.

[KerbMav]

Well energy can not be "used up", of course! It has to go somewhere, and the only place it has to, is the spaceship.

As I said, didnt it go into the exhaust to "stop" the particles?

[robly18]

I always just thought of the orberth effect as this: When you're firing prograde, you're increasing your altitude in the opposite point of your orbit. Therefore you're not increasing it anywhere else, increasing the efficiency. If you do it at the periapsis, since you're increasing what is already the highest point, it becomes even more effective because you,re not needing to go that extra mile per hour to increase up to the apoapsis. However, if you pointed, say, away from the barycenter, you'd be increasing your orbit more uniformly, thus not increasing it nearly as much as if you'd just spent all that Delta V on firing prograde.

Maybe I was wrong? I don't know.

[Mr Shifty]

A rocket is like a gun <snip>

This is the correct explanation.

Still puzzles me - the fuel had the same kinetic energy as the rocket while still in the tanks - wasnt the energy used do stop that motion? How can it be transfered onto the rocket?

The fuel has chemical energy stored in its molecular bonds that it releases through combustion. The Oberth effect basically is a way to maximize the transfer of the chemical energy of the propellant to the kinetic energy of the rocket. If you're going faster, more of the chemical energy is used to accelerate the rocket rather than the propellant.

[Stratoroc]

A practical demonstration would be playground swings. throwing your legs to get a push works best when you're nearest the ground moving fastest, you can do whatever you want at the top of your arc and it won't change the swing much at all

[Mephane]

As I said, didnt it go into the exhaust to "stop" the particles?

To the contrary. When their speed is reduced, their energy also is (physicists would rather word this the other way round though). What you are probably thinking of is impulse. Transfer of energy and transfer of impulse are two completely different beasts, and can go hand in hand or in opposite directions, depending on the situation.

[lazarus1024]

This is the correct explanation.

The fuel has chemical energy stored in its molecular bonds that it releases through combustion. The Oberth effect basically is a way to maximize the transfer of the chemical energy of the propellant to the kinetic energy of the rocket. If you're going faster, more of the chemical energy is used to accelerate the rocket rather than the propellant.

This.

There is no practical demonstration you can do with swings, or hammocks or anything like that. None of that is the Oberth effect or anything remotely like it.

The closest you can kind of come is hop on a skate board and throw a medicine ball off. Next, push the skateboard up to a bit of speed and throw the medicine ball off opposite the direction of travel. Doing this results in a bigger push of the you/skateboard system. Oberth effect. Visual/sensory difference in the push though might not be overly noticable as the difference in energy of the system isn't all that high as the velocities involved are relatively low.

[Frederf]

OK, this site takes a different approach.

To summarize what I understood:

A rocket is like a gun, although we have to switch the model around, as in the rocket being the barrel and the fuel/exhaust being the projectile.

The rocket shoots out fuel/exhaust and its speed increase is equivelant to the recoil of the gun.

Now would the gun travel at the muzzle velocity of the projectile, the bullet would just be released hanging dead in the air (and dropping down if inside a gravitational field/fired on e.g. earth).

Now it says that because the rocket put energy into the fuel by expelling it yet the exhaust's "backwards" speed is lower by the amount the rocket is going forward, their is a difference in energy that has to be left somewhere because energy cannot be destroyed. So the Oberth effect is the transfer of the kinetic energy of the exhaust onto the rocket.

Still puzzles me - the fuel had the same kinetic energy as the rocket while still in the tanks - wasnt the energy used do stop that motion? How can it be transfered onto the rocket?

I tried to think of the Oberth effect like this a while ago and it just wasn't intuitive. The obvious question comes up what happens when your rocket is going faster than the exhaust? By the explanation that energy is divided between the rocket and its exhaust it would stand to reason that the maximum amount of energy to the rocket is achieved when the minimal amount is left in the exhaust. This is when the exhaust is stationary. Such is not the case as the Oberth effect is uninterrupted at any increase of speed, well beyond the exhaust velocity. I believe the problem with this is focusing on a scalar quantity, energy. Transferring the whole thought process over to a vector quantity, momentum, bears more fruit.

Relying on an external concept of stationary will mislead you. Instead, consider the center of mass of the rocket-fuel system. The momentum and position of the system are fixed and can be set to zero safely. The only effect observed by burning the rocket is on the energy of the system by separating the components of the system. This doesn't explain the Oberth effect however. The Oberth effect is noting that the separation acceleration from speed N to N+1 increases the energy of the system more when N is larger.

My intuition about the Oberth effect is just that the faster I go, the less time gravity has to affect me over any given distance, so the less gravity drags me down.

Conversely, the faster I'm falling, the less extra speed I will have built up when I get down to periapsis.

Gravity, orbits, etc. are all distractions. Oberth effect is an extremely basic consequence of physics and applies to a single object accelerating in free space in a straight line. It doesn't even have to be a rocket with reaction mass. It all comes down to how a fixed change in momentum (or speed for a constant-mass object) results in a variable change in kinetic energy depending on the momentum or speed to which the change was applied.

A practical demonstration would be playground swings. throwing your legs to get a push works best when you're nearest the ground moving fastest, you can do whatever you want at the top of your arc and it won't change the swing much at all

I'm not sure this is applicable. To apply the energy of the swinger must be changed by applying an impulse at high speed to increase KE better than at low. Legs and pushing aren't so well behaved because your ability to make an impulse is impaired by how long you are in contact with the thing to shove off of, the angles, etc.

[miracmert]

This.

There is no practical demonstration you can do with swings, or hammocks or anything like that. None of that is the Oberth effect or anything remotely like it.

The closest you can kind of come is hop on a skate board and throw a medicine ball off. Next, push the skateboard up to a bit of speed and throw the medicine ball off opposite the direction of travel. Doing this results in a bigger push of the you/skateboard system. Oberth effect. Visual/sensory difference in the push though might not be overly noticable as the difference in energy of the system isn't all that high as the velocities involved are relatively low.

There is also no relation between Einstein's relativity theory and what is used as an example (Having to do something difficult and the time passes slow while it is pretty much faster when you are enjoying something) however examples from daily life makes it easier for people who are interested in science but they don't have the knowledge base for it. If we make it less scary for people than more people interested in science we get and more educated people we get in the future

And by the way, I didn't understand medicine ball example since I don't know the meaning for it even though I looked up the dictionary. May you explain that example a little more?

[Cronos988]

How is the exhaust "standing still" when the rocket is faster? Isn't the relative speed between the rocket and it's exhaust always the same? It might be standing still in relation to any other frame of reference, depening on the speed of the rocket and the energy released by the burn, but why does that matter?

I mean it's perfectly obvious that if energy is a function of mass times speed squared, inreasing the speed from 100 to 101 increases the energy way more than increasing it from 0 to 1. I just don't see how the bullet/cannon example makes this any easier to explain. Does this have something to do with relativity, perhaps? The exhaust also gets heavier as it gets faster, and flinging someting heavier gives you more energy.

[SkyRender]

You don't really need anything fancy to observe the Oberth Effect, as the Oberth Effect effectively states that it's easier to go faster when you're already moving fast. Even here on the ground, you can observe its effects quite frequently: consider how much longer it takes for a vehicle to go from a full stop to top speed than it does to go from halfway at top speed to top speed. Mind you, it's less obvious here on the surface of a planet since we have a number of factors (air resistance, surface friction, etc.) slowing us down. But it's still there nonetheless.

[someoneON]

Dont complicate things. Wikipedia explains it very simple, its all in "useful" mechanical work. Read that paragraph. Here i quote:

"But when the rocket moves, its thrust acts through the distance it moves. Force acting through a distance is the definition of mechanical energy or work."

So by traveling faster, your thrust acts through more distance (in the same time) as it would when traveling slower.