A Washington Post article talked about getting to interstellar rocketry by slingshotting around the Sun. I understood the idea of slingshotting required a planet or a moon– something that was orbiting something else because you are taking some of that acceleration of the planet for your rocket. Indeed this is called “Gravity Assist,” so what is this slingshotting around the Sun? Well, this is where you get the Oberth Effect. The idea is you go really fast by getting close to the sun, burning your fuel, and you will go much faster even after you slow down again having left the proximity of the Sun.
But after reading and rereading the wikipedia article, talking with my mates, watching a youtube video I still did not get an intuitive grasp of it– in fact it seems to break the rule of conservation of energy. How could blasting your fuel while going faster make you go faster? I think I have an approach to see how it makes sense, in general, but does not bring us to equations.
A rocket works by throwing exhaust out the back at high speed, thus pushing the rocket forward. For a given amount of fuel, you throw it out (relatative to the rocket) at a certain speed and certain weight to become the exhaust thus pushing the rocket forward with a certain amount of force.
Let’s say the fuel is thrown out the back at a certain speed, V. If you do this while standing still, the fuel goes one way at speed almost V and the rocket goes the other. If the rocket is moving at speed V, then throws the burnt fuel out the back (say it does it all at once for simplicity), then exhaust will be standing still and the rocket moving faster than it was.
The first case is like in a swimming pool you push against a floaty to go one way– the floaty goes the other way. The second case can be seen as you are in a swimming pool and you push just as hard, but you are pushing against the wall. The wall goes nowhere, and you go faster across the pool.
You would push against a wall if you could. And by speeding up the rocket you are effectively doing that. Same force, but the floaty goes the other way vs the wall which stays still. When you push against the wall you go faster in the direction you want to go.
In the rocket case, we have to get the rocket to go the speed of the exhaust V, and not take any energy to do that. The way the rocket is speed up is by “borrowing speed” by accelerating towards the sun. The sun accelerates both the rocket and the fuel it will burn equally– this is that tricky thing in high school physics where a feather and a bowling ball fall at the same rate (if there is no air resistance). So the sun pulls both in, the rocket and fuel are going really fast, say V, then the rocket blasts out the rocket fuel leaving it behind, and then decelerates as it leaves the sun. If the rocket got up to speed V, then the net effect is that instead of the rocket throwing its exhaust backwards, it would go nowhere, thus being the wall it pushed against.
The rocket decelerates as it leaves because of the gravity of the sun is pulling it back, but it is getting less force because it weighs less because it burned fuel. Again, this is that tricky bit about the feather and bowling ball. So the Sun applied more force to the rocket+fuel on the way in than on just the rocket on the way out. Another way of seeing how this effect works.
At least this makes sense to me. The effect is caused by the sun accelerating the rocket+fuel, then when the rocket blasts out its spent fuel, the exhaust is not going in the opposite direction as much, thereby offering more of a push to the rocket, even after it is decelerated.