How does the Oberth effect work?
In: Physics
Rockets are momentum engines. A certain amount of fuel adds a certain amount of momentum to the spacecraft. Momentum is just a simple Mass * Velocity so if your spacecraft remains roughly the same mass then a certain change in momentum is really a change in velocity
But orbits are defined by the energy that a spacecraft has, and energy is Mass * Velocity^2 so a certain change in momentum/velocity has a different effect on the energy of the spacecraft depending on how fast its going
Burning a rocket engine when the spacecraft is traveling quickly ends up adding a lot more energy to the spacecraft for the same fuel as you would get by burning the engine when traveling slowly. The Oberth Effect is the process of making a close approach to a planet so its gravity pulls the spacecraft in and gets it going wayyyy faster briefly and then burning the engine to make use of the significantly higher speed to get more energy from its fuel so it can change its orbit with less fuel. The downside is that it requires the course to actually pass near planets so it can make a journey significantly longer while allowing it to be done with way less fuel
Just to be clear to anyone, the Oberth effect is that when you are at the periapsis of your orbit, spending fuel is more efficient, than spending it at the apoapsis.
This is because the fuel has potential energy, that transforms into kinetic energy, when it’s getting closer to the gravitational center it’s orbiting. By spending fuel, when more of the fuel’s energy is kinetic, the rocket gains more kinetic energy total from spending fuel. EDIT: I forgot to add that rocket engines have a maximum speed, at which they propel spent fuel outwards, and because of that can make use of the “faster” fuel in a lower orbit, because the difference in velocity of the fuel is larger.
You can also see that it has to be this way, because of thermodynamics. Energy can’t be destroyed, only transferred or changed. The exhaust from the fuel is either in a high orbit or a low orbit, depending on where it has been spent.
Fuel in a higher orbit obviously has more energy, so it has to give less to the rocket.
This is also called a “Powered Fly-by.” As a spacecraft enters the gravitational field of an object (normally a planet, star, or moon), the spacecraft accelerates due to the gravity. The pilot fires thrusters in the same direction in order to “fall faster” into the gravitational field. This allows them to attain a higher speed than just firing those thrusters alone. (They will not be falling toward the planet. Rather, they’re “falling” *past* the planet.)
Spacecraft will use this powered flyby to “slingshot” themselves off the gravitation field. This is how we sent some of our spacecraft to Mars. We used a powered flyby with the Moon’s gravity, taking advantage of the Oberth Effect. These powered flyby’s are much more fuel efficient that just flying/accelerating in empty space. Sometimes, a pilot will even some fuel to slow the spacecraft down in order to fall into the gravitational field correctly so they can use less fuel overall to attain the speed and direction (velocity) desired.
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