# Reaction Mass Creation

#### metastable

Given a large enough energy source (nuclear?)... can I build a rocket that creates its own reaction mass via electron-positron pair production, and then derive thrust by accelerating the charged particles out the back with a magnetic nozzle?

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#### anorlunda

Mentor
Gold Member
For pair production you need at least the following minimum conditions.
https://en.wikipedia.org/wiki/Pair_production said:
The photon must have higher energy than the sum of the rest mass energies of an electron and positron (2 × 0.511 MeV = 1.022 MeV) for the production to occur. The photon must be near a nucleus in order to satisfy conservation of momentum, as an electron-positron pair producing in free space cannot both satisfy conservation of energy and momentum. Because of this, when pair production occurs, the atomic nucleus receives some recoil.
Although a nuclear reaction might produce some such photons, it also produces lots of heat that just serves to melt the spacecraft. You not only need an energy source, you need a system which produces only high energy photons near nuclei, and does not do much else. You also need a way to get the electrons and positrons to the nozzle without much interaction with anything else.

So, with my engineer's hat on, I think energy is the least difficult part of the problem.

#### metastable

Can I recover some of the energy spent creating the pairs by accelerating them out the back at a slightly converging angle into some sort of bell / chamber, which is designed such that when the annihilations occur, there is a high probability of the gamma rays interacting with the walls of the chamber?

#### etudiant

Gold Member
There have been speculative propulsion systems using a very high power particle accelerator as the impulse generator.
Obviously there are a few engineering concerns when trying to shrink a more powerful than CERN device into a spacecraft, but conceptually, it does offer tremendous specific impulse from all electric propulsion.,

#### Rive

Given a large enough energy source (nuclear?)...
That energy source will be about converting mass to energy. You want to convert that energy back to mass. Try guessing the efficiency...

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