Creating Reaction Mass with Electron-Positron Pair Production for Rocket Thrust

[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?

 

[anorlunda]

For pair production you need at least the following minimum conditions.

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]

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...

 

[anorlunda]

The OP question has been patiently and adequately answered.

Thread closed.