If photons can be used to accelerate solar sails, why can't they be ejected from a spacecraft and be used as an "engine?"

Question

I'm not really sure why or how, but photons apparently have momentum. This goes against my very limited sense of physics, as they have no mass at all. How is this possible?

At any rate, I came to wonder--if photons have momentum and can propel solar sails, why can't they be used in a way that's similar to rocket engines? Or can they? I got this idea because of the "equal and opposite reaction" law. Does this apply to light or not? Would the "light engines" be large laser pointers?

I know this is probably a ridiculous idea and completely impractical, but is it possible?

Answer 1

They can, but would be less efficient than solar sails.

First, you would have to add the mass of the light-producing equipment and whatever powered it to your spacecraft, meaning you need more force to get the same acceleration.

Second you lose a factor of two in the rate of change of momentum, because a light sail is designed to be reflective. This means a photon of momentum $p$ incident on a reflective sail gives $\sim 2p$ of momentum to the spacecraft, whereas simply ejecting the same photon from the spacecraft would give a momentum boost of just $p$.

Of course sails can have other problems (e.g. difficult to use when you are far away from any light source).