# B Can light generate propulsion?

1. Sep 23, 2016

### RandyD123

2. Sep 23, 2016

### Staff: Mentor

3. Sep 24, 2016

### RandyD123

So if I had a flashlight in space and I turned it on, I could see it moving relative to me?

4. Sep 24, 2016

### Staff: Mentor

Yes, if you are in free fall and you turn on a flashlight and then let go of it, it will accelerate slightly. However, the operative word here is "slightly", as the force involved is extremely small - it's a good exercise to try calculating it for yourself, but for reasonable assumptions about the size and power of a flashlight I'm getting a micro-newton or thereabouts, which is well and thoroughly negligible.

The real appeal of light propulsion is that you can use a fixed power source to illuminate a solar sail, so the propulsion system doesn't have to accelerate itself and its fuel supply. You attach the sail to the payload, and then send it on its way using a powerful earth-based laser.

5. Sep 24, 2016

### sophiecentaur

. . . or light from a nearby star.

6. Sep 24, 2016

### sophiecentaur

Another advantage of light propulsion is that, if you use an on-board torch (X1000000, say), you are imparting momentum to your craft without using up any of its mass which is what always happens with a rocket engine. Even Ion Drive engines end up depleting their mass.

7. Sep 24, 2016

### jbriggs444

Ummm, shining a light out the back does deplete mass. The required energy has a mass equivalent.

8. Sep 24, 2016

### houlahound

9. Sep 25, 2016

### sophiecentaur

True, in principle but there is a factor of c2 in there somewhere. Not sure where the photon momentum (hc/λ) would affect the effective ratio of ejected mass and ejected photons but the mass defect would still be tiny.

10. Sep 25, 2016

### jbriggs444

Yes, but there is a trade-off. The thrust per unit energy is also tiny.

The sweet spot for exhaust velocity depends on the energy density of your fuel. A photon drive is in the sweet spot for an antimatter fuel source. With less energy density than that, a lower exhaust velocity is better -- eject the expended fuel as reaction mass.