Re: Laser as Reactionless Propulsion

Ross A. Finlayson (
Tue, 20 Apr 1999 17:53:50 -0400

Billy Brown wrote:

> Ross A. Finlayson wrote:
> > Does shooting a laser from a laser generator not generate opposite force
> > of the beam output?
> Lasers have recoil, just like anything else - it is very small, because the
> beam has very little mass, but it is measurable.

OK, I understand this. That lasers have recoil I did not know.

> > Do photons from a laser upon impact of a reflective surface cause an
> > inertial reaction, no matter how infinitesimal?
> Yes, it does. These two forces are exactly equal, so your idea:
> > If lasers have no "kick" upon firing, and photons from a coherent laser
> > do cause an inertial reaction, then lasers could be used as reactionless
> > propulsion.
> unfortunately won't work. You can use a stationary laser to push a moving
> object, but you can't use the laser/mirror pair to move itself. Darn,
> another great idea shot down by those nasty conservation laws!
> > As a craft propelled by such moved along, it's trail would be a beam of
> > coherent light flying at the speed of light away from the engine.
> As an interesting note, you CAN get thrust by simply firing the laser out
> the back of your spaceship. Unfortunately, we currently can't build lasers
> that move enough energy to make this practical - the thrust would be far too
> low for even the most modest of needs.
> Now, if we could make very powerful, compact, lightweight lasers, and power
> them with a nuclear energy source, we would have a very efficient means of
> propulsion (and this is not a new idea, so neither of us can lay claim to
> it).
> Billy Brown, MCSE+I

Correct, I have also heard of this. Another old idea is to use a ground-based laser to shoot up projectiles into orbit.

The idea of shooting a laser for thrust is still an interesting one. This would appear to require no reaction mass, only power, as opposed to minimal reaction mass, using ions. So, lasers can be the source of reaction propulsion with no reaction mass propellants.

In terms of usable long range thrust capabilities, it would appear that a Bussard ramjet might be more efficient, ie, using a magnetic scoop to funnel interstellar hydrogen into the the reaction chamber for ion propellant, collecting the nearly zero reaction mass available along the way.

It would seem likely that before we developed any interstellar travel mechanism using conventional slower than light travel, there will be some advances that allow dimensional travel, but that is beyond the realm of conventional science. These things are much "easier said than done."

Ross F.

Ross Andrew Finlayson
"C is the speed of light."