How practical would be a rocket that would use laser light as a reactive mass, given there is a very efficient electric energy source or storage?
closed as off-topic by JBH, elemtilas, Confounded by beige fish., L.Dutch♦ Apr 16 at 11:33
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The thrust you gain from using lasers in this way would be extremely low. By the way, light wouldn't be reactive mass since it is massless by definition.
How low? From a comment in the physics stack:
The rule of thumb that is usually quoted is that it takes a megawatt of power beamed to a vehicle per kg of payload while it is being accelerated to permit it to reach low earth orbit. If lasers were very efficient at delivering power to a target through the atmosphere, the military would be using them as weapons, not just as missile guide markers, as they do now.
Also, if you are generating energy in space through solar panel, you are getting zero net thrust at worst, and double what you would get just from the lasers at best.