Sci-fi Laser : Ironman

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.

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...although some obtain propulsion directly from light pressure acting on a light sail structure, and at low altitude heating air gives extra thrust

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Wolfman29 said:

By the way, photons don't have mass. They have energy. While there is a mass-energy equivalence, it's irrelevant in the case of a photon, since they never slow down (which is the only thing that contributes to mass-energy equivalence in matter). Light exerts pressure though because it has momentum (it has energy, velocity, etc.). Weird to think that something without mass can have momentum, but it can!

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Wolfman29 said:

Radiation pressure comes from transfer of momentum! That's what happens when a photon strikes a mirror and bounces off of it. It decreases in wavelength slightly, and thus loses momentum (VERY SLIGHTLY).

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Wolfman29 said:

We all agree photons carry energy. That's a thing.

The energy is calculated via (I think?) the de Broglie relation: E = hv, where h is Planck's constant and v is the wavelength.

Then, we apply the relativistic definition of energy: E^2 = p^2 c^2 + m^2 c^4. Setting m = 0 (because photons have no momentum), we have:

E = pc

Now, we know E = hv, so we instead have
hv=pc.

Solving for momentum, we have
p = hv/c.

Turns out, this expression is experimentally verified.

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Wolfman29 said:

We figure it out by the reflectivity of the material

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