This brief note is about Beamed Energy Propulsion (BEP), which is, actually, a rocket science. However, you dont have to be a rocket scientist in order to understand what it is about, it is simple and I will explain it to you in next few paragraphs.
It is all about rockets. What everyone knows about them? On Earth motion is based on pushing from the medium, wherever the motion takes places: cars pushing from pavement by wheels, swimmers pushing from water by limbs, etc. Rockets fly in space and space is mostly a vacuum, it is empty. Rockets move in space using reactive principle, i.e. exhausting hot gases (backward) and pushing away from exhaust in opposite direction (forward). Rockets have to carry all their fuel and burning agent onboard, because there is nothing in space that can be used for burning fuel (like air for burning gasoline in automobiles on earth).
Rockets cargo space is very expensive. Why? Because with so many elements onboard: fuel, oxidizer, cryogenics, tanks, lines, you name it, there is a small (and very expensive room) for the payload. If we could only find a way to separate the heavy energy source from the rocket, the gain in efficiency of such rocket will be tremendous.
Energy can be delivered to the rocket from remote external source using light or, say, laser, x-ray, microwave high-power beams. With its mirrors, rocket will collect and focus that beams on its "fuel". Any solid matter can be a fuel. When high-power beam of photons is focused on a solid material, the material evaporates and ionizes instantaneously. The energy density in focused high-power beam exceeds hundreds to millions times one in the heat of burning hydrogen. Thus, beam-driven rocket will remain a rocket, it will be pushing from its own exhaust, but the energy of this exhaust is much higher, and the rocket itself is much lighter, comparing to hydrogen burners.
Beam-driven rocket is equipped with beam-collecting optics (i.e. mirrors) and some relatively lightweight solid fuel. That is it: no more tanks, oxidizer, cryogenics, nozzles, lines, - the rest will be cargo. Such rocket will be a subject of 4P Principle, formulated by the founder of laser propulsion, Arthur Kantrowitz: Payload, Propellants and Photons, Period!
Can efficiency of beam-driven rocket be compared to hydrogen burner? Yes, this can be done with scientific precision! For example, current price per lb. of a payload delivered to low earth orbit is $10,000. Calculations (repeated many time by independent researchers worldwide) show that with the laser propulsion space delivery price will be reduced to $100 per pound of a payload! It is a hundredfold profit!
In addition to laser beams, other types of energy transfers using microwave, x-ray and even particle beams are getting explored for BEP. The number of in-lab demonstrations of BEP principles grows every year, and the day when we will achieve in-space BEP demonstration becomes inevitable. Of course, there are still a plenty of scientific and engineering problems which should be solved before BEP will bring us its benefits. However, the days for first practical use of beamed-energy propulsion are near.
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