Beamed Energy Space Propulsion: an Introduction
Beamed Energy Propulsion or, in brief, BEP, is a part of rocket science. However, you dont need to be a rocket scientist to get its idea, it is very simple, and I will explain it to you in the next several paragraphs.
Rocket science is of course about rockets. What is so special about rockets? Motion on Earth is based on pushing from the environment, wherever the motion takes place. For example, cars use wheels to push from the pavement, birds use wings to push from air, and so on. Space is empty, so in order to fly in space every rocket has to use reaction principle, i.e. burn fuel, form exhaust gases and push from that exhaust in opposite direction. There is no air in space, so if you need to burn fuel, you have to carry burning agent together with the fuel onboard. Hence rockets has to carry onboard their fuel (hydrogen), oxidizer (oxygen) and everything that needed to burn hydrogen and keep the motion in desired direction (rocket engine).
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.
Such separation will be done, if we could beam energy towards the rocket from a remote power station. The rocket will collect the beam and focus it on a “fuel”. High-power beam of photons (laser, microwave, x-ray, whatever) when focused on solid matter (even slightly) evaporate and ionize such matter in an instant. I.e. the energy density in such focus is much tighter, than in the heat generated by burning hydrogen. The beam-riding rocket will remain the rocket, it still will need to push from its own exhaust, but this will be much more energetic exhaust, comparing to hydrogen burners! Also, such rocket will be much lighter.
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!
Laser propulsion, i.e. use of high-power lasers for satellite launches and in-space transportation is the most developed today branch of BEP. Various types of laser propulsion have been demonstrated in field and by many research groups in lab. Microwave propulsion is another relatively well explored part of BEP. Much less is known about potential of x-rays and particle beams for BEP. Overall, beamed-energy propulsion remains a field of future technology, where a lot of interesting development will happen in the next several decades. Still, it is quite clear, that in the future a great part of space transportation will be driven by high-power photonic beams.