Slashdot Mirror
Tokyo University's "Microwave Rocket"
LiftOp writes "Apparently a group from Tokyo University's Department of Advanced Energy has
used a high-power microwave beam to heat the air beneath a model rocket
, sending it skyward (well, two meters).
Dr. Kimiya Komurasaki, who led the group, seems to be quite a
directed energy
buff; when the rocket eventually gets beyond the air level, a conventional motor could be used to send it further."
Ah, well the point of it is simple. A rocket normally requires a LOT of lift of energy precisely because it needs to lug tons and tons of fuel up with it as it moves a long. By dropping those fuel-pounds, or at least some of them, you can carry a lot more stuff... More stuff into space is a Good Thing.
A propulsion system such as this can provide a tremendous reduction in required energy.
Conventional rockets, which carry their own fuel are large consumers of energy, as not only must they lift a payload into space but all the fuel as well. The total weight of a rocket including fuel is given by an exponential function known as the rocket equation. Stated simply, a rocket of mass m0 requires fuel of mass m1 to lift it; that fuel of mass m1 requires more fuel of mass m2 to lift it; the fuel of mass m2 requires fuel of mass m3; and so on, ad infinitum. The rocket equation is given by
m = m0 exp(Vf/Vex)
where m is the total required mass, m0 is the mass of the payload, Vf is the final velocity, and Vex is the exhaust velocity of the combusting fuel.
This exponential increase in initial mass can be huge. For example a low earth orbit requires a change in velocity, Vf, of about 8 km/s. Kerosine and liquid oxygen provide an exhaust velocity of about 2.5 km/s. Thus, m/m0 = exp(Vf/Vex) = 24.5. It would take 25 times the original weight of a given payload mostly in fuel to achieve a low earth orbit with kerosine and liquid oxygen! Assuming a payload of 1000 kg and an energy density of 10^7 J/kg for the fuel, the total energy would be E = (25*10^3 kg)(10^7 J/kg) =~ 250 GJ!
The wonderful thing about rockets that don't carry fuel with them is that there is no exponential dependency on initial mass. The energy required is simply the orbital energy, given by half the gravitational potential energy (derivation mercifully omitted) of the payload, given by E = -(G m0 M)/2r. The energy in this case, omitting concerns of efficiency, would be
E = (6.67*10^-11 Nm^2/kg^2)(5.98*10^24 kg)(1000 kg)/(2*6400 km) =~ 30 GJ
The savings in energy is almost a factor of ten!
Michael.
P.S. - Lots of derivations late at night. Be merciful in the event of errors.
Linux : Mac