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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
Correct me if i'm wrong, but doesn't using microwave radiation to heat the air consume a lot of energy than burning solid fuel?
:-)
Yes, I would imagine it does.
If so, wheres the applicable purpose?
Unfortunately when building a rocket to go into space, most of the fuel is spent CARRYING FUEL UP. That's just plain uneconomical. So when one is only lifting the actual payload (and perhaps some small reflector or whatever) there's a *HUGE* energy savings.
There's also the issue of reliability/stress. Things which are being thrown into orbit at high velocities have to be engineered very well to survive the trip. Mass must be shaved, redundancies might be cut to lower costs, etc. Building things on the ground is much easier in these respects... redundancy is much less limited, much less stress is on teh equipment, it's much easier to diagnose repair (because it doesn't have to be all micro-sized, etc). Also miniturizing things can considerably inflate their cost. So keeping as much of the equipment on the ground as you can is a good idea.
There's also safety issues... Most rockets use very dangerous explosive fuels, some of which are environmentally unfriendly. As long as a poor bird doesn't stray into the beam, this should be able as environmentally friendly as possible.
One last answer is that it allows most of the launch system to be reused between launches... Disposable rockets can't do this, and the Space Shuttle doesn't even re-use that much of it's mass... just some of the more complicated bits.
There are of course downsides to this technique (what if the spacecraft drifts off the beam, or the beam is obscured, etc) however I believe that they will eventually be overweighed by the enormous benefits.
It's a good question and not all the answers are obvious. I can't wait for the day where most of the work launching stuff into space is done from the ground.
Cheers,
Justin
Warning: I am not a physicist yet, but I almost have my degree. I also work at the Jet Propulsion Lab