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."

Comments

high energy ?

[ddd2k]

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? If so, wheres the applicable purpose?

Footfall

[mattsucks]

Reminds me of the book "Footfall", by Larry Niven and Jerry Pournelle. The climax of that book has a space vessel launched with atomic bomb explosions as the propulsive force. check it out.

Re:Footfall

[Anonymous Coward]

Man, that I lived to be old enough to snap at this....

Google for the Orion Project - 50's concept of using atomic explosions to lift spacecraft

microwave

[savuporo]

Microwaves (~100GHZ range ) get through the air almost without energy loss, thats what makes Solar Power Satellites concept feasible at all. I dont remember which, but one Japanese semiconductor corp is planning to put up small sats to beam power to handheld devices via microwave.

Re:Rocket Equation

[WolfWithoutAClause]

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.

That's not actually true at all. For a few reasons:

a) some (a lot) of the energy ends up in moving the exhaust around (for example you have to throw it downwards to get thrust from it- unlike the case where you carry the fuel with you the propellent has to end up moving downwards, since it was stationary when you met it).

b) gravity losses- until you reach orbit you're falling all the time; it costs energy keeping you up until then.

c) aerodynamic drag getting there (a lot, since you need be in the atmosphere to collect your fuel, and you're trying to achieve orbital velocity -mach 25 or so there; normal rockets get out of the atmosphere sharpish to minimise that problem)

d) no air at orbital altitudes; so you need some on-board fuel to circularise your orbit at the very least.

Also, many remotely powered vehicles have high exhaust velocities. That means that the amount of power left in the exhaust is many times greater than in the vehicle (energy efficiency is inversely proportional to exhaust velocity, roughly).

Frankly, I don't see any easy way to work out how energy efficient a system like that would be; it's trajectory specific. It may be more efficient, but certainly not an order of magnitude; and it's quite possible that it's less efficient particularly if the exhaust velocity is very high.