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

Re:high energy ?

[QuantumFTL]

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 :-)

Re:high energy ?

[dschuetz]

There are of course downsides to this technique (what if the spacecraft drifts off the beam)

From a great special-issue Scientific American a few years back, I think I have an answer for this.

Some of the "heated air" approaches (using microwaves or lasers) depend on a convex reflective surface under the spacecraft, which focuses the energy just below it. If the spacecraft tilts, or drifts to the side, the light from the laser, hitting the underside, gets reflected in a slightly different place. In fact, the simple geometry of the craft's underbelly guarantees that the focal point shifts just enough, in the same direction as the drift, that the next energy burst will nudge the spacecraft back onto the beam.

So it's sort of self-correcting.

And, remember, All Things Serve The Beam. (sorry, couldn't resist).

Re:Rocket Equation

[Michael.Forman]

Good points.

However, the original post is a first-order approximation of the required energy. It is meant to highlight that one system's required fuel weight is exponential while the other's is not. Let's not let excessive details (which apply similarly to both vehicles) obfuscate the elegant simplicity of the rocket equation.

Your statement that a reduction in fuel by a factor of ten is not possible is very wrong. For a given orbit, provided exhaust velocities are equal, a rocket which does not carry its fuel will always use less fuel. How much less? Well, given that one has an exponential dependence on fuel and the other does not, it stands to reason that for high orbits a factor of ten could be conservative.

A better way for you to argue against a power savings is to point out that there's no way super heated air could ever achieve the exhaust velocity equal to that of kerosine and liquid oxygen. Thus the linear function may for a range of orbits be more expensive than an exponential function.

I knew this was a large subject, when I started it, but wanted present the concept of the exponential rocket equation. I'm working on a full write up on my site and should have something up by the next slashback.

Michael.