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?

Re:high energy ?

[clambake]

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.

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:high energy ?

[QuantumFTL]

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

Reply: 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.

Of course they would build a system that was dynamically stable (they would be incredibly foolish not to) but do not be fooled, any dynamically stable system has its limits! Hitting a pocket of turbulence or something may disrupt the path of the spacecraft enough that even the corrective abilities of the underbelly are not enough. Of course active tracking from the ground can help this, but a significantly large bump can still possibly put the spacecraft into a state where it is unlikely it will recover.

I've used "optical tweezers" here at Cornell that use a similar self-correcting effect to trap small particles in a laser beam (a cell for instance) and you can shake around the solution quite a bit without dislodging it, but sure enough, move things a little too fast and poof, you reach the limit. Due to constraints of geometry, in the microwave system, the corrective force is only a small fraction of the overall propulsive force, and even a small intermittent problem with the flight control surfaces could take the craft off the beam.

Just because something's "self correcting" doesn't mean it won't break.

Cheers,
Justin

Warning: I am not a physicist yet, I'll have my degree in a year :)

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.

Didn't someone do this before?

[Oriumpor]

Am I not correct in assuming that someone fired either a laser or a microwave beam at an object on a tether that looked similarly conical as this object, and made it move (In a lab mind you, horizontally, on a string... but I thought the concept was proven already.)

The only problem was the projected G Forces were just too much for the human body, from what I remember.

The best part of this engine?

[clambake]

That one is easy, fresh popcorn for the onlookers!

Rocket Equation

[Michael.Forman]

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.

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.

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.

Leik Myrabo

[krysith]

Leik Myrabo has been working on beam powered rockets etc. for years. "The Future of Flight" was published in 1985. He has done more work in this area than NEone else on the planet. He is currently working for RPI. Links: http://www.rpi.edu/dept/mane/deptweb/faculty/membe r/myrabo.html http://www.lightcrafttechnologies.com/technology.h tml (I apologize if the urls dont come out properly. Slashdot formatting is still an arcane science to a newbie like myself. Dammit, Jim, I'm a physicist, not a webmaster!)

Laser powered rocket

[n1ywb]

This microwave rocket sounds totally pussy compared to the frikkin LASER powered rocket I saw on Discovery (or was it TLC? I never watch TLC anymore since it's all Trading Spaces now.)

Anyway the laser "rocket" is actually a very lightweight aluminum puck about a foot in diameter, with a some funky curves. They shoot high powered laser pulses up its ass and that superheats the air underneith it, the expansion of which propells the rocket upwards. The pulses fire at about 500Hz so the damn thing sounds like a pulsejet. But at last check it reached an altitude of 71 meters and a flight time of 12.7 seconds. Microwave rocket eat your heart out! :)