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 ?

[PD]

Concave surfaces. Convex mirrors won't focus

Re:high energy ?

[dschuetz]

Concave surfaces. Convex mirrors won't focus

D'oh!

Thanks. Obviously, that was posted before I finished my morning cola.

Re:high energy ?

[WolfWithoutAClause]

Yes, but even having 10x more fuel still only costs 1% of the cost of the rocket. So you need bigger engines to carry the fuel up- but the thrust:weight ratio of rocket engines are typically 100:1, so one hundred tonnes of fuel is carried by 1 tonne of engines.

And because the dry mass of the rocket is the expensive bit, you haven't pushed up the cost of the vehicle by much.

And air is actually a much worse propellent than rocket fuel; you need many times more energy than the better rocket fuels to get the same thrust. That and the fact that airbreathing vehicles make you stay in the atmosphere for longer means you get much more drag- and drag goes up with the square of your velocity- mach 25 is needed for orbit, and people complain about Concorde being inefficient at mach 2!

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 :)

Re:high energy ?

[xluap]

I think the microwave energy goes throug the air without much heating. When it hits the rocket it is bundled and at the focal point of the radiation the field strenght is so high the air at that point ionizes and the energy of the microwave is dissipated there as heat. The expansion of the heated air propulses the rocket.

Re:high energy ?

[Dylan Zimmerman]

Well, one cool thing is that we would be able to launch rockets without using any fuel that we can't find more of. Billions of joules of sunlight pour onto the Earth every day. We just have to collect it and find a way to use it.

This as opposed to fossil fuels, which take quite a bit of time to produce. They are far more efficent than most forms of energy storage, but they are very hard to make, and therefore, not easily renewable. Of course, the sun will eventually burn out, but we might as well grab all of the energy that we can now because if we don't use it, it just goes out into space.

Re:high energy ?

[casius_longinus]

What about making a jet out of the same technology? A rocket requires pretty tremendous amounts of thrust, and the air thins out at higher altitudes. I could see the need for a compressor to ensure that there is enough air to heat; otherwise, it would lose power in the upper stratosphere. I like the idea of of electrical energy. Now all they need is a smaller nuclear power plant, and you could have a plane that flies forever. Apparently, there were some nuclear powered airplane experiments back in the 1950s.

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

Re:Footfall

[hubie]

The idea of nuclear pulsed propulsion goes all the way back to Stanislaw Ulam in the 1950's. An investigation into the idea was called Project Orion and was headed by Freeman Dyson. One brief writeup can be found here.

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.

Re:Didn't someone do this before?

[ahfoo]

If you're referring to the SciAm article on these things a few years back, I don't recall anything about excessive G-force.
I believe the problem they had there was the lack of a suitably powerful laser. They were using C02 lasers in the SciAm tests I believe.
What I wonder is if the deuterium flouride lasers the military has suggested for the MIRACL program would be of use. It seems that the problem with those was that they took a long time to recharge after lasing so perhaps not.
Probably what we need is a particle beam, but that will make for an interesting passenger experience.
Here, you just buckle in and I'll fire her up. You'll be flying through the atmosphere in no time.

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

[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.

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.

Re:Rocket Equation

[PD]

If you're vaporizing things, you have great flexibility in your choice of fuels. I wonder how vaporized Uranium would compare to heated air?

Re:Rocket Equation

[WolfWithoutAClause]

Your statement that a reduction in fuel by a factor of ten is not possible is very wrong.

What statement? I said your calculation that you would only need 10% of the energy was bogus; and I stand by that. In externally powered airbreathing rockets as we are discussing here, the energy and the fuel are unrelated; or are only related by the trajectory you've chosen to use.

For a given orbit, provided exhaust velocities are equal, a rocket which does not carry its fuel will always use less fuel.

Trivially true. However, with an airbreathing rocket the last point on its orbit where it burns atmosphere is necessarily within the atmosphere and hence it's orbit will still intersect the atmosphere, and you can't achieve a stable orbit; so you need to carry some fuel with you (there are a few sneaky ways around that, for example doing a slingshot around the moon, but they are often at best rather awkward). Additionally, the whole time you are in the atmosphere you are fighting drag; you absolutely cannot ignore that effect.

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.

No, not really. Superheated air can go faster than kerosene and LOX, if you heat it enough. But the molecular mass of nitrogen (air is mostly nitrogen) is pretty high. The exhaust velocity that can be achieved for a particular energy is inversely related with molecular mass, so air is fairly poor in that regard; by way of contrast kerosene has lots of hydrogen in it, and so requires less energy for a given exhaust velocity.

Energy or fuel mass isn't really the problem anyway. More important is the dry mass of your rocket, since that's the bit that costs the most (it turns out surprisingly that fuel is much cheaper than the cost of the vehicle, fuel is less than 1% of the total cost, fabrication of the metal bits and so forth are much more expensive).

Re:Rocket Equation

[WolfWithoutAClause]

Apart from being radioactive, it's a heap of junk ;-) (Sorry!)

The problem is that because it is such a heavy atom when you heat it, it ends up moving very slowly. By conservation of momentum, your rocket ends up moving much more slowly than if you had used hydrogen, or even nitrogen for the same mass of fuel (the fuel would be much denser, but still, it turns out to be a big, big loss).

Re:Rocket Equation

[WolfWithoutAClause]

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.

I don't think I'm going to hold my breath on this. You don't appear to know the difference between energy and fuel, and waving your hand like:

Let's not let excessive details (which apply similarly to both vehicles) obfuscate the elegant simplicity of the rocket equation.

Really doesn't cut it when conventional rockets leave the atmosphere as soon as possible to avoid drag, whereas airbreathing vehicles necessarily sit right in the draggy atmosphere the whole time they are thrusting.

The other factor you've forgotten is thrust:weight ratios. Conventional rocket engines have 100:1 thrust:weight ratios, airbreathing rockets are lucky to get 10:1. That means that the dry weight of the vehicle is 10x higher. That greatly offsets any reduction in fuel use- and LOX/Kerosene fuel is dirt cheap (under $1/kg), whereas the airbreathing mass is typically very expensive. So you've typically swapped cheap fuel for expensive hardware, not good.

Re:Rocket Equation

[Anonymous Coward]

wtf? wolfwithoutaclue again?

if you expell an atom with twice the mass at half the velocity the net momentum transfer is the same.

what you need to do is approach this problem from the angle of conservation of energy and then use the conservation of momentum. given that a 'heavy' and 'light' fuel have the same stored energy, the ratio of exhaust momentum is inversely proportional to the ratio of the molecular velocities.

dont post again until youve derived it.

Re:Rocket Equation

[Michael.Forman]

Sir, it's bad form for a scientist to tell people that they don't know what they're talking about. If I am incorrect, exercise the option of teaching rather than insulting.

Anyway, since I don't want you holding (or refusing to hold your breath), I did a little research. For those, who are still interested, below is a wonderful set of links to websites that discuss the rocket equation. The second is my favorite. Enjoy!

Teachin' Science
Rocket Equation Applet
Wolfram

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.

Re:microwave

[sigep_ohio]

I dont remember which, but one Japanese semiconductor corp is planning to put up small sats to beam power to handheld devices via microwave.

that sounds dangerous. i don't want the same microwaves that cook my food(or even waves in the same spectrum) being beamed from over head just so i don't have to replace the batteries in my pda. it is stuff like this that makes me trust technology less and less.

i know that was off topic, but it is my two cents.

on topic, perhaps they could use a swarm of these(say 3-4) and tether a long carbon nanotube in between. it may be a cheap and easy way to get the space elevator started. and they both should be feasible by around the same time in the future too.

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!)

Re:Leik Myrabo

[JohnFluxx]

do <a href="http://...">click here </a>

Re:Leik Myrabo

[krysith]

Thanks JohnFluxx. Let's try that again:
click here
click here
And BTW, this ~is~ the laser powered rocket n1ywb mentioned below. Myrabo pops up on the Discovery channel every couple of years. Unfortunately, his progress has been fairly slow. I read his book back when I was in Junior High. His big problem is using lasers. Both lasers and microwaves have their problems. I think it is a good idea, though, and I wish Dr. Myrabo luck. I recommend the book to NEone who is seriously interested in the problem of how to put things in orbit cheaply and safely.

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! :)

Yes it has been done already

[BigDaddyMike]

http://www.space.com/businesstechnology/technology /laser_propulsion_000705.html

I remember reading about this several years ago in a magazine. This is just one of the many articles ou there on the subject. I know they have sent the object up a couple hundred feet untethered.

There was also another article that used microwave power to gernerate a sort of force field around the nose of a space craft to remove drag and possible the shockwave. (Somethig spike was the name). Just some cool technologies that ould be nice to have on my car!

Troll??

[Deflagro]

Dude i don't kmnow why this was set as Troll...i think it's funny as hell. I'm against the american hypocrisy but that Sahaf guy is hilarious. Not a very good propaganda spinner.

+1 Funny!

Re:Troll??

[Tuxinatorium]

I agree. Some people just don't have a sense of humor.

Re:Troll??

[Anonymous Coward]

Some people just don't have a sense of humor.

Sorry, that joke is worn out and will be replaced soon.

Microwaves are problematic

[WolfWithoutAClause]

The big problem with microwaves is focusing them down. For this to work you need most of the microwave energy you are pointing at the vehicle to make it to the vehicle.

Normally when microwaves are used for sending signals, for example satellite TV or point-point transmission, most of the power ends up missing the receiver, but the receiver amplifies the signal using power from the mains.

When you have a rocket, there's obviously no power cord ;-), so you need the vehicle to catch as much of the microwaves as possible.

However, there's a law of nature that says that the maximum focusing you can do is determined by the size of the transmitter antenna and the wavelength of the microwaves you are sending.

So:

angle of transmitted beam (in radians) = 1.22 * wavelength/diameter of antenna

wavelength of 100Ghz microwaves = 300,000,000/100,000,000,000 = 3mm (1/8 of an inch)

Let's make the diameter of transmitter = 10 m (say). (This is a huge transmitter, ~30 feet diameter, reasonably big).

This gives the angle of the beam to be: 1.22 * 0.003/10 = 0.000366 radians

At 200km, this means that the beam is: 200,000 * 0.000366 = 73m wide (~200 feet)

That means that your rocket would have to be 200 feet in diameter to capture most of this energy. Which is very likely to be heavy.

The only way to improve on that is to go to a higher frequency, difficult from 100 Ghz, or make the transmitter bigger, 100 meters (~300 yards?) and bring the rocket down to 7.3m (21 yards) diameter, but it's still all a bit awkward. Oh yeah, and you can't use 'synthetic aperture techniques'- you can't have a few widely spaced transmitters 100m apart, it turns out that doesn't work, you get huge sidebeams that suck away your power and it never reaches the vehicle, and those sidebeams are hazardous.

Still, it might be good for climbing up some way through the atmosphere, and then using a conventional rocket the rest of the way.

Feel free to play with the numbers, I'm not saying it absolutely doesn't work, it certainly works at low altitudes, I'm just trying to demonstrate the difficulties with microwave beamed power in this context.

Re:Microwaves are problematic

[Anonymous Coward]

You don't need to use a single 10m antenna.
By using a phased array of smaller antennae
you can synthesize a smaller beam pattern.
Near the ground (near field) you will need the
single transmitter, but once in the far-field regime you can switch to a phased array.

Radar and radio-astronomy use phased arrays
frequently, so as a space propulsion system
it should be OK too, but you will need a high
(maybe Hawaii) site to get the atmospheric phase stability good enough.

Re:Microwaves are problematic

[Anonymous Coward]

Still, it might be good for climbing up some way through the atmosphere, and then using a conventional rocket the rest of the way.

Why not just use a balloon? Like a big bagel full of floaty with the rocket hung from cables and pointed through the hole in the bagel. Float...float...float...KablaMMO IT'S ROCKET TIME!

Re:Microwaves are problematic

[mikegroovy]

What about Left-Handed Materials?http://www.aip.org/enews/physnews/2000/s plit/pnu476-1.htm Perhaps they would be capable of focusing the beam a little tighter? ... and once the "Rocket" is high enough, why not use more than one transmitter... have the transmitters arranged in consecutive circles. As long as the rocket is high enough then there will be more vectors for the microwaves to approach. For that matter maybe they could setup a microwave interferometer, have multiple larger frequencies converge into a single tighter frequency.. Aim the beams so that the interferance pattern occurs right under the Rocket. Riding on something that is being blasted with high energy could be just as dangerous as riding on something stuffed with explosives(Traditional rocket) Gee Space is dangerous!

Re:Microwaves are problematic

[WolfWithoutAClause]

What about Left-Handed Materials?

Nah. Doesn't work. You have to be approximately 1 wavelength away for that to work, in this case we would be millions of wavelengths.

and once the "Rocket" is high enough, why not use more than one transmitter... have the transmitters arranged in consecutive circles.

Provided the rings are within a wavelength of each other; yup. Otherwise, nope; you get a horrible diffraction grating effect and most of the power slops around and probably takes out bystanders.

phtttt

[savage_panda]

Microwaves work on principle of heating up water molecules in food. I'd imagine it works on similar principle in expanding the air mostly by exciting the water molecules within it. If so, I would think that water molecules are much more abundant at sea level then a few miles up, and non existant past the troposphere. Making microwave not practical for any long distance lifting. I'd imagine it'd be more practical to build a launch pad on top of a fairly high mountain and fire rockets from there.

network!

[utexaspunk]

hey- wouldn't it be cool if they built a network of microwave cannons and juggled passengers in rockets all over the world?

Microwaves to lauch objects? NOT

[LinearBob]

Not long ago I remember reading about a "directed energy weapon" intended for military use in non-lethal "crowd control." This crowd control device is actually a very short wavelength, high power microwave beam radiated from an antenna on the roof of a vehicle. It is supposed to produce a buring pain in the skin of those in the beam, but is supposed to be a non-lethal because, while it feels like a burn (and perhaps the microwave beam could cause REAL burns) the pain induced supposedly can be stopped simply by getting out of the beam. My recollection is that this military microwave device operates at something like 70 GHz with several kilowatts of power (which is why this thing is mounted on a vehicle -- the vehicle provides the electrical power required to generate the microwave beam.) If anyone else remembers this, please post a URL or two where the details can be found.

So, what is the difference between this microwave beam weapon and a microwave beam rocket launcher except where the high power microwave beam happens to be aimed? I suspect that if one happened to be hit by scattered microwave energy from the microwave rocket launcher, that expereince could be downright painful, or even worse, depending on how much scattered microwave energy one happened to intercept.

As for the idea of microwave power beams coming down to earth from orbiting satellites; I cannot imagine a more efficient or militarily desirable "Death Ray." Think about a solar powered satellite dumping a few thousand megawatts of microwave power (at 100 GHz or so) onto a 100 foot by 100 foot area somewhere on the surface of the earth. Such a beam would probably be lethal to anything living it hits, and there would be no residual ionizing radiation, no radioactive fallout, or toxic chemical pollution left after the microwave beam "Death Ray" was shut off.

There must folks in the Pentagon rubbing their hands in glee, because I doubt that there are many weapons able to kill people by the hundreds or even by the thousands in a matter of seconds that leave no hazardous residues after their use.

I suggest that high power microwave beams like these be kept in places where they can do no harm, preferably, in laboratories.