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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."
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.
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
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!)
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!
-73, de n1ywb
www.n1ywb.com
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