In a study published this month in the Journal of Spacecraft and Rockets, researchers led by the University of Tsukuba have demonstrated wireless power transmission via microwaves for a free-flying drone and determined the efficiency of this process.
Previous analyses of this kind were carried out decades ago and mostly considered microwaves of a low frequency (a few gigahertz; GHz). Given that the power transmission efficiency increases as the operating frequency is raised, the team behind this latest research used microwaves with a relatively high frequency (28 GHz). The team’s drone weighed roughly 0.4 kilograms and hovered for 30 seconds at a height of 0.8 meters above the source of the microwave beam.
The researchers measured the efficiencies of the power transfer through the beam (4%), the capture of microwaves by the drone (30%), the conversion of microwaves to electricity for propulsion (40%), and other relevant processes. Based on this information and an analytical formula, they calculated the overall power transmission efficiency in their experiment to be 0.43%. For comparison, in a previous study, the team measured the total transmission efficiency for a fixed-position (rather than free-flying) drone to be 0.1%.
“These results show that more work is needed to improve the transmission efficiency and thoroughly evaluate the feasibility of this propulsion approach for aircraft, spacecraft, and rockets,” explains Shimamura. “Future studies should also aim to refine the beam-tracking system and increase the transmission distance beyond that demonstrated in our experiment.”
Although microwave-powered rocket propulsion is still in its early stages, it could someday become a superior way to launch rockets into orbit given the high onboard-fuel demands of conventional propulsion techniques.
SOURCES- University of Tsukuba, Journal of Spacecraft and Rockets – 28 GHz Microwave-Powered Propulsion Efficiency for Free-Flight Demonstration
Written by Brian Wang, Nextbigfuture.com
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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So these guys basically just had a microwave oven with the door removed.
They weren't bothering to optimize the energy transmission, that's all. They were just spewing it in the direction of the drone, with most of it missing.
With an optimized system you'd be looking at more like 65% efficient, end to end.
Yes, the weight of the nuclear reactor was a serious drawback to the program.
Of course, nobody had ever spent too much effort making reactors light before, at best it needed to be able to be carried in a large submarine. So I imagine that this was an issue that could be improved if anyone sank resources into it.
But that's off topic. The question is: can a microwave driven rectenna and electric thermal rocket be made with a high thrust to weight? And I see no actual information on that anywhere.
Interesting.
You obviously can't use rectennas for microwave powered rocketry, unless it's operating in zero g and at extremely low thrust to weight. You need a very simple system, with direct application of the power, to pull off a launch from Earth's surface.
But, let's say you contrive to have a microwave transparent engine, (Fused quartz? 'Transparent aluminum'?) and seed the propellant with something that would absorb the microwaves. You could track the engine from a ground based phased array, dumping power into the fuel. The array size is perfectly feasible if you're limiting yourself to, say, ground to orbit. (Well, a series of arrays, under the track.)
But, how do you power the pumps? OK, run a conventional liquid fueled engine with turbo pumps. Then dump hydrogen into the combustion chamber, post combustion. Maybe on the outer ring of the injector? Transpiration cooling for the engine bell?
The conventional propellant powers the onboard systems, the microwaves dump more energy into the flow.
Probably want a higher than normal expansion ratio, because you're dumping heat into the exhaust, it's not cooling as it expands, maybe even heating.
A microwave powered afterburner!
Sorry no numbers: Just got back from hiking Zion, and I'm really sleep deprived from the flight home.
Nuclear thermal thrust to weight is pretty bad. At best I think MITEE had a T/W of 30?
The idea is that you have electrical power. YOu can use this to run electric motors. Or you can use it to run electric thermal rocket engines. See the other comments where we discuss electrically heated thermal hydrogen rockets.
Dan Lantz,
Do you have some commentary on the 0.4% power transfer efficiency?
Because that kind of destroys the case for solar power satellites unless these guys are clowns who have no idea what they were doing.
The nuclear rocket programs, which got as far as making near full sized rockets and running them at full power for some time (radioactive pollution? what's that?) showed what could be done with just hot hydrogen.
And this wasn't theoretical, they were made, with accessories and pumps and cooling and all that. One model ran for a total of 1 engine hour. WHich isn't that impressive for a diesel, but for a 250 kN rocket producing 1.2 GW it isn't bad.
https://en.wikipedia.org/wiki/NERVA
Anyway, they recorded 862 seconds ISP, So I think that converts to 8500 Ns/kg. Proven, with late 1960s materials tech and control systems.
I think it's safe to say that 1000s isn't an unrealistic goal for a modern design.
The idea is that instead of carting say Hydrogen and Oxygen which burns with an ISP of up to 450 seconds, you instead only need take the hydrogen and your exhaust is just hot hydrogen (ISP up to 1000 seconds in the NERVA tests).
Put 1000s in your rocket equations and all sorts of things become much better.
https://strout.net/info/science/delta-v/
I get that your rocket will reach orbit, single stage, with only 60% of the launch mass being fuel.
NB: NERVA used a nuke to heat the hydrogen, not electrical power or microwaves, but as far as the hydrogen knows it's the same thing. Except of course a microwave receiver should be enormously lighter than a nuke, and somewhat easier to get flight clearance.
That was what I was really curious about: the microwave/laser receiver to absorb the microwaves or laser pulses. 1. How large? 2. What material would be appropriately durable/strong enough?
And also, how to steer the rocket? By constantly monitoring and changing the aim and energy level of the beam? What's that feedback loop look like? Rocket sensors to ground sensors with nanosecond adjustments (I'm guessing) for precision control of the beam?
Thanks for breaking everything down. Sounds like an engineering nightmare, especially considering how the guidance systems would have to be modified. I know waveforming and beamforming antennas and receivers have come a long way, but not on anything approaching these energy levels.
By the way, I've read your posts for years. You've got a wicked smart mind. Way above my reasoning abilities.
I appreciate how you break it down for guys like me…
By my count, the wavelength in air(n = 1.05) for 28 GHz is 10 mm, so no. It's still close, though, and it was probably a directional antenna.
80cm, does that technically count as near field for that frequency?
If you can better match up the power system for the drone with the received frequencies, you can drop a fair bit of conversion mass for aerial applications, but for small moving target stuff laser PV seems easier.
Kind of a typical J-Fluff piece. Lot of research, plenty of write up, and precious little to show for it, really.
Thing is, it really ought to be considered in the context of PV powered ion thruster technology, which has gotten to nearly 50% PVe to ThrustE conversion efficiencies. One certainly hopes that the infrared or microwave might be improved by 100×, you know? But when. And for what parasitic mass penalty?
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
For those more interested in a contrasting discussion, To "Tangential" I posted a 3 part open-ended thought stream. Might be worth clicking on. I should have put it at the main level, but … alas. GoatGuy
The killer is the beam transmission efficiency of 4%. Unless you can lift that by an order of magnitude, this is an interesting experiment but not practical.
(part 3):
But then there's the original physics … higher ISP if the density of the REM is lower per unit volume.
So use (H₂) gas (stored cryogenically) pure.
No oxidizer.
No O₂.
What does this bring?
WAY larger tankage than H₂O.
Higher packaging mass, insulation, exotic-materials parasitics.
But what is realized?
Well, for one, 5000 °K monoatomic H plasma has a theoretical thrust limit near 7000 Ns/kg (maybe more!). Very nice. ½ the fuel for the same orbital vectoring maneuvers. Less than ½, when back-calculating the parasitics of ½ the fuel TIMES ½ the mass, and … etc.
Which can be 'used up' with that microwave collector, boiler, plasma maker, electrical cabling and power electronics.
Might turn out to be a wash.
If it is a wash though, then what's the point?
In theory for 'station keeping' (i.e. punting around a satellite which is already more-or-less where it is supposed to be, but needs vernier adjustment of its orbital parameters), decoupling the energy-production from the RE mass can be quite good: the REM is optimized for miserly use-up over long periods of time. The energy source can be PV or something equally unchallenging and light weight. Win-win.
But longer term, we need a 500 ESP thruster which can generate 300 mega-newtons, for atmospheric magnificence.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
(Part 2):
Now here is where the microwaves-to-heat-to-thrust action takes place.
If you comport 'up there' just a whole lot of water (not H₂ and O₂, cryogenic), a big inverse radiator to capture both heat and convert some to electrical power, one can vaporize the water, shock a plasma thru it heating it to maybe 2500 ° K, and shoot it out a nozzle, conventional style. Or hundreds of littler nozzles. Whatever.
What do you get?
Ostensibly, the tankage is hugely simplified, not being cryogenic at near absolute zero (H₂) conditions. And ridiculously bulky to boot. (remember the external shuttle tank?)
That's good.
And the 're-heating' requirement to get it to steam, pre-plasmification is finite and not overly ornerous. So, with lower tankage requiremenst, less mass, then there's less parasitic mass to thrust forward. More room for payload, or an overall lighter weight projectile.
Sounds good.
But then there is the concentrated microwave receiver. Mmmm… 100's of m², one thinks, dealing with yellow-hot temperatures. Or hotter. And then, how to turn a bunch into electrons, to make plasma, to get real meaninful thrust above 3000 Ns/kg? Mmmm… back to parasitics again. All that collector shît.
All that, and then the parasitics of boilers, tubes, ionizers, plasma generators, waste-heat-of-production sloughing. Arrgghhh!
Back to the drawing board.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅ (more coming)
Well … an admirer! Excellent (and ⊕1 to you). Seriously, it is a good question to muse upon.
A rocket needs to
• Lift its reaction mass
• Lift its energy source
• Lift its REM accelerator
• Lift its airframe, tankage, stage sep stuff
• Lift its fire suppression and failure escape doodads
• Lift its payload
• Lift the 'capsules and crâhp' that contain payload
• Lift any additional fuel/REM to make a dry landing a la Musk
Conventional binary rocket fuel is both REM and energy source.
Fuel and oxidizer.
Mass and fire.
The burning generates heat and light.
Heat causes the REM to gain a lot of PV/T (pressure × volume ÷ temperature)
Which, faced with a carefully obstructed and vented hole…
Shoots that all out the underside, to generate thrust.
Thrust which is rather limited by specific mass, temperature, pressure, physics.
So, for instance, one can theoretically realize about 4500 newton-seconds per kg of H₂ and O₂.
Because its burned density is low ("18").
At the other end, rocket geeks are lucky for 3300 Ns/kg for O₂ and kerosene.
But KerOx requires only one part (ox) to be cryogenic. Kero is like water.
This'll be long, so I'll post more as replies-to-self
Just saying, GoatGuy (part 1)
You're right; it's a research program pitch, which makes total sense for University people. In any case, power beaming doesn't help much with rockets; you will still have to carry the remass to get to orbit, since air will become thinner and thinner as the ship climbs.
This is where I miss GoatGuy.
How much of a rocketship's mass could be eliminated and how much launch cost reduction is possible if liquid/solid fuel isn't required to initiate a launch (yes, I realize fuel would still be needed to complete the launch and guide the ship back)?
VS.
How much money would be required to transmit enough microwave/laser energy at pitiful efficiencies? I know we have powerful laser weapons on airplanes capable of disabling incoming missiles. Would these massive lasers be any good for prodding a ship into space?
Yep. I didn't get the feeling they were testing fuel heating on a small rocket, as it is usually depicted/understood. It sounded like electrical power transfer via microwaves, but I might be wrong.
The article is surprisingly vague in details, and the PDF is behind a paywall.
Beam power, either microwave or laser, can be used to augment the ISP of rockets. Could be used to prewarm the fuel. The requirement is that the weight of the mechanism for collecting the power must be less than the amount of fuel saved.
I think laser might be better because of their higher power density.
0.43% transfer efficiency, in a lab setting, 80 centimeters above the microwave source, for 30 seconds… That's not the best investor pitch.
Am I missing something here? How does one go from transmitting electricity to a drone to microwave powered rocket propulsion? The drone seems to be receiving energy to power its electric motors/rotors. How does that translate to providing thrust for a launch vehicle?