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Crunching the numbers on NASA’s flying saucer

Google "impossible thrust" right now and you'll probably stumble upon talk of a breakthrough interstellar propulsion system that violates the laws of physics.

Only it doesn't.

A NASA team (informally known as “Eagleworks”) has been chasing down possible implementations of David Froning’s Quantum Interstellar Ramjet, which featured in Arthur C. Clarke's 1986 novel, "The Songs of a Distant Earth."

Describing their quest in 2011, Harold White and his colleagues talked about building a very sensitive test rig, capable of measuring a force equivalent to the weight of one ten-thousandth of a gram under earth gravity. This would be the test rig for a prototype “Quantum Vacuum Plasma Thruster (QVPT).” Thankfully, they shortened the name to Q-thruster.

The Q-thruster looks attractive for interplanetary and interstellar missions because it does not need “reaction mass.” A conventional rocket generates thrust by firing reaction mass out of its exhaust. When it runs out reaction mass, such as fuel and oxidant, it stops thrusting. A Q-thrusted spacecraft would not be weighed down with a whole lot of deadweight, in the form of reaction mass. Which should open the way to bigger payloads.

Juts like a normal thruster, the Q-thruster fires stuff out of its exhaust. But Q-thruster's reaction mass comes from the quantum vacuum. Which is one of the weird things predicted by quantum electrodynamics. According to quantum electrodynamics, particles such as electrons and atoms are constantly flickering in and out of existence. On average, flickering in equals flickering out. In cancels out. It's weird, which is another word for "quantum."

By stealing particles as they blink into existence, the Q-thruster supposedly pushes against the quantum vacuum. It electrically charges particles as they pop into existence, and then fires them out through a nozzle before they have a chance to pop back out of existence.

In 2006, Eagleworks researcher Paul March, along with Andrew Palfreyman, published the results of testing and modelling on a Mach-Lorentz Thruster (MLT). This is a donut-shaped arrangement of coils and capacitors: The perfect shape for a flying saucer. Still, the American Institute of Physics took March and Palfreyman seriously enough to publish their paper. The really interesting thing was that March and Palfreyman got way more thrust than they expected: 0.1 Newton per kilowatt of input power*.

Had they invented a real live quantum ramjet? This was a job for Eagleworks.

If March & Palfreyman's gadget worked, a Q-thruster based on their design could deliver payloads in the tens or hundreds of tonnes to Jupiter’s orbit in 35 days. Given a suitable source of electricity, a Q-thrusted ship could reach Proxima Centauri in about 30 years.

It all hinged on the numbers. Eagleworks were looking for 0.1 ~ 0.4 N/kW of static thrust.

After building their super-sensitive measuring widget, Eagleworks tested a device called a Cannae thruster. According to its designer, the Cannae is a reactionless thruster, but not necessarily a Q-thruster. (No. Apparently the name was not inspired by Scotty’s: “Ya cannae change the laws o’ physics, Jim”.)

Last week the Eagleworks team reported at the American Institute of Aeronautics and Astronautics’s 50th Joint Propulsion Conference, that while testing the Cannae, they might have demonstrated “an interaction with the quantum vacuum virtual plasma”.

Their Cannae thrusted. But. So did a modified Cannae that shouldn’t. Which is weird. Which is what we’d expect from anything quantum. Including a Q-thruster. Could the Cannae be a Q-thruster?

A Wired blogger got himself completely tangled up. “NASA Validates ‘Impossible’ Space Drive," he wrote.

Actually, no.

It was not impossible.

If the Cannae really can thrust against the quantum vacuum, that's just quantumly weird. Which would be normal. For any kind of quantum thruster. Especially a QVPT.

And no.

NASA did not validate anything.

Wired's David Hambling reckons the Cannae test validates the claims of Roger Shawyer, a British entrepreneur with another reactionless thruster concept. Which it does not. Shawyer himself told Hambling the Cannae cannae be compared with his EMDrive. For a start off, the Cannae has a low-Q resonator, whereas Shawyer's Thruster has a high-Q resonant cavity. Trying to make a low-Q device work where you need a high-Q gizmo is a little bit like trying to pull away from a set of traffic lights with your car in top gear.

The Cannae can't tell us anything about the Mach-Lorentz Thruster either. The Cannae is a tuned cavity. In plain English, a metal box with microwaves bouncing around inside it. Its general outline is not unlike a conventional rocket engine. It looks absolutely nothing like a flying saucer. Which the Mach-Lorentz Thruster does. Fed with 28 Watts at 935 Megahertz, the Cannae pushed out 40 micronewtons (0.0014 N/kW) of thrust. With 15 Watts at 1933 Megahertz it produced 91 micronewtons (0.0054 N/kW).

The Mach-Lorentz Thruster worked at frequencies between two and four Megahertz. According to March and Palfreyman’s 2006 paper, it produced almost twenty times more specific thrust than the Cannae. And that's not the end of the Cannae's problems. The practical performance of a Q-thrusted spacecraft will depend on its thrust-to-mass ratio. A microwave generator suitable for the Cannae would be heavier than a high-frequency power generator suitable for the Mach-Lorentz Thruster. Which means that in the unlikely event that both prove capable of thrusting against the quantum vacuum, the Cannae will need a higher thrust to power ratio than the Mach-Lorentz Thruster, if it is to match the MLT's performance.

There are two things. One. March and Palfreyman couldn’t explain why the Mach-Lorentz Thruster produced more thrust than they expected. That doesn't mean it is a real live Q-thruster. They were trying to measure 3 millinewtons. That is such a tiny force we simply cannot rule out instrumentation problems. The Eagleworks Cannae test is even more problematic in that respect. The strongest force they measured was only 0.09 millinewtons. In both cases, the obvious next step is to scale up by a factor of one thousand. In the case of the MLT, that is not at all difficult. They need a kilowatt-scale high-frequency generator. Lots of readily available devices could be modified to do the job. To put it bluntly: There is still no evidence of the Q-thruster's technical feasibility.

And the other thing. We know that quantum electrodynamics is a model of the natural world. A successful one. But a model, nonetheless. Some physicists think the underlying universe is actually smooth, and it only looks quantum because we can't yet drill far enough down into the underlying reality. If they are right, maybe the Q-thruster really is impossible. How will we find out? Scale up those tiny prototypes, big enough to create a strong effect in the laboratory. A prototype that can produce 100 Newtons of thrust is going to be far more credible that a tiny model that makes 0.00009 of one Newton.

I am fascinated by the web reaction to the Eagleworks presentation. It seems to me that the possibility of a Q-thruster has created more excitement than real-live proof ever could. Too many people think the Q-thruster proves the laws of physics are wrong. Which means too many people need to brush up on their physics.

In any case, this is high-priority research. Progress in space exploration is moving far too slowly. A practical Q-thruster, as envisaged by the Eagleworks team, would be a genuine game-changer.

The Q-thruster would be perfect for long-haul missions. It's an ideal manouevring thruster. It doesn't seem to replace a conventional rocket for launching from earth into space. It's just the thing for just about everything else. But in spite of Wired might think, right now, it is pie in the sky.


Kevin Cudby is a Wellington-based Freelance Writer and Parametric Modelling Consultant who loves writing about cool new technology. Email him to discuss your requirements: hello {a}

* For constant thrust, thruster power input would increase as the space-craft's velocity increases.

Crunching the numbers on NASA's flying saucer

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