My last post on the NASA "quantum vacuum plasma thruster" was mainly about the shoddy theory behind it - like how there's no such thing as a "quantum vacuum plasma".

But you could argue: hey, if the gizmo actually works, isn't that good enough?   

Unfortunately, the experiment has problems too.  In brief:

1.  They tested a device that was designed to work and one that was designed not to work.  They both worked.

2.  They tested the devices in a "vacuum chamber", but they didn't take the air out.

3.  They didn't carefully study all possible causes of experimental error... like their devices heating the air.

In a bit more detail:

1.  Their device, called the Cannae drive, was invented by a guy named Guido Fetta.  You can see a picture below.  It's not complicated!  It's a hollow container made of metal, about 11 inches in diameter and 4-5 inches long.  You pump radio waves in one end. At the other end, a copper wire serves as an antenna.  This lets you measure the radio waves bouncing around inside the container, and adjust their frequency until you hit a resonance.  Then this thing is supposed to generate thrust, for some unknown reason.

Fetta thought this device would work if you carve slots on one side of the flat part.  The NASA guys tried a version with slots and one without slots.  They claim both versions generate a thrust of 22-48 micronewtons when they pump 17-28 watts of radio waves into them:

Thrust was observed on both test articles, even though one of the test articles was designed with the expectation that it would not produce thrust. Specifically, one test article contained internal physical modifications that were designed to produce thrust, while the other did not (with the latter being referred to as the "null" test article).

So, basically they found evidence against Fetta's idea: the slots make no difference.  It's like giving someone a placebo and finding it works just as well as the drug you're testing.

They also tried a resistor instead of their device.  They claim this produced no thrust.  This rules out some possibilities of experimental error... but not others. 

For example, if parts of their flat metal can get hot and create air currents, that might create the force they saw.  It's a tiny force, less you'd get from 5 milligrams of mass pushing down due to gravity.

2.  Their paper goes into great detail about the "vacuum chamber" their experiment was done in - but in the abstract to the paper, they say they didn't remove the air.  This is important because of the issue of air currents. 

It's also just weird.  In their paper they say:

To simulate the space pressure environment, the test rig is rolled into the test chamber. After sealing the chamber, the test facility vacuum pumps are used to reduce the environmental pressure down as far as 5x10E-6 Torr. Two roughing pumps provide the vacuum required to lower the environment to approximately 10 Torr in less than 30 minutes. Then, two high-speed turbo pumps are used to complete the evacuation to 5x10E-6 Torr, which requires a few additional days. During this final evacuation, a large strip heater (mounted around most of the circumference of the cylindrical chamber) is used to heat the chamber interior sufficiently to emancipate volatile substances that typically coat the chamber interior walls whenever the chamber is at ambient pressure with the chamber door open. During test run data takes at vacuum, the turbo pumps continue to run to maintain the hard vacuum environment. The high-frequency vibrations from the turbo pump have no noticeable effect on the testing seismic environment.

They're working really hard to get a good vacuum, right?  But in their abstract they say:

Testing was performed on a low-thrust torsion pendulum that is capable of detecting force at a single-digit micronewton level, within a stainless steel vacuum chamber with the door closed but at ambient atmospheric pressure.

At ambient atmospheric pressure?   What's the point of the fancy vacuum chamber?  A sentence in their conclusions gives a clue.  Talking about future plans, they say:

Vacuum compatible RF amplifiers with power ranges of up to 125 watts will allow testing at vacuum conditions which was not possible using our current RF amplifiers due to the presence of electrolytic capacitors.

So it seems they couldn't actually test their device in a vacuum.

3.  If you're trying to find some small effect, checking the ways you could have screwed up is the most important thing.  The device they're testing is simple, but the test apparatus itself is very complicated, and lots of things could go wrong. 

Their paper should have a big section on this, but it doesn't.  Instead it has a section on how if the gizmo works, you could scale it up and do great things:

Figure 23 shows a conservative 300 kilowatt solar electric propulsion roundtrip human exploration class mission to Mars/Deimos. Figure 24 shows a 90 metric ton 2 megawatt (MW) nuclear electric propulsion mission to Mars that has considerable reduction in transit times due to having a thrust to mass ratio greater than the gravitational acceleration of the Sun (0.6 milli-g’s at 1 AU). Figure 25 shows the same spacecraft mass performing a roundtrip mission to the Saturn system spending over a year around two moons of interest, Titan and Enceladus.

This is called 'counting your chickens before the eggs have hatched'.

I would need to be more of an expert than I am to imagine all the things that could go wrong with their experiment.  But just so you see what I mean, here's one thing they do mention:

one visible effect to the seismic environment is the periodic (about one-third to one-quarter Hertz) perturbation created by the waves from the Gulf of Mexico (about 25 miles southeast of Johnson Space Center), especially on windy days.

The thrust they're measuring is so small that waves in the ocean 25 miles away could screw up the experiment!  They tried to deal with this... but it goes to show, you can't revolutionize physics until you carefully check all the sources of error.

I thank Greg Egan and +Matt McIrvin for their help, but of course they're not to blame for any mistakes I made. 

The paper I'm talking about was published here:

• David Brady, Harold White, Paul March, James Lawrence and Frank Davies, Anomalous thrust production from an RF test device measured on a low-thrust torsion pendulum, 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference,  

Unfortunately it's not free except for the abstract.  Luckily someone has liberated the paper and put a free version here:

Beware: the abstract in the paper is different than the abstract on the NASA technical report server here:

This is where they say they didn't remove the air from the vacuum chamber.

There's a website about Guido Fetta's company and his device:

• Cannae Drive,

It says:

The Cannae Drive is a resonating cavity with design features that redirect the radiation pressure exerted in the cavity to create a radiation pressure imbalance on the cavity. This differential in radiation pressure generates an unbalanced force that creates thrust. The cavity is accelerated without use of propellant. Don't believe it? Study the theory.  Replicate our numerical models.  Review our experimental results.  And draw your own conclusions.

Unfortunately, when I click on the links to theory, numerical models or experimental results, I get:

404 - Article not found

+Hamilton Carter pointed out another paper by the NASA team, which explains the wild optimism behind this experiment:

• Dr. Harold “Sonny” White, Paul March, Nehemiah Williams, and William O’Neill, Eagleworks Laboratories: advanced propulsion physics research,

They write:

NASA/JSC is implementing an advanced propulsion physics laboratory, informally known as "Eagleworks", to pursue propulsion technologies necessary to enable human exploration of the solar system over the next 50 years, and enabling interstellar spaceflight by the end of the century. This work directly supports the "Breakthrough Propulsion" objectives detailed in the NASA OCT TA02 In-Space Propulsion Roadmap, and aligns with the #10 Top Technical Challenge identified in the report. Since the work being pursued by this laboratory is applied scientific research in the areas of the quantum vacuum, gravitation, nature of space-time, and other fundamental physical phenomenon [sic], high fidelity testing facilities are needed. The lab will first implement a low-thrust torsion pendulum (<1 uN), and commission the facility with an existing Quantum Vacuum Plasma Thruster. To date, the QVPT line of research has produced data suggesting very high specific impulse coupled with high specific force. If the physics and engineering models can be explored and understood in the lab to allow scaling to power levels pertinent for human spaceflight, 400kW SEP human missions to Mars may become a possibility, and at power levels of 2MW, 1-year transit to Neptune may also be possible. Additionally, the lab is implementing a warp field interferometer that will be able to measure spacetime disturbances down to 150nm.  Recent work published by White suggests that it may be possible to engineer spacetime creating conditions similar to what drives the expansion of the cosmos. Although the expected magnitude of the effect would be tiny, it may be a “Chicago pile” moment for this area of physics.

The "Chicago pile" was the experiment that demonstrated a nuclear chain reaction. 

#spnetwork doi:10.2514/MJPC14  #cannae_drive #physics  
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