Many readers of this post will have watched the video demonstration of Elon Musk and Tesla's so-called "Flying Roadster." Of course, the invitees to the demonstration were 100% hard-core Tesla/Musk faithful, no skepticism was apparent. And, frankly, it was an impressive feat of engineering, regardless of whether, in the end, it has anything to do with a production Roadster.
The premise is that the promised cold-gas thruster package for the new Roadster is a rehash of the reaction controls Lockheed put on the NF-104A Starfighter in 1963. The video says Cold War engineers already learned the hard way that it can't work. The conclusion is right, but a surprising amount of the supporting story is wrong.
Start with what's right, because it's the important part. A wing flies by pushing a lot of air down slowly. A thruster does the opposite: it throws a small amount of gas out very fast. Thrust is just mass flow times exhaust velocity, ~F = \dot{m} v_e~, but the power needed goes as ~\tfrac{1}{2}\dot{m} v_e^2~. Fast and small is the expensive way to make thrust. That's why helicopters have big rotors.
Let's toss in some numbers. Rocket people (of which I am conspicuously not one) rate efficiency by specific impulse, ~I_{sp}~: how many seconds one pound of propellant produces one pound of thrust. Kerosene and oxygen gets around 300 seconds. Cold compressed air gets about 60. To hover a 4,500 lb car, you need 4,500 lb of thrust, so:
~\dot{m} = \frac{F}{I_{sp}} = \frac{4{,}500\text{ lb}}{60\text{ s}} = 75\text{ lb/s}~
The video gets this right. Now the tank. They posit a 300-liter carbon-fiber pressure vessel at 10,000 psi where the back seat used to be. At that pressure air is about 550 kg/m³ (it isn't an ideal gas anymore), so the tank holds roughly 165 kg, or about 360 lb. The video says 180–200 lb, which is low by almost half. Allowing for the air you can't get out, call it 290 lb usable:
~t = \frac{290\text{ lb}}{75\text{ lb/s}} \approx 4\text{ seconds}~
So the video's "two to three seconds" should be more like four. Either way, that's a hop, not a flight.
Refilling is where it gets ugly. The minimum work to compress that much air to 10,000 psi is about 25 kWh, closer to 35 - 40 kWh with real compressors. That's a big bite out of even a large EV battery, and a compressor small enough to ride along would take hours. All for four more seconds of hover. The video is correct here and, if anything, understates the case. Running some quick numbers:
| Average battery power used for compression and cooling | Refill time |
|---|---|
| 3 kW | About 17 hours |
| 5 kW | About 10 hours |
| 10 kW | About 5 hours |
Of course, one wouldn't always need to refill an empty tank to completely full, but you can see the issue, I'm sure.
Now the parts I had trouble with:
Early on we're told 10,000 psi is "triple the chamber pressure of a military sniper rifle." It's the other way around. A .308 runs about 62,000 psi, so 10,000 psi is about one-sixth. Later, the thruster exhaust at 2,000 feet per second is "the exact muzzle velocity of an M16." An M16 round leaves the barrel north of 3,000 ft/s. These are easy things to look up.
The bigger problem is the hook itself - the video relies heavily on NASA's experimention in the 1960s. You can read all about it here. The NF-104A did not use cold-gas thrusters. Its reaction controls ran on hydrogen peroxide decomposed over a silver catalyst: hot steam, not cold air. The video admits this around the nine minute mark, which contradicts its own opening. So the "1963 proof" isn't really about the technology in question.
Then there's the long buildup about Joule-Thomson cooling freezing valves at –100°F. Valve and regulator icing is real (ask any diver who's had a regulator free-flow in cold water). But simple throttling through a valve cools the gas by tens of degrees, not hundreds. The deep cold happens in the nozzle, where heat becomes velocity, and in the tank as it empties. And moisture icing is a known problem with known fixes: dry gas, heaters, the right seals.
That buildup leads into Chuck Yeager's December 10, 1963 crash in NF-104A 56-0762. The video gives a dramatic and mostly accurate account: rocket lit at 35,000 feet, a steep zoom past Mach 2, a flat spin, and an ejection with terrible burns when the seat's rocket motor struck his helmet. Most accounts put that flight's peak near 101,600 feet rather than the 108,700 quoted. That higher altitude was achieved on Yeager's first flight of the day. The crash occurred on his second.
But here's the thing. By the video's own telling, the crash happened because the nose thrusters didn't have the authority to stop a pitch-up into a spin. No valve froze. Ten minutes of setup about frozen valves, and then the crash it builds to has nothing to do with them.
The road-hazard section has the same problem. Ground "suck-down" under a wide, flat body is a real phenomenon from the VTOL era, and the Harrier description is accurate. But "a Claymore mine aimed at every windshield within 50 yards" is dramatized, and I could find nothing on Soviet T-55 tanks with rockets vaulting trenches.
The video also leaves out two points that cut the other way. First, 10,000 psi carbon tanks are already on public roads. Hydrogen fuel cell cars like the Toyota Mirai carry them, crash-certified. Having spent a career around testing and inspection, I'd say a COPV (Composite Overwrapped Pressure Vessel) behind the seat is an engineering problem, not a death sentence. Second, as originally pitched, the thrusters were mainly about acceleration, braking, and cornering, not hovering. That takes a fraction of the car's weight in thrust, and the air lasts much longer.
So the verdict? The Roadster won't fly in any meaningful sense, it will only hop. The video is right about that. Compressed gas is magnificent for nudging a satellite and pretty lousy for holding up two tons of electric car. But the video gets there with a misleading history, a couple of flatly wrong comparisons, and a lot of scary adjectives. The physics was enough on its own. The drama wasn't needed, though, in this age of content creators looking for eyeballs, I guess it's understandable.
Ps: If anyone has a good source on the T-55 claim, send it my way. I'd love to be wrong about that one.
Labels: aviation, engineering, physics, Tesla, Elon Musk
