Elon Musk answered a physics critique of the Tesla Roadster’s thrusters on Sunday with a line from Monty Python. Yann LeCun, who shared the 2018 Turing Award and was Meta’s chief AI scientist until last year, had worked out on X that a car that is half compressed air by mass gets about five seconds of useful thrust, and called the flying Roadster “cute demo, but totally impractical.” Musk’s reply, in full: “I fart in your general direction.”
The exchange sits under Musk’s October 15 post of the Roadster kickflipping over a Semi, captioned “Turns out the future does have flying cars.” By Monday morning that video had 76 million views.
Five seconds is also the figure Tesla’s engineers gave Top Gear for one fill of the tanks after the reveal.
LeCun Puts the Roadster’s Useful Thrust at About Five Seconds
LeCun’s Saturday post puts compressed air’s specific impulse under 50 seconds, so even a car that is half air by mass gets under 10 seconds of thrust at a thrust-to-weight ratio above one, more likely about five. The car, he wrote, must be a light shell packed with tanks.
He had started the thread on Friday with two words, “Hydrogen peroxide?”, a guess that the plumes came from a monopropellant. Tesla’s Lars Moravy had already said on the livestream that the gas is air, and EVXL’s thruster story on Friday morning covered the tanks that hold it. LeCun’s Saturday post took the correction and ran the numbers from there.
Okay, so it's just compressed air.
Yann LeCun (@ylecun) October 17, 2026
Easier.
But crappy specific impulse (less than 50 seconds).
Even assuming that 50% of the mass of the car is the compressed air, that gives you less than 10s of useful thrust (with thrust-to-weight ratio larger than 1.0). Probably more like 5 seconds, actually.
So the car must be an empty, lightweight shell containing big compressed air tanks.
Cute demo, but totally impractical.
But of course, you knew that.
Musk’s reply came Sunday at 10:41 AM Eastern, with a wind emoji, and had 3.2 million views by Monday. The line is the French soldier’s taunt from Monty Python and the Holy Grail. LeCun took it literally that afternoon: a fart from a high-pressure belly might propel you upward and forward, he wrote, and that is exactly how the Roadster is powered, “a 700 bar, 600 liter, 3 second, noisy fart.”
Musk’s own platform sided with the professor in between. Twenty-one minutes after the fart post, a user asked Grok, X’s chatbot, to check the math, and it replied that “LeCun’s physics holds up,” putting useful hover or hop time at about five seconds and calling the impractical verdict accurate.
In a later post he said he owns two Teslas and does not criticize real achievements, Musk’s included, but does expose marketing stunts that are not the achievements they are claimed to be. Thrusters might make a car hover for a couple of seconds if the buyer accepts a 400-liter air tank in place of the passenger seat, he wrote, but they are not a path to flying cars, any more than SpaceShipOne was a path to orbit. He added that he is anti-Musk on politics, citing what he called the spread of disinformation through X.
Tesla’s Own Numbers Give One Fill About Five Seconds of Thrust
Moravy said on the livestream that the Roadster carries 600 liters (159 gallons) of compressed air, and Sawyer Merritt’s list puts one fill at about five seconds of thrust. Drive Tesla weighs that air at about 250 kilograms (551 pounds), and Hagerty says a refill takes six to seven hours.
Those figures are in EVXL’s Roadster rundown from Friday, and LeCun’s shell-full-of-tanks description is not far from Tesla’s configuration. The Thrust version carries a 200-liter tank in its trunk, and the Rocket, the only version Tesla says can fly, adds a 400-liter tank where the passenger seat would be. Moravy told Hagerty the carbon fiber vessels can hold up to 10,000 psi (690 bar), which matches LeCun’s 700 bar. Electrek reports that Tesla’s own Roadster page keeps the Rocket off public roads.
The two five-second figures measure different things. LeCun’s is thrust strong enough to lift the car, and Tesla’s is the run time of one fill however the air is spent, so they line up more by coincidence than by calculation.
Musk had already put the Roadster in its place early Sunday. Optimus and Cybercab will be “the vast majority of Tesla’s value,” he wrote, “But Roadster is super cool, so we’re doing it anyway.” Google’s Sundar Pichai, replying to the flight video within 20 minutes of it going up, wrote that “the FSD version of this will be something.”
The Atlantic’s Ian Bogost argued on Monday that the flying car works better as a fantasy than as a product, under the headline “The Flying Car Must Never Be Built.” He notes that Musk closed the reveal by paraphrasing Peter Thiel’s 2011 Founders Fund line, “We wanted flying cars, instead we got 140 characters,” which the fund has since removed from its manifesto.
EVXL’s Take
LeCun’s physics is right and his gotcha is empty, because Tesla’s own engineers told Top Gear after the reveal that one fill buys about five seconds. The number was never hidden. Reading the show as a stage act is fair on the record, though. Tesla closed its August 2021 AI Day with a dancer in a Tesla Bot suit, and Bloomberg reported that employees offsite were steering many of the Optimus units at the October 2024 We, Robot event. The Roadster at least flew on its own hardware. The stretch is in Musk’s caption: a car that hops for five seconds and then needs six hours on a compressor is not a flying car, and I’d file “the future does have flying cars” with robotaxis next year. A fart joke is a fine answer to a professor who calls your demo impractical. It is a poor answer to the question Electrek asked on Thursday and nobody at Tesla has taken up, which is what 250 kilograms of air at up to 10,000 psi does in a crash. Musk can out-meme LeCun all week. He still owes buyers with $250,000 reservations a sentence about the tanks.
Sources: Elon Musk on X, Yann LeCun on X, Elon Musk on X, Yann LeCun on X, Elon Musk on X, Sundar Pichai on X, Grok on X, The Atlantic, Hagerty, Drive Tesla, Electrek, Bloomberg
EVXL uses automated tools to support research and source retrieval. All reporting and editorial perspectives are by Haye Kesteloo.
