Yann LeCun vs. Mr. Musk on the Roadster's Cold-Gas Thrusters

On October 17 and 18, 2026, Yann LeCun and Mr. Musk argued on X about how the new Tesla Roadster’s thrusters work. LeCun’s numbers are his estimates. Tesla’s product page describes “advanced cold-gas thrusters” and does not publish a tank pressure, a burn time, or a specific impulse.

This page is the receipt. The demo, the trim table, and the 600-liter reporting are on the Roadster reveal. The post-reveal sentences, including the carrier comparison, are on the claims log.

The post he was answering

The status 2111153802006888765 is not a post by Mr. Musk. It is LeCun’s, at 17:54 UTC on October 16, 2026:

@elonmusk Hydrogen peroxide?

That reply points at Mr. Musk’s post 2110913107094606330, 01:57 UTC the same day:

Turns out the future does have flying cars

Label: Primary for those words. “The future does have flying cars” is his sentence about the demo. It is not a flight duration, and it is not a certification.

LeCun’s next post in the thread, the compressed-air reply below, is a reply to his own “Hydrogen peroxide?” line.

Compressed air, in LeCun’s estimate

At 16:37 UTC on October 17, 2026, LeCun wrote:

Okay, so it’s just compressed air. 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.

Label: Expert estimate, not a measurement. Specific impulse “less than 50 seconds,” “less than 10s of useful thrust,” and “Probably more like 5” are his calculations under his assumption that half the car’s mass is compressed air. This desk did not weigh a car or time a burn. “Cute demo, but totally impractical” is his judgment.

The Times of India (updated October 17, 2026) reported that reply, including the specific-impulse line and “Cute demo, but totally impractical.” The quotation on this page is the post.

“I fart in your general direction”

At 14:41 UTC on October 18, 2026, Mr. Musk replied:

@ylecun I fart in your general direction 💨

Label: Primary for that sentence. It is a taunt, the line spoken by the French soldier in Monty Python and the Holy Grail (1975). It is not a technical answer, and it does not concede or contest the specific-impulse estimate.

The 700 bar, 600 liter, 3 second joke

At 17:25 UTC the same day, LeCun replied:

@elonmusk From a high-pressure belly, your fart might propel you upward and forward. Come to think of it, that’s exactly how your car is powered: A 700 bar, 600 liter, 3 second, noisy fart 😆 [Hopefully without the methane and the smelly compounds]

Label: His joke, with his figures mixed into it.

The 3 seconds are his. They are not a Tesla burn-time specification.

The 600 liters match a figure outlets already attributed to Tesla’s engineering chief at the reveal. TechCrunch reported 10 thrusters fed by 600 liters of compressed air. The Verge reported 10 nozzles and two tanks totaling 600 liters. WIRED reported four thrusters. Those accounts, and the disagreement on the count, are on the reveal page. This page does not re-adjudicate them. LeCun’s joke uses 600 liters. It does not cite those outlets.

The 700 bar is his. tesla.com/roadster, fetched October 19, 2026, describes “advanced cold-gas thrusters” and does not state a pressure in bar or in psi. The TechCrunch, Verge, and WIRED accounts already used on the reveal page do not print 700 bar. A search the same day found secondary writeups that mention about 10,000 psi, including one that attributes that pressure to a 2018 remark by Mr. Musk. This desk did not retrieve that 2018 post. It is not adopted here as a Tesla specification. In the October 18 joke, 700 bar is LeCun’s number.

“I even own two Teslas”

At 18:03 UTC, LeCun replied to @spiroskaye, who had answered the compressed-air post. The reply is not a direct answer to Mr. Musk:

I’m definitely not anti-achievement and never criticized real achievements, including by Elon. I even own two Teslas.

I do expose marketing stunts that, while cool on the surface, are not the achievements they are claimed to be.

Compressed air thrusters might give you acceleration boosts in any direction. They might even make your car hover for a couple of seconds if you accept replacing the passenger seat by a 400 liter high-pressure air tank. But they are not a path to actual flying cars. The same way Burt Rutan’s SpaceshipOne was never going to be a path to orbital flights (just bunny hops out of the atmosphere).

I am anti-Musk when it comes to politics. His political actions, the massive dissemination of disinformation through X and other channels, and his support for authoritarian regimes and racist ideas have the effect of weakening democracy and weakening human decency and social progress around the world.

Label: Primary for what he wrote. “I even own two Teslas” and “marketing stunts” are his sentences. The “couple of seconds” and the 400 liter tank are a further estimate, separate from the 600 liters outlets reported. The political sentences are his characterization. They are not a finding of this page.

The carrier sentence, the same afternoon

At 15:16 UTC on October 18, between the Monty Python reply and LeCun’s comeback, Mr. Musk quote-posted @XFreeze:

The new Tesla Roadster accelerates faster than a fighter jet taking off from an aircraft carrier

The quoted post, @XFreeze at 08:04 UTC, is the one that prints “>80 kN (~18,000 lbf)” and the F-16 comparison. Label: The carrier sentence is Mr. Musk’s claim. The kilonewton lines stay @XFreeze’s. Tesla’s page does not publish them. The claims log already separates those lines from Pratt & Whitney’s published F100 thrust. This page does not repeat that comparison.

What the product page does say

Fetched October 19, 2026, tesla.com/roadster says:

Advanced cold-gas thrusters use all-electric actuators to offer precise thrust, completing micro adjustments within a fraction of a second. This helps stabilize the vehicle in flight.

The same page says: “We already put Roadster in a rocket. Now we put a rocket in Roadster.” The limited-release copy says cold-gas thrusters in the rear propel the car “from 0-60 in under 1 second.” The invite-only copy says thrusters around the perimeter support “forward and upward acceleration as well as lateral rotational movement. So yes, it can fly.” A note says the Rocket trim is “Not approved for public road use.”

Label: Company product-page claims. They describe cold-gas thrusters and a short control response. They do not state specific impulse, tank pressure, liters, or seconds of useful thrust.

The 2018 flight that put a first-generation Roadster on a rocket is a different car and a different page: Falcon Heavy and Starman.

On the map

The October 15 demo is the reveal. Later sentences from Mr. Musk are the claims log. The company is Tesla. The rocket company on the product page is SpaceX.

Sources

Checked October 19, 2026.

A direct fetch of x.com returned 403. The status links were read through fxtwitter on October 19, 2026. The links in the table are the x.com URLs. No Reuters, AP, or Financial Times story of the October 18 exchange was used. The posts are the record of that exchange.