You might recall that around six months ago, that he was planning an option package for the next Tesla Roadster that would replace the rear seats with equipment for a SpaceX-developed system of rocket thrusters. Initially, Musk described the system as “dramatically improve acceleration, top speed, braking & cornering,” and followed up with “Maybe they will even allow a Tesla to fly…” Now it seems . Which is sort of nuts.

I mean, “nuts” is pretty relative when we’re actually talking about a car with a spacecraft-like reaction control system, so keep that in mind. But you can see where his head is at with this tweet:

Based on this, it seems like he’s definitely getting more excited by the flying/hovering part than anything else. He brought this up back in November as well, with a bit of a hedge:

Now, it’s worth remembering that the fundamental idea of using some kind of reaction control jets on a car is not really a new idea, as such, at least for enhancing handling and even safety. Why, and real, established companies like as traction improvement and safety systems for motorcycles. Doing something like this I think is actually possible, especially if you happen to own a rocket company, like someone we know does.

The sorts of thrusters Elon seems to be contemplating are what are known as cold gas thrusters, and they’ve been used to help spacecraft orient and maneuver themselves for decades. They’re called “cold gas” thrusters because nothing gets ignited—it’s just gas stored at high pressures that get vented out in controlled bursts to provide thrusts.

It’s basically the same technology as the fire extinguisher used by noted fictional robot Wall-E to maneuver in space.

that the cold-gas thrusters he’s thinking of will be using compressed air as the propellant, which does mean that an on-board compressor could be used to refill/recharge the high-pressure thruster tanks. But even so, the amount of propellant needed to move a 3,000+ pound car means that in the available area the tanks would take up, you’d be measuring the amount of time you could be thrusting in terms of seconds before the tanks would need to be recharged.

Now, if we’re talking very short controlled bursts for assisting with cornering and grip and handling, that’s probably fine, but Elon seems to have other goals in mind, as suggested by this Twitter exchange:

Ah. He still wants to fly. See, that’s the problem here. The basic idea of a cold gas reaction control system for a sports car is really compelling and exciting, and could have some genuine, real-world advantages.

Making a car that can hop a dozen feet in the air for ten seconds or so is, well, kind of stupid.

Elon getting hung up on the hovering/hopping part is what makes him so frustrating, sometimes. He has a good concept, but he’s fixated on the silly crap he’s seen in some movie that he thinks is cool. It’s why the Model X ended up with those cool but complicated and impractical Falcon doors instead of a useful, much cheaper sliding door or something, and it’s also why the Roadster may include an option to let your car hop up onto a picnic table and then get stuck there for a half hour as it re-compresses more air to get down.

Some people have already been doing the math to figure out what sort of flight time such a system could give:

This seems pretty plausible: about 10 seconds. and gotten even shorter results:

Well, assuming 50kg of nitrogen with an ISP of 60, fired from a Roadster with a total mass including nitrogen of 1350kg... One g of acceleration would apply 22.2m/s of delta V in...

About two and a quarter seconds. So you could float the car for that long, if the thrusters could keep the car balanced.

So, a little over two seconds. Everyone’s having to do a lot of guessing about the specs and capabilities here, but regardless it’s safe to say that whatever the eventual system turns out to be, there’s really only enough room for enough gas for very, very short hops.

Which brings up the big question: who the hell wants this? What are you going to do with a car that can hop up a few feet for a few seconds, and then land? Get over the occasional curb? Try to impress some lowriders? Quickly jump over a possum in the road?

I mean, this wouldn’t be a flying car. At best, it’d be a hopping car, something that even fewer people have been demanding than a flying car.

And we haven’t even addressed the cost of such an option. Cold gas thrusters themselves aren’t particularly complex, but the pressure vessels for storing the gas are. Elon describes a “composite over-wrapped pressure vessel” which would be expensive and subject to a hell of a lot of safety testing, because in any sort of wreck that could damage the vessel, it would very quickly become a bomb.

It’s frustrating to see Musk take a genuinely interesting idea—applying reaction control jets to automotive handling—but then push it to its most showy and useless application. No one wants a car that can hop around, uselessly, for very short jumps. Unless you’re superstitious enough and concerned enough about your mother’s spine that you I can’t see what possible use lifting thrusters would have.

I reached out to Tesla for comment, but, shocker, they’re not commenting on Elon’s crazy Tweets. Probably a smart move.