How Did the Tesla Roadster Fly? Here’s How It Works

The most staggering technology introduced on the new Tesla Roadster is a propulsion system pulled directly from SpaceX—the rocket, satellite and artificial intelligence company also owned by Elon Musk. In the Roadster shown at the event, four metal nozzles sit just above the rear bumper and look for all the world like the exhaust tips on your standard supercar (think C8 Corvette but spread farther apart). But this is an EV, so those pipes are not venting exhaust gases, but high-pressure air, primarily nitrogen, that’s stored in a massive black spherical tank that sits behind the roadster’s two front seats. The ball is a composite-overwrapped pressure vessel (COPV), and it’s connected by two pipes that feed the four nozzles, which function as cold gas thrusters (CGT).

Starts With COPV

As indicated in the name, the COPV is a highly pressurized container that’s wrapped in composite. In the case of Roadster, the vessel is a very large black sphere, about the size of a large exercise ball you sit on at the gym, wrapped in the distinctive shiny black weave of carbon-fiber composite.

You can think of the COPV like the fuel tank in a compressed natural gas (CNG)- or liquid propane gas (LPG)-powered vehicle, except the nitrogen mixture in the COPV is stored at a much higher pressure, up to 10,000 psi, similar to the pressures in modern hydrogen fuel-cell vehicles. Here the vessel looks like a huge-diameter scuba cylinder with near hemispherical ends, and we’ve heard that the volume is about 200 liters, which is a lot of gas to have at 10,000 psi.

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The pipes leading away from the COPV send highly pressurized nitrogen to CGT nozzles shown below.

COPVs are not new technology. Highly pressurized tanks have been part of modern rocketry since the beginning. At first they were made of steel, then aluminum and even titanium. In the race to make the vessels as light and as strong as possible, the NASA Space Shuttle team pioneered the use of metal-lined vessels wrapped in various forms of composite. Key to maintaining these high internal pressures is the overwrap—a continuous wrapping of the lowest-weight, highest-strength composite material available, including Kevlar and carbon fiber. Continuous wrapping, versus overlaying individual strips or strands, is key to delivering the highest possible strength, while the metal liner inside is utilized to guard against gas permeability. Earlier this year, SpaceX purchased Hexagon Masterworks, which manufactures COPVs, to bring this vital technology in-house (SpaceX rockets use multiple COPVs).

Valves, Actuators, and Pipes

At the opposite ends of the sphere, secured by a 12 large bolts, is a large flange that connects to a large pipe surrounded by two piston-like actuators. The beefiness of these two flanges and the number of safety-wired nuts attest to the high pressures within the COPV, as do the actuator motors, which are apparently pulled from the ankle of the Tesla Optimus robot. These actuators control the valves within the pipe that regulate the pressure of the gas exiting the COPV at supersonic speeds. This gas shoots through the two long pipes that split and terminate into four CGT nozzles.

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Highly pressurized nitrogen shoots out of the four cold gas thruster nozzles at supersonic speed.

Exits Via CGT

No, CGT does not stand for Carrera Gran Turismo. In this case, it means cold gas thruster, and the nozzles serve to direct the high-pressure gases exiting the COPV.

According to Tesla’s vice president of vehicle engineering Lars Moravy, as with the equal-length heads in an exhaust manifold, it’s critical that all the tubes and valves exiting the COPV be exactly the same length. Any difference in length creates an uneven path for the exiting gas, which means uneven supply to the CGT nozzles. In the forward thrust scenario, such an imbalance could induce unintended yaw. For instance, gas exiting quicker on the left two nozzles could yaw the vehicle to the right, and vice versa.

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Roadster uses a 100+kWh battery pack comprised of Tesla’s cylindrical 4680 cells, with revised chemistry and improved cooling.

Exotic Metals

Given the high pressures of the system and the Roadster’s requirement to minimize mass, the long tubes and thruster nozzles are made of titanium, while the splitter pipes between them on each side are made of Inconel.

So How Fast Is the Thrust Roadster?

Fabulously wealthy car collectors chasing that ultimate sub-one-second 0–60-mph time know only too well that today’s crop of super-mega-hypercars are limited by the maximum accelerative force that can be delivered through tires alone. Thrust will determine the winners of the ultimate drag race.

Our back-of-the-napkin math suggests that the two rear motors good for 1 MW of power (1,341 hp) should be able to contribute about 32.3 kN of forward force (7,261 pounds-force), thanks in large part to the Roadster’s ducted sucker fan arrangement pressing them to the pavement with over 2 g.

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Roadster’s drive unit is derived from the legendary three-motor system pioneered in Plaid, but it uses two motors instead of three and is lighter, more compact, and significantly more powerful.

That leaves just 19.8kN (4,451 lb) that the thrusters must contribute in order for a 3,750-pound Roadster to hit 60 mph in Tesla’s claimed 0.8 second. We know nothing about the geometry of the CGT throats (nor about the Roadster’s drag coefficient or frontal area), so it’s impossible to say how long meaningful thrust can be maintained or how fast the car will be traveling at that point, but a few seconds seems reasonable, by which point it should be traveling at least 135 to 180 mph with the motors still pulling.

What About the Spinning and Flying?

During launch demonstrations, the Roadster Rocket hovered, spun, and flew, thanks to the an additional, larger-capacity COPV that tripled the total amount of gas available for thrust to 600 liters. Six additional CGT nozzles were required to get the Roadster off the ground for its jumps, flips, and barrel rolls. Four CGTs, positioned just behind the wheels and pointing down provide vertical thrust, while two more on the nose of the car, pointing left and right, provide sideways yaw thrust.

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Where to Get the Rocket Fuel?

While new and exiting in the automotive world, COPV and CGT are pretty well understood technologies and comparatively low-power ways to get around, especially when compared to the chemical reactions that power solid rocket boosters. Theoretically, the compressed gases needed to power the the CGTs in the Roadster could be supplied by existing industrial gas suppliers (we were thinking such companies as Air Liquide, Airgas, and Linde Gas). But then on stage at the event, Elon equated the gas used in Roadster’s COPV CGT system as essentially air and intimated that the vehicle would be able to capture, compress, and store its own rocket fuel, all of which sounds too good to be true. In fact, your terrestrial-transportation experts are struggling somewhat to divine the useful purpose of a car that can levitate, spin, and fly short distances, but if adding a third dimension to automotive travel is the next big trend, we stand ready to embrace it. Stay tuned for more big updates on Tesla Roadster.