Elon Musk has floated the idea of an electric plane again. The Tesla CEO appeared on the Joe Rogan podcast Thursday night, after agreeing that he would speak on the live broadcast if Tesla Model 3 production reached 5,000 cars per week. During the podcast, where he smoked weed and chatted about super-smart machines, Musk also said that “the exciting thing to do would be a vertical takeoff and landing supersonic jet of some kind,” noting that he’d discussed the idea with “friends and girlfriends.”

“The interesting thing about an electric plane is you want to go as high as possible, so you need a certain energy density in the battery pack, because you have to overcome gravitational potential energy,” Musk said. “The energy you use in cruise is very low, and then you can recapture a large amount of your gravitational potential energy on the way down. So you really don’t need any kind of reserve fuel if you will, because you have…the energy of height.”

Musk has suggested as far back as 2009 that he wants to build an electric plane. In June 2017, he said the technology was not yet ready as it requires around 400 watt-hours of energy per kilogram of plane, a figure that would require a big shift in airframe technology. Musk also said that while 400 was suitable, around 500 watt-hours per kilogram would be ideal. Tesla car batteries offer around 250 watt-hours per kilogram. Some analysts claim that batteries could reach such a density within around five or 10 years.

Other companies have been working on the electric plane in the meantime. Siemens’ battery-powered aircraft broke the speed record in April 2017 by hitting 200 mph, although in a plane weighing just 110 pounds. The company claimed at the time that electric planes will be able to carry 100 passengers over 1,000 kilometers by around 2030.

“The way to think about a plane is it’s a force balance,” Musk explained. “So a plane that is not accelerating is a neutral force balance. The force of gravity, lift force of wings, then you’ve got the force of one of the thrusting device, propellor turbine or whatever it is, and the resistance of the air. The higher you go the lower the air resistance. Air density drops exponentially, but drag increases with the square, and exponential beats the square. The higher you go, the faster you’ll go with the same amount of energy. At a certain altitude, you can go supersonic with quite a lot less energy per mile than an aircraft at 35,000 feet. Because it’s just a force balance.”

However, when asked if Musk would consider creating such a vehicle, he responded that “I have a lot on my plate.”

“The electric airplane isn’t necessary right now,” he explained. “Electric cars are important. Solar energy is important. Stationary storage of energy is important. These things are much more important than creating an electric supersonic VTOL. Also the plane’s naturally, you really want that gravitational energy density for an aircraft, and this is improving over time, so it’s important that we accelerate the transition to sustainable energy.”

Musk noted that Tesla’s goal to transition the world onto electric cars was more important because it’s more of a pressing concern for the planet.

“Why run this crazy experiment where we take trillions of tons of carbon from underground and put it into the atmosphere and oceans?” Musk said. “This is an insane experiment. This is the dumbest experiment in human history.”