Elon Musk said in 2017 that SpaceX's Starship could ferry passengers between any two cities on Earth in under an hour.

Musk tweeted on Wednesday that the experience of travelling on one of these Earth-to-Earth flights would feel a lot like riding Space Mountain.

As a result, passengers wouldn't be able to move around during the flight and all the seats would be coach class, Musk said.

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While answering people's questions about SpaceX's Starship launch system on Twitter, Elon Musk compared the experience of slipping the surly bonds of Earth to Disney's Space Mountain ride.

Musk was initially responding to a question about how many passengers Starship could take on one trip to Mars — to which his answer was roughly 100, although he said it could take up to 1,000 on shorter trips as passengers wouldn't need cabin space.

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More Twitter users chimed in with their own questions, including how many people the rocket could carry on Earth-to-Earth trips. Musk first proposed the idea of city-to-city travel by a suborbital rocket in 2017, declaring that passengers could go "anywhere on Earth in under an hour."

Here is a promotional video SpaceX put out in September 2017:

One Twitter user asked whether passengers would be able to move around during the flight.

"That would be unwise," answered Musk. "Probably needs a restraint mechanism like Disney's Space Mountain roller coaster. Would feel similar to Space Mountain in a lot of ways, but you'd exit on another continent."

In an earlier tweet, Musk said that for Earth-to-Earth trips, the rocket is "basically an ICBM" — an intercontinental ballistic missile — travelling at "Mach 25." Mach number refers to how much faster than sound an object is travelling.

A supersonic Concorde jet plane. AP

For context, the Concorde travelled at around Mach 2, meaning the Starship is significantly quicker than the supersonic airplane, which was grounded in 2003.

And forget the idea of luxury. Musk said all seats on his BFR would be "coach," with "no toilets, pilot area or food galley needed" during the flights, most of which would be up to 20 minutes long.

This story has been updated.