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Rocket fuel itself might be slightly cheaper than jet fuel, but there are a lot of problems with using rockets for passenger flight.

Rockets are not very efficient at carrying payloads, so it would take a lot more rocket fuel than jet fuel to get the same amount of cargo and passengers from one place to another. Also, rockets are very expensive to build and maintain compared to jets. They are designed to travel supersonic or hypersonic (faster than the speed of sound). The only passenger plane to do this, the Concorde, had lots of problems with finding viable air routes due to the noise from the sonic boom it creates.