The supersonic-jet maker Aerion recently announced that the flight-service provider Flexjet has placed a firm order for 20 examples of the Aerion AS2 supersonic jet—for which the company began taking orders last year—making Flexjet the first fleet operator for the plane, the first publically available supersonic jet since commercial supersonic travel ended in 2003.  

Each jet is priced at $120 million, for a total order potential of $2.4 billion. Aerion says the AS2 will carry 8 to 12 passengers in…