It turns out supersonic aircraft are just like buses: You wait years for one, and then two come along at once.
Almost eight years after Concorde touched down for the last time, plane 
manufacturers and designers at the Paris Air Show have unveiled rival 
"son of supersonic" concepts.
The first, aircraft manufacturer EADS's Zero Emission Hyper Sonic 
Transport (ZEHST), will be powered by biofuel made from seaweed, 
carrying passengers above the Earth's atmosphere and dramatically 
cutting flight times.
EADS says the plane's three engine types -- conventional jets, rockets 
and supersonic "ramjets" -- will allow it to speed at Mach 4, around 
5,000 kmh.
That would cut the journey time from Paris to Tokyo from its current 11 hours to less than 2.5 hours.