SpaceX has revealed why it lost an Air Force space mission contract to United Launch Alliance — the rocket it used for the bid isn't ready.

The Air Force awarded the STP-3 mission to Centennial-based ULA last week, selecting it over the competing bid from Elon Musk's Space Exploration Technologies Inc.

Details of the bids are kept confidential, but Elon Musk’s company said in an email Monday that the Air Force STP-3 mission's performance requirements led SpaceX to propose using a Falcon Heavy rocket, a large launch vehicle that SpaceX not yet test flown.

As a result, ULA’s winning bid was to launch the Air Force’s mission on an Altas V rocket at a cost of $191 million.

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