NASA needs good weather to launch Ares I-X because engineers need to see the rocket flight all the way through first stage separation, which will mark the end of the test.

The 1-x will climb about 40km (25 miles) into the sky during the powered phase of its flight, continuously measuring vehicle aerodynamics, controls and performance of the rocket's first stage.

Military aviation experts said the fighter? Known as the J-20? Appeared to be conducting high-speed taxi tests, which are usually the final stage before a test flight.