The Ordinary convergent exhaust nozzle is a perfectly satisfactory and efficient device for converting pressure in the jet pipe into velocity, so long as the pressure ratio across it remains below about 2.0; or, put another way, so long as the velocity at exit from the nozzle is not theoretically greater than that of sound at the jet temperature prevailing. If the pressure ratio is greater than this however, full expansion cannot take place and some losses in thrust occur. The pressure ratio across a jet pipe on modern jet engines is of the order of on the test bed, so already under these conditions some losses occur. The amount is insignificant however, but in flight at supersonic speeds the pressure ratio across the jet pipe may become very large, and in this case important losses result.