The specification had also encouraged the use of the new Rolls-Royce Goshawk evaporatively cooled inline engine; many of the submissions produced by various aviation companies in response accordingly featured the Goshawk engine.[5] However, the Goshawk engine proved to be unreliable (mainly due to its over-complex and underdeveloped cooling system) and unsuited to use on fighter aircraft, this outcome correspondingly stalled development of the aircraft intended to use it.[5] A further stumbling point for many of the submitted designs was the placement of the machine gun breeches in arm's reach of the pilot. At the same time, the development of monoplane fighters such as the Hawker Hurricane and Supermarine Spitfire casted doubt over the future viability of the requirement altogether.[5]