 Halford's engine was developed, and emerged as the Halford H.1.By April 1941, design work on the engine had been completed, and a prototype H.1 engine performed its first test run one year later.The low power output of the early jet engines had meant that only twin-engined aircraft designs were considered to be practical during the early stages of development; however, as more powerful jet engines were quickly developed, particularly Halford's H.1 (later known as the de Havilland Goblin), the practicalities of single-engined jet fighter were soon realised.