Complex sentence: The Gyron Junior was a two-fifths output scaling down of the Gyron, starting as Project Study number 43 in 1954; the first prototype Junior ran in August 1955.
Split into simpler sentences: The Gyron Junior was a two-fifths flow scale version of the existing Gyron engine. It started as Project Study number 43 in 1954 and the first prototype ran in August 1955.