Complex sentence: In 1945 the establishment re-opened as a research centre known as The Explosives Research and Development Establishment, or ERDE; and was in existence until 1977, when it became the Propellants, Explosives and Rocket Motor Establishment, Waltham Abbey, or PERME Waltham Abbey.
Split into simpler sentences: In 1945 the establishment re-opened as a research centre known as The Explosives Research and Development Establishment, or ERDE. In 1977 it became the Propellants, Explosives and Rocket Motor Establishment, Waltham Abbey, or PERME Waltham Abbey.