The earliest rocket projectile weaponry used in the Second World War?
Was it the German six-barrelled 15cm Nebelwefer 41?
When an infantry battalion encountered Its devastating fire would try to neutralised it as quickly as possible. It was generally deployed by darting in and out of prepared positions, being towed by a half-track vehicle.
A ten-barrelled self-propelled version mounted on a lightly armed semi-track vehicle was also developed.
I know that the British established a Projectile Development Establishment at Fort Halstead in November 1939 and that projectile weapons were used by the Royal Navy and in Anti-Aircraft Units.