He played an important part in preparations for the war, in his studies on gas and internal combustion engines, the science of flame and explosions and in his study of the elastic hysteresis of metals.

At outbreak of war, Hopkinson was himself mobilised. The next four years were the culmination of his life's work. He threw into the war effort all his inventiveness, initiative and untiring energy. He developed the idea of implanting blisters on ships hulls to absorb the force of explosions from mines, as well as designing bombs and equipment for aircraft.