Hajile

Hajile was an experimental project developed by the British Admiralty's Directorate of Miscellaneous Weapons Development (DMWD) during the final years of Second World War for slowing the landing of air-dropped supplies with retrorockets.

The project was initiated by a request from the Army for a method of dropping heavy equipment and vehicles from aircraft at high speed, retaining the materiel's terminal velocity for as long as possible in order to minimise drift and damage from anti-aircraft gun batteries. It was further required that the materiel suffer only minimal or no damage from landing, and once dropped be ready to deploy within minutes.

The high falling speed ruled out parachutes, so the DMWD came up with the idea of loading the drops onto a platform surrounded with cordite rockets. These would fire at the last instant to decelerate the materiel to a safe landing speed. The initial test produced the project's codename; as the rockets' exhaust engulfed the apparatus in a plume of smoke and fire, an attending officer, Captain G. O. C. "Jock" Davies, remarked "Look at it! It's Elijah in reverse", referring to the biblical prophet's ascension to Heaven in a "chariot of fire".

Once testing began, a number of problems became apparent. The most immediate was that of how to get the rockets to fire at exactly the right instant. Too early and the platform would pick up enough speed again to cause damage to the load. Too late and the deceleration would be ineffective. The solution settled upon was a plumb-bob, which would dangle below the platform and activate the rockets when it hit the ground. However, the implementation of this idea was complicated by the fact that the weight of the plumb-bob would have to be carefully calibrated, heavy enough not to be blown back into the underside of the platform by the extreme upward winds during the fall, yet still sensitive enough to react immediately on hitting "fuzzy" terrain such as heather or long grass.

...
Wikipedia