Hajile was a "Deceleration Device" — in other words, the world's first retro-rocket. The Admiralty's Directorate of Miscellaneous Weapon Development was actually working on a request from the Army, who wanted a better way than parachutes of getting heavy equipment like jeeps safely and quickly from an aircraft drop to the ground.
DMWD's answer was to put the equipment in a cradle fitted with a number of rockets that would fire a few metres above the ground, bringing the cargo to a soft landing, A plummet on a wire would trigger the firing. Unfortunately, things never quite went exactly to plan...
The difficulty was probably mainly the unreliability of rockets at that time, not helped by a nest of them being required to fire simultaneously. The project was fairly well dogged by spectacular misadventures, The 1970's BBC series :The Secret War" (with some cinematic licence, though with film of the original experiments) dramatized this quite nicely. Visualize:
Shot of a large block of concrete hanging from a tall crane. It drops. Before it reaches the ground, it is completely hidden by smoke and flame. The smoke clears, to show the block buried almost up to its midpoint in the ground. [Voice-over announcer:] "Not entirely successful, so they added a little more gunpowder..."
Another shot, same crane, another block, same sequence. When the smoke clears, the block is buried in the ground again, not quite as far. [Announcer:] "So, they added still more gunpowder..."
Another block drops from the crane. Once again it is enveloped in smoke and flame. All is hidden for a tantalizing time. Then, sailing up out of the top of that massive cloud comes a great concrete block...
The land-based trials shown by the BBC (and filmed, incidentally, by well-known motorsport photographer Louis Klemantaski) were actually preceded by some over water. These were thought to be safer...
The early sea trials were held at "HMS Birnbeck" — otherwise the pier at Weston-Super-Mare. The idea was to have the block dropped nearby — from a Lancaster bomber — while those on the pier filmed the action. The first drops were too far away to be recorded properly, so they asked the pilot to aim closer. This apparently put the pilot on his mettle, as he aimed the block from 2000 feet above.
The observers on the pier at first gazed up with open mouths, but quickly realized it would be a better idea to start running back down the pier — hard! The block crashed squarely through the roof of DMWD's engineering shop. Luckily no-one was hurt, though there were some shattered nerves.
Unhappily, though there were many more experiments, they never really got the scheme behaving reliably. Rockets would misfire, and even their jets could erratically bounce off the ground below, sometimes turning the whole assemblage upside down. They eventually got close, but the end of the war was approaching, and Hajile never was commissioned. On D-Day itself there was one final incident when someone accidentally hit the firing switch of a Hajile unit surrounded by the staff. Klemantaski himself was blinded for several days by a blast of sand.
Hajile goes to Mars!