A series of plans found in the German archives show the bunker of Eperlecques. An attached sketch of M.Y. Delefosse shows the current state of the Bunker with regard to the initial project.
It essentially was an assembly plant for V2 rockets. The latter were assembled in the northern part , as we can see on the map (drawings of M.Y. Delefosse.) This part was serviced by two railroads, linking Calais to Saint-Omer.
Between these two railways was a garage for the road vehicles having access to the same bunker in order to unload there. This side of the building, although very damaged by the later bombardments, is still visible to this day. More in the South was the assembly and quality control hall of V2 rockets. In the last part was planned the installation of five liquid oxygen groups compressors.
Once drawn up vertically in the gallery, the rocket was transferred and filled with its fuels and combustive. The rocket so equipped was launch-ready thus crossed the corridor and the 17.5 metre tall revolving door, whose location we can still see. Finall,the firing took place.
We must mention these are chicanes on the side walls of this corridor. These chicanes are still visible, they were certainly intended to break the shock wave which had to penetrate into the corridor during rockets'launch time.
After the bombing of the Bunker of Eperlecques on 27th August 1943, German military authorities became aware of the vulnerability of their construction site. It had indeed become impossible to use the Bunker of Eperlecques as unique base to assemble and launch rockets.
In November 1943: the works start again
We must urgentlyfind a solution to finish the works; at least the south part of the building which had virtually escaped the allies' bombs.
Floss, the engineer, then had the extraordinary idea to keep building using "the tortoise technique: making a 5 metres thick shell to then pour concrete inside at the level where the constructions had reached then to lift this shell by hydraulic jacks, shielding the construction site from bombardments.
Thanks to this technique the current Bunker was built. The roof, realized in several sectors, was lifted by successive stages, and we can still see to this day the concrete strata which were poured after every rising to form the outside wall. This part of the bunker had so risen 22 metre hight, which created inside a volume sufficientto install the liquid oxygen factory, and could also be used as a staff shelter and storage point for the rocket elements, combustives and fuel required for propulsion.
German do not abandon...
We can suppose that the German technicians had not completely given up using the bunker of Eperlecques, even in its reduced format, as a completely autonomous base for the assembly and launch of some rockets, while being, according to the official recommendations, just a liquid oxygen manufacturing factory.
The presence of several openings, at the top of the exit rockets corridor, to allow, without a doubt, the evacuation of gases which risked accumulating in this corridor, validates this point of view.
It is towards the same period that, in front of the impossibility to realize in Eperlecques a unique building allowing to both assemble rockets and produce liquid oxygen, it was decided to transfer these assembly and launch sites under the dome of Wizernes, a former limestone quarry, so that factories or launching site were shielded from the allied bombardments.
... In spite of bombs
A tallboy bomb directly fell on the North side of the building, the other one fell 27 metres near the South side. On the North side we can still see the impact inside, but the bomb hardly damaged the building. It doubtless shook it and the resulting concrete crater was quickly repaired by the Germans, because at the time of the liberation by the allied troops, the coffering which had been used for this repair was still in place.