When a passing rain-cloud is spotted by satellite, the DGN is sent to intercept. Looking like an enormous black wigwam suspended from a huge bicycle tyre, the DGN is actually half airship, half tent.
In action, hot air from ground level enters at the base of the machine, under the edge of the tent, and rises rapidly up the 500m diameter ‘skirt’. Inlet guide vanes at the base impart a swirl to the rising air which accelerates as the up-current reaches the throat at the top of the skirt, 500m above the ground. The rotation of the air imparts an opposite torque to the DGN, which, despite its size, completes one revolution every 4 minutes.
Suspended from the base of the skirt are weights which abrade the ground surface as they are dragged round. The resulting dust, entrained in the airflow, enters the skirt where the steadily increasing centrifugal force separates out the larger fragments and drops them back to earth via spill tubes which dangle at regular intervals round the rim.
As the rising hot air/ dust mix emerges from the throat at the top of the skirt, it resembles a tornado and this roaring column ascends to cloud level passing centrally through the tyre (actually a hydrogen-filled balloon) with good clearance. As the vast machine rotates, it propels itself forward by the simple expedient of putting a ‘foot’ onto the ground… located just inboard of the guide vanes, the feet are hydraulically-operated rams about which the whole craft swivels. A computer deploys and retracts the feet smoothly so progress actually comprises a series of controlled hops. Stopping is achieved by retracting the abrader weights, so the skirt drops, reducing hot air intake and associated uplift. Progressively putting all the feet on the ground then brings rotation to a halt and the vessel ‘sits’ in a stable fashion.
Once a cloud has been seeded by the DGN it only takes a matter of hours for rain to start falling. The speed of droplet coalescence depends on a number of factors including cloud density, droplet size, temperature etc. but there is always a component of the dust which is of the right size to do the job. This contrasts with naturally occurring dust which is often too fine to seed rain quickly, which then falls far away.
The path of the DGN looks like a gravel runway after its passage. Because the fine dust has been removed and the ground levelled, it makes a good cross-terrain all-weather route.
