Base bleed
Base bleed is a system used on some artillery shells to increase their range, typically by about 20%-35%.
Most (50%-60%) of the drag on an artillery shell comes from the nose of the shell, as it pushes the air out of its way at supersonic speeds. Shaping the shell properly can reduce this drag but it is difficult to remove.
However, another powerful source of drag is the low-pressure area left behind the shell due to its blunt base. Base bleed can reduce this drag without extending the base of the shell. Instead, a small ring of metal extends just past the base, and the area in the rear of the shell is filled with a small gas generator. The gas generator provides little to no thrust, but simply fills the vaccum in the area behind the shell with an inflow of gas, dramatically reducing the drag. The only disadvantages are a small loss of accuracy due to the somewhat more turbulent airflow , and in older shells a small loss in explosive payload due to some of the space inside the shell being taken up by this mechanism.
History
The principles were developed in Sweden in the mid 1960s, by the Swedish National Defence Research Institute and the Artillery bureau at the Kungliga Materielförvaltningen (later the Defence Materiel Administration). Their goal was to increase the range of coastal artillery. By 1966, it had been concluded that a small, slow-burning charge at the base of the projectile would alleviate the low pressure behind the shell, hence increasing the range by lessening the difference between the pressure due to aerodynamic drag on the nose of the shell and the low pressure behind the base. The first full-scale tests took place in 1969 with modified 10.5 cm steel shells with excellent results, and the Swedish patent was filed in 1971.
The concept was quickly implemented into the 7,5 cm sjömålsgranat m/66 (7.5 cm anti-shipping shell m/66) used in the 7,5 cm tornpjäs m/57 fixed coastal artillery gun, and then rapidly into all anti-shipping shells in the Swedish military.
Since the gas generator for the 12 cm sjömålsgranat m/70 (12 cm anti-shipping shell m/70), used in the 12 cm TAP m/70 fixed coastal artillery gun, was to be manufactured by a company in the USA, the classification secret was removed from the patent. Shortly thereafter, the international rights were sold, ending up with the Space Research Corporation (SRC), owned by Gerald Bull.
At the end of the 20th century, the technology was generally available. Since base bleed extends the range by a percentage, it is only really useful on longer range artillery where an increase of approximately 5–15 kilometres (3.1–9.3 mi) can be achieved. Until the late 1980s the small gains in range were not considered worthwhile for field artillery. Base bleed shells are becoming more common in units equipped with modern artillery which have far greater range than the old ones, but are only used at maximum ranges due to the higher cost per shell.