Base bleed

Base bleed or base burn (BB)[1] is a system used on some artillery shells to increase range, typically by about 20–35%. It expels gas into the low pressure area behind the shell to reduce base drag (it does not produce thrust; if it did it would be a rocket-assisted projectile). Since it extends the range by a percentage, it is more useful on longer range artillery where an increase of approximately 5–15 kilometres (3.1–9.3 mi) can be achieved, and it also was found that the reduced turbulence gave the projectiles a more consistent trajectory, resulting in tighter grouping and efficient shelling more than 40 km away.

Simplified diagram of a base-bleed artillery shell reducing turbulent vortices
Diagram of a base bleed unit. The top diagram shows the bottom of the shell and the location of the gas vents. The bottom diagram is a cut-away view showing the gas generator mechanism.

Base bleed technology was developed in Sweden in the mid-1960s but took some time to spread and find its niche between cheaper classical ordnance and even more expensive rocket-assisted projectiles. It is now a fairly common option.[2][3]

Function

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, creating energy-consuming shock waves. Shaping the shell properly can reduce this drag.

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 or profiling it as in sabot projectile. 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 fills the vacuum in the area behind the shell with an inflow of gas, dramatically reducing drag.

The reduced drag means that a longer range can be achieved or that a smaller initial velocity of the shell is required to reach a given range; more of the kinetic energy of the shell can be allocated to its mass if deemed appropriate, potentially firing heavier shells. The cost is a little space to fit the gas generator in the casing.

History

The principles were developed in Sweden in the mid-1960s by the Försvarets forskningsanstalt (abbreviated FOA) and the Artillery bureau at the Kungliga Materielförvaltningen (later the Försvarets Materielverk (FMV)) while working on a rocket-assisted projectile called "reatil". Their goal was to increase the range of coastal artillery.[4] 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 granted to FOA in 1971 although both application and patent were classified. Since the development was done, the patent was transferred to FMV for procurement to the armed forces of Sweden.

The concept was quickly implemented into the 7.5 cm sjömålsgranat m/66 (7.5 cm anti-ship shell m/66) used in the 7.5 cm tornpjäs m/57 fixed coastal artillery gun, and then rapidly into all anti-ship shells in the Swedish military.

Since FMV was to contract a company in the US to manufacture the gas generator for the 12 cm sjömålsgranat m/70 (12 cm anti-ship shell m/70), used in the 12 cm TAP m/70 fixed coastal artillery gun, the classification secret was removed from the patent. Shortly thereafter the international rights were sold, eventually ending up with the Space Research Corporation (SRC), then owned by aeronautical engineer Gerald Bull.

By the end of the 20th century, the technology was generally available world-wide.

See also
XM1128 showing bleed propellant cavity (red)[1]

References
  1. Nguyen, Ductri (30 August 2011). "M1128 Insensitive Munition High Explosive Base Burn Projectile" (PDF). Department of the Army. Office of the Project Manager for Combat Ammunition Systems. Archived from the original on 13 May 2015. Retrieved 7 April 2023.{{cite web}}: CS1 maint: bot: original URL status unknown (link) (Public domain Public domain)
  2. "155 mm HE ER FB-BB (OFd M3-DV)". MSM GROUP. Retrieved 29 July 2022.
  3. YAKOUT, HASSAN; ABDEL-KADER, MOHAMED S. (14–16 May 1991). ASSESSMENT OF ERFB-BB PROJECTILE (PDF). FOURTH ASAT CONFERENCE. CAIRO, EGYPT: MILITARY TECHNICAL COLLEGE. MF-366. Archived (PDF) from the original on 2019-04-29.
  4. Grenander

Bibliography
  • Gibson, Chris (2023). "It's All About that Base". The Aviation Historian (42): 122–129. ISSN 2051-1930.
  • Grenander, Gunnar (1987). Vapenlära för armén (in Swedish). Stockholm: Liber. ISBN 91-38-09025-2.
  • Hansson, Lars (2008). ERSTA - Från svarvspån till byggnadsminne (in Swedish). Nyköping: LAH Bunkertours. ISBN 978-91-977297-0-3.
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