Howitzers Use Laser Ignition
Brent D. Johnson
Ever since Napoleon taught the world the lessons of artillery, the field gun has been the centerpiece of battlefield strategy. The simplicity of this weapon has changed little over the years. In fact, most modifications have focused on targeting and fire control issues rather than on the fundamental design of the weapon. However, in the most recent generation of the self-propelled howitzer, the critical breech-mounted laser ignition system of the 155-mm Crusader XM297 has undergone significant alterations.
Breech-mounted laser units were used in numerous live fire exercises on cannons such as the Paladin M284 (top) and the Crusader XM297 (bottom).
Traditionally, these field guns have used a black powder charge ignited by a primer as a propellant. Henry Kerwien, the systems project engineer for laser ignition of artillery
cannons at the US Army Armament Research Development & Engineering Center, is involved in the development of a breech-mounted laser ignition system that uses a rugged-ized Nd:YAG from Kigre Inc. to ignite the propelling charge. He said the laser beam is directed through an optical window in the breech of the cannon, and the energy from the laser beam is directed at the propellant charge, causing it to ignite.
Prior to this development, a pyrotechnic primer -- a cartridge loaded with energetic material that expels hot gases and particles into the propelling charge -- was used. One problem with primers is that they must be replaced each time the gun is fired. Thus, they must be resupplied to the front lines of battle to sustain combat operations, posing a significant environmental hazard and logistics burden.
A laser igniter has neither of these drawbacks. Chris Hardy, chief engineer at Kigre, said that the laser will last a long time if it can survive the mechanical stress. However, recoil from the gun produces shock and g load that are unprecedented for most laser applications. He said the researchers tested a variety of resonator and pump chamber designs to ensure that they could withstand the shock.
Engineers at Kigre made a proof-of-principle preliminary prototype design through a Small Business Innovation Research project and showed that it could work reliably on a modified Paladin M284 cannon. They researched the beam profile to get the best energy density. The optical window that delivers the laser beam to the target needed a low beam divergence of 10 mrad. Ultimately, they arrived at a mechanical solution using a solid-state Nd:YAG. They used a Gaussian profile with a wavelength of 1064 nm at 10 J, which resulted in a 5-mm spot diameter on the primer target and which gave them the ignition characteristics they desired.
The pump chamber materials included KK1 filter glass, a diffuse reflector produced by Kigre that is based on its FD dual-lamp pump chamber series. This provided a high-efficiency exchange between lamps and rods.
The final design withstood the high shock and vibration and met the solarization and nuclear requirements of the military, while significantly increasing the reliability and fire rate of the weapon. The breech-mounted laser is under consideration for use on the lightweight towed 155-mm howitzer as well as on the new superlightweight Atlas 155-mm cannon system.
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