Enhanced Design of the Large Compressed Gas Gun Facility for Increased Launch Velocity, 18-9258Printer Friendly Version
Inclusive Dates: 04/17/01 - 08/23/01
Background - The United States Air Force recently raised its low-altitude operational speed for some aircraft to 550 knots. This increased speed necessitated a redesign and certification of F16 canopies for bird-strike survivability. Most of SwRI's prior work in bird-strike testing has been conducted at speeds below 300 knots. SwRI's first attempts at launching a bird at 550 knots showed the difficulties involved. Bird-strike testing at the Institute is performed with the Large Compressed Gas Gun Facility at the SwRI Ballistics Range. This gun, 33 feet in length with a bore of 10 inches, can launch relatively large, heavy projectiles at speeds near Mach 1. The projectile used in these tests is a four-pound chicken. The difficulties in this project were launching a highly fluid and frangible projectile intact, and separating that projectile from the gun.
Approach - The team's approach was to develop a sabot (bore rider) that carried the bird down the gun to the muzzle at which point a "stripper" stops the sabot, enabling the bird to fly freely from the gun at 550 knots. Three problems had to be addressed: 1) the gun muzzle needed to be plugged by the sabot at the stripper, 2) the bird had to remain intact, and 3) there had to be minimal and unintrusive secondary debris from the sabot launched into the target area.
Accomplishments - Calculations were performed to identify the major forces when the sabot hits the stripper and the bird is released. These calculations gave the team a starting point for choosing a new material to use inside the sabot to center the bird in the gun barrel. However, these calculations, while providing insight, were much too simplistic to describe the complex dynamics of sabot arrest and bird release. Iterative experimentation was required to develop an acceptable solution. More than thirty test shots were performed over a two-month period. Variables investigated included placement of sabot components and types of material for those components, as well as glue type and curing time. The team produced a sabot capable of launching the bird in one piece, and which plugged the gun breech and launched minimal secondary debris. A 0.25-inch aluminum plate inside the sabot plugged the muzzle reliably. Placing the bird in a canvas bag inside a cardboard tube supported by and protruding from the front of the sabot maintain bird integrity while exiting the muzzle. The use of a polyethylene foam and a secondary debris stripper reduced the amount of debris down range.