Professional Engineering

Extreme products call for extreme testing. The products being designed and built by the defence industry, which have to endure some of the harshest environments imaginable, certainly fit that bill.

From fighter planes to body armour, many of these products are intended for use where there is a very real risk of damage by bullets, shrapnel or explosive blasts. To make sure the range of materials used in these products is suitably robust, a great deal of computer modelling is carried out. But the ultimate test, given that lives could be at stake in these scenarios, is to simulate these very high-speed impacts. 

At BAE Systems’ Filton site near Bristol, as part of the Advanced Technology Centre, is the High Strain Rate Test Facility. Inside this specially designed building is the machinery that, under very controlled conditions, creates the experience of being shot at by using gas guns.

The facility started out 20 years ago with one relatively small gas gun, but has grown to host five guns of varying size and power. “As we developed the facility, we built a range of gas guns,” says David Townsend, executive scientist at the facility. “The biggest fires projectiles at 4km a second. At the other end of the range, where we’re using heavier and slower projectiles, that one can fire 20 to 30kg at 100m per second.” As the five guns share some common parts, only two can be used at any one time.

Gas guns work on the principle of pumping gas, usually helium, into a chamber until very high pressure levels are reached. The pressure is released directly onto a sabot – effectively an open carrier for the projectiles – which is propelled through the gun’s barrel until it hits a stop close to the barrel’s opening. At this point the sabot is halted and the projectiles continue forward, into the material sample. The trigger mechanism consists of a diaphragm made of oxygen-free copper, which is pushed to its specified breaking point during the build-up to each firing. Having burst to allow the gas to shoot towards the sabot, a new one is installed for each shot.

The largest gun, which has been in use for 15 years, is a two-stage design. “Essentially it is two guns back-toback,” says Townsend. “The first generates movement through a build-up of high pressure gas, and that feeds into a piston.” This further amplifies the speed with which the sabot is sent forward. 

The gas used is helium, which is stored at 7,000bar before release. This provides enough energy for speeds of up to 4km a second, which equates to just less than 9,000mph. This is sufficient for most of the centre’s testing needs, says Townsend, adding that 1,500 shots are fired per year, on about 100 different test subjects. 

However, faster speeds are possible for certain experiments. “To get higher speeds we would use hydrogen, if we wanted to look at collisions caused by space debris,” says Townsend.