I catch the first whiff of explosives as we step through the doorway. A long and windowless corridor stretches into the distance. At its centre, a metal pipe – around a hand’s width in diameter – is mounted horizontally between the grey concrete walls. After several metres, the pipe joins a rectangular block of steel, its components clamped together with enormous nuts and bolts.
“It’s known as the fastest gun in the west,” says Chuck Cornelison, head of Nasa’s Ballistic Range Complex and my guide to the facility.
This cannon, at Nasa Ames in Silicon Valley, California, was built in the mid-1960s for the Apollo Moon missions. It was designed to simulate atmospheric re-entry by firing models of spacecraft down the tube towards a target.
Although from a bygone age, the gun is still very much in demand. Recent projects have included assessing SpaceX’s Dragon during its descent towards splashdown and simulating how Nasa’s newest manned spacecraft, the Orion capsule, will fare during its return to Earth.
The gun itself is about 45m [150ft] long and shoots 4cm- [1.5inch] diameter scale models of the craft – or “launch packages” as they are known. Strictly speaking, it’s a “two stage light gas gun”. Gunpowder is used to propel a piston through a hydrogen-filled tube. The piston compresses the hydrogen, which pushes the projectile along the barrel at velocities of up to 8km/s.
“It’s a handful of milliseconds from start to finish,” says Cornelison. When the gun is fired the team have to keep a respectable distance away in an underground bunker. “Even down in that control room, you hear a bang” he exclaims, “and you can feel the building shudder.”
No wonder. As Cornelison guides me around the facility, I am allowed to clamber inside the final section of the gun and look back towards the barrel. I crouch next to the shielding that prevents the projectiles flying out of the end and it is sobering to see the pulverised, blackened remains of the target. Apparently, some projectiles are completely vaporised when they smash into it.
As a result, the team need to collect as much information as possible in the fractions of a second that it takes for a model to zip down the tube. And this is carried out in an adjoining section of the gun that looks like an elongated yellow submarine with windows. “There are 16 imaging stations,” says Cornelison, describing the windows, where the engineers take freeze-frame pictures of the projectiles in flight.
If a gun that fires model spacecraft sounds decidedly old-fashioned, then the cameras positioned along the side really bring home the heritage of the facility. They are definitely from the pre-digital era. “This is old school,” Cornelison admits. At each station there is an 8 by 10 [inch] sheet of film and electro-optical shutter mechanisms with 40 nanosecond exposure time. “You can still get better detail with film,” he says.
These photos are used alongside careful timings to reconstruct the model's flight path. This is then fed into a computer, which calculates the aerodynamic properties of the spacecraft.
It is a truly impressive machine but, in the age of sophisticated computer modelling, isn’t it a little…well…primitive, I ask.
“The computer models have really grown immensely,” says Cornelison. “However, it’s a very complicated problem, there are a lot of different phenomena that occur – depending on the vehicle’s shape and the type of atmosphere you’re entering. And so there’s still a need to do tests like this to provide benchmark points that you can validate that the [computer] models are predicting reality.”