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An Introduction to the Ballistic Range Complex

by Chuck Cornelison
Updated: September 5, 2007

As mentioned earlier, one of the facilities (the HFFAF) within the Complex is an aeroballistic range (see figure 1). Perhaps the easiest way to picture an aeroballistic range is to think of it as being the opposite of a wind tunnel. In a typical wind tunnel you mount a small scale model inside a big tube called a test section, blow air over it and use various devices to measure aerodynamic properties such as lift, drag, and pitching moment. In the aeroballistic range, a large gun is used to launch small a scale model into a test section containing still air. This aeroballistic test section has many windows and reference wires. As the model flies through the test section it is photographed. These photos along with the time history of the model’s flight can be used to reconstruct the model’s flight path (trajectory). This information is then fed into a sophisticated computer code, which calculates the aerodynamic properties of the model. So, in a wind tunnel a model is held still while air is blown over it. Whereas, in an aeroballistic range the air is still and the model flies through it. Both types of test facilities are used to determine aerodynamic properties. An aeroballistic range is particularly useful for studying very high-speed flight. In fact, practically all of NASA’s spacecraft that have entered an atmosphere (such as Mercury, Gemini, Apollo, Shuttle, Viking, and Galileo) have had some testing performed at Ames’ Ballistic Range Complex. The information gathered in these tests was crucial to the successful design of these vehicles.

Diagram of the
Hypervelocity Free-Flight Aerodynamic Facility

Figure 1: Sketch of the Hypervelocity Free-Flight Aerodynamic Facility

The Ames Vertical Gun Range (AVGR) is one of the other operational facilities within the ballistic range complex (see figure 2). This facility also uses a gun to launch particles at high speeds, but for this type of testing we are most interested in what happens when the particle hits a target, and not so much interested in what it does during its flight. The gun is mounted on what is basically a large hinge so that the impact angle can be varied from horizontal to vertical (0 to 90 degrees). The types of particles that can be launched include spheres, cylinders, irregular shapes, and clusters of many small particles. The particles can be metallic (i.e. aluminum, copper, iron), mineral (i.e. quartz, basalt), glass (i.e. Pyrex), or plastic (i.e. lexan, nylon). These capabilities are important because crater size, shape and the way material is ejected from the crater during an impact event is closely related to such things as impact angle, particle shape, composition, velocity etc. Scientists use experimental data they obtain from the AVGR when they to go out to a crater site such as the Chicxylub (pronounced “chicks-ee-lube”) crater in the Yucatan peninsula. By comparing the crater characteristics to their laboratory tests, they can develop a good idea as to the size, speed, composition and impact angle of the particle that crashed into the earth at this location some 65 million years ago, and quite possibly triggered the extinction of the dinosaurs

Picture of the Ames Vertical Gun Range

Figure 2: Ames Vertical Gun Range

A discussion of the ballistic range wouldn’t be complete if I didn’t mention a few words about some of the guns we use in these facilities. For very high speed testing we use what are called two-stage light-gas guns. These guns are capable of launching particles at speeds in excess of 26,000 ft/sec (18,000 mph) which in metric terms is equal to 8 km/s. We have several different guns with barrels ranging in size form 0.28 to 1.50 inches in diameter. This allows us to launch models that are only a fraction of an inch to models that are an inch or more in diameter and several inches in length. Figure 3 shows some of the models we’ve tested over the years.

Display of several aeroballistic models

Figure 3: Examples of Aeroballistic Models

A typical light-gas gun has four main components: a launch tube (also called the gun barrel), a high pressure coupling (sort of looks like a giant funnel), a pump tube (which is usually 3 to 5 time larger in diameter and length than the launch tube), and a gun powder chamber. In order to launch a model the first step is to load it into the launch tube. Unless the model to be tested is a simple cylinder (which has the same diameter as the gun barrel) it must be place in a carrier called a sabot. A typical sabot is a plastic cylinder that is cut along its length into four equal pieces (similar to carrot sticks), and has a cavity machined in its center to match the shape of the model. The sabot aligns and supports the model during launch. The launch package (sabot and model) is loaded into the launch tube (gun barrel). A rupture disk is placed behind the launch package and the launch tube is connected to the high-pressure coupling. Next, the coupling is connected to the pump tube. At the opposite end of the pump tube a plastic piston is installed, and this end of the pump tube is connected to the gun powder chamber. The pump tube is evacuated (air removed using a vacuum pump) and then filled with hydrogen. A gunpowder charge is installed in the gun powder chamber. To fire the gun the powder is ignited. The sudden release of chemical energy buy the burning gun power causes the piston to accelerate down the pump tube. As the piston moves it compresses (squeezes) the hydrogen, increasing its pressure and temperature. Once the pressure reaches a certain value, the rupture disk bursts open and the high-pressure hydrogen pushes on the base of the sabot. The launch package is accelerated down the gun barrel. Once the launch package exits the gun barrel, the sabot pieces separate from the model, which then flies down the test section.  As the model flies past the various photographic stations, its picture is taken and its time of arrival is recorded.

In the Aeroballistic Facility, the test section has 16 photo stations each with two views. After the model passes the last the photo station, it slams into a 2-ft thick wall of polyethylene (a type of plastic). For slow speed shots the model becomes embedded in the wall. For high speed shots the model vaporizes.  In the Vertical Gun Range there are 3 photo stations, each with a single view. After the model passes the last photo station it enters an impact chamber and slams into the target of interest. This impact event is recorded using high-speed video and other photographic techniques.

There is the potential for danger in the ballistic range facilities because we use hydrogen, explosives and high voltage electricity. However, we have very good safety practices and we avoid taking unnecessary risks. In fact, these facilities have been in operation for over 40 years and there have been no serious injuries of damage to equipment.

 
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