When an artillery shell explodes near an armored vehicle, the shock waves can damage or disrupt critical electronic and mechanical parts inside. To help U.S. Army researchers predict how well their equipment will survive such blasts, three engineering students at Johns Hopkins have designed and built an air-powered hammering device that can repeatedly deliver up to 45,000 pounds of force to a slab of metal.

The U.S. Army Research Laboratory plans to use this invention to check the accuracy of computer models that are used to forecast what will happen to armored vehicle equipment when it is hit by even more powerful forces on a real battlefield. In the past, Army researchers have validated their computer results by using a hand-held nail gun capable of delivering up to 35,000 pounds of force. But the nail gun is inconsistent and difficult to adjust. Last fall, Army researchers asked mechanical engineering students at Hopkins to devise a safe, reusable and more reliable device that could pound the target with even greater force.

The 22-year-old seniors--Kevin Turner, of Brielle, N.J.; Ryan Thimatariga, of Lutherville, Md.; and Hal von Brockdorff, of Garden City, N.Y.--recently finished building the explosive loading simulator and successfully tested it in a university laboratory. The cannon-like device features a cylindrical breech chamber that collects pressurized air and a narrow barrel that houses a projectile. When a valve is opened, the change in air pressure sends the tip of the aluminum-and-plastic projectile hammering into a 2-inch-thick aluminum slab. A safety plate prevents the entire projectile from leaving the barrel. A sensor on the target, connected to an oscilloscope, measures the force as a function of time.

Mechanical engineering students Ryan Thimatariga, Hal von Brockdorff and Kevin Turner test the explosive load simulator they built for a senior design project.

The explosive load simulator was one of 12 projects completed this year by undergraduate teams in the Engineering Design Project course taught by Andrew F. Conn, a Hopkins graduate with more than 25 years of experience in public and private research and development. Each team of two or three students, working within a budget of up to $8,000, had to design a device, purchase or fabricate the parts, and assemble the final product. Corporations, government agencies and nonprofit groups provided the assignments and funding.

In the past, Conn's students have developed a "safer" handgun that does not fire in the hands of an unauthorized user; an infrared mouth-held device that allows a paraplegic to operate a computer from a bed; an automatic wheelchair brake; a bicycle helmet that offers more protection than commercial headgear; and a wheelchair lift powered by a van's exhaust.

This air-powered projectile strikes an electronic sensor that measures force.

The explosive load simulator project was sponsored by Morris Berman, a mechanical engineer with the Army Research Laboratory's Composites and Lightweight Structures Branch in Adelphi, Md. Although more testing must be done, Berman said he was impressed by the students' design for the high-pressure pounding device. "This looks like a neat way of accomplishing the task," Berman said. "Having been an engineering student, it would have been nice to have been able to work on a project like this that would actually be used in the real world instead of just designing something on paper. The Army hopes to use this device to build confidence in the computer models we use to predict how well parts and equipment will perform in armored vehicles on the battlefield."

Before building the load simulator, the students had to figure out the best way to generate the hammering force requested by the Army. "We threw out a lot of ideas about how to move the projectile," Turner recalled. "One option was to use springs. But we decided it would be better to use compressed air. Then we had to build something to pump the air into, and then transfer it into useful energy."

Thimatariga added, "It was a really good experience. It gave us a chance to apply the things we had learned in our classes over the previous three years."

Von Brockdorff agreed. "It was a chance to put to work many of the things we were learning about in our classes here at Johns Hopkins."