Project HARP, short for High Altitude Research Project, was a joint project of the United States Department of Defense and Canada‘s Department of National Defence created with the goal of studying ballistics of re-entry vehicles at low cost; whereas most such projects used expensive (and failure-prone) rockets, HARP used a non-rocket spacelaunch method based on a very large gun to fire the models to high altitudes and speeds.
Started in 1961, HARP was created largely due to lobbying from Gerald Bull, a controversial but highly successful ballistics engineer who went on to head the project. Bull had developed the high-speed gun technique while working on anti-ballistic missile (ABM) and intercontinental ballistic missile (ICBM) research at CARDE in the 1950s, shooting models of high-speed interceptor missiles from guns as opposed to building supersonic wind tunnels, which would be much more expensive. The ABM project eventually ended without delivering a working system, but Bull was convinced the rocket systems he had developed had potential and started looking for other ways to use the technology.
HARP was such a development. The U.S. was in the process of testing newer ICBM systems and required repeated tests of newer re-entry vehicles. Bull suggested that the program could be run for considerably less money if the test vehicles were lofted from a large gun, as opposed to using rockets. This would also allow the test program’s schedule to be greatly accelerated, as repeated firing was easy to arrange, compared to rockets. The key concept was the use of an oversized gun firing an undersized vehicle mounted in a sabot, allowing it to be fired with relatively high acceleration. Test electronics were potted in a mix of sand and epoxy, proving more than capable of withstanding the rigors of launch.
The project was based on a flight range of the Seawell Airport in Barbados, from which shells were fired eastward toward the Atlantic Ocean. Using an old U.S. Navy 16 inch (406 mm) 50 caliber gun (20 m), later extended to 100 caliber (40 m), the team was able to fire a 180 kilogram slug at 3,600 meters per second (13,000 km/h), reaching an altitude of 180 kilometers. The program was canceled shortly after this. The politics of the Vietnam War (then in its fifth year) and soured Canada/U.S. relations played their role in the project’s cancellation. The project received just over 10 million dollars during its lifetime.
During this time, many houses and buildings within a radius of 5-10+ miles developed cracks in their concrete reinforced walls, other damage such as cracked toilets, sinks and household infrastructure were invariably damaged when the HARP gun was fired. In fact the vibrations could easily be felt miles away and may well have felt like a minor earthquake. After the project was canceled the gun remained for years and rusted parts may still be on the site abandoned. A couple of used barrels and what appeared to be an unused barrel were also left there.
Bull’s ultimate goal was to fire a payload into space from a gun, and many have suggested that the ballistics study was offered simply to gain funding. While the speed was not nearly enough to reach orbit (less than half of the 9 km/s delta-v required to reach Low Earth Orbit), it was a major achievement at much lower cost than most ballistic missile programs.