Project HARP: 1960s Space Cannon, Ballistic Innovations and Legacy

In 1966, on the southeastern coast of Barbados, the US Army, in partnership with the Canadian Government, conducted tests of a massive artillery gun as part of Project HARP (High Altitude Research Project). This gun, with a 36-meter-long barrel formed by welding two 16-inch naval guns end-to-end, was designed to launch projectiles to extreme altitudes. Loaded with over half a metric ton of black powder, the barrel required significant reinforcement and careful adjustment to ensure accuracy. Its white paint indicated its scientific purpose, not military, as it was a "space cannon" capable of launching a projectile to an altitude of 180 kilometers, nearly halfway to the International Space Station's orbit and holding the record for the highest altitude projectile ever launched.

The Genesis of Project HARP

The project's history is a fascinating tale of ballistic science, geopolitical strategy, and ultimately, tragedy. The remnants of these experimental guns still lie on the beaches of Barbados, visible from Grantley Adams International Airport's runway 09.

Project HARP was a collaborative effort between the Canadian Government, funding research at McGill University, and the US Army’s Ballistics Research Laboratory. The US Army sought data on upper atmosphere conditions, which was highly valuable in the era of the SR-71, but gathering this data with sounding rockets was prohibitively expensive.

While Canada had ample open spaces for such a project, McGill Dean Donald Mordell envisioned a low-cost space launch system capable of orbiting satellites for a fraction of traditional costs. Barbados was chosen for several strategic reasons:

• Location: As the easternmost Caribbean island, firing from its southeastern shore over the sea provided an essentially unlimited fallout area for debris.
• Tracking Infrastructure: Its location along the launch path of Cape Canaveral meant existing tracking infrastructure was nearby.
• Rotational Speed: Situated just 13 degrees north of the equator, Barbados benefited from increased rotational speed, reducing the energy required for launches.
• Atmospheric Research: The region was of significant interest for atmospheric research, being the "nursery" of hurricanes affecting the Gulf of Mexico.
• Local Support: In the 1960s, Barbados was a low-income country dependent on sugar exports. The prospect of a burgeoning space launch industry was met with enthusiasm by the government, which struck a deal with McGill University.

Ballistic Science and Project HARP's Innovations

To achieve its objectives, Project HARP required an extremely powerful gun with precise control over the explosive force.

Projectile Design

The primary concern was projectile design. A lower mass projectile achieves higher muzzle velocity, but this must be balanced with the ballistic coefficient (a measure of resistance to air resistance). The ballistic coefficient is calculated by dividing the projectile's mass by the drag coefficient multiplied by its cross-sectional area. To maximize this, Project HARP used narrow, dense, dart-like projectiles with sabot rounds.

• Sabot Rounds: These rounds are narrower than the gun barrel, minimizing cross-section and maximizing ballistic coefficient.
• Sealing: A seal with the gun barrel is necessary for propellant pressure to act on the projectile. Sabots increase effective projectile weight, so they must be as light as possible.
• Pusher Plate: The main body of a pusher plate was made from lightweight aluminum. To prevent the missile's fins from sinking into the soft aluminum under immense pressure, a high-strength steel insert was added.
• Alignment: Laminated plywood cutouts between each fin helped keep the projectile aligned in the barrel.
• Seal Material: Polyethylene was used to form a tight, barrel-scratch-resistant seal between the sabot and the barrel.

Muzzle Velocity and Propellant Management

The altitude reached is ultimately determined by the muzzle velocity. Once the projectile leaves the barrel, atmospheric drag begins to reduce its kinetic energy. Using gunpowder as an energy source presented challenges: too much energy too quickly could over-pressure and damage the gun.

• Pressure Distribution: The key to ballistic performance lies in optimizing the gun pressure distribution. Early methods used copper or lead plates to assess pressure, but the piezoelectric crystal sensor revolutionized this.
• Piezoelectric Sensors: These sensors, which generate an electric current when mechanical stress is applied, provided real-time pressure measurements, allowing scientists to develop optimized pressure gradients and maximize the total energy released without exceeding the gun's maximum pressure.
• Barrel Length: Extending the barrel length allowed more time for pressure to act on the projectile. However, as propellant burns and volume increases, pressure diminishes. An optimal barrel length was crucial.
• Propellant Maximization: Project HARP used large 50-kilogram black powder bags, rammed into the barrel with a hydraulic press. These bags had ignition charges sewn into their backs, facing the primer, and only ignited when the initial flame front reached them at sufficient temperatures. A large air gap was included between the final bag and the sabot base.

Addressing Propellant Ignition Issues

Early tests with this configuration resulted in higher-than-predicted peak pressure and lower-than-expected muzzle velocity. Inspection of the polyethylene disk revealed unburnt black powder, indicating incomplete energy release and a larger problem:

• The Problem: The primer ignited the first bag, creating a pressure front that forced the column of bags towards the sabot, while hot gas flowed around the edges. The flame front lagged behind the pressure front, which compressed the bags, impeding the flame. The column then impacted the sabot at high speed, sometimes causing damage. The flame front would then catch up, creating a high-pressure region that traveled backward, wasting kinetic energy.
• HARP Flyer: Testing solutions on the only 16-inch, 120-foot gun was impractical. An instrumented barrel on rails, dubbed the "HARP Flyer" (due to its tendency to fly off its mountings during overpressure tests), was used for interior ballistics tests.
• Spacers: A solution involved placing light wooden spacers between the black powder bags. These prevented bag movement and allowed hot gas to reach and ignite bags further down the barrel. This method lowered peak pressure and increased the total energy released.
• Electrical Ignition: The final configuration included electrically fired squibs to simultaneously ignite all bags.

Projectile Development and Payloads

Project HARP developed various projectile designs and payloads. One notable payload was tri-methyl-aluminum (TMA), a liquid chemical pumped from the missile by a nitrogen-driven piston, triggered by a timer. Once outside the rocket, TMA reacted with oxygen in the ozone layer, forming a luminescent tracer cloud that glowed in the dark, allowing ground observers to analyze wind shear in the upper atmosphere.

Ambitions Beyond Atmospheric Research

While Project HARP provided a cheap and reliable method for studying the upper atmosphere, the Canadian leadership had loftier ambitions: a massive gun capable of launching satellites into space. This envisioned gun would have a 32-inch bore diameter, twice that of Project HARP's largest gun, with thick steel walls to withstand increased pressure.

The main challenge was developing a multistage, high-mass-fraction rocket system capable of firing from the gun and accurately placing a satellite into orbit.

Martlet 4 Rocket

The Martlet 4 was designed for this purpose. It was a full-bore design, eliminating the need for a sabot, with only a polyethylene disk to protect the rocket's base from hot gases.

• Challenges: During launch, the rocket would experience extreme compressive loads, causing it to bulge and create friction with the barrel. In one test, a fiberglass rocket got stuck, and gas pressure burst through its middle, expelling its internals like a giant buckshot cannon.
• Testing: Numerous tests with inert propellant stand-ins were conducted. Longitudinal stripes were painted on test projectiles to examine friction wear. For these tests, the gun was moved to an ice and snow-covered lake north of McGill's Montreal Campus, allowing horizontal firing and easy recovery of projectiles.
• Martlet 3C: Another design, the Martlet 3C, was slightly smaller than the gun bore, with the space between the barrel and rocket casing filled with a dense fluid to prevent friction.
• Optimal Design: Ultimately, a full-bore fiberglass rocket with a Teflon coating was deemed the best option.
• Propellant Construction: Active propellant grain layers were laminated in a hydraulic press around a star-shaped mandrel. After drying, an inhibitor coating was applied, end casings and nozzles were fitted, and the fiberglass casing was wrapped around the solid propellant and cured—an extremely cheap construction method.
• Martlet 3E: This led to the single-stage Martlet 3E, incorporating flip-out stabilization fins, capable of placing a 25-kilogram payload to an apogee of 450 kilometers (above the ISS's orbit). However, without horizontal velocity, these payloads would simply fall back to Earth.
• Multistage for Orbit: Achieving orbit required a multistage system like the Martlet 4. Scaling such a system was difficult, requiring G-hardened guidance and control systems feeding into a central computer controlling cold gas thrusters. High acceleration tests showed these systems could survive up to 5000 Gs. Staging necessitated gradually decreasing diameters, revisiting the fluid support method.

The End of Project HARP and Gerald Bull's Fate

The joint US-Canadian program was time-limited to three years, and the engineers faced pressure to develop unguided multistage rockets based on earlier saboted systems. The program ultimately ended on June 30, 1967, before its ambitions could be fully realized. Managing the bureaucracy between a university, a government, and a military, each with its own priorities, proved too difficult.

The chief engineer behind Project HARP, Gerald Bull, sought new patrons for his expertise. This path led to his demise. He began selling and smuggling weapons to the South African government, ignoring a United Nations arms embargo, which resulted in a six-month US prison sentence. After release, he continued these activities, incurring another fine. He then moved to Brussels.

In 1981, the Iraqi government, led by Saddam Hussein, approached Bull. For a $25 million payment, Bull applied his Project HARP knowledge to "Project Babylon," aiming to build an artillery weapon capable of firing from Iraq into Israel and Iran. This was a tactically useless scheme, proven ineffective in World War II and even less so in the era of guided missiles. Bull reportedly knew of its ineffectiveness but used it to continue his research. However, the Israeli government viewed it differently, and Bull was assassinated as he unlocked his Brussels home door, found by police with a briefcase containing $20,000.

Legacy and Future Endeavors

Other projects, like Project SHARP (Super High Altitude Research Project), an even larger light gas-powered gun, were tested in California in the 1990s. Recognizing that no projectile can exceed the velocity of the propellant, Project SHARP used lightweight hydrogen gas, achieving a muzzle velocity of 7 km/s. However, the estimated $1 billion funding needed to scale it up never materialized, and the system was ultimately used to test subscale hypersonic scramjet designs.

Today, SpinLaunch is attempting to continue where Project HARP left off, using a completely different kinetic launch system to replace expensive first-stage rockets. While the engineering challenges are significant, the underlying physics are sound.