The probe used a discarding four quadrant centre sabot which consisted of an aluminum crown, a plastic base and a obdurator disk. The probe body consisted of a discarding aluminum nose cone to permit payload ejection, a hollow steel main body to carry the payload and an aluminum tail boom with four fins for stability. The payload compartment of the 5.1 probe was 48 mm in diameter, 225 mm long and could carry a 0.9 kg (2 lb.) payload to 76 km.
The greatest advantage of the 5-1 probe was its simplicity and its reliability. I had an almost perfect flight record with hundreds of successful flights.. Launched at a velocity of 1646 m/s (5400 ft/sec) the 5-1 probes regularly achieved altitudes of 76 km. It was interesting to note that the 5-1 probe was capable of covering much of the upper atmosphere including all of the ozone layer.
The typical flight profile of a 5-1 probe was remarkably simple. After the probe was prepared it was loaded into the gun launcher with a propellant charge and the gun was elevated to vertical. When all ground systems were ready the gun was fired and the probe was launched at a velocity of about 1646 m/s. At launch two mechanical fuse timers were activated. The first was a 60 second timer that released the main nose cone retaining pins and the second was a 120 second timer that activated the payload eject charge.
Immediately after launch the sabot was discarded and the probe began its glide to apogee. The probe was acquired by radar tracking systems about 20 seconds after launch and was tracked throughout the remainder of the flight. At t+60 seconds the first mechanical fuse timer fired a small charge that blew out the two nose cone retaining pins. After this event the nose cone was only held to the main body by two small shear pins. At 120 seconds the probe was near apogee and the second timer fired the payload ejection charge. This charge pushed out the payload canister, which then pushed off the nose cone. The nose cone shear pins went and both the nose cone and the payload were ejected at a velocity of approximately 91 m/s (300 ft/sec). Free of the main body the payload was deployed and in some cases a small nose cone parachute was deployed. Once ejected ground crews could track the experiments until their conclusion.
One of the prime advantages of the 5-1 probes was their incredible reliability. This reliability extended to the vehicle impact zone. The HARP 5 inch gun probes experienced such a small and consistent impact zone that flights were regularly conducted from the Highwater, Quebec launch site, which was adjacent to the US border, without worry of vehicles impacting on US territory. The major impact zone for the 5 inch gun probes was a circle 3 km (2 miles in diameter) with its centre only 3 km (2 miles) downrange of the gun. Within this circle all probe bodies fell. On many firings the ejected nose cone was equipped with its own 1 m (3 foot) parachute. The parachuted nose cone would fall at a rate of no more then 6 m/s (20 ft/sec). This not only reduced its safety hazard but also allowed it to be tracked by radar to yield additional wind data. Payloads such as chaff did not pose a significant safety hazard. Other payloads such Metsondes were flown with a 6 foot aluminized parachute to both increase descent time and to permit radar tracking.
by Richard K Graf
Gross mass: 10 kg (23 lb).
Payload: 0.90 kg (1.98 lb).
Height: 1.16 m (3.80 ft).
Span: 0.13 m (0.42 ft).