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The Martlet 2 series were the primary 16" gun-launched sub-orbital flight vehicles used during the High Altitude Research Program (HARP). Martlet 2's were used to conduct extensive research at altitudes of up to 180 km with some 200 flights being conducted between 1963 and 1967. The very low cost per flight, about $3,000, made it ideal for a wide variety of applications.. Typical mission payloads included chemical ejection to produce an observable atmospheric trail and assorted sensors with multi-channel telemetry.
The Martlet 2 series was a remarkably effective and reliable launch vehicle with a nearly perfect operational flight reliably record. The primary advantages of the Martlet 2 was its versatility and its very low costs which allowed hundreds of flights with a wide variety of payloads to be flown.
VEHICLE DESCRIPTION
The Martlet 2 series primarily consisted of the 2A, 2B, 2C vehicles. These were similar in size and appearance, differing only in the mission-related modifications and materials used in their construction. The typical Martlet 2 was a dart shaped vehicle 13 cm (5 inches) in diameter and varying in length from 1.27 m to 1.68 m (50" to 66"). The vehicle was stabilised with four fixed fins which were welded to the main body and bevelled to induce spin. The normal flight weight of a Martlet 2 was about 84 kg (185 lb). with an all up shot weight, including the sabot, of about 190 kg (420 lb).
The sub-calibre design of the Martlet 2 vehicles required the sabot to increase the shot diameter to the 16" bore of the gun system. The Martlet 2 sabot design used a pusher plate and discardable petal arms to support and stabilise the vehicle during launch. The original pusher plate was a full bore aluminium disk with a smaller steel load-spreading disk directly beneath the vehicle. This plate was manufactured slightly oversized and was forced into the gun with a hydraulic jack to insure a constant shot start pressure. To prevent the gun gases from escaping past the vehicle and insure that a good seal was maintained, a polyethylene obdurator disk was inserted behind the pusher plate. The original pusher plate assembly weighed in at about 50 kg (110 lb). Later versions of the pusher plates retained the same basic configuration although they were somewhat lighter.
The sabot petal arms were used to insure that the vehicle remained in a stable upright orientation during the gun launching. Four petal arms were used, each occupying a quarter of the bore circumference . The original petal arms were made from sections of laminated plywood, machined to size. Later versions were made from aluminium and were much lighter.
During a launch, after the vehicle and sabot exited the muzzle, atmospheric pressure would peel the petal arms away from the vehicle body, and the pusher plate would drop behind, freeing the vehicle for ballistic flight to apogee. Typically the sabot sections would fall within 6.5 km of the gun launcher.
PAYLOAD CONFIGURATIONS
The primary scientific mission of the Martlet 2 series was to explore the meteorological, chemical and physical conditions of the upper atmosphere at altitudes above those attained by balloons and below those of satellites. At the time there was an acute need for detailed atmospheric information to support the design of high altitude supersonic aircraft and new missile systems. This mission was accomplished by a variety of experiments which included atmospheric measurement and atmospheric modification techniques. The Martlet 2 vehicles were designed to carry multiple payloads during a single vehicle flight, although the low cost per flight made it practical to fly single objective missions.
The Martlet 2 gun launched glide probes differed greatly from sounding rocket-launched experiments, since they derived their entire propulsive impulse from the gun launcher. In this manner the entire vehicle could be dedicated to carrying payloads. There was no need for a integral rocket motor. The Martlet 2 vehicles generally consisted of two primary components: the main body , which was typically used for ejectable payloads, and the nose assembly, which typically contained electronic telemetry instrumentation.
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The Martlet 2 nose cone payloads included magnetometers, temperature sensors, electron density measurements and even a Langmuir probe. For the electronic instrumentation to function properly after gun launching it was necessary to harden all of the circuitry prior to launch. This was typically accomplished by casting the entire circuit in a block of epoxy which prevented the components from moving and being damaged. In this way the Martlet 2 regularly carried complicated electronic instrumentation subjected to gun launching loads of up to 15,000 g's.
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Since the Martlet 2 derived its entire propulsive impulse from the gun launcher, the altitude achieved was solely dependent on the gun launch velocity. Early theoretical calculations indicated that with the original 16" L45 gun the Martlet 2 should reach an apogee of about 125 km. With the gun extended to a 16" L86 configuration, an apogee of some 200 km would be attainable.
Early Martlet 2 flights tended to fall short of the theoretical altitudes, mostly due to poor performance of the 16" gun system. As the HARP program proceeded, there was a concentrated effort to improve the launch performance of the gun system. The improvements included: lengthening of the gun barrel; use of customised propellant grains; and unique charge loading and charge ignition techniques. These greatly improved both the performance and reliability of the of the system. Before long the launch velocity was increased from early velocities of 1400 m/s (4500 ft/sec) to nearly 2100 m/s (7000 ft/sec).
The Martlet 2 regularly achieved altitudes in the range of 160 km when fired from the 16" L86 gun with flight times in the range of 320 seconds. Typically the vehicle would re-enter and splash down within 80 km of the gun launcher. Three decades later, a Martlet 2C launched from the 16"L86 gun stills holds the world altitude record for a sub-orbital gun-launched projectile. On November 19, 1966 a Martlet 2C was fired to an altitude of 180 km with a flight time of over 400 seconds.
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Launches: 200. Payload: 84 kg. to a: 180 km altitude suborbital trajectory. Total Mass: 190 kg. Core Diameter: 0.1 m. Total Length: 1.7 m. Development Cost $: 9.00 million. in 1962 average dollars. Launch Price $: 0.00 million. in 1965 price dollars. Total Development Built: 200.
A Martlet 2C launched from the 16 inch L86 gun established the world altitude record for a sub-orbital gun-launched projectile. A Martlet 2C was fired to an altitude of 180 km with a flight time of over 400 seconds.
The cancellation came only a few months before an orbital 2G-1 could be flown. Martlet 2's were used to conduct extensive research at altitudes of up to 180 km with some 200 flights being conducted between 1963 and 1967. The very low cost per flight, about $3,000, made it ideal for a wide variety of applications.. Typical mission payloads included chemical ejection to produce an observable atmospheric trail and assorted sensors with multi-channel telemetry.