Martlet 3.

Martlet-3A Drawing

Sounding rocket.

Single stage, gun-launched vehicle.

Launches: 1. First Launch Date: 1963-09-05. Last Launch Date: 1963-09-05. Apogee: 150 km (90 mi). Liftoff Thrust: 0 N ( lbf). Total Mass: 200 kg (440 lb). Core Diameter: 0.13 m (0.42 ft). Total Length: 1.70 m (5.50 ft).

Martlet 2G.

Martlet 2G

Sounding rocket. Status: Hardware 1967.

This derivative of the Martlet 2 gun-fired suborbital space probe achieved a higher scientific payload through use of a lighter sabot. 12 were flown before the program was ended.

Although the Martlet 2 series had adequate payload capacity for its primary scientific mission, it was understood from the first flights that the heavy pusher plate, petal arms, and sabot represented a sub-optimal vehicle configuration . About half of the all-up launch weight consisted of the sabot which was promptly discarded after launch. If more of the launch mass was retained after launch a larger vehicle with a greater payload capacity would result - without an increase in the launch weight and without the need to prototype a new gun propellant configuration.

As the HARP Program proceeded a low-priority development program was conducted for a new vehicle with a much lighter centre sabot. This vehicle was designated the Martlet 2G and its design was substantially based on the probes flown on the smaller 5" and 7" HARP guns.

The Martlet 2G had the same launch weight as the earlier Martlet 2's but nearly twice the flight weight and a substantially increased payload capacity. The flight envelope of the Martlet 2G was intended to be identical to the earlier Martlet 2 with a maximum theoretical apogee of 200 km. Only 12 operational Martlet 2G's were flown. Although none of these flights were high altitude tests, the basic concept of the Martlet 2G vehicle configuration was proven.

The Martlet 2G was a dart shaped vehicle 2.27 m (89.5") long and 18 cm (7") in diameter with a flight weight of 159 kg (350 lb). In the place of the earlier pusher plate and petal arms the Martlet 2G used a much lighter centre sabot. This consisted of four blocks that were attached to the vehicle near the centre of balance with each block occupying 90 degrees of the bore circumference. The Martlet 2G sabot was about 46 cm (18") long and was manufactured from three components:

• A sabot crown, made of aluminium and mated to the vehicle body using buttress threads.
• A sabot base, made of polycarbonate and machined slightly oversized so that it could be force-fitted into the bore.
• A polythene obdurator, to seal the gun gases.

After the vehicle had exited the gun muzzle, aerodynamic forces would peel the sabot sections away from the body, freeing it for ballistic flight to apogee. With this type of sabot the tail end of the vehicle was allowed to trail in the gun gases during the launch. It was found that the short duration exposure to the gun gases posed no substantial hazard to vehicle's flight performance.

by Richard K Graf

Martlet 3A.

Martlet-3A Sabot

Sounding rocket.

The Martlet 3A was the first serious attempt to produce a sub-calibre, gun-launched, rocket-assisted, vehicle for the 16 inch gun system. The basic design criteria for the Martlet 3A was to gun launch a vehicle containing a rocket motor that could provide a velocity boost equal to or greater then the initial gun-launch velocity.

The theoretical performance of the Martlet 3A was for an 18 kg payload to be carried to an altitude of some 500 km at gun-launch accelerations of 12-14,000 g’s and gun launch velocities in the range of 2100 m/sec (similar to the Martlet 2 series maximum launch parameters).

Martlet 3 Series - General

The HARP Project’s Martlet 3 series of gun-launched vehicles were all single-stage, gun-boosted rockets. This series began as an attempt to extend the peak altitude of a gun-launched high altitude probe by adding a rocket stage to a vehicle of the same mass as the Martlet 2.

The HARP Project engineers recognised early in the program that if they designed a new vehicle which included a high mass-fraction rocket for second stage boost, but with the same launch mass and velocity as the Martlet 2, the added boost from the rocket motor would provide a significant increase in both altitude and payload. It was also recognised that this would be the first step towards the development of the multi-stage Martlet 4 series - a gun-launched satellite vehicle.

The development of gun-launched rockets during the HARP Project began at a time when rocketry itself was still in its infancy. It was not at all uncommon in the early sixties for new rocket designs to fail during test flights. To complicate matters there was practically nothing previously known about the dynamics of solid rocket motors during the accelerations and stresses of gun launching.

The only serious attempt to develop a gun-launched rocket prior to HARP was during World War 2 when the first Rocket-Assisted Projectile (RAP) was produced. The RAP used a very small rocket motor which added only a few hundred feet per second to the shell’s velocity. This small velocity increase was of little interest to HARP.

The development of gun-launched rockets during HARP began by exploring the most fundamental technical aspects of the designs. The rapid pace of the of development of the various Martlet 3 vehicles is a credit to the ingenuity of the HARP staff, particularly considering the financial constraints imposed by the limited funding. The Martlet 3A and 3B vehicles were a remarkably successful test bed. A wide variety of techniques were tested in a relatively short time.

Perhaps the most important development stemming from the Martlet 3A and 3B vehicles was the hydrostatic containment technique used to prevent the rocket motor grain from deforming during gun launching.

During the early Martlet 3A test flights it was found that an unsupported, case bonded, core burning, rocket motor grain would mechanically fail at gun launch loads of about 5000-6000 g’s, which was only half of the desired gun launch loading for an ideal vehicle configuration. During this failure the rocker motor grain would first deform and then extrude into the central cavity and out of the rocket nozzle. This would render the motor effectively useless as a propulsion stage and had to be addressed if any serious uses of Gun-launched Rockets were to be realised .

The obvious solution was to switch from a core burner to a solid end burner rocket motor. End burners were successfully tested and it was found that they could withstand full gun launching loads (12,000 + g’s). Unfortunately end burners have an undesirable performance for the HARP application, including a lower thrust and a longer burn time, and they had to be abandoned.

After considerable effort it was found that by filling the central cavity with a fluid of equal density to the rocket fuel grain, and then plugging the rocket nozzle, the rocket motors could be launched at the desired loads. By filling the core of the rocket motor with fluid, the fluid and the rocket grain would perform as a single mass of unified density under load and the fuel grain would not deform. This technique would become HARP’s primary method of supporting rocket motors during gun launching.

By mid-1964 the focus of gun-launched rocket motor development had shifted from sub-calibre rockets to the Martlet-3E and Martlet-3D full-bore rockets. The development of full-bore rockets, as with the sub-calibre rockets, began with the most basic fundamentals and progressed rapidly to operational vehicles.

The basic design developed for all HARP full-bore rocket vehicles consisted of a Fiberglas bore-riding airframe with flip-out fins for aerodynamic stability and a propellant mass fraction of 0.80.

Full-bore rockets were developed for two gun systems during HARP. The rocket vehicles developed for the portable 7 inch HARP guns were primarily intended to serve as high altitude atmospheric sounding probes similar to the Martlet 2. The full bore rockets developed for the 16 inch gun system were intended to be part of the Martlet-4 gun-launched satellite vehicle.

Martlet 3A

The Martlet 3A was the first serious attempt to produce a sub-calibre, gun-launched, rocket-assisted, vehicle for the 16 inch gun system. The basic design criteria for the Martlet 3A was to gun launch a vehicle containing a rocket motor that could provide a velocity boost equal to or greater then the initial gun-launch velocity.

The theoretical performance of the Martlet 3A was for an 18 kg payload to be carried to an altitude of some 500 km at gun-launch accelerations of 12-14,000 g’s and gun launch velocities in the range of 2100 m/sec (similar to the Martlet 2 series maximum launch parameters).

After launch the rocket motor ignition was delayed 14 seconds while the vehicle coasted in ballistic flight, as the Martlet 2, before the rocket stage was ignited. During development this allowed the vehicle’s structural and launch problems to be separated from rocket ignition and flight problems.

The Martlet 3A was similar in appearance to the Martlet 2 series. The Martlet 3A rocket motor used a standard 6 inch (15 cm) diameter by 40 inch (1.0 m) long double-base, cast-propellant, core-burning rocket motor grain. The rocket fuel grain was case bonded to the heavy aluminium motor case with a 7 inch (18 cm) exterior diameter. The overall length of the vehicle was 72 inches (1.83 m) with a straight taper nose cone and four simple fins for stability.

The Martlet 3A employed a pusher-plate and petal-arm sabot similar to that of the Martlet 2 series. The notable difference was the use of a steel load-spreading disk that transferred the launch loads to the rim of the motor case rather than the entire base of the vehicle. It was intended that the launch loads would be transferred from the pusher plate to the vehicle’s airframe and then to the rocket motor fuel grain through the case bonding. There was limited support provided at the base of the motor by the nozzle assembly.

The Martlet 3A design program began in the spring of 1963 with test flights beginning in September. Early Martlet 3A test flights were less then successful. At launch loads of 5000-6000 g’s the rocket motor fuel grain would fail as described previously.

The Martlet 3A set a world record as the largest rocket launched from a gun.

by Richard K Graf

Martlet 3E.

Martlet 3E

Sounding rocket.

The Martlet 3E vehicle was designed to take advantage of the portability of the HARP 7 inch guns. Unlike the big fixed 16 inch guns the 7 inch HARP guns, were portable and could be relocated to conduct launches from a wide variety of sites. It was soon determined that a gun-launched rocket vehicle for the 7 inch gun would have a similar performance to the Martlet 2 glide probe launched from the fixed 16 inch guns. Launch costs would also be about the same.

The Martlet 3E was a full-bore, rocket-assisted, Fiberglas airframe vehicle 88.5 inches (2.25 m) long and 7.15 inches (18 cm) in diameter. The vehicle used six flip-out fins for stability, a straight tapered nose cone, and the recently-developed hydrostatic containment technique to support the rocket grain during launch. The launch weight of the Martlet 3E vehicle was 135 pound (61 kg) without payload, of which 94 pound (43 kg) was rocket fuel.

The Martlet 3E was initially designed to be launched at a velocity of 1200 m/sec (4000 ft/sec) from the HARP 7 inch guns with a 12 second ignition delay and a seven second rocket motor burn time. The specific impulse of the rocket motor was 280 sec./vacuum. The theoretical performance of the 3E would have allowed a 20 kg payload to be lofted to an altitude of some 250 km - well in excess of the Martlet 2 vehicles performance envelope. Higher launch velocities would have allowed heavier payloads or higher altitudes to be realised.

Considerable research went in to the development of the Martlet 3E with numerous test launches. Individual test programs were conducted to address design concerns with the rocket motor grain hydrostatic-support technique, the structural integrity of the motor’s Fiberglas airframe throughout the launch cycle (including case wear), the new flip-out fins, and other aspects.

Most of the development flights of the Martlet 3E were conducted using surplus 155 mm smooth-bore guns (6.25 inch) in place of the 7 inch (7.17 inch actual bore diameter) guns. There were only a limited number of 7 inch guns available. The 155 mm smooth-bore guns were available in larger numbers and were expendable if development problems occurred. Once the primary design problems of the 3E were worked out it was intended to scale the design up from the 6.25 inch to 7.17 inch. The final high altitude vertical flight testing was to be conducted with the 7 inch gun systems.

With the launch costs of the Martlet 3E in the same range as a Martlet 2 vehicles it was intended that once the 3E became operational it would replace the Martlet 2 as the primary atmospheric sounding vehicle for the HARP Program. The use of the 3E over the Martlet 2 would allow portable soundings to be conduced all over the world. This would also free up the 16 inch gun for the future development and operation of a gun-launched satellite vehicle.

by Richard K Graf

Payload: 20 kg (44 lb). to a: 250 km altitude suborbital trajectory. Total Mass: 155 kg (341 lb). Core Diameter: 0.18 m (0.59 ft). Total Length: 2.15 m (7.05 ft).

Martlet 4.

Martlet 4

Status: Development ended 1966.

The Martlet 4 was ultimate goal of the HARP program - a gun-launched orbital launch vehicle. Two versions were considered: a preliminary version with two solid propellant upper stages, and a later model with two liquid propellant upper stages. Payload of the liquid propellant version would have reached 90 kg. The initial version was in an advanced stage of suborbital flight test when the HARP program was cancelled in 1967.

MARTLET 4

The Martlet 4 was a full-bore, multi-stage, gun-launchable rocket. Although the Martlet 4 could have been used to launch heavy sub-orbital payloads, it was primarily designed as a satellite launcher.

THE HARP ORBITAL PROGRAM

The HARP orbital program was not part of the original HARP mandate of exploring the upper atmosphere. It was not until 1964, when agreements between the Canadian and the US governments permitted stable funding over the following three years, that HARP was able to seriously consider an orbital program. Even though the technical aspects of the Martlet 4 development progressed relatively smoothly, the HARP orbital program met with criticism from its inception, with much of it being unfounded, uninformed and undue. To defend itself the HARP staff applied considerable efforts to rebut these often insidious attacks, which came steadily over a period of years from certain quarters. This criticism, along with external political pressures and competing research programs, led to repeated funding delays during the development of the Martlet 4. Even the extraordinary technical efforts of the HARP staff could not overcome this external pressure. With these considerations in mind it was not surprising that HARP was not able to successfully gun-launch a satellite, although they were more then successful in proving the feasibility of a low-cost, gun-launched satellite system.

During the last year of the HARP program, when it became clear that further funding was not forthcoming, and that the goals of the Martlet 4 program were not to be realised, full efforts were diverted to developing a Martlet 2G-2 orbital vehicle (GLO-1A). It was felt that if a satellite - any satellite, no matter now small - could be successfully gun-launched, that it then would be possible to encourage further funding, either public or private, which would permit the orbital goals of the HARP program to be realised. Unfortunately time and fate were against HARP and the project was closed down on June 30 1967, only a few months before an orbital 2G-2 could be flown.

The Martlet 4 program began in the spring of 1965 with extensive parametric studies which showed that meaningful payloads could be launched into low Earth orbit from the 16 inch L86 HARP gun on the Barbados flight range using a full bore, 3 stage rocket vehicle.

MARTLET 4 VEHICLE GENERAL DESCRIPTION

The original Martlet 4 design parameters called for a vehicle with three solid rocket stages able to launch a payload between 25 and 50 pounds into low earth orbit from the 16 inch L86 gun on Barbados with an all up shot weight on the order of 2000 pounds

The original Martlet 4 vehicle was only 29 feet long, which was quite small for a satellite launcher, and puts it in the size category of a typical sounding rocket. The many advantages of gun-launching allowed the Martlet 4 to be capable of orbiting a small satellite while retaining launch costs similar to that of a sounding rocket.

The first stage of the Martlet 4 (Martlet 4A stage) was a solid rocket motor 156 inches long with a fuel weight of 1620 pounds and an all up weight 1960 pounds Attached to the base of the first stage were a set of 6 flip-out fins. These fins were folded flat and in-line with the vehicle's body while the vehicle was in the gun barrel. They popped out to a 45 degree angle at muzzle exit. The fins were chamfered to allow aerodynamic forces to create a vehicle spin rate of about 4.5 to 5.5 rotations per second, providing gyroscopic stability for the remainder of the flight. The fixed geometry of the gun barrel and the very stable and predictable flight path of all gun-launch vehicles eliminated the need for any guidance corrections prior to the Martlet 4's first stage rocket motor burn.

A slightly modified version of the Martlet 4A stage was also intended for use as the Martlet 3D vehicle. The Martlet 4A stage holds the worlds record for the largest rocket motor launched from a gun.

The second stage of the Martlet 4 (Martlet 4B stage) was a solid rocket motor 52 inches long with a fuel weight of 400 pounds and an all up weight of 500 pounds . The Martlet 4B stage design was little more then a shortened Martlet 4A stage that was optimised for its role.

Between the second and third stages was the Attitude Control Module. After the first stage had burned out and separated from the rest of the vehicle, the Attitude Control Module was used reorient the vehicle to pre-programmed pitch and yaw angles relative to the horizon, prior to the ignition of the second and the third rocket stages. Immediately prior to second stage ignition the module was deactivated and the second stage thrust occurs without active guidance. After the second stage had burned out the empty stage was retained and the Attitude Control Module was reactivated so that the vehicle could be properly orientated for third stage thrust. Just prior to third stage ignition the second stage, with the Attitude Control Module attached, was released and the third stage thrust also occurred with out active guidance.

The third stage of the Martlet 4 (Martlet 4C stage) was a solid rocket motor 48 inches long with a fuel weight of 160 pounds and an all up weight of 200 pounds . Early versions of Martlet 4C stage design called for a sub-calibre rocket motor some 12 inches in diameter but this concept was soon abandoned and all subsequent versions use a full-bore16 inches diameter motor

The satellite insert motor was considered the fourth stage of the Martlet 4. This motor was fixed to the satellite payload and its nozzle pointed in the direction of travel during launch. As the vehicle was gyroscopically stabilised by the flip-out fins immediately after launch the satellite retained its relative orientation throughout the launch sequence and was pointing in the proper direction at the first apogee when the insert burn occurred.

The nose cone was a simple straight cone and allowed payloads of up to 48 inches in length to be carried.

MARTLET 4 MISSION PROFILE

The Martlet 4 mission began with the preparation and pre-flight testing of the gun-launcher and the launch vehicle. The simplicity and the small size of this launch system greatly reduced the preparation time for the gun-launcher and the vehicle to only a few hours. This was particularly noteworthy when comparing the Martlet 4 to a conventional satellite launching rocket with a similar capacity which could require weeks or even months to prepare. The fast preparation time for a gun-launched orbital flight was one of the greatest advantages of a gun-launched satellite system and it would not have been uncommon for the HARP launch facility on Barbados to have been capable of launching four to six Martlet 4 vehicles per DAY when required. As well, due to the very high gun-launch velocities, Martlet 4's could have been launched in adverse weather conditions that would have prevented a rocket-based satellite launcher from flying.

After the preparations were complete the Martlet 4 vehicle and the gun propellant were loaded into the gun-launcher and the gun was elevated to the launch angle. With the gun loaded and ready there was a short countdown to insure that all systems were still "go" and the down range danger zone was clear. The firing of the gun was a simple mater of sending an electrical signal to the gun-launcher at the appropriate moment to ignite the gun propellant. The actual gun-launch cycle took only a fraction of a second with the Martlet 4 leaving the launcher at about 1370m/s. Immediately upon leaving the gun-launcher the flip-out fins pop into position. Aerodynamic forces acting on the fins spun up the vehicle, providing gyroscopic stability throughout the remainder of the flight.

After gun-launch the vehicle coasted upward and down range for 45 seconds prior to first stage ignition. This First Coast Phase allows the vehicle to rise to an altitude of 26.5 km were the air had thinned to a fraction of sea level pressure. The delay in the first stage ignition also allowed the flight controllers ample opportunity to determine if the vehicle was in stable flight and if all of the primary systems were operating properly prior to allowing first stage powered flight. During the First Coast Phase air drag and gravity will have reduced the vehicles velocity from 1370 m/s at launch to about 1072 m/s.

First stage ignition occurs at T+45 seconds into the flight. The first stage burn would have taken about 20 seconds and increased the vehicle's velocity to 3520 m/s. At burnout the vehicle's altitude was 40 km. Immediately after first stage burnout the stage separated from the rest of the Martlet 4 and then re-entered the atmosphere and impacted in the ocean.

No active guidance was used between the gun-launching and the first stage's powered flight. One of the prime technical advantages of a gun-launcher was that the fixed geometry of the gun barrel and very high launch velocities allow for highly accurate predictions of a gun-launched vehicle's flight path. The flight path of a gun-launched vehicle was so predictable, in fact, that there was no need for active guidance prior to the first stage burn of any gun-launched rocket. This held true for multi-stage vehicles such as a Martlet 4 and for single-stage rocket-assisted vehicles such as the Martlet 3D and 3E.

The second stage ignition was delayed until T+65 seconds. During the 10 second delay between the first and second stage burns the Attitude Control Module was activated. Any corrections to the vehicles attitude, relative to Earth's horizon, were made. The Attitude Control Module was then deactivated just prior to second stage ignition. The second stage burn was to have taken about 20 seconds and increased the vehicle's velocity to 5790 m/s and its altitude to 78.4 km. Following second stage burnout the second stage was NOT separated but was retained until just prior to third stage ignition.

The third stage ignition occurred at T+490 seconds into the flight. Between second stage burnout and third ignition was a Third Coast Phase of 395 seconds. During this coast phase the Attitude Control Module was activated periodically and allowed to reorient the vehicle to insure the proper alignment for the third stage burn. The Third Coast Phase also allowed time for the vehicle to gain significant altitude and to travel far down range on its sub-orbital trajectory.

Immediately prior to third stage ignition the Attitude Control Module was deactivated and the burned-out second stage, with the Attitude Control Module attached, was jettisoned from the remainder of the vehicle.

By T+509 seconds the third stage had burned out. By this time the vehicle's velocity was 7653 m/s and its altitude was 428 km. After the burnout the third stage was separated to re-enter and burn up in the atmosphere.

The intended final orbit was nearly circular with a perigee of about 425 km and an apogee of about 430 km. Before the satellite was able to settle into its final orbit a fourth and final rocket motor burn was required. The orbital insert motor was incorporated into the satellite and was not ejected after use. At third stage burnout the orbital insertion motor was orientated with its nozzle pointing in the direction of travel. As the gyroscopically stabilised satellite came around the Earth it was pointing in the appropriate direction for the orbital injection burn. The orbital injection burn occurred at the first apogee some 45 minutes after the vehicle was gun-launched.

The accuracy of the final orbit was highly dependent on the timing of the orbital injection burn with greater accuracy being achieved by the command firing of the injection burn by ground controllers.

ATTITUDE CONTROL MODULE

The attitude control module for the Martlet 4 vehicle was a remarkable piece of engineering. Consider for a moment the state of the available technologies in 1965 when work on this critical component began. In 1965 solid state electronics was still an infant technology with few complicated integrated circuit chips available and no significant CPU's. The gyroscopes typically used for missile and rocket guidance control were not suitable for use in a gun-launched vehicle. The Martlet 4 project engineers were faced with the daunting task of developing an entirely new type of guidance system which would be suitable for this unique launch application.

From the beginning of the HARP program their use of big guns to launch payloads into space attracted great interest from a wide variety of specialised industries. To solve the problem of the Martlet 4 guidance system the development program was expanded to include interested industrial contributors. The primary HARP development group (McGill University, the US Army's Ballistics Research Laboratory, and the Harry Diamond Laboratory) designated Aviation Electric Limited of Montreal, Canada, as the prime contractor to develop a gun-launchable guidance system suitable for the Martlet 4 mission profile.

Almost from the onset of the program it was recognised that the delicate mechanical gyroscopes typically used for rocket guidance at the time were unsuitable for the high g-loads of gun-launching. The foremost goal of the Attitude Control Module research team was to create new methods of determining the pitch, yaw and roll rate of the vehicle.

The final concept for the Martlet 4 Attitude Control System was simple yet effective. The Martlet 4's Attitude Control System was primarily an analogue unit that gathered information from several sensors, compared it to pre-set reference angles and then fired cold gas thrusters at the appropriate moment to correct the vehicles pitch and yaw angles for the next motor burn. During flight this module would be activated periodically to let the system correct the vehicle's attitude and then turned off to conserve propellant as the vehicle coasted to the next rocket motor ignition event.

The first piece of necessary information acquired by the system was the exact spin rate of the vehicle. The spin rate sensor was a simple accelerometer developed by Aviation Electric Limited. As the vehicle rotated in flight the sensor determined the roll rate and provided a reference pulse for the other subsystems.

The pitch angle of the vehicle was determined by two infrared sensors. Actually miniature infrared telescopes, the sensors looked out the side of the Attitude Control Module, one perpendicular to the vehicle's axis, and the other at an angle. On each rotation of the vehicle the "Earth Pulse Width Comparator" would analyse the signals from both sensors and a pitch angle relative to the local horizon was generated.

The yaw angle of the vehicle was determined by sun sensors. The sun height sensor was mounted looking out and forward of the Attitude Control Module. On each rotation the "Sun Pulse Height Comparator" circuit read the information from the sun height sensors and determined the pitch angle of the vehicle relative to the sun. This comparator obviously required a pre-programmed sun angle relative to the time of day the vehicle was launched to serve as a reference for the proper yaw angles to be determined.

These three pieces of information were fed in to the "Switching Logic and Nozzle Firing" circuit. The Switching Logic module compares the sensor information and determined if there was a misalignment in the vehicle's attitude. When a misalignment was identified, the system determined which thrusters to fire and the timing of the thruster pulse to correct the vehicle's attitude for the next rocket motor burn.

Once the operating principles of the Attitude Control Module were established the development and flight testing of the system's components began. In 1965, when this critical component of the Martlet 4 was under development, the HARP project's engineers already had many years of experience in the design and development of gun safe electronics. Therefore the development of the guidance system's components proceeded relatively smoothly. The primary components of the Attitude Control Module were assembled and bench tested, and then they were flight-tested to insure that they could withstand the stresses of gun-launching.

The flight testing of the components for the Attitude Control Module was carried out primarily from the HARP test range in Highwater, Quebec. To increase turn around time and reduce costs much of the flight testing was carried out using the small 155 mm (6.25 inch ) smooth bored guns. The components were packaged into full-bore slugs and fired either vertically, with recovery via parachute, or horizontally, with recovery using the over-the-ice technique.

Components for the Attitude Control Module were gun-launch tested to proof loads of 10,000 g's which was about twice that experienced by the Martlet 4 during launch and provided a considerable safety factor. The launch forces experienced by the Martlet 4 were actually fairly soft for a gun-launch vehicle considering the some HARP vehicles regularly carried telemetry packages after launch loads in the range of 50,000 g's.

To be considered qualified for this application components would be gun-launched, recovered and compared to the pre-launch calibrations to determine if any gun-launch acceleration-related changes had occurred. Typical components would be test launched several times before the design was considered flight qualified to insure a high degree of system reliability. Once the primary components of the Attitude Control Module were individually proven, entire ACM assemblies were packaged in a 6.25 inch airframe and flown to insure the assembly would function in flight as a complete unit.

With the development of the Martlet 4 rocket stages incomplete it was not possible to test the Attitude Control Module under actual flight conditions. It was considered that the testing procedures for this assembly insured a high degree of confidence in the Attitude Control Modules reliability and no major concerns during actual flight conditions were anticipated.

DEVELOPMENT OF THE MARTLET 4 PROPULSION STAGES

The development of the Martlet 4 rocket stages was a unique endeavour in the history of rocketry and it was the first time that anyone had attempted to develop such a large multi-stage gun-launchable rocket vehicle. The Martlet 4's development began soon after the developmental flights of the Martlet 2A and 2B and closely followed the successes of the 7 inch full bore Martlet 3E development program. The development of the Martlet 4 rocket stages, and that of other Martlet 4 subsystems, ran in parallel to the development of the Attitude Control Module.

The original design concept for the Martlet 4 first stage (Martlet 4A) consisted of welded steal, sheet metal, motor case with threaded end enclosures. The motor case was slightly sub calibre and used narrow brass bore riders at either end and a thin rolled metal external discarding solid sabot between the bore riders as a ware surface. The all up weight of the motor was 1160 pounds with a fuel weight of 1450 pounds . The solid rocket grain was 114 inches long with a 7-point star-shaped progressive burning core. The heavy motor case, necessary to handle the gun-launching loads, permitted a chamber pressure of about 1500 psi at motor burn out with a vacuum specific impulse in excess of 250 sec.

The original motor design was successfully launched on several tests although the very tight machine tolerances needed for this type of construction proved to be quite impractical. Following the success of the Martlet 3E, the Martlet 4A motor case was redesigned to use a Fiberglas motor case with metallic end enclosures.

The advantages of a Fiberglas over a steel motor case were many. Foremost was the weight savings and the associated improvement in the stage mass fraction. The Fiberglas motor case was simpler to manufacture as the end enclosures could be fixed directly to the motor grain and the Fiberglas wrapped in place, improving the overall structural integrity. The problem of motor case wear during launch was addressed by both designing the motor case with sufficient tolerances to permit the expected wear and to liberally coat the motor case and the gun bore with a silicon lubricant.

Development trials for the Martlet 4A began in the fall of 1966 with tests proceeding into early 1967. The majority of the early work was conducted on the Highwater, Quebec test range where the structural integrity of the Martlet 4A motor during gun-launching was proven. Prior to the abrupt end of the HARP project in July 1967, soft recovery trials and flight testing had been planed for the winter of 1967/1968. After this the Martlet 4A stage would have been declared operational for use with the Martlet 4 and as the Martlet 3D vehicles.

Final specifications of the original Martlet 4A stage called for a Fiberglas cased motor 156 inches long with 6 flip-out fins at the base for aerodynamic stability. The overall weight of the motor stage was 1960 pounds with a fuel weight of 1620 pounds yielding a mass fraction of 0.82. At nearly one ton the Martlet 4A holds the worlds record for being the largest rocket motor ever fired from a gun.

The development of the Martlet 4 second (Martlet 4B) and third (Martlet 4C) rocket stages was to follow the development of the Martlet 4 first stage. The two upper stages of the Martlet 4 were designed to simply be shorter versions of the Martlet 4A first stage. The substantial difference between the upper stages of the Martlet 4 and its first stage were the thickness of the motor cases and the rocket nozzle design. The case walls of the Martlet 4 first stage had to be thick enough to not only support itself but to support the mass of the upper stages and the payload as well. The case wall of the second stage could be thinner as it only needed to support itself , the third stage and the payload. The third stage wall could be even thinner since it only had the satellite payload and itself to support. The progressive thinning of case walls of the upper stages reduced the inert mass of the rocket stages and improved the stage mass fractions. The changes to the rocket nozzles were minor and took into account the different thrust profiles of the motors and the altitudes at which they were used.

MARTLET 4 GUN BALLISTICS

Weighing in at over a ton the Martlet 4 vehicle was considerably heavier then any of the other 16 inch Martlet vehicles although it was in the same weight range as a standard 16 inch military shell. The very heavy shot weight of the Martlet 4 required that an entirely new propellant charge be developed specifically for the Martlet 4 and research was performed to develop a propellant charge that would provide the highest possible launch velocity from the 16 inch Barbados gun.

Throughout the HARP project there was a continuous effort to develop improved propellant charges for all of the HARP guns in order to increase the launch velocities of the various Martlet vehicles. This work was covered in detail in the section on the HARP guns. Much of this work concentrated on the 16 inch gun and the Martlet 2 series and culminated in an a very special propellant charge.

The Multi-Point Spaced Ignition Charge for the Martlet 2 vehicles solved many of the problems encountered earlier in the program with erratic burn rates and was the basis for the Martlet 4 propellant charge. The Multi-Point Spaced Ignition Charge for the Martlet 2 series consisted of about 900 pounds of M8M propellant divided into eight propellant bags. These bags were loaded into the gun in four sets of two bags with spacers to keep the bag pairs separated by about 18 inches . With a conventional charge the propellant was ignited at the breach end and the flame front progresses from one bag to the next. With the Multi-Point Spaced Ignition Charge each bag was ignited simultaneously which produced a very even ignition and helped to stabilise the pressure curve.

The standard Martlet 2 charge was not suitable for the much heavier Martlet 4. The primary reason for this was that the propellant grain size and the burn rate of the propellant was matched by the lighter shot mass of the Martlet 2. The lighter Martlet 2 accelerated down the bore much quicker then the heavier Martlet 4. The high burn rate of the standard Martlet 2 propellant charge would have generated too much propellant gas too soon, resulting in an overpressure and the destruction of the gun.

An ideal charge for the Martlet 4 would have to produce the same volume of propellant gas as the Martlet 2 charge but at a much slower rate to match the slower acceleration of the heavier Martlet 4. The solution was to increase the size of the individual propellant pieces while using a similar total charge weight. The standard 0.220 inch web propellant grains for the Martlet 2 application were replaced with a much larger 0.405 inch web grain. These much larger propellant grains reduced not only the number of individual propellant pieces in the charge but the initial surface area of the charge as well. The smaller total surface area of the propellant pieces reduced the rate at which propellant gas was generated and allowed the Martlet 4 vehicle to accelerate down the barrel without overpressurising the gun.

MARTLET 4 UPGRADES

With a payload of only 50 pounds to LEO the original Martlet 4 had a less then stellar payload capacity and efforts were made to increase the payload flexibility of the Martlet 4 satellite launching vehicle.

The most obvious and simplest upgrade to the Martlet 4 performance would have been to increase the initial gun-launch velocity of the Martlet 4 vehicle. As the gun-launcher was a ground-based propulsion system any increase in the initial gun-launch velocity would translate into an increase in the payload to orbit mass with out any changes to the Martlet 4 vehicle.

The most obvious means of increasing the gun-launch velocity would have been to increase the gun-launcher's propellant charge. The propellant charge developed for the Martlet 4 vehicle was considered to be the maximum charge practical for the 16 inch HARP guns and it would not have been possible to safely increase the quantity of propellant without endangering the safety of the gun. With this in mind the next best way to increase the gun-launch velocity would be to lengthen the gun-launcher. A longer barrel extended the time the propellant gasses had to push on the vehicle which resulted in a faster vehicle velocity at muzzle exit.

The original 16 inch naval gun installed on Barbados had already been extended from 61 feet to 120 feet (L45 to L86) in early 1965 by welding a second barrel onto the end of the first, doubling the length of the original gun. Plans existed to double the length of the three 16 inch HARP guns yet again to about 240 feet. To this end the 16 inch gun in Highwater, Quebec was lengthened from 120 feet to 172 feet (from L86 to L126) by adding a third barrel section to it. Plans for a fourth barrel section which would extend the gun to 240 feet (L150) were never realised. Studies were also conducted for extending the HARP 16 inch guns to up to 480 feet although this may not have been practical due to the necessary support structures needed to maintain an exact barrel alignment. Lengthening the Barbados gun from 120 feet to 240 feet could have increased the Martlet 4 payload mass by about 20% depending on the desired final orbit. Other studies to improve gun propulsion performance included replacing a standard charge with a travelling charge and a unique secondary charge concept. This consisted of two charges separated by a piston. The piston prevented the first charge from igniting the second one, which was accelerated down the barrel with the projectile. The second charge would only be ignited only after the bore pressure from the first charge had dropped to a safe level.

Studies were also conducted on larger bore guns. From these studies preliminary designs for guns with 32 inch and 64 inch bores were considered as well as a concept to replace the 16 inch Barbados gun with a 24 inch barrel gun using the same mountings. These larger guns would have launched proportionately larger Martlet 4 type vehicles with considerable increases in payload capacities and masses. None of these concepts were ever realised.

Another approach which HARP pursued to improve the performance of the Martlet 4 vehicle concentrated on improving the performance of the Martlet 4 propulsion stages. It was considered that the most practical means would be to replace the solid propellant rocket motors with liquid propellant rocket motors.

Liquid propellant rocket motors were studied for the two upper stages of the Martlet 4. Based on existing 16 inch gun performance and maintaining a similar launch mass, liquid propellant upper stages could have theoretically increased the Martlet 4's mass to orbit to some 200 pounds It was considered that for all 16 inch Martlet 4 designs that the first stage would always remain a solid propellant rocket.

There were many fuel combinations which could have been used in these liquid rocket motors. Due to the relatively small sizes of these motors it was possible to consider several very exotic propellant combinations, including using fluorine as an oxidiser, which could have provided incredibly high performances. This was balanced by the need for a motor that was both simple to manufacture and would operate reliably after gun-launching. For the initial upgrades at least, it was decided to use common fuel combinations

The final two propellant combinations which were seriously considered for the initial vehicle upgrade were hydrazine as the fuel with either nitrogen tetroxide or liquid oxygen as the oxidiser. Hydrazine/ nitrogen tetroxide, a very common room temperature hypergolic propellant combination, would have specific impulse of some 320 seconds in vacuum in this application. The hydrazine and liquid oxygen combination would have a much higher specific impulse of about 370 seconds in vacuum.

The hydrazine/liquid oxygen propellant's superior performance would, of course, be the preferred choice.. The primary concern was that the optimal stage design allowed only minimal tank insulation. This would have permitted only a 20 minute handling window between the oxidiser tank fill and launch. Beyond that point the cryogenic liquid oxygen would have warmed up to the point that venting and refilling of the tank would be required. Therefore any delays in launch operations would also require the difficult and time-consuming task of unloading the vehicle from the gun to vent and refill the oxidiser tanks.

The Martlet 4's liquid propellant second stage was 73 inches long with 435 pounds of propellants and an all up-weight of 537 pounds . The liquid propellant third stage for the Martlet 4 was only 27 inches long with 103 pounds of propellants and an all up weight of 128 pounds .

The basic motor design for the second stage was quite simple. To simplify the engine design a regenerative cooling system was not used and instead the rocket motor was immersed in the Hydrazine fuel tank. The fuel tank surrounded the engine's combustion chamber and bell nozzle with the base of the fuel tank extending down to the base of the bell nozzle. The fuel and oxidiser tanks were separated by a flexible membrane. During the gun-launching this flexible membrane allowed the oxidiser to literally ride on top of the fuel without the two coming in contact with each other and eliminating the need for separate, heavy, rigid propellant tanks.

The Primary method of pressurising the propellant tanks was to apply regulated helium gas from a sphere of high pressure helium which was mounted on top of the oxidiser tank. An alternative to this would have been to use a pyrotechnic pressurisation system. The pyrotechnic pressurisation system consisted of a simple Solid Propellant Gas Generator (SPGG) which was to be mounted inside the upper enclosure of the oxidiser tank. The SPGG cartridge generated pressurising gas at a specific rate which would provide a fairly constant tank pressure and a consistent propellant feed rate. The SPGG would have provided a savings in weight and complexity although the pressuring gas it generated was very hot, which could have created secondary problems in some vehicle configurations. The pressurising gas was applied to the propellant tanks immediately prior to the ignition of the motors.

The third stage motor configuration was somewhat different then the second stage. At only 27 inches long the third stage's rocket motor dominated the design. As with the second stage, the third stage's fuel tank surrounded the combustion chamber and nozzle . Unlike the second stage, the third stage's fuel tank did not extend past the top of the combustion chamber. The flexible membrane separating the propellants was attached to the top of the combustion chamber and extended to the edge of the upper enclosure. The oxidiser tank was roughly cone-shaped on the bottom with the hemispherical end enclosure at the top. The helium pressurising tank was a toroidal, mounted inside the fuel tank, and surrounded the combustion chamber.

The combined performance improvements noted above had the potential of increasing the payload of the Martlet 4 from an initial 50 pounds to some 200 pounds . With a 50 pounds payload to low earth orbit the Martlet 4 had a marginal performance even by 1960's standards. With a 200 pounds payload, even by today's standards, the Martlet 4 would have been capable of satisfying many orbital applications.

by Richard K Graf

LEO Payload: 23 kg (50 lb). to: 425 km Orbit. at: 13.00 degrees. Liftoff Thrust: 47.400 kN (10,656 lbf). Total Mass: 1,300 kg (2,800 lb). Core Diameter: 0.42 m (1.37 ft). Total Length: 8.54 m (28.01 ft).

GLO-1B.

GLO-1B

Status: Development ended 1966.

When compared to the early Martlet 4 designs the GLO-1B was a considerably more sophisticated vehicle with many of the shortcomings of it's predecessor having been addressed. Not long after the original HARP project ended the major assets of the project were acquired by the projects management, Dr. Gerald Bull in particular. The HARP Program became the Space Research Corporation (SRC) with the intention of resurrecting the HARP orbital program. Over the years a much improved and considerably more sophisticated Martlet 4 was developed and given the name of GLO-1B.

In general the GLO-1B was similar in appearance to the Martlet 4 although there were several modifications to the original design intended to improve the vehicle's performance. The major differences were that the overall length of the vehicle was shortened and the launch mass was reduced. The satellite payload mass was similar to the Martlet 4 with an initial minimum 50 pound satellite. Improvements in the gun-launch velocity alone (such as lengthening the gun from 120 feet to 240') could have been increased the payload mass to nearly 100 pounds The rocket motor cases were to be made from aluminium rather then Fiberglas to simplify manufacturing.

The first stage of the GLO-1B was shortened from 156 inches to 92 inches . The fuel weight was reduced to 1100 pounds and the overall weight to 1308 pounds . The GLO-1B used the same basic six flip-out fin design of the Martlet 4. The general design of the rocket motors changed little. The most notable exception was a modification to the rocket nozzle assembly. The rocket motors of the Martlet 4 used a very conventional design with the rocket nozzle extending out of the base of the motor. With the GLO-1B the base of the rocket nozzle assembly was flush with the bottom of the rocket motor case and the throat of the nozzle projects into the motor and was surrounded by the rocket fuel. This helped reduce the overall length of the motor, and greatly reduced the complexity of the inter-stage adapters, as the motors could literally be stacked one on top of the other.

The second stage of the GLO-1B was reduced from 52 inches to 51 inches . This motor used an internally positioned rocket nozzle similar to the first stage and retained a similar propulsion profile to the Martlet 4B stage.

The third stage of the GLO-1B was shortened from 48 inches to 18 inches with a fuel weight 140 pounds and an overall weight of 165 pounds . The third stage's rocket motor case was spherical with an internal rocket nozzle that protruding only slightly from the case. There was also a liquid propellant motor considered for this stage, similar to the Martlet 4 third stage, which would have provided higher performance and allowed a heavier satellite to be flown.

The insert motor was considered the fourth stage and was incorporated into the satellite payload in the same manner as the Martlet 4.

One of the most notable differences between the Martlet 4 and the GLO-1B was the elimination of the Attitude Control Module between the second and third stages. A dedicated Attitude Control Module was eliminated by modifying the flight profile of the GLO-1B vehicle. The Martlet 4 vehicle used the Attitude Control Module to insure the pitch and yaw of the vehicle was within set parameters prior to the ignition of the second and third stages. With the GLO-1B this was simplified by modifying the mission sequence which made it necessary to provide attitude control for the third stage only.

The first stage of the GLO-1B was unguided and, in the same manner as the Martlet 4, relied on both spin stabilisation and the fixed geometry of the barrel to insure a predictable flight path prior to first stage ignition. The Martlet 4 flight profile specified a delay between the first stage burnout and second stage ignition during which the Attitude Control Module was activated to insure the vehicles orientation was correct prior to the second stage burn. The GLO-1B eliminated this delay and the need to re-orient the vehicle by igniting the second stage immediately after first stage burn out and separation, which allowed the second stage to share the first stage's orientation.

A simplified attitude control system was incorporated into the satellite payload and was used to correct the vehicle's attitude prior to the third stage burn only. As part of the satellite payload it could also be used to orient the satellite vehicle for the orbital insertion burn to insure that a precise orbit was achieved. Once in a final orbit the attitude control system could be used to reorient the satellite for functions such as antenna or sensor pointing.

Along with the GLO-1B launch vehicle an initial satellite vehicle was designed which would fit into the vehicle's 40 inch long ogive nose cone. The initial satellite's payload was little more then a transmission repeater which was similar to many amateur satellites in orbit today and would have demonstrated that the launch system worked as planned. The basic sub-systems of the satellite would have been a beacon transmitter, a command receiver, a command logic module, an attitude control module and an active repeater/transponder.

The satellite body had an overall length of 24 inches . The primary payload section of the satellite was a decagon 14.5 inches across and 9 inches high. The base of the main section had an integrated 12 inches diameter high gain dish antenna and the exterior was covered in solar cells to provide electrical power. The upper section of the satellite was an octagon 8 inches across and 15 inches long. The first 3 inches of this section was the primary battery compartment which provided power when the satellite was in the Earth's shadow. The remaining 12 inches of this section was the satellite insert motor. The exterior of this section was also covered in solar cells.

Although limited in capacity this initial satellite would have provided a useful function while demonstrating the capability of the GLO-1B to orbit satellites. As with the Martlet 4 it was recognised that improvements to the initial gun-launch velocity and the use of a liquid propellant third stage would have nearly doubled the mass of the satellite

DISCUSSION

During the era when HARP, and later the Space Research Corporation, were developing the Martlet 4, and later the GLO-1B, there was still a prevalent attitude in the space launch industry that satellites and launchers should be ever bigger. New smaller capacity launchers, such the GLO-1B, met with little interest. By the late 1980's and the early 1990's it was realised that the costs of huge satellites was becoming unmanageable and policies such as NASA's smaller, faster, better began to gain acceptance. Today attitudes have changed substantially and satellites in the range of 50-200 pounds are once again considered useful tools.

A published report in 1972 indicated that the launch costs for a GLO-1B would be in the range about $88,000 per flight. In year 2000 dollars this was about $360,000 a flight. With a minimal payload of 50 pounds the launch costs of a GLO-1B would be about $7200 per pound which was within the pricing range of many current launchers. Modest improvements to the gun-launcher, such as lengthening the barrel, could have increased the mass of the GLO-1B's satellite to about 100 pounds . When considering a satellite mass of 100 pounds the proportional launch costs come down to about $3600 per pound which was substantially less then current launchers. Previously mentioned improvements in the vehicle design, particularly the use of liquid propellant second and third stages for the GLO-1B, could have increased the satellite mass even further to some 200 pounds Due to the small size and simplicity of the liquid rocket motors for the Martlet 4/GLO-1B it was considered that there would be little, if any, increase in vehicle costs for the liquid rocket stages. The use of liquid rocket stages in the GLO-1B could reduce launch costs even further to the range of about $1800 per pound. Even though these figures were rudimentary and extrapolated from relatively old information they do show the tremendous low cost potential of gun-launched satellite systems.

The ability of the HARP orbital program to launch low cost satellites was well known and was usually the sole attribute discussed when gun-launched satellites were compared to conventional satellite launchers. The real potential of gun-launched satellites was not only their ability to provide low cost launches, but also in their ability to launch a vast numbers of satellites a year. Even though the individual HARP satellites had a modest mass, the system's ability to launch 200 or more satellites a year certainly made up for the smaller payload.

As an example, a 200 pound payload launched 200 times a year results in a total launch mass of 40,000 pounds or 20 tons per year. This was the equivalent of more then four Ariane 4 launches, nearly ten Delta 2 launches or one launch of Russia's heavy lift Proton rockets. When additional capacity was required one or more additional guns could be economically installed at any appropriate site around the world. A single 16 inch HARP gun-launcher would be capable of supplying nearly all of the bulk consumables (fuels, water, breathing gasses) needed by the International Space Station or Mir each year.

Perhaps one of the most attractive uses of the high launch volume of a gun-launched system would be the ability to assemble a satellite platform in orbit by docking several gun-launched satellites together. In this manner a satellite platform of any size could be constructed and if a particular module fails, or a system upgrade was required, a replacement module could be quickly launched. A platform of this type could be operational for many decades as failed systems could be replaced, thrusters could be refuelled and the platform could be expanded on orbit to fulfil new requirements.

The 16 inch gun system was considered by HARP to be the smallest attractive bore size for a satellite launcher and plans for larger bore launchers, with larger satellite payloads, were under consideration. Larger gun systems could have launched satellites with masses from 500 kg to 1000 kg while still maintaining the low launch costs and high volumes of the 16 inch gun system.

CONCLUSION

Even though the HARP orbital programs never actually culminated in the successful launch of a gun-launched satellite, they successfully demonstrated the potential of gun-launchable satellite systems. Had the development of the Martlet 4 or the GLO-1B gun-launched satellite system proceeded as expected it was quite likely that today the skies would be littered with satellites and that at least a few of the grand dreams of the space colonisation from the 1970's may have come true.

by Richard K Graf

LEO Payload: 23 kg (50 lb). to: 425 km Orbit. at: 13.00 degrees. Liftoff Thrust: 32.000 kN (7,193 lbf). Total Mass: 900 kg (1,980 lb). Core Diameter: 0.42 m (1.37 ft). Total Length: 5.10 m (16.70 ft).