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Aviation History
1988
1988 - 0938.PDF
TECHNOLOGY Hughes advances SDI mini-projectile WASHINGTON D.C. A mini-projectile weighing only 3kg that will intercept a nuclear warhead in space will be ground-tested by Hughes, towards the end of next year, reports Julian Moxon. The lightweight exo- atmospheric advanced pro jectile (Leap) is a project to develop a kinetic-kill vehicle (a missile that destroys a target by force of impact alone) using technologies beyond those now being devel oped for Phase 1 of the Strate gic Defence Initiative. If chosen for development by the SDI Office, such a vehicle could be a part of either a space-based or ground-based kinetic-kill interception system. A Leap mockup exhibited at a recent SDI symposium in Washington D.C. provided a good idea of the impressive technology advances being made in SDI research. Leap demands microminiaturised, nuclear-hardened electronics, small but powerful ultra-fast reaction rocket thrusters (and the associated valves for high speed switching), an advanced infrared detector, and a miniature inertial measure ment unit. The pace of development has surprised even Hughes, which is also involved in developing the ground- launched high endoatmos- pheric defence interceptor (Hedi), a larger, ground- launched kinetic-kill vehicle designed for late mid-course, and terminal defence against incoming warheads. "At the time we designed Hedi," a Hughes engineer told Flight, "we would have thought the technology needed for Leap was impos sible to achieve in the near future." The company now plans to ground-test a proto type Leap in November 1989, and is under pressure from the SDI Office to bring the project forward—possibly with a view to incorporating Leap tech nologies into Phase 1 kinetic- kill-vehicle systems. Leap was originally designed to be fired from an electromagnetic railgun, hence the need for smallness. Railgun technology is lagging, however, so planning now Leap ftopj is roughly 14in long and 8in diameter and uses a fibre- optic gyro f centre) inertial sensor and focal-plane array infrared seeker (above). The computer frightj fits behind the mirror includes the use of more- conventional, chemically fuelled rockets to boost Leap to the interception area from the space-based "garage" that would house a suite of kinetic- kill vehicles. A Leap projectile would close on its target at about 48,000km/hr, placing extreme demands on the terminal guidance system. In a space- based mode, Leap would be fired from a garage at the general area of a nuclear warhead, initial targeting having been pre-computed by other SDI sensors. At an undisclosed range, Leap's own terminal guidance system would guide the missile (known in SDI parlance as a "smart rock") to a direct hit. Critical Leap technologies indentified by Hughes include the optics necessary to view the target, the algorithms that ensure the missile ignores the target's exhaust plume and aims instead for the warhead, and the computer that controls the projectile. The "eye" of the Hughes system is a staring (as opposed to a scanning) focal- plane array containing 16,384 mercury cadium telluride elements. These collect infrared emissions focussed by the parabolic mirror at the front of the missile, and convert them to electronic signals. Hughes claims a significant advantage over its rivals in the field, and has demonstrated a Leap-sized focal-plane array at its Missile Systems facility in California. Video signals from the focal-plane array, and location signals from the inertial measurement unit (a Litton fibre-optic component weighing only 6oz) are processed in a microcomputer located on both sides of a disc that fits directly behind the mirror. Hughes says that signal processing is carried out at 4 • 1 million operations a second, a rate similar to that found in today's much larger optically guided missiles. Hughes has developed and successfully tested algorithms that will prevent the missile's infrared seeker from aiming for the hot exhaust plume of the target instead of the cooler warhead. Accuracy has already been tested against simulated solid- and liquid- fuelled rockets, from all aspects, says the company. Guidance commands are issued to two sets of thrusters, under development by Marquart. Located at the e.g. of the projectile are four larger bipropellant thrusters (pro- pellant is stored in tanks running the length of the missile) that will translate the body laterally in any direc tion, according to the combi nation in which they are oper ated. To prevent drift, all four thrusters would be operating at once but at varying thrusts, so that fine control of flight direction is achieved as the missile approaches the target. For attitude control, 12 much smaller (sin-long) thrusters arranged in four three-plane sets (for roll, pitch, and yaw) are located at the front of the missile, just behind the computer. With a response time of around 1-4 millisec, these can be switched at an extremely high rate to aim the projectile as it homes in. Leap ground testing will involve hovering the missile using its thrusters to check the accuracy of the guidance system. The present $50- million, 27-month SDI contract, awarded at the end of last year, calls for ground testing only. There are no plans for flight testing as yet. "That depends on whether we're successful," says Hughes. "So far, things have worked out extremely well." 34 FLIGHT INTERNATIONAL, 9 April 1988
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