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XRM-86 PWN-4A Exos

The Exos was manufactured by the University of Michian with the cooperation of the National Aeronautics and Space Adninistration under sponsorship of ths Air Force Cambridge Research Center. The Exos sounding rocket, originally designated RM-86 and later PWN-4, was developed to fill the needs of upper-air research for an economical vehicle carrying a 40-l1b payload to a 500-mile altitude. The Exos sounding rocket is a three-stage, solid-propellant sounding rocket capable of carrying 40 lb of instruments to a 300-mile altitude. It was developed by The University of Michigan with the cooperation of NACA under sponsorship of the Air Force Cambridge Research Center.

The first two stages, Honest John prime mover and Nike-Ajax booster, are fin-stabilized, while the third stage, a Thiokol Recruit, employs a flared skirt. The first stage separates at burnout and the remainder of the rocket coasts for 25 seconds. The second stage is ignited by timer and batteries and at burnout the pressure drop signals third-stage ignition. A speed of over Mach 10 is achieved at third-stage burnout. Aerodynamic heating is countered by a blunt nickel nose tip, Inconel nose cone, and inner radiation reflector. Acceleration at third-stage burnout is about 175 g's.

Two flight tests verified the performance predictions. The preparation, assembly, and launching of an Exos can be done by only a small crew with modest equipment, and the total cost of the vehicle is under $20,000. It was a simple and economical means of lifting moderate payloads to the 300-mile region. While the first flights were carried out from a modified Honest John launcher which uses a short rail, Exos was easily adaptable to zero-length launchings.

Changes in payload weight will alter the predicted peak altitude about one mile per pound. Since the third stage operates as a low-performance rocket (mass ratio less than 3) and the 40-lb payload is only about 1/5 of the burned-out weight, the changes in payload may be quite large without greatly affecting the performance or operation of the rocket.

In staging the vehicle, the optimum trajectory from a thrust and drag standpoint is not utilized because of the overriding consideration of aerodynamic heating. Thus the long coast period between first and second stages is designed to permit the rocket to attain an altitude of reduced density and bring the heating within tolerable limits during the high-speed portion of the flight.

The first Exos was flight-tested at NACA, Wallops Island, Va., on June 26, 1958. It was instrumented for measurement of flight conditions by Air Force Cambridge Research Center. The third stage weighed 467 lb. The launch angle was 75°. Takeoff,first-stage burning, and separation were normal. Visual tracking was lost during the coast period when the rocket entered a cloud. Radar beacon tracking failed at takeoff. Radar skin tracking failed at the time of visual loss. Telemeter reception failed at the time of second-stage ignition, Long-range radar tracking near Boston was good, but only three points were recorded. These points were too close together to establish an accurate trajectory; however, they were near the trajectory peak and verified that the rocket attained an altitude in the neighborhood of 240 miles. On the basis of weight and launch angle, the predicted peak was 260 miles. Therefore the vehicle was deemed to have operated successfully and substantially as predicted.

The second Exos was flight-tested at NACA, Wallops Island, Virginia, on September 25, 1958. It was instrumented for measurement of nose cone and skin temperatures, and acceleration in three planes by the Air Force Cambridge Research Center. The third stage weighed 464-1/2 lb. The launch angle was 80°. Operation through final-stage burnout was normal. Radar beacon operated for one minute, which will enable a drag-free trajectory to be computed. Long-range radar tracking near Boston was very good. Preliminary reduction of these data yields a peak altitude of 296 miles. On the basis of weight and launch angle, the predicted peak was 288 miles.

The vehicle was fired once from Eglin Air Force Base. This unsuccessful flight was believed caused by launcher interference.