RESEARCH INSTRUMENTATION

Previous X-planes had recorded all of the research data onboard, mainly because telemetry systems were in a very early state of development and bandwidth was very limited. Nevertheless, several earlier programs did telemeter a small amount of data to the ground in real time. It was decided early on that all X-15 data would continue to be recorded onboard the aircraft, although much more extensive use of telemetry would also be made. The reason for recording everything onboard was "to eliminate the risk of data loss and degradation inherent in radio-frequency telemetry links." This took on more significance for the X-15 program since the airplane would frequently be out of range of the antennas at Edwards, and would have to rely on the new and untried High Range installations at Beatty and Ely.-11

Initially, the instrumentation centered on the aero-thermo environment that the researchers intended the X-15 to investigate. When the follow-on experiments began to arrive, more of the instrumentation and recording capacity shifted to support non-aero-thermo investigations. A group of researchers from the HSFS, Langley, and Lewis-with limited input from the WADC and AFFTC-came up with the initial requirement for between 1,000 and 1,100 data points. Per the original specification, instrumentation was limited to 800 pounds and 40 cubic feet, and could use up to 2.25 kilowatts of power. The approved design included 1,050 instrumented points (588 thermocouples, 64 strain gages, 28 control surface position indicators, 136 aerodynamic surface pressures, 22 basic flight parameters (angle of attack, etc.) and 212 airplane condition monitors). By contrast, the X-2 had used only 15 thermocouples and a few electrical pressure transducers, and carried only 550 pounds of research instrumentation.-12!

In mid-1957, the NACA asked the Air Force to modify the X-15 specification to double the amount of research instrumentation carried by the airplanes. Given that North American had already frozen the design by that time, this came as something of a shock. In order to keep the airplane’s weight (and hence performance) from being too seriously degraded, numerous structural and subsystem details were redesigned to save weight.-13

When the X-15 emerged from North American, it could carry 1,300 pounds of research instrumentation, the majority of which were installed in a removable elevator in the instrumentation compartment just aft of the cockpit. Engineers designed the cabling so that they could remove the entire elevator from the airplane, which allowed them to perform all pre – and postflight calibrations more easily. Originally, the engineers intended this feature to remove the instrumentation from possible ammonia contamination, but NASA seldom used it for that purpose. Within the airframe itself, all of the wiring and tubing were routed through the fuselage side tunnels.-4-

In addition to the instrumentation compartment, North American installed small amounts of equipment in the nose of the airplane, in a center-of-gravity compartment located between the oxidizer and fuel tanks, and in the rear fuselage. The main instrumentation compartment and the nose compartment were pressurized and temperature-controlled. The center-of-gravity

compartment was temperature-controlled but unpressurized, and the rear fuselage area was insulated against high temperatures but was otherwise uncontrolled. Individual instruments and equipment were shock-mounted or hard-mounted as necessary; hard mounting was preferred because it saved weight and space.[5]

In many respects, X-15 development occurred at an awkward time. Modern data-processing systems were in their infancy, but they promised to offer a substantial improvement over the largely mechanical systems that had preceded them. However, the simple fact was that they were not ready. This forced the instrumentation engineers to rely on oscillographs and precision photographic recorders for the aircraft instead of modern magnetic tape recorders. Most of the rationale was simple: these devices were available from commercial sources or from NACA stock, lessening the cost of an already over-budget program. The program could also procure and test them within the time available before the first flight.

However, they came with some handicaps. The time associated with processing data from an oscillograph system, especially when large quantities of data were involved, was long and tedious compared to that required for data from magnetic tape systems. The instrumentation community debated this problem at length, but finally decided that the 15,000 data points expected to be collected on each flight would not result in processing times that would be detrimental to the planned flight schedule. It was also a fact that during 1956-1957, a costly, time-consuming development program would have been required to obtain a fully automatic magnetic tape system that could withstand the X-15 environment.-^

Despite the "design" instrumentation list, as manufactured the first two airplanes each had 656 thermocouples, 112 strain gages, 140 pressure sensors, and 90 telemeter pickups. The thermocouples were 30-gage chromel-alumel leads that were spot-welded to the inside surface of the skin. The leads connected to 20-gage extensions that were routed to the signal­conditioning equipment and recorders. The use of 20-gage extensions was necessary to reduce circuit resistance in the thermocouple loops and to minimize measurement errors due to resistance changes caused by the large temperature variations along the wire. Since the thermocouples were inaccessible after the airplanes were constructed, North American designed the installation to function for the life of the airplane and require no maintenance. A silicone – impregnated fiberglass braid covered the leads and extensions, and those in close proximity to the skin used an outer sleeve of unimpregnated fiberglass. The silicone impregnation slowly sublimated during repeated exposure to elevated temperatures, but retained its electrical insulating properties. Its use, however, created a potential problem since tests showed that out – gassing could result in an explosion if the temperature quickly rose to 1,200°F for the first time. NASA eliminated this hazard on the X-15 by gradually building up to the maximum Mach number during the course of the envelope-expansion program.-71

The first two airplanes used Bakelite strain gages, but these lost their effectiveness as structural temperatures increased. Consequently, North American completed X-15-3 with Micro-Dot weldable-type strain gages designed for use at higher temperatures. The static pressure taps consisted of 0.3125-inch-outside-diameter tubing installed flush with the outside surface of the skin. A study was made of the lag effects of a tube-connected system, and it was determined that 0.25-inch tubing with lengths as great as 40 feet was acceptable for gradual maneuvers and steady-state data at altitudes up to 100,000 feet.-81

either side. Because of installation difficulties, no instrumentation was located near the integral propellant tanks. Similarly, North American did not initially install any pressure instrumentation in the horizontal stabilizers due to the difficulty of running tubing to this location. However, the company did install some strain gages in the horizontal, with the wiring running through the pivot point. As part of a loads study late in the program, North American manufactured a new set of horizontal stabilizers with electrical pressure transducers, loads sensors, and thermocouples. Toward the end of the flight program, researchers also installed instrumentation in the wing-tip pods and ventral stabilizer on some flights.-191

Precision NACA recorders that employed servo-repeater systems to position a light source on moving film recorded angle-of-attack and angle-of-sideslip data provided by the ball nose. Similar devices recorded the attitude-angle outputs from the stable platform. Electrical transducers sensed all other data. A central patch panel in the main instrumentation compartment collected the data, routed it to appropriate signal conditioners, and then sent it to recording oscillographs and the telemetry set. The NACA-developed photo-oscillographs were capable of recording 36 channels each. Recording speeds could be varied from 0.25 inch per second to 4.0 inches per second, resulting in recording times ranging from 56 minutes to only

3.5 minutes using 70-foot film magazines. The photo-oscillographs used a blue-sensitive polyester-based thin film with the trade name Cronar®. A variety of 16-mm motion picture cameras photographed portions of the pilot’s instrument panel, and the wings and empennage during flight.1101

This block diagram shows the basic interrelationship of the various pieces of research instrumentation carried on the X-15. The exact instrumentation varied considerably between airplanes, and flight to flight. Late in the program the X-15-3 received a much more modern PCM telemetry system. (NASA)

Recorder limitations restricted the number of installed sensors that could be recorded simultaneously. A 12-channel oscillograph recorded 40 thermocouples per channel at 1-second intervals, and four manometer-oscillographs recorded up to 96 pressure transducers. A NACA – designed aneroid-type 24-cell film-recording manometer similar to those used in previous flight programs recorded the surface pressures. Again, recorder limitations restricted the number of pressure measurements that could be recorded simultaneously. The exact data recorded often differed on each flight as researchers and engineers connected different sensors to the recorders and telemetry system. A single switch in the cockpit turned on all of the recorders, and an event switch allowed the pilot to mark the recording when something significant occurred.[11]

Two separate cockpit instrument panels were supplied with each of the first two airplanes: one for the initial low-speed flights using the XLR11 engines and the nose-mounted flight-test boom, the other for hypersonic flights using the XLR99 engine and the ball nose. NASA significantly revised the instrument panel in the first two airplanes early in the flight program based on pilots’ comments that the original panel was difficult to scan under all flight conditions, especially when they were wearing the MC-2 full-pressure suit. As initially completed, X-15-3 had an instrument panel identical to the XLR99 panels manufactured for the first two airplanes. However, when North American rebuilt the airplane following its XLR99 ground explosion, the Air Force decided to incorporate the Minneapolis-Honeywell MH-96 adaptive flight-control system, and this necessitated a unique instrument panel. NASA subsequently replaced this panel late in the flight program with a set of vertical-tape displays developed by Lear-Siegler. All of the instrument panels were in a constant state of flux as various switches and indicators were added to almost any available location in the cockpit to support the various experiments and data requirements for any given flight. Every attempt was made to keep the critical displays and switches in constant locations between the three airplanes (at least as much as possible given the radical difference in X-15-3), and twice the program created a "standard X-15" cockpit arrangement and brought the airplanes into compliance. This greatly eased the problems associated with keeping the simulator accurate, and made life much easier for the pilots and flight planners.-1121

Initially, the X-15s used a pulse-duration modulation (PDM) telemetry system that researchers considered state of the art when they selected it. However, the system was insufficient for many types of data that researchers wanted to view on the ground (particularly the biomedical parameters), and the AFFTC Human Factors Subcommittee requested the installation of a more sophisticated FM-FM telemetry system. Initially, the FRC objected to the proposed change because of the size and volume requirements of such a system. However, on 2 December 1960 Paul Bikle stated that he favored the installation of a FM-FM system if it fit into the space then used by the existing North American telemetry system. By then, the state of the art allowed the Air Force to purchase a 12-channel FM-FM system for use in the biomedical package. NASA subsequently installed this system in the X-15s as needed to support biomedical work, and the first flights took place in late 1961.-131

In May 1967, NASA installed a modern pulse-code modulation (PCM) system in X-15-3. The first flight (3-58-87) for the new system was on 26 April 1967 with Bill Dana at the controls. By all accounts, the new system worked well and provided a great deal more bandwidth than the old PDM and FM-FM telemetry systems. It appears that NASA never updated the other two aircraft to PCM.1141