When the Air Force approved the X-15 program in December 1954, the aviation medical community in the United States had already embarked on research into the physiological effects of weightlessness in anticipation of technological developments that would permit manned space flight. This research consisted largely of obtaining physiological measurements from human subjects while they flew Keplerian trajectories in a modified Lockheed F-94C Starfire and attempted to perform certain psychomotor tasks. The maximum duration of weightlessness in these early experiments was 30-40 seconds, and it was difficult to separate the subject’s responses to the weightless state from his responses to the pre – and post-trajectory accelerations. Nausea and vomiting, for example, occurred in the majority of test subjects in these experiments, probably due to the relatively rapid transitions between hypergravic and hypogravic states. Despite the amount of uncertainty surrounding the experiments, some medical researchers nevertheless concluded that weightlessness would induce nausea and vomiting in most people.
The Air Force widely reported this conclusion, which greatly influenced several early manned space studies.-161
Because the X-15 flight profiles would provide longer periods of weightlessness than were possible with lower-performance aircraft, acquiring physiological data became an early objective of the program. As North American finalized the X-15 configuration, there were additional reasons for monitoring biomedical data from a safety standpoint. Since the cockpit of the X-15 was engineered for a 3.5-psi differential between the inside of the cockpit and the outside atmosphere (or lack thereof), it was not considered feasible to use a breathable atmosphere. At sea level, oxygen accounts for approximately 20% of the normal atmospheric pressure of 14.7 psi. A breathable atmosphere therefore requires 20% of the normal 14.7 psi-about 3 psi of oxygen in the 3.5-psi cockpit.-171 Engineers considered the problems of combustion and fire associated with this 86% oxygen atmosphere to be insurmountable. As a result, North American pressurized the X-15 cockpit and ventilated the full-pressure suit with nitrogen, supplying breathing oxygen to the helmet area only. A neck seal (MC-2 suit) or a face seal (A/P22S-2 suit) separated the breathing space from the remainder of the suit. In order to prevent the leakage of nitrogen into the breathing space, technicians adjusted the breathing-oxygen regulator to deliver oxygen at a pressure 1 inch of water higher than the suit pressure.-181
The maintenance of this helmet-suit pressure differential was vital, since otherwise the pilot could develop an insidious hypoxia that would lead to serious impairment or unconsciousness. The suit pressure regulator maintained 3.5 psi in the suit, the same pressure maintained in the cockpit. In the event of loss of cockpit pressure, the suit would automatically inflate, thus preventing a catastrophic decompression of the pilot. It was not particularly unusual for the suit to inflate partially as the X-15 cabin differential changed during exit or reentry. The suit/cabin-pressure differential provided an indication of the proper functioning of the suit and suit-pressure regulator, and the cabin-pressure regulator. In the event of failure of the suit regulator or breathing-oxygen regulator, the pilot could select the emergency oxygen system that pressurized both the suit and helmet with oxygen; this was the same emergency system used during ejection.-1191
North American designed and fabricated the original biomedical instrumentation system as part of the basic X-15 contract. The system monitored eight parameters, including the ECG, oxygen flow rate, helmet/suit-pressure differential, cabin/suit-pressure differential, and pilot skin temperature, from four locations. The helmet/suit-pressure differential also served as an excellent respirometer; since the breathing space in the helmet was relatively small, the pilot’s respiration produced pressure fluctuations that the 0-0.5-psi transducer could follow, providing a real-time indication of respiratory rate. The ECG, oxygen flow rate, and skin temperatures were recorded using an onboard oscillograph recorder. The two pressure differentials were sent to the ground via PDM telemetry and displayed in the control room, along with cabin pressure (obtained via vehicle, not biomedical, instrumentation) on a heated stylus stripchart recorder. The first physiological and environmental data recorded on the X-15 program were obtained during flight 1-6-11 on 6 May 1960 with Bob White at the controls.-1291
One of the first things that researchers assessed on the X-15 was whether the cockpit environment adequately protected the pilot. The requirement was to keep the pilot’s skin temperature below 100°F even though the aircraft’s outer skin could heat to nearly 1,000°F. In fact, researchers found that the cockpit outer wall was reaching 750°F, but the cockpit itself remained in a temperature range between 36°F and 81°F. The Air Force removed the skin- temperature sensors after the first round of flights when it became obvious that the mission was not exposing the pilot to any significant thermal stress.1211
The original biomedical signal-conditioning package, mounted in the instrumentation compartment behind the cockpit, was 5 by 6.5 by 11.5 inches in exterior dimensions and weighed 11 pounds. In general, this instrumentation functioned well, but as researchers gained experience they corrected minor deficiencies and simplified the system. For example, the original ECG used five electrodes that simulated the clinical I, II, III, and V4 leads. However, since the ST segment and T(minus) wave changes are essentially uninterpretable under dynamic conditions, a one-channel ECG gave just as much information as a multi-channel system. Therefore, the ECG was simplified to a three-electrode configuration (two 0.75-inch-diameter stainless-steel screen mid-axillary leads and a reference electrode on the lower abdomen). A silicone potting compound ring surrounded the metal mesh electrode, a conductive paste assured good contact with the skin, and pressure-sensitive adhesive secured a plastic cap over each electrode to keep it in place.
Beginning with flight 2-18-34 on 12 September 1961, the Air Force installed a Tabor Instruments amplifier on the ejection seat. Amplifying the signals closer to the pilot resulted in much better data.1221
Researchers found the pilot’s heart rate during flight usually increased from the normal 70-80 beats per minute to 140-150 per minute, but with no apparent physiological effect. One interesting finding, later confirmed on Mercury flights, was that the pilot’s heart rate decreased during the period of zero g. The reduction, however, was not great (to about 130 beats per minute). The respiration rate followed similar trends, increasing to three or four times the resting rate, but researchers considered this less meaningful because talking influences the respiratory system and has a poor dynamic response rate in any case. On almost all flights, there was a large peak in respiration rate during the powered portion of the flight when the pilots tended to breathe rapidly and shallowly.
During the initial phases of the flight program, researchers only installed the biomedical package in the X-15 on a non-interference basis. As a result, it frequently did not work correctly since technicians had not allocated sufficient time to its installation and checkout. Most of the difficulties were traced to shorts and broken wires. Although the biomedical team coordinated with the Air Force and NASA, the next flight frequently suffered the same problems. As a result, in early 1961 the Air Force and NASA assembled a dedicated team to work on biomedical issues, and the system became much more reliable.
The researchers requested that the biomedical instrumentation package fly on all XLR99 flights that expanded the envelope. In a meeting held at the FRC on 2 December 1960, the Air Force and NASA agreed that the acquisition of physiological data was important from both a flight-safety and research perspective. However, NASA did not make the acquisition of biomedical data mandatory, mainly because it did not want to have to cancel a flight because the biomedical package failed.-1251
After Paul Bikle approved the concept of installing an FM-FM telemetry system dedicated to the biomedical package, the Air Force awarded a $79,000 contract to the Hughes Aircraft Company to develop and manufacture the system.-126 The system included an FM radio transmitter rather than a hardwire link to the aircraft telemetry transmitter. This was done to demonstrate the feasibility of the radio link in order to permit the mobility expected on future spacecraft. The final Bendix TATP-350 unit measured 9.30 by 3.95 by 0.70 inches and weighed 0.84 pound. The first unit was delivered in March 1962, and the third and final unit was delivered in July 1962. Researchers demonstrated the system during flights of the JTF-102A using MC-2 suits, and in an unusual test the system received a telemetered ECG from a free-falling parachutist.-1271
NASA installed the new FM-FM system in all three X-15s during the summer of 1962. This installation permitted the ECG to be telemetered and displayed in the control room along with the helmet/suit – and cabin/suit-pressure differentials, cabin pressure, and two axes of aircraft acceleration (vertical and longitudinal). The system multiplexed these signals onto a single FM channel and then displayed them on a six-channel Sanborn stripchart recorder in the control room.-126
In 1962, the AFFTC began providing biomedical system expertise to the Dyna-Soar program. Because of severe space and weight limitations on the X-20, miniaturization was an absolute necessity. Based on its success in providing the X-15 FM-FM system, the Air Force awarded Hughes Aircraft a contract to fabricate two prototype signal-conditioning units designed around X-20 requirements. Researchers first used the system to monitor environmental and physiological data during dynamic simulations on the centrifuge at NADC Johnsville. When North American rebuilt X-15-2 into its advanced configuration, the company installed a Hughes signalconditioning system to test the new system prior to its use on the X-20. After the cancellation of the Dyna-Soar program in December 1963, Hughes modified the design to incorporate interchangeable modules that could meet a variety of requirements. Hughes repackaged the system so that all of the modules were a common size and used identical connectors.-1291
The final package measured 4.0 by 3.5 by 0.7 inches and weighed only 0.6 pound. Like the earlier
Hughes system, this package used an FM transmitter link to the aircraft telemetry system, although a hardwire link could also be used if needed. For X-15 use, the package provided the ECG, helmet/suit – and cabin/suit-pressure differentials, Korotkoff sounds, and partial pressure of oxygen in the breathing space. When required, researchers could substitute modules that provided the partial pressure of CO2 and an impedance pneumogram.
During the summer of 1963, the Air Force again modified the biomedical instrumentation, this time by adding a blood-pressure monitoring system developed by the Air Force School of Aerospace Medicine at Brooks AFB, Texas. The system used an occlusive cuff crystal microphone to determine arterial pressure in the upper arm. An electro-pneumatic programmer that cycled once per minute automatically inflated the cuff. During deflation of the cuff, a microphone detected the Korotkoff sounds, and a display showed these sounds simultaneously with the cuff pressure trace and the ECG. The system could be turned on or off by the pilot and had a fail-safe feature that could dump the pressure in the cuff in the event of a power failure, preventing a tourniquet effect from the cuff. Surprisingly, the first few flights of the package did not yield meaningful blood-pressure information, since the pilot’s pressures exceeded the maximum reading available on the instrumentation. The school modified the cuff inflation pressure to allow up to 240 millimeters of mercury to obtain useful systolic pressure end-points.-131
The school had used the blood-pressure monitoring system for in-flight studies using conventional jet aircraft for some time, and it had been very reliable in service. However, the initial use in the X-15 was not completely satisfactory because of a generally inadequate signal-to – noise ratio. The X-15 environment proved to be particularly severe; the combination of frictional noise from the suit, vibration, acoustic noise, and pilot movement artifacts produced a high background noise and a low signal-to-noise ratio. Researchers experimented with several filters and amplifiers, and eventually found a satisfactory combination.-32
In addition to the ECG and various cockpit and suit pressures and temperatures, the researchers added blood pressure, skin temperature (on the calf, abdomen, forearm, and axilla), respiratory rate, radiation, and partial pressures of oxygen and carbon dioxide in the helmet. A $47,000 Bendix Aviation mass spectrometer determined the partial pressure of oxygen, carbon dioxide, carbon monoxide, nitrogen, and water vapor. A linear pneumotachometer provided by Spacelabs, Inc., under a $23,000 Air Force contract341 furnished a rapid method for determining the total oxygen consumed by measuring changes in the pilot’s breathing rate.35
As researchers evaluated the biomedical data during the flight program, it became apparent that the initial objective of obtaining data on the physiological response to weightlessness was not feasible using the X-15. The duration of weightlessness (3-4 minutes) was too short and the pilot’s responses were conditioned by too many uncontrollable variables that occurred simultaneously for any conclusions to be made concerning the physiological response to weightlessness. In addition, the manned space programs initiated shortly after the X-15 began its flight program provided a much longer weightlessness duration without the attendant stresses of having to fly the airplane; this portion of the X-15 data was instantly obsolete. Nevertheless, the X-15 data provided researchers a unique opportunity to observe the basic physiological responses of pilots in manned vehicles flying exit and reentry profiles-something that Mercury did not, since the astronaut was simply along for the ride during those periods.-1361