Almost the End
On 13 October 1960, the government established the Aeronautics and Astronautics Coordinating Board (AACB) to coordinate various activities between the Department of Defense and NASA. The deputy administrator of NASA and the assistant secretary of the DDR&E served as cochairmen of the AACB; initially this meant Hugh Dryden and Herbert F. York, respectively. In an indirect way, the Research Airplane Committee that was created in 1954 to manage the X-15 program fell under the auspices of the AACB. However, given that the X-15 program existed prior to the creation of the AACB, the board had little direct impact on the program. The Research Airplane Committee continued to function much as it always had until sometime in 1965.
The AACB Aeronautics Panel began discussing the issue of continued funding for the X-15 in early 1966. Charles W. Harper from NASA made a good case for continuing Air Force funding for the X-15 since both the HRE and delta-wing projects were of potential value to the Air Force as well as to NASA. Both projects were part of a joint national hypersonics program organized in May 1965 by John Becker from NASA Langley and R. E. Supp from the Air Force Systems Command. Becker and Supp made a presentation to the Aeronautics Panel on 13 June 1966 showing that the HRE and delta-wing projects would be the principal users of the X-15 after the end of 1967, although a number of other experiments also continued. After a brief discussion, the Aeronautics Panel endorsed these projects and recommended that the AACB develop a cost-sharing plan that would allow the X-15 program to continue.-1376
The next meeting of the AACB on 5 July 1966, in fact, would influence the X-15 program greatly, but not the way the Aeronautics Panel had expected. Instead, the meeting essentially defined the date the X-15 program would end. In rejecting the recommendation of the Aeronautics Panel, the AACB indicated that the two most important approved Air Force experiments (20 and 24) would conclude at the end of 1967, and the AACB saw little need for continued Air Force support of the program past that date. Beginning on 1 January 1968, the program would become the
responsibility of NASA exclusively.13771
Rather quickly, however, it became apparent that the planned completion of the two Air Force experiments would run well into 1968. Consequently, at the 24 August 1967 meeting of the AACB, the participants attempted to work out some compromise that would allow the X-15 program to continue. The agreement changed little on the surface. From a monetary perspective, NASA agreed to begin funding the sustaining engineering contracts the Air Force maintained with North American, Reaction Motors, and the other original contractors. Both agencies concluded it was easier to allow the Air Force contracts to continue than to terminate them and restart them as NASA contracts. Instead, NASA would reimburse the Air Force for the cost of the contracts. The FRC agreed to continue its maintenance responsibilities for the airplanes and most of their systems, while the AFFTC agreed to continue maintenance of the carrier aircraft, rocket engines, and other systems it had been responsible for.13781
The largest change was the dissolution of the Research Airplane Committee that had guided the X-15 program since the signing of the original 1954 memorandum of understanding. The X-15 Joint Operations Committee and the X-15 Joint Program Coordinating Committee that had reported to the Research Airplane Committee would now report to the Aeronautics Panel of the AACB.13791
All in attendance agreed the X-15 program would continue at least through the middle of 1968. How long the program would continue after that depended upon the status of the Air Force experiments and the NASA funding situation. On 26 October 1967, the Air Force and NASA signed a new memorandum of understanding, replacing the original 1954 MoU that had governed the X – 15 program for 13 years. Charles W. Harper (NASA deputy associate administrator for the Office of Advanced Research and Technology) worked with Thomas C. Muse (assistant director OSD,
DDR&E) to get the new agreement signed by Dr. John S. Foster (director, DDR&E) and Dr. Robert C. Seamans, Jr. (NASA deputy administrator). The new MoU reestablished Air Force responsibility for X-15 costs, and spelled out the specific responsibilities of the two organizations. However, instead of ending with a statement of national priority, the new MoU contained the ominous proviso, "funds permitting." To most NASA managers, this meant that NASA would still have to face up to the total funding of the X-15 program as soon as the last two Air Force experiments ended.13801
Charles Harper and his boss at the Office of Advanced Research and Technology, Mac Adams, made one last effort to find funds for the program during the fall of 1967. They solicited help from the NASA Office of Manned Spaceflight (OMSF) because both the HRE and the delta-wing projects would produce new technology for the Space Shuttle. The attempt failed, however, because the OMSF was already having trouble promoting the space shuttle concept and did not want to add to its problems by supporting a potentially attractive-sounding alternative.13811
The accident involving Mike Adams underscored the concerns long expressed privately by Paul Bikle and others regarding the high costs and risks associated with extending the X-15 program. In the discussions that followed the accident, Bikle convincingly speculated on the enormous costs of the HRE flight program involving years of delay in getting started, malfunctions, and repairs. In December 1967, the Air Force and NASA both agreed to abandon the HRE flight program and to terminate the X-15 program at the end of 1968. On 13 March 1968 the Air Force announced that it would allow its X-15 funding to expire at the end of the year, but that it would continue to support flight tests to the "completion of Air Force IR [241 and WTR [201 experiments."13821
NASA allocated $1,500,000 for X-15 operations in FY68, with the Air Force contributing another $777,000. It appeared the program could save $150,000 by not returning X-15A-2 to flight status, and by flying a minimum number of other flights using X-15-1. The first six months of 1969 would require approximately $400,000 to catalog and dispose of spare parts, ground equipment, and prepare the two remaining vehicles for shipment to museums. The X-15 program would transfer some parts and ground equipment to other programs, and scrap the remainder.-138^
1968 FLIGHT PERIOD
The X-15 program would only fly another eight missions. During 1968, Bill Dana and Pete Knight took turns flying X-15-1. However, even within NASA, not everyone was certain the flights were worth the risk and $600,000 cost.-1384
X-15A-2 returned to Edwards on 27 June 1968. On 15 July, a series of nondestructive load and thermal tests began on the instrumented right wing in the FRC High Temperature Loads Calibration Laboratory. The airplane would remain grounded forever.-1385
Nevertheless, during the first part of 1968 the AFFTC and FRC worked together to see if there was sufficient interest to extend the program. By October 1968, they had surveyed the current users of the airplane and potential future researchers, and found some programs that could likely benefit from the X-15 being available. Two of the Air Force experiments (20 and 24) might need more time, especially the WTR launch monitoring, which would require extraordinary luck to get the X-15-1 and an ICBM in the air at the same moment. The groups investigating the impingement heating on the last flight of X-15A-2 also would have been happy to keep that airplane flying, since they had little other means of conducting experiments to understand the problem.-1385
Technically, NASA had already canceled the HRE flight program, but most everybody acknowledged that the ramjet experiments could also benefit from flight testing. However, NASA was a bit gun-shy after the bad experience on X-15A-2, and the flight ramjet development was running well behind schedule. Several other programs within the defense community were studying advanced propulsion concepts (ramjets, turbo-ramjets, or similar engines), and most of them potentially could have used the X-15 as a platform if it was still flying. There was even some talk about reviving the delta-wing concept that had been canceled after the loss of X-15-3.-1387
Despite this minor interest, in the end the AFFTC report concluded that "no known overpowering technological benefits will be lost if [the X-15] program ends on 31 December 1968." It noted that there was a firm requirement for the completion of the two Air Force experiments, and that "many USAF/USN technological activities [were] underway or planned for the Mach 4-6 regime," but the report failed to identify any specific requirements for the use of the small black airplanes.
It noted that "the future value of the X-15 as a hypersonic test capability should be more evident by mid-late 1969" and that the "option to use X-15 resources after 1969 should be protected."
Bill Dana completed the 199th—and as it turned out the last-X-15 flight, reaching Mach 5.38 and 255,000 feet on 24 October 1968. The program made 10 attempts to launch the 200th flight, but maintenance and weather problems forced cancellation every time. The attempt on 12 December actually got airborne (1-A-142), but the X-15 inertial system failed before launch. On 20 December 1968, things looked dismal, but everybody geared up for an attempt. Bill Dana began taxiing an F-104 for a weather flight, but John Manke noted that snow was falling-at Edwards! Manke recalled Dana before he took off and canceled the mission. Later that afternoon, technicians at the FRC demated X-15-1 from the NB-52A for the last time. After nearly 10 years of flight operations, the X-15 program ended.
By the end of the program, the two remaining airplanes were tired. In absolute terms, they were still young airframes-just 10 years old and with only about 10 flight hours each. The total free – flight time for all three airplanes was only 30 hours, 14 minutes, and 57 seconds. Even counting all the time spent under the wing of the two NB-52s, the total barely reached 400 hours. Despite early Air Force estimates of 300-500 flights, that had not been the original idea. Bob Hoey remembers asking North American project aerodynamicist Edwin W. "Bill" Johnston how long North American expected the airplanes to last. Johnston responded that the company had "expected that each airplane would only see 5 or 6 exposures to the design missions [i. e., Mach 6 or 250,000 feet]." They did much better.
The X-15s accumulated much more flight time than most of the high-performance X-planes, and the environment they flew in was certainly extreme. They frequently experienced dynamic pressures as high as 2,000 psf, and as low as (essentially) 0 psf. The airframes endured accelerations ranging from -2.5 g to over +8.0 g. Temperatures varied from -245°F to over 1,200°F. It had been a rough life.
In addition, NASA tested the airplanes-a lot. After each flight, NASA removed, disassembled, and thoroughly checked almost every system. Then each was reinstalled and tested some more. If the technicians noted any anomalies they made the appropriate repairs and retested. Milt Thompson wrote, "[M]y personal opinion is that we wore the airplanes out testing them in preparation for flight." The space shuttle would suffer much the same fate.
It is interesting to note that although the X-15 is generally considered a Mach 6 aircraft, only two of the three airplanes ever exceeded Mach 6, and then only four times. On the other hand, 108 flights exceeded Mach 5 (not including the four Mach 6 flights), accumulating 1 hour and 25 minutes of hypersonic flight. At the other end of the spectrum, only two flights were not supersonic (one of these was the first glide flight), and 14 others did not exceed Mach 2. It was a fast airplane. Similarly, there were only four flights above 300,000 feet (all by X-15-3), but only the initial glide flight was below 40,000 feet.