THE FOLLOW-ON PROGRAM

During the early 1960s, the X-15 was the only platform that could realistically carry a useful payload above the Earth’s atmosphere and return. Researchers had been making use of various sounding rockets that provided relatively inexpensive access to the upper atmosphere from a variety of locations around the world. However, in general these rockets had very small payload capabilities, could not provide much in the way of power or controlled flight, and were usually not recoverable. On the other hand, although the X-15 was very limited in where it could fly (over southern California and Nevada), it could provide a fair amount of power, it was at least somewhat controllable for aiming purposes, and, most importantly, it was recoverable.

EXPERIMENT ACCOMMODATIONS

Although the X-15 provided some internal space for experiments, many researchers wanted specific views of the world outside or to have their experiments located away from the "noise" of the airplane. This gave rise to several modifications that ultimately affected all three X-15s.

Wing-Tip Pods

Several experiments (particularly a proposed micrometeorite collection system) had to be located outside the flow field of the X-15 and, it was hoped, outside the zone of contamination from the ballistic control-system thrusters. The most obvious location was the wing tips.

There were several preliminary designs for the wing-tip pods. Initially North American wanted to give the pods a rectangular cross-section, since it would be easier to package the various experiments in them. However, after considering both normal and potential emergency operations in terms of the effects of stability and control, heating, drag, and turbulence, the engineers decided to use circular-cross-section pods constructed of Inconel X. The pods were 8 inches in diameter and 58 inches long, and could weigh a maximum of 96.2 pounds, although the program exceeded this limit on numerous occasions.-1561

There was some initial discussion about equipping the pods with an emergency jettison system in case the micrometeorite collection system stuck open, but the final design had the collection equipment simply burning off during reentry if that occurred. Researchers tested the wing-tip pods in the supersonic and hypersonic wind tunnels at the Jet Propulsion Laboratory (JPL) on 24­25 October 1962 to verify that they had no adverse effect on the airplane. On 5 November 1962, North American tested the configuration in its low-speed wind tunnel to verify that the landing characteristics were not affected. The test results proved to be satisfactory.-1571

Wing tip pods were developed for X-15-1 to house experiments that needed to be located outside the flow field of the X-15. The pods were 8 inches in diameter, 58 inches long, and could weigh a maximum of 96.2 pounds. The first flight (1-50-79) with the pods was on 15 October 1964 with Jack McKay at the controls. Similar pods were later manufactured for X-15-3. (NASA)

Initially, North American manufactured a single set of the pods for X-15-1. NASA installed the modifications necessary to use the wing-tip pods, including the attachment points and wiring routed through the wing, on X-15-1 during March 1964. The first flight (1-50-79) with the pods was conducted on 15 October 1964 with Jack McKay at the controls. The flight reached Mach 4.56 and 84,900 feet, and the pods did not seem to have any major adverse effect on the handling of the airplane. Subsequently, however, some pilots complained that the pods seemed to introduce a buffet at load factors significantly below the previous buffet boundary. Researchers installed accelerometers in the pods to verify this, but failed to uncover any evidence of buffeting. However, the redistribution of mass due to the pod installation appeared to result in a 17-cps vibration tied to the wing-bending mode that was excited by some maneuvers and gusts, which likely explained what the pilots felt.[58]

Researchers subsequently determined that having only one set of pods put an unreasonable constraint on scheduling experiments, and decided to manufacture a second, easily removable set of pods. NASA modified the third airplane to carry wing-tip pods, and could switch the pods between X-15-1 and X-15-3 as needed to support the experiments and flight schedule.

Frequently the rear compartment on one or both pods contained cameras aimed at various parts of the airplane (usually the ablative panels on the stabilizers) or one of the experiments in the tail-cone box. At some point, the pods on X-15-1 also received a small set of drag braces for unspecified reasons (probably an attempt to cure the vibration problem). Despite the original intent, and the best efforts of all involved, the wing-tip pods did not put the experiments outside the contamination zone of the ballistic control thrusters. Residue from the hydrogen peroxide would render several experiments useless. The pods were also inside the nose shock-wave interference zone at certain angles of attack, further hampering some experiments.-1591