The Third Industry Conference

November 1961 saw the first industry conference held in three years (NASA had held previous conferences in 1956 and 1958). The classified conference at the FRC featured 24 papers from 56 authors, including 4 X-15 pilots, and was attended by 442 people. Of the authors, 5 came from North American, 37 from various NASA centers, 13 from the Air Force, and 1 from the Navy. The attendees represented virtually every major aerospace contractor in the country, all of the NASA centers, several universities, the various military services, and the British Embassy.-154

At the time the papers for the conference were prepared, the program had made 45 flights during the 29 months since the initial X-15 flight. The first of these was a glide flight, and of the subsequent powered flights, 29 had used the XLR11 engines and 15 used the XLR99. A maximum altitude of 217,000 feet (flight 2-20-36) and a velocity of 6,005 feet per second (flight 2-21-37) had been achieved.155

Researchers had already accomplished quite a bit of analysis on aerodynamic heating, one of the primary research objectives of the X-15. Several theoretical models had been developed to predict heating rates, but little experimental data were available to validate them since it was uncertain whether wind tunnels were capable of realistically simulating the conditions. The X-15 provided the first real-world experience at high Mach numbers in a well-instrumented, recoverable vehicle. Data from the X-15 showed that none of the models were completely accurate, although all showed some correlation at different Mach numbers. The data showed that the wind tunnels were reasonably accurate.156

A particular area of interest to researchers was how the boundary layer transitioned at different Mach numbers and angles of attack. Researchers used two methods to detect laminar and turbulent areas on the airplane in flight. The first was to use thermocouple data reduced to heat – transfer coefficients, which showed a much higher level of heat transfer in a turbulent boundary layer than in a laminar one. The second method was to use temperature-sensitive "DetectoTemp" paint applied over large areas of the airplane. In general, NASA applied the paint to the left side of the airplane, and the thermocouples were on the right side.157

The first use of the paint was on 4 August 1960 for flight 1-9-17, which was the XLR11 maximum speed attempt. The results were promising inasmuch as the paint established a semipermanent pattern of contrasting colors at different temperature levels. The pattern retained on the wing and vertical stabilizer after the flight clearly indicated all of the heat-sink locations and areas of high heating. For instance, the internal spars and ribs stood out as heat sinks, while areas such as the expansion joints on the wing leading edge stood out in the color pattern as concentrated heating areas. Researchers decided that they could use the paint to collect qualitative temperature data, particularly in small areas that were not equipped with thermocouples.-1581

One of the notable discoveries made using the paint was that patterns indicated high – temperature, wedge-shaped areas originating at the wing leading-edge expansion joints and extending for a considerable distance rearward. The 0.080-inch-wide expansion joints appeared to result in a turbulent flow during the entire flight, producing 1,000°F temperatures in an 8-inch wedged-shaped area behind them. The measured heat-transfer data on the other wing supported this view, offering "a classic example of the interaction among aerodynamic flow, thermodynamic properties of air, and elastic characteristics of structure." Although the rates were well within the limits of the airframe, engineers installed small 0.008-inch-thick Inconel X shields over the expansion joints in an attempt to minimize the interference. Flights with these covers showed that the turbulent wedges still existed, although they were smaller, and researchers theorized that they would be present for shorter periods on each flight.159

The conclusion drawn from this was that the "boundary layer transition, which may be produced by such discontinuities in the surface of a high-speed vehicle, would be extremely difficult to predict. As yet, for the X-15, there has not been established parametric correlation which would allow the prediction of the transition location on the wing a priori. Under these circumstances, it would seem that conservative estimates of transition should still be required."169

To show how the preflight estimates and flight data correlated, the authors presented data for one thermocouple on the lower surface of the right wing about 1.4 feet from the leading edge at mid-semispan. For the high-speed flight profile, the measured data indicated an all-turbulent flow with a high skin-heating rate and high maximum temperature. The calculated skin temperature agreed quite well during the high heating period, but slightly overestimated the measured value near its peak and during a period of cooling just afterwards. A close look at the trajectory during this period of disagreement showed a high angle of attack, and researchers believed the differences were due to their inability to properly predict the local flow conditions.

For a high-altitude mission, however, this point of the wing appeared to experience laminar flow, at least at times. An all-turbulent flow prediction resulted in a higher temperature than was actually measured during the exit phase of the trajectory, greater cooling during the ballistic portion, and an overestimate of the maximum temperature during reentry. The assumption of laminar flow during the latter part of the exit phase resulted in better agreement between the measured and calculated data. Researchers noted, however, that one of the turbulent wedges originating on a wing leading-edge expansion joint might affect the thermocouple in question. Researchers did not understand exactly what might cause the location to go laminar, but theorized that either the turbulent wedge vanished or its lateral spread was delayed.161

The wing leading-edge expansion slots produced problems in addition to the wedge-shaped boundary layer issue. On one flight the area directly behind the expansion slots buckled. One reason for this was that the fastener spacing directly behind the slot was wider than on other sections of the leading edge, providing less support for the area. It was also determined that the original segmentation of the leading-edge heat sink did not adequately relieve the thermal compression loads. The skins at the expansion slots acted as a splice plate for the solid heat-sink bar, and as a result buckled in compression. Engineers made several changes to solve this problem. The shield installed over each expansion slot to help the boundary layer problem minimized the local hot spot, but engineers also added a fastener near each slot and three additional expansion slots (with shields) in the outboard segments of the leading edge. This presented some concern since North American had designed the original expansion slots with shear ties to prevent relative displacement of the leading edge, and it was not cost-effective to provide shear ties for the new slots because the entire wing structure would have required modification. A structural analysis showed that sufficient shear stiffness was present in the leading edge to meet the design requirements without shear ties, but engineers expected some relative displacement at the three new slots. Actual flight tests showed that this displacement averaged about 0.125 inch. Overall, the modifications prevented any serious leading-edge buckling, although minor distortions continued throughout the flight program.-162

The X-15 program was one of the first to employ temperature-sensitive paint that established a semipermanent pattern of contrasting colors at different temperature levels. The paint clearly showed the different heating loads absorbed by the hot-structure airframe. In general, NASA applied the paint to the left side of the airplane; the thermocouples were on the right side. (NASA)

The conclusion drawn from the available data was that "when the boundary layer is known to be either laminar or turbulent, the skin temperatures can be predicted with reasonable accuracy." The problem was to figure out what the boundary layer would do under different flight conditions.163

The effect of temperature is not linear, and at Mach 6 the heating load on the X-15 was eight times that experienced at Mach 3. Unsurprisingly, the front and lower surfaces of the aircraft experienced the highest heating rates. During the conference, researchers discussed several intriguing aspects of the temperature problems. One was surprising, given that the program had always worried about high temperatures: "The first temperature problem occurred on the side­fairing panels along the LOX tank before the X-15 was first flown. Pronounced elastic buckles appeared in the panels as a result of contraction when the tank was filled for the first time."

Adding a 0.125-inch expansion joint to the tunnel fairing near the wing leading edge relieved the buckling.1164!

However, after a Mach 4.43 flight (2-13-26) on 7 March 1961, several permanent 0.25-inch buckles formed in the outer sheet of the fairing between the corrugations near the edge of a panel. Since the panel only carried air loads (not structural loads), the buckles did not seriously affect structural integrity. During the flight, the panels that buckled had experienced temperatures between 490°F (near the wing leading edge) to 590°F (near the front of the fairing). On this particular flight, the pilot shut down the engine prior to propellant depletion, leaving about 20% of the liquid oxygen in its tank. The maximum temperatures occurred after shutdown, and it was theorized that the cold tank (-260°F), together with the high outer-skin temperatures, resulted in large thermal gradients that caused the buckles. These gradients were higher than had been calculated for the original design, since the estimates had assumed propellant depletion on all flights. Based on this experience, engineers added four expansion joints in the fairing ahead of the wing that allowed a total expansion of slightly over 1 inch. This modification appeared to prevent any further buckling.!1661

Researchers expected the surface irregularities produced by the buckles to cause local hot spots during high-speed flights. To investigate this, NASA covered the buckled areas with temperature – sensitive paint for flight 2-15-29. The results from the Mach 4.62 flight showed that the maximum temperature in the buckle area was essentially the same as in the surrounding areas with no evidence of local hot spots. The researchers went back to their slide rules to come up with revised theories.-11661

Other heating problems experienced during the early flight program included hot airflow into the interior of the airplane, which caused unexpected high temperatures around the speed brake actuators, and loss of instrumentation wires in the wing roots and tail surfaces. In a separate incident, cabin pressure forced the front edge of the canopy upward, allowing hot air to flow against and damage the seal. NASA resolved the canopy problem by attaching a shingle-type strip to the fuselage just ahead of the canopy joint to prevent airflow under the edge of the canopy. A similar problem developed in the nose landing-gear compartment: a small gap at the aft end of the nose-gear door was large enough to allow the airstream to enter the compartment and strike the bulkhead between the nose-gear compartment and the cockpit. This stream caused a local hot spot that melted some aluminum tubing used by the pressure-measuring system on flight 2­17-33. During the Mach 5.27 flight, the bulkhead heated to 550°F, high enough to scorch the paint and generate some smoke inside the cabin. It was a potentially catastrophic problem, but fortunately no significant damage resulted. In response, engineers added an Inconel compression seal to the aft end of the nose-gear door and installed a baffle plate across the bulkhead.11671