B-70 Signature Reduction

In what was perhaps the first true "follow-on" experiment, in 1961 researchers used X-15-2 to test a coating material designed to reduce the infrared emissions of the North American B-70 Valkyrie bomber. One of the complaints frequently voiced against the B-70 was that it was a large target. Although the concept of "stealth" (a term not yet applied to the idea) was not far advanced in 1960, engineers at Lockheed and North American both understood that reducing the radar and infrared signatures of strategic aircraft would at least delay their detection by the enemy. For instance, Lockheed specifically intended the shape and materials of the A-12/SR-71 Blackbird to lower its radar signature. North American conducted several detailed studies into the infrared and radar signatures of the B-70 to provide a basis for reduction attempts.-68

During the very short YB-70 development period, the Air Force directed North American to investigate ways to reduce the probability that the B-70 would be detected. The company made preliminary investigations into applying various radar absorbing materials to the airframe, particularly the insides of the air intakes. However, most of the North American effort appears to have concentrated on reducing the infrared signature of the aircraft. Exhausting cool air around the J93 engines was one means of reducing the infrared signature of the B-70.

As part of its research, North American developed a "finish system" (i. e., paint) that provided a low emittance at wavelengths used by Soviet infrared detecting devices, and allowed most of the excess heat to be radiated from the surface in wavelengths that were not normally under surveillance. The finish used a low-emittance basecoat with an organic topcoat that was transparent to energy in the 1-6-micron range. The topcoat was strangely opaque and highly emissive at wavelengths between 6 and 15 microns. This finish was relatively invisible to infrared detecting equipment and still allowed the skin to radiate excess heat overboard to maintain its structural integrity.-^

was applied a 1-mil-thick mixture of 85% Ferro Enamaling no. AL-8 Frit and 15% Hommel no. 5933 Frit. The Type II basecoat was a mixture of 40% Hanovia silver resinate and 60% Hanovia L. B. coating no. 6593 applied 0.004 mil thick. The topcoat was a mixture of 74% 3M Kel-F no. 2140, 24% 3M Kel-F no. 601, and 2% Al2O3 applied 1 mil thick. Most probably, the topcoats would have been opaque silver instead of the white finish used on the two XB-70A prototypes.-1701

The finish system was somewhat difficult to apply to an aircraft as large as the B-70, but the engineers expected that further development would yield improvements in the process. The most difficult problem was that the underlying surface had to be highly polished prior to applying the basecoat. In addition, the basecoat of both finishes had to cure at 750°F, while the topcoat of the Type II finish had to cure at 1,000°F (creating almost a ceramic finish). Accelerated environmental tests indicated that the surface would prove durable on the stainless-steel sections of the B-70, but its long-term adhesion to titanium appeared to be weak. Both finishes were relatively immune to exposure to hydraulic fluid, fuels, oils, and other substances encountered during operational service.-1711

To obtain real-world flight experience, the Type I coating was applied to one panel on the vertical stabilizer of X-15-2 in March 1961 and flown on flight 2-13-26 by Bob White. Since Inconel X is a type of stainless steel, the test was relatively representative of the proposed B-70 installation.

No observable physical changes occurred during the Mach 4.43 flight, during which the aircraft’s exterior reached 525°F, and the engineers made no attempt to measure the infrared qualities of the coating during this single flight. It was made simply to determine whether the coating would survive the aero-thermo environment, and appears to have been successful.-721