MiG-25 Series

The MiG-25 was a special case. Originating in the late 1950s as a response to the ambitious Lockheed A-ll project,* the aircraft that was to become the MiG-25—still referred to inside the OKB as the Ye-155— •The Lockheed А-ll project would lead to the YF-12A interceptor and the SR-11A reconnaissance aircraft. The existence of the project was disclosed by President Johnson on 23 February 1964—but in fact it dated back to 1959, and the Soviets were

helped the Soviet aerospace industry to make great strides forward. And at the time technology was already progressing by leaps and bounds. Immediately after the first aircraft broke the sound barrier, everyone was already talking about level flight at Mach 3! And every­one knew that to reach that speed, another barrier had to be broken: the heat barrier.

On the MiG-19 at Mach 1.3 in 0° C (32° F) ambient air temperature, the airflow temperature at the nose reached 72° C (161.6‘ F). On the MiG-21 at Mach 2.05 that temperature increased to 107′ C (224.6° F). At Mach 3 it would hit 300° C (572° F) The basic material used in air­craft manufacture, duralumin, could withstand temperatures of up to 130° C (266° F), but there were no semiconductors capable of surviv­ing over 65° C (149° F). The new barrier seemed truly impassable. "The eyes are scared but the hands work,” goes an old Russian saying – one the OKB engineers seemed to take to heart. Some started to make computations, others set out to visit suppliers, and in a short time the project started to take shape.

The engine was the first priority A. A. Mikulin and S. K. Tuman – skiy, his closest colleague, proposed an immediate answer: one derived from the 15K, an axial flow turbojet designed for a winged missile. The two engine manufacturers quickly developed the compressor, the com­bustion chamber, and the afterburner. They read the temperatures all along the gas channel and developed an adjustable-area nozzle. To obtain an exact fuel/air ratio for engine ratings subject to quick changes, the hydromechanical fuel metering valve was replaced by an electronic fuel control unit.

With the engine development seemingly well in hand, the time had come to deal with the airframe. The engineers’ task was to create an aircraft whose flight envelope would be quite unusual—especially in terms of speed and ceiling—and one that would be equipped with many new systems. After testing several models in the TsAGI wind tunnels, one was selected. The next step was to choose the materials.

The forced abandonment of duralumin left only one option: titani­um, which Lockheed used for the A-ll project. On the engineering drawings, the fuselage and the wing center section were to be used as built-in fuel tanks Theoretically, those tanks could be made of duralu­min because they were to be filled with a cold fluid; their walls would only warm up to dangerous levels once the tanks were empty But to build such structures, rivets and sealer cement that could withstand high temperatures were vital—and they did not exist. Moreover, titani­um was veiy difficult to machine, and cracks often formed after weld­ing. Was steel a viable alternative?

At the same time, an unexpected obstacle cropped up: a shortage of qualified riveters. Few people wanted to do this unrewarding, unpleasant work. With welded steel, rivets would not be necessary. A number of steelworks cast high-quality, easy-to-weld steel that obviated the need for cement. Moreover, since World War II many welding schools had opened all over the country.*

After weighing the alternatives, Mikoyan made up his mind: the new aircraft would be made of steel. Everyone at the design office, the metallurgical industry’s research institutes, and the specialized test lab­oratories went to work developing strong, corrosion – and heat-resistant, steels; new titanium-aluminum alloys for the less sensitive parts; and innovative machining, casting, stamping, and welding tools. Research was also conducted into microscopic metallurgy in a welding bath; the tendency of metal and welded assemblies to crack at different tempera­tures; the interaction of basic and added materials; crystallization laws; and crystallization process control for hard-to-weld materials. As fast as those problems were solved, all of the factory workshops were upgrad­ed to use the new technologies, spot welding and seam welding, auto­matic or manual. All riveters were turned into welders.

A high-quality steel is three times more solid than duralumin but also three times heavier, so in order not to add weight to the aircraft’s structure every structurally significant item had to be three times thin­ner. This forced the engineers to reconsider matters such as the strength of materials, aeroelastic stability, aerodynamic flutter, and so on. The whole process was as complicated as the shift from the antique wood airplane to the modem all-metal aircraft. Any move forward hap­pened step by step, and workers constantly had to become acquainted with new methods for assembling panels and parts.

To start, only three wing structures were built. The first two were rejected because they did not withstand particularly severe static tests. The pessimists—and they were numerous—thought that the welded built-in fuel tanks would not hold out or that every landing would prompt disastrous cracks. The plexiglass of the canopy was so outdated that it melted. The hydraulic fluid decayed, and tires as well as rubber sealing rings lost their elasticity Everything had to be questioned, adapted, or modified.

But eventually all the pieces of the jigsaw fell into place, and it became possible to build the first prototype. The technological results speak for themselves:

1. Material: structure made of tempered steel, 80 percent of the air­frame weight; titanium alloys, 8 percent; structurally significant items made of D19 heat-resistant aluminum alloy, 11 percent

2. Assembly method: spot welding and seam welding, 50 percent (weld spot > 1,400,000); argon arc welding, 25 percent (4,000 m

•As early as the 1930s the Soviets had developed many forms of welding. During World War II the scholar Ye. P. Patone invented automatic welding methods that quintupled tank production.

[13,000 feet] of weld bead), fusion welding and inert gas welding

1 5 percent, assembly with bolts and rivets, 23 5 percent

The welded fuel tanks took up 70 percent of the fuselage vol­ume The seal was secured by welds whose reliability can be judged by the following anecdote over one full year of welds—whose dis­tance was equivalent to that between Moscow and Gorki (450 km [280 mi]) —only one or two insignificant leaks were detected The repair was no problem and, most important could be made by field maintenance personnel

The thermal problems were not completely setded for all of that A full range of air-air and air-fuel exchangers, as well as turbine cooler units and other similar systems had to be developed in order to lower the temperature of the air bled into the engine compressor from 700° C (1292° F) to the -20 C (-4 F) that had to be maintained near the elec­tronic bay access door—and keep in mind that aircraft systems them­selves emit a lot of heat Even if the pilot’s head was protected by fresh air sent by special nozzles the canopy was far too hot to touch

The engine bay was insulated by a heat shield made of silver-plat­ed steel Gosplan allocated 5 kg (11 pounds) of silver per aircraft—not a single ounce more The silver was 30 microns thick and its absorption factor was between 0 03 and 0 05 Other metals were tested such as gold and rhodium but they were far too expensive even if their absorp­tion factors were satisfactory. The 5 percent of heat absorbed by this silver-plated steel lining was held in fiberglass blankets to prevent it from escaping toward the fuel tanks. Even coatings made of basalt fibers were tested

All of the big secrets of the MiG-25 are summed up above, and it takes just a few lines On 16 March 1965 the world learned that Fedotov had topped the SR-71 records with a certain Ye-266, this was the some­what spunous designation sent to the FAI authorities to have the MiG – 25 records ratified On twenty-one subsequent occasions, the FAI was notified of record attempts made by the Ye-266 or the Ye-266M In 1993 nine of the records set by the MiG-25 in 1967, 1973 1975, and 1977 still stand