In the mid-sixties, the CIA and the US Air Force received the latest A-12 and SR-71 reconnaissance aircraft. These machines, unified in terms of the main part of the units, were distinguished by extremely high flight performance, which made it possible to effectively solve the main tasks. However, reaching the maximum speed at the level of M = 3, 3 and a flight altitude of more than 25 km turned out to be an extremely difficult task, which required fundamentally new design solutions and technologies.
Circle of problems
The development of the A-12 and SR-71 projects was carried out in the Lockheed division with the unofficial name Skunk Works. The creation of new aircraft started with research and development and the search for optimal technical solutions. At this stage, it was established what problems the "three-fly" aircraft is facing. Then the search for suitable technologies began.
Aerodynamics became one of the central issues. Flying at a speed of approx. M = 3 has its own characteristics, which must be taken into account when forming the appearance of the aircraft. However, reaching such speeds was also difficult. This required special engines that could work equally effectively at all speed modes.
At the required flight speeds, the problem of thermal loads should have fully manifested itself. It was required to protect the glider from overheating, deformation and possible destruction. With all this, the aircraft had to be distinguished by high strength, since at operating speeds even the simplest maneuvers were associated with overloads.
A separate requirement concerned the aircraft's visibility to the enemy. By that time, the leading countries had managed to build a developed radar network for airspace control, which made the issue of reducing radar signature urgent. This problem had to be taken into account when developing the airframe.
Finding solutions to expected problems has proven difficult and slow. Work on the A-12 aircraft for the CIA started in 1957 and continued for several years. During this time, the general concepts and approaches to the design have changed several times. The first flight of the prototype aircraft was completed only in 1962. The SR-71 reconnaissance aircraft for the Air Force was developed on the basis of an already finished vehicle, which made it possible to significantly speed up the work.
Special glider
The solution to the main part of the expected problems was directly related to the design of the airframe and general aircraft systems. After a long search, it was possible to find the optimal version of the aerodynamic appearance. The "tailless" scheme was considered the best one with developed inflows in the bow and central part of the fuselage and a pair of keels. The applied scheme made it possible to obtain high lift and improve the flow at all speeds. In addition, the bending moment in the bow sharply decreased.
The special contours of the airframe made it possible to partially scatter signals from the radar. In some parts of the airframe, where the design allowed it, there were parts made of radio-absorbing materials. However, reducing the visibility was not the main task of the project, and there were other factors that partially neutralized all design achievements in this area.
The issues of thermal protection, weight and strength were solved with the help of titanium and its alloys. The glider consisted of 85% of them. Other parts were made from heat-resistant steels, ceramics, etc. The glazing of the cockpit canopy was made of quartz glass. For mechanical and thermal strength, it was connected to the airframe using ultrasonic welding.
According to calculations, during the flight, the average temperature of the skin should have reached 260 ° C, the maximum at the leading edges - up to 400 ° C. In this regard, numerous pipelines were provided in the airframe for circulating fuel, removing excess heat and preheating the fuel.
The titanium structure retained its strength when heated - but changed dimensions. At cruising speed, the A-12 and SR-71 lengthened by several inches. This problem was taken into account in the design and provided for special gaps in the skin, internal structures and even in the fuel system. As a result, fuel literally oozed out of the plane on the ground, but after acceleration the leak stopped. Also, part of the cladding was made of corrugated sheets.
Record engine
The A-12 and SR-71 aircraft used the unique hybrid engines of the JT11D / J58 family from Pratt & Whitney. Their design combined turbojet and ramjet engines with the possibility of joint or alternating operation. Maximum thrust, depending on modification, 20-25 thousand pounds; afterburner - 32.5 thousand pounds.
The core of the J58 engine was a turbojet unit housed inside a ramjet unit with a fan assisted. The air intake was equipped with a movable central body, and there was also a set of hatches and flaps to control the incoming flow. The air intakes were controlled in accordance with the flight modes using a separate computer.
At sub- and supersonic speeds, the air intake cones were in the forward position and optimized the flow at the engine inlet. With an increase in altitude and speed, they were displaced backward. At speeds above M = 3, the air flow was divided between the ramjet and turbojet engines, which created 80 and 20 percent. thrust, respectively.
The J58 engine used JP-7 special jet fuel based on kerosene. Under normal conditions, it was distinguished by an increased viscosity, but when heated, it did not differ from standard compositions. The fuel was also used as part of the cooling systems for the skin, cockpit, instrument compartments, etc. It was used as a working fluid in nozzle control hydraulics. The heated liquid immediately entered the engine and burned out.
The engine was started by means of the injection of the so-called. The starting fuel is liquid triethylborane (TEB), which ignites on contact with air. Each J58 had its own TEB tank for 16 engine / afterburner starts. The motors used a special silicone grease optimized for high temperatures. At temperatures below zero Celsius, this composition hardened, which made it difficult to operate the equipment.
Big price
The Skunk Works department and related enterprises have successfully solved all the tasks and created aircraft with unique high flight characteristics. However, this took several years and significant financial expenses, and the resulting aircraft were distinguished by high production costs and operational complexity.
The development of the project and the search for all the necessary technologies took several years. The launch of production was also associated with certain problems. For example, in a memoir by Skunk Works chief Ben Rich mentions the difficulty of obtaining titanium. The United States did not have such raw materials, which is why it had to arrange a whole operation to purchase it from the USSR through shell companies.
In the interests of the CIA, 15 aircraft of the main modifications were built. The Air Force received 32 units. The contract with the Air Force provided for the cost of one SR-71 at the level of 34 million dollars (more than 270 million at current prices), and the production program turned out to be a record expensive for its time.
The operation also turned out to be difficult and expensive. Preparation for the flight took several days. After each flight, the aircraft required 650 different checks and procedures that took several hours. After 25, 100 and 200 hours of flight, a thorough inspection with partial disassembly was required, for which several working days were allocated. The engines were sent to the bulkhead after 200 hours of operation, and after 600 hours - for overhaul.
Shortly before the decommissioning of the SR-71, it was openly reported that the flight hour of such an aircraft costs approx. 85 thousand dollars. The operation of one machine annually cost at least 300-400 million.
Nevertheless, the CIA and the Air Force received a special tool with the highest performance. A-12 and SR-71 could operate at altitudes of at least 25-26 km and develop speeds up to M = 3, 3, which for many years saved them from the air defense of a potential enemy. During the operation, the CIA lost 6 of its A-12s, the Air Force - 12 SR-71 units. At the same time, there were no combat losses.
Technological breakthrough
Operation of the A-12 aircraft lasted only a few years - until 1968, the Air Force used its SR-71 until 1998, and NASA wrote off the equipment a year later. The aircraft of two models and several modifications, having a special design based on advanced technologies, could show outstanding tactical and technical characteristics. However, for the same reason, they were prohibitively expensive and complex. By the time they were abandoned, more convenient and effective means of reconnaissance appeared.
A direct replacement for the A-12 / SR-71 never appeared - the niche of reconnaissance aircraft was long and firmly occupied by spacecraft. As a result, new models of aviation technology with comparable characteristics have not yet appeared in the United States. However, the projects of high-speed aircraft from Skunk Works have created a serious scientific, technical and technological basis for the further development of military and civil aviation. Some solutions proposed in the past are still actively used.
