In the previous parts, we got acquainted with two fairly progressive front-line bomber projects. Both of them were distinguished by their originality, innovative ideas, and were arranged around a pair of powerful AL-7F engines. What was the reason for the failure of the venerable aircraft designers?
Today we already know that of the entire spectrum of military turbojet engines created at that time, with afterburner thrust from 5 to 10 tf, they successfully passed all the troubles of painful refinement and only two became serial: R11F-300 and AL-7F. All other power plants, for one reason or another, did not leave the "infancy" age, or did not give the necessary traction.
Let's analyze the main technical data of R11F-300 and AL-7F. First of all, the closeness of the parameters is striking (the temperature of the gases in front of the turbine is 1175 … 1200 K, the degree of pressure increase in the compressor is 8, 6 … 9, 1, the specific consumption in the non-afterburner mode is 0.93 … afterburner - 2, 03 … 2, 04 kg / kgf • s). This is not surprising: after all, they belong to the same generation of domestic military turbojet engines. Let us note the second important feature: the afterburner thrust of the AL-7F is approximately twice that of the R11F-300, and the ratio for the non-afterburner thrust is 1.6: 1. The masses of the engines are also approximately 2: 1 (2010 kg for the AL-7F and 1040 kg for the R11F-300).
When the engine was operating at a mode close to the afterburner "maximum", the AL-7F "ate" about 4 tons of kerosene per hour of operation, and two such engines - twice as much. The engine of a supersonic aircraft operated at a mode close to the "maximum" when flying at a speed of about 900 … 1000 km / h.
Thus, to obtain a flight range of about 3000 km on an airplane with two AL-7Fs, it was necessary to burn … about 24 tons of kerosene! Let's estimate the takeoff weight of the aircraft if the proportion of fuel being refueled is 35 … 40% of the bomber's mass: we get 60 … 68 tons. But the total eight tons of thrust is hardly enough to accelerate such a heavy car to 1000 km / h. It is also necessary to add "gas", which means to increase fuel consumption. Thus, a vicious circle is obtained with an almost unlimited increase in the mass of a front-line bomber. In other words, it was simply impossible to satisfy all the requirements of the Air Force for a front-line bomber (in terms of range and flight speed) with two AL-7Fs.
Even less realistic were the requirements for obtaining a supersonic range of the order of 1700 km - after all, two AL-7Fs consumed almost 40 tons of kerosene per hour of operation in this mode! How impressive it looks when comparing this mass with the maximum takeoff weight of the serial Il-28, which was 20 tons. The doubling of the takeoff weight was perceived by the insufficiently competent leadership ("this is not a front-line bomber") as excessive. Meanwhile, neither A. N. Tupolev, nor S. V. Ilyushin did not unreasonably increase the mass and size characteristics of their aircraft - this was led by the stubborn logic of the development of aviation technology. Achievement of a qualitatively new property - supersonic flight speed - had to be paid.
But there was also a second way, well known to Alexander Sergeevich Yakovlev since the war. If it was not possible to increase the engine power, he resorted to his own recipe: he minimized the size of the aircraft, improved its aerodynamics in every possible way, reduced the payload to a minimum, and even sometimes sacrificed strength. Having decided to create his own version of a front-line bomber, A. S. Yakovlev initially relied not on the super-powerful AL-7F engine, but on the Mikulinsky AM-9, which was a further development of the AM-5 engine, which was installed on the Yak-25 long-range interceptor fighter.
The first attempt to create a jet front-line bomber OKB A. S. Yakovlev made it back in the summer of 1954, after completing the construction of the "125B" aircraft. This machine was developed as a variant of the "125" reconnaissance aircraft, and that, in turn, was designed on the basis of the Yak-25M long-range interceptor (the first with this name, in the OKB it had the designation "120M"). During the specified period of time, OKB-300 A. A. Mikulina proposed the AM-9A engine to aircraft designers, which was, in essence, a modification of the AM-5 with an afterburner and an additional compressor stage. The main advantage of the AM-9A was its relatively low weight (700 kg) with a thrust of about 3250 … 3300 kgf in forced mode (for comparison, the VK-1F had a thrust of 3380 kgf on afterburner, but its weight was 1280 kg). Another advantage of the Mikulin novelty was the diameter of the "forehead", which was only 660 mm (for the VK-1F it was almost twice as large). Taken together, these two factors made it possible to achieve a solid increase in speed for an aircraft, the dimensions and aerodynamic shapes of which would be close to those of the IL-28.
But A. S. Yakovlev installed AM-9A engines on the Yak-25M aircraft, which had a much smaller size, a normal flight weight of less than 10 tons and, moreover, had a wing with a sweep angle of 45 °. It was assumed that at high altitude, the car could easily overcome the speed of sound. However, the idea of getting by with "little blood" did not justify itself. Due to the relatively thick wing, the aircraft remained subsonic (more precisely, transonic), moreover, the AM-9A developers faced a number of difficulties and were forced to transfer the refinement to Ufa.
Designers from the OKB A. S. Yakovlev had to return to the time-tested AM-5 engines. The project of a front-line bomber based on the Yak-25 received the code 125B, but was more often called the Yak-125B. The main purpose of the aircraft was to make a nuclear strike against especially important targets in the operational depth of the enemy's defense in conditions of strong air defense. The combat load was placed in a bomb bay located in the middle of the fuselage, in connection with which the chassis underwent significant refinement. The rear support was moved behind the bomb bay, and the front, due to a sharp increase in the load on it, was completely changed, making it two-wheeled. (A similar chassis scheme was used on all subsequent front-line bombers of this design bureau.) Taking into account the experience of working on the scout and the need to install a radio bomb sight, the navigator's cabin was placed in the nose of the aircraft. The view from it was provided by a spherical plexiglass nose cap, eight side windows, and a flat silicate glass window was made from below to work with the OPB-P5 optical bomb sight (the glazing was refined during the tests). The entrance hatch of the navigator's cockpit was located on top. To drop the special cargo, a panoramic radar sight "Rubidiy" RMM-2, an automatic optical sight OPB-11r and a long-distance radio station RSB-70M were additionally installed. The aircraft received special bombardment weapons, RDS-4 product control systems and heating of the bomb bay. In the explanatory note to the draft design of the Yak-125B aircraft, the following was noted: “Thanks to the successful combination of aircraft parameters, it was possible to obtain a project of a light bomber carrying a load of 1300 kg, having a range of 2400 km, having the flight characteristics and maneuverability of a modern fighter and equipped with all the necessary equipment for combat flights in any weather and at night. Designing a bomber based on a serial fighter will greatly facilitate its launch into serial production. " The small size and weight of the vehicle limited the bomber's maximum "payload" weight to 2000 kg, and the normal weight to 1300 kg. The last "non-circular" number is due to a simple consideration - that is how much the domestic tactical bomb RDS-4 weighed.
The experienced Yak-125B, which was supposed to be designated Yak-25B in the series, was tested in 1955. The tests were carried out at once in two stages of the "Program of joint factory and state tests of special equipment of the Yak-25 carrier aircraft (factory code Yak-125B)." The program of the second stage of testing provided for an assessment of the aircraft's capabilities for the transportation and combat use of the RDS-4 product. The first flight under the joint test program was carried out exactly on one of the points of stage II.
Since the Yak-125B project provided for the use of not only RDS-4 bombs, some design changes were made for testing, which made it possible to suspend FAB-1500 bombs in an inert design without fuses. The bombing was carried out from altitudes of 7000-14000 m at flight speeds of 800-1035 km / h. In such flights, the flight data of the aircraft with the bomb load were checked, and the performance of the bomber weapons was evaluated. The test report does not contain data on the accuracy of bombing, but estimates of the behavior of the aircraft on the combat course, as directly affecting the accuracy of bombing, are given. It turned out that when the bomb bay doors were open at medium altitudes, the aircraft had a tendency to roll along the roll and lose speed. According to the crew, bombing from heights close to the practical ceiling (13,500 m) was more comfortable.
In total, 30 flights were performed under the program of stage I. 10 flights were performed under the stage II program, incl. three “shaking” ones with real products and two flights with product simulators to evaluate the operation of the electric heating system of the compartment. In "shake" flights, maneuvering was performed in the aerobatic zone with the performance of accelerations, decelerations, turns and hills with a maximum overload of +4, 5 g. In fact, the maneuvering of the aircraft with a real load in a combat situation was simulated. In general, the aircraft and its special bomber armament passed the tests.
Nevertheless, a number of comments were made on the aircraft and armament. The installation of an ejection seat in the navigator's cockpit was recognized as unsuccessful. The seat could be set in a normal position for bombing. In the normal position, the navigator could not work with the sights, and in the bombing position, work with the side panels was not provided. The navigator on the combat path unfastened the seat belts and moved forward on the seat. This working position ensured work with all devices installed in the navigator's cockpit, but the possibility of ejection was excluded. The principal requirement of the customer, stipulated at the meeting of the mock-up commission, was the possibility of a standard suspension in the bomb bay of conventional bombs of 250, 500 and 1500 kg caliber. OKB-115 did not fulfill this requirement. The bomb bay heating system worked reliably, but the air temperature in its lower part was lower than permissible due to poor thermal insulation of the doors. But all these remarks were easy to eliminate.
In general, Yakovlev made a good light bomber capable of carrying nuclear weapons and at the same time having half the mass compared to the Il-28 with approximately the same normal payload and flight range! It seemed that A. S. Yakovlev worked. But, having reached a maximum speed of 1080 km / h, the plane never became supersonic. And in the mid-fifties of the last century, the military was carried away by the next stage of the race for the specter of speed and, not without reason, believed that all combat aircraft of the next generation were to become supersonic. Therefore, there was no need to talk about the adoption of the Yak-125B and putting it into series. However, A. S. Yakovlev did not insist. Moreover, the accident that occurred with the "125" aircraft during factory tests postponed the transfer of the aircraft to the Air Force Research Institute of the Air Force until May 1955, when some AM-9 problems (however, now it was already called RD-9B) remained behind. It became possible to return to a more powerful power plant and, taking into account the experience gained during the development of an experienced front-line bomber and reconnaissance aircraft, create a vehicle that is really in demand by the Air Force.
