An engine for fast, high-altitude MiG-25 aircraft. They were developed in the 1960s.
R-15
AMNTK Soyuz (Tumansky), Russia (development)
OKB of A. A. Mikulin, Russia (development)
MMPP Salyut, Russia (production)
The single-shaft R-15 engine was developed specially for flight at high altitude and at a speed exceeding M=2.5. The basic variant of the engine was designed by OKB-300 of S. K. Tumansky, further development was continued in cooperation with OKB of A. A. Mikulin. Initially, it was intended for use with the strategic nuclear cruising missile Tu-121 with a range of almost 4000 km. This missile was first launched in August 1959, but next year the project was abandoned, because the main interest was shifted to ballistic missiles. Specially for this type of devices, the 15K propulsion plant (also referred to as R-15K-300 or KR-15-300) with afterburner thrust of 10000 kp was developed. The first engines produced in series were mounted into “giant” reconnaissance drones Tu-123 Jastreb (DBR-2). These originated by a modification of the Tu-121 missiles in 1960-61, were deployed on 23rd May 1964 and 52 of them had been produced until 1972. Regarding the thermodynamic processes, the R-15 was close to scramjets. This was corresponded by the supercharger outlet pressure, which was under static conditions very low even for that time period. The supercharger compression was compensated by scramjet compression and by a special uncontrollable nozzle. Because of this, the thrust at high altitudes and speeds of about M=2.5 was significantly higher than the one of usual engines – it was close to 20 tons (almost 200 kN). Long lifetime was not expected, and the guaranteed service time without failure was determined to be only 50 hours. When related to the Tu-121 missile, the guaranteed service time is sometimes stated at only 15 hours, while the engine was able to run continually on full afterburner for 3 hours.
In 1957, a prototype of the E-150 fighter plane was designed on drawing boards. The use of the previous engines, modified and designated as R-15-300(M), was planned. It took 18 months to finish the engines, and the first E-150 prototype did not take off until 8th July 1960. Beginning in 1962, the second E-150 prototype tested the upgraded R-15M-300 engines with improved high-altitude performance. The direct successor of the E-150, the E-152, was flown since 21st May 1961, also with one R-15-300(M) engine (attention, the E-152A had two R-11F-300 engines). In the FAI system, the plane was referred to as E-166, and the engine as R-166. The lifetime already became a difficulty – service time of only several dozen hours only sufficed for ground tests and trial flights, but for mass-produced aircraft, the engine had to be tuned up and much more reliable. Even though, the R-15-300 engines and modifications collected on the E-150, E-152 and E-152M planes a significant amount of flying hours, including flights at high altitudes and at high speeds. The E-151, the armed variant of the E-150 driven by the same engine, was only built as a mockup.
The R-15, as the only available engine with the required parameters, was chosen for propulsion of the E-155, the MiG-25 prototype. To tell the truth, it was not the only engine of this performance level. The Rybinsk design bureau led by P. A. Kolesov designed and tested even more powerful RD17-16 engines at the same time. However, only a handful of these engines were built, and it was assigned for use with the M-52 supersonic bomber, which, in the end, has never flown. Lyulka also worked on an engine of the same thrust class, but the project was only on the drawing board at that time.
For the use with the E-155 aircraft, the R-15B-300 (Izdeliye 15B) variant was developed, with a number of design changes in comparison to the original 15K engines from the drones and the R-15-300 engines from the test planes. In a short time, the designers performed a replacement of the original simple hydromechanic fuel intake control system, optimized for operation at a constant thrust, by a fully governable electronic system (for the first time on a Soviet combat aircraft), type RRD-15B with a spare mechanic system. The work on the electronic governor was led by S. Chiekunov. The problem of the original hydromechanic control became evident already with the E-150 and E-152 planes, where the fuel flow rate was often being changed in a wide range of 150 – 15000 kg per hour. The system was not designed for so different flow rates. For the E-155, the accuracy of RPM control of 0.2 % was required, and furthermore, the system had to be bound with air inlet governor.
The afterburner chamber was the biggest one that has been built by that time. Furthermore, a new, three-position nozzle had to be designed. The five-stage supercharger was also modified, less tending to **** (pumpaz – don’t know what it is, sorry - Utchoud), and the tube-annular combustion chamber, now working at a 50°C higher temperature was tuned for operation at a higher altitude. The turbine has one stage. The special T-6 (T – Toplivo) kerosene with high boiling point is used as fuel. The fuel can be replaced by T-7P and RT (Reaktivnoye Toplivo). Each engine has its own fire detector with ionization sensors and a chlorine-fluorine-carbon (CFC) extinguisher. The RPM at maximum power were 7000.
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The rear edge of the engine has a respectable diameter of almost 150 cm
The final version of the R-15B-300 and especially its electronic control system was tested intensively in 1963-64 under the Tu-16LL flying laboratory first. The results, however, did not provide all the necessary information, because the flight conditions offered by the T-16 plane were far of those the engine was optimized for. The first R-15B-300 only had a lifetime of 25 hours! The E-155 took off for the first time in March 1964, and in 1965-75 the planes of this type established 21 world records. In the FAI system, the plane was referred to as E-266 and the engine as R-266, that everything in order to confuse the “western” enemy. The lifetime of the engines of the first serial MiG-25 aircraft was designated as 150 hours, for the following series it was increased to 750 hours of operation. Continuous afterburner operation was initially set to 3 minutes. Later it was increased to 8 and then to 40 minutes. The nozzles are in their rear parts turned slightly toward each other due to aerodynamic purposes, and are so close to each other that their profiles blend together. Therefore, 3 segments had to be removed from each nozzle and replaced by a separating block, attached onto the fuselage. Because of significant heating of the engine surfaces, the aircraft had to be equipped with a high-quality thermal shielding. The engines were used on older MiG-25 aircraft and they proved to be sufficiently reliable even under demanding climate conditions ranging from cold Russia to hot Egypt. The designers of the MiG-25 were finally awarded the Lenin prize, and Fedor Shukhov got it for the R-15B-300 propulsion plant.
The unsuccessful T-37 aircraft project was also based on these engines. The twin-engined Tu-128 2R-15B-300 interceptor, as well as the four-engined Tu-125 supersonic bomber, also remained in the project stage.
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Engine mounting and placement of fuel tanks with a capacity of 15 tons of special kerosene in the MiG-25 aircraft.
However, the R-15B-300 engines always restricted the MiG-25 performance (cruise speed, range, ...). In flight above M=2.8 they had a tendency to uncontrollable over-revving, they overheated and could have caught fire. After M=3.2 flights (e.g. over Israel at that time) they had to be replaced. Soon after the MiG-25 was deployed, more powerful and more economical propulsion plants were demanded. This could have been achieved even by a not very drastic change of the already available engines.
The new R-15BF2-300 (Izdeliye 65M) by Shukhov and Rotmistrov designers had one more supercharger stage added, and new materials used in the combustion chamber and in the turbine allowed operation at higher temperature. While the outer dimensions were maintained, the compression ratio was increased from 4.75 to 4.95, and fuel consumption was lowered. Thrust increased by about a quarter and would have allowed flights at 3500 kph. Since 1973, the engines were tested on two E-155M (MiG-25M) planes, which achieved several records in climb rate and ceiling. Although the increase of aircraft performance was proved and the engines were fully exchangeable with the previous versions, the serial production of the R-15, lasting for over 10 years, would have been unnecessarily complicated. However, the main issue was that the USSR began changing priorities at that time and extreme speed was no longer as important as before. Furthermore, Koliesov’s PS-30F (D-30F) engines for the planned MiG-31 offered similar performance, but had significantly lower fuel consumption.
The last version produced in series was the R-15BD-300 with modified equipment (gearbox, ...), lifetime designated to be 1000 hours and with “equal” performance as its predecessor (see the note below the chart of technical data). The engine was mounted to new series of MiG-25 beginning in the late 1970s, namely at least to these versions: PD, BM and conversions of older aircraft to the versions PDS and RBF. The older MiG-25 planes, whose engines had reached the end of their service time, were already becoming the new R-15BD-300 engines. The later series of the R-15BD-300 had a slightly higher thrust.
One more engine variant was to originate, the R-15BV-300 (V – Vysotnyj), with improved power at high altitudes. The engine would have allowed flight at unprecedented speed of M=3.5 (over 3700 kph). In the mid 1960s, the engine was considered in conjunction with the E-155PA (MiG-25PA) interceptor. However, the project was not continued any further.
Table:
Type
Length (mm)
Intake diameter (mm)
Maximum diameter (mm)
Dry weight (kg)
Overall compression
Maximum temperature in front of the turbine (°C)
Maximum temperature behind the turbine (°C) - 820 (in flight), 800 (start-ups)
Maximum thrust (kp)
- with afterburner (kp)
- afterburner alt=15 km, M=1.8
- afterburner alt=11 km, M=2.4
Specific fuel consumption – maximum (kg/kp/h)
- with afterburner (kg/kp/h)
Unless specified, thrust and fuel consumption are at alt=0, M=0.