HINDUSTAN FIGHTER HF-24 MARUT
PART I: BUILDING INDIA'S JET FIGHTER
BY K CHATTERJEE
Conceiving the Marut
The pursuit of self-reliance in aviation, led the Air Staff, to issue a requirement for a home-made multi-role fighter aircraft during the mid-1950s. The building of HF-24, or Marut (Spirit of the Tempest), as this aircraft was to be called, was the first attempt of its kind anywhere outside the major powers. At the time of the Marut's conception, the domestic aviation industry's only design experience amounted to the HT-2, a prop trainer. Whatever aircraft manufacturing capability existed resulted from the license production of the Vampire FB Mk.52s and T Mk.55s. To have considered building a Mach 2.0 capable aircraft, given such limited capabilities bordered on audacity.
The Marut was conceived to meet an Air Staff Requirement (ASR), that called for a multi-role aircraft suitable for both high-altitude interception and low-level ground attack. The specified performance attributes called for a speed of Mach 2.0 at altitude, a ceiling of 60,000 feet (18,290 m) and a combat radius of 500 miles (805 km). Furthermore, the ASR demanded that the basic design be suitable for adaptation as an advanced trainer, an all-weather fighter and for 'navalization' as a shipboard aircraft. It was directed that this aircraft be developed within the country. As an aside, it might be worth noting that the design philosophy and ASR for the current Light Combat Aircraft (LCA) is quite similar.
The task of meeting the ASR was assigned to Hindustan Aircraft Limited (now HAL Bangalore). However, in 1955 no infrastructure capable of supporting the programme existed in India. And there was scant appreciation on the part of the government of the technological hurdles that would need to be surmounted. In response to an Indian invitation, Kurt Tank (of Focke-Wolf fame) and his assistant Engineer Mittelhuber, arrived in Bangalore in August 1956. As head of the design team it was Kurt Tank who would give the ASR shape and substance.
Hindustan Aircraft, in 1956, possessed only three senior Indian design engineers and the entire design department boasted only 54 personnel. The prototype shop had 60 people on staff and the entire strength of the production engineering department amounted to just 13. Worse yet, no hangar space was available for the construction of prototypes, no machine shop existed for prototype engineering, and there were no test equipment, structural test rigs or a flight test laboratory.
In fact, the Hindustan Aircraft complex lacked even a suitable runway from which the new aircraft could begin flight testing. The entire infrastructure had to be built from scratch. Much effort was invested in building up a viable design and testing infrastructure. And by the time the first prototype of the Marut commenced its flight test programme in 1961, Hindustan Aircraft employed 18 German design engineers, a design department possessing 150 personnel, a prototype shop with 631 personnel including 39 supervisors, and a production engineering department with more than 100 personnel.
Building the Marut
Work on designing the new "Hindustan (Indian) Fighter" commenced in June 1957 and the aircraft was awarded the HF-24 designation. A full scale representation (wooden glider) of the projected fighter was ready by early 1959, less than a year after the last mock-up conference on 10 April 1958. A test program was initiated with this glider on 1 April 1959 by Wg. Cdr. Suri and Wg. Cdr. (later Gp. Cpt. retd.) Kapil Bhargava. The two-seat glider was towed by a C-47 Dakota and usually released at altitudes of between 12,000 and 15,000 feet (3660 and 4570 m). By the end of this phase of the programme on 24 March 1960, the glider had completed 78 flights. Assembly of the first HF-24 prototype (HF-001) began in April 1960, and eleven months later, on 11 March 1961, powered taxiing trials were initiated.
After a comprehensive three month ground test programme, HF-001, with the late Wg. Cdr. (later Grp. Cpt.) Suranjan Das at the controls, flew for the first time on 17 June 1961. This aircraft's first official flight took place a week later on June 24th, in the presence of the then Defence Minister, V.K. Krishna Menon. By then HF-001 had IAF roundels applied to it and had been assigned the serial number BR 462. By November 1961, a structural test airframe had been completed and was subjected to extensive structural and functional tests in rigs designed and fabricated at Bangalore. On 4 October 1962, a second prototype (BR 463) joined the flight development programme. The flight development team was headed by Wg. Cdr. Das who in turn was assisted by Sqn. Ldr. I.M. Chopra, largely responsible for stability trials, Sqn. Ldr. W.M. Tilak for armament and instrumentation development, and Sqn. Ldr. Dey whose concern was primarily with power plant development.
Search for a Suitable Engine
The design of the HF-24 had been based around the availability of the 8170 lbs. (3705 kg) afterburning Orpheus BOr 12 engine. Unfortunately, the British requirement for this powerplant was discarded and the Indian Govt. declined to underwrite its continued development. In retrospect, this was a very shortsighted decision on the part of the Indian Government. The manufacturer had asked for £13 million as development costs, not a large sum even by the standards of the 1960s. And the Government's decision not to underwrite the costs of the BOr 12 development was to haunt the Marut programme for ever. In the even that the BOr 12 was no longer an option, the design team was forced to adopt the non-afterburning 4850 lbs. (2200 kg) Orpheus 703 for the initial and interim version of the fighter. India now initiated what was to prove a lengthy and frustrating search for an alternative power plant to the Orpheus BOr 12.
In 1961 the MoD approached the Soviet Government with a view to the acquiring the Tumansky RD-9F, a small-diameter axial-flow afterburning engine used in the MiG-19SF fighter. Six RD-9Fs were imported late in 1961 and bench-tested at Bangalore. Discussions were held in Moscow during July 1962 over licence manufacture of this turbojet for the HF-24. In the end this came to nothing. The RD-9F was finally rejected in 1963 on the grounds that it was prone to surging. Furthermore, its overhaul life was unacceptably short and there seemed little likelihood of its compressor being developed beyond its Mach 1.4 stress limit, and negotiations with the Soviet Union were accordingly discontinued.
Rather than shelve the entire Marut programme, the Indian Government decided to order 18 pre-production aircraft powered by the Orpheus 703 in late 1962. The pre-production batch was followed by 62 similarly powered production examples, despite the lAF's initial reluctance. The IAF's reluctance stemmed from the fact that Orpheus 703 powered Marut offered only marginal improvement on the Hunter's performance. And the air force's reluctance contributed to the aircraft's protracted delivery schedules.
After the Tumansky RD-9F was rejected, the E-300 turbojet, designed under Egyptian Government contract by Ferdinand Brandner (an Austrian repatriate from the Soviet Union), was considered. The E-300 was a relatively simple lightweight engine with a nine-stage compressor and a two-stage turbine. The engine was expected to afford 10,580 lbs. (4800 kg) thrust with 40% afterburning augmentation. In 1962 it was believed that a version with a smaller afterburner, the EL-300 affording 9240 lbs. (4355 kg) thrust, could be developed for the HF-24.
So on 2 November 1964, a collaboration agreement was signed in Cairo. Twenty months later, in July 1966, the Indian Government furnished a specially modified pre-production Marut, designated HF-24 Mk 1 BX, to participate in the Egyptian engine development programme. This aircraft had a modified fuselage capable of accepting either the Orpheus 703 or the EL-300. Test flying of the HF-24 Mk 1 BX with one E-300 and one Orpheus began at Helwan, Egypt, on 29 March 1967 with Sqn. Ldr. I.M. Chopra at the controls. Until the EL-300 program came to a standstill after the June 1967 Arab-Israeli conflict, the HF-24 Mk I BX had completed 106 hours of flight testing in 150 flights. The EL-300, unfortunately, failed to live up to earlier expectations and with two power plants of this type installed, the HF-24 Mk 1 BX was unable to exceed Mach 1.1.
The Indian Government eventually became disenchanted with the entire programme and on 1 July 1969, the Indian test team was recalled from Egypt. The test aircraft, however, was presented to the Egyptian Govt. Meanwhile Bristol Siddeley proposed in 1964 to marry the high-pressure inner spool of the Pegasus to the Indian-manufactured Orpheus 703, thereby theoretically offering a performance similar to that anticipated from the abandoned Orpheus BOr 12. But the Indian Govt. was again reluctant to underwrite development costs. The issue was also somewhat confused by Indian hopes that the US government would provide support for the HF-24 programme. In 1964 U.S. technological aid was formally requested to help with the development of a suitable power-plant for what was to be the HF-24 Mk 2. In the event, the possibility of US aid was delayed by Indian negotiations with the Soviet Union for a MiG-21 manufacturing licence and was finally abandoned as a result of the Indo-Pakistan conflict in September 1965.
Marut joins the IAF
The first pre-production HF-24 Mk1 (BD-828) made its initial flight in April 1963 and was joined by BD-829 and BD-830 within the year. Two of the pre-production Maruts were handed over to the IAF at a ceremony on 10 May 1964 at Bangalore and taken over by the IAF's Aircraft & Armament Testing Unit (AATU). Joined by more pre-production Maruts, these aircraft underwent service and weapon system trials (the latter at the Armament Firing Wing at Jamnagar) for nearly three years before being suitably updated.
No.10 Flying Daggers Squadron which re-formed on 1 April 1967 became the first unit to be equipped with India's first indigenous combat aircraft. Of the 18 pre-production Maruts, three were retained by HAL for equipment and avionics development work. Two more were retained as test-beds for an experimental reheat system developed be the Gas Turbine Research Establishment at Bangalore. One became the HF-24 Mk 1 BX. The remaining 12 were handed over to the IAF. Close liaison between the IAF and the Hindustan Aircraft developed during the Marut' s service evaluation. The aircraft was progressively modified after it became clear that the IAF intended to employ it, in the lo-lo attack role.
As part of the modifications for the the ground attack role the planned Ferranti AIRPASS radar and provision for AAMs were deleted. The target ranging radar was also deleted and the Ferranti ISIS (Integrated Strike and Interception System) two-axis rate gyro gun-sight was standardized. About 1800 test flights had been completed by the time the first series production Mk1 aircraft was flown on 15 November 1967.
The Marut entered operational service with a conventional all-metal semi-monocoque fuselage area-ruled in the region of the wing trailing edge. The wings were of conventional torsion-box construction and carried hydraulically-actuated ailerons and trailing-edge flaps. Provision was made for manual reversion of both the ailerons and elevators, the rudder being manually operated at all times. The variable-incidence tail-plane was hydraulically operated with electrical back-up, and in the event of a hydraulic failure the trimmer switch actuated the electrical system and the correct degree of incidence was set manually. The hydraulically-operated cheese-type air brakes were mounted in the lower fuselage aft of the main wheel wells.
The engine air intakes, which feature non-adjustable shock cones, fed two Orpheus 703 turbojets which were installed side-by-side in the rear fuselage. Internal fuel capacity is 654 Imp gal (2 962 1) and was housed by a main fuselage collector tank, a wing center-section supply tank and two integral wing tanks. The pilot was accommodated on a Martin-Baker Mk S4C zero-altitude ejection seat beneath an aft-sliding blister canopy.
The cockpit was pressurized to a differential of 3.5 lb./sq in (0.25 kg/cm2) between 24,000 and 40,000 feet (7315 and 12,190 m). While much of the instrumentation was of British origin, it also included items from France, Sweden and the USA. Standard equipment included DFA 73 D/F, TA and RA Bendix receiver and 12-channel VHF system. Two VHF antennae were located in the fiberglass-reinforced dielectric fin tip and the radio compass was housed in the fiberglass dorsal airing.
The armament comprised 4 powerful 30mm Aden Mk.2 cannon with 130 rds/gun and an internally-housed MATRA Type 103 rocket launcher modified to accommodate 50 68-mm unguided rockets in 10 rows of five rockets each. The aircraft had four underwing hardpoints each stressed to carry 1000 lbs. (454-kg) bombs, napalm canisters, Type 116 SNEB rocket launchers, clusters of T10 air-to-ground rockets, or 100 Imp Gal (454 L) drop tanks. When production ceased, the Marut had around 80% indigenous content and all components were being manufactured by Hindustan Aeronautics.
During the early years Maruts with the IAF suffered from one major problem, namely servicability - resulting mainly from the non-availability of spares. These chronic shortages affected the Marut fleet between 1965 and 1968, however as production picked up the situation improved markedly. But the aircraft had teething troubles that were not solved until 1970, and only a very meticulous reporting of problems, and the professionalism of the pilots and engineers, prevented any fatalities from occurring.
Brian De Magray, from No.10 Squadron, recalls that gun vibrations were so excessive that the gun-sightings had to be harmonized after every two gun-firing sortie. Vibrations during four-gun firings also caused the canopy to jettison. Wg. Cdr. Tilak lost his canopy twice. The reason for this was that the external jettison switch located in the nose was often electrically activated during trials. This problem was fixed before the Maruts found themselves at war, much to the relief of the pilots.
There is wide consensus about excellent handling characteristics of the aircraft. Most pilots who have flown the aircraft describe it as pleasant to fly and excellent for aerobatics with fine control responses. And its ability to out-accelerate the Hunter led one pilot to describe the Marut, with undisguised affection, as the Hunter Mk.II ! The Marut offered a stable gun platform and packed a formidable punch. While the Marut's pilots expressed an understandable desire for more thrust than the Orpheus 703 offered, they were unanimous in their view that the aircraft proved itself a thoroughly competent vehicle for the low-level ground attack profile.
The Marut was a robust aircraft with extremely good visibility for the pilot, and was aerodynamically one of the cleanest fighters of its time. With a very stable platform, the aircraft required no artificial augmentation or auto-stabilization. The Marut was originally to have been stressed to 10.5g but 8g was eventually considered to be adequate for the production model. The controls were provided with artificial feel and were effective over the entire speed range, the top limit being 620 knots (1149 km/in) IAS at sea level.
Throughout the December 1971 hostilities, the Marut squadrons enjoyed extremely high serviceability rates (in contrast to the late 1960s), this undoubtedly owed much to an improved spares situation and the original design's emphasis on ease of maintenance. It should also be noted that from January '71 onwards, an improved version of the Marut with a lengthened wing cord (giving it greater wing area and hence greater lift), numerous cockpit changes and a sophisticated ISIS gunsight, started entering squadron service.
Maruts constantly found themselves under heavy and concentrated fire from the ground during their low-level attack missions. On at least three occasions, Maruts regained their base after one engine had been lost to ground fire. On one of these, a Marut returned to base without escort on one engine, from about 150 miles (240 km) inside hostile territory. On another occasion, Wg. Cdr. Ranjit Dhawan, flying his Marut through debris that erupted into the air as he strafed a convoy, felt a heavy blow in the rear fuselage of the aircraft, the engine damage warning lights immediately glowing and one engine cutting. Fortunately, the Marut attained a safe and reasonable recovery speed on one engine. Consequently, Dhawan had no difficulty in flying his crippled fighter back to base. Another safety factor was the automatic reversion to manual control in the event of a failure in the hydraulic flying control system, and there were several instances of Maruts being flown back from a sortie manually.
Conversion to the Marut was a straightforward affair and future pilots received 10 hours ground instruction before first flight. Most pilots converting to the Marut came from Mystere, Hunter or Gnat units and until the HF-24 Mk.1T (two-seat conversion trainer) became available in 1975, had to check out on the Hunter T Mk.66. The Marut eventually equipped three IAF Squadrons. No.10 Squadron was the first to convert in April 1967, the No.220 in May 1969 and the No.31 in March 1974. Of the 145 Marut produced, 130+ entered squadron service.
Given the limited number of Marut units, most Marut squadrons were considerably over-strength for the duration of their lives. According to Brian de Magray, at peak strength No.10 Squadron had on charge 32 Maruts! Although the squadron probably did not hold a unit-establishment of more than 16.
With the completion of Kurt Tank's tenure in 1967, responsibility for Marut development had passed to S.C. Das and an all-Indian team which produced the Mk.1T tandem two-seat trainer. The two prototypes of the trainer (BD 888 and 889) were the 46th and 47th Marut airframes, and the first of these was flown on 30 April 1970 by the then chief test pilot, Wg. Cdr. R.D. Sahni.
The essential difference between the single and two-seat versions of the Marut was the removal of the MATRA rocket pack featured in the former to provide space for the second cockpit. The minimal airframe changes required for the Mk.1T resulted in low development costs and almost total spares interchangeability. The second prototype Mk.1T was flown in March 1971, and the first of these entered squadron service in early 1975.
The search for a suitable engine continued even after the Maruts went full ops. In September 1966, the MoD announced that flight testing had begun on the third pre-production aircraft (as HF-005) designated Mk.1A with an afterburning Orpheus 703 with an 18% greater boost than the original at 5,720 lb. (2 595 kg). By 1970, two more Maruts, designated Mk.1R, were brought into the afterburner development trials. Unfortunately, the programme suffered a severe setback when, on 10 January 1970, the first of two Mk.1R prototypes (HF-032) being flown by Gp. Cpt. Suranjan Das crashed just after take-off.
India's foremost test pilot was the unfortunate victim of this crash. At the time it was rumored that one of the engines had completely failed and that there may have been a partial failure of the second engine. However, the official inquiry attributed the accident to malfunction of the canopy locking system. The Mk.1R prototype had been fitted with a hinged clamshell-type canopy in place of the earlier sliding canopy, and the failure of the locks and the sudden opening of the canopy, resulting in rapid decay of speed at a critical stage, proved fatal.
This set the programme so back, that the final stages of the flight test programme, using the second Mk.1R (BD 884), were achieved only in 1973. This airframe had a modified wider aft fuselage. The Orpheus 703 afterburning system had progressed to provide a 27% boost, giving 6160 lbs. (2 794 kg) of thrust, but the performance increment that it provided the Marut was insufficient to result in a production order for the Mk 1R. By the late 1970s, HAL entered into discussion with Rolls-Royce about using the Turbomeca Adour twin-spool after burning turbofan to power the Marut. The projected Adour-powered fighter was designated as the Marut Mk 2.
The Rolls-Royce RB.153 was considered for a while, but Hindustan Aeronautics was neither able to accept the terms of the proposed contract nor, at the time, was ready to consider the major redesign of the fuselage that adoption of the RB.153 would have entailed. In the event that by the early 1980s, the Air Staff requirements for a TASA (Tactical Attack and Strike Aircraft) and a DPSA (Deep Penetration and Strike Aircraft) were fulfilled by foreign aircraft, the need for a upgraded Maruts became somewhat superfluous. And by the mid-1980s enough Jaguars and MiG-23BN/27s were joining the IAF, that the Marut programme no longer remained viable.
No.10 Squadron gave up its Maruts in August 1980 and by the following year enough MiG-23BNs were available to allow No.220 Squadron to begin conversion. The last unit to give up its Maruts was No.31 Sqn, whose aircraft were finally withdrawn in mid-1990. And so ended the saga of India's first, and until the LCA flies, India's only home- grown fighter programme. In retrospect, despite its audacity, the Marut progamme helped lay the infrastructure for an Indian aviation industry.
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