LCA-AF Mk.2's Fuselage Design Details Emerge, Plus BVRAAMs, LRAAMs & ASMs Powered By SFDR

The release of global RFIs on January 19, 2019 by India’s state-owned Hindustan Aeronautics Ltd (HAL) for the supply of three separate aircraft fuselage assembly jigs (one unit of each type) meant for fabricating the front, centre and rear fuselage sections of a single full-scale engineering-test model of the projected LCA-AF Mk.2 multi-role combat aircraft (MRCA) is indicative of what the DRDO’s Bengaluru-based Aeronautical Development Agency (ADA) had proposed back in 2014, and which can be viewed here:

https://pdfs.semanticscholar.org/fd95/a760d06858325ba702bd98d15dfffbede300.pdf

It can thus be safely inferred that ADA has passed on the Design Applicability List (DAL) and Standard of Preparation (SOP) documentation to HAL for the full-scale engineering-test model of the projected LCA-AF Mk.2, which in turn will be used for optimising the design and location of the various mission avionics sub-systems/LRUs and related wiring harnesses, hydraulic pipings, accessories, and skin-level distributed apertures for sensors of the integrated EW suite, inclusive of radar warning receivers-cum-jamming emitters, laser warning receivers, and missile approach warning systems.

Simply put, jig-less assembly aims at reducing or eliminating the need for product-specific jigs during airframe assembly by developing new assembly concepts, models, tools and procedures. In the new concept of jig-less aircraft assembly, the end locators are replaced by transferring the holes directly on to the part. These holes are made by high-precision machines. In this approach, all the parts will have at least two holes so that one part can be assembled with the adjacent part. These holes are termed as ‘key holes’. In this method, the tooling elements (end locators) are eliminated and only the jig is used as the main assembly structure. This approach reduces the tool manufacturing time, reduces the product-dependent fixture and also increases the accuracy of the assembled product. It also results in a reduction of the number of tools and joints (rivets), and subsequently weight as well. And going hand-in-hand with jig-less assembly is the practice of using modular tooling solutions.

The most common tooling technology for aircraft assembly used today is conventional tooling consisting of steel beams that are welded together. Such toolings are tailored for a specific tooling operation. Since conventional tooling is designed to a specific application, each assembly has its own dedicated tooling, or product-specific tooling. In addition, when building a complex product like a MRCA, the final design is forced by changes that immediately affect the tooling design. This necessitates modifying the tooling/ locating holes or shimmed pick-ups to new locations. Hence, the preferred solution today is modular tooling, which is built for a dedicated purpose as well, but the surrounding system and distance supports are designed from a toolbox of modular components. The framework of modular tooling is screwed together. It thus helps in reducing the cycle-time of assembly, thereby enhancing the production rate. Modular tooling also helps to design jigs/ fixtures that will meet a group of components.

The front fuselage assembly jig for the LCA-AF Mk.2 will be made up of various modules like the composite radome, floor assembly, canopy, windscreen, starboard and portside air-intakes, twin canards, starboard and portside wing leading-edge root extensions. The assembly will comprise approximately 1,300 individual parts, out of which 40% are estimated to besheet metal, 30% being machined parts and the remaining 30% being made of carbon-fibre composites (CFC). The length of the assembly is 6,100mm, with a maximum width of about 1,100mm. The assembly will comprise 23 stations, and the entire assembly will weigh some 500kg. The component assembly will comprise the following attachment points: 1) Bulkheads for cockpit avionics 2) Nose landing gear pivot 3) Nose landing gear jack-point 4) Nose landing gear up-lock 5) Windscreen 6) Canopy 7) Wing leading-edge root extensions 8) Air intakes 9) Radome 10) Starboard side fixed air-to-air refuelling probe 11) Longerons 12) RLG-INS and GPS receiver mounting structure 13) Cockpit floor 14) Shear walls 15) Inclined bulkhead 16) Doors/hatches 17) Canards 18) Mooring points 19) Hauling points 20) Ejection seat brackets 21) Symmetry check-points 22) System installation brackets 23) Air data probes: Nose air data probe (NADP)/side air data probe (SADP)/Angle-of-attack/Angle-of- Side-slip (AOSS) locating points 24) Equipment bay housing the environmental control system (ECS), digital flight-control computer (DFCC), mission avionics, and an on-board oxygen generation system (OBOGS).

The centre fuselage assembly jig will be made up of modules such as: air-ducts and side-skins, spine, main landing gear bay, shear wall, fuel tanks, and a dividing wall structure. The assembly will comprise approximately 1,600 parts, out of which 40% will be sheet metal, 30% will be machined parts and remaining 30% are to be made of CFCs. The length of the centre fuselage is about 4,500mm with a maximum width of some 2,100mm. The centre fuselage assembly will comprise 15 stations and the it will weigh about 900Kg. Since the centre fuselage section needs to be coupled with the front and rear fuselage sections, the inter -changeability (ICY) media are critical. The centre fuselage assembly will comprise the following: i) Bulkheads 2) Trouser duct 3) Circular duct 4) Side-skin structure 5)Top-skin structure 6) Fuel Bay side-walls 7) Longerons 8) Shear walls 9) Doors and hatches 10) Fuel tanks 11) Main undercarriage 12) Spine structure 13) Wing pick-up points 14) Landing gear doors 15) Gun-bay structure (meaning the Gryazev-Shipunov GSh-23 cannon has been repositioned to the starboard side of the upper centre fuselage) 16) Lifting points 17) Other systems installation inter-changeability requirements 18) Symmetry points.

The Rear fuselage assembly jig will be made up of modules like the engine bay doors, spine and fin attachment, and brake parachute. This assembly will comprise approximately 900 parts, out of which 30% will be sheet metal, 40% will be machined parts and remaining 30% will be made of CFCs. The length of this section will be about 3,300mm with a maximum width of about 1,800mm. The rear fuselage assembly will comprise eight stations and the total shell-weight will be some 400Kg. The rear Fuselage needs to be coupled with the centre fuselage and hence the ICY media are critical. The assembly will include the following: 1) Bulkheads 2) Shear walls 3) Floors 4) Engine mounts 5) Doors 6) Wing attachment points 7) Covers/Hatches 8) Fin attachment points 9) Spine structure 10) Hauling attachment 11) Tie beam 12) Shroud 13) Trailing-edge extension 14) Elevon inboard actuator attachment 15) Symmetry points.

OEMs expected to respond to the RFIs will likely include Airbus Military Aircraft, BAE Systems, Leonardo Group’s Alenia Aeronautica subsidiary, Boeing, Dassault Aviation, Lockheed Martin, SaabTech and Russian Aircraft Corp. Subject to a contract award taking place by the year’s end, the three different assembly js will begin arriving at HAL by 2022, following which another three years will be required for optimising the design and location of the various mission avionics sub-systems/LRUs and related wiring harnesses, hydraulic pipings, accessories, and skin-level distributed apertures for EW sensors. Consequently, only by late 2026 will be it possible to roll out the first flying prototype of the LCA-AF Mk.2. Add to this 2,000 hours of flight-tests with four flying prototypes over a period of three years. So, for all intents and puposes, the LCA-AF Mk.2 will not enter series-production until 2030.

In another development, the successful ground-launch and flight-testing on February 8, 2019 of a solid-fuel ducted ramjet (SFDR) as the propulsion sustainer component that has been indigenously developed by the MoD-owned Defence R & D Organisation’s (DRDO) Hyderabad-based Defence R & D Laboratory (DRDL) for a futuristic beyond-visual-range air-to-air missile (BVRAAM) meant to destroy hostile MRCAs, makes India the sixth country after the US, Russia, France, South Africa and China to have achieved success in this arena of rocket propulsion.

The DRDL-developed SFDR on the DRDO-developed BVRAAM, which is initially using the missile-body of the indigenously developed Astra Mk.1 BVRAAM, comes attached to a solid-fuel rocket booster to propel the BVRAAM to speeds at which SFDR can start operating.

The SFDR cycle is the same as the ramjet cycle except that the fuel exists in solid form within the chamber and the stoichometry of combustion is controlled by the regression rate of the fuel. The fuel is not a propellant in the solid rocket motor sense, but a pure fuel, inert without external oxidizer much like in a hybrid rocket motor. A wide range of fuels can be used from polymers such as PMMA or PE to long-chain alkanes such as paraffin or cross-linked rubbers such as HTPB. Because the fuel exists in the solid form, inclusion of solid metals is significantly easier than in a liquid-fuelled ramjet. SFDRs also offer significant advantages such as: Extremely simple compared with liquid-fuelled rockets or ramjets since, in its simplest form, a SFDR is basically a tube with a fuel grain cast in it; higher fuel density in the solid phase for pure hydrocarbons and even higher if metal additives are used; easy inclusion of metal fuels such as boron, magnesium or beryllium, which raise the heat of combustion and/or the density and therefore the density impulse capability compared with liquid ramjets; solid fuel acts as an ablative insulator, allowing higher sustained combustion chamber exit temperature levels (and hence specific thrust) with less complexity; fuel is stored within the combustion chamber, allowing for more efficient packaging and higher mass fractions than liquid ramjets; and no need for pumps, external tankage, injectors or plumbing for fuel delivery.

The earliest known SFDR Patent was filed in the US by Cordant Technologies Inc as far back as 1959. The patent can be viewed here:

https://patentimages.storage.googleapis.com/d1/dc/62/4b8ae22672ea4d/US3173249.pdf

While the solid-fuel booster rocket/SFDR combination has been a tried-and-tested means of propulsion, it has limitations of maximum engagement ranges against agile and manoeuvring targets like manned combat aircraft, which can bleed the energy of the BVRAAM. To improve terminal-stage manoeuvring, present-day BVRAAMs use multi-pulse solid-fuel rocket motors, with the second pulse-firing taking place only in the terminal stages after the BVRAAM’s Ka- or Ku-band active seeker has locked on to its target. The MBDA-developed Meteor BVRAAM was the first to do away with the need for multi-pulse solid-fuel rocket motors by incorporating a throttleable ducted rocket (TDR) version of a solid-fuel ramjet developed by Germany-based Bayern-Chemie. The TDR functions as an extended sustainer with variable thrust being generated by a solid propellant, but can sustain high thrust-levels for far longer periods since it acquires its oxidiser from the air. However, both BVRAAMs and LRAAMs using SFDR have a limitation: ramjet motors are heavier and take time to reach maximum speed in the initial phases, and are thus less agile. Consequently, BVRAAMs and LRAAMs using SFDRs will be less effective against agile MRCAs, but the LRAAM will be highly effective against lumbering combat-support platforms like AEW & CS aircraft and aerial refuelling tankers.

Elsewhere in the world, Russia’s Vympel JSC (now part of Tactical Missiles Corp) had by the early 1990s completed developing the SFDR-powered RVV-AE-PD BVRAAM, which hosts rear-mounted lattice-type aerodynamic control surfaces. However, this BVRAAM has not yet been series-produced.

Instead, Russia has opted for a two-stage solid-propellant rocket propulsion system for its Novator R-172S LRAAM.

In the US, Raytheon had in the 1990s developed a SFDR-equipped FRAAM variant of the AIM-120 AMRAAM. But like the RVV-AE-PD, the FMRAAM has not yet entered series-production.

Throughout the late 1970s and 1990s, Somchem of South Africa pursued R & D on SFDR-based propulsion under Project Integral, with the objective being to power both BVRAAMs and MR-SAMs (developed by KENTRON) using SFDRs. The latter, known as SAHV, was first displayed in 1995 with a rounded glass nose-section housing an IIR guidance system.  To avoid over-heating of the IIR seeker during the MR-SAM’s Mach 2.3 flight to its target, the SAHV’s nose-section was fitted with a cap that was jettisoned only in the final stages of the engagement.

Making its debut at the Airshow China 2018 expo in Zhuhai last November was the HD-1, China’s first anti-ship missile using a solid-fuel ramjet combustor and a SFDR—both developed by CASC’s No.4 Institute. Developed and built by the Guangdong Hongda Mining Co, the 2,200kg HD-1’s maiden flight-test took place in October 2018. The HD-1, like the still-classified YJ-18 anti-ship missile, employs a tandem single-stage solid-propellant rocket booster—as opposed to wraparound boosters to reduce drag—for missile launch and acceleration to a forward velocity suitable for efficient operation of the ramjet’s intake system, which comprises four air-intakes arranged in an ‘X’ configuration around the missile body. Tapered control surfaces are mounted on the intake housings near the nozzles. The HD-1 has a length of 8.3 metres, with a missile body and booster diameter of 375mm and 650mm, respectively. Cruise speed is quoted as 2,716kph, while terminal cruise speed being 4,321kph, with the missile cruising at altitudes of up to 15km (49,212 feet) and performing sea-skimming manoeuvres at altitudes between 16 and 32 feet. The missile’s maximum range is 290km.

Coming back to the DRDO’s efforts to develop a BVRAAM or LRAAM using SFDR, the principal challenge remains the development of an internal data-link that will receive targetting cues from a friendly AEW & CS platform, since the AESA-MMR of the MRCA launching such an LRAAM will not be able to detect or track hostile combat-support platforms flying at distances beyond 300km. Furthermore, if the LRAAM is tasked with the destruction of hostile combat-support platforms, then the LRAAM will have a launch-weight of more than 250kg. However, if the idea is to develop a BVRAAM like the Meteor, then a maximum engagement range of 150km will suffice, while the launch-weight will have to be kept down to 220kg at most.