Hypersonic Technology Demonstration Vehicle (HSTDV)
The air-breathing Hypersonic technology demonstrator vehicle can cruise at a speed of Mach 6. The Scramjet technology program was one of the most ambitious programs of advanced countries like USA, Russia, China, France, Australia and Japan. Worldwide it is a closely guarded technology and the detailed information including fabrication technology is not available in the open literature. The Indian Space Research Organisation (ISRO) has worked on the development of the technology and has successfully tested a system in 2016.
Defence Research and Development Organisation (DRDO) conducted a test of this system 12 June 2019. India has successfully conducted the maiden flight test of its indigenously developed fired Hypersonic Technology Demonstrator Vehicle (HSTDV) off Odisha Coast. The HSTDV was developed by DRDO. Defence Research and Development Organisation (DRDO) carried out the trials from Dr Abdul Kalam Island in the Bay of Bengal. While it can be used to launch small satellites at a much lower cost, it will be used for launching long-range cruise missiles. It will help in the development of a hypersonic cruise missile system.
DRDO successfully demonstrated the hypersonic air-breathing scramjet technology with the flight test of Hypersonic Technology Demonstration Vehicle (HSTDV) at 1103 hours from Dr APJ Abdul Kalam Launch Complex at Wheeler Island, off the coast of Odisha 07 September 2020.
The hypersonic cruise vehicle was launched using a proven Agni solid rocket motor, which took it to an altitude of 30 kilometres (km), where the aerodynamic heat shields were separated at hypersonic Mach number. The cruise vehicle separated from the launch vehicle and the air intake opened as planned. The hypersonic combustion sustained and the cruise vehicle continued on its desired flight path at a velocity of six times the speed of sound i.e., nearly 02 km/second for more than 20 seconds. The critical events like fuel injection and auto ignition of scramjet demonstrated technological maturity. The scramjet engine performed in a text book manner.
The parameters of launch and cruise vehicle, including scramjet engine was monitored by multiple tracking radars, electro-optical systems and Telemetry Stations. The scramjet engine worked at high dynamic pressure and at very high temperature. A Ship was also deployed in the Bay of Bengal to monitor the performance during the cruise phase of hypersonic vehicle. All the performance parameters have indicated a resounding success of the mission.
With this successful demonstration, many critical technologies such as aerodynamic configuration for hypersonic manoeuvers, use of scramjet propulsion for ignition and sustained combustion at hypersonic flow, thermo-structural characterisation of high temperature materials, separation mechanism at hypersonic velocities etc. were proven.
Raksha Mantri Shri Rajnath Singh congratulated DRDO on this landmark achievement towards realising Prime Minister Narendra Modi’s vision of Atmanirbhar Bharat. He also spoke to the scientists associated with the project and congratulated them on this great achievement. India is proud of them, he added.
Secretary Department of Defence R&D and Chairman DRDO Dr G Satheesh Reddy congratulated all the Scientists, Researchers and other personnel related with HSTDV mission for their resolute and unwavering efforts towards strengthening Nation’s defence capabilities. On this successful demonstration, the country enters into the hypersonic regime paving way for advanced hypersonic Vehicles.
Defence minister Rajnath Singh congratulated the DRDO immediately after the test and praised their efforts to indigenously build a scramjet engine. He said that it is a landmark achievement towards realising the vision of ‘Atmanirbhar Bharat’ (self-reliant India). The DRDO, in its statement, called it significant milestone towards a ‘Sashakt Bharat’ (Empowered India) and ‘Atmanirbhar Bharat’ (Self-reliant India).
Vice President M. Venkaiah Naidu called 27 August 2020 for the nurturing of entrepreneurial talent among the youth of the nation to make India ‘Atmanirbhar’ in the time to come. He said that we must tap into the entrepreneurial talent and technological skill of every citizen of the nation and harness our local resources to attain self-reliance and to serve the humanity at large. Calling for the creation of a Sashakt Bharat, a Swabhimani Bharat, and an Atmanirbhar Bharat that Vinoba ji and Gandhi ji had envisioned, the Vice President said that India’s concept of self-reliance is not about being ultra-nationalist and protectionist but to become a more significant partner in global welfare.
Airbreathing propulsion engines have several advantages over expendable rockets, namely, they do not require stored oxidixer, which results in smaller and less costly launch vehicles. In addition, airbreathing engines don't have to rely strictly on engine thrust but can utilize available aerodynamic forces, thus resulting in far greater maneuverability. This can also manifest itself in greater vehicle safety since missions can be aborted much easier.
Alternatives to rocket propulsion systems include a combination of gas turbine jet engines, ramjets, scramjets and rockets that can be integrated into a combined cycle airbreathing propulsion system. Advanced turbojet engines, such as found in fighter aircraft, rely on compressing the air, injecting the fuel into it, burning the mixture, and expanding the combustion products through the nozzle to provide thrust at much higher specific impulses (Isp) than rocket engines. Turbojets are used to power conventional airplanes and cruise missiles, but are currently materials limited to Mach 2-3 so as to prevent overheating and damage to the turbine blades.
At this point another form of propulsion engine, called a ramjet, takes over. This is in lieu of undertaking an expensive development of high-temperature gas turbine blade materials technology to increase the maximum upper limit to approximately Mach 3-4. The ramjet engine operates by using a specially designed inlet to scoop up the ram air, slow it down and then compress it while the vehicle is flying through the atmosphere. Fuel is injected into the air, mixed with it, combusted and then expanded through the nozzle to provide thrust in a similar fashion to the turbojet. Ramjet engines operate most efficiently at vehicle speeds beyond Mach 2-3. A ramjet can be readily integrated into a turbojet engine. The turbojet by itself would operate from take-off to ramjet takeover, and the ramjet would then power the vehicle to its velocity limit of about Mach 6. Above this limit the combustion chamber temperature becomes very high, causing the combustion products to dissociate, which in turn reduces vehicle thrust.
To operate at still higher vehicle speeds, supersonic combustion ramjets, or scramjets as they are called, would be employed. Again, fuel is injected, mixed and combusted with the air, but at supersonic speeds, thus necessitating a different fuel injection scheme than that used by the ramjet. As the vehicle continues to accelerate into the upper atmosphere, rocket engines may be required to supplement the scramjet engine(s) for Mach numbers above 10-12.
Such hypersonic vehicles are, of course, subject to extreme temperature fluctuations within the vehicle's envelope of performance. Specifically, the leading edges, flight control surfaces and a substantial portion of the external surfaces of such vehicle support structures, or frames, as well as the internal construction associated with engines necessary to power the vehicle require that thermal design parameters incorporate means for ensuring structural survivability during short periods of high heat flux.
However, existing insulative systems are limited in the maximum allowable temperature (or heat flux) at the outer surface (mostly below about 1600 deg. C.). There exists a need to provide adequate thermal protection to hypersonic vehicles in the event of a high heat load event that combines the most desirable attributes of the insulative thermal protection systems. Such a system ideally also realizes other positive attributes such as cost and weight reduction.
A Technology Demonstration Project was sanctioned by Defence Research and Development Organisation (DRDO) in March 2001 for ‘Design and Development of Hypersonic Technology Demonstrator Vehicle’ (HSTDV) by Defence Research and Development Laboratory (DRDL). DRDL undertook a feasibility study in September 2003, which included a study on design and development of scramjet engine. The study found that the temperature encountered in the scramjet engine combustor was of the range equivalent to 2227-2527°C.
DRDL therefore identified two high temperature resistant materials viz Nimonic C-263 and Niobium C-103, for possible use in the development of the engine. DRDL found that C-263 was the suitable material which could sustain for 20 seconds flight duration. The maximum temperature resistance capability for C-103 material was found to be 1200°C, which could be enhanced only up to 1370°C through coating technique.
Initially, the preliminary design of Scramjet engine was carried out considering two approaches – Single Wall & Double Wall type construction. For achieving the required design with single wall type construction resulted into heavy weight penalty. Hence, double wall type construction was considered and it was found that double wall construction for weight optimum design with stringent constraints of deflection and allowable stress is a feasible solution (Report no.DRDL/DOFS/ASD/HSTDV-01/18 dated 13th July 2004). Subsequently, a single module double wall configuration of size 550 mm width X 250 mm height X 2885 mm length with C-103 Niobium based alloy and C-263 Nimonic alloy as outer wall was analysed and design finalised. Among all refractory materials, Niobium alloys (C-103 is one of the Niobium alloys) were found to be ductile, lighter weight, possessing good fabricability. C-103 material for the combustor chamber of the scramjet engine is a reliable material for high temperature application.
In September 2005, Ministry of Defence sanctioned a project for ‘Development of Scramjet Engine and Engine Integrated Airframe’ at an estimated cost of Rs. 48.65 crore as part of the HSTDV project, to be taken up by DRDL, Hyderabad. The aim of the project was to design, fabricate and carryout testing of scramjet engine. Scramjet engine is subjected to very high temperature. DRDL identified C-103 material as High Temperature Resistant Material (HTRM) for inner layer of the engine and C-263 for the outer layer. Requirement of C-103 material, which has a shelf life of 10 years, was accordingly projected for development of five scramjet engines. However, keeping in view the anticipated design changes and high cost involved, the Special Purchase Committee (SPC) held in May 2006 recommended procurement of C-103 material for development of only three scramjet engines. In July 2007, DRDL accordingly procured a quantity of 1329 Kg of HTRM worth Rs. 4.83 crore which was received between October 2007 and October 2008. A quantity of 3660 Kg of C-263 material was also procured between December 2007 and February 2008 at a cost of Rs. 1.76 crore, for use in the project.
DRDO stated "C-103 material procured was intended to be used for fabrication of single module-double wall Scramjet engine. The material was procured after the recommendations of two experts committee under the Chairmanship of Dr. AR Acharya, Group Director, VSSC and Dr. Baldev Raj, Director, IGCAR, Kalpakkam during scramjet engine design and fabrication reviews. The committees had cleared the design and fabrication methodology using C-103 materials. C-103 is a strategic material being widely used in the international scenario for high temperature and high speed engine development programmes for which the lead time in procurement is high. Hence, conscious decision was taken to procure C-103 material."
It was observed in March 2012 that the feasibility study carried out in 2003 had specifically brought out that C-103 material can resist temperature only up to 1370°C whereas the temperature generated in the scramjet engine combustor would range up to 2527°C. Despite this known limitations, DRDL procured 1329 Kg of C-103 material. During the process of development, DRDL used only 107 Kg of the C-103 material and found that it could not withstand the high temperature beyond five seconds and therefore, the balance material was not further used. When enquired about the justification for procurement of the material, DRDO HQ stated that due to severe oxidation problem/change in engine combustor design, C-103 material could not be used and C-263 material alone has been used for the scramjet engine development, although usage of C-103 material had limitation as the temperature experienced is more than 2300°C, yet considering the ground test data it was expected that the same had potential for longer duration tests of the order of 100 seconds and 200 seconds with suitable anti-oxidation coating techniques.
The temperature in the engine is of the order of 2,500 degrees because the hot flame is actually coming out of the engine. But even though the gas temperature is 2,500 degrees, the metal temperature will be around 1,000 degrees for the 20 second duration. It takes time for the metal to heat up. So, it was not a wrong decision to select this material. Even now the temperature predicted will be 1,000 degrees and this material will withstand. But after procuring the material, when DRDO attempted to use this material for fabrication, it got into a number of fabrication issues. The original design was to have an external structural layer of C-263 with C-103 inside which is exposed to the heat. The combination of these two materials faced some welding problem. This had never been done earlier. This material had to be given silicide coating without which the properties were going to drop. This coating application process was not readily available for this configuration.
The 2,500 degrees is the gas temperature. Maybe some people were unable to clarify the technical point. The gas temperature is 2,500 degrees, but the material which is holding that gas is not crossing 1,000 or 1,100 degree centigrade. So this material is still valid and even in the world nowhere there is a material which can withstand this kind of 2,500 degree centigrade temperature.
Welding trials such as TIG welding, Electron Beam Welding were carried out. As these two materials are dissimilar it resulted in the formation of brittle intermetallic compounds in the weld joints. Hence, this scheme did not meet the requirement. As an alternate approach, the diffusion and explosive bonding were carried out to bond C-103 & C-263. Out of this, explosive bonding was found to be suitable at coupon level. Several complexities were encountered to manufacture full scale Scramjet engine because of large shape and size (rectangular cross section, length 2.8m, width 550mm), joining complexities, requirement of large size vacuum heat treatment facility, inadequate manufacturing facilities. Hence, fabrication of full scale engine using C-103 and C-263 was found to be complex and difficult.
Mid-course technical design changes are very much a part of design and development process. This modified design reduced the stresses and deflections as the width of engine is reduced from 550mm to less than 225mm. The two module engine configuration using C-263 also met the design requirements, in spite of C-263 being inferior to C-103 in terms of thermal properties. It may be noted that C-263 has been considered for limited short duration of 20 seconds for present HSTDV mission, however for long duration flight (about 600 sec), C-103 along with regenerative cooling will be a better option. For present HSTDV mission, based on this modified design, the Scramjet engine has been fabricated using C-263 alone and more than 60 ground tests have been conducted successfully. The flight worthy Scramjet engine has been realized (weight=330kg) and thermo-structurally qualified.