RLV-TD was developed by ISRO under the RLV Technology Demonstration Programme. The Technology Demonstration Programme consists of development of hypersonic rocket with air-breathing engines and the reusable launch vehicle.
Engines
In January 2006, ISRO completed the design, development and tests of Scramjet at its Vikram Sarabhai Space Centre in Thiruvananthapuram. During the ground tests, stable supersonic combustion with an inlet Mach number 6 was demonstrated for 7 seconds. On 3 March 2010, ISRO successfully conducted the flight test of its new sounding rocket ATV-D01 from Satish Dhawan Space Centre in Sriharikota. ATV-D01 weighed 3 tonnes at lift-off and was the heaviest sounding rocket ever developed by ISRO at the time. It was mounted with a passive Scramjet engine. The rocket flew for 7 seconds, achieved Mach number 6 + 0.5 and dynamic pressure 80 + 35 kPa.
Reusable launch vehicle
In January 2012, the design of ISRO's reusable launch vehicle was approved by the "National Review Committee" and clearance was granted to build the vehicle. The vehicle was named "Reusable Launch Vehicle-Technology Demonstrator". ISRO aims to bring down the cost of payload delivery to low Earth orbit by 80% from existing $20,000/kg to $4,000/kg. The RLV-TD was developed with an objective to test various aspects such as hypersonic flight, autoland, powered cruise flight, hypersonic flight using the air-breathing enginepropulsion and "Hypersonic Experiment". A series of four RLV-TD test flights are planned by ISRO: HEX, LEX, REX, and SPEX. A team of 750 engineers at Vikram Sarabhai Space Centre, National Aeronautical Laboratory, and Indian Institute of Science worked on the design and development of RLV-TD and the associated rocket. RLV-TD underwent 120 hours of wind tunnel, 5,000 hours of computational fluid dynamics and 1,100 runs of flight simulation tests. RLV-TD has mass of 1.75 tonnes, wingspan of 3.6 meters and overall length of 6.5 meters. The vehicle had 600 heat-resistant tiles on its undercarriage and it features delta wings and angled tail fins. Total cost of the project was. Future planned developments include testing an air-breathing propulsion system, which aims to capitalise on the oxygen in the atmosphere instead of liquefied oxygen while in flight.
The Hypersonic Flight Experiment, first of the five test flights was conducted on 23 May 2016. The Sub-orbital test flight was launched at 07:00 IST from Satish Dhawan Space Centre in Sriharikota. The test flight lasted for 770 seconds, reached maximum altitude of and covered a distance of Sriharikota, steered itself to an on-target splashdown to land at a designated spot in the Bay of Bengal. Not designed to float, the vehicle disintegrated on impact with water and was not recovered. RLV-TD was mounted on top of and launched aboard a single stagesolid fuelbooster that was derived from strap-on boosters flown on India's Polar Satellite Launch Vehicle. By the time, the booster consumed its solid propellant in 91 seconds, the RLV-TD separated from its boost stage and peaked to a sub-orbital altitude of about. The heat shield, guidance, navigation and control algorithms were tested at hypersonic speed by accurately steering the vehicle during the descent stage. For the descent back, the test vehicle was programmed to pitch its nose up, exposing silica tiles on its underside and reinforced carbon–carbon nose cap to the airflow into the thick lower layers of the atmosphere. The RLV-TD endured high temperatures of descent through the atmosphere due to its thermal protection system. All stages of the test flight were tracked by ground station at the launch site and a shipborne terminal. All the mission objectives were met and technologies like autonomous navigation, guidance and control, reusable thermal protection system and descent mission management were successfully validated.
Future developments
The second experiment in RLV-TD series would be an air-drop test where the prototype will be equipped with landing gear to perform an autonomous landing on a runway. The RLV-TD is being designed and built at the Vikram Sarabhai Space Centre. Its navigational equipment was supplied by the ISRO Inertial Systems Unit in Thiruvananthapuram and ISRO's Satellite Applications Centre in Ahmedabad. The RLV-TD's landing gear was supplied by Hindustan Aeronautics Limited. ISRO plans to carry out the landing experiment on a military runway at Challakere in Chitradurga district. The flight article will be carried to an altitude of 3 km by a helicopter and released to land autonomously at the airstrip. LEX is scheduled for mid 2020. In other experiments, ISRO intends to carry out an orbital re-entry experiment and a Scramjet Propulsion Experiment using a reusable carrier vehicle. ISRO Chief K. Sivan stated that the agency had many experiments planned for the RLV-TD, but would only focus on them from 2019, as it was occupied with other missions.
