by Anantha Krishnan

BANGALORE: The Defence Research and Development Organisation (DRDO), in September this year successfully flight-tested the Hypersonic Technology Demonstrator Vehicle (HSTDV) using the indigenously developed scramjet propulsion system.

Termed as a giant leap in indigenous defence technologies, the initial success of this hypersonic air-breathing scramjet technology has given missile scientists the confidence to undertake R&D into much more complex areas as they progress into the next phase.

Onmanorama interacted with the HSTDV team, capturing the history of the program and the challenges involved in realizing some of the complex technologies set to serve as the building block for next generation hypersonic vehicles.

Through this three-part series, we bring to you a glimpse of the early years of India’s HSTDV dream, the current state, and the road ahead.

A Decade of Work

It was in early 2000 that DRDO initiated work on HSTDV in order to be on par with developed nations in the field of hypersonics. During this period the United States, Russia and Australia were among the developed nations deeply involved in hypersonic missions.

The primary aim was to develop an autonomous hypersonic cruise vehicle, which can cover a long distance in a short time.

Hypersonic cruise vehicles are very different from ballistic missiles in the sense, the trajectory is difficult to predict.

Accordingly, feasibility studies were carried out in 2004 at the Defence Research and Development Laboratory (DRDL), Hyderabad. This feasibility study formed the base for developing the hypersonic vehicle.

Dr S Panneerselvam, a pioneer in the field of aerodynamics, was the first Project Director of the HSTDV project. He and a team of five scientists worked to conceptualize the shape and size of the HSTDV Cruise Vehicle (CV) using computational tools. The team was multidisciplinary, with expertise in streams of engineering involving aerodynamics, structures, controls, and scramjet propulsion.

Many new technologies were developed during the development cycle. Subsequently, a cruise vehicle was indigenously realized in DRDO.

There were several scientists who played the role of mentors during the HSTDV design work. Late A K Kaushik (retired DRDL scientist), Prof H S Mukunda (ex-Prof, IISc), late Prof P J Paul (ex-Prof, IISc), Prof M A Ramaswamy (ex-Prof, IISc), Dr Y R Mahajan (ex-ARCI), Dr T S Prahlad (former Director, NAL), Dr Abdul Majeed (Ex-VSSC), Dr Kota Harinaryana (former Program Director, LCA), Prof G R Shevare (ex-Prof, IIT, Bombay), N V Kadam (former Associate Director, DRDL), S K Ray (former Director, RCI) and Dr Rajaram Nagappa (ex-Prof, MIT) were the pillars who guided a dedicated team of DRDO scientists towards the realization of the design.

Challenges Galore

Initially, the HSTDV CV was conceptualized. The CV works in a specific window of hypersonic regime (altitude: 29-34 km and at 5.8-6.5 Mach).

A launch vehicle (LV) was required to carry the CV to the desired conditions. It was planned to design the CV with available materials (titanium, nickel-based alloys and aluminium).

“Those days DRDO’s well-proven Agni-I missile was the best suited booster for HSTDV CV and it was chosen for the LV. During the ascent phase of the LV, due to high kinetic heating within the atmosphere, the temperature on the exposed CV was exceeding the capability of materials. There was a need for a protective system for the HSTDV CV to deal with the kinetic heating during the LV ascent phase. Seven-metre -long and one-metre diameter size split protective fairings were designed to keep the CV within the allowable temperature,” said a scientist, who is in the team.

The LV was going through very high dynamic pressure of around 5 bar, which is a very huge number for a launch vehicle.

The scientists had to conduct several heating studies and experiments to clear the LV. The control capability of the LV was another bottleneck. Extensive modifications of the launch vehicle control surfaces and wind tunnel and aero-thermal testing were carried out to qualify the LV.

Within the one-meter diameter restrictions of the Agni-I launch vehicle, the cruise vehicle’s wing had to be folded like a small baby in the womb. This required an unfolding mechanism at high speed and it was implemented.

“The protective fairings have to be separated when the injection conditions for cruise vehicle have been achieved. This was massive bottleneck as it needed a large number of pyro-mechanisms to be operated within a few milliseconds simultaneously so that the panels open at hypersonic speed,” recalled the scientist.

Finally, several iterations and brain storming sessions with experts in the country provided major design inputs that came to the rescue of the team.

“Very limited testing is possible for fairing separation. The entire sequence of separation was studied through CFD (Computational Fluid Dynamics) and it was tested successfully in the Terminal Ballistics Research Laboratory (TBRL), Chandigarh. So far the most developed nations have not attempted such separation mechanisms at relatively high dynamic pressures,” said another team member.

New Technologies

In the CV, the design process was newly evolved and it was the first of its kind in DRDL, Hyderabad. Eventually, a lot of technological base was developed in the areas of hypersonic aerodynamics, aero-thermodynamics, intake aerodynamics and scramjet propulsion.

“The design was a purely indigenous effort. With the CV being non-axisymmetric and since we were addressing it for the first time, there were thermal buckling issues, which were sorted out tweaking the design of hot structures,” recalled the scientist.

The materials for fuel injection posed a fresh set of challenges due to the high combustion temperatures. The scientists had to constantly make multiple iterations, changing the material and construction.

The HSTDV team says that most of the technologies for the mission were developed afresh, as its design aspects were completely different from the various missiles developed by DRDO.

“In other missiles, there is a certain decoupling between airframe and engine. But the CV needs an aero-propulsion-structure-integrated design. The engine–airframe attachment was another major technical challenge we had to address. We attempted several schemes before finalizing the right one,” added the scientist.