IIT Madras achieves milestone in VTOL tech using hybrid rocket thrusters for next-gen flight systems

Chennai: Indian Institute of Technology Madras (IIT Madras) researchers have taken India one step closer to developing a Vertical Take-off and Landing (VTOL) Aircraft and Unmanned Aerial Vehicle (UAV) with Hybrid Rocket Thrusters.

In a sophisticated experiment that combined a real-time hybrid rocket thruster with a virtual simulation, the researchers achieved the necessary velocity for ‘soft landing’, a crucial feature for all craft ranging from planetary landing of an unmanned or manned exploration module to terrestrial landing of a VTOL aircraft.

Importance of hybrid rocket motor

Touchdown velocity is an important parameter to ensure safe vertical landings.

Researchers studied the feasibility of using the hybrid rocket motor for vertical landing platforms, as they are less complicated and safer compared to liquid engines. Moreover, hybrid rocket systems are gaining popularity due to their inherent safety and throttling capabilities, apart from the possibility of combining the advantages of liquid and solid rocket engines.

Currently, VTOL systems are complex and need high maintenance. Therefore, the researchers conceived the concept of a hybrid rocket thruster-powered platform as part of a system development study to bring out an effective propulsion unit for achieving VTOL in aircraft and UAVs.

The findings of this research were published in a paper, co-authored by Prof. PA Ramakrishna, Dr Joel George Manathara and Anandu Bhadran of IIT Madras, in the reputed peer-reviewed International Journal of Aeronautical and Space Sciences.

Vertical take-off benefits minimum infrastructure

Elaborating on the need for technologies such as VTOL, Prof. PA Ramakrishna, Department of Aerospace Engineering, IIT Madras, said, “Vertical take-off and landing, as the name suggests, will enable an aircraft to take off and land vertically, eliminating the need for infrastructure like long runways. Because of this, VTOL capability will enable access to remote locations and rugged terrains where long runways and large airports are difficult to establish. Currently, Helicopters are the system operating on those terrains; however, they face limitations in terms of speed, range, and efficiency compared to a fixed-wing aircraft.”

Prof. PA Ramakrishna added, “Once the VTOL system reaches the Technology Readiness Level (TRL) for commercial application, it will be a game-changer in both civil and military aviation. VTOL will help to decentralise air transport to multiple locations rather than a single big airport or an airbase. Such decentralisation brings significant strategic and operational advantages. This study is a precursor towards the development and demonstration of a proof-of-concept for a hybrid rocket-powered VTOL platform”

Commenting on how this research can be implemented in real-world applications, Prof. PA Ramakrishna said, “Because of the positive outcomes of this study, an experimental study of the landing platform with multiple degrees of freedom could be carried out as a future study. Once attitude stabilisation of the platform is achieved, a hardware-in-the-loop simulation for landing without the assumption of an attitude-stabilised system could be carried out. This would be a step closer to realising a VTOL platform with hybrid rocket thrusters. With validated performance and control strategies, the developed VTOL technology can be extended to fixed-wing UAVs as an initial implementation. This not only serves as a proof-of-concept for broader aerospace applications but could also attract attention from industries interested in next-generation air mobility solutions”

Easy integration into aerospace vehicles

Highlighting the unique aspects of this research, Dr Joel George Manathara, Associate Professor, Department of Aerospace Engineering, IIT Madras, said, “The use of hybrid rocket thrusters for VTOL itself is a unique and novel aspect of this research. Our work paves the way to move beyond the conventional approaches by demonstrating the viability of hybrid rocket propulsion – which offers a balance of safety, simplicity, and control – for VTOL applications.”

A significant contribution is the development of a hybrid rocket fuel that needs only compressed air as the oxidiser. This makes VTOL systems easier to integrate with aerospace vehicles, especially in scenarios where compressed air is readily available.

Dr Joel George Manathara added, “Another interesting aspect of this work is the unique Hardware-in-the-Loop Simulation (HILS) framework that IIT Madras researchers have developed. HILS combines physical hardware testing with simulations to offer a cost-effective way to quickly develop complex systems. While typical HILS setups involve a servo motor or microcontroller as the hardware interacting with the simulation, we pushed the boundaries to integrate a real, live-firing hybrid rocket motor directly into our HILS loop. This ensures that our testing framework is incredibly accurate, effectively reducing the leap from this technology to a functional prototype, bringing us much closer to practical application.”

How does the hybrid rocket motor function?

Further, Anandu Bhadran, Researcher, IIT Madras, said, “Hybrid rocket motors have been regaining attention in the last few decades, and the throttling capabilities of hybrid rocket motors are being studied for various applications. In a previous study, we also established closed-loop thrust control and throttling capability of our hybrid rocket motor and that thruster was tested in this study within a HILS framework to assess the feasibility of vertical soft landing using a hybrid rocket thruster. The hybrid rocket motor demonstrated the ability to dynamically respond to the thrust demands generated by the control system governed by the guidance algorithm. Even with a basic controller, the preliminary results were promising, highlighting the reliable and versatile thrust modulation capability of the thruster. These findings reinforce the potential of hybrid rocket propulsion in VTOL and other precision-controlled aerospace systems.”

In the hot flow test, a touchdown velocity of 0.66 m/s was obtained for the system. The system achieved the desired objective with air as the oxidiser, which also contributes to its overall safety.

The HILS simulation approach facilitated a series of cold and hot flow tests within a simulated VTOL environment, enabling the fine-tuning of a PID controller to achieve a desired landing profile

A landing algorithm based on velocity tracking and a proportional-integral-derivative (PID) controller was developed and used in the HILS simulation. The velocity-tracking algorithm incorporated a three-segmented velocity profile, which was conceptualised to be versatile and adaptable. The PID controller was employed to monitor and correct the velocity profile of the system based on the feedback for a seamless landing process. Cold flow tests and pure numerical simulation also showed that a safe touchdown is achievable with hybrid rocket thrusters.

The hardware-in-the-loop simulation demonstrates a successful soft landing of the platform with a touchdown velocity of less than 1 m/s. The system achieved the desired objective with air as the oxidiser, which also contributes to its overall safety.

In this research, the system model used in the simulation had only one degree of freedom. Now that the hardware-in-the-loop simulation study gave promising outcomes towards the objective of developing a hybrid rocket thruster-powered VTOL platform, an experimental study of the landing platform with multiple degrees of freedom could be carried out.

The attitude stabilisation of the experimental VTOL platform with multiple thrusters is currently in progress. Once attitude stabilisation of the platform is achieved, a hardware-in-the-loop simulation for landing without the assumption of an attitude-stabilised system would be carried out. This phase of the study would be a step closer to realising a VTOL platform with hybrid rocket thrusters.