Vertical take-off and landing (VTOL) of aerial vehicles such as aircraft like helicopters and manned or unmanned aerial vehicles like drones is always a challenge. The soft landing of such vehicles is studied in detail, as a proper landing is important for the safety of the passenger, the safe recovery of the vehicle and equipment in drones, and the safety of mission-critical equipment in a planetary landing module.
Current VTOL systems typically rely on liquid rocket engines for planetary landings and a combination of multiple propulsion units for VTOL fighter aircraft. These systems are complex and demand significant maintenance. In this study, the authors, Dr. Anandu Bhadran, Dr. Joel George Manathara, and Prof. P. A. Ramakrishna from the Department of Aerospace Engineering, Indian Institute of Technology (IIT) Madras, Chennai, India, have studied the feasibility of using a hybrid rocket motor as a propulsion unit for VTOL systems.
Hybrid rocket motors are chosen because they are less complex and safer when compared to liquid engines. They use liquid/gaseous oxidiser and solid fuels, which combine the advantages of liquid rocket engines and solid rocket motors. Because of their inherent safety and throttling capabilities, hybrid rocket systems are gaining popularity.
In the case of the VTOL platform with hybrid rocket thrusters, a touchdown velocity of approximately one metre per second was achieved in a previous study at Korean Aerospace University with open-loop throttling of the thruster. However, open-loop throttling may not ensure consistent performance under off-nominal conditions. Therefore, in this study, a closed-loop controller was used. A closed-loop controller actively responds to disturbances and ensures a soft landing even in off-nominal conditions.
For the oxidiser, compressed air was chosen. This is because air is a safer oxidiser and will push the hybrid rocket system to its operational limits. Also, as there is no need to carry the oxidiser onboard, it results in reduced weight of the VTOL system.
A hardware-in-the-loop simulation (HILS) framework was used to study the feasibility of using hybrid rocket motors for VTOL systems. HILS is a useful, cost-effective and flexible tool for developing and testing complex systems. It integrates real-time physical components (hardware) with a virtual representation (simulation). The versatile nature of HILS has made it a prominent technique for controller design, prototyping, and system development in various fields. This study used a real-time hybrid rocket thruster in the hardware part of the HILS simulation. A single thruster with closed-loop thrust control was used to carry out the hardware-in-the-loop simulation. A landing algorithm based on velocity tracking and a proportional-integral-derivative (PID) controller was developed and used in the HILS. The PID controller was employed to monitor and correct the velocity profile of the system based on the feedback for a seamless landing process.
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.
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.
The following is an image of the hardware part of the simulation during a test:
Prof. Luigi T. DeLuca, a retired Professor from Politecnico di Milano, Milan, Italy, acknowledged the importance of the work done by the authors with the following comments: “The authors offer a welcome contribution to the literature on hybrid rocket engines (HREs). Although HREs were the first rocket configuration studied by famous pioneers (such as Robert H. Goddard in the USA and Hermann Oberth in Germany) at the very beginning of modern astronautics, they still suffer from a limited Technology Readiness Level (TRL) for large-scale configurations. The paper correctly identifies a propulsive application (soft landing) that fits well with the peculiar HRE feature of throttleable low accelerations thanks to the low fuel regression rate. For the selected mission, this provides a gentler action compared to solid propulsion and a simpler system compared to liquid propulsion. In this general framework, the paper proposes and implements an innovative “hardware-in-the-loop” simulation technique, in conditions of maximum simplicity and versatility, using only air as oxidizer. Although not yet definitive, the results are very encouraging, making the overall approach prone to further long-range developments. The authors deserve full credit and should be congratulated for this interesting step forward in hybrid propulsion using a novel experimental methodology.”
Article by Akshay Anantharaman
Click here for the original link to the paper
