06 November 2024
In August 2023, Chandrayaan-3’s Vikram lander touched down softly on the lunar surface, demonstrating its precise landing technology. As we continue to celebrate this triumph, it’s exciting to consider how emerging technologies might shape the future of landings - both in space and on Earth.
Whether it’s a lunar lander gracefully descending on the Moon’s surface, a Mars explorer touching down on the Red Planet, or a cutting-edge vertical takeoff and landing (VTOL) aircraft on Earth, the ability to land softly and safely is a complex yet crucial challenge.
Historically, liquid rocket engines have been the go-to for achieving VTOL capabilities in planetary vehicles, largely due to their ability to provide controllable thrust. However, VTOL applications on Earth necessitate a safer option. Enter hybrid rockets, which could prove to be a superior alternative for VTOL operations within Earth’s atmosphere and in space, offering advantages that extend well beyond just safety.
In another post, I had described my PhD student Anandu Bhadran’s work on establishing the thrust controllability of hybrid rocket motors. Leveraging on that, we embarked on a journey to show that hybrid rocket motors can be used for soft landings. Beyond simulations, we wanted to demonstrate soft landing using hybrid rocket motors. However, we did not have the bandwidth, in terms of time and resources, to develop a complete platform for this.
Hence, Anandu, along with Prof. Ramakrishna and I, delved into an innovative approach to this problem – we demonstrated the practical feasibility of using hybrid rocket thrusters in landing platforms with a technique called hardware-in-the-loop simulation (HILS). We reported our studies in the International Journal of Aeronautical and Space Sciences.
But first, what exactly is hardware-in-the-loop simulation? HILS is a powerful method used in system development and testing, where real physical components interact with computer-simulated elements in real-time. This approach allows engineers to test complex systems under realistic conditions without the need for full-scale prototypes or expensive field trials. It’s like having one foot in the virtual world and one in reality, giving us the best of both for testing and refining our designs.
In this research, we took HILS to its extreme. It is usually, the control algorithm implemented on a microprocessor, an actuator controlled by the microprocessor, etc., that forms the hardware part of HILS. In our case, beyond the controller sitting on a real microprocessor operating in real-time, we also had the hybrid rocket motor sitting on a test stand and firing hot. The measured thrust feeds into the simulation to propagate the lander motion, and the controller regulates this thrust depending on how the lander is behaving in the simulation world.
Although liquid rocket engines are the usual choice for soft landing applications, in our study, we focused on using hybrid rocket thrusters. Hybrid rockets, which use a combination of solid fuel and liquid or gaseous oxidiser, offer an intriguing middle ground between solid and liquid rocket engines. They’re simpler and potentially safer than liquid engines, yet offer more control possibilities than solid rockets. This makes them an attractive option for landing systems where precise thrust control is essential.
Our HILS setup, like any other HILS, consisted of two main parts: a hardware module and a simulation module. The hardware module included a real, lab-scale hybrid rocket motor. (As described in a previous post, Anandu developed a hybrid rocket motor using a wax-aluminium fuel and compressed air as the oxidiser. This choice of propellants is particularly interesting for its potential applications in aircraft, where compressed air could be sourced from the main engines, reducing the need for separate oxidiser storage.)
The simulation module, on the other hand, modelled the dynamics of a vertical landing platform. This virtual platform was assumed to start its descent from a stable hover at 50 meters altitude, with the goal of achieving a soft landing. The key to success lies in precisely controlling the thrust of the hybrid rocket to follow a predetermined velocity profile during descent.
One of the main contributions of our work was the development of a three-segmented velocity profile for the landing sequence. This profile allows flexibility – the landing approach can be adapted as per the mission requirements. We implemented a Proportional-Integral-Derivative (PID) controller to regulate the oxidiser flow to the rocket motor, effectively controlling its thrust to match the desired velocity profile.
The beauty of our HILS approach is that it allowed us to fine-tune the control system and test its performance under realistic conditions without the risks, costs, and time associated with full-scale flight tests. We could iterate quickly, adjusting parameters and algorithms based on real-time feedback from the physical rocket motor.
Through a series of tests, including cold flow tests (using only the oxidiser flow without combustion) and hot fire tests (with the rocket motor fully operational), we were able to refine our control strategy. The final result? A successful simulated landing with a touchdown velocity of less than a meter per second – well within the typical safe range for soft landings.
By demonstrating the feasibility of using hybrid rocket thrusters for precise landing control, we’ve opened up new possibilities for future VTOL aircraft designs and space exploration missions.
Here is a graphical abstract of our work:
Looking ahead, there’s still much to explore. Our current study focused on vertical motion only, assuming a stable attitude throughout the descent. The next objective is to expand this to include attitude control, dealing with the complexities of managing multiple thrusters to maintain stability while landing. We’re also interested in scaling up the system and investigating how these hybrid rocket thrusters might perform in larger, more capable landing platforms.
(Anandu’s PhD thesis, of which this work forms a part, is an exciting intersection of propulsion, control technology, and systems engineering.)