Electronic Systems! Or, how to control a hybrid rocket.Follow article
In the design of amateur research rockets, several otherwise isolated groups of components need to work in tandem (e.g., the different stages of a rocket). Additionally, aboard a rocket, many parameters need to be monitored for safety (such as temperature near the combustion chamber). To monitor these parameters, and other factors such as location requires a lot of telemetry data that needs real-time processing at the launchpad. Finally, there are procedures that need to occur automatically for before flight (such as the filling of nitrous oxide, an oxidiser fluid).
These are some of the problems that need addressing. A good grasp of how rockets work, which requires collaboration between multiple team members. To undertake this during undergraduate studies helps to develop good practice in scientific and engineering principles that are useful later in life. This is one of the goals of an engineering project of a similar scale to an experimental hybrid rocket; which members at Loughborough Space are currently working on.
Rocket sketch. “Beacon” is planned to be a two-stage rocket
Beacon will be an experimental, low-cost “hybrid” rocket that is due to launch around Summer 2022 (provisional). It will feature retractable canards to aid landing, amongst other features.
Electronic systems are an easy, weight-efficient way to ensure all procedures involved in and related to a rocket, pre-, inter-and post-flight, are done in time and with accuracy.
A whistle-stop tour of hybrid rockets
To understand the functions of any electronics aboard the rocket/on the ground, a basic understanding of hybrid rocketry is required.
Rockets can be split into three categories: solid-fuel, liquid-fuel and hybrid. Solid fuel rockets have oxidiser and fuel mixed as one solid lump. Liquid fuels have separate fuel and oxidiser, both in the liquid state. Hybrid rockets have a solid fuel (usually a plastic (HDPE)), and a liquid oxidiser (nitrous oxide). Hybrid rockets have simpler plumbing than a liquid rocket, with an energy-dense fuel that results in a smaller combustion chamber, reducing weight. This makes them ideal for a university-level project.
Simplified model of a hybrid rocket engine (from “The Science and Design of the Hybrid Rocket Engine”, Richard M. Newlands)
The parts used in Chimera – the engine used to power our rocket, Beacon. (Top) oxidiser tank. (Bottom) combustion chamber
There are many parts of a hybrid rocket that work with each other to ensure a safe flight, as with multiple procedures. After the engine is filled via a set of piping (‘Oxidiser Loading System’), an igniter causes the ignition to occur in the combustion chamber. The increase in temperature breaks a nylon disk that separates oxidiser from fuel, and atomises through an injector plate, allowing the oxidiser to travel through the combustion chamber. This ignites and accelerates the combustion of the fuel core. The mixture of fuel and oxidiser better mixes in the gap between the fuel and nozzle (called the ‘post-combustion chamber’), allowing for more complete combustion. This results in faster gases leaving the nozzle, giving the rocket thrust. To ensure the rocket can then move faster and higher, its weight needs to be minimised.
Isogrid pattern on combustion chamber body, an example of weight minimisation (so the rocket can fly farther). Expected to be fully produced by mid-June 2021
To ensure that the rocket is working as expected, temperature and pressure data (captured using thermocouples and pressure transducers respectively) need sending back to ground, which needs to be analysed in real-time. Additionally, to track the rocket, altitude and GPS tracking is required. From this, other data such as rocket speed can be calculated at the launchpad. Additionally, once the rocket reaches close to apogee (the maximum height), the rocket engine (1st stage) needs to decouple from the 2nd stage. This will be fitted with more experimental features, such as retractable fins (canards) that act to guide the stage for landing, a camera for streaming capability, and a small solid-rocket motor for propulsion.
2nd stage concept art
In addition to the electronics requirements for the rocket/engine itself, before the rocket can be flown, its rocket engine needs extensive testing to validate calculations and models on (for example) thrust, to ensure it works (and safely at that), and to meet criteria for the UK Rocketry Association’s Team Project Support (TPS – a scheme used to aid the development of research rockets, source launch locations in the UK, and cover insurance). Electronic measurement equipment – such as load cells – are used to measure the forces sustained by the engine in flight. This requires electronics on the launch-pad to be versatile, and easy to change: one day GPS data may be tracked for rocket fire, and another day thrust (load cell) data may be needed for a static test.
(Left) static test rig - rocket engine would be clamped to the rig and a 100 kg load cell. (Right) close-up view of the Oxidiser Loading and measurement (brown) equipment in the test rig configuration
Additional to the testing and operation of the engine in-flight, are the checks and procedures pre-flight. Temperature and pressure of the engine are monitored on the ground in the lead-up to launch in case any abnormalities may occur, which would require the engine to be purged of oxidiser (‘vented’) and the flight to be aborted. Filling the oxidiser tank with nitrous oxide is automatic, so no person has to be near the rocket for flight, in case of catastrophic failure. This Oxidiser Loading System consists of a stepper motor connected to a valve that can be opened or closed. This allows the oxidiser to flow through a tube connected to the piping shown in green, into the oxidiser tank of the rocket engine.
Summary of (most) the equipment used
Previous systems were proposed using several Arduino Mega microcontroller boards to control the different operations of the rocket engine (telemetry/communications, on launch-pad, and on-rocket systems). This was to re-use equipment from previous smaller, land-based rocket engine tests. However, there were limitations in computing power. For example, live-streaming capability would be difficult with Megas (due to bandwidth). The Arduino setup has fewer use-cases for large-scale projects and is therefore limited in the long term. As such, we are switching to a Raspberry Pi-based system.
Summary of on-launchpad systems
A Pi in the rocket’s second stage will act as the ‘brain’ of the rocket. This would transmit GPS, altitude/latitude/longitude and camera data to the ground using a Long Range radio transceiver (LoRa). Additionally, data from the first stage (oxidiser temperature, combustion chamber pressure) is sent to this Pi, which is also transmitted. This data is grouped into packets that are then sent down in discrete time intervals to the ground. Finally, the rocket Pi will be controlling actions such as separation of 1st and 2nd stages (the main rocket engine and experimental module respectively), and ignition of the solid-fuel motor in the 2nd stage..
Summary of on-rocket systems
On the ground, a second Pi is responsible for receiving and processing all the data, which is sent to a third Pi. This Pi is akin to ‘mission control’. It is connected to a touchscreen and displays the data in a human-readable format and has a number of buttons for arming the engine before launch, initiating launch procedures, and emergency stop (venting). Of course, a ‘big red’ (what would normally be an emergency stop) button is used to start launch, since why not? Ideally, only the button to arm the electronics and to start the launch procedure would ever need pressing. However, an emergency stop is required in case any abnormalities occur.
Prototype mission control with mini-output, originally for a smaller test hybrid engine
The electronic systems chosen, result in low-energy demand, which results in lightness due to fewer batteries (important in rocketry), safety (of the rocket, of team members, of British airspace, and of the public), and expected reliability. Given the difficulty of the task, a lot of assembly work and is yet to be completed (although many of them have been tested), the launchpad user interface (called 'Cassi') is in development, and the engine/rocket in production until July 2021 (although several iterated finite element analyses have shown its theoretical viability), and full-scale engine testing over Summer 2021. However, over the past year, additional needs have been established. For example, most of the electronics are currently hard-wired on prototype boards. However, this is both difficult and results in high-maintenance, unreliable equipment. As such, a move to using PCBs (printed circuit boards) to attach all electronics is planned over the next few months.
Love and Rockets!