Ground Stuctures is responsible for the design and manufacture of the Mini-Maxwell test stand and engine.
The Mini-Maxwell engine consists of 4 main pieces:
These are sealed together via Polytetrafluoroethylene (PTFE) O-rings. PTFE is a temperature and chemical resistant material, which is essentially chemically inert (not chemically reactive – it is thermodynamically unstable, but decomposes at a slow or negligible rate).
The pieces are held together by a series of M8 bolts, which simultaneously attach the engine to the thrust plane of the test-stand – more about this in the ‘Test stand’ section.
There is a gap between the injector adapter and injector – this is set to be skilled with a Viton (fluoropolymer elastomer and synthetic rubber compound) rubber gasket (essentially, a fluorinated hydrocarbon rubber product, which is compatible with hydroxytryptophan, or HTP for short).
This is a piece that allows the test stand flexible tubing to connect the interface with the engine, stepping down from 1/4 inch to 1/8 inch tubing.
Injector adaptor – Injector interface (the connection between the two pieces) features Viton, providing appropriate sealing between the oxidiser and fuel inlets. The temperature that has been simulated – 260 ℃ – makes the PTFE or other perfluorinated polymer suitable in thermal and chemical compatibility. As the Injector adaptor will have direct and prolonged contact with the oxidiser substance, it has to be chemically compatible (not react with it). For this reason, passivated stainless steel is used.
This is the most complex piece to manufacture. It will be used to essentially inject the fuel and the oxidiser into the combustion chamber.
The piece features two inlets (where chemical substances – the fuel and the oxidiser – are pushed in to the injector) and three outlets (where the fuel and oxidiser are pushed out of the injector, into the combustion chamber). The fuel inlet is divided in two fuel outlets, while the oxidiser inlet does not change.
The central oxidiser line is on the same line as the nozzle – hence minimising the contact between the oxidiser substance and the combustion chamber.
The two fuel outlets are set at 45 degrees, pointing inwards – here, the streams of the fuel collide with the stream of the oxidiser, cancelling momentum and carrying the fuel-oxidiser mixture unhindered (freely, with no further obstacles) through the engine nozzle.
The Injector piece has prolonged direct contact with the oxidiser, HTP, so it must be made of a compatible metal – one that does not react with it. Stainless steel is a good choice, provided it has undergone passivation. Passivation is a finishing process, with the purpose of preventing corrosion in stainless steel. It involves using nitric acid or citric acid to remove contaminants (iron) from the surface of the metal, which creates a protective oxide layer that is less likely to chemically react with other elements.
This piece is made up of the top face of the injector outlet-face, the cylindrical chamber, and the nozzle piece at the bottom. The chamber, along with the nozzle, will take the majority of the heat generated during combustion. It is therefore made of copper. The oxidiser, HTP, is not compatible with copper, however as the firing time is short and the contact with the surface of the piece is low, the contact between HTP-copper is negligible.
The engine supports a maximum of two firings – hence the inner surface will be inspected for erosion prior to a second firing.
This is the lower face of the combustion chamber; it features the ‘throat’ and nozzle of the engine. This piece, according to the simulations, will receive the highest thermal load, especially around the throat. The thickness of the piece around the throat should allow for greater heat capacity – which is why the base material is copper, due to temperatures around the nozzle. The thrust plate will be fixed below the nozzle piece, below the widened rim and secured with M8 bolts.
The test-stand will be used for the testing of the engine. It comprises the engine feed, pressurization gas feed, fuel/oxidiser feed, all held together by the frame. Each of these pieces and their manufacturing process will be described below.
The frame is the main body of the test stand. All the other stages will be mounted to it. It is required to resist the stresses of multiple fires of engine, as well as allow for the straightforward mounting/unmounting of all the attached parts/mechanisms.
It is made of the following materials:
The manufacturing plan will follow the steps outlined below:
These are two feeds which will be attached on the left and right facing plates of the frame. The feeds contain tanks of HTP and ethanol respectively, which will mix in the engine combustion chamber and provide thrust once ignited.
Both pieces are required to handle the 17 bars of pressure, for the duration of the engine preparation and fire. The feeds will need to be exposed to highly reactive chemicals and thus the materials and preparation methods should be carefully considered.
The materials used to manufacture the fuel/oxidizer feeds are:
The manufacturing plan will follow the outlined steps:
This piece is mounted at the top of the test-stand frame and will pressurise the entire feed system. It will be required to withstand the 17 bar pressure during both testing and firing. No leaks or structural weak points can be included.
The materials used for manufacturing are:
The manufacturing will follow the steps outlined below:
The engine feed is the final stage between the test stand and engine; it allows for the fuel and oxidizer to be in the correct locations before entering the engine. As the other feeds, pressure capability is of highest concern, as well as ability to withstand chemicals passing through them.
The materials used in manufacturing the engine feed are:
The manufacturing will follow the outlined steps:
The Electronics division focuses on the following tasks:
The electronics system is aimed to work wirelessly over RF communication, via two RF modules connected to 2 teensy 4.1s which act as computer-circuit microcontrollers.
The teensy module in the engine connects to all sensors through its analogue input pins and to the valves through digital output pins.
The pressure transducers’ data are passed through operational amplifiers to boost their signal.
The valves are controlled through 2 high-low power relays, which are activated by the teensy’s output signals.
These relays are, in turn, powered and connected to 2 12V batteries in series.
5 out of the 9 valves are normally closed valves, which require a 24V current to activate; the other 4 are normally open, which need 12V.
The necessity for two 12V batteries is the current consumption by all valves and relay boards.
Both relay boards are powered by a 12V supply, and draw a combined 400mA when all valves are activated.
The 12V valves operate with 10W and require 3.5A when all activated.
This high current circuit requires a use of higher amp-hour power sources – which we cannot achieve through connections to mains.
Hence, the circuit consists of 2 high amp-hour 12V batteries which are connected in series to power the 24V valves and used individually to power the relays 12V valves.
The teensy, RF transceiver, sensors and corresponding amplifiers are powered by 5V with a total current draw of less than 1A, which we have decided to provide via a buck converter to step down the 12V of one battery to 5V.
The Propulsion Team brings together both engineers and chemists to share their expertise in the respective fields in order to ensure the successful and safe hot fire.
As the rocketry industry is inherently related to the use of hazardous propellants, the chemists in our team make sure that the relevant steps are being taken in order to reduce the risks related to chemicals handling. That is being done by filling in the COSHH forms and risk assessments corresponding to each activity involving work with chemicals, such as oxygen cleaning, passivation with citric acid, passivation testing, and firing the engine. On top of that, Propulsions have written up the Procedures Handbook, describing each activity related to the preparation of the parts for assembly as well as the steps undertaken before and after firing the engine and in case of emergencies. We have also looked into the requirements of the PPE and needed materials to be compatible with HTP to make sure that the chemist operating the engine during the hot fire is safe in case of any emergencies.
The engineering part of Propulsions runs the simulations and performs calculations to estimate the most optimal parameters for firing the engine. That includes thrust and specific impulse, mass flow rates of oxidiser and fuel, (which in the case of MiniMaxwell will be used in a ratio 3:1) and the operating maximum temperatures during the fire (259oC for a 2s run). Another important factor is the pressure simulations, considering the chamber pressure, safety factor and maximum stress, as well as the feed system pressure drop. All of that helps us to predict the outcome of the hot fire and ensure that the design of the engine is optimal. In collaboration with Electronics, we also work on the Piping and Instrumentation Diagrams to depict the connections between each part of the system, and how the valves operate.
The Piping and Instrumentation Diagram describes the system of valves and sensors, and their position within the feed systems. It allows for the development of firing procedure and protocol as well as the calculation of power requirements for the electrical components during the different stages of firing.
To simulate the temperature in the engine, we used the Bartz equation, to estimate the convection coefficient of the exhaust flow in a rocket nozzle at a local cross-sectional area. We are then able to calculate the heat flux by multiplying the convection coefficient by the temperature difference. To calculate the heat flux on converging diverging sections of the nozzle we calculated the heat flux at certain intervals and then averaged them. A thermal simulation with a run time of 2 seconds was then completed on SolidWorks.
This block diagram outlines the steps to be undertaken before firing the engine, and how to proceed in case of system failure, to make the engine safe to approach. Those steps should be undertaken by a chemist in full PPE in collaboration with the Electronics Team, operating the state machine if possible. The safety procedures are implemented and outlined clearly in the Procedures Handbook.
