1. Introduction

Project Sirius was my final-year Group Design Project as part of the MEng Aerospace Engineering programme at the University of Southampton.

The project formed part of a multi-year effort to develop and experimentally fire a regeneratively cooled bipropellant rocket engine using High-Test Peroxide (HTP) and turpentine. The engine featured a 3D-printed Inconel combustion chamber and nozzle and was designed to produce approximately 1.5 kN of thrust. Our objective during the 2019-2020 academic year was to prepare the engine and its supporting systems for their first full-scale firing campaign.

2. Engine Architecture

The Sirius engine uses High-Test Peroxide (HTP) as an oxidiser and turpentine as fuel. HTP is an increasingly attractive propellant choice for spacecraft and small launch vehicles, offering a safer and more environmentally friendly alternative to conventional hypergolic propellants.

During operation, liquid peroxide first flows through the regenerative cooling channels integrated within the combustion chamber and nozzle walls. This process removes heat from the engine and pre-heats the oxidiser before it is injected into the catalyst decomposition chamber. Inside the decomposition chamber, a manganese oxide-based catalyst decomposes the HTP, generating hot gaseous oxygen and steam.

The decomposed oxidiser is then accelerated through a reducer before entering the combustion chamber. Turpentine is injected downstream through a swirl injector, where it mixes with the hot decomposition products and ignites spontaneously. The resulting combustion gases expand through the converging-diverging nozzle to generate thrust.

The engine is supported by a dedicated propellant delivery system, instrumentation and control software responsible for supplying propellants, regulating flow and acquiring test data during experimental testing.

3. Project Context

Project Sirius consisted of several parallel development streams that collectively aimed to prepare the engine for its first full-scale firing campaign.

Across the team, work spanned catalyst qualification, injector development, propellant delivery hardware, control software, wet testing, nozzle repair studies and preparations for the planned firing campaign.

My work focused primarily on catalyst validation and oxidiser injection system development, where I took full ownership of the design and validation activities. I also redesigned combustion-system hardware and manufactured a large share of the catalyst inventory required for the planned firing campaign. I later contributed to the investigation of a critical manufacturing defect discovered in the engine nozzle shortly before the planned testing date. Alongside these technical responsibilities, I acted as lead editor for the group’s final report and project video.

The chapters that follow detail each of these contributions in turn, starting with catalyst validation.

4. Catalyst Validation

4.1. Developing a Representative Validation Methodology

One of the most important technical questions facing the project was whether the internally developed catalyst could reliably decompose HTP at the operating conditions required by the full-scale engine.

Previous teams had demonstrated promising catalytic performance, but validation at the target catalyst bed loading had not yet been achieved. To address this challenge, I redeveloped the validation methodology using a scaling approach the project team referred to as the tubule model.

The model treated each injector hole as an individual flow region with its own area of influence within the catalyst bed. By preserving local catalyst loading and injector configuration, a smaller decomposition chamber could reproduce the key operating conditions of the full-scale engine.

4.2. Redesigning the Validation Hardware

With no CAD records available for the inherited small-scale injectors from previous project years, I first had to measure their geometry directly, using digital callipers and a microscope, which revealed hole dimensions that differed substantially from those of the full-scale injector.

Rather than scaling the hardware geometrically, the design process therefore focused on preserving the parameters that govern catalyst performance: catalyst bed loading, catalyst bed length and injector characteristics. These requirements drove the redesign of both the injector and nozzle.

The injector was developed to replicate the oxidiser distribution and local catalyst loading of the full-scale chamber, while the nozzle was optimised using thermodynamic performance predictions generated with NASA CEA to achieve the required mass flow rate within the pressure limits of the available laboratory equipment at the university test facilities.

The resulting hardware provided a representative test platform that allowed catalyst performance to be validated.

4.3. Experimental Validation Campaign

Following the design and manufacturing effort of the small-scale thruster, multiple catalyst decomposition tests were conducted over two separate test campaigns at the university.

The first test campaign reached a maximum bed loading of approximately 38 kg·s⁻¹·m⁻², short of the 50 kg·s⁻¹·m⁻² target, as achieving a higher bed loading would have required nitrogen line pressures beyond the safe operating limits of the propellant delivery rig. To reach the target bed loading without exceeding these pressure limits, a sleeve insert was introduced to reduce the cross-sectional area of the catalyst bed, increasing bed loading for a given mass flow rate.

Performance was evaluated using chamber-pressure measurements, chamber-temperature measurements and mass-flow calculations derived from test data. The second campaign, conducted with the reduced-area chamber, achieved bed loadings approaching 60 kg·s⁻¹·m⁻² while producing decomposition temperatures above 600°C, demonstrating complete peroxide decomposition and validating the catalyst for use in the full-scale engine.

This milestone removed one of the major technical uncertainties associated with the planned firing campaign, providing confidence in the decomposition chamber design ahead of full-scale testing.

5. Oxidiser Injection System Development

While the catalyst validation programme focused on proving catalyst performance, it also highlighted the importance of achieving uniform oxidiser distribution across the catalyst bed.

The oxidiser injector inherited from previous project iterations used a different geometry from the validation hardware and provided a less uniform flow distribution than desired. This limited the applicability of previous testing campaigns and introduced uncertainty when extrapolating results to the full-scale engine.

To address this issue, I redesigned the oxidiser injector used in the full-scale engine, with the objective of improving flow distribution while maintaining consistency with the validation hardware and ensuring the design remained manufacturable.

5.1. Design Objectives

The oxidiser injector plays a critical role in determining how peroxide is distributed across the catalyst bed. Non-uniform injection can lead to localised overloading, uneven decomposition and reduced catalyst utilisation, making injector design a key factor in overall engine performance.

The redesign was therefore driven by four primary objectives:

• Improve oxidiser distribution across the catalyst bed.
• Maintain consistency between the sub-scale and full-scale systems.
• Achieve a geometry that could be reliably manufactured.
• Provide sufficient structural rigidity while minimising unnecessary thickness and mass.

These requirements often competed with one another. Increasing injector plate thickness improved stiffness but constrained the achievable injector geometry and increased component mass, while thinner plates enabled smaller injector features but were more susceptible to deformation under pressure.

The final design therefore required balancing hydraulic performance, structural integrity and manufacturability.

5.2. Injector Design and Analysis

To meet these requirements, I redesigned the full-scale oxidiser injector from first principles.

The new design adopted a hexagonal hole distribution pattern, ensuring that each injector element influenced a similar area of catalyst bed. Compared to the inherited design, this provided a significantly more uniform distribution of oxidiser across the decomposition chamber. The final injector incorporated 109 orifices with a diameter of 0.7 mm and was designed around a pressure drop of approximately 3.5 bar.

Beyond the hole pattern itself, plate thickness was a second key design variable, given the trade-off between rigidity and geometric flexibility previously noted. To identify an appropriate value, I combined analytical plate-deflection calculations and finite element analysis models.

Balancing these factors led to the selection of a 4 mm thick injector plate, providing sufficient structural rigidity while maintaining the desired injector geometry and remaining practical to manufacture with available methods.

During manufacture, four drill bits snapped while producing the orifices, leaving the corresponding holes on one side of the plate slightly oversized. Rather than requiring a full re-manufacture, the affected plate was oriented so that these defects faced the oxidiser manifold rather than the catalyst chamber, allowing the injector to remain fit for use within the required tolerances.

5.3. Experimental Characterisation

Following manufacture, the injector performance was experimentally characterised through cold-flow testing of the small-scale thruster injector using deionised water.

Mass flow rate measurements were collected across a range of operating pressures and used to determine the injector discharge coefficient. The results were then compared against the assumptions used during the design process.

The measured discharge coefficient came out approximately 17% higher than the value assumed during the design process. This discrepancy was likely caused by measuring pressure upstream of the injector rather than immediately before the orifice plate, on a rig with narrow tubing not designed for the flow rates involved. As a result, the reading may not have reflected the true stagnation pressure at the injector face.

A follow-up test using the full-scale injector and the full-scale propellant delivery system was planned to resolve this discrepancy, but could not be carried out due to the COVID-19 closure of the University. The injector’s hydraulic performance was therefore characterised at small scale but not conclusively verified at full scale.

6. Combustion System Optimisation

Catalyst decomposition and oxidiser distribution were critical to engine operation, but the flow conditions at which the decomposed oxidiser entered the combustion chamber also influenced combustion performance.

Previous experimental work within the Sirius programme indicated the existence of an optimal combustion chamber entry velocity for the decomposition products. Achieving this target required redesigning the reducer section connecting the catalyst decomposition chamber and combustion chamber.

6.1. Reducer Redesign

To achieve the desired inlet conditions, I redesigned the reducer using conservation-of-mass calculations together with thermodynamic properties obtained from NASA CEA analysis.

The objective was to accelerate the decomposed oxidiser to the target velocity while remaining compatible with the selected fuel injector and overall engine geometry. By modifying the reducer outlet area, the flow velocity entering the combustion chamber could be adjusted without introducing significant changes elsewhere in the propulsion system.

Although relatively simple in appearance, the component played an important role in defining the flow conditions entering the combustion chamber and therefore contributed directly to combustion performance.

I produced the engineering drawings for the redesigned reducer and coordinated its manufacture, resulting in a completed part ready for integration into the full-scale engine.

7. Catalyst Manufacturing

The engine catalyst was based on manganese oxides deposited onto gamma-alumina support pellets.

Following the successful validation of the catalyst decomposition chamber, the next challenge was manufacturing sufficient catalyst for the planned full-scale firing campaign.

While catalyst development had been conducted at laboratory scale in previous years, the full-scale engine required close to a kilogram of catalyst pellets to completely fill the decomposition chamber. Scaling production from experimental batches to engine-scale quantities therefore became an important part of the project.

7.1. Scaling Production

Catalyst production involved a lengthy process that included calcination, chemical impregnation, thermal treatment and quality assurance testing.

Over the course of the project, I personally manufactured about 580 g of catalyst across eight production batches, representing more than half of the catalyst inventory ultimately prepared for the full-scale engine. Each batch was weighed after every processing step to monitor consistency, with the resulting quality data used to track process repeatability throughout the campaign.

The data revealed that batches produced using fresh impregnation solution consistently achieved higher and more repeatable catalyst uptake than those produced early in the project using solution reused from previous batches, whose concentration had noticeably reduced with each use. Based on this finding, only virgin solution was used for the remainder of the campaign.

Producing eight batches also required careful planning around the limited furnace capacity available at the university’s high-temperature laboratory. Calcination and thermal treatment cycles were lengthy, reaching temperatures of up to 500°C. Coordinating batch schedules around this constraint made personal time management a challenge.

Each batch was also verified experimentally through HTP drop tests on sample pellets, which I helped carry out, confirming consistent catalytic reactivity across batches before the catalyst was cleared for use.

By the end of the manufacturing campaign, the catalyst inventory was ready for installation in the full-scale decomposition chamber.

8. Investigation of a Critical Nozzle Manufacturing Defect

Only weeks before the planned firing campaign, a major issue was discovered during testing of the engine cooling system.

Water introduced into the regenerative cooling channels was found leaking directly through the nozzle throat wall. Initial investigations revealed that the problem was far more severe than a simple sealing issue, with a significant proportion of the cooling flow escaping through the nozzle structure itself. This immediately became one of the highest-priority technical risks facing the project.

8.1. Root Cause Investigation

Further testing and design reviews suggested that the defect originated from porosity within the additively manufactured Inconel nozzle.

The issue was concentrated around the throat region, where wall thicknesses approached the practical manufacturing limits of the metal additive manufacturing process used to produce the component. As a result, coolant was able to pass through the nozzle wall rather than remaining within the intended cooling circuit.

With a firing campaign approaching, the challenge was no longer simply understanding the defect, but identifying a practical method of mitigating it without requiring a complete redesign and remanufacture of the engine.

8.2. Evaluation of Repair Strategies

Following the discovery of the defect, I joined the investigation effort and contributed to the assessment of several potential repair strategies.

The options considered included:

• Re-manufacturing the nozzle.
• Hot Isostatic Pressing (HIP).
• Ceramic sealants.
• Alternative coating technologies.
• Operating the engine without modification.

Each option was evaluated against manufacturing complexity, schedule impact, technical risk and expected performance. While a complete re-manufacture would likely eliminate the defect, it was ultimately impractical within the available project timeline and budget.

Following this trade study, the team selected barrier coatings as the preferred repair strategy, and I was tasked with identifying and shortlisting suitable candidate compounds ahead of testing.

8.3. Experimental Evaluation of Ceramic Coatings

Having already worked extensively with the university’s high-temperature furnaces during catalyst manufacture, I worked alongside another team member to evaluate the shortlisted candidate coatings experimentally. This involved assessing adhesion, application behaviour and performance under thermal cycling representative of engine operating conditions. The objective was to identify a coating capable of sealing the porous regions while remaining mechanically stable during operation.

The testing programme identified a Zirconium-oxide based coating as the most promising candidate.

The coating outperformed the alternative solutions evaluated in both adhesion and thermal performance, and was therefore selected as the preferred mitigation strategy.

Although the investigation established a credible path forward and significantly improved confidence in the nozzle hardware, the repair could not be fully verified before the university’s closure. As a result, the team’s near-term objective was scaled back to a monopropellant firing only, reserving bipropellant operation until further verification could be carried out.

9. Preparing for Full-Scale Testing

By the final stages of the project, the major engine subsystems had been validated and manufactured, and the focus shifted towards integrating these developments and preparing the propulsion system as a whole for full-scale testing.

In parallel, the wider team completed development and wet testing of the propellant delivery system, allowing the fuel and oxidiser supply systems to be operated safely and reliably ahead of engine testing.

Together, these activities helped establish the technical, operational and safety foundations required for a full-scale firing campaign.

9.1. Test Campaign Preparation

Preparing for engine testing required significantly more than simply completing the hardware.

The team worked on a range of activities including safety documentation, test planning, logistics, operating procedures and reviews required to support an external firing campaign. These activities were essential to ensure that the propulsion system could be tested safely and effectively once hardware development was complete.

In parallel, preparations continued for testing at an external facility capable of supporting peroxide rocket engine operations.

Following the assessment of several candidate facilities, a suitable external test site was secured within one of the UK’s most historic rocket propulsion centres. The location provided dedicated test infrastructure, nearby propellant storage facilities and the safety provisions required for liquid rocket engine operations. Detailed test procedures, risk assessments and firing logistics were subsequently developed in preparation for the campaign.

9.2. Technical Documentation

Alongside the engineering work, I contributed to the technical documentation required by the external test facility ahead of the firing campaign, including information on the team, propulsion hardware and test procedures.

As part of this effort, I produced a schematic diagram of the propellant delivery system reflecting its configuration at the time, which was used to support the facility’s review of the proposed test campaign.

10. European Space Propulsion Conference 2020

As part of the project’s external communication activities, I authored the abstract submitted to the European Space Propulsion Conference (ESPC 2020).

The submission was accepted, providing an opportunity to present the progress achieved by the Sirius programme to the wider propulsion community. Acceptance of the abstract provided external recognition of the technical work completed throughout the project.

11. Key Outcomes

Although the planned firing campaign was ultimately cancelled as a result of the COVID-19 pandemic, the project achieved several significant technical milestones and substantially increased the readiness of the propulsion system for future testing.

Key outcomes of the work described in this article included:

• Successful validation of the catalyst decomposition chamber above the target operating bed loading.
• Redesign and small-scale hydraulic characterisation of the full-scale oxidiser injector system, though full-scale verification was prevented by the COVID shutdown of the university.
• Manufacture of about 580 g of catalyst, representing more than half of the inventory required for engine testing.
• Optimisation of combustion-chamber inlet conditions through reducer redesign.
• Contribution to diagnosing a major manufacturing defect in the nozzle and developing a viable repair strategy.
• Preparation of the propulsion system and supporting infrastructure for full-scale testing.
• Acceptance of the project for presentation at the European Space Propulsion Conference (ESPC 2020).

Collectively, these activities removed or significantly reduced several of the major technical risks associated with the engine and brought Sirius within reach of its first full-scale firing campaign. Although testing was ultimately prevented by circumstances outside the team’s control, the project concluded with a propulsion system that was considerably more mature, better characterised and closer to operational readiness than at the start of the academic year.

The video below provides a full overview of the project and the progress achieved by the team as of May 2020:

12. Conclusions

Project Sirius provided a rare opportunity to contribute to the development of a complete liquid rocket propulsion system encompassing propulsion design, experimental testing, additive manufacturing, catalyst chemistry, fluid systems and systems engineering.

My work focused on catalyst validation, oxidiser injector development, catalyst manufacturing and combustion system optimisation. My most significant achievement was the successful validation of the catalyst decomposition chamber beyond the full-scale design operating point, an outcome that took three years of development across successive project teams to reach.

The experience provided invaluable insight into the challenges of taking a rocket engine from development and validation towards full-scale experimental testing.