71st International Astronautical Congress (IAC) – The CyberSpace Edition, 12-14 October 2020. Copyright ©2020 by the International Astronautical Federation (IAF). All rights reserved.

[KEYNOTE] IAC-20-C4-7-1

The Synergetic Air-Breathing Rocket (SABRE) - Development Status Update

Feast, S.

Reaction Limited, Building F5, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK [email protected]

Abstract This paper provides an overview and development status update on the Synergetic Air-Breathing (SABRE), currently under development at in the UK. SABRE is a new class of propulsion system which incorporates elements of both air-breathing and rocket technologies with light-weight, high performance heat exchangers. The resulting engine is a high efficiency rocket system, capable of using air as its oxidiser during the initial ascent phase of the launch up to hypersonic (Mach 5) flight speeds in the atmosphere. Beyond this point, SABRE can transition to operate as a pure liquid fuelled rocket engine, using (LOX) from tanks on- board the launch vehicle to continue to provide high thrust propulsion at higher speed and altitude. The air-breathing capability offered by SABRE provides a significant mass saving which enables a number of architectural and design benefits, leading to a practical, low-cost reusable aircraft-like launch vehicle. This brings many positive operational attributes compared to present-day launch vehicles (with vertical take-off and landing), such as improved reliability, increased launch cadenced, robust mission abort capability, greater re-entry cross range and reduced ground operations complexity. The full development of SABRE and its prospective vehicle applications is a complex engineering undertaking, requiring a multitude of sub-systems and technologies. However, due to the modular system architecture of SABRE, many of the technology development activities can be performed in parallel and can also significantly capitalise on the substantial experience and heritage of the existing space launch and aerospace industry, including their well-established supply chain. The overall SABRE development programme follows an agile aerospace development process with a carefully structured incremental development logic, designed to mature the overall system technology to an acceptable level in order to achieve operational readiness with minimum cost and risk. Some new and novel enabling technologies are introduced into the SABRE design (most notably the high performance light-weight heat exchangers) and early de-risk of these have been the focus of the SABRE development programme to date. Reaction Engines have already undergone an extensive and successful development and test programme to de-risk the heat exchanger technology – most recently (in 2019) the air pre-cooler was successfully ground tested under Mach 5 equivalent conditions, becoming a world first achievement in this area of technology. In addition to an update on the SABRE programme, this presentation will also address the key challenges and opportunities associated with the development of the various SABRE technologies

Keywords: (SABRE, Development, Hypersonic, Precooled, Air-breathing, Rocket)

Acronyms/Abbreviations 1. Introduction

Commercial-Off-The-Shelf (COTS) The SABRE engine has been under development by Air-breathing core ground development (DEMO-A) Reaction Engines for over 30 years. During this time a Rocket systems ground development (DEMO-R) significant understanding of its applicability to Space launch has been made, together with a strong maturity Nacelle systems ground development (DEMO-N) of its overall design and performance. Hypersonic Test Bed (HTB) Today, the SABRE programme is receiving High-Temperature Heat Exchanger Test (HTX) significant interest and support from government (LH2) agencies and industry, having demonstrated many of the Liquid Oxygen (LOX) critical technologies previously considered to be a major Return-to-Launch-Site (RTLS) challenge for the overall concept. SABRE is now Single Stage to-Orbit (SSTO) progressing into a new phase, focusing towards integrated development and testing for many of its Synergetic Air-Breathing Rocket Engine (SABRE) major systems and building the supply chain for the full Technology Readiness Level (TRL) product engine. This paper sets out to both introduce the Two-Stage-to Orbit (TSTO) SABRE system as well as provide an overview and status update for the current development programme.

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2. SABRE Engine Overview To achieve the required cooling, an air pre-cooler (heat exchanger) is placed between the air-intake and 2.1 SABRE Operational Overview core engine. The precooler reduces and maintains the intake air temperatures at near constant conditions, SABRE is a combined cycle air-breathing and rocket providing a fixed design point for the core engine. The engine. The design of SABRE is intended to address the precooler also allows continuous operation of the air- limitations of any single propulsion system by breathing core engine beyond the conventional combining , and rocket engine operating limits imposed on conventional gas-turbines technologies, integrated together into a single engine (~Mach 2.5) throughout the entire air-breathing portion nacelle. The main capability SABRE offers is the ability of the ascent trajectory up to Mach 5. to operate as an air-breathing engine for part of the During air-breathing operation, the SABRE cycle ascent, reducing the overall weight of oxidiser carried uses its liquid hydrogen fuel as a heat sink, the cooling on the vehicle, giving a significant improvement in the requires a heat exchanger that is both lightweight, as overall mass ratio of the launch system it is used upon. well as able to extract multiple MW of heat required to reduce the stagnation temperature of captured air at Mach 5 from 1350 K down to low temperatures. A simplified representative overview of the SABRE thermodynamic cycle is presented in figure 2, highlighting the main fluid streams and heat exchangers used for thermal management. The air pre-cooler operates as part of a closed-loop cooling system, which forms an integral part of the core engine thermodynamic power cycle. The cooling loop Fig. 1. SABRE Engine Installed Layout uses high pressure helium as its working fluid and coolant inside the air pre-cooler. The helium rejects heat SABRE has two separate modes of operation during through a separate heat exchanger with the cold the launch ascent trajectory. At take-off from a runway cryogenic liquid hydrogen fuel (already on the vehicle), and up to flight speeds of around Mach 5, SABRE which acts as the overall a heat sink for the cycle. operates in ‘air-breathing mode’, taking in atmospheric The helium is continuously circulated around the air through a conical intake as a source of oxidiser for closed loop between the high-temperature pre-cooler the hydrogen fuel. Beyond Mach 5, the air-breathing (heat source) and very low temperature LH2 heat systems shut down and the intake cone translates to a exchanger (Heat sink). The large temperature difference fully closed position. The engine then transitions over to around the helium loop is used thermodynamically, as a a pure liquid fuelled rocket engine using a separate on- heat engine, as a means of generating internal work; the board supply of liquid oxygen as the source of oxidiser. high-temperature helium downstream of the pre-cooler In ‘rocket mode’ SABRE has higher thrust to continue is expanded through a turbine which drives the core to accelerate the launch vehicle to orbital or near-orbital engine turbo-. In addition to this, the helium velocity, depending on the mission and vehicle design. is also used in a similar way to drive the fuel pump and This results in a single integrated propulsion system that helium circulator turbines. This results in a highly can serve across the entire flight envelope from stand- efficient engine cycle with all combustion gasses being still to orbital insertion speeds with potential to achieve directed for propulsive power out the engine nozzle. Single Stage-to-Orbit (SSTO) or be used as part of a Two-Stage-To-Orbit (TSTO) horizontal launch architecture.

2.2 SABRE Functional Overview

During air-breathing mode, supersonic air is captured by the intake and slowed to sub-sonic speeds inside the engine nacelle, allowing conventional jet- engine compressor technology to be used within the air- breathing core engine. However during this process, the high kinetic energy of the air is converted to heat, due to slowing and compression through the intake and at high Mach numbers this requires significant cooling prior to Fig. 2. Representative SABRE thermodynamic cycle ingestion into the core engine.

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During air-breathing mode, the fuel flow required Reaction Engines has already performed an for air cooling (i.e. for the helium cycle heat rejection) extensive development and test programme to de-risk exceeds the flow actually required for stoichiometric the precooler and other SABRE heat exchanger combustion with the core air stream (i.e. the equivalence technologies – a recent (2019) example being the pre- ratio is greater than 1). The air intake is sized at Mach 5 cooler (‘HTX’) demonstration under Mach 5 equivalent and captures additional airflow than the core engine can conditions, which is discussed in section 5.4.2. handle at lower Mach numbers, the excess is passed down the bypass duct and burned with the excess LH2 through bypass burners, centred around the outside perimeter of the core engine and base of the nacelle. This also produces valuable additional thrust between approximately Mach 2 and 4, as well as providing dynamic control of the intake pressure recovery throughout the air-breathing flight regime. The overall principle of the SABRE operation is summarised in Figure 3, which show the key steps with numbered descriptions below the figure.

Fig. 4. Reaction Engines Air Pre-cooler Heat Exchanger

Reaction Engines is also developing alternative forms of compact high-pressure heat exchangers for the heat rejection system, based on micro-channel plate

Fig. 3. SABRE Engine Functional Overview designs, with incredibly high surface area to volume. At lower flight speeds and for example from take- 1. High speed air entering the engine heats up as it off, when the intake air temperature is much lower, the decelerates to sub-sonic speeds (1250K at Mach 5) heat input to the cycle by the precooler is substituted by a separate heat exchanger and pre-burner, to top up the 2. The air pre-cooler cools the high temperature intake temperature of the helium. The ability to ‘throttle’ air to near-constant compressor inlet conditions between these two systems provides a near constant the 3. The core engine compresses the airflow and rejects heat input to the cycle and hence a single turbine inlet heat to the LH2 fuel via a closed helium circuit design point. Additionally this significantly simplifies the overall cycle performance modelling of the engine. 4. The bypass burners and nozzles accelerate excess airflow to high velocity (fuelled by excess H2 from the 3. Benefits of SABRE core) 3.1 Propulsion Performance Comparison 5. A common nozzle is employed for air-breathing and rocket modes of operation The SABRE engine class can be compared against 6. A separate rocket engine is employed for exo- other engine types to highlight its relative performance atmospheric flight from Mach 5 to orbital velocities and operational benefits. Figure 5 and 6 show and installed thrust-to-weight ratio over a Mach

range from zero to Mach 10. It can be seen that the SABRE is based on well-established physics and combination of the Air-breathing and Rocket systems technology from the aerospace industry - predominantly offered by the SABRE system provide a good air- from and rocket engine systems. The breathing specific impulse across the entire air- primary novel feature of the engine is the integration of breathing phase, giving it a fuel efficiency comparable these conventional systems with ultra-lightweight and to jet engines, however SABRE also has a very compact heat exchangers to extract heat from the competitive thrust-to-weight ratio across all operating incoming air and the key technology that enables this modes compared to other air-breathing engine types. synergy between the air-breathing and rocket engine is These performance benefits are captured in a single the introduction of the air-pre-cooler. In particular, the integrated engine design, making SABRE a very ability to combine air-breathing functionality with a attractive and practical solution for accelerating a rocket engine provides significant overall system weight winged vehicles from zero-forward speed on the savings by reducing the oxidiser required to be carried runway, up to the high altitude and velocities required on-board the vehicle during launch. for Space launch.

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3.2 Impact on Access to Space

The majority of space launch vehicles have been developed almost entirely around the rocket. The technology for all modern rocket engines has principally originated from ballistic missiles, and the fundamental design of space launch vehicles has not changed over many decades (Fig.8). Rocket engine performance is largely dictated by thermodynamic system design, component efficiencies, and the energy potential for the given propellant combination. Overall launch vehicle performance is predominantly driven by the achievable mass fraction, which is dictated by structural materials. Owing to very marginal performance gains in all of these areas, the overall performance and cost for these Fig. 5. SABRE specific impulse (ISP) vs systems is now approaching its fundamental limit. Furthermore, despite their high-level of technical maturity and use of state-of-the-art materials, all rockets using practical design and propellant combinations rely on multi-stage architectures with complex integration in order to deliver a meaningful payload to Earth orbit.

Fig. 8. A selection of space launch vehicle designs over the past 60 Years

Fig. 6. SABRE thrust-to-weight ratio vs Mach number Multi-staging of vertical systems leads to operationally-complex launcher designs, which unlike As discussed, the ability to pre-cool and use any other high-energy transportation system, are largely atmospheric air as the oxidiser during the initial part of based around single-use or ‘expendable’ system the ascent results in an air-breathing engine with high elements. air-breathing thrust and an overall reduced propellant Recently, partial system reusability has emerged, consumption (especially when compared to a pure whereby the larger booster stages of some rockets rocket only based system). An example SABRE launch (typically the most expensive part) are recovered. This vehicle ascent trajectory showing the two distinct provides a positive impact by reducing recurring launch phases for ‘air-breathing’ and ‘rocket’ modes is shown costs, but comes with considerable compromises on in figure 7. payload performance and increased operational complexity. As such, launch prices still remain a significant part of the overall cost for a space mission. Whilst significant cost reductions can be made through hardware reuse, additional improvements can be made within the operational aspects, for example; turnaround time, reliability and mission abort capabilities. SABRE offers improvements in all of these areas by enabling the practical design of winged launch vehicles with ‘aircraft-like’ horizontal take-off and landing operations. The overall weight saving achieved by introducing air-breathing has significant benefits to overall launch Fig. 7 Example SABRE launch vehicle ascent trajectory vehicle design and can be usefully exploited to incorporate systems such as wings and undercarriage for

IAC-20-C4.7.1 Page 4 of 12 71st International Astronautical Congress (IAC) – The CyberSpace Edition, 12-14 October 2020. Copyright ©2020 by the International Astronautical Federation (IAF). All rights reserved. partial or full vehicle recovery as well as enabling more 4. SABRE Engine Applications streamlined aircraft-like operations. As such, SABRE will provide a step change for Space access, improving 4.1 SABRE Launch Vehicles and enabling many disruptive features that are considered highly beneficial to the next generation of A prospective SABRE launch vehicle is essentially a launchers; system reusability, improved mission rocket launch vehicle that has been reconfigured into an operability, increased system reliability, and robust aeroplane and consists of a number of features governed abort strategies (back to launch site or to a suitable by its functional and operational constraints: downrange site) also with the potential to recovery an upper-stage/payload on abort. • Large quantities of cryogenic liquid hydrogen fuel Ultimately the overall aspiration for the space require large fuel tanks (owing to its low density), industry is frequent, reliable, and low-cost access to which require thermal insulation (similar to a rocket). Earth orbit. This is crucial to enabling economically- • High Mach number flight in the atmosphere requires a sustainable space operations and future growth of slender airframe. commercial and industrial space-based activities. Hence, the need for a reliable, low-cost Reusable • Pitch trim necessitates that the centre of mass is Launch Vehicle (RLV) is a critical and underpinning closely-matched with the centre of pressure. This leads part of a future space transport infrastructure. The to a centrally-mounted payload bay splitting the development of horizontally launched, winged launch forebody and aftbody propellant tanks (with the heaviest vehicles and introduction of aircraft-like operations to (LOX) propellant closest inboard). Space transportation will significantly reduce the • Wing-tip mounted engine nacelles, driven by the pitch recurring operational costs currently incurred by Space launch. The resulting reduction in cost/kg to orbit will trim issue above, and to mitigate exhaust plume transform the current launch market and stimulate true impingement effects on the vehicle aftbody. economic growth for the Space industry as a whole. • A low wing (and carry-through structure) allowing Additionally there will likely emerge many unforeseen dorsal payload deployment and to minimise in main- downstream applications of space enabled technologies. gear undercarriage weight.

3.3 SABRE Scalability An example of the resulting configuration for a large SABRE launch vehicle is shown in Figure 9. Design The SABRE concept is scalable and can be used studies at Reaction Engines have shown that this launch across a range of launch vehicle classes for launching vehicle concept has the potential to deliver a 15-tonne small, medium or large payloads. Due to the modular payload to Low-Earth-Orbit (LEO) with Single Stage- nature of the main engine systems, the overall engine to-Orbit (SSTO) capability. However, early variants of a thrust level can be adjusted through design by SABRE vehicle upon entry to service may operate with multiplying the number of air-breathing cores within reduced system performance and technology level, each nacelle, and the number of nacelles on the vehicle. potentially as part of a Two Stage-to-Orbit (TSTO) architecture. In this case, the SABRE vehicle would 3.4 Wider Benefits and Opportunities operate as a winged reusable 1st stage and travel on a sub-orbital trajectory. This would provide the majority The SABRE programme builds on a wide of the velocity required to reach orbit. The vehicle knowledge base from past aerospace and propulsion would then deploy a small 2nd stage to complete the developments, however with many unique opportunities ascent. If expendable, this stage would be relatively for innovation, collaboration and the development of small and low-cost compared with existing upper stages. new skills and knowledge across agencies, industry and academia. The development of SABRE technology will also create a high degree of spin-out into adjacent markets, such as high-speed terrestrial aviation, energy and automotive/motorsport sectors. Reaction engines is already identifying multiple wider applications of its heat exchanger technology and has formed a dedicated part of the business known as Applied Technologies, to address the demand and interest within these adjacent markets and stimulate new ideas and processes that can also feed back into the SABRE programme. Fig. 9 Example SABRE launch vehicle concept

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Initial SABRE vehicle applications can be realised in multi-stage systems enabling high launch cadence with winged booster reusability. However ultimately SABRE stands as the only propulsion concept (using near-term technologies) that offers a potential solution to future generation, fully reusable SSTO launch vehicles whilst being scalable and to support a wider range of multi-stage vehicle architecture. A high-level example of the operational concept (shown for a TSTO mission architecture) is presented in Fig. 11. SABRE Systems Breakdown figure 10 below. Due to the modular system architecture of SABRE, the development activities can largely be performed in parallel for each system group. SABRE can also capitalise on the substantial experience and heritage of the existing space launch and aerospace industry supply chain. It is perceived that the introduction of SABRE will be highly disruptive to the existing space launch market through enabling a new generation of launch vehicles with significantly improved operational characteristics,

Fig. 10. Example SABRE TSTO Vehicle CONOPS such as higher reliability and lower operating costs. As such, the development programme needs to capture a

number of wider strategic, industrial, commercial and 5. SABRE Development Programme political considerations in its approach and undertaking. 5.1 SABRE Development Programme Overview Some of the key primary drivers that shape the The full development of SABRE and its application development programme include: launch vehicle is a complex and expensive undertaking, requiring a multitude of sub-systems. Each of these will 1. Identify a development route that minimises follow the typical aerospace development process, cost/risk/schedule. involving: system and component design and analysis, 2. Minimise the amount of new technology developed. component performance and rig testing, and system 3. Concentrate on the areas of highest technical risk integration with validation in an environment relevant to first. the product architecture. As such, the development programme will require an extensive range of technical 4. Maximise the amount of ground testing and only disciplines working together across multiple teams. resort to flight test when ground testing is not Several novel enabling technologies are also being possible (supports point 1). introduced into the SABRE design, most notably the 5. Exploit SABRE’s modularity by developing the high performance light-weight heat exchangers, for core, nacelle, and rocket systems in parallel which early de-risking tests have been the main focus of (supports point 1). the SABRE development programme to date. 6. Involve the wider international aerospace industry As shown in figure 11, the SABRE engine for their expertise and ability to attract external architecture is broken down into 3 main sub-system funding. groups; 7. Ensure that the SABRE specification is sufficiently 1. Air-breathing core systems broad to meet government and commercial 2. Rocket systems requirements. 3. Nacelle systems 8. Identify spinoff opportunities to stimulate wider funding for the programme. This makes it possible to structure a development programme with multiple activities running in parallel. This also provides a sensible approach to risk by simultaneously working on critical sub-systems and technologies to improve their individual readiness levels, whilst maturing the overall integration design.

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5.2 Air-Breathing Core Engine Development technology programme progressing well as Reaction Engines move into the next phase of the programme. A major advantage of SABRE is that the air- The DEMO-A programme will ultimately be breathing core engine can be entirely ground tested, as complemented by an incremental upgrade programme the nacelle systems (air-intake, pre-cooler and bypass) termed ‘DEMO-A+’ to mature the subsystems provide near-constant compressor inlet conditions to the technology readiness level (TRL) to that necessary to core throughout the entire air-breathing trajectory. As embark on the development of a flight (product) such, the core development programme is largely prototype, as illustrated in figure 13. centred on a ground-based development rig termed ‘DEMO-A’, shown in figure 12. This serves (initially) The DEMO-A+ objectives are to: as a technology development platform to demonstrate • Develop a flight integrated scaled SABRE air- scaled core subsystem performance, and then moving breathing core demonstrator towards an initial scaled system cycle ground demonstrator. • Bring all sub-systems design close to final SABRE performance specification The DEMO-A objectives are to: • Bring all core technologies to TRL 5 • Develop core engine technologies • Develop and verify performance models and control logic • Demonstrate controlled start-up, shutdown • Demonstrate stable operation over a range of throttle conditions • Identify cycle tolerance to component inefficiencies • Demonstrate SABRE Cycle in a scaled ground based configuration • Achieve component TRL 4/5 at a relevant scale Fig. 13. DEMO-A+ Progression

The overall main technical challenges for the air- breathing core engine are listed below:

• High pressure turbomachinery - efficiency, axial bearing load, tip clearance control, rotordynamics • Gearbox - possibly required for main turbo- compressor (high power/speed) • Bearings - space compatible, reusability • Shaft seals - high pressure & speed, low leakage • Hydrogen embrittlement • Heat exchangers - lightweight, high pressure & temperature, low leakage, reusability Fig. 12. DEMO-A air-breathing core development rig • Preburner - stability, equivalence ratio range, temperature uniformity As it can be seen in figure 12, the DEMO-A experimental development rig does not resemble a • Valves - actuation rates, control range and tightly integrated layout as per the final flight engine repeatability design. Instead the DEMO-A design is intended to • Flexible bellows - high pressure & temperature, provide an open architecture for ease of access to many large diameter of the sub-systems, allowing for incremental test and • Core integration and control, stable start-up and replacement, providing a high degree of flexibility shut-down towards an incremental build and test approach. Reaction Engines are working to address many of these The DEMO-A development rig has now challenges and are actively collaborating with multiple successfully completed a Preliminary Design Review partners and suppliers to deliver appropriate solutions. (PDR) with ESA with many of the sub-component

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5.3 Rocket Engine Previous experimental work has been undertaken by Reaction Engines and Airborne Engineering Ltd, The SABRE rocket engine is a high performance, examining a number of different nozzle designs, cryogenic LH2/LOX design. (including adaptive nozzle Expansion/Deflection Reaction Engines has carried out initial concept design concepts, single and dual throats) in order to mature the and analysis of the rocket system in order to specify the understanding of feasibility and performance of the overall rocket system performance and requirements as SABRE dual mode nozzle concept, as highlighted in well as outlining the technology development needs. figure 15.

Fig. 15. Previous integrated air-breathing/rocket nozzle experiments (REL/Airborne)

5.4 Nacelle Systems

The Nacelle has the task of capturing and conditioning the air-flow into the core engine and bypass system. The Nacelle systems comprise of the overall containment aeroshell, which creates the boundary between the external and internal air flow; the intake, which captures air at both sub-sonic and super- sonic flight speeds and decelerates it to sub-sonic. The air-precooler, which cools the high enthalpy air at Fig. 14. Initial SABRE dual-mode nozzle concept elevated flight speeds; and the bypass , which burn the excess hydrogen (resulting from the excess The main technical challenges for the SABRE rocket hydrogen mass flow required for cooling, above the are listed below: need for stoichiometric combustion in the core), using additional air mass flow captured by the intake. The • Staged combustion, high pressure, closed cycle, nacelle systems also account for the overall support and LOX/LH2 Rocket engine thrust structures, including the structural interfaces with • Reusability Aspects the airframe. • / Turbo-pump seals and bearings The main technical challenges for the SABRE nacelle • Dual-mode integrated Air-breathing/Rocket Nozzle are listed below:

• Precooler mass, life, cost Existing experience and test facilities already exist within the industry which can be leveraged for its • Intake , aero-elasticity, variable development. It is anticipated that the entire geometry mechanism, re-entry sealing development of the rocket system will be sub- • Bypass burner equivalence ratio range, nozzle contracted, but managed by the overall engine system construction, variable geometry mechanism integrator. • Acoustic coupling of air intake and bypass burner Initial development work will be centred on system requirements, integrated nozzle technologies, and long • Nacelle shell construction, CMC shingles, life technologies for reusability. This will culminate in a insulation, cooling ground-based development rig termed ‘DEMO-R’, which will mature the sub-systems TRL to that • Thrust Structure MMC struts, assembly/joining necessary to embark on the development of a flight (product) prototype.

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Several of the nacelle technologies are already being Within the context of the SABRE development developed on the ground (figure 16). This work is programme, this flight test vehicle is termed the primarily focused around materials and structures ‘Hypersonic Test Bed’ (‘HTB’). development using thermos-structural rigs, however the air intake and bypass burner aero-thermal design is also 5.4.1 HTB Studies being progressed through a combination of CFD and wind-tunnel testing. Reaction Engines have undertaken an initial design studies into the HTB system architecture and operations. These studies have predominantly focused on the experimental nacelle feasibility and design, however have also included an initial investigation into a number of airframe concepts, with initial analysis of the overall vehicle performance and sizing. The initial objectives of the HTB are primarily to de- risk nacelle systems in flight at a Mach number and altitude that is representative of a SABRE launch vehicle air-breathing trajectory. This would be achieved Fig. 16. Nacelle development activities through an incremental development and flight test programme, using a small unpiloted (automatic, but not Initial research and experimental aerodynamic autonomous) reusable vehicle with horizontal take-off testing of the intake and bypass system will culminate in and landing capability. a ground-based experiment termed ‘DEMO-N’, as well In order to keep the technical complexity, costs and as a parallel nacelle materials and structures technology risks as low as possible, the proposed initial flight test development programme. vehicle would meet a ‘minimum function’ design. It There are however several critical areas of the would purely satisfy the needs of the nacelle nacelle design which cannot be satisfactorily de-risked development programme and would not represent a through simulation and ground testing alone (at a ‘prototype’ or ‘Y-plane’ extensively targeting the representative scale) and necessitate experimental product vehicle validation. Instead it would be testing in real flight conditions. These areas are considered as a bespoke test vehicle or ‘X-plane’ that summarised in Figure 17 and are considered to be a would bridge the TRL gap between technology crucial element of the nacelle development programme. development and prototype, i.e. it would serve as a test Reaction Engines have explored a number of platform to develop key design features and operational possibilities for flight testing of the nacelle systems. aspects of critical sub-systems and technologies at an Owing to the high flight speed conditions during the air- appropriate scale and in a relevant environment. breathing phase of the ascent trajectory, the solution Where the design allows, the HTB would make currently perceived to bring the most value to the extensive use of Commercial-Off-The-Shelf (COTS) programme out of the options considered, is the hardware in order to minimise the development development of a bespoke flight test vehicle. This is timescales and costs, and only use bespoke hardware predominantly based on the lack of availability for where systems do not already exist or are required due existing ground facilities to supply a large airflow (total to shortfalls in performance of existing options. A tunnel flow >5000kg/s) at Mach 5 (1.5km/s) and high primary example of COTS use is in the experimental enthalpy (1250K), and the relative cost that would be nacelle, where the air-breathing core, can be entailed in commissioning new facilities. supplemented by an existing gas turbine engine (fig.18).

Fig. 17. In-flight development needs for the Nacelle

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5.4.2 HTX Programme

Reaction Engines has previously successfully built and demonstrated the air pre-cooler system operating at low ‘cryogenic’ temperatures. More recently however, Reaction Engines has focussed on extending this demonstration by producing a new precooler test article, Fig. 18. HTB Experimental Engine concept with the intent of proving its operation across the full temperature range that it would experience across the Ideally where possible, the selection of this donor jet full air-breathing ascent phase up to Mach 5. This engine would consider the air flow and physical phase of the pre-cooler testing is referred to as the High envelope required for a future scaled air-breathing core Temperature Heat Exchanger (HTX) Programme. derived from the DEMO-A+ programme, to provide a The overall aim of the HTX programme was to potential reconfiguration or replacement nacelle for a demonstrate precooler functional operation at elevated small scale fully integrated air-breathing flight air inlet temperatures and at heatant and coolant fluid demonstration as an evolution of the HTB platform. flow conditions representative of the Mach 5 design Both single and twin nacelle concepts are currently point. The main technical objectives of the HTX being examined for the HTB vehicle concept (fig. 19), Programme were to obtain: which are currently undergoing detailed trades in relation to performance, operability and cost. • Measurement of the precooler performance • Verification of the precooler mechanical design • Validation of aerodynamic and performance models

The heat exchanger test article core and the containment structure used to direct the high- temperature airflow through the core are shown Figure 20. The test site used a repurposed J79 Jet engine to provide hot exhaust gas to around 1000 degrees C with a mass flow between around 4 to 11.5 kg/s. To achieve the range of air-inlet flows and temperatures, the J79 was run at a range of different conditions, with and without afterburner running and compressor bleed mixing with the downstream flow. The test rig also required the full development of a high-pressure helium circulator operating between 0.9 to 1.5kg/s.

Fig. 19. Example Early HTB Vehicle Concepts

Although the primary purpose of the HTB is for nacelle technology development, an extension to the HTB programme may provide potential opportunities to support some aspects of both the air-breathing core and rocket engine development programmes. For example, having developed a flight test platform that is capable of high Mach/Altitude flight, it may be possible to use this capability for in-flight testing of nozzle aerodynamics and air breathing/rocket transition and as previously discussed the platform could also be used for in-flight demonstration of SABRE by substituting the experimental nacelle COTS gas turbine with a flight integrated version of the DEMO-A+ core. Furthermore, the HTB may potentially also be used to develop many of the future launch vehicle airframe technologies (e.g. materials, structures, re-entry) in advance of a full-scale prototype SABRE launch vehicle. Fig. 20. HTX test article core and containment structure

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6. SABRE Development Roadmap

A high-level integrated SABRE development roadmap is presented in Figure 22. This highlights the three main streams of technology development: air- breathing core, rocket engine, and nacelle. These are carried out in parallel, with the HTB flight test programme bridging the nacelle development with the final integrated prototype SABRE. It is anticipated that the integrated prototype engine will undergo a series of ground qualification tests followed by flight testing on a prototype airframe, in order to demonstrate performance, safety, reliability and lifetime, and to achieve certification for the full product launch vehicle. Fig. 21. HTX test site in Colorado, US The product launch vehicle architecture has been matured over the last few decades by Reaction Engines, The programme was a significant challenge for the and has formed a central part of a continuous update to teams involved, with the full design and fabrication of the SABRE system requirements. Technical concepts the test article hardware being performed in the UK, and the operational feasibility of future SABRE- whilst a dedicated test facility was being prepared by powered launch vehicles are currently under our US team over in Colorado. The teams worked development by key potential agency and industry incredibly hard throughout all phases, constantly stakeholders. It is anticipated that continued needing to innovate across all areas of the programme to engagement and collaboration on SABRE will leverage meet the challenging timescales and objectives. the necessary fundraising and industrial support to The overall test programme achieved all objectives it initiate a parallel vehicle development programme. It is set out to and in October 2019 the test team achieved currently anticipated that a mature product launch the full Mach 5 equivalent conditions, which represents vehicle will be fully operational by the 2040s with a critical milestone in demonstrating the precooler potential entry to service with Early Operating technology and has provided key technical information Capability (EOC) in the early-mid 2030s. as Reaction Engines now moves into the next phase of its development programme.

Fig. 22. High-level SABRE development roadmap

IAC-20-C4.7.1 Page 11 of 12 71st International Astronautical Congress (IAC) – The CyberSpace Edition, 12-14 October 2020. Copyright ©2020 by the International Astronautical Federation (IAF). All rights reserved.

7. Next Steps Conclusions

The major next steps for the SABRE programme are Reaction Engines is continuing to rise to the summarised below: challenge by maturing and de-risking the critical SABRE technologies through a carefully structured • Continue with the current DEMO-A design to development and test programme. The SABRE achieve CDR and conclude supporting technology programme is positively supported through close programmes. engagement with UKSA, ESA and other national agencies who are providing technical review as well as • Plan DEMO-A+ programme and harmonise with positive guidance around the future development potential HTB flight testing strategy. strategy.

• Conclude initial studies around HTB options and Reaction Engines has demonstrated a strong work with potential stakeholders to review and commitment to the SABRE programme delivery, with down-select a preferred system concept and all recent major programme milestones passed and are forward programme strategy. progressing well in the air-breathing core development and test programme. This has also helped to establish • Continue to refine SABRE launch vehicle key industrial partnerships and suppliers for the onward application studies to support the final SABRE programme. product engine requirements. The SABRE programme will unlock the future of Reaction Engines is also currently undertaking and low-cost and reliable access to space, as collaborative supporting a number of SABRE vehicle studies with studies with key stakeholders work towards optimising key strategic partners, to examine and develop future SABRE for the next generation of reusable launchers. SABRE launch vehicle concepts and define the final product SABRE engine requirements. The SABRE development programme is also These studies are aiming to optimise the scale and offering wider opportunities for technology spin-out performance for both initial entry into service as well as into a number of high value adjacent markets, including ensuring the SABRE design is scalable to meet the high-speed terrestrial flight as well as energy generation needs of both the future commercial and institutional and automotive applications, stimulating a much launch markets - involving detailed trades around the broader downstream impact to global society. launch service and operational concepts and the impact on the predicted recurring and non-recurring system Acknowledgements costs system. These studies are also bring with them a positive Reaction Engines would like to thank the IAF and focus on identifying the future partners and the supply the Space Propulsion Technical Committee for the chain for the development of the full product engine and opportunity to provide this Keynote paper into the 2020 vehicle systems. IAC conference. Reaction Engines would also like to acknowledge both the UK Space Agency and ESA, who have been extremely supportive and continue to provide both expertise in technical review of the SABRE programme, as well as positive guidance around our forward plans. Additionally Reaction Engines would like to acknowledge the ever growing interactions and collaborations with other agencies, industrial partners and suppliers who are engaging with the SABRE development programme and our future studies. Finally a strong acknowledgment is due to the dedication and passion of all the teams and individuals working at Reaction Engines, for carrying the SABRE project forwards and pursuing an exciting vision and journey towards enabling future low-cost and reliable Fig. 23 Wide range of SABRE launch vehicle studies access to space.

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