SABRE (rocket engine)

<h2 id="history">History</h2>
The precooler concept evolved from an idea originated by Robert P. Carmichael in 1955.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a> This was followed by the <a href="/facts/Liquid_air_cycle_engine/ySDzpQd0">liquid air cycle engine</a> (LACE) idea which was originally explored by <a href="/facts/General_Dynamics/LoTpPuU6">General Dynamics</a> in the 1960s as part of the <a href="/facts/US_Air_Force/7MxWBYlc">US Air Force</a>'s <a href="/facts/Aerospaceplane/2b59V3fR">aerospaceplane</a> efforts.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a>
The LACE system was to be placed behind a supersonic air intake which would compress the air through ram compression, then a <a href="/facts/Heat_exchanger/CUzlMnBG">heat exchanger</a> would rapidly cool it using some of the <a href="/facts/Liquid_hydrogen/Vpk3Gb8P">liquid hydrogen</a> fuel stored on board. The resulting liquid air was then processed to separate the <a href="/facts/Liquid_oxygen/qnI0KDTb">liquid oxygen</a> for combustion. The amount of warmed hydrogen was too great to burn with the oxygen, so most was to be expelled, giving useful thrust, but greatly reducing the potential efficiency.
Instead, as part of the <a href="/facts/HOTOL/do33C6Pf">HOTOL</a> project, the liquid air cycle engine (LACE) based <a href="/facts/RB545/do33C6Pf">RB545</a> engine was developed with more efficient cycle. The engine was given the Rolls-Royce name "Swallow".<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a> In 1989, after funding for HOTOL ceased, Bond and several others formed <a href="/facts/Reaction_Engines_Limited/X5lDckBh">Reaction Engines Limited</a> to continue research. The RB545's precooler had issues with <a href="/facts/Hydrogen_embrittlement/J3JCu2Y6">embrittlement</a> and excess liquid hydrogen consumption, and was encumbered by both patents and the UK's <a href="/facts/Official_Secrets_Act/hCz5uYyL">Official Secrets Act</a>, so Bond developed SABRE instead.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a>
In 2016 the project received £60m in funds from the UK government and ESA for a demonstrator involving the full cycle.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a>
In July 2021 the UK Space Agency provided a further £3.9m for continued development.<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a>

<h2 id="concept">Concept</h2>
Like the <a href="/facts/RB545/do33C6Pf">RB545</a>, the SABRE design was neither a conventional <a href="/facts/Rocket_engine/XT5gaFAb">rocket engine</a> nor a conventional <a href="/facts/Turbojet/tr3vYndL">jet engine</a>, but a hybrid that used air from the environment at low speeds/altitudes, and stored <a href="/facts/Liquid_oxygen/qnI0KDTb">liquid oxygen</a> at higher altitude. The SABRE engine "relies on a heat exchanger capable of cooling incoming air to −150 °C (−238 °F), to provide oxygen for mixing with hydrogen and provide jet thrust during atmospheric flight before switching to tanked liquid oxygen when in space."
In air-breathing mode, air would enter the engine through an inlet. A bypass system would then direct some of the air through a precooler into a compressor, which would inject it into a combustion chamber where it would be burnt with fuel, the exhaust products then accelerated through nozzles to provide thrust. The remainder of the intake air would continue through the bypass system to a ring of flame holders which act as a ramjet for part of the air breathing flight regime. A helium loop would be used to transfer the heat from the precooler to the fuel and drive the engine pumps and compressors.

<h3>Inlet</h3>
At the front of the engine, the concept designs proposed a simple translating axisymmetric <a href="/facts/Inlet_cone/4sEVTE72">shock cone inlet</a> which would compress and slow the air (relative to the engine) to subsonic speeds using two shock reflections. Accelerating the air to the speed of the engine would incur <a href="/facts/Ram_drag/kJzC06AK">ram drag</a>. As a result of the shocks, compression, and acceleration the intake air would be heated, reaching around 1,000 °C (1,830 °F) at Mach 5.5.
<a href="/facts/Bayern-Chemie/MPejVU7V">Bayern-Chemie</a>, through ESA, had undertaken work to refine and test the intake and bypass systems<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a>

<h3>Precooler</h3>
As the air would enter the engine at <a href="/facts/Supersonic/D3VpzmlC">supersonic</a> or <a href="/facts/Hypersonic/QUsQbhur">hypersonic</a> speeds, it would become hotter than the engine can withstand due to compression effects.<a class="footnote-ref" id="fnref:18" href="#fn:18">18</a> <a href="/facts/Jet_engines/CJjFpeqt">Jet engines</a>, which have the same problem but to a lesser degree, solve it by using heavy <a href="/facts/Copper/I8PEE8oX">copper</a> or <a href="/facts/Nickel/KwMCHYRP">nickel</a>-based materials, by reducing the engine's <a href="/facts/Overall_pressure_ratio/dFVVlj2X">pressure ratio</a>, and by throttling back the engine at the higher airspeeds to avoid melting. However, for a <a href="/facts/Single-stage-to-orbit/4qPbu35V">single-stage-to-orbit</a> (SSTO) spaceplane, such heavy materials are unusable, and maximum thrust is necessary for orbital insertion at the earliest time to minimise <a href="/facts/Gravity_drag/mGTumK3J">gravity losses</a>. Instead, using a gaseous <a href="/facts/Helium/yRul93TG">helium</a> coolant loop, SABRE would dramatically cool the air from 1,000 °C (1,830 °F) down to −150 °C (−238 °F) in a counterflow heat exchanger while avoiding <a href="/facts/Liquid_air/soClHXAx">liquefaction</a> of the air or blockage from freezing water vapour. The counterflow heat exchanger would also allow the helium to exit the engine at a sufficiently high temperature to drive pumps and compressors for the liquid hydrogen fuel and helium working fluid itself.
Previous versions of precoolers such as HOTOL put the hydrogen fuel directly through the precooler. SABRE would insert a helium cooling loop between the air and the cold fuel to avoid problems with <a href="/facts/Hydrogen_embrittlement/J3JCu2Y6">hydrogen embrittlement</a> in the precooler.
The dramatic cooling of the air created a potential problem: it would be necessary to prevent blocking the precooler from frozen water vapour and other air fractions. In October 2012, the cooling solution was demonstrated for 6 minutes using freezing air.<a class="footnote-ref" id="fnref:19" href="#fn:19">19</a> The cooler would consist of a fine pipework heat exchanger with 16,800 thin-walled tubes,<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a> and would cool the hot in-rushing atmospheric air down to the required −150 °C (−238 °F) in 0.01 s.<a class="footnote-ref" id="fnref:21" href="#fn:21">21</a> The ice prevention system had been a closely guarded secret, but REL disclosed a <a href="/facts/Methanol/EIfpoNCg">methanol</a>-injecting 3D-printed de-icer in 2015 through patents, as they needed partner companies and could not keep the secret while working closely with outsiders.<a class="footnote-ref" id="fnref:22" href="#fn:22">22</a><a class="footnote-ref" id="fnref:23" href="#fn:23">23</a><a class="footnote-ref" id="fnref:24" href="#fn:24">24</a>

<h3>Compressor</h3>
Below five times the <a href="/facts/Speed_of_sound/dYtbt1nf">speed of sound</a> and 25 kilometres of altitude, which are 20% of the speed and 20% of the altitude needed to reach <a href="/facts/Orbit/nD2TPySG">orbit</a>, the cooled air from the precooler would pass into a modified turbo-<a href="/facts/Gas_compressor/I0vV0Tx2">compressor</a>, similar in design to those used on conventional jet engines but running at an unusually high <a href="/facts/Overall_pressure_ratio/dFVVlj2X">pressure ratio</a> made possible by the low temperature of the inlet air. The compressor would feed the compressed air at 140 <a href="/facts/Atmospheres_(unit)/aTbYALII">atmospheres</a> into the combustion chambers of the main engines.<a class="footnote-ref" id="fnref:25" href="#fn:25">25</a>
In a conventional jet engine, the turbo-compressor is driven by a <a href="/facts/Gas_turbine/JJJNwXrG">gas turbine</a> powered by combustion gases. SABRE would drive the turbine with a helium loop, which would be powered by heat captured in the precooler and a preburner.<a class="footnote-ref" id="fnref:26" href="#fn:26">26</a>

<h3>Helium loop</h3>
The 'hot' helium from the air precooler would be recycled by cooling it in a heat exchanger with the liquid hydrogen fuel. The loop would formsm a self-starting <a href="/facts/Brayton_cycle/jH9TTdoK">Brayton cycle</a> engine, cooling critical parts of the engine and powering turbines. The heat would pass from the air into the helium. This heat energy would then be used to power various parts of the engine and to vaporise hydrogen, which would then be burnt in <a href="/facts/Ramjet/wFschau9">ramjets</a>.<a class="footnote-ref" id="fnref:27" href="#fn:27">27</a><a class="footnote-ref" id="fnref:28" href="#fn:28">28</a>

<h3>Combustion chambers</h3>
The combustion chambers in the SABRE engine would be cooled by the oxidant (air/liquid oxygen) rather than by liquid hydrogen<a class="footnote-ref" id="fnref:29" href="#fn:29">29</a> to further reduce the system's use of liquid hydrogen compared with <a href="/facts/Air-fuel_ratio/l2BGvlyw">stoichiometric</a> systems.

<h3>Nozzles</h3>
The most efficient atmospheric pressure at which a conventional <a href="/facts/Propelling_nozzle/GDstAvRx">propelling nozzle</a> works is set by the <a href="/facts/Geometry/wTQf5ffQ">geometry</a> of the <a href="/facts/De_Laval_nozzle/5LcDcs69">nozzle bell</a>. While the geometry of the conventional bell remains static the <a href="/facts/Atmospheric_pressure/wJLMSaVG">atmospheric pressure changes with altitude</a> and therefore nozzles designed for high performance in the lower atmosphere lose efficiency as they reach higher altitudes. In traditional rockets this is overcome by using multiple stages designed for the atmospheric pressures they encounter.
The SABRE engine would have to operate at both low and high altitude scenarios. To ensure efficiency at all altitudes a sort of moving, <a href="/facts/Expanding_nozzle/7qdodhK5">expanding nozzle</a> would be used. First at low altitude, air-breathing flight the bell would be located rearwards, connected to a toroidal combustion chamber surrounding the top part of the nozzle, together forming an <a href="/facts/Expansion_deflection_nozzle/b9DJaCMj">expansion deflection nozzle</a>. When SABRE later transitions into rocket mode, the bell would be moved forwards, extending the length of the bell of the inner rocket combustion chamber, creating a much larger, high altitude nozzle for more efficient flight.<a class="footnote-ref" id="fnref:30" href="#fn:30">30</a>

<h3>Bypass burners</h3>
Avoiding <a href="/facts/Liquid_air/soClHXAx">liquefaction</a> would improve the efficiency of the engine since less <a href="/facts/Entropy/s52IXeFz">entropy</a> would be generated and therefore less liquid hydrogen would be boiled off. However, simply cooling the air would need more liquid hydrogen than could be burnt in the engine core. The excess would be expelled through a series of burners called "spill duct <a href="/facts/Ramjet/wFschau9">ramjet</a> burners",<a class="footnote-ref" id="fnref:31" href="#fn:31">31</a><a class="footnote-ref" id="fnref:32" href="#fn:32">32</a> that would be arranged in a ring around the central core. These would be fed air that bypasses the precooler. This bypass ramjet system was designed to reduce the negative effects of drag resulting from air that would pass into the intakes but would not be fed into the main rocket engine, rather than generating thrust. At low speeds the ratio of the volume of air entering the intake to the volume that the compressor could feed to the combustion Chambers would be at its highest, requiring the bypassed air to be accelerated to maintain efficiency at these low speeds. This distinguished the system from a <a href="/facts/Air_turborocket/koKP4mUu">turboramjet</a> where a turbine-cycle's exhaust is used to increase air-flow for the ramjet to become efficient enough to take over the role of primary propulsion.<a class="footnote-ref" id="fnref:33" href="#fn:33">33</a>

<h2 id="development">Development</h2>

Tests were carried out in 2008 by Airborne Engineering Ltd on an <a href="/facts/Expansion_deflection_nozzle/b9DJaCMj">expansion deflection nozzle</a> called STERN to provide the data needed to develop an accurate engineering model to overcome the problem of non-dynamic exhaust expansion. This research continued with the STRICT nozzle in 2011.
Successful tests of an oxidiser (both air and oxygen) cooled combustion chamber were conducted by EADS-Astrium at <a href="/facts/Lampoldshausen/J3xaqX5s">Institute of Space Propulsion</a> in 2010.
In 2011, hardware testing of the heat exchanger technology "crucial to [the] hybrid air- and liquid oxygen-breathing [SABRE] rocket motor" was completed, demonstrating that the technology is viable.<a class="footnote-ref" id="fnref:34" href="#fn:34">34</a><a class="footnote-ref" id="fnref:35" href="#fn:35">35</a> The tests validated that the heat exchanger could perform as needed for the engine to obtain adequate oxygen from the atmosphere to support the low-altitude, high-performance operation.<a class="footnote-ref" id="fnref:36" href="#fn:36">36</a><a class="footnote-ref" id="fnref:37" href="#fn:37">37</a>
In November 2012, Reaction Engines announced it had successfully concluded a series of tests that prove the cooling technology of the engine, one of the main obstacles towards the completion of the project. The <a href="/facts/European_Space_Agency/L8YRNV0C">European Space Agency</a> (ESA) evaluated the SABRE engine's precooler heat exchanger, and accepted claims that the technologies required to proceed with the engine's development had been fully demonstrated.<a class="footnote-ref" id="fnref:38" href="#fn:38">38</a><a class="footnote-ref" id="fnref:39" href="#fn:39">39</a><a class="footnote-ref" id="fnref:40" href="#fn:40">40</a>
In June 2013 the United Kingdom government announced further support for the development of a full-scale prototype of the SABRE engine,<a class="footnote-ref" id="fnref:41" href="#fn:41">41</a> providing £60M of funding between 2014 and 2016<a class="footnote-ref" id="fnref:42" href="#fn:42">42</a><a class="footnote-ref" id="fnref:43" href="#fn:43">43</a> with the ESA providing an additional £7M.<a class="footnote-ref" id="fnref:44" href="#fn:44">44</a> The total cost of developing a test rig was estimated at £200M.<a class="footnote-ref" id="fnref:45" href="#fn:45">45</a>
By June 2015, SABRE's development continued with The Advanced Nozzle Project at Westcott. The test engine, operated by Airborne Engineering Ltd., was used to analyze the <a href="/facts/Aerodynamics/Cn6Lnl3H">aerodynamics</a> and performance of the advanced nozzles that the SABRE engine would use, in addition to new manufacturing technologies such as the 3D-printed propellant injection system.<a class="footnote-ref" id="fnref:46" href="#fn:46">46</a>
In April 2015, the SABRE engine concept passed a theoretical feasibility review conducted by the U.S. <a href="/facts/Air_Force_Research_Laboratory/QGBMMKKa">Air Force Research Laboratory</a>.<a class="footnote-ref" id="fnref:47" href="#fn:47">47</a><a class="footnote-ref" id="fnref:48" href="#fn:48">48</a><a class="footnote-ref" id="fnref:49" href="#fn:49">49</a> The laboratory was to reveal two-stage-to-orbit SABRE concepts shortly afterwards, as they considered that a single-stage-to-orbit Skylon space plane was "technically very risky as a first application of SABRE engine".<a class="footnote-ref" id="fnref:50" href="#fn:50">50</a>
In August 2015 the <a href="/facts/European_Commission/Cc5JgbEI">European Commission</a> competition authority approved UK government funding of £50 million for further development of the SABRE project. This was approved on the grounds that money raised from <a href="/facts/Private_equity/DpqoqjBH">private equity</a> had been insufficient to bring the project to completion.<a class="footnote-ref" id="fnref:51" href="#fn:51">51</a> In October 2015 British company <a href="/facts/BAE_Systems/W1wLXh5x">BAE Systems</a> agreed to buy a 20% stake in the company for <a href="/facts/Pound_sign/1w2ueXso">£</a>20.6 million as part of an agreement to help develop the SABRE hypersonic engine.<a class="footnote-ref" id="fnref:52" href="#fn:52">52</a><a class="footnote-ref" id="fnref:53" href="#fn:53">53</a>
In 2016, Reaction CEO Mark Thomas announced planned to build a quarter-sized ground test engine, given limitations of funding.<a class="footnote-ref" id="fnref:54" href="#fn:54">54</a>
In September 2016 agents acting on behalf of Reaction Engines applied for planning consent to build a rocket engine test facility at the site of the former <a href="/facts/Rocket_Propulsion_Establishment/PLMoXvA8">Rocket Propulsion Establishment</a> in Westcott, UK<a class="footnote-ref" id="fnref:55" href="#fn:55">55</a> which was granted in April 2017,<a class="footnote-ref" id="fnref:56" href="#fn:56">56</a> and in May 2017 a <a href="/facts/Groundbreaking/aF7z7Sgr">groundbreaking</a> ceremony was held to announce the beginning of construction of the SABRE TF1 engine test facility, expected to become active in 2020.<a class="footnote-ref" id="fnref:57" href="#fn:57">57</a><a class="footnote-ref" id="fnref:58" href="#fn:58">58</a> However, development of the TF1 facility was since quietly dropped, and the site was taken on by aerospace and defence group <a href="/facts/Nammo/PFbM8xw6">Nammo</a>.<a class="footnote-ref" id="fnref:59" href="#fn:59">59</a>
In September 2017 it was announced the United States <a href="/facts/DARPA/Uf9sarhT">Defence Advanced Research Projects Agency</a> (DARPA) had contracted with Reaction Engines Inc. to build a high-temperature airflow test facility at Front Range Airport near Watkins, Colorado.<a class="footnote-ref" id="fnref:60" href="#fn:60">60</a> The DARPA contract was to test the Sabre engine's pre-cooler heat exchanger (HTX). Construction of the test facilities and test articles began in 2018 with testing focusing on running the HTX at temperatures simulating air coming through a subsonic intake travelling at Mach 5 or around 1,800 °F (1,000 °C) beginning in 2019.<a class="footnote-ref" id="fnref:61" href="#fn:61">61</a><a class="footnote-ref" id="fnref:62" href="#fn:62">62</a>
The HTX test unit was completed in the UK and sent to Colorado in 2018, where on 25 March 2019 an <a href="/facts/F-4/epGAnaxt">F-4</a> GE <a href="/facts/J79/xa7dWaIN">J79</a> turbojet exhaust was mixed with ambient air to replicate Mach 3.3 inlet conditions, successfully quenching a 420 °C (788 °F) stream of gases to 100 °C (212 °F) in less than 1/20 of a second. Further tests simulating Mach 5 were planned, with temperature reduction expected from 1,000 °C (1,830 °F).<a class="footnote-ref" id="fnref:63" href="#fn:63">63</a><a class="footnote-ref" id="fnref:64" href="#fn:64">64</a> These further tests were successfully completed by October 2019.<a class="footnote-ref" id="fnref:65" href="#fn:65">65</a><a class="footnote-ref" id="fnref:66" href="#fn:66">66</a><a class="footnote-ref" id="fnref:67" href="#fn:67">67</a>
The successful HTX test was thought to maybe lead to spin-off precooler applications which could be developed before a scalable SABRE demonstrator was completed; suggested uses were to expand <a href="/facts/Gas_turbine/JJJNwXrG">gas turbines</a> capabilities, in advanced <a href="/facts/Turbofan/x8XWVw19">turbofans</a>, hypersonic vehicles, and industrial applications.<a class="footnote-ref" id="fnref:68" href="#fn:68">68</a>
In March 2019, the <a href="/facts/UK_Space_Agency/fTSBbRcI">UKSA</a> and ESA preliminary design review of the demonstrator engine core confirmed the test version to be ready for implementation.<a class="footnote-ref" id="fnref:69" href="#fn:69">69</a>
In 2019, Airborne Engineering conducted a test campaign on subscale air/hydrogen injectors for the SABRE preburners.<a class="footnote-ref" id="fnref:70" href="#fn:70">70</a>
In 2020, Airborne Engineering conducted a test campaign on an "HX3 module" (preburner to helium loop heat exchanger).<a class="footnote-ref" id="fnref:71" href="#fn:71">71</a>
In 2022, a Foreign Comparative Testing of Reaction’s precooler heat exchanger was performed. The testing was successfully completed by the company’s US subsidiary (Reaction Engines Incorporated – REI) and the US Air Force Research Laboratory (AFRL). “The FCT test program greatly expanded the demonstrated capabilities of our engine precooler technology”, said REI’s director of engineering, Andrew Piotti. “During these recent tests, the precooler successfully achieved our objective of over 10 megawatts of transferred thermal energy from the high-temperature airflow, which is three times higher than our previous test program.”<a class="footnote-ref" id="fnref:72" href="#fn:72">72</a>

<h3>Engine</h3>
Due to the static thrust capability of the hybrid rocket engine, the vehicle could <a href="/facts/Takeoff/kMbvceUo">take off</a> under air-breathing mode, much like a conventional <a href="/facts/Turbojet/tr3vYndL">turbojet</a>.<a class="footnote-ref" id="fnref:73" href="#fn:73">73</a> As the craft would ascend and the outside air pressure drop, more and more air would be passed into the compressor as the effectiveness of the ram compression drops. In this fashion the jets would be able to operate to a much higher altitude than would normally be possible.
At Mach 5.5 the air-breathing system would become inefficient and would be powered down, replaced by the on-board stored oxygen which would allow the engine to accelerate to orbital velocities (around Mach 25).<a class="footnote-ref" id="fnref:74" href="#fn:74">74</a>

<h2 id="evolution">Evolution</h2>
<h3>RB545</h3>
Designed for use with HOTOL.
The engine had no air-breathing static thrust capability, relying on a rocket trolley to achieve takeoff.

<h3>SABRE</h3>
Designed for use with Skylon A4.
The engine had no air-breathing static thrust capability, relying on RATO engines.

<h3>SABRE 2</h3>
Designed for use with Skylon C1.
The engine had no static thrust capability, using LOX until the air-breathing cycle could take over.

<h3>SABRE 3</h3>
Designed for use with Skylon C2.
This engine included a fuel rich preburner to augment the heat recovered from the airstream used to drive the helium loop, giving the engine static thrust capability.

<h3>SABRE 4</h3>
SABRE 4 was no longer a single engine design, but a class of engines, e.g. a 0.8–2 MN (180,000–450,000 lbf; 82–204 tf) instance of this engine would have been used with SKYLON D1.5, a 110,000–280,000 lbf (0.49–1.25 MN; 50–127 tf) for a USAF study into a partially reusable TSTO.

<h2 id="performance">Performance</h2>
The designed <a href="/facts/Thrust-to-weight_ratio/lgAXJLRJ">thrust-to-weight ratio</a> of SABRE is fourteen compared to about five for conventional jet engines, and two for <a href="/facts/Scramjet/2PdT7gKc">scramjets</a>.<a class="footnote-ref" id="fnref:75" href="#fn:75">75</a> This high performance is a combination of the denser, cooled air, requiring less compression, and, more importantly, the low air temperatures permitting lighter alloys to be used in much of the engine. Overall performance is much better than the <a href="/facts/RB545/do33C6Pf">RB545</a> engine or scramjets.
<a href="/facts/Fuel_efficiency/URhdQ7Wl">Fuel efficiency</a> (known as <a href="/facts/Specific_impulse/JKGgw2I4">specific impulse</a> in rocket engines) peaks at about 3500 <a href="/facts/Specific_impulse/JKGgw2I4">seconds</a> within the atmosphere.<a class="footnote-ref" id="fnref:76" href="#fn:76">76</a> Typical all-rocket systems peak around 450 seconds and even "typical" <a href="/facts/Nuclear_thermal_rocket/p2uSHk3l">nuclear thermal rockets</a> at about 900 seconds.
The combination of high <a href="/facts/Fuel_efficiency/URhdQ7Wl">fuel efficiency</a> and low-mass engines permits an SSTO approach, with air-breathing to Mach 5.14+ at 28.5 km (94,000 ft) altitude, and with the vehicle reaching orbit with more payload mass per take-off mass than just about any non-<a href="/facts/Nuclear_pulse_propulsion/K9W09ulf">nuclear</a> launch vehicle ever proposed.
The precooler adds mass and complexity to the system and is the most aggressive and difficult part of the design, but the mass of this heat exchanger is an order of magnitude lower than has been achieved previously. The experimental device achieved heat exchange of almost 1 GW/m3. The losses from carrying the added weight of systems shut down during the closed cycle mode (namely the precooler and turbo-compressor) as well as the added weight of Skylon's wings are offset by the gains in overall efficiency and the proposed flight plan. Conventional launch vehicles such as the <a href="/facts/Space_Shuttle/7BoyetEv">Space Shuttle</a> spend about one-minute climbing almost vertically at relatively low speeds; this is inefficient but optimal for pure-rocket vehicles. In contrast, the SABRE engine permits a much slower, shallower climb (thirteen minutes to reach the 28.5 km transition altitude), while breathing air and using its wings to support the vehicle. This trades gravity drag and an increase in vehicle weight for a reduction in propellant mass and a gain from aerodynamic lift increasing <a href="/facts/Payload_fraction/h23VrNen">payload fraction</a> to the level at which SSTO becomes possible.
A hybrid jet engine like SABRE needs only reach low <a href="/facts/Hypersonic/QUsQbhur">hypersonic</a> speeds inside the <a href="/facts/Stratosphere/taBra67V">lower atmosphere</a> before engaging its closed cycle mode, whilst climbing, to build speed. Unlike <a href="/facts/Ramjet/wFschau9">ramjet</a> or <a href="/facts/Scramjet/2PdT7gKc">scramjet</a> engines, the design is able to provide high thrust from zero speed up to Mach 5.4,<a class="footnote-ref" id="fnref:77" href="#fn:77">77</a> with excellent thrust over the entire flight, from the ground to very high altitude, with high efficiency throughout. In addition, this static thrust capability means the engine can be realistically tested on the ground, which drastically cuts testing costs.<a class="footnote-ref" id="fnref:78" href="#fn:78">78</a>
In 2012, REL expected test flights by 2020, and operational flights by 2030.<a class="footnote-ref" id="fnref:79" href="#fn:79">79</a>

<h2 id="see-also">See also</h2>

<ul><li><a href="/facts/Altitude_compensating_nozzle/LBQMDjE7">Altitude compensating nozzle</a></li>
<li><a href="/facts/Avatar_(spacecraft)/G3NXFgfE">Avatar (spacecraft)</a> – India's air-breathing spaceplane</li>
<li><a href="/facts/Liquid-propellant_rocket/TAonrVDn">Liquid-propellant rocket</a></li>
<li><a href="/facts/Precooled_jet_engine/PfGMWtvB">Precooled jet engine</a></li></ul>

<h2 id="resources">Resources</h2>
<ul><li><a href="https://web.archive.org/web/20110615104534/http://www.reactionengines.co.uk/tech_docs/JBIS_v57_22-32.pdf">"The Skylon Space plane"</a> (PDF). <a href="/facts/Reaction_Engines_Limited/X5lDckBh">Reaction Engines Limited</a>. Archived from <a href="http://www.reactionengines.co.uk/tech_docs/JBIS_v57_22-32.pdf">the original</a> (PDF) on 15 June 2011.</li>
<li><a href="https://web.archive.org/web/20110615133300/http://www.reactionengines.co.uk/tech_docs/The%20SKYLON%20Spaceplane-Progress%20to%20Realisation%2C%20JBIS%2C%202008.pdf">"The Skylon Spaceplane: Progress To Realisation"</a> (PDF). <a href="/facts/Reaction_Engines_Limited/X5lDckBh">Reaction Engines Limited</a>. Archived from <a href="http://www.reactionengines.co.uk/tech_docs/The%20SKYLON%20Spaceplane-Progress%20to%20Realisation,%20JBIS,%202008.pdf">the original</a> (PDF) on 15 June 2011.</li>
<li><a href="https://web.archive.org/web/20110615104428/http://www.reactionengines.co.uk/tech_docs/JBIS_v56_108-117.pdf">"A Comparison of Propulsions Concepts for SSTO Reusable launchers"</a> (PDF). <a href="/facts/Reaction_Engines_Limited/X5lDckBh">Reaction Engines Limited</a>. Archived from <a href="http://www.reactionengines.co.uk/tech_docs/JBIS_v56_108-117.pdf">the original</a> (PDF) on 15 June 2011.</li></ul>
<h2 id="external-links">External links</h2>

Wikimedia Commons has media related to SABRE (rocket engine).

<ul><li><a href="https://www.reactionengines.co.uk/sabre">Official website</a></li>
<li>Volpicelli, Gian M. (12 October 2015). <a href="https://www.wired.co.uk/magazine/archive/2015/11/start/rocket-engine-planes-space">"This rocket engine could soon put planes in outer space"</a>. <a href="/facts/Wired_UK/zpYlV116">Wired UK</a>.</li>
<li>Tony Osborne (4 May 2017). <a href="http://aviationweek.com/technology/reaction-engines-begins-test-facility-construction">"Reaction Engines Begins Test Facility Construction"</a>. Aerospace Daily & Defense Report. Aviation Week.</li>
<li><a href="https://www.youtube.com/watch?v=srZzRVNj6uI">TF1 time lapse – August 2018</a></li>
<li><a href="https://web.archive.org/web/20110615124053/http://www.reactionengines.co.uk/images/sabre/library/sabre_cycle_m.jpg">Archive of Reaction Engines flow diagram for the SABRE cycle</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">"SABRE – Synergetic Air Breathing Rocket Engine". Reaction Engines Limited. Archived from the original on 19 December 2018. Retrieved 18 December 2018. <a href="https://www.reactionengines.co.uk/sabre" target="_blank">https://www.reactionengines.co.uk/sabre</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">"A Comparison of Propulsions Concepts for SSTO Reusable launchers" (PDF). Reaction Engines Limited. pp. 114, 115. Archived from the original (PDF) on 15 June 2011. Retrieved 2 August 2010. <a href="https://web.archive.org/web/20110615104428/http://www.reactionengines.co.uk/tech_docs/JBIS_v56_108-117.pdf" target="_blank">https://web.archive.org/web/20110615104428/http://www.reactionengines.co.uk/tech_docs/JBIS_v56_108-117.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">"Flight Applications". Reaction Engines. <a href="https://www.reactionengines.co.uk/sabre/flight-applications" target="_blank">https://www.reactionengines.co.uk/sabre/flight-applications</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">"Alan Bond Interview". Vimeo. Retrieved 19 December 2017. <a href="https://vimeo.com/45136248" target="_blank">https://vimeo.com/45136248</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Foust, Jeff (11 November 2024). "Spaceplane developer Reaction Engines goes bankrupt". SpaceNews. Retrieved 28 January 2025. <a href="https://spacenews.com/spaceplane-developer-reaction-engines-goes-bankrupt/" target="_blank">https://spacenews.com/spaceplane-developer-reaction-engines-goes-bankrupt/</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">"Skylon Users' Manual" (PDF). Reaction Engines Limited. 18 January 2010. pp. 4, 3. Archived from the original (PDF) on 18 April 2016. Retrieved 2 August 2010. <a href="https://web.archive.org/web/20160418035934/http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf" target="_blank">https://web.archive.org/web/20160418035934/http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">"UK's Sabre space plane engine tech in new milestone". BBC News. 8 April 2019. <a href="https://www.bbc.co.uk/news/science-environment-47832920" target="_blank">https://www.bbc.co.uk/news/science-environment-47832920</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">"The Sensitivity of Precooled Air-Breathing Engine Performance to Heat Exchanger Design Parameters" (PDF). Reaction Engines Limited. 29 March 2007. p. 189. Archived from the original (PDF) on 23 June 2013. Retrieved 9 August 2010. <a href="https://web.archive.org/web/20130623100631/http://www.reactionengines.co.uk/tech_docs/JBIS_v60_188-196.pdf" target="_blank">https://web.archive.org/web/20130623100631/http://www.reactionengines.co.uk/tech_docs/JBIS_v60_188-196.pdf</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">"Skylon Users' Manual" (PDF). Reaction Engines Limited. 18 January 2010. pp. 4, 3. Archived from the original (PDF) on 18 April 2016. Retrieved 2 August 2010. <a href="https://web.archive.org/web/20160418035934/http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf" target="_blank">https://web.archive.org/web/20160418035934/http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">"Reaction Engines has reached a number of high profile milestones". Reaction Engines. 2018. Retrieved 8 April 2019. <a href="https://www.reactionengines.co.uk/about/story-so-far" target="_blank">https://www.reactionengines.co.uk/about/story-so-far</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">"Liquid Hydrogen as a Propulsion Fuel, 1945–1959". NASA History Division. Archived from the original on 16 April 2015. Retrieved 1 July 2009. <a href="https://web.archive.org/web/20150416163620/http://www.hq.nasa.gov/pao/History/SP-4404/ch7-13.htm" target="_blank">https://web.archive.org/web/20150416163620/http://www.hq.nasa.gov/pao/History/SP-4404/ch7-13.htm</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">"The Sensitivity of Precooled Air-Breathing Engine Performance to Heat Exchanger Design Parameters" (PDF). Reaction Engines Limited. 29 March 2007. p. 189. Archived from the original (PDF) on 23 June 2013. Retrieved 9 August 2010. <a href="https://web.archive.org/web/20130623100631/http://www.reactionengines.co.uk/tech_docs/JBIS_v60_188-196.pdf" target="_blank">https://web.archive.org/web/20130623100631/http://www.reactionengines.co.uk/tech_docs/JBIS_v60_188-196.pdf</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">"News Channel - Homepage - flightglobal.com". FlightGlobal.com. Retrieved 19 December 2017. <a href="http://www.flightglobal.com/articles/2009/02/23/322955/secret-files-reveal-us-interest-in-uk-hotol-spaceplane.html" target="_blank">http://www.flightglobal.com/articles/2009/02/23/322955/secret-files-reveal-us-interest-in-uk-hotol-spaceplane.html</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">"A. Bond". daviddarling.info. Retrieved 8 August 2010. <a href="http://www.daviddarling.info/encyclopedia/B/Bond.html" target="_blank">http://www.daviddarling.info/encyclopedia/B/Bond.html</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">"Funding flows for UK's 'revolutionary' Sabre rocket engine". Science. BBC. 12 July 2016. Retrieved 12 July 2016. <a href="https://www.bbc.com/news/science-environment-36773074" target="_blank">https://www.bbc.com/news/science-environment-36773074</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">"Reaction Engines secures new UK Government funding for Space Access Programme". <a href="https://www.gov.uk/government/news/reaction-engines-secures-new-uk-government-funding-for-space-access-programme" target="_blank">https://www.gov.uk/government/news/reaction-engines-secures-new-uk-government-funding-for-space-access-programme</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">"BAYERN-CHEMIE concludes agreement with European Space Agency on the further development of SABRE engine". 20 April 2017. <a href="https://bayern-chemie.com/bayern-chemie-concludes-agreement-with-european-space-agency-on-the-further-development-of-sabre-engine/" target="_blank">https://bayern-chemie.com/bayern-chemie-concludes-agreement-with-european-space-agency-on-the-further-development-of-sabre-engine/</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
<li id="fn:18">"UK's Sabre space plane engine tech in new milestone". BBC News. 8 April 2019. <a href="https://www.bbc.co.uk/news/science-environment-47832920" target="_blank">https://www.bbc.co.uk/news/science-environment-47832920</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></li>
<li id="fn:19">Marks, Paul (October 2012). "Die Erben der Concorde" (in German). New Scientist. Archived from the original on 24 November 2012. Retrieved 10 December 2012. In English <a href="https://web.archive.org/web/20121124100307/http://www.new-scientist.de/inhalt/hyperschallflugzeuge-die-erben-der-concorde-a-860705.html" target="_blank">https://web.archive.org/web/20121124100307/http://www.new-scientist.de/inhalt/hyperschallflugzeuge-die-erben-der-concorde-a-860705.html</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
<li id="fn:20">Guy Norris (7 April 2019). "Reaction Engines Pre-Cooler Passes Mach 3.3 Test". Aviation Week & Space Technology. <a href="https://aviationweek.com/space/reaction-engines-pre-cooler-passes-mach-33-test" target="_blank">https://aviationweek.com/space/reaction-engines-pre-cooler-passes-mach-33-test</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></li>
<li id="fn:21">Amos, Jonathan (28 November 2012). "BBC News – Skylon spaceplane engine concept achieves key milestone". Bbc.co.uk. Retrieved 1 July 2013. <a href="https://www.bbc.co.uk/news/science-environment-20510112" target="_blank">https://www.bbc.co.uk/news/science-environment-20510112</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></li>
<li id="fn:22">Norris, Guy. "Reaction Engines Reveals Secret Of Sabre Frost Control Technology[permanent dead link‍]" Aerospace Daily & Defense Report, 8 July 2015, p. 3 Similar article[dead link‍]
 <a href="https://awin.aviationweek.com/ArticlesStory.aspx?id=ae0e5600-45e8-4259-9000-9f9c2d14c448" target="_blank">https://awin.aviationweek.com/ArticlesStory.aspx?id=ae0e5600-45e8-4259-9000-9f9c2d14c448</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></li>
<li id="fn:23">"The Skylon Spaceplane's 3D Printed Injector" <a href="http://www.engineering.com/3DPrinting/3DPrintingArticles/ArticleID/10407/The-Skylon-Spaceplanes-3D-Printed-Injector.aspx" target="_blank">http://www.engineering.com/3DPrinting/3DPrintingArticles/ArticleID/10407/The-Skylon-Spaceplanes-3D-Printed-Injector.aspx</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></li>
<li id="fn:24">"Helping the Skylon Spaceplane Reach Orbit with a 3D Printed Injector Mechanism" <a href="http://3dprint.com/81151/skylon-spaceplane-3d-printing/" target="_blank">http://3dprint.com/81151/skylon-spaceplane-3d-printing/</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></li>
<li id="fn:25">"SABRE: how it works". Reaction Engines Limited. Archived from the original on 26 July 2013. Retrieved 29 November 2012. <a href="https://web.archive.org/web/20130726014458/http://www.reactionengines.co.uk/sabre_howworks.html" target="_blank">https://web.archive.org/web/20130726014458/http://www.reactionengines.co.uk/sabre_howworks.html</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></li>
<li id="fn:26">"SABRE: how it works". Reaction Engines Limited. Archived from the original on 26 July 2013. Retrieved 29 November 2012. <a href="https://web.archive.org/web/20130726014458/http://www.reactionengines.co.uk/sabre_howworks.html" target="_blank">https://web.archive.org/web/20130726014458/http://www.reactionengines.co.uk/sabre_howworks.html</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></li>
<li id="fn:27">"Skylon Users' Manual" (PDF). Reaction Engines Limited. 18 January 2010. pp. 4, 3. Archived from the original (PDF) on 18 April 2016. Retrieved 2 August 2010. <a href="https://web.archive.org/web/20160418035934/http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf" target="_blank">https://web.archive.org/web/20160418035934/http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></li>
<li id="fn:28">"Reaction Engines Ltd – Frequently Asked Questions". Reactionengines.co.uk. Archived from the original on 2 June 2015. Retrieved 1 July 2013. <a href="https://web.archive.org/web/20150602004107/http://www.reactionengines.co.uk/faq.html#q5" target="_blank">https://web.archive.org/web/20150602004107/http://www.reactionengines.co.uk/faq.html#q5</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></li>
<li id="fn:29">"The rocket that thinks it's a jet". UK Space Agency. 19 February 2009. Retrieved 5 November 2015. <a href="http://www.bristol.ac.uk/news/2009/6200.html" target="_blank">http://www.bristol.ac.uk/news/2009/6200.html</a> <a href="#fnref:29" class="footnote-back-ref">↩</a></li>
<li id="fn:30">"Nozzle arrangement for an engine". <a href="https://patents.google.com/patent/US20150101337A1/en" target="_blank">https://patents.google.com/patent/US20150101337A1/en</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></li>
<li id="fn:31">"Skylon Users' Manual" (PDF). Reaction Engines Limited. 18 January 2010. pp. 4, 3. Archived from the original (PDF) on 18 April 2016. Retrieved 2 August 2010. <a href="https://web.archive.org/web/20160418035934/http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf" target="_blank">https://web.archive.org/web/20160418035934/http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf</a> <a href="#fnref:31" class="footnote-back-ref">↩</a></li>
<li id="fn:32">"Reaction Engines Ltd – Frequently Asked Questions". Reactionengines.co.uk. Archived from the original on 2 June 2015. Retrieved 1 July 2013. <a href="https://web.archive.org/web/20150602004107/http://www.reactionengines.co.uk/faq.html#q5" target="_blank">https://web.archive.org/web/20150602004107/http://www.reactionengines.co.uk/faq.html#q5</a> <a href="#fnref:32" class="footnote-back-ref">↩</a></li>
<li id="fn:33">"Travelling at the edge of space: Reaction Engines and Skylon in the next 20 years". University of Strathclyde. Archived from the original on 10 March 2012. Retrieved 9 August 2010. <a href="https://web.archive.org/web/20120310093203/http://ewds.strath.ac.uk/space/OnDemandSeminar/tabid/4560/articleType/ArticleView/articleId/288/Travelling-at-the-edge-of-space--10-March-2010.aspx" target="_blank">https://web.archive.org/web/20120310093203/http://ewds.strath.ac.uk/space/OnDemandSeminar/tabid/4560/articleType/ArticleView/articleId/288/Travelling-at-the-edge-of-space--10-March-2010.aspx</a> <a href="#fnref:33" class="footnote-back-ref">↩</a></li>
<li id="fn:34">Reaction Engines Limited (28 November 2012). "The biggest breakthrough in propulsion since the jet engine" (PDF). Reaction Engines Limited. Archived from the original (PDF) on 7 December 2012. Retrieved 28 November 2012. <a href="https://web.archive.org/web/20121207125042/http://www.reactionengines.co.uk/press_release/PRESS%20RELEASE%20-%20THE%20BIGGESTBREAKTHROUGH%20IN%20PROPULSION%20-%2028%20November%202012.pdf" target="_blank">https://web.archive.org/web/20121207125042/http://www.reactionengines.co.uk/press_release/PRESS%20RELEASE%20-%20THE%20BIGGESTBREAKTHROUGH%20IN%20PROPULSION%20-%2028%20November%202012.pdf</a> <a href="#fnref:34" class="footnote-back-ref">↩</a></li>
<li id="fn:35">Thisdell, Dan (1 September 2011). "Spaceplane engine tests under way". Flightglobal News. Retrieved 4 November 2015. <a href="https://www.flightglobal.com/news/articles/spaceplane-engine-tests-under-way-361501/" target="_blank">https://www.flightglobal.com/news/articles/spaceplane-engine-tests-under-way-361501/</a> <a href="#fnref:35" class="footnote-back-ref">↩</a></li>
<li id="fn:36">Reaction Engines Limited (28 November 2012). "The biggest breakthrough in propulsion since the jet engine" (PDF). Reaction Engines Limited. Archived from the original (PDF) on 7 December 2012. Retrieved 28 November 2012. <a href="https://web.archive.org/web/20121207125042/http://www.reactionengines.co.uk/press_release/PRESS%20RELEASE%20-%20THE%20BIGGESTBREAKTHROUGH%20IN%20PROPULSION%20-%2028%20November%202012.pdf" target="_blank">https://web.archive.org/web/20121207125042/http://www.reactionengines.co.uk/press_release/PRESS%20RELEASE%20-%20THE%20BIGGESTBREAKTHROUGH%20IN%20PROPULSION%20-%2028%20November%202012.pdf</a> <a href="#fnref:36" class="footnote-back-ref">↩</a></li>
<li id="fn:37">Thisdell, Dan (1 September 2011). "Spaceplane engine tests under way". Flightglobal News. Retrieved 4 November 2015. <a href="https://www.flightglobal.com/news/articles/spaceplane-engine-tests-under-way-361501/" target="_blank">https://www.flightglobal.com/news/articles/spaceplane-engine-tests-under-way-361501/</a> <a href="#fnref:37" class="footnote-back-ref">↩</a></li>
<li id="fn:38">Reaction Engines Limited (28 November 2012). "The biggest breakthrough in propulsion since the jet engine" (PDF). Reaction Engines Limited. Archived from the original (PDF) on 7 December 2012. Retrieved 28 November 2012. <a href="https://web.archive.org/web/20121207125042/http://www.reactionengines.co.uk/press_release/PRESS%20RELEASE%20-%20THE%20BIGGESTBREAKTHROUGH%20IN%20PROPULSION%20-%2028%20November%202012.pdf" target="_blank">https://web.archive.org/web/20121207125042/http://www.reactionengines.co.uk/press_release/PRESS%20RELEASE%20-%20THE%20BIGGESTBREAKTHROUGH%20IN%20PROPULSION%20-%2028%20November%202012.pdf</a> <a href="#fnref:38" class="footnote-back-ref">↩</a></li>
<li id="fn:39">Svitak, Amy (29 November 2012). "ESA Validates SABRE Engine Technology". Aviation Week. Archived from the original on 19 October 2013. Retrieved 8 December 2012. <a href="https://web.archive.org/web/20131019151400/http://www.aviationweek.com/Blogs.aspx?plckPostId=Blog:04ce340e-4b63-4d23-9695-d49ab661f385Post:64760201-0b8f-4e1c-bd24-6bea77962435" target="_blank">https://web.archive.org/web/20131019151400/http://www.aviationweek.com/Blogs.aspx?plckPostId=Blog:04ce340e-4b63-4d23-9695-d49ab661f385Post:64760201-0b8f-4e1c-bd24-6bea77962435</a> <a href="#fnref:39" class="footnote-back-ref">↩</a></li>
<li id="fn:40">"Skylon Assessment Report" (PDF). UK Space Agency. April 2011. Retrieved 26 April 2015.[permanent dead link‍] <a href="http://ukspaceagency.bis.gov.uk/assets/ukspaceagency/docs/skylon-assessment-report-pub.pdf" target="_blank">http://ukspaceagency.bis.gov.uk/assets/ukspaceagency/docs/skylon-assessment-report-pub.pdf</a> <a href="#fnref:40" class="footnote-back-ref">↩</a></li>
<li id="fn:41">"UK pledges fresh support for revolutionary space engine" Archived 3 July 2013 at the Wayback Machine SEN. 27 June 2013. Retrieved 16 July 2013. <a href="http://www.sen.com/news/uk-pledges-fresh-support-for-revolutionary-space-engine.html" target="_blank">http://www.sen.com/news/uk-pledges-fresh-support-for-revolutionary-space-engine.html</a> <a href="#fnref:41" class="footnote-back-ref">↩</a></li>
<li id="fn:42">"UK government excited by 'disruptive' Sabre engine" BBC. 16 July 2013. Retrieved 16 July 2013. <a href="https://www.bbc.co.uk/news/science-environment-23332592" target="_blank">https://www.bbc.co.uk/news/science-environment-23332592</a> <a href="#fnref:42" class="footnote-back-ref">↩</a></li>
<li id="fn:43">"UK earmarks £60m for super-fast space rocket engine". The Guardian. London. 16 July 2013. <a href="https://www.theguardian.com/science/2013/jul/16/60m-space-rocket-engine" target="_blank">https://www.theguardian.com/science/2013/jul/16/60m-space-rocket-engine</a> <a href="#fnref:43" class="footnote-back-ref">↩</a></li>
<li id="fn:44">"Futuristic British Space Plane Engine to Get Flight Test in 2020" space.com 18 July 2013. Retrieved 18 July 2013. <a href="http://www.space.com/22004-skylon-space-plane-rocket-engine.html" target="_blank">http://www.space.com/22004-skylon-space-plane-rocket-engine.html</a> <a href="#fnref:44" class="footnote-back-ref">↩</a></li>
<li id="fn:45">"UK government excited by 'disruptive' Sabre engine" BBC. 16 July 2013. Retrieved 16 July 2013. <a href="https://www.bbc.co.uk/news/science-environment-23332592" target="_blank">https://www.bbc.co.uk/news/science-environment-23332592</a> <a href="#fnref:45" class="footnote-back-ref">↩</a></li>
<li id="fn:46">"BAE Systems and Reaction Engines to develop a ground breaking new aerospace engine". 2 November 2015. Archived from the original on 18 October 2014. <a href="https://web.archive.org/web/20141018192749/http://www.reactionengines.co.uk/news_updates.html" target="_blank">https://web.archive.org/web/20141018192749/http://www.reactionengines.co.uk/news_updates.html</a> <a href="#fnref:46" class="footnote-back-ref">↩</a></li>
<li id="fn:47">Black, Charles (16 April 2015). "Revolutionary rocket engine passes U.S. Air Force feasibility test". sen.com. Retrieved 7 May 2015. <a href="http://sen.com/news/revolutionary-rocket-engine-passes-u-s-air-force-feasibility-test" target="_blank">http://sen.com/news/revolutionary-rocket-engine-passes-u-s-air-force-feasibility-test</a> <a href="#fnref:47" class="footnote-back-ref">↩</a></li>
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</ol>

SABRE (rocket engine) open-in-new

SABRE (rocket engine)