View source for SABRE (rocket engine)

{{short description|Synergetic Air Breathing Rocket Engine - a hybrid ramjet and rocket engine}}
{{Use dmy dates|date=October 2019}}
{{Use British English|date=March 2019}}
{{Infobox rocket engine
 |image                      = SABRE engine designed for Skylon spaceplane, 1990s. (9660572897).jpg
 |caption = A version of the SABRE engine designed in the 1990s
 |name                       = SABRE
 |country_of_origin          = [[United Kingdom]]
 |designer                   = [[Reaction Engines Limited]]
 |purpose                    = [[Single-stage-to-orbit]]
 |predecessor                = [[RB545]]
 |associated                 = [[Skylon (spacecraft)|Skylon]]
 |status                     = [[Research and development]]
 |type                       = liquid
 |fuel                       = [[liquid hydrogen]]<ref name="names">{{cite web|url=http://www.reactionengines.co.uk/downloads/REL_Engine_Names.pdf |title=Reaction Engines Limited Engine Names
|publisher=[[Reaction Engines Limited]] |url-status=dead |archive-url=https://web.archive.org/web/20120305234315/http://www.reactionengines.co.uk/downloads/REL_Engine_Names.pdf |archive-date=5 March 2012 }}</ref>
 |oxidiser                   = [[Air]] or [[liquid oxygen]]
 |cycle                      = Combined cycle [[precooled jet engine]] and [[closed cycle rocket engine]]
 |thrust(SL)                 = Approx. {{convert|1960|kN|abbr=on}}{{citation needed|date=May 2018}}
 |thrust(Vac)                = Approx. {{convert|2940|kN|abbr=on}}{{citation needed|date=May 2018}}
 |specific_impulse_vacuum    = {{convert|460|isp}}<ref name="user"/>
 |specific_impulse_sea_level = {{convert|3600|isp|tsfc+km/s}}<ref name="user"/>
 |thrust_to_weight           = Up to 14 (atmospheric)<ref name="precooled"/>
}}
'''SABRE''' ('''Synergetic Air Breathing Rocket Engine'''<ref name="SABRE_page">{{cite web|url=https://www.reactionengines.co.uk/sabre |title=SABRE – Synergetic Air Breathing Rocket Engine|publisher=[[Reaction Engines Limited]] |access-date=18 December 2018 |url-status=live |archive-url=https://web.archive.org/web/20181219091110/https://www.reactionengines.co.uk/sabre |archive-date=19 December 2018 }}</ref>) is a concept under development by [[Reaction Engines Limited]] for a [[hypersonic]] [[precooled jet engine|precooled]] hybrid [[Engine#Heat engine|air-breathing]] [[rocket engine]].<ref name="compare">{{cite web | title=A Comparison of Propulsions Concepts for SSTO Reusable launchers | url=http://www.reactionengines.co.uk/tech_docs/JBIS_v56_108-117.pdf | publisher=[[Reaction Engines Limited]] | pages=114, 115 | access-date=2 August 2010 | url-status=dead | archive-url=https://web.archive.org/web/20110615104428/http://www.reactionengines.co.uk/tech_docs/JBIS_v56_108-117.pdf | archive-date=15 June 2011 | df=dmy-all }}</ref><ref name=lapcat>{{Cite web
  |title=Flight Applications
  |publisher=[[Reaction Engines]]
  |url=https://www.reactionengines.co.uk/sabre/flight-applications
}}</ref> The engine is being designed to achieve [[single-stage-to-orbit]] capability, propelling the proposed [[Skylon (spacecraft)|Skylon]] [[spaceplane]] to low Earth orbit. SABRE is an evolution of [[Alan Bond (engineer)|Alan Bond]]'s series of [[liquid air cycle engine|LACE]]-like designs that started in the early/mid-1980s for the [[HOTOL]] project.<ref>{{cite web|url=https://vimeo.com/45136248|title=Alan Bond Interview|website=Vimeo|access-date=19 December 2017}}</ref>

The design comprises a single [[combined cycle]] rocket engine with two modes of operation.<ref name="user">{{cite web | url=http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf | title=Skylon Users' Manual | date=18 January 2010 | publisher=[[Reaction Engines Limited]] | pages=4, 3 | access-date=2 August 2010 | url-status=dead | archive-url=https://web.archive.org/web/20160418035934/http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf | archive-date=18 April 2016 | df=dmy-all }}</ref> The air-breathing mode combines a turbo-[[Gas compressor|compressor]] with a lightweight air [[Precooled jet engine|precooler]] positioned just behind the [[inlet cone]]. At high speeds this precooler cools the hot, ram-compressed air, which would otherwise reach a temperature that the engine could not withstand,<ref name=precooling>{{cite web |url=https://www.bbc.co.uk/news/science-environment-47832920 |title=UK's Sabre space plane engine tech in new milestone|website=BBC News|date= 8 April 2019}}</ref> leading to a very high [[Overall pressure ratio|pressure ratio]] within the engine. The compressed air is subsequently fed into the rocket [[combustion chamber]] where it is ignited along with stored [[liquid hydrogen]]. The high pressure ratio allows the engine to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed and allow a very lightweight engine—essential for reaching orbit. In addition, unlike the LACE concept, SABRE's precooler does not [[Liquid air|liquefy the air]], letting it run more efficiently.<ref name="precooled">{{cite web | url=http://www.reactionengines.co.uk/tech_docs/JBIS_v60_188-196.pdf | title=The Sensitivity of Precooled Air-Breathing Engine Performance to Heat Exchanger Design Parameters | date=29 March 2007 | publisher=[[Reaction Engines Limited]] | page=189 | access-date=9 August 2010 | url-status=dead | archive-url=https://web.archive.org/web/20130623100631/http://www.reactionengines.co.uk/tech_docs/JBIS_v60_188-196.pdf | archive-date=23 June 2013 | df=dmy-all }}</ref>

After shutting the inlet cone off at [[Mach number|Mach]]&nbsp;5.14, and at an altitude of 28.5&nbsp;km,<ref name="user" /> the system continues as a [[staged combustion cycle (rocket)|closed-cycle]] high-performance rocket engine burning [[liquid oxygen]] and [[liquid hydrogen]] from on-board fuel tanks, potentially allowing a hybrid [[spaceplane]] concept like [[Skylon (spacecraft)|Skylon]] to reach orbital velocity after leaving the atmosphere on a steep climb.

An engine derived from the SABRE concept called [[Reaction Engines Scimitar|Scimitar]] has been designed for the company's [[Reaction Engines A2|A2]] [[Supersonic transport|hypersonic passenger jet]] proposal for the [[European Union]]-funded [[LAPCAT]] study.<ref>{{cite web |url=https://www.reactionengines.co.uk/about/story-so-far |title=Reaction Engines has reached a number of high profile milestones|website=Reaction Engines|date=2018|access-date= 8 April 2019}}</ref>
{{toclimit|3}}

==History==
The precooler concept evolved from an idea originated by Robert P. Carmichael in 1955.<ref>{{cite web |url=http://www.hq.nasa.gov/pao/History/SP-4404/ch7-13.htm |title=Liquid Hydrogen as a Propulsion Fuel, 1945–1959 |publisher=NASA History Division |access-date=1 July 2009 }}</ref> This was followed by the [[liquid air cycle engine]] (LACE) idea which was originally explored by [[General Dynamics]] in the 1960s as part of the [[US Air Force]]'s [[aerospaceplane]] efforts.<ref name="precooled" />

The LACE system was to be placed behind a supersonic air intake which would compress the air through ram compression, then a [[heat exchanger]] would rapidly cool it using some of the [[liquid hydrogen]] fuel stored on board. The resulting liquid air was then processed to separate the [[liquid oxygen]] 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.{{Citation needed|date=August 2011}}

Instead, as part of the [[HOTOL]] project, the liquid air cycle engine (LACE) based [[RB545]] engine was developed with more efficient cycle. The engine was given the Rolls-Royce name "Swallow".<ref>{{cite web|url=http://www.flightglobal.com/articles/2009/02/23/322955/secret-files-reveal-us-interest-in-uk-hotol-spaceplane.html|title=News Channel - Homepage - flightglobal.com|website=FlightGlobal.com|access-date=19 December 2017}}</ref> In 1989, after funding for HOTOL ceased, Bond and several others formed [[Reaction Engines Limited]] to continue research. The RB545's precooler had issues with [[Hydrogen embrittlement|embrittlement]] and excess liquid hydrogen consumption, and was encumbered by both patents and the UK's [[Official Secrets Act]], so Bond developed SABRE instead.<ref>{{cite web |url=http://www.daviddarling.info/encyclopedia/B/Bond.html | title=A. Bond | work=daviddarling.info |access-date=8 August 2010 }}</ref>

In 2016 the project received £60m in funds from the UK government and ESA for a demonstrator involving the full cycle.<ref name="bbc_12July2016">{{cite news|title=Funding flows for UK's 'revolutionary' Sabre rocket engine|url=https://www.bbc.com/news/science-environment-36773074|access-date=12 July 2016|work=Science|publisher=[[BBC]]|date=12 July 2016}}</ref>
In July 2021 the UK Space Agency provided a further £3.9m for continued development.<ref>{{cite web | url=https://www.gov.uk/government/news/reaction-engines-secures-new-uk-government-funding-for-space-access-programme | title=Reaction Engines secures new UK Government funding for Space Access Programme }}</ref>

==Concept==

Like the [[RB545]], the SABRE design is neither a conventional [[rocket engine]] nor a conventional [[turbojet|jet engine]], but a hybrid that uses air from the environment at low speeds/altitudes, and stored [[liquid oxygen]] at higher altitude. The SABRE engine "relies on a heat exchanger capable of cooling incoming air to {{convert|-150|C}}, 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 enters the engine through an inlet. A bypass system directs some of the air through a precooler into a compressor, which injects it into a combustion chamber where it is burnt with fuel, the exhaust products are accelerated through nozzles to provide thrust. The remainder of the intake air continues 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 is used to transfer the heat from the precooler to the fuel and drive the engine pumps and compressors.

===Inlet===

At the front of the engine, the concept designs propose a simple translating axisymmetric [[Inlet cone|shock cone inlet]] which compresses and slows the air (relative to the engine) to subsonic speeds using two shock reflections. Accelerating the air to the speed of the engine incurs [[ram drag]]. As a result of the shocks, compression, and acceleration the intake air is heated, reaching around {{convert|1000|C|F}} at Mach{{nbsp}}5.5.

[[Bayern-Chemie]], through ESA, have undertaken work to refine and test the intake and bypass systems<ref name=BayernChemieSabreDevelopment>{{Cite web|url=https://bayern-chemie.com/bayern-chemie-concludes-agreement-with-european-space-agency-on-the-further-development-of-sabre-engine/|title=BAYERN-CHEMIE concludes agreement with European Space Agency on the further development of SABRE engine|date=20 April 2017 }}</ref>

===Precooler===

As the air enters the engine at [[supersonic]] or [[hypersonic]] speeds, it becomes hotter than the engine can withstand due to compression effects.<ref name=precooling/> [[Jet engines]], which have the same problem but to a lesser degree, solve it by using heavy [[copper]] or [[nickel]]-based materials, by reducing the engine's [[Overall pressure ratio|pressure ratio]], and by throttling back the engine at the higher airspeeds to avoid melting. However, for a [[single-stage-to-orbit]] (SSTO) spaceplane, such heavy materials are unusable, and maximum thrust is necessary for orbital insertion at the earliest time to minimise [[gravity drag|gravity losses]]. Instead, using a gaseous [[helium]] coolant loop, SABRE dramatically cools the air from {{cvt|1000|C}} down to {{cvt|−150|C}} in a counterflow heat exchanger while avoiding [[liquid air|liquefaction]] of the air or blockage from freezing water vapour. The counterflow heat exchanger also allows 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 inserts a helium cooling loop between the air and the cold fuel to avoid problems with [[hydrogen embrittlement]] in the precooler.

The dramatic cooling of the air created a potential problem: it is 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.<ref>{{cite web| last=Marks| first=Paul| title=Die Erben der Concorde| url=http://www.new-scientist.de/inhalt/hyperschallflugzeuge-die-erben-der-concorde-a-860705.html| publisher=[[New Scientist]]| access-date=10 December 2012| language=de| date=October 2012| url-status=dead| archive-url=https://web.archive.org/web/20121124100307/http://www.new-scientist.de/inhalt/hyperschallflugzeuge-die-erben-der-concorde-a-860705.html| archive-date=24 November 2012| df=dmy-all}} [https://translate.google.com/translate?hl=en&sl=de&u=http://www.new-scientist.de/inhalt/hyperschallflugzeuge-die-erben-der-concorde-a-860705.html&prev=/search%3Fq%3D%2522mark%2Bford%2522%2Bestec%2Bsabre%26start%3D10%26hl%3Den%26safe%3Doff%26sa%3DN%26tbo%3Dd%26rls%3Dcom.microsoft:en-US:%257Breferrer:source%257D%26biw%3D1200%26bih%3D1805&sa=X&ei=4GHGUJmcIvH44QSckoHoCQ&ved=0CDEQ7gEwADgK In English]</ref> The cooler consists of a fine pipework heat exchanger with 16,800 thin-walled tubes,<ref name=AvWeek7apr2019/> and cools the hot in-rushing atmospheric air down to the required {{cvt|−150|C}} in 0.01{{nbsp}}s.<ref>{{cite news| last=Amos |first=Jonathan |url=https://www.bbc.co.uk/news/science-environment-20510112 |title=BBC News – Skylon spaceplane engine concept achieves key milestone |publisher=Bbc.co.uk |date=28 November 2012 |access-date=1 July 2013}}</ref> The ice prevention system had been a closely guarded secret, but REL disclosed a [[methanol]]-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.<ref>Norris, Guy. "[https://awin.aviationweek.com/ArticlesStory.aspx?id=ae0e5600-45e8-4259-9000-9f9c2d14c448 Reaction Engines Reveals Secret Of Sabre Frost Control Technology]{{dead link|date=September 2017|bot=InternetArchiveBot|fix-attempted=yes}}" ''[[Aviation Week & Space Technology|Aerospace Daily & Defense Report]]'', 8 July 2015, p. 3 [https://awin.aviationweek.com/ArticlesStory.aspx?id=ae0e5600-45e8-4259-9000-9f9c2d14c448 Similar article]{{dead link|date=September 2017}} <!--paywall: http://aviationweek.com/technology/reaction-engines-reveals-inner-secret-sabre-propulsion-technology --></ref><ref>"[http://www.engineering.com/3DPrinting/3DPrintingArticles/ArticleID/10407/The-Skylon-Spaceplanes-3D-Printed-Injector.aspx The Skylon Spaceplane's 3D Printed Injector]"</ref><ref>"[http://3dprint.com/81151/skylon-spaceplane-3d-printing/ Helping the Skylon Spaceplane Reach Orbit with a 3D Printed Injector Mechanism]"</ref>

===Compressor===
Below five times the [[speed of sound]] and 25 kilometres of altitude, which are 20% of the speed and 20% of the altitude needed to reach [[orbit]], the cooled air from the precooler passes into a modified turbo-[[Gas compressor|compressor]], similar in design to those used on conventional jet engines but running at an unusually high [[overall pressure ratio|pressure ratio]] made possible by the low temperature of the inlet air. The compressor feeds the compressed air at 140 [[Atmospheres (unit)|atmospheres]] into the combustion chambers of the main engines.<ref name="how">{{cite web |url=http://www.reactionengines.co.uk/sabre_howworks.html |title=SABRE: how it works |publisher=Reaction Engines Limited |access-date=29 November 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130726014458/http://www.reactionengines.co.uk/sabre_howworks.html |archive-date=26 July 2013  }}</ref>

In a conventional jet engine, the turbo-compressor is driven by a [[gas turbine]] powered by combustion gases. SABRE drives the turbine with a helium loop, which is powered by heat captured in the precooler and a preburner.<ref name="how" />

===Helium loop===

The 'hot' helium from the air precooler is recycled by cooling it in a heat exchanger with the liquid hydrogen fuel. The loop forms a self-starting [[Brayton cycle]] engine, cooling critical parts of the engine and powering turbines.{{Citation needed|date=January 2011}} The heat passes from the air into the helium. This heat energy is used to power various parts of the engine and to vaporise hydrogen, which is then burnt in [[ramjet]]s.<ref name="user" /><ref name="Reactionengines.co.uk">{{cite web |url=http://www.reactionengines.co.uk/faq.html#q5 |title=Reaction Engines Ltd – Frequently Asked Questions |publisher=Reactionengines.co.uk |access-date=1 July 2013 |url-status=dead |archive-url=https://web.archive.org/web/20150602004107/http://www.reactionengines.co.uk/faq.html#q5 |archive-date=2 June 2015  }}</ref>

===Combustion chambers===

The combustion chambers in the SABRE engine are cooled by the oxidant (air/liquid oxygen) rather than by liquid hydrogen<ref name="uksa">{{cite web | url=http://www.bristol.ac.uk/news/2009/6200.html | title=The rocket that thinks it's a jet | date=19 February 2009| publisher=[[UK Space Agency]] | access-date=5 November 2015 }}</ref> to further reduce the system's use of liquid hydrogen compared with [[Air-fuel ratio|stoichiometric]] systems.

===Nozzles===
The most efficient atmospheric pressure at which a conventional [[propelling nozzle]] works is set by the [[geometry]] of the [[De Laval nozzle|nozzle bell]]. While the geometry of the conventional bell remains static the [[Atmospheric pressure#Altitude atmospheric pressure variation|atmospheric pressure changes with altitude]] 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 must operate at both low and high altitude scenarios. To ensure efficiency at all altitudes a sort of moving, [[Expanding nozzle]] is used. First at low altitude, air-breathing flight the bell is located rearwards, connected to a toroidal combustion chamber surrounding the top part of the nozzle, together forming an [[expansion deflection nozzle]]. When SABRE later transitions into rocket mode, the bell is 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.<ref>{{Cite web|url=https://patents.google.com/patent/US20150101337A1/en|title=Nozzle arrangement for an engine}}</ref>
[[File:SABRE_Rocket_nozzle_mode.png|thumb|186x186px|Nozzle in Rocket mode]]

===Bypass burners===

Avoiding [[Liquid air|liquefaction]] improves the efficiency of the engine since less [[entropy]] is generated and therefore less liquid hydrogen is boiled off. However, simply cooling the air needs more liquid hydrogen than can be burnt in the engine core. The excess is expelled through a series of burners called "spill duct [[ramjet]] burners",<ref name="user" /><ref name="Reactionengines.co.uk"/> that are arranged in a ring around the central core. These are fed air that bypasses the precooler. This bypass ramjet system is designed to reduce the negative effects of drag resulting from air that passes into the intakes but is not 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 can feed to the combustion chamber is at its highest, requiring the bypassed air to be accelerated to maintain efficiency at these low speeds. This distinguishes the system from a [[Air turborocket#Types|turboramjet]] 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.<ref>{{cite web | title=Travelling at the edge of space: Reaction Engines and Skylon in the next 20 years | url=http://ewds.strath.ac.uk/space/OnDemandSeminar/tabid/4560/articleType/ArticleView/articleId/288/Travelling-at-the-edge-of-space--10-March-2010.aspx | publisher=[[University of Strathclyde]] | access-date=9 August 2010 | url-status=dead | archive-url=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 | archive-date=10 March 2012 | df=dmy-all }}</ref>

==Development==
[[File:SABRE ENGINE.jpg|thumb|The latest SABRE engine's design.]]
Tests were carried out in 2008 by Airborne Engineering Ltd on an [[expansion deflection nozzle]] 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 [[Lampoldshausen|Institute of Space Propulsion]] 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.<ref name="precooltest" /><ref name="fg20110901">{{cite news |last=Thisdell| first=Dan |title= Spaceplane engine tests under way |url=https://www.flightglobal.com/news/articles/spaceplane-engine-tests-under-way-361501/ |access-date=4 November 2015 |newspaper=Flightglobal News |date=1 September 2011}}</ref> 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.<ref name="precooltest" /><ref name="fg20110901" />

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 [[European Space Agency]] (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.<ref name="precooltest">{{cite web |url=http://www.reactionengines.co.uk/press_release/PRESS%20RELEASE%20-%20THE%20BIGGESTBREAKTHROUGH%20IN%20PROPULSION%20-%2028%20November%202012.pdf |title=The biggest breakthrough in propulsion since the jet engine |publisher=[[Reaction Engines Limited]] |date=28 November 2012 |author=Reaction Engines Limited |access-date=28 November 2012 |url-status=dead |archive-url=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 |archive-date=7 December 2012  }}</ref><ref name="avEsa">{{cite web| last=Svitak| first=Amy| title=ESA Validates SABRE Engine Technology| url=http://www.aviationweek.com/Blogs.aspx?plckPostId=Blog:04ce340e-4b63-4d23-9695-d49ab661f385Post:64760201-0b8f-4e1c-bd24-6bea77962435| publisher=[[Aviation Week]]| access-date=8 December 2012| date=29 November 2012| archive-date=19 October 2013| archive-url=https://web.archive.org/web/20131019151400/http://www.aviationweek.com/Blogs.aspx?plckPostId=Blog:04ce340e-4b63-4d23-9695-d49ab661f385Post:64760201-0b8f-4e1c-bd24-6bea77962435| url-status=dead}}</ref><ref name=esar>{{cite web | url=http://ukspaceagency.bis.gov.uk/assets/ukspaceagency/docs/skylon-assessment-report-pub.pdf | title=Skylon Assessment Report | date=April 2011 | publisher=[[UK Space Agency]] | access-date=26 April 2015 }}{{dead link|date=September 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

In June 2013 the United Kingdom government announced further support for the development of a full-scale prototype of the SABRE engine,<ref>[http://www.sen.com/news/uk-pledges-fresh-support-for-revolutionary-space-engine.html "UK pledges fresh support for revolutionary space engine"] SEN. 27 June 2013. Retrieved 16 July 2013.</ref> providing £60M of funding between 2014 and 2016<ref name = "UKgovinvestBBC">[https://www.bbc.co.uk/news/science-environment-23332592 "UK government excited by 'disruptive' Sabre engine"] [[BBC]]. 16 July 2013. Retrieved 16 July 2013.</ref><ref>{{cite news| url=https://www.theguardian.com/science/2013/jul/16/60m-space-rocket-engine | location=London | work=The Guardian | title=UK earmarks £60m for super-fast space rocket engine | date=16 July 2013}}</ref> with the ESA providing an additional £7M.<ref>[http://www.space.com/22004-skylon-space-plane-rocket-engine.html "Futuristic British Space Plane Engine to Get Flight Test in 2020"] space.com 18 July 2013. Retrieved 18 July 2013.</ref> The total cost of developing a test rig is estimated at £200M.<ref name="UKgovinvestBBC"/>

By June 2015, SABRE's development continued with The Advanced Nozzle Project at Westcott. The test engine, operated by Airborne Engineering Ltd., is being used to analyze the [[aerodynamics]] and performance of the advanced nozzles that the SABRE engine will use, in addition to new manufacturing technologies such as the 3D-printed propellant injection system.<ref name='newsupdates Nov 2015'>{{cite web | url=http://www.reactionengines.co.uk/news_updates.html | title=BAE Systems and Reaction Engines to develop a ground breaking new aerospace engine | date=2 November 2015 | url-status=dead | archive-url=https://web.archive.org/web/20141018192749/http://www.reactionengines.co.uk/news_updates.html | archive-date=18 October 2014 | df=dmy-all }}</ref>

In April 2015, the SABRE engine concept passed a theoretical feasibility review conducted by the U.S. [[Air Force Research Laboratory]].<ref>{{cite news |last=Black| first=Charles |title=Revolutionary rocket engine passes U.S. Air Force feasibility test| url=http://sen.com/news/revolutionary-rocket-engine-passes-u-s-air-force-feasibility-test |access-date=7 May 2015 |work= sen.com |date=16 April 2015}}</ref><ref>"[http://www.theengineer.co.uk/aerospace/news/arfl-confirms-feasibility-of-reaction-engines-sabre-engine-concept/1020216.article ARFL confirms feasibility of Reaction Engines’ SABRE engine concept]"</ref><ref>"[http://spacenews.com/afrl-gives-seal-of-approval-to-british-air-breathing-engine-design/ AFRL Gives Seal of Approval to British Air-breathing Engine Design]"</ref> 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 is "technically very risky as a first application of SABRE engine".<ref>{{cite web |url=https://www.space.com/32115-skylon-space-plane-engines-air-force-vehicle.html |title=US Military Set to Unveil Concepts Based on Skylon Space Plane Tech |website=space.com|date=3 March 2016 |access-date= 8 April 2019}}</ref>

In August 2015 the [[European Commission]] 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 [[private equity]] had been insufficient to bring the project to completion.<ref>{{cite web|title=State aid: Commission approves £50 million UK support for the research and development of an innovative space launcher engine|url=http://europa.eu/rapid/press-release_IP-15-5495_en.htm|website=Europe.eu|publisher=European commission|access-date=8 September 2015}}</ref> In October 2015 British company [[BAE Systems]] agreed to buy a 20% stake in the company for [[Pound sign|£]]20.6 million as part of an agreement to help develop the SABRE hypersonic engine.<ref>{{cite news |url=http://aviationweek.com/technology/bae-takes-stake-reaction-engines-hypersonic-development |title=BAE Takes Stake in Reaction Engines Hypersonic Development |last1=Norris |first1=Guy |date=1 November 2015 |website=aviationweek.com |publisher=Aviation Week & Space Technology |access-date=1 November 2015}}</ref><ref>{{cite news |last1=Hollinger |first1=Peggy |last2=Cookson |first2=Clive |url=https://www.cnbc.com/2015/11/02/bae-systems-to-pay-206m-for-20-of-space-engine-group.html |title=BAE Systems to pay £20.6m for 20% of space engine group |work=CNBC |date=2 November 2015 |access-date=5 November 2015 }}</ref>
In 2016, Reaction CEO Mark Thomas announced plans to build a quarter-sized ground test engine, given limitations of funding.<ref>{{cite news |url=http://aviationweek.com/technology/reaction-engines-refines-hypersonic-engine-demonstrator-plan |title=Reaction Engines Refines Hypersonic Engine Demonstrator Plan |last1=Norris |first1=Guy |date=21 September 2016 |website=aviationweek.com |publisher=Aviation Week & Space Technology |access-date=26 September 2016}}</ref>

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 [[Rocket Propulsion Establishment]] in Westcott, UK<ref>{{cite web|url=https://publicaccess.aylesburyvaledc.gov.uk/online-applications/files/DFB7DF52C99227B18D89F8A79B37D276/pdf/16_03478_APP-APPLICATIONFORMNOPERSONALDATA-1596950.pdf|url-status=dead |title=Application Form No Personal Data |date=January 2018 }}</ref> which was granted in April 2017,<ref>{{cite web|url=https://publicaccess.aylesburyvaledc.gov.uk/online-applications/files/9BFFC6D62D889B4C57727C889A3513B2/16_03478_APP-DECISION_NOTICE-1702440.rtf|title=Decision Notice|url-status=dead |date=January 2018  }}</ref> and in May 2017 a [[groundbreaking]] ceremony was held to announce the beginning of construction of the SABRE TF1 engine test facility, expected to become active in 2020.<ref>{{cite web|url=https://www.reactionengines.co.uk/reaction-engines-begins-construction-uk-rocket-engine-test-facility/|title=Reaction Engines begins construction of UK rocket engine test facility – Reaction Engines|date=4 May 2017|website=ReactionEngines.co.uk|access-date=19 December 2017|archive-url=https://web.archive.org/web/20171222051252/https://www.reactionengines.co.uk/reaction-engines-begins-construction-uk-rocket-engine-test-facility/|archive-date=22 December 2017|url-status=dead}}</ref><ref>{{cite web|url=http://www.oxfordmail.co.uk/news/15268287.Space_plane_test_facility___39_up_and_running_by_2020__39_/|title=Space plane test facility 'up and running by 2020'|website=Oxford Mail|date=5 May 2017 |access-date=19 December 2017}}</ref> However, development of the TF1 facility has since been quietly dropped, and the site has now been taken on by aerospace and defence group [[Nammo]].<ref>{{cite web|url=https://westcottspacecluster.org.uk/nammo-take-on-new-building-at-westcott/|title=Nammo take on new building at Westcott|date=19 April 2022|website=westcottspacecluster.org.uk|access-date=24 April 2022|archive-url=https://web.archive.org/web/20220419115749/https://westcottspacecluster.org.uk/nammo-take-on-new-building-at-westcott/|archive-date=19 April 2022|url-status=live}}</ref>

In September 2017 it was announced the United States [[DARPA|Defence Advanced Research Projects Agency]] (DARPA) had contracted with Reaction Engines Inc. to build a high-temperature airflow test facility at Front Range Airport near Watkins, Colorado.<ref>{{cite web|url=https://www.reactionengines.co.uk/reaction-engines-awarded-darpa-contract-to-perform-high-temperature-testing-of-the-sabre-precooler/|title=Reaction Engines Awarded DARPA Contract to Perform High-Temperature Testing of the SABRE Precooler – Reaction Engines|date=25 September 2017|website=ReactionEngines.co.uk|access-date=19 December 2017|archive-url=https://web.archive.org/web/20170928232712/https://www.reactionengines.co.uk/reaction-engines-awarded-darpa-contract-to-perform-high-temperature-testing-of-the-sabre-precooler/|archive-date=28 September 2017|url-status=dead}}</ref> The DARPA contract is 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{{nbsp}}5 or around {{convert|1800|F|C|-2}} beginning in 2019.<ref>{{Cite web|url=https://aviationweek.com/defense-space/space/reaction-begins-building-us-hypersonic-engine-test-site|title=Reaction Begins Building U.S. Hypersonic Engine Test Site &#124; Aviation Week Network|website=aviationweek.com}}</ref><ref>{{Cite news|url=https://www.bbc.com/news/science-environment-47585433|title=Super-fast engine set for key tests|first=Jonathan|last=Amos|work=BBC News|date=15 March 2019}}</ref>

The HTX test unit was completed in the UK and sent to Colorado in 2018, where on 25 March 2019 an [[F-4]] GE [[J79]] turbojet exhaust was mixed with ambient air to replicate Mach{{nbsp}}3.3 inlet conditions, successfully quenching a {{cvt|420|C}} stream of gases to {{cvt|100|C}} in less than 1/20 of a second. Further tests simulating Mach{{nbsp}}5 were planned, with temperature reduction expected from {{cvt|1000|C}}.<ref name=precooling/><ref name=AvWeek7apr2019>{{cite news |url= https://aviationweek.com/space/reaction-engines-pre-cooler-passes-mach-33-test |title= Reaction Engines Pre-Cooler Passes Mach 3.3 Test |date= 7 April 2019 |author= Guy Norris |work= Aviation Week & Space Technology}}</ref> These further tests were successfully completed by October 2019.<ref>{{cite web |title=REACTION ENGINES TEST PROGRAMME FULLY VALIDATES PRECOOLER AT HYPERSONIC HEAT CONDITIONS |url=https://www.reactionengines.co.uk/news/reaction-engines-test-programme-fully-validates-precooler-hypersonic-heat-conditions |website=Reaction Engines |access-date=18 February 2020}}</ref><ref>{{cite web |title=Reaction Engines' precooler tech demo chills 1,000°C air in less than 1/20th of a second |url=https://www.theregister.co.uk/2019/10/22/reaction_engines_precooler_demo_mach_5/ |website=The Register |access-date=18 February 2020}}</ref><ref>{{cite web |title=Skylon's SABRE Engine Passes a Big Test |url=https://www.universetoday.com/143810/skylons-sabre-engine-passes-a-big-test/ |website=Universe Today |date=26 October 2019 |access-date=18 February 2020}}</ref>

The successful HTX test could lead to spin-off precooler applications which could be developed before a scalable SABRE demonstrator is completed; suggested uses are to expand [[gas turbine]]s capabilities, in advanced [[turbofan]]s, hypersonic vehicles, and industrial applications.<ref name=AvWeek15May2018>{{cite news |url= http://aviationweek.com/defense/turbojet-runs-precursor-hypersonic-engine-heat-exchanger-tests |title= Turbojet Runs Precursor to Hypersonic Engine Heat Exchanger Tests |date= 15 May 2018 |author= Guy Norris |work= Aviation Week & Space Technology}}</ref>
In March 2019, the [[UK Space Agency|UKSA]] and ESA preliminary design review of the demonstrator engine core confirmed the test version to be ready for implementation.<ref>{{Cite web |title=ESA greenlight for UK's air-breathing rocket engine |url=https://www.esa.int/Enabling_Support/Space_Engineering_Technology/ESA_greenlight_for_UK_s_air-breathing_rocket_engine |access-date=2024-03-02 |website=www.esa.int |language=en}}</ref>

In 2019, Airborne Engineering conducted a test campaign on subscale air/hydrogen injectors for the SABRE preburners.<ref>{{Cite web|url=https://www.ael.co.uk/projects/preburner-injector-test-rig.html|title=Preburner Injector Test Rig|access-date=9 December 2020}}</ref>

In 2020, Airborne Engineering conducted a test campaign on an "HX3 module" (preburner to helium loop heat exchanger) <ref>{{Cite web|url=https://www.ael.co.uk/projects/hx3-single-module-test-rig.html|title=HX3 Single Module Test Rig|access-date=9 December 2020}}</ref>

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.”<ref>{{cite web |last1=Sampson |first1=Ben |title=Reaction and US Air Force progress air breathing engine technology testing |url=https://www.aerospacetestinginternational.com/news/engine-testing/reaction-and-us-air-force-progress-air-breathing-engine-technology-testing.html |website=Aerospace Testing International |date=4 January 2023 |publisher=Mark Allen Group Limited |access-date=6 January 2023}}</ref>

===Engine===

Due to the static thrust capability of the hybrid rocket engine, the vehicle can [[takeoff|take off]] under air-breathing mode, much like a conventional [[turbojet]].<ref name="user" /> As the craft ascends and the outside air pressure drops, more and more air is passed into the compressor as the effectiveness of the ram compression drops. In this fashion the jets are able to operate to a much higher altitude than would normally be possible.

At Mach{{nbsp}}5.5 the air-breathing system becomes inefficient and is powered down, replaced by the on-board stored oxygen which allows the engine to accelerate to orbital velocities (around Mach{{nbsp}}25).<ref name="how" />

==Evolution==
===RB545===
Designed for use with HOTOL.

The engine had no air-breathing static thrust capability, relying on a rocket trolley to achieve takeoff

===SABRE===
Designed for use with Skylon A4.

The engine had no air-breathing static thrust capability, relying on RATO engines.

===SABRE 2===
Designed for use with Skylon C1.

The engine had no static thrust capability, using LOX until the air-breathing cycle could take over.{{citation needed|date=December 2020}}

===SABRE 3===
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.

===SABRE 4===
SABRE 4 is no longer a single engine design, but a class of engines, e.g. a {{convert|0.8|-|2|MN|lbf tf|abbr=on}} instance of this engine is used with SKYLON D1.5, a {{convert|110000|-|280000|lbf|MN tf|abbr=on}} for a USAF study into a partially reusable TSTO.

==Performance==

The designed [[thrust-to-weight ratio]] of SABRE is fourteen compared to about five for conventional jet engines, and two for [[scramjet]]s.<ref name="compare" /> 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 [[RB545]] engine or scramjets.

[[Fuel efficiency]] (known as [[specific impulse]] in rocket engines) peaks at about 3500&nbsp;[[Specific impulse#Units|seconds]] within the atmosphere.<ref name="user" /> Typical all-rocket systems peak around 450 seconds and even "typical" [[nuclear thermal rocket]]s at about 900&nbsp;seconds.

The combination of high [[fuel efficiency]] and low-mass engines permits an SSTO approach, with air-breathing to Mach{{nbsp}}5.14+ at {{convert|28.5|km|feet|abbr=on}} altitude, and with the vehicle reaching orbit with more payload mass per take-off mass than just about any non-[[Nuclear pulse propulsion|nuclear]] launch vehicle ever proposed.{{citation needed|date=July 2013}}

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&nbsp;GW/m<sup>3</sup>. 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 [[Space Shuttle]] 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&nbsp;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 [[payload fraction]] to the level at which SSTO becomes possible.

A hybrid jet engine like SABRE needs only reach low [[hypersonic]] speeds inside the [[Stratosphere|lower atmosphere]] before engaging its closed cycle mode, whilst climbing, to build speed. Unlike [[ramjet]] or [[scramjet]] engines, the design is able to provide high thrust from zero speed up to Mach{{nbsp}}5.4,<ref name="SABRE_page"/> 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.<ref name="compare" />

In 2012, REL expected test flights by 2020, and operational flights by 2030.<ref>{{cite web|url=https://www.flightglobal.com/news/articles/in-focus-british-engineers-crack-secret-of-reusab-379609/|title=IN FOCUS – British engineers 'crack secret of reusable spaceplane'|date=29 November 2012|website=FlightGlobal.com|access-date=19 December 2017}}</ref>

==See also==
{{columns-list|colwidth=30em|
* [[Altitude compensating nozzle]]
* [[Avatar (spacecraft)]] – India's air-breathing spaceplane
* [[Liquid-propellant rocket]]
* [[Precooled jet engine]]
}}

==References==
{{Reflist|30em}}

==Resources==
* {{cite web
 | title=The Skylon Space plane
 | url=http://www.reactionengines.co.uk/tech_docs/JBIS_v57_22-32.pdf
 | publisher=[[Reaction Engines Limited]]
|archive-url=https://web.archive.org/web/20110615104534/http://www.reactionengines.co.uk/tech_docs/JBIS_v57_22-32.pdf
 |archive-date=15 June 2011 }}
* {{cite web
 | title=The Skylon Spaceplane: Progress To Realisation
 | url=http://www.reactionengines.co.uk/tech_docs/The%20SKYLON%20Spaceplane-Progress%20to%20Realisation,%20JBIS,%202008.pdf
| publisher=[[Reaction Engines Limited]]
 | archive-url=https://web.archive.org/web/20110615133300/http://www.reactionengines.co.uk/tech_docs/The%20SKYLON%20Spaceplane-Progress%20to%20Realisation%2C%20JBIS%2C%202008.pdf
 | archive-date=15 June 2011
 | url-status=dead
 | df=dmy-all
 }}
* {{cite web
 | title=A Comparison of Propulsions Concepts for SSTO Reusable launchers
 | url=http://www.reactionengines.co.uk/tech_docs/JBIS_v56_108-117.pdf
 | publisher=[[Reaction Engines Limited]]
|archive-url=https://web.archive.org/web/20110615104428/http://www.reactionengines.co.uk/tech_docs/JBIS_v56_108-117.pdf
 |archive-date=15 June 2011 }}

==External links==
{{Commons category|SABRE (rocket engine)}}
* {{Official website|https://www.reactionengines.co.uk/sabre}}
* {{cite magazine | title=This rocket engine could soon put planes in outer space | date=12 October 2015 | url=https://www.wired.co.uk/magazine/archive/2015/11/start/rocket-engine-planes-space | magazine=[[Wired UK]]| last=Volpicelli | first=Gian M. }}
* {{cite news |url= http://aviationweek.com/technology/reaction-engines-begins-test-facility-construction |title= Reaction Engines Begins Test Facility Construction |date= 4 May 2017 |author= Tony Osborne |work= Aerospace Daily & Defense Report |publisher= Aviation Week}}
* [https://www.youtube.com/watch?v=srZzRVNj6uI TF1 time lapse – August 2018]
* [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]

{{Reaction Engines}}

{{DEFAULTSORT:Reaction Engines Sabre}}
[[Category:Rocket engines using hydrogen propellant]]
[[Category:Rocket engines of the United Kingdom]]
[[Category:Jet engines]]
[[Category:Ramjet engines]]
[[Category:Proposed engines]]
[[Category:Rocket engines using the staged combustion cycle]]