SABRE
Encyclopedia
SABRE is a concept by Reaction Engines Limited
for a hypersonic
precooled hybrid air breathing rocket engine
for propelling the proposed Skylon launch vehicle into low Earth orbit
(LEO). SABRE is the logical continuation of Alan Bond
's series of liquid air cycle engine
(LACE) and LACE-like designs that started in the early/mid-1980s for the HOTOL
project.
The design comprises a single combined cycle
rocket engine with two modes of operation.
The air breathing mode combines a turbo-compressor
with a lightweight air precooler positioned just behind the inlet cone
. At high speeds this precooler cools the hot, ram compressed air leading to an unusually high pressure ratio
within the engine. The compressed air is subsequently fed into the rocket combustion chamber
where it is ignited with stored liquid hydrogen
. The high pressure ratio allows the engine to continue to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed which gives a very lightweight engine — essential for reaching orbit. In addition, unlike the LACE concept that preceded it, SABRE’s precooler does not liquefy the air letting it run more efficiently.
After shutting the inlet cone off at Mach
5.14, 28.5 km altitude, the system continues as a closed cycle
high performance rocket engine combusting liquid oxygen
and liquid hydrogen
from on-board fuel tanks allowing Skylon to reach orbital velocity after leaving the atmosphere on a steep climb.
An engine derived from the SABRE concept called Scimitar
has been designed for the company’s A2
hypersonic passenger jet
proposal for the European Union
-funded LAPCAT
study.
Alan Bond says that the technology readiness level
of the SABRE engine is, as of May 2009, 2-3. A project called the Technology Demonstration Programme, funded through private finance as well as a European Space Agency
grant, began in February 2009 with the aim of validating key aspects of the engine by the end of 2011. The company anticipates this will bring the system to a technology readiness level of 4-5.
(LACE) idea which was originally explored by Marquardt
and General Dynamics
in the 1960s as part of the US Air Force's aerospaceplane
efforts.
In an operational setting with LACE, the 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 out the liquid oxygen for burning in the engine. The amount of warmed hydrogen was too great to burn with the oxygen, so most was to be simply dumped overboard (nevertheless giving useful thrust.)
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 embrittlement
, relatively high liquid hydrogen consumption, patents and Official Secrets Act
, so Bond went on to develop SABRE in its place.
, the SABRE design is neither a conventional rocket engine
nor jet engine, but a Precooled Hybrid Air Breathing Rocket Engine that burns liquid hydrogen fuel combined with an oxidant of either compressor-fed gaseous air or stored liquid oxygen fed using a turbopump
.
At the front of the engine a simple translating axisymmetric shock cone inlet
slows the air to subsonic speeds using just two shock reflections.
Part of the air then passes through a precooler into the central core, with the remainder passing directly through a ring of bypass ramjet
s. The central core of SABRE behind the precooler uses a turbo-compressor
run off the same gaseous helium
loop Brayton cycle
which compresses the air and feeds it into four high pressure combined cycle rocket engine combustion chambers.
/hypersonic
speeds, it becomes very hot due to compression effects. The high temperatures are traditionally dealt with in jet engines by using heavy copper
or nickel
based materials, by reducing the engine's pressure ratio
, and by throttling back the engine at the higher airspeeds to avoid melting. However, for an SSTO craft, such heavy materials are unusable, and maximum thrust is necessary for orbital insertion at the earliest time to minimise gravity losses
. Instead, using a gaseous helium
coolant loop, SABRE dramatically cools the air from 1000 °C down to -140 °C in a heat exchanger while avoiding liquefaction
of the air or blockage from freezing water vapour.
Previous versions of precoolers such as HOTOL put the hydrogen fuel directly through the precooler, but inserting a helium cooling loop between the air and the cold fuel avoids problems with hydrogen embrittlement
in the air precooler.
However, the dramatic cooling of the air raised a potential problem: it is necessary to prevent blocking the precooler from frozen water vapour and other fractions. A suitable precooler, which rejects condensed water before it freezes has now been experimentally demonstrated.
, similar in design to those used on conventional jet engines but running at unusually high pressure ratio
made possible by the low temperature of the inlet air. This feeds the compressed air at very high pressure into the combustion chambers of the main engines.
Unusually for jet engines, the turbo-compressor is powered by a gas turbine
running on a helium loop, rather than off combustion gases as in a conventional jet engine. Thus, the turbo-compressor is powered by waste heat collected by the helium loop.
The loop forms a self-starting Brayton cycle
engine, and is used to both cool critical parts of the engine, but also to power turbines and numerous miscellaneous parts of the engine.
The heat passes from the air into the helium. This heat energy is not entirely wasted, it is in fact used to power the various parts of the engine, and the remainder is used to vapourise hydrogen, which is burnt in ramjet
s.
At Mach 5.5 the jets become inefficient and are powered down, and stored liquid oxygen/liquid hydrogen is used for the rest of the ascent in the separate rocket engines; the turbopump
s are powered by the helium loop from the heat produced by the preburner.
Unusually the combustion chambers in the SABRE engine are to be cooled by the oxidant (air/liquid oxygen) rather than by liquid hydrogen to further reduce the systems overuse of liquid hydrogen compared to stoichiometric
.
The most efficient atmospheric pressure at which a conventional propelling nozzle
works at is set by the geometry
of the bell
. While the geometry of the conventional bell remains static the atmospheric pressure changes with altitude and therefore nozzles designed for high performance in the lower atmosphere will significantly drop in efficiency as they reach higher altitudes. This is overcome in traditional rockets through the multistage
process so engines designed for different atmospheric pressures can be used during the most appropriate stages of flight. An SSTO
engine must use the same nozzles however. As part of the work on the Technology Development Programme tests were carried out on an expansion deflection nozzle
called STERN to overcome the problem of non-dynamic exhaust expansion. The successful experiment means that this technology is likely to be incorporated into the final SABRE design.
is generated and therefore less liquid hydrogen is boiled off. However, even simply cooling the air needs more liquid hydrogen than can be burnt in the engine core. The excess is dumped overboard through a series of burners – "spill duct ramjet
burners" which 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 doesn’t get fed into the main rocket engine, rather than generating appreciable thrust of their own. 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 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.
The Sabre engine "relies on a heat exchanger capable of cooling incoming air to -140 C, to provide liquid oxygen (LOX) for mixing with hydrogen to provide jet thrust during atmospheric flight before switching to tanked LOX when in space." The fall 2011 test program will validate that the critical heat exchanger technology can perform as needed for the engine to obtain adequate oxygen from the atmosphere to support the low-altitude, high-performance operation.
s. This high performance is a combination of the cooled air being denser and hence requiring less compression, but more importantly, of the low air temperatures permitting lighter alloy to be used in much of the engine. Overall performance is much better than the RB545
engine or scramjets.
The engine gives good fuel efficiency peaking at about 3500 seconds within the atmosphere. Typical all-rocket systems are around 450 at best, and even "typical" nuclear thermal rockets only about 900 seconds.
The combination of high fuel efficiency and low mass engines means that a single stage to orbit approach for Skylon can be employed, with air breathing to mach 5.14+ at 28.5 km altitude, and with the vehicle reaching orbit with more payload mass per take-off mass than just about any non-nuclear
launch vehicle ever proposed.
Like the RB545, the precooler idea adds mass and complexity to the system, normally the antithesis of rocket design. The precooler is also the most aggressive and difficult part of the whole SABRE design. The mass of this heat exchanger is an order of magnitude better than has been achieved previously; however, experimental work has proved that this can be achieved. The experimental heat exchanger has achieved heat exchange of almost 1 GW/m³, believed to be a world record. Small sections of a real precooler, referred to as modules, now exist.
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 would appear to be heavy, yet the gains in overall efficiency more than make up for this. These losses are greatly offset by the different flight plan. Conventional launch vehicles such as the Space Shuttle
usually start a launch by spending around a 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, air breathing and using wings to support the vehicle, giving far lower fuel usage before lighting the rockets to do the orbital insertion.
, a hybrid jet engine can utilise air to create combustion saving on propellant weight and therefore increasing payload fraction
.
Ramjet
s and Scramjet
s must spend a significant amount of time within the lower atmosphere to build speed to reach orbital velocity creating issues with extremely high drag leading to intense heating and the subsequent weight and complexity of required thermal protection. A hybrid jet like SABRE needs only reach low hypersonic
speeds inside the 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 5.5, 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 easily tested on the ground, which drastically cuts testing costs.
Reaction Engines Limited
Reaction Engines Limited is a British aerospace company based in Oxfordshire, England.- History & personnel :Reaction Engines was founded in 1989 by Alan Bond and Richard Varvill and John Scott-Scott...
for a hypersonic
Hypersonic
In aerodynamics, a hypersonic speed is one that is highly supersonic. Since the 1970s, the term has generally been assumed to refer to speeds of Mach 5 and above...
precooled hybrid air breathing rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
for propelling the proposed Skylon launch vehicle into low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
(LEO). SABRE is the logical continuation of Alan Bond
Alan Bond (rocket developer)
Alan Bond is Managing Director of Reaction Engines Ltd and associated with Project Daedalus, Blue Streak missile, HOTOL, Reaction Engines Skylon and the Reaction Engines A2 hypersonic passenger aircraft.- Career :...
's series of liquid air cycle engine
Liquid air cycle engine
A Liquid Air Cycle Engine is a type of spacecraft propulsion engine that attempts to increase its efficiency by gathering part of its oxidizer from the atmosphere...
(LACE) and LACE-like designs that started in the early/mid-1980s for the HOTOL
HOTOL
HOTOL, for Horizontal Take-Off and Landing, was a British air-breathing space shuttle effort by Rolls Royce and British Aerospace.Designed as a single-stage-to-orbit reusable winged launch vehicle, it was to be fitted with a unique air-breathing engine, the RB545 called the Swallow, to be...
project.
The design comprises a single combined cycle
Combined cycle
In electric power generation a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators...
rocket engine with two modes of operation.
The air breathing mode combines a turbo-compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
with a lightweight air precooler positioned just behind the inlet cone
Inlet cone
Inlet cones are a component of some supersonic aircraft. They are primarily used on ramjets, such as the turboramjets of the SR-71 or the pure ramjets of the D-21 Tagboard and Lockheed X-7...
. At high speeds this precooler cools the hot, ram compressed air leading to an unusually high pressure ratio
Overall pressure ratio
In aeronautical engineering, the term overall pressure ratio is defined as the ratio of the stagnation pressure as measured at the front and rear of the compressor of a gas turbine engine...
within the engine. The compressed air is subsequently fed into the rocket combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...
where it is ignited with stored liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...
. The high pressure ratio allows the engine to continue to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed which gives a very lightweight engine — essential for reaching orbit. In addition, unlike the LACE concept that preceded it, SABRE’s precooler does not liquefy the air letting it run more efficiently.
After shutting the inlet cone off at Mach
Mach number
Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...
5.14, 28.5 km altitude, the system continues as a closed cycle
Staged combustion cycle (rocket)
The staged combustion cycle, also called topping cycle or pre-burner cycle, is a thermodynamic cycle of bipropellant rocket engines. Some of the propellant is burned in a pre-burner and the resulting hot gas is used to power the engine's turbines and pumps...
high performance rocket engine combusting liquid oxygen
Liquid oxygen
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...
and liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...
from on-board fuel tanks allowing Skylon to reach orbital velocity after leaving the atmosphere on a steep climb.
An engine derived from the SABRE concept called Scimitar
Reaction Engines Scimitar
The Reaction Engines Scimitar is a derivative of the SABRE engine technology, but intended for airliners , rather than space launch applications. Consequently, most of the Scimitar engine technology is similar to Sabre but designed for much longer life...
has been designed for the company’s A2
Reaction Engines A2
|-See also:-External links:* *...
hypersonic passenger jet
Supersonic transport
A supersonic transport is a civilian supersonic aircraft designed to transport passengers at speeds greater than the speed of sound. The only SSTs to see regular service to date have been Concorde and the Tupolev Tu-144. The last passenger flight of the Tu-144 was in June 1978 with its last ever...
proposal for the European Union
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
-funded LAPCAT
LAPCAT
LAPCAT was a 36 month European FP6 study to examine ways to produce engines for a Mach 4-8 hypersonic aircraft. The project ended in April 2008...
study.
Alan Bond says that the technology readiness level
Technology Readiness Level
Technology Readiness Level is a measure used by some United States government agencies and many of the world's major companies to assess the maturity of evolving technologies prior to incorporating that technology into a system or subsystem...
of the SABRE engine is, as of May 2009, 2-3. A project called the Technology Demonstration Programme, funded through private finance as well as a European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
grant, began in February 2009 with the aim of validating key aspects of the engine by the end of 2011. The company anticipates this will bring the system to a technology readiness level of 4-5.
History
The precooler concept is due to an idea originated by Robert P. Carmichael in 1955. This was followed by the liquid air cycle engineLiquid air cycle engine
A Liquid Air Cycle Engine is a type of spacecraft propulsion engine that attempts to increase its efficiency by gathering part of its oxidizer from the atmosphere...
(LACE) idea which was originally explored by Marquardt
Marquardt
Marquardt Corporation was one of the few aeronautical engineering firms that was dedicated almost solely to the development of the ramjet engine. Marquardt designs were developed through the 1940s into the 1960s, but the ramjet never became a major design and the company turned to other fields in...
and General Dynamics
General Dynamics
General Dynamics Corporation is a U.S. defense conglomerate formed by mergers and divestitures, and as of 2008 it is the fifth largest defense contractor in the world. Its headquarters are in West Falls Church , unincorporated Fairfax County, Virginia, in the Falls Church area.The company has...
in the 1960s as part of the US Air Force's aerospaceplane
Aerospaceplane
The US Air Force's aerospaceplane project encompassed a variety of projects from 1958 until 1963 to study a fully reusable spaceplane. A variety of designs were studied during the lifetime of the project, including most of the early efforts on liquid air cycle engines and even a nuclear-powered...
efforts.
In an operational setting with LACE, the system was to be placed behind a supersonic air intake which would compress the air through ram compression, then a heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
would rapidly cool it using some of the liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...
fuel stored on board. The resulting liquid air was then processed to separate out the liquid oxygen for burning in the engine. The amount of warmed hydrogen was too great to burn with the oxygen, so most was to be simply dumped overboard (nevertheless giving useful thrust.)
In 1989, after funding for HOTOL ceased, Bond and several others formed Reaction Engines Limited
Reaction Engines Limited
Reaction Engines Limited is a British aerospace company based in Oxfordshire, England.- History & personnel :Reaction Engines was founded in 1989 by Alan Bond and Richard Varvill and John Scott-Scott...
to continue research. The RB545
RB545
The RB545 was an air-breathing rocket engine that was proposed to propel a British space shuttle to orbit using a single stage. Rolls-Royce was involved; while British Aerospace worked on the vehicle.-Design:...
's precooler had issues with embrittlement
Hydrogen embrittlement
Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen...
, relatively high liquid hydrogen consumption, patents and Official Secrets Act
Official Secrets Act
The Official Secrets Act is a stock short title used in the United Kingdom, Ireland, India and Malaysia and formerly in New Zealand for legislation that provides for the protection of state secrets and official information, mainly related to national security.-United Kingdom:*The Official Secrets...
, so Bond went on to develop SABRE in its place.
Design
Like the RB545RB545
The RB545 was an air-breathing rocket engine that was proposed to propel a British space shuttle to orbit using a single stage. Rolls-Royce was involved; while British Aerospace worked on the vehicle.-Design:...
, the SABRE design is neither a conventional rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
nor jet engine, but a Precooled Hybrid Air Breathing Rocket Engine that burns liquid hydrogen fuel combined with an oxidant of either compressor-fed gaseous air or stored liquid oxygen fed using a turbopump
Turbopump
A turbopump is a gas turbine that comprises basically two main components: a rotodynamic pump and a driving turbine, usually both mounted on the same shaft, or sometimes geared together...
.
At the front of the engine a simple translating axisymmetric shock cone inlet
Inlet cone
Inlet cones are a component of some supersonic aircraft. They are primarily used on ramjets, such as the turboramjets of the SR-71 or the pure ramjets of the D-21 Tagboard and Lockheed X-7...
slows the air to subsonic speeds using just two shock reflections.
Part of the air then passes through a precooler into the central core, with the remainder passing directly through a ring of bypass ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
s. The central core of SABRE behind the precooler uses a turbo-compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
run off the same gaseous helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
loop Brayton cycle
Brayton cycle
The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the airbreathing jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber...
which compresses the air and feeds it into four high pressure combined cycle rocket engine combustion chambers.
Precooler
As the air enters the engine at supersonicSupersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
/hypersonic
Hypersonic
In aerodynamics, a hypersonic speed is one that is highly supersonic. Since the 1970s, the term has generally been assumed to refer to speeds of Mach 5 and above...
speeds, it becomes very hot due to compression effects. The high temperatures are traditionally dealt with in jet engines by using heavy copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
or nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
based materials, by reducing the engine's pressure ratio
Overall pressure ratio
In aeronautical engineering, the term overall pressure ratio is defined as the ratio of the stagnation pressure as measured at the front and rear of the compressor of a gas turbine engine...
, and by throttling back the engine at the higher airspeeds to avoid melting. However, for an SSTO craft, such heavy materials are unusable, and maximum thrust is necessary for orbital insertion at the earliest time to minimise gravity losses
Gravity drag
In astrodynamics and rocketry, gravity drag is a measure of the loss in the net performance of a rocket while it is thrusting in a gravitational field...
. Instead, using a gaseous helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
coolant loop, SABRE dramatically cools the air from 1000 °C down to -140 °C in a heat exchanger while avoiding liquefaction
Liquid air
Liquid air is air that has been cooled to very low temperatures so that it has condensed to a pale blue mobile liquid. To protect it from room temperature, it must be kept in a vacuum flask. Liquid air can absorb heat rapidly and revert to its gaseous state...
of the air or blockage from freezing water vapour.
Previous versions of precoolers such as HOTOL put the hydrogen fuel directly through the precooler, but inserting a helium cooling loop between the air and the cold fuel avoids problems with hydrogen embrittlement
Hydrogen embrittlement
Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen...
in the air precooler.
However, the dramatic cooling of the air raised a potential problem: it is necessary to prevent blocking the precooler from frozen water vapour and other fractions. A suitable precooler, which rejects condensed water before it freezes has now been experimentally demonstrated.
Compressor
Below 28.5 km, the cooled air from the precooler passes into a reasonably conventional turbo-compressorGas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
, similar in design to those used on conventional jet engines but running at unusually high pressure ratio
Overall pressure ratio
In aeronautical engineering, the term overall pressure ratio is defined as the ratio of the stagnation pressure as measured at the front and rear of the compressor of a gas turbine engine...
made possible by the low temperature of the inlet air. This feeds the compressed air at very high pressure into the combustion chambers of the main engines.
Unusually for jet engines, the turbo-compressor is powered by a gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
running on a helium loop, rather than off combustion gases as in a conventional jet engine. Thus, the turbo-compressor is powered by waste heat collected by the helium loop.
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
Brayton cycle
The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the airbreathing jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber...
engine, and is used to both cool critical parts of the engine, but also to power turbines and numerous miscellaneous parts of the engine.
The heat passes from the air into the helium. This heat energy is not entirely wasted, it is in fact used to power the various parts of the engine, and the remainder is used to vapourise hydrogen, which is burnt in ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
s.
Engines
Due to the static thrust capability of the hybrid rocket engines, the vehicle can take off under air breathing mode without assistance much like conventional turbojets. 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 alone drops. In this fashion the jets are able to operate to a much higher altitude than would normally be possible.At Mach 5.5 the jets become inefficient and are powered down, and stored liquid oxygen/liquid hydrogen is used for the rest of the ascent in the separate rocket engines; the turbopump
Turbopump
A turbopump is a gas turbine that comprises basically two main components: a rotodynamic pump and a driving turbine, usually both mounted on the same shaft, or sometimes geared together...
s are powered by the helium loop from the heat produced by the preburner.
Unusually the combustion chambers in the SABRE engine are to be cooled by the oxidant (air/liquid oxygen) rather than by liquid hydrogen to further reduce the systems overuse of liquid hydrogen compared to stoichiometric
Air-fuel ratio
Air–fuel ratio is the mass ratio of air to fuel present in an internal combustion engine. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich...
.
The most efficient atmospheric pressure at which a conventional propelling nozzle
Propelling nozzle
A propelling nozzle is the component of a jet engine that operates to constrict the flow, to form an exhaust jet and to maximise the velocity of propelling gases from the engine....
works at is set by the geometry
Geometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....
of the bell
De Laval nozzle
A de Laval nozzle is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass-shape...
. While the geometry of the conventional bell remains static the atmospheric pressure changes with altitude and therefore nozzles designed for high performance in the lower atmosphere will significantly drop in efficiency as they reach higher altitudes. This is overcome in traditional rockets through the multistage
Multistage rocket
A multistage rocket is a rocket that usestwo or more stages, each of which contains its own engines and propellant. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or...
process so engines designed for different atmospheric pressures can be used during the most appropriate stages of flight. An SSTO
Single-stage-to-orbit
A single-stage-to-orbit vehicle reaches orbit from the surface of a body without jettisoning hardware, expending only propellants and fluids. The term usually, but not exclusively, refers to reusable vehicles....
engine must use the same nozzles however. As part of the work on the Technology Development Programme tests were carried out on an expansion deflection nozzle
Expansion deflection nozzle
The expansion-deflection nozzle is an advanced rocket nozzle which achieves altitude compensation through interaction of the exhaust gas with the atmosphere, much like the plug and aerospike nozzles.- Description :...
called STERN to overcome the problem of non-dynamic exhaust expansion. The successful experiment means that this technology is likely to be incorporated into the final SABRE design.
Ramjets
Avoiding liquefaction improves the efficiency of the engine since less entropyEntropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
is generated and therefore less liquid hydrogen is boiled off. However, even simply cooling the air needs more liquid hydrogen than can be burnt in the engine core. The excess is dumped overboard through a series of burners – "spill duct ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
burners" which 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 doesn’t get fed into the main rocket engine, rather than generating appreciable thrust of their own. 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 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.
Test program
, hardware testing of the "heat exchanger technology crucial to [the] hybrid air- and liquid oxygen-breathing [Sabre] rocket motor" is under way. This is an important step in the Sabre development process, to demonstrate to investors that the technology is viable. Tests are scheduled to continue through December 2011.The Sabre engine "relies on a heat exchanger capable of cooling incoming air to -140 C, to provide liquid oxygen (LOX) for mixing with hydrogen to provide jet thrust during atmospheric flight before switching to tanked LOX when in space." The fall 2011 test program will validate that the critical heat exchanger technology can perform as needed for the engine to obtain adequate oxygen from the atmosphere to support the low-altitude, high-performance operation.
Performance
The designed thrust/weight ratio of SABRE ends is high—up to 14—compared to about 5 for conventional jet engines, and just 2 for scramjetScramjet
A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...
s. This high performance is a combination of the cooled air being denser and hence requiring less compression, but more importantly, of the low air temperatures permitting lighter alloy to be used in much of the engine. Overall performance is much better than the RB545
RB545
The RB545 was an air-breathing rocket engine that was proposed to propel a British space shuttle to orbit using a single stage. Rolls-Royce was involved; while British Aerospace worked on the vehicle.-Design:...
engine or scramjets.
The engine gives good fuel efficiency peaking at about 3500 seconds within the atmosphere. Typical all-rocket systems are around 450 at best, and even "typical" nuclear thermal rockets only about 900 seconds.
The combination of high fuel efficiency and low mass engines means that a single stage to orbit approach for Skylon can be employed, with air breathing to mach 5.14+ at 28.5 km altitude, and with the vehicle reaching orbit with more payload mass per take-off mass than just about any non-nuclear
Nuclear pulse propulsion
Nuclear pulse propulsion is a proposed method of spacecraft propulsion that uses nuclear explosions for thrust. It was first developed as Project Orion by DARPA, after a suggestion by Stanislaw Ulam in 1947...
launch vehicle ever proposed.
Like the RB545, the precooler idea adds mass and complexity to the system, normally the antithesis of rocket design. The precooler is also the most aggressive and difficult part of the whole SABRE design. The mass of this heat exchanger is an order of magnitude better than has been achieved previously; however, experimental work has proved that this can be achieved. The experimental heat exchanger has achieved heat exchange of almost 1 GW/m³, believed to be a world record. Small sections of a real precooler, referred to as modules, now exist.
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 would appear to be heavy, yet the gains in overall efficiency more than make up for this. These losses are greatly offset by the different flight plan. Conventional launch vehicles such as the Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
usually start a launch by spending around a 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, air breathing and using wings to support the vehicle, giving far lower fuel usage before lighting the rockets to do the orbital insertion.
Advantages
Unlike traditional rocket engines, and like other types of air breathing jet engineJet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
, a hybrid jet engine can utilise air to create combustion saving on propellant weight and therefore increasing payload fraction
Payload fraction
In aerospace engineering, payload fraction is a common term used to characterize the efficiency of a particular design. Payload fraction is calculated by dividing the weight of the payload by the weight of the otherwise empty aircraft when fully fueled...
.
Ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
s and Scramjet
Scramjet
A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...
s must spend a significant amount of time within the lower atmosphere to build speed to reach orbital velocity creating issues with extremely high drag leading to intense heating and the subsequent weight and complexity of required thermal protection. A hybrid jet like SABRE needs only reach low hypersonic
Hypersonic
In aerodynamics, a hypersonic speed is one that is highly supersonic. Since the 1970s, the term has generally been assumed to refer to speeds of Mach 5 and above...
speeds inside the lower atmosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
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 5.5, 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 easily tested on the ground, which drastically cuts testing costs.
See also
- Single-stage-to-orbitSingle-stage-to-orbitA single-stage-to-orbit vehicle reaches orbit from the surface of a body without jettisoning hardware, expending only propellants and fluids. The term usually, but not exclusively, refers to reusable vehicles....
- Precooled jet engine
- Hybrid rocketHybrid rocketA hybrid rocket is a rocket with a rocket motor which uses propellants in two different states of matter - one solid and the other either gas or liquid. The Hybrid rocket concept can be traced back at least 75 years....
- Liquid-propellant rocket
- Altitude compensating nozzleAltitude compensating nozzleAn altitude compensating nozzle is a class of rocket engine nozzles that are designed to operate efficiently across a wide range of altitudes.- Conventional designs :...
External links
- http://www.reactionengines.co.uk/sabre.html