Liquid air cycle engine
Encyclopedia
A Liquid Air Cycle Engine (LACE) is a type of spacecraft propulsion
engine that attempts to increase its efficiency by gathering part of its oxidizer from the atmosphere
. A liquid air cycle engine uses cryogenic hydrogen fuel to liquify the air.
In a LOX
/LH2 bipropellant rocket the liquid oxygen
needed for combustion is the majority of the weight of the spacecraft on lift-off, so if some of this can be collected from the air on the way, it might dramatically lower the take-off weight of the spacecraft.
LACE was studied to some extent in the USA
during the late 1950s and early 1960s, and by late 1960 Marquardt had a testbed system running. However, as NASA
moved to ballistic capsules during Project Mercury
, funding for research into winged vehicles slowly disappeared, and LACE work along with it.
, where intake ramp
s create shock wave
s that compress the air. The LACE design then blows the compressed air over a heat exchanger
, in which the liquid hydrogen
fuel is flowing. This rapidly cools the air, and the various constituents quickly liquefy. By careful mechanical arrangement the liquid oxygen can be removed from the other parts of the air, notably water
, nitrogen
and carbon dioxide
, at which point it can be fed into the engine as usual. The hydrogen is so much lighter than oxygen that the now-warm hydrogen is often dumped overboard instead of being re-used as fuel, at a net gain.
The use of a winged launch vehicle allows using lift
rather than thrust
to overcome gravity, which greatly reduces gravity losses.
One issue with the LACE system is that in order to appreciably reduce the mass of the oxygen carried at launch, a LACE vehicle needs to spend more time in the lower atmosphere to collect enough oxygen to supply the engines. This leads to greatly increased vehicle heating and drag losses, which therefore increases fuel consumption to offset the drag losses and the additional mass of the thermal protection system. This increased fuel consumption offsets somewhat the savings in oxidizer mass; these losses are in turn offset by the higher Isp (Specific impulse
) of the air-breathing engine. Thus, the engineering trade-offs involved are quite complex, and highly sensitive to the design assumptions made.
Other issues are introduced by the relative material and logistical properties of LOX
versus LH2.
LOX is quite cheap; LH2 is nearly two orders of magnitude more expensive.
LOX is dense (1.141 kg/L), whereas LH2 has a very low density (0.0678 kg/L) and is therefore very bulky. (The extreme bulkiness of the LH2 tankage tends to increase vehicle drag by increasing the vehicle's frontal area
.) Finally, LOX tanks are relatively lightweight and fairly cheap, while the deep cryogenic nature and extreme physical properties of LH2 mandate that LH2 tanks and plumbing must be large and use heavy, expensive, exotic materials and insulation. Hence, much as the costs of using LH2 rather than a hydrocarbon fuel may well outweigh the Isp benefit of using LH2 in a Single-Stage-To-Orbit rocket, the costs of using more LH2 as a propellant and air-liquefaction coolant in LACE may well outweigh the benefits gained by not needing to carry as much LOX on-board.
Most significantly, the LACE system is far heavier than a pure rocket engine having the same thrust (air-breathing engines of almost all types have relatively poor thrust-to-weight ratios compared to rockets), and the performance of launch vehicles of all types is particularly affected by increases in vehicle dry mass (such as engines) that must be carried all the way to orbit, as opposed to oxidizer mass that would be burnt off over the course of the flight. Moreover, the lower thrust-to-weight ratio of an air-breathing engine as compared to a rocket significantly decreases the launch vehicle's maximum possible acceleration, and increases gravity losses
since more time must be spent to accelerate to orbital velocity. Also, the higher inlet and airframe drag losses
of a lifting, air-breathing vehicle launch trajectory as compared to a pure rocket on a ballistic launch trajectory introduces an additional penalty term
into the rocket equation known as the "Air-Breather's Burden."
The "air-breather's burden" term implies that unless the lift-to-drag ratio
and the acceleration of the vehicle as compared to gravity
are both implausibly large for a hypersonic
air-breathing vehicle, the advantages of the higher Isp of the air-breathing engine and the savings in LOX mass are largely lost.
Thus, the advantages, or disadvantages, of the LACE design continue to be a matter of some debate.
during the late 1950s and early 1960s, where it was seen as a "natural" fit for a winged spacecraft project known as the Aerospaceplane
. At the time the concept was known as LACES, for Liquid Air Collection Engine System, or ACES for Air Collection and Enrichment System. Both Marquardt
and General Dynamics
were involved in the research, and by late 1960 Marquardt had a testbed system running that was capable of running a 275 lbf (1.2 kN) thrust engine for minutes at a time. However, as NASA
moved to ballistic capsules during Project Mercury
, funding for research into winged vehicles slowly disappeared, and LACE along with it.
Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
engine that attempts to increase its efficiency by gathering part of its oxidizer from the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
. A liquid air cycle engine uses cryogenic hydrogen fuel to liquify the air.
In a LOX
Lox
Lox is salmon fillet that has been cured. In its most popular form, it is thinly sliced—less than in thickness—and, typically, served on a bagel, often with cream cheese, onion, tomato, cucumber and capers...
/LH2 bipropellant rocket the 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:...
needed for combustion is the majority of the weight of the spacecraft on lift-off, so if some of this can be collected from the air on the way, it might dramatically lower the take-off weight of the spacecraft.
LACE was studied to some extent in the USA
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
during the late 1950s and early 1960s, and by late 1960 Marquardt had a testbed system running. However, as NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
moved to ballistic capsules during Project Mercury
Project Mercury
In January 1960 NASA awarded Western Electric Company a contract for the Mercury tracking network. The value of the contract was over $33 million. Also in January, McDonnell delivered the first production-type Mercury spacecraft, less than a year after award of the formal contract. On February 12,...
, funding for research into winged vehicles slowly disappeared, and LACE work along with it.
Principle of operation
Conceptually LACE works by compressing and then quickly liquefying the air. Compression is achieved through the ram-air effect in an intake similar to that found on a high-speed aircraft like ConcordeConcorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...
, where intake ramp
Intake ramp
An intake ramp is a rectangular, plate-like device within the air intake of a jet engine, designed to generate a shock wave to aid the inlet compression process at supersonic speeds. The ramp sits at an acute angle to deflect the intake air from the longitudinal direction...
s create shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
s that compress the air. The LACE design then blows the compressed air over 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...
, in which 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 is flowing. This rapidly cools the air, and the various constituents quickly liquefy. By careful mechanical arrangement the liquid oxygen can be removed from the other parts of the air, notably water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, at which point it can be fed into the engine as usual. The hydrogen is so much lighter than oxygen that the now-warm hydrogen is often dumped overboard instead of being re-used as fuel, at a net gain.
The use of a winged launch vehicle allows using lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...
rather than thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
to overcome gravity, which greatly reduces 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...
One issue with the LACE system is that in order to appreciably reduce the mass of the oxygen carried at launch, a LACE vehicle needs to spend more time in the lower atmosphere to collect enough oxygen to supply the engines. This leads to greatly increased vehicle heating and drag losses, which therefore increases fuel consumption to offset the drag losses and the additional mass of the thermal protection system. This increased fuel consumption offsets somewhat the savings in oxidizer mass; these losses are in turn offset by the higher Isp (Specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
) of the air-breathing engine. Thus, the engineering trade-offs involved are quite complex, and highly sensitive to the design assumptions made.
Other issues are introduced by the relative material and logistical properties of LOX
Lox
Lox is salmon fillet that has been cured. In its most popular form, it is thinly sliced—less than in thickness—and, typically, served on a bagel, often with cream cheese, onion, tomato, cucumber and capers...
versus LH2.
LOX is quite cheap; LH2 is nearly two orders of magnitude more expensive.
LOX is dense (1.141 kg/L), whereas LH2 has a very low density (0.0678 kg/L) and is therefore very bulky. (The extreme bulkiness of the LH2 tankage tends to increase vehicle drag by increasing the vehicle's frontal area
Drag equation
In fluid dynamics, the drag equation is a practical formula used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid....
.) Finally, LOX tanks are relatively lightweight and fairly cheap, while the deep cryogenic nature and extreme physical properties of LH2 mandate that LH2 tanks and plumbing must be large and use heavy, expensive, exotic materials and insulation. Hence, much as the costs of using LH2 rather than a hydrocarbon fuel may well outweigh the Isp benefit of using LH2 in a Single-Stage-To-Orbit rocket, the costs of using more LH2 as a propellant and air-liquefaction coolant in LACE may well outweigh the benefits gained by not needing to carry as much LOX on-board.
Most significantly, the LACE system is far heavier than a pure rocket engine having the same thrust (air-breathing engines of almost all types have relatively poor thrust-to-weight ratios compared to rockets), and the performance of launch vehicles of all types is particularly affected by increases in vehicle dry mass (such as engines) that must be carried all the way to orbit, as opposed to oxidizer mass that would be burnt off over the course of the flight. Moreover, the lower thrust-to-weight ratio of an air-breathing engine as compared to a rocket significantly decreases the launch vehicle's maximum possible acceleration, and increases 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...
since more time must be spent to accelerate to orbital velocity. Also, the higher inlet and airframe drag losses
Parasitic drag
Parasitic drag is drag caused by moving a solid object through a fluid medium . Parasitic drag is made up of many components, the most prominent being form drag...
of a lifting, air-breathing vehicle launch trajectory as compared to a pure rocket on a ballistic launch trajectory introduces an additional penalty term
into the rocket equation known as the "Air-Breather's Burden."
The "air-breather's burden" term implies that unless the lift-to-drag ratio
Lift-to-drag ratio
In aerodynamics, the lift-to-drag ratio, or L/D ratio, is the amount of lift generated by a wing or vehicle, divided by the drag it creates by moving through the air...
and the acceleration of the vehicle as compared to gravity
are both implausibly large 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...
air-breathing vehicle, the advantages of the higher Isp of the air-breathing engine and the savings in LOX mass are largely lost.
Thus, the advantages, or disadvantages, of the LACE design continue to be a matter of some debate.
History
LACE was studied to some extent in the United States of AmericaUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
during the late 1950s and early 1960s, where it was seen as a "natural" fit for a winged spacecraft project known as the 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...
. At the time the concept was known as LACES, for Liquid Air Collection Engine System, or ACES for Air Collection and Enrichment System. Both 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...
were involved in the research, and by late 1960 Marquardt had a testbed system running that was capable of running a 275 lbf (1.2 kN) thrust engine for minutes at a time. However, as NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
moved to ballistic capsules during Project Mercury
Project Mercury
In January 1960 NASA awarded Western Electric Company a contract for the Mercury tracking network. The value of the contract was over $33 million. Also in January, McDonnell delivered the first production-type Mercury spacecraft, less than a year after award of the formal contract. On February 12,...
, funding for research into winged vehicles slowly disappeared, and LACE along with it.
See also
- Air-augmented rocketAir-augmented rocketAir-augmented rockets use the supersonic exhaust of some kind of rocket engine to further compress air collected by ram effect during flight to use as additional working mass, leading to greater effective thrust for any given amount of fuel than either the rocket or a ramjet...
- RB545RB545The 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:...
- Reaction Engines SABRE - a precooled jet engine that cools but does not liquefy the air
- ScramjetScramjetA scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...