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A liquid air cycle engine (LACE) is a type of
spacecraft propulsion Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric e ...
engine that attempts to increase its efficiency by gathering part of its oxidizer from the
atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
. A liquid air cycle engine uses
liquid hydrogen Liquid hydrogen () is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecule, molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point (thermodynamics), critical point of 33 Kelvins, ...
(LH2) fuel to liquefy the air. In a liquid oxygen/liquid hydrogen
rocket A rocket (from , and so named for its shape) is a vehicle that uses jet propulsion to accelerate without using any surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entirely ...
, the liquid oxygen (LOX) 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 The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
moved to ballistic capsules during
Project Mercury Project Mercury was the first human spaceflight program of the United States, running from 1958 through 1963. An early highlight of the Space Race, its goal was to put a man into Earth orbit and return him safely, ideally before the Soviet Un ...
, funding for research into winged vehicles slowly disappeared, and LACE work along with it. LACE was also the basis of the engines on the British Aerospace HOTOL design of the 1980s, but this did not progress beyond studies.


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
Concorde Concorde () is a retired Anglo-French supersonic airliner jointly developed and manufactured by Sud Aviation and the British Aircraft Corporation (BAC). Studies started in 1954, and France and the United Kingdom signed a treaty establishin ...
, where intake ramps create
shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...
s that compress the air. The LACE design then blows the compressed air over a
heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...
, in which the
liquid hydrogen Liquid hydrogen () is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecule, molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point (thermodynamics), critical point of 33 Kelvins, ...
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 is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
,
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
and
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
, at which point the liquid oxygen can be fed into the engine as usual. It will be seen that heat-exchanger limitations always cause this system to run with a hydrogen/air ratio much richer than stoichiometric with a consequent penalty in performance and thus some hydrogen is dumped overboard.


Advantages and disadvantages

The use of a winged launch vehicle allows using lift rather than
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
to overcome gravity, which greatly reduces gravity losses. On the other hand, the reduced gravity losses come at the price of much higher
aerodynamic drag In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...
and aerodynamic heating due to the need to stay much deeper within the atmosphere than a pure rocket would during the
boost phase A ballistic missile goes through several distinct phases of flight that are common to almost all such designs. They are, in order: * boost phase when the main boost rocket or upper stages are firing; * post-boost phase when any last-minute change ...
. 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 during the remainder of the launch. 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
specific impulse Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine, such as a rocket engine, rocket using propellant or a jet engine using fuel, generates thrust. In general, this is a ratio of the ''Impulse (physics), ...
, ''I'', 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 LH. LOx is quite cheap; LH is nearly two orders of magnitude more expensive. LOx is dense (1.141 kg/L), whereas LH 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 LH mandate that LH tanks and plumbing must be large and use heavy, expensive, exotic materials and insulation. Hence, much as the costs of using LH rather than a
hydrocarbon fuel In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may ...
may well outweigh the ''I'' benefit of using LH in a single-stage-to-orbit rocket, the costs of using more LH 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 In mathematics, the term "almost all" means "all but a negligible quantity". More precisely, if X is a set (mathematics), set, "almost all elements of X" means "all elements of X but those in a negligible set, negligible subset of X". The meaning o ...
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 \frac into the rocket equation known as the ''air-breather's burden''. This term implies that unless the lift-to-drag ratio (''L''/''D'') and the acceleration of the vehicle as compared to gravity (''a''/''g'') are both implausibly large for a hypersonic air-breathing vehicle, the advantages of the higher ''I{{sub, sp'' 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 America The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 states and a federal capital district, Washington, D.C. The 48 contiguo ...
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''. The liquified air and some of the hydrogen is then pumped directly into the engine for burning. When it was demonstrated that it was relatively easy to separate the oxygen from the other components of air, mostly nitrogen and carbon dioxide, a new concept emerged as ACES for ''Air Collection and Enrichment System''. This leaves the problem of what to do with the leftover gasses. ACES injected the nitrogen into a ramjet engine, using it as additional working fluid while the engine was running on air and the liquid oxygen was being stored. As the aircraft climbed and the atmosphere thinned, the lack of air was offset by increasing the flow of oxygen from the tanks. This makes ACES an ejector ramjet (or ramrocket) as opposed to the pure rocket LACE design. Both Marquardt Corporation and General Dynamics were involved in the LACES research. However, as
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
moved to ballistic capsules during
Project Mercury Project Mercury was the first human spaceflight program of the United States, running from 1958 through 1963. An early highlight of the Space Race, its goal was to put a man into Earth orbit and return him safely, ideally before the Soviet Un ...
, funding for research into winged vehicles slowly disappeared, and ACES along with it.


See also

* Air-augmented rocket * RB545 * Reaction Engines SABRE - a
precooled jet engine A precooled jet engine is a concept that enables jet engines with turbomachinery, as opposed to ramjets, to be used at high speeds. Precooling restores some or all of the performance degradation of the engine compressor (by preventing rotating sta ...
that cools but does not liquefy the air *
Scramjet A scramjet (supersonic combustion ramjet) is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully b ...


References


External links


Liquid Air Cycle Rocket Equation
Spacecraft propulsion Single-stage-to-orbit Rocket engines using hydrogen propellant