The precooled jet engine is a concept that enables
jet engines
A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, water jet, and hybrid propulsion, the term typicall ...
with
turbomachinery
Turbomachinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor, a compressor transfers energy from ...
, 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 stall/choking/reduced flow), as well as that of the complete gas generator (by maintaining a significant combustor temperature rise within a fixed turbine temperature limit), which would otherwise prevent flight with high ram temperatures.
For higher flight speeds precooling may feature a
cryogenic fuel
Cryogenic fuels are fuels that require storage at extremely low temperatures in order to maintain them in a liquid state. These fuels are used in machinery that operates in space (e.g. rockets and satellites) where ordinary fuel cannot be used, d ...
-cooled
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 conta ...
before the air enters the compressor. After gaining heat and vapourising in the heat exchanger, the fuel (e.g. H
2) burns in the
combustor
A combustor is a component or area of a gas turbine, ramjet, or scramjet engine where combustion takes place. It is also known as a burner, combustion chamber or flame holder. In a gas turbine engine, the ''combustor'' or combustion chamber is f ...
. Precooling using a heat exchanger has not been used in flight, but is predicted to have significantly high thrust and efficiency at speeds up to Mach 5.5. Precooled jet engine cycles were analyzed by Robert P. Carmichael in 1955.
Pre-cooled engines avoid the need for an air
condenser because, unlike
liquid air cycle engine
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 liquid hydrogen (LH2) fuel to liquefy the a ...
s (LACE), pre-cooled engines cool the air without
liquefy
In materials science, liquefaction is a process that generates a liquid from a solid or a gas or that generates a non-liquid Phase (matter), phase which behaves in accordance with fluid dynamics.
It occurs both nature, naturally and artificially. ...
ing it.
For lower flight speeds precooling can be done with mass injection, known as WIPCC (water injection precompressor cooling) This method has been used for short duration (due to limited coolant capacity) increases to an aircraft's normal maximum speed. "Operation Skyburner", which gained a world speed record with a
McDonnell Douglas F-4 Phantom II
The McDonnell Douglas F-4 Phantom II is an American tandem two-seat, twin-engine, all-weather, long-range supersonic jet interceptor and fighter-bomber originally developed by McDonnell Aircraft for the United States Navy.Swanborough and B ...
, and the Mikoyan Ye-266 (
Mig 25
The Mikoyan-Gurevich MiG-25 (russian: Микоян и Гуревич МиГ-25; NATO reporting name: Foxbat) is a supersonic interceptor and reconnaissance aircraft that is among the fastest military aircraft to enter service. Designed by ...
).
Both used a water/alcohol spray to cool the air ahead of the compressor.
Precooling (as well as combustion chamber water injection) is used at the lowest flight speeds, i.e. during take off, to increase thrust at high ambient temperatures.
Advantages and disadvantages of using precooler heat exchangers
One main advantage of pre-cooling is (as predicted by the
ideal gas law
The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first stat ...
) for a given
overall pressure ratio
In aeronautical engineering, overall pressure ratio, or overall compression ratio, is the ratio of the stagnation pressure as measured at the front and rear of the compressor of a gas turbine engine. The terms ''compression ratio'' and ''pressure ...
, there is a significant reduction in compressor delivery temperature (T3), which delays reaching the T3 limit to a higher Mach number. Consequently, sea-level conditions (corrected flow) can be maintained after the pre-cooler over a very wide range of flight speeds, thus maximizing net thrust even at high speeds. The compressor and ducting after the inlet is subject to much lower and more consistent temperatures, and hence may be made of light alloys. This reduces the weight of the engine, which further improves the thrust/weight ratio.
Hydrogen is a suitable fuel because it is liquid at deeply cryogenic temperatures, and over its useful range has a very high total
specific heat capacity
In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...
,
including the latent heat of vapourisation, higher than water.
However, the low density of
liquid hydrogen
Liquid hydrogen (LH2 or LH2) 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 cooled below its critical point of 33 K. However, for it to be in a fully l ...
has negative effects on the rest of the vehicle, and the vehicle physically becomes very large,
although the weight on the undercarriage and wing loading may remain low.
Hydrogen causes structural weakening in many materials, known as
hydrogen embrittlement
Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed ...
.
The weight of the precooler adds to the weight of the engine, thereby reducing its
thrust to weight ratio.
Passing the intake air through the precooler adds to the inlet drag, thereby reducing the engine net thrust, and so reducing the thrust to weight ratio.
Depending on the amount of cooling required, despite its high thermal capacity, more hydrogen may be needed to cool the air than can be burnt with the cooled air. In some cases, part of the excess hydrogen can be burnt in a ramjet with uncooled air to reduce this inefficiency.
Unlike a LACE engine, a precooled engine doesn't need to liquefy the oxygen, so the amount of cooling is reduced as there is no need to cover of fusion of the oxygen and a smaller total temperature drop is required. This in turn reduces the amount of hydrogen used as a heat-sink, but unable to be burnt. In addition a condenser isn't required, giving a weight saving.
History of precooling using heat exchangers
Robert P. Carmichael in 1955 devised several engine cycles that used liquid hydrogen to precool the inlet air to the engine before using it as fuel.
Interest in precooled engines saw an emergence in the UK in 1982, when
Alan Bond created a precooled air breathing rocket engine design he called SATAN. The idea was developed as part of the
HOTOL
HOTOL, for Horizontal Take-Off and Landing, was a 1980s British design for a single-stage-to-orbit (SSTO) spaceplane that was to be powered by an airbreathing jet engine. Development was being conducted by a consortium led by Rolls-Royce and ...
SSTO spaceplane
A spaceplane is a vehicle that can fly and glide like an aircraft in Earth's atmosphere and maneuver like a spacecraft in outer space. To do so, spaceplanes must incorporate features of both aircraft and spacecraft. Orbital spaceplanes te ...
project, and became the Rolls-Royce RB545. In 1989, after the HOTOL project was discontinued, some of the
RB545
HOTOL, for Horizontal Take-Off and Landing, was a 1980s British design for a single-stage-to-orbit (SSTO) spaceplane that was to be powered by an airbreathing jet engine. Development was being conducted by a consortium led by Rolls-Royce and B ...
engineers created a company, Reaction Engines Ltd, to develop the idea into the
SABRE engine, and the associated
Skylon spaceplane.
In 1987, N Tanatsugu published "Analytical Study of Space Plane Powered by Air-Turbo Ramjet with Intake Air Cooler." part of Japan's
ISAS (now
JAXA
The is the Japanese national air and space agency. Through the merger of three previously independent organizations, JAXA was formed on 1 October 2003. JAXA is responsible for research, technology development and launch of satellites into orb ...
) study into an Air-Turbo Ramjet (ATR, later
ATREX
The ATREX engine (Air Turbo Ramjet Engine with eXpander cycle) developed in Japan is an experimental precooled jet engine that works as a turbojet at low speeds and a ramjet up to mach 6.0.
ATREX uses liquid hydrogen fuel in a fairly exotic sin ...
after the addition of an expander cycle) intended to power the first stage of a
TSTO
A two-stage-to-orbit (TSTO) or two-stage rocket launch vehicle is a spacecraft in which two distinct stages provide propulsion consecutively in order to achieve orbital velocity. It is intermediate between a three-stage-to-orbit launcher and a hy ...
spaceplane
A spaceplane is a vehicle that can fly and glide like an aircraft in Earth's atmosphere and maneuver like a spacecraft in outer space. To do so, spaceplanes must incorporate features of both aircraft and spacecraft. Orbital spaceplanes te ...
. ATREX was superseded by the Preecooled Turbojet (PCTJ) and Hypersonic Turbojet studies. A liquid nitrogen precooled hydrogen burning test engine was flown at
Mach 2 at
Taiki Aerospace Research Field
Taiki Aerospace Research Field is a facility of the Japan Aerospace Exploration Agency. It is located within the of Taiki, Hokkaido, Japan.
It is used, among other things, for scientific balloon launches. Rocket launch pad of Interstellar ...
in September 2010.
[{{cite conference , title= Development status of the hypersonic turbojet engine for Mach 5 flight in JAXA (IAC-11.C4.5.1) , authors= Kobayashi, H and Taguchi, H and Kojima, Takayuki and Harada, K and Okai, K and Hongoh, M and Arai, T and Sato, T , date= 6 October 2011 , conference= 62nd International Astronautical Congress 2011, IAC 2011 , location= Cape Town, South Africa , volume= 8 , pages= 6655–6659]
See also
*
Air turborocket
The air turborocket is a form of combined-cycle jet engine. The basic layout includes a gas generator, which produces high pressure gas, that drives a turbine/compressor assembly which compresses atmospheric air into a combustion chamber. This ...
*
ATREX
The ATREX engine (Air Turbo Ramjet Engine with eXpander cycle) developed in Japan is an experimental precooled jet engine that works as a turbojet at low speeds and a ramjet up to mach 6.0.
ATREX uses liquid hydrogen fuel in a fairly exotic sin ...
*
Compressor map A compressor map is a chart which shows the performance of a turbomachinery compressor. This type of compressor is used in gas turbine engines, for supercharging reciprocating engines and for industrial processes, where it is known as a dynamic co ...
*
HOTOL
HOTOL, for Horizontal Take-Off and Landing, was a 1980s British design for a single-stage-to-orbit (SSTO) spaceplane that was to be powered by an airbreathing jet engine. Development was being conducted by a consortium led by Rolls-Royce and ...
*
Hydrogen vehicle
*
Hydrogen-cooled turbo generator
A hydrogen-cooled turbo generator is a turbo generator with gaseous hydrogen as a coolant. Hydrogen-cooled turbo generators are designed to provide a low- drag atmosphere and cooling for single-shaft and combined-cycle applications in combination ...
*
Intercooler
*
Liquid air cycle engine
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 liquid hydrogen (LH2) fuel to liquefy the a ...
*
Lockheed CL-400 Suntan
Suntan was the code-name of a prototype reconnaissance aircraft program, with the goal of creating a much faster and higher-altitude successor to the U-2, enabled by the use of liquid hydrogen (LH2) as aircraft fuel. The program was cancelled ...
*
Reaction Engines A2
The Reaction Engines Limited LAPCAT Configuration A2 (called the LAPCAT A2) is a design study for a hypersonic speed jet airliner intended to provide, long range, high capacity commercial transportation.
The aircraft was designed, ...
*
SABRE (rocket engine)
SABRE (Synergetic Air Breathing Rocket Engine) is a concept under development by Reaction Engines Limited for a hypersonic precooled hybrid air-breathing rocket engine. The engine is being designed to achieve single-stage-to-orbit capability, ...
*
Skylon (spacecraft)
Skylon is a series of concept designs for a reusable single-stage-to-orbit spaceplane by the British company Reaction Engines Limited (Reaction), using SABRE, a combined-cycle, air-breathing rocket propulsion system. The vehicle design is fo ...
References
Gas turbines
Jet engines
Hydrogen technologies