Section 2.3 - Combustion Engines
A. Air-Breathing Engines
The next group of methods normally involve using a planet's (usually the Earth's) atmosphere as a supply of oxygen to support combustion with a fuel carried on the vehicle. They differ in the details of how the incoming air flow and combustion is managed. It should be noted that some vehicle concepts, such as the National Aerospaceplane (NASP) of the 1990's, or the current British Skylon would integrate more than one method in a single engine. This is referred to as a Combined Cycle Engine. The same general engine concepts could be used in a reducing atmosphere, such as Hydrogen or Methane, with Oxygen as the carried fuel, or in a sufficiently powerful nuclear engine with any atmosphere. In the latter case the nuclear engine is used to drive a compressor or heat the incoming gas flow.
Fundamental to understanding the operation of air-breathing engines is the concept of mass conservation. The mass of incoming air at the Inlet does not change in total amount going through the engine, although pressure and temperature vary. The exception is the point where fuel is added, or in some types, where condensed air flow is removed. Heat can be added or removed by combustion or heat exchangers, but otherwise the mass flow stays the same. From that constant mass flow, changes in the other conditions of the gas flow can be calculated, and performance derived.
In practice, the flow through an engine can involve many fan blades and turbulence, so that computer simulations are not fully accurate. Real engines also operate at extremes of temperature and stress. Therefore engine development typically includes extensive testing on static test stands, in wind tunnels, and in flight attached to aircraft, to determine actual performance and durability.
Type: Gas Flow by Fuel-Atmosphere Combustion
Description: The fanjet is the standard type of jet engine found on passenger aircraft and military aircraft. The original form of the engine, the turbojet, has a series of turbine compressor stages to compress the incoming air flow. This is followed by a combustor where fuel is added and burned, creating a hot gas. The gas is then expanded through a turbine which is connected by a shaft to the compressor. The expanded gas emerges at high velocity from the back of the engine.
The modern fanjet adds a fan which is also driven by the turbine. All of the airflow goes through the fan, but only a part goes into the compressor. The air which does not go into the compressor is said to have 'bypassed' the compressor. The 'bypass ratio' is the ratio of bypass air to combustor air. Generally higher bypass ratio engines are more fuel efficient, in units of thrust divided by fuel consumption rate, because they increase the mass flowing through the engine. In general, engines that operate at higher speeds are designed with lower bypass ratios. This is due to more combustion needed to increase thrust against the higher inlet drag from the higher velocity.
Typical modern performance values are engine thrust-to weight ratios (T/W) of 6:1 for large subsonic engines, trending towards about 10:1 for high performance military jets. Fuel efficiency is measured in units of thrust divided by mass flow rate. In English units this is pounds divided by pounds per second, or just seconds, and is termed 'specific impulse'. In SI units this is Newtons per kilogram per second, which has the units of meters per second. In some propulsion systems, such as chemical rockets, the SI unit corresponds to the actual exhaust jet velocity. In the case of air- breathing propulsion it is not, the velocity result is just an indicator of engine efficiency. In English units the performance of subsonic engines is about 10,000 seconds, trending to about 7000 seconds for supersonic military engines. Fanjets and turbojets operate up to about 3.5 times the speed of sound (M=3.5). Extra thrust can be generated with an Afterburner, which burns more fuel after the turbine stage of the engine. This comes at the cost of lower fuel efficiency.
Although the maximum altitude and velocity of a fanjet is limited compared to Earth orbit velocity, the effect on payload can be much larger. This is because compared to a conventional rocket, it can avoid most of the drag, pressure, and gravity losses near the ground. Conventional rocket payload is typically a small fraction of total mass, so small reductions in losses can produce large relative increases in payload.
Status: In common use on aircraft for aircraft propulsion. For space launch, the B-52 bomber and L-1011 aircraft have been used to carry the Pegasus three stage solid rocket to 35,000 ft altitude. The B-52 uses 8 fanjet type engines for propulsion. The Stratolaunch system is under development, using parts from two 747 aircraft with 6 high bypass fanjet engines, and carrying a large rocket. Numerous paper studies have been made of using aircraft as carriers for rocket stages.
- Carrier Aircraft - A conventional jet aircraft is used to carry a separate rocket to an altitude of roughly 9000 meters and velocity of 240 m/s, after which the rocket ignites and finishes reaching orbit.
- Booster Jets - A set of military fighter engines are attached to a rocket as separate strap-on boosters, or a connected booster ring. They can loft a rocket to about 15 km altitude and 480 m/s velocity, after which they either parachute land or do a powered vertical landing.
Type: Gas Flow by Fuel-Atmosphere Combustion
Description: In this method a multi-stage fan compresses the incoming air stream, which is then mixed with fuel, burned and exhausted. The compressor is driven by a gas generator/turbine. In a fanjet, the incoming air is compressed and heated by the compressor stages, then mixed with fuel and run through the turbine stages. At higher velocities the air gets hotter in compression since it has a higher incoming kinetic energy. This leads to a higher turbine temperature. Eventually a turbine temperature limit is reached based on the material used, which sets a limit to the speed of the engine. In the turbo-ramjet the compressor is driven by a gas generator/turbine set instead, which use on-board propellant for their operation. Since the gas generator is independent of the flight speed, it can operate over a wider range of Mach numbers than the fanjet ( to Mach 6 vs. to Mach 3).
Description: In this method, the incoming air stream is decelerated to subsonic velocity relative to the engine via a shaped inlet, mixed with fuel, then accelerated again out an exit nozzle. Conceptually it is the simplest form of jet engine, because it has no fans or turbines. The incoming air is moving at the vehicle velocity entering the engine. After burning the fuel, the air is hotter and can expand to a higher velocity out the nozzle. This sets up a pressure difference that leaves a net thrust. Ramjets cannot operate at zero speed, but they can reach somewhat higher limits than an engine with rotating machinery (range Mach 0.5 to about Mach 8).
- Air Augmented Rocket - This is a form of combined cycle engine. Since ramjets do not function at zero speed, an internal rocket chamber in the engine is used for initial thrust. By entraining air flow, the thrust level can be augmented at lower speeds. Once ramjet speeds are reached, the engine functions in ramjet mode, using the rocket chamber as a fuel injector. At the upper end of ramjet function, the engine transitions back to pure rocket mode.
- Wikipedia article: Air-augmented Rocket
Description: This is similar to how a ramjet functions. The incoming air stream is compressed by shock waves, mixed with fuel, and expanded against the engine or vehicle body. The difference is the airstream remains supersonic relative to the vehicle. The ramjet requirement to slow the airstream to subsonic speed becomes inefficient at higher velocities. Even though the gas is moving supersonically relative to the vehicle, the sidewise expansion can act on the vehicle if the slope of the nozzle is low enough. Thus the vehicle can fly faster than the exhaust gas moves. Scramjets may provide useful thrust up to about Mach 15, or 60% of Earth orbital speed.
The very high velocities lead to extreme heating of vehicle parts. High compression and expansion efficiency is needed to get a positive net thrust, since the energy added by the fuel becomes small relative to the kinetic energy of the air flow. Thus development of working scramjets has proved difficult. Scramjets also do not function at zero velocity, so some other method is needed to get to their starting point. Therefore complete vehicles will need a combined engine system.
Status: Scramjet engine components and small scale versions have been tested with mixed success.
29 Inverted Scramjet
Alternate Names: Buoyant Scramjet
Description: Ramjet and scramjet vehicles would prefer Hydrogen as a fuel because it gives higher performance, and can be used for cooling vehicle parts heated by the high velocity in air. Unfortunately Hydrogen is also very low density, which leads to relatively heavy vehicle structure due to large tanks. This method inverts the problem by using a series of balloons or a lightweight pipe supported in the atmosphere. They contain Hydrogen, and the vehicle carries Oxygen in its tanks. Oxygen is about 16 times denser than Hydrogen, so the tank size is much reduced.
Status: This is only a concept at present.
30 Laser-Thermal Jet
Description: In this method a laser beam is focused on and absorbed by a heat exchanger on the vehicle, or creates a laser-sustained plasma. The hot gas is then exhausted for thrust. By not requiring fuel, it is potentially efficient. The drawback is it requires a very powerful laser to be feasible even for small vehicles. Powerful lasers are currently expensive. Another limitation is the distance over which the beam can maintain focus.
Status: Minimal experiments have been done in a laboratory.
- Myrabo, L. N. "Concept for Light-Powered Flight", AIAA paper number 82-1214 presented at AIAA/SAE/ASME 18th Joint Propulsion Conference, Cleveland, Ohio, 21-23 June 1982.
B. Internally Fueled Engines (Rockets)
Conventional rockets function by expelling a gas at high velocity in a desired direction. By conservation of momentum (a physical law), the remainder of the vehicle will gain velocity in the opposite direction. Rockets have been the principal method of space transport to date because they only require internally stored fuel, and can thus operate in a vacuum. Since they propel the vehicle, the fuels are also called Propellants. Rockets can be sorted into types by how many propellants they have and what physical form the propellants are stored in. The former category includes single fuel Monopropellant, two fuel Bipropellant, and the rarely used three fuel Tripropellant types. The category includes gaseous, solid, liquid, and hybrid - part solid and part liquid.
Thrust is mass flow rate times exit or Exhaust Velocity. To get the most use from a finite amount of fuel, you want to minimize flow rate and maximize exit velocity. Therefore the gas should be as hot as possible and have low molecular weight. That in turn drives the choice of chemical reaction and fuels to use. There are numerous fuel combinations that are possible, but a relatively few that have a combination of high energy and other desirable characteristics such as density, safety, and low corrosion. There are several ways to get the gas hot: catalytic decomposition, combustion, or external heating. The first two are grouped under chemical rockets, and the latter are categorized by how the gas gets heated. Chemical rockets generally use a combination of combustion chamber and expansion nozzle, as that is a very efficient way to direct the gas flow at high velocity. Rocket engines will function with a surrounding atmosphere, but that impedes the gas flow, so they generate lower thrust. The lost thrust can be approximated by the local exterior pressure times the area of the nozzle exit.
Rockets are generally less efficient than air-breathing engines in terms of momentum per fuel mass, since the latter can use oxygen from the air as part of the fuel. This increases the combustion energy per carried fuel mass. Air breathing engines also increase the mass flowing through the engine via the Nitrogen component of air, and additional un-combusted air flow using engine-driven fans. Both rockets and air-breathing engines involve similar design principles, as they both use combustion and hot gas flow to get reaction forces.
History - The earliest reference to using expelled mass for reaction force is the Greek Archytus around 400 BC.  This used external heating to generate steam. The first known use of internal chemical energy is in 1232 AD in China by fire arrows using gunpowder as fuel. The idea may have been transmitted to Europe by the Mongols, where experiments and use for fireworks occurred in the 13-15th centuries. Experiments began in the 18th century as a transport method rather than explosive device, although military use continued. Notably, rockets used in a battle in 1812 were recorded in the US national anthem.
The use of rockets to reach space was proposed by Konstantin Tsiolkovsky in 1898. Robert Goddard built experimental solid and liquid fuel rockets starting in 1915. A book published by Hermann Oberth in 1923 influenced the formation of rocket societies where groups pursued their development.  The German government pursued the development of rockets to deliver explosives by sub-orbital trajectories from 1937 to 1945. Subsequently the scientists and their hardware and data went to the United States and the USSR, where they helped develop sub-orbital ballistic missiles and orbital transport, both first flying in 1957. The first orbital rockets were essentially identical to ballistic missiles, but have since diverged. By 1963 Liquid Hydrogen/Liquid Oxygen propellants were in use, which is still the highest energy fuel mix commonly used. Liquid and solid chemical rockets are by far the most common space transport method, and are now built in a number of countries by both governments and private companies.
Design - The non-fuel mass of a rocket stage can be grouped into engines, tanks, and "other". Rocket engines can produce 400-1000 N/kg of engine mass in thrust, which is many times larger than the 9.8 N/kg force of gravity. For liftoff from the Earth, you want approximately 1.2-1.5 times the vehicle takeoff weight in thrust, so the engine component then has a mass of about 1.3-3% of the total vehicle. A large tank, such as the Shuttle External Tank, can weigh 4% of the fuel weight, but other tanks can range up to 10% of the fuel weight. 'Other' includes plumbing, parachutes, landing gear, heat shields, guidance systems, and such non-propulsion parts. It can range from 1% up to 10% of the total weight.
Older materials and components required 15% of the total liftoff mass to be vehicle other than fuel, assuming a single flight operating life. Modern materials require about 10% of the total mass for a many flight operating life. Structures tend to get heavier at the rate of 10% for each factor of 10 in life. This comes from the fatigue life of materials under load cycles (flights) as a function of stress. Lower stress and longer life requires thicker and heavier structural parts to distribute the load over. So a 100-use structure will be about 20% heavier than a one-use structure.
31 Solid Rocket
Description: A solid rocket consists of a high-strength casing, a nozzle, and a precast solid propellant grain which burns at a pre-designed rate. The grain is a mixture of materials containing both fuel and oxidizer, so combustion can proceed without any external action once it is ignited. Modern solid propellants have a formulation close to the following: About 15% by weight organic fuel, usually a type of rubber, about 20% by weight aluminum powder (which acts as a metallic fuel), and about 65% ammonium perchlorate (NH3ClO4), which is the oxidizer. About 1-2% epoxy is added to the powders to hold them together. The epoxy, being an organic material, is also part of the fuel. Solid propellants burn from the surface of the precast grain. Therefore the shape of the grain at ignition, and shape as it burns away, determines the thrust level.
Advantages of solid rockets are short preparation time to launch and long term storage, compared to cryogenic fuels like liquid Oxygen. Disadvantages include relatively low exhaust velocity (2.6-3 km/s), and no easy way to turn it off or control it once ignited. They are often used as a booster first stage since the relatively dense fuel (1.35 g/cc) lowers the area of the vehicle. This is an advantage during the first two minutes of flight, where aerodynamic drag is important. When aluminum is used in the propellant, part of the end product is aluminum oxide, which is an excellent abrasive. Thus nozzle erosion is a significant effect that must be accounted for, in addition to the high temperatures.
Solid rockets are simpler in the sense of having few moving parts, but the entire motor casing that surrounds the fuel grain must withstand the operating pressure. In liquid rockets with fuel pumps, the propellant tanks see hydraulic and acceleration loads, which are typically lower, and only the pumps and combustion chamber see the full operating pressure.
Status: In common use for rocket stages, particularly the strap-on booster stage.
32 Hybrid Rocket
Description: The hybrid rocket consists of a solid fuel grain and a liquid oxidizer. One combination is rubber for the fuel and liquid oxygen for the oxidizer. The fuel is in the form of a hollow cylinder or perforated block. The oxidizer is sprayed onto the fuel and the material is ignited. By not being self-supporting in combustion, the fuel part can be treated as non- hazardous when being made and shipped. Only when on the launch pad and the oxidizer tank is filled is there a hazardous combination. With only a single liquid to handle, the hardware is relatively simple in design. Hybrid rockets are intermediate in performance compared to solid and full liquid engines.
33 Liquid Rocket
Type: Fuel/Oxidizer Combustion by Combustion Gas exhaust
Description: In a liquid rocket, the propellant ingredients are forced into a combustion chamber, where they burn, and which leads to a converging-diverging nozzle. The flow becomes sonic at the narrow part of the nozzle, then continues to accelerate in the diverging part of the nozzle, reaching about 1.5-2 times the speed of sound by converting temperature and pressure via expansion into a directed flow. A variety of propellant combinations have been used, including mono- bi-, and even tri-propellants. Monopropellants typically use catalytic decomposition for heating. The most common form of liquid rocket uses a separate fuel and oxidizer, which are mixed and burned in the combustion chamber. Many bipropellant mixtures are possible, but the highest energy-to-mass ratio mix in common use is from 1 part Hydrogen to 6 parts Oxygen. This produces mostly steam with a little leftover Hydrogen, which lowers the average molecular weight and thus increases the average molecule velocity. This propellant mix can reach about 4.7 km/s exhaust velocity under the best conditions.
Some propellant mixes will burn on contact, and so do not require an ignition source. These are called Hypergolic propellants. Some liquid propellants are liquid at room temperature and can be stored for long periods in a tank. These are referred to as Storable. Others, including Hydrogen, Methane, and Oxygen, are only liquid at very low temperatures. These are referred to as Cryogenic.
Status: This is the most common form of launch propulsion used to date to put things into Earth orbit.
Variations: There are multiple combinations of liquid engine types that are possible and have been used. They can be sorted by what propellant combination is used, how the propellants are delivered into the combustion chamber, and how the resulting hot gas is expanded out of it.
- Variations by composition - The following tables list some oxidizers and fuels, and combinations thereof. It is not an exhaustive list, and some ingredients have practical issues such as storage temperature, human toxicity, corrosiveness, or chemical instability. Rocket propellants by their nature contain a lot of chemical energy, and that energy can cause unintended reactions. Actual engine performance depends on factors such as chamber and exit pressures, so the table values should only be used for general comparisons. Kerosine is a mixture of compounds derived from petroleum, and Rocket Propellant 1 (RP-1) is a standardized type of Kerosine specified as a rocket fuel. Therefore it does not have an exact formula, and it is given as an approximate average value. It also does not have a well defined melting and boiling point, which is given as a range defined by distillation of its components.
|Chemical Name||Formula||Melting Point (K)||Boiling Point (K)||Density (kg/m3)|
|Chemical Name||Formula||Melting Point (K)||Boiling Point (K)||Density (kg/m3)|
|Kerosine (RP-1)||~CnH1.95n n=~14||200||478-528||810|
Exhaust Velocities assume chamber pressure of 6.9 MPa and exit pressure of 0.1 MPa (Earth sea level) with optimum nozzle expansion.
|Oxidizer||Fuel||Mass Ratio (O/F)||Exhaust Velocity (m/s)|
|Hydrogen peroxide (90%)||Kerosine||7.0||2912|
- Variations by fuel feed - Some engine designs use a pump to feed the propellants into the combustion chamber, others use a pressurized tank. Large engines may use a combination of tank pressurization to prevent cavitation of the pump inlet and pumps to reach chamber inlet pressure. The fuel must enter the chamber at a higher pressure than the combustion pressure in a steady state engine. Non-steady state pulsed ignition engines are possible, but not generally used. Steady state engined deliver more continuous thrust. Pumps require a lot of power to operate, and generally use the same fuel as the rocket. In Gas Generator systems a portion of the fuel flow is used to create hot gas which drives a turbine to run the pump. The hot gas is then vented. In Staged Combustion systems, the hot gas is not completely burned, and is fed into the combustion chamber. This is more efficient but also more complex.
- Variation by nozzle type - Most rocket engines to date use a bell-shaped nozzle, where the gas flow is on the inside, surrounded by structure which directs the flow. An alternate design called an Aerospike nozzle inverts this arrangement, with the structure on the inside in the form of a wedge or cone, and the gas flow surrounding it. The outer edge of the gas flow is contained by the surrounding atmosphere. Since that automatically adjust for pressure differences, it is a type of altitude compensating nozzle. The benefit to compensation is that nozzle exit area in an atmosphere represents a thrust loss. A truncated cone can integrate better with some vehicle shapes, and the larger chamber and nozzle area can lower heat flux.
- Variation by cooling type - The high energy combustion in liquid rocket engines can exceed the melting point of most structural materials. Therefore methods to prevent this are needed in all but the smallest engines, which simply use high temperature alloys and radiate the heat away. In one method, the fuel is run through channels in the rocket engine walls to keep it from overheating. This also recovers some of the energy that would otherwise be lost. A method in recent development by Orbitec injects a counterflow vortex of one propellant ingredient along the inside of the engine walls, which is mixed with the other ingredient at the back and the hot gases then flow in the forward direction down the core of the chamber. The unburned ingredient protects the structure with a layer of cool gas.
- Cooper, Larry P. "Status of Advanced Orbital Transfer Propulsion", Space Technology (Oxford), v 7 no 3 pp 205-16, 1987.
- Godai, Tomifumi "H-II Rocket: New Japanese Launch VehicleÊ in the 1990s", Endeavour , v 11 no 3 pp 116-21, 1987.
- Wilhite, A. W. "Advanced Rocket Propulsion Technology Assessment for Future Space Transportation", Journal of Spacecraft and Rockets, v 19 no 4 pp 314-19, 1982.
34 Gaseous Thruster
Description: In this method the propellant is introduced in gas form to the chamber. It may be a mono-propellant (a single gas) or a bi-propellant combination. Due to high tank mass this is usually used for small auxiliary thrusters. By using direct pressure from the tank to make the propellant flow, it can be very simple.
35 Mechanically Augmented Thruster
Description: The velocity of the exhaust gases are increased by placing the thrusters on the end of rotating arms. This can add 2-3 km/s to the exhaust velocity based on structural limits. It requires some external energy input to maintain the rotation of the arms, since the thrust opposes their rotation. Full electric thrusters generally have higher performance than this method, so they are preferred.
Status: Concept only, since there are better options.
- NASA, Brief History of Rockets, web page 15 Apr 2012
- For a history of the rocket societies, see Frank Winter Prelude to the Space Age, Smithsonian Inst Press, 1983