Introduction

All flight vehicles require a propulsion system to sustain flight, the only exception being a glider or a sailplane. The term propulsion means creating a force to propel some system forward. For example, a propulsion system for an aircraft, which is air-breathing, consists of a mechanical power source (i.e., an engine of some kind) and a propulsor capable of doing work on the air to increase its momentum and converting this engine power into a propulsive force. This can be done using a jet efflux, as shown in the figure below, or by a propeller’s action on the air. Increasing the time rate of change of momentum of the fluid as it goes through the propulsion system produces the thrust to propel the aircraft forward.

In the case of a rocket motor, which is not air-breathing, thrust is produced by expelling high-speed gases that are a product of the combustion of a fuel and oxidizer out of a nozzle, as shown in the figure below. Sufficient thrust is needed to overcome the vehicle’s weight, give the payload the required energy, and ultimately accelerate the payload into orbit. In each case, the principles of thrust production are obviously in accordance with Newton’s laws and the conservation of mass, momentum, and energy.

Types of Propulsion Systems

An aircraft propulsion system must produce at least enough thrust to balance the drag of the airplane when in flight and drive the aircraft forward at the required flight velocity; drag is typically an order of magnitude less than the aircraft’s weight. However, the thrust produced must exceed the drag of the airplane for it to accelerate to a higher airspeed and/or to climb to a higher altitude. Therefore, an aircraft propulsion system must have at least some excess thrust (or power) capability for takeoff, etc., and more than that required for straight-and-level flight. Likewise, the rocket motor(s) on a launch vehicle must create enough thrust to overcome the entire weight of the vehicle initially, then progressively build up enough excess thrust as fuel is burned off for the rocket to reach an orbital velocity and altitude.

Not all flight vehicles are created equally, requiring different propulsion systems. The choice of which propulsion system to use for a given aircraft design depends primarily on the aircraft’s intended airspeed or Mach number, as suggested in the figure below. For example, a propeller and engine combination or a turboprop might power a low-speed transport aircraft. In contrast, a turbojet or turbofan may power a fighter jet capable of supersonic flight. A piston engine and propeller combination will likely power smaller general aviation airplanes. Turboshaft engines usually power helicopters. Airliners spend most of their flight time at cruise conditions operating at one airspeed and almost constant engine thrust; turbofans’ high efficiency and low fuel burn are attractive for these types of airplanes.

For higher supersonic speeds and flight Mach numbers, turbojets are more attractive. Military airplanes need significant amounts of excess thrust to accelerate quickly, such as during combat maneuvers, and to overcome the high drag associated with operations at transonic and supersonic flight speeds. For this reason, military aircraft typically have significant margins of excess thrust. They may also employ afterburners to create much excess thrust, at least for short amounts of time. Compared to air-breathing engines, rockets also typically operate at very high thrust levels as well as high pressures and temperatures, but for relatively short amounts of time.

In summary, the needed propulsion on a flight vehicle can be achieved by using at least one of the following systems:

• Propeller and engine combination, such as a reciprocating (piston) engine.
• Turbojet, which is a basic jet engine producing pure jet thrust.
• Turbofan, which is a jet engine with a bypass fan that directs air around the engine core.
• Turboprop, a turbojet engine driving a propeller from a power turbine with little jet thrust.
• Turboshaft in which all shaft power goes to a reduction gearbox and transmission system, such as used on helicopters.
• Rocket engine, which, unlike the preceding types, is not air-breathing.

Each propulsion system is different in terms of its functional design, but the purpose in each case is to convert fuel into a propulsive force to move a flight vehicle. This goal is crucial because, to a large extent, any flight vehicle’s performance capabilities are determined by the thrust from the propulsion system and the quantity of fuel it takes to produce that thrust.

Piston Engines

A reciprocating piston internal combustion engine driving a propeller is often used to power low- to moderate-performance airplanes. Several manufacturers continue to produce piston engines suitable for aircraft use, including Lycoming, Teledyne Continental, and Rotax. The propeller is attached directly to the engine’s crankshaft, which spins and produces forward thrust, as shown in the figure below. The propeller may be of the fixed-pitch type for low-performance airplanes and the variable-pitch (or constant speed) type for higher-performance airplanes.

The advantages of this type of propulsion system are that it is robust and relatively inexpensive while also giving a reasonable propulsive efficiency in terms of the combined engine and propeller efficiency. Supercharging or turbocharging may be used to increase the power of a piston engine and so maintain power output to higher flight altitudes. A modern propeller has excellent propulsive efficiency and relatively low noise. However, a piston engine and propeller combination become increasingly heavy for larger power outputs and higher altitude operations above 15,000 ft. In this case, there is usually a preference for using a turboprop if the use of a propeller is still desired.

Turbojet, Turbofans & Turboshaft Engines

Out of all the engine types used on aircraft, the turbojet, turbofan, and turboshaft are the most frequently confused. In a turbojet engine, as shown in the schematic below, the exhaust gases are expended at high velocity through a nozzle at the rear of the engine, this process producing all the thrust. The compressor stage is driven by the turbine stage, which brings the intake air to the pressures and temperatures needed to support combustion.

Turbofan

In a turbofan engine, one or more large fans are mounted at the front, as shown in the figure below. The fan increases the net mass flow through the engine, which gives more thrust, but the fan also expels the flow at a lower exit or jet velocity, which is a more efficient way of producing this thrust. In a turbofan engine, the fan produces much of the net thrust from the engine, the remainder from the jet thrust developed through the engine’s core and exit nozzle.

In reference to turbofan engines, the term “bypass ratio” or BPR is often used, a higher bypass ratio being attractive in terms of thrust-producing efficiency. The bypass ratio (BPR) is the area or mass flow through the fan divided by the area or mass flow through the core itself, i.e.,

(1)  

A BPR of 5, for example, means that 5 units of mass per unit time of air goes through the “cold” bypass stream for every one unit going through the “hot” engine core.

Typical BPR values on a modern turbofan engine range from 8 to 11 and have progressively increased over the last three decades by improved turbofan engine design. In this regard, it is much more efficient to create thrust by accelerating a large mass flow of air through the fan and expelling it at a lower velocity versus accelerating a lower mass flow of air at a high velocity. For this reason, turbofan engines are more commonly used than turbojets, e.g., on commercial airliners, because of their better efficiency and lower fuel consumption.

Turboprops

Turboprops are closely related to turbofans in terms of operational principle because both transfer energy from a power turbine to do work on a bypass flow stream. As shown in the figure below, with a turboprop, the turbine stage delivers power to a shaft that drives a propeller. The propeller is often coupled through a gearbox to reduce the propeller speed and keep the tips of the propeller blade from becoming supersonic. As a result, there is little energy in the exhaust of this type of engine, and so little or no jet thrust is produced.

With a turboprop engine, little jet thrust is produced, the energy of the hot gasses being used to drive turbine and a shaft to which a propeller is attached.A turboprop is a very efficient way of producing propulsive thrust on an aircraft because it has a very high effective bypass ratio. Nevertheless, the propulsive characteristics of the propeller itself must be carefully considered relative to the overall system performance, which is usually limited to flight Mach numbers of less than 0.5.

Turboshafts

Turboshaft and turboprop engines are very similar in that both are designed to deliver nearly all of their power to a shaft rather than producing jet thrust, although some small jet thrust may still be produced at the exhaust. The main difference in their design is at the power or compressor turbine stage. Although in most turboshaft designs, the compressor turbine (gas generator) and power section are mechanically separate, referred to as a free power turbine, as shown in the figure below, the advantage is that they can rotate each at different speeds appropriate to the conditions of use. Turboshaft engines are often used to power helicopters, but they have also been used on tanks and ships.

Air-Breathing Propulsion Fundamentals

Consider the general air-breathing propulsive device, as shown by the control volume in the figure below, which is moving through the air with velocity . Air is entrained into the front of the device, which then does work on the air (as a byproduct of the combustion of fuel) to increase its momentum and kinetic energy. Finally, the exhaust is then ejected into the slipstream with a higher “jet” velocity .

If the mass flow of air into the device is denoted by and the mass flow rate of fuel is , then conservation of momentum applied to the flow gives the thrust produced as

(2)  

Now if it assumed that then

(3)  

i.e., the thrust is equal to the time rate of increase in momentum of the air as it passes through the propulsive device, and so it proportional to the increase in velocity of the flow .

The increased velocity of the flow also appears as a gain in kinetic energy, which is irrecoverable, i.e., a power loss, as given by

(4)  

It is clear that from Eq. 3 that the production of thrust depends on both the mass of flow being moved through the device as well as its increase in velocity. So, at least in principle, the same amount of thrust can be generated by accelerating a larger mass of flow by a smaller jet velocity or accelerating a smaller mass of flow at a larger jet velocity.

Quantifying Propulsive Efficiency

In general, a propulsion system’s overall efficiency can be viewed in terms of producing the needed thrust for a given power and the fuel needed to produce that power. However, the relative efficiency of the device is essential, i.e., the aerodynamic efficiency of creating a useful propulsive force by doing work on the air. The efficiency in producing thrust is always related to the kinetic energy of the exit flow from the propulsion system, which is the lost energy per unit time left in the slipstream or wake.

Relative Efficiency

In terms of quantifying the efficiency of thrust production, consider the useful power supplied by the engine (to propel the aircraft forward at ), which will be the product of the force produced by the propulsive device and the velocity (true airspeed) of the aircraft, i.e.,

(5)  

This latter equation would pertain to a ground-reference frame; in the frame moving with the vehicle the thrust does no work. The relative propulsive efficiency can now be defined as

(6)  

Recall that based on conservation of momentum then the thrust is given by Eq. 3 so substituting in the previous equation gives

(7)  

which after some simplification gives

(8)  

where always . Interestingly enough, matching the exhaust speed and the vehicle airspeed gives optimum efficiency, at least in theory but obviously not in practice because no thrust would be produced.

Notice that a low value of for a given thrust can only be achieved by having a large mass flow rate through the engine, which is exactly what a turbofan does. Therefore, it is more efficient to create thrust by accelerating a large volume of air at a lower velocity versus a lower volume of air at a high velocity. The same argument applies to a propeller, which is why it is a “high bypass” device, and so it has good propulsive efficiency. Nevertheless, the efficiency is always less than 100% because of an increase in kinetic energy of the exit (jet) flow, an inevitable loss that is a byproduct of creating thrust. Also, it will be apparent that the propulsive efficiency increases with increasing airspeed.

Specific Fuel Consumption

An aircraft’s performance characteristics are highly influenced by the engine’s fuel consumption, which can be quantified in terms of specific fuel consumption (SFC). The BSFC (brake-specific fuel consumption) is the fuel weight used per brake unit of power that is generated, which is a measure of efficiency. In the case of a thrust-producing engine, the fuel consumption is quantified by using the thrust-specific fuel consumption (TSFC), which is measured in terms of the weight of fuel used per unit thrust per unit time, again a measure of efficiency. Usually, the time period for which the TSFC and the BSFC values are quoted is 1 hour. For turboshaft or piston engines, the shaft power produced must be used by a device (such as a propeller or fan) to do work on the air, so the propeller efficiency also comes into consideration in the determination of the overall efficiency of the engine and propeller as a system.

Needless to say, over the many decades of their continuous development, the SFC of aircraft engines has improved (reduced) markedly, which is summarized in the figure below. Remember that these values measure engine thrust-producing fuel efficiency, so the reductions shown are significant and commensurate with advances in engineering technology in general.

The efficiency of several propulsive devices is shown in the figure below in terms of their specific fuel consumption, i.e., the fuel used per unit thrust per unit time, which is inversely proportional to propulsive efficiency. Therefore, the higher the efficiency, the lower the specific fuel consumption. Notice that “bypass” is a method of increasing mass flow through the engine, giving a lower exit or jet velocity. The bypass ratio (BPR) is the ratio of the mass flow through the fan relative to the mass flow through the engine’s core.

As an example of what has just been concluded in the preceding equations, a turbofan engine (which has a high BPR) has much better fuel efficiency (i.e., lower fuel burn rate) than a turbojet because the fan stage helps to increase mass flow through the engine, but without increasing the net jet velocity () substantially. A higher BPR also helps to decrease jet noise because of the lower values of , the noise increasing quickly with increasing jet velocity. This characteristic is precisely why high-bypass turbofans are used on most modern airliners rather than turbojets; their efficiency is better than 60%, and they have relatively low noise. In fact, by similar arguments, a propeller and engine combination has a higher propulsive efficiency (about 70%) because it produces a high mass flow rate through the propeller, and the downstream is relatively small. On a turbojet, is always relatively high, and the propulsive efficiency of this type of engine may be only 20%.

Nevertheless, it is not always possible to have a propulsion system that is well-suited for the flight conditions in which it operates and is also highly efficient. This issue means that engineers designing new aircraft often have to perform studies to decide on the relative benefits of using one propulsion system over another, depending on the cruise speed and flight Mach number of the aircraft and other design factors. Engineers often call these trade studies where the relative merits of one concept are traded off against the relative merits of another.

Afterburning

A supersonic aircraft generally requires a turbojet with an afterburner, the afterburner giving additional jet thrust to propel the aircraft through the high drag conditions during transonic flight, i.e., to go through the “sound barrier” or to maintain high supersonic speeds. Afterburning, which is sometimes called “reheat,” is achieved by injecting additional fuel into the jet pipe nozzle, which significantly increases the jet velocity and the thrust but also has the disadvantage of very high fuel consumption and low propulsive efficiency. The photograph below shows the SR-71 Blackbird with both engines on full afterburner, which allowed the aircraft to accelerate through transonic conditions and maintain high supersonic airspeeds.

Rocket Propulsion Fundamentals

Rocket engines are used for spacecraft and rockets, which are non-airbreathing, so they must carry both the fuel and the oxidizer during flight. The photograph below shows a rocket engine under test, in this case one that uses hydrogen and oxygen as propellants, which gives a very clean exhaust and the production of lots of steam. Rocket engines may also power some types of high-speed aircraft. The first aircraft to break the speed of sound, the Bell X-1, was powered by a rocket engine, as was the first hypersonic aircraft, the X-15. Rocket engines are extremely powerful and operate at high pressures and temperatures, but only for relatively short periods.

The principle of thrust generation for a rocket engine is from the reaction force associated with accelerating a mass of gas at high velocity, the gas being a byproduct of combustion of the fuel and the oxidizer, and so increasing the momentum of the ejected gas. The force on the rocket engine is then opposite to the direction of the gas exit velocity, which is a result in accordance with Newton’s third law. Notice that there is no external ambient mass flow into the engine, such as there would be with an air-breathing engine.

Using the principles of conservation of momentum, then the thrust produced by the rocket engine will be

(9)  

where is the equivalent average exit velocity and is the propellant net mass flow rate. Notice that there is no incoming momentum.

Therefore, the higher the mass flow of fuel and oxidizer into the motor and the higher the exit velocity from the nozzle, then the higher the thrust will be. Rocket motors typically have extremely high mass flow rates and exit velocities compared to air-breathing engines. However, all of the exhaust flow in this case appears as a gain in kinetic energy, which is irrecoverable, i.e.,

(10)  

which means that a rocket is much less efficient than an air-breathing engine in terms of thrust production.

In the case of rocket motors, it is possible to define their efficiency in terms of a specific impulse, which is the thrust produced divided by the propellant mass flow rate. Specific impulse, given the symbol , and having the units of seconds, is inversely proportional to the specific fuel consumption. For rockets, values of typically range from 280 to 465 seconds, depending on the fuel and oxidizer used and the rocket motor design.

As shown in the figure below, an air-breathing engine has a much larger specific impulse than a rocket, typically by one order of magnitude. This latter characteristic is because an air-breathing engine uses ambient air as the oxidizer for combustion, which does not have to be carried, which results in lower kinetic energy losses and so uses far less energy to generate a given amount of thrust than a rocket. The most efficient engines are turbofans and also turboprops because they accelerate a large mass flow rate of air at relatively low velocity.

Electric Propulsion

The engine used to drive a propeller is usually a piston engine or turboshaft engine. Recently, however, engineers have been experimenting with electric propulsion systems, including hybrid systems, meaning that some other engine is used to augment the electric system.

Compared to a piston engine or a gas turbine engine, an electric motor has certain advantages, including that it is small, compact, and has relatively few moving parts. It is also relatively inexpensive and quiet compared to an internal combustion engine, and of course, it produces no emissions and has a smaller carbon footprint. As a point of reference, electric motors will convert about 60% of the electrical energy (which is obtained from the electrical grid in some form) into useful power. In comparison, conventional engines using fossil fuels (e.g., AvGas or JET-A ) only convert about 20% of the energy stored in the fuel into useful work and power.

However, using electric engines for aircraft propulsion has many challenges. The main challenge is storing all the energy needed for flight, which needs batteries. However, batteries are heavy and bulky, as shown in the figure below, and must also be connected together and to the motor using heavy cables. Fossil fuels have a much higher energy storage per unit weight, called energy density, compared to what can be stored in batteries, about twenty times. Battery life is also reasonably short because of the chemical and thermal stresses inside the battery induced by the relatively fast discharge cycles associated with the power draw required for flight. Battery packs tend to be expensive to replace, the replacement cycle being significantly shorter than the time between overhauls of an internal combustion engine.

A hybrid propulsion system can give an electric aircraft much greater flexibility. In this case, another type of engine and generator complements the electric system and gives added power when needed. For example, such hybrid systems could be paired with a hydrogen fuel cell, gas turbine, or even a diesel engine, which is then coupled to a generator. The consequence is that a hybrid system could give an electric aircraft significantly better flight range and/or endurance, using the hybrid system for takeoff and the smaller electric motors for the cruise.

For now, only relatively small electric-powered aircraft are being designed, which may be able to fly for perhaps up to a maximum of an hour or so. Electric vertical takeoff and landing (eVTOL) aircraft are also being developed, primarily to transport goods and people in an urban environment, i.e., as an air taxi for increased urban mobility. The AgustaWestland Project Zero was the world’s first eVTOL aircraft, which was based in a tiltrotor concept. Developed as a technology demonstrator, and was used to investigate all-electric propulsion and other advanced technologies.

The photograph below shows another type of eVTOL concept vehicle using rotors for vertical lift and a propeller for forward thrust. While many such eVTOL aircraft concepts are now being considered, it remains to be seen whether such aircraft will become technically and economically viable. Another concern is how their flight operations can be effectively and safely integrated into the national airspace system.

Summary & Closure

The production of thrust is essential for all flight vehicles, a glider or sailplane being an exception. A propulsion system consists of an engine of some kind that creates a work and hence a force to propel the vehicle forward. Air-breathing engines include reciprocating piston engines driving a propeller, turboprops, turbojets, and turbofans. In each case, the purpose of the engine is to do work on the air to increase the momentum of the flow and so create a reaction force, i.e., the propulsive thrust. An aircraft propulsion system must produce at least enough thrust to balance the aerodynamic drag of the airplane at the required flight airspeed and altitude. Rocket motors are not air-breathing and can operate outside of the atmosphere. However, the principle of thrust generation is the same, i.e., the thrust is equal to the time rate of change of momentum of the gases exiting the nozzle.

The efficiency of thrust production of an engine is also essential, which is directly correlated with the quantity of fuel required to produce thrust and the aerodynamic efficiency of the propulsive system. Generally speaking, it is more aerodynamically efficient to produce thrust by having large mass flow rates with lower exit velocities. To this end, turbofans and turboprops are attractive options to power many airplanes. While electric propulsion concepts for aircraft are still very much in their infancy, they will likely see more widespread use in the coming decades, at least for smaller aircraft or VTOL concepts used for urban mobility.