4 Anatomy of Aircraft & Spacecraft

Introduction

Even to the layperson, it is evident that many different aircraft types will comprise the aviation spectrum. However, most aircraft have many common features, i.e., a fuselage, wings, tail surfaces, powerplant, undercarriage, etc. Larger aircraft tend to need more systems, and their airframe designs are commensurately more complex. To this end, aircraft types are placed into different categories and classes that relate to the relative complexity of their engineering design as well as their flight operations, e.g., the skill levels needed in their piloting and maintenance.

The U.S. Federal Aviation Administration (FAA), a division of the Department of Transportation (DoT), formally classifies aircraft according to categories, the primary categories being:

  1. Airplanes, e.g., commercial airliners.
  2. Rotorcraft, e.g., a helicopter.
  3. Lighter-than-air, e.g., a hot air balloon.
  4. Gliders, e.g, a sailplane.

For example, it is logical that it will take more engineering effort to design the airframe and the multitude of different systems used on a commercial airliner than for a glider, and such a design should also be held to a higher standard of airworthiness because it carries many passengers. Spacecraft, however, are not classified in the manner done for aircraft, i.e., there are no designated categories of spacecraft. Therefore, the name “spacecraft” can vary greatly depending on its mission, from simple probes to highly complex vehicles carrying astronauts. In comparison to aircraft, spacecraft have unique engineering challenges, such as dealing with the harsh environment of space and navigating in zero gravity, which require specialized skills and knowledge.

Objectives of this Lesson

  • Understand the essential parts of an airplane and their functions.
  • Know about the four fundamental forces involved in flight: lift, weight, thrust, and drag.
  • Appreciate the functionality of flight controls on an airplane.
  • Recognize the different types of wings, tails, undercarriage, and other components that can be used for airplane designs.
  • Have a general appreciation for different types of spacecraft, including multi-stage rockets and satellites.

Anatomy of Flight Vehicles

Both aircraft and spacecraft must be designed for structural strength and lightness and will be comprised chiefly of shell-like monocoque or semi-monocoque structures. The overarching principle for all types of flight vehicle design is to use lightweight materials and minimal structure while ensuring adequate performance in terms of payload and fuel-carrying capability, robustness, durability of the airframe, ease of maintenance and repair, etc.

Airplanes and spacecraft are designed and built from components comprising a series of assemblies and sub-assemblies, each usually having different functions. Still, the components must also be integrated to create a fully functioning flight vehicle. Component integration is a challenging aspect of all types of flight vehicle design, not just from a structural perspective; the engineers who do this work require broad multi-disciplinary skills and significant experience.

Aircraft Anatomy

As shown in the figure below, for a relatively simple general aviation aircraft, the airframe consists of five main groups of structural sub-assemblies, namely:

  1. The fuselage, i.e., the main body of the aircraft.
  2. The wings, which produce the lift on the aircraft.
  3. The empennage, which consists of horizontal and vertical stabilizers.
  4. The flight control surfaces, i.e., the ailerons, elevator, rudder, and flaps.
  5. The undercarriage or landing gear.
A diagram of a small airplane, broken into sections.
The structure of an a simple general aviation type of airplane can be decomposed into five main sub-assemblies, the fuselage, wings, empennage, flight controls, and undercarriage.

Each assembly must be designed for functionality and to carry the local loads imposed upon it as well as the loads transferred to and from each of the other sub-assemblies. The entire airframe and its components are joined together using rivets, bolts, and other fasteners. In some cases, welding, adhesives, or other bonding techniques may also be used instead of mechanical fasteners. The engine and propeller are, of course, another major sub-assembly that will be connected to the airframe before the airplane can fly.

Larger and heavier aircraft have the same primary components but are usually multi-engined and so have more complex airframe components and systems. For example, the diagram below shows a commercial airliner where the additional components used on this aircraft include spoilers, leading-edge slats, and an all-flying “trimmable” horizontal stabilizer. The fuselage will also be pressurized, which adds to the needed systems.

A white airplane with various control surfaces color coded.
Compared to a smaller general aviation airplane, the components of a multi-engine commercial airliner include slats, multi-segment flaps, spoilers, and multi-segmented control surfaces for low-speed and high-speed flight.

The structural weight of each component is also an important consideration, which also affects costs. The weight of an airplane design generally grows disproportionately quickly with its increasing wingspan (i.e., the so-called “square-cube law“). For larger “jumbo” airliners, the detailed structural design becomes increasingly challenging to keep the airframe weight from becoming too heavy to the point that it limits the useful load-carrying capability of the aircraft. i.e., the sum of the payload and fuel.

Wings

The primary function of the wings is to provide the needed lift to overcome the aircraft’s weight. To create this lift, the wing shape is carefully designed such that the wings create the needed lift but also minimize the associated drag. The design of these components must balance conflicting requirements, such as lift, drag, stability, control, and structural efficiency, while also considering manufacturing and maintenance costs. Weight and drag are often considered as the “killers” of aircraft performance, so low airframe weight and low aerodynamic drag are key to achieving good aircraft performance.

As illustrated in the figure below, wings are made of spanwise spars and stringers to carry shear forces and bending moments imposed by the aerodynamic lift loads, with crosswise ribs to give the wing its basic planform and cross-sectional shape. The ribs also carry torsion (i.e., twisting) loads. The wings may contain fuel tanks and other systems because they are mostly hollow, shell-type structures.

An illustration of a wing without the skin and with various components labelled.
Basic internal structure of an airplane wing, consisting of spanwise spars and stringers, and crosswise ribs to define the wing profile,

This internal wing structure is covered with a thin skin that is riveted to the internal structure. A rivet is an inexpensive and lightweight form of mechanical fastener that is bucked (i.e., deformed) into place, thereby pinning the entire structure together. This type of airframe construction method is called a semi-monocoque stressed-skin design. Besides the ribs, the skin itself carries much of the torsion loads imposed on the aircraft’s structure caused by the aerodynamic loads.

Two cross sections of airplane wings next to each other.
Detail of the “stressed-skin” structure of an airplane wing, which is designed to be extremely strong but also lightweight. The skin is attached to the underlying structure by using many small bucked rivets.

Like all aircraft structures, wings are designed not only for aerodynamic functionality but also for lightness, strength, durability, and ease of maintenance. In addition, wings are designed and built to be extremely strong to ensure that they can carry the normally expected flight loads plus a generous safety margin and unexpected loads such as from gusts in the atmosphere, heavy landing loads, etc.

The consideration of cyclic loadings as they may impact the weakening or fatigue of the metallic wing structure must also be considered, i.e., the tendency to develop structural cracks or other related problems that can weaken the structure over thousands of flight cycles. To this end, regions of the structure that cause high local stress concentrations should be minimized during the design.

Durability and repairability of the structure are also important in that everyday use should not cause significant wear and tear or damage that requires frequent repair. However, if repairs are needed, they should be readily performed at almost any airport without returning the aircraft to the factory. In this regard, metallic structures are readily repaired, although composite structure repair may require special facilities and tooling.

Empennage

The other aerodynamic lifting surfaces on the airplane are the horizontal and vertical tails, collectively known as the empennage, a French word with its origin in the name for “tail feathers,” i.e., as related to the flights used for an arrow. The empennage may be conventional or may be in a “T” or an “H” configuration; a “V” tail is much less common.

The horizontal tail primarily gives the aircraft stability in pitch in the horizontal plane, i.e., in the nose-up/nose-down direction about the pitch axis. This flight surface is called a horizontal stabilizer. In addition, the horizontal tail or horizontal stabilizer may be trimmable or “all-flying,” which means that the entire lifting surface is mounted to pivot (pitch) nose-up or nose-down to create pitching moments to balance or trim the aircraft in pitch. A trimmable tail also helps to minimize drag during flight and reduce the control forces.

The vertical tail gives the aircraft directional or “weather-cock” stability, i.e., in the nose-left/nose-right direction in a vertical plane about the yaw axis, and is called a vertical fin or vertical stabilizer. Airplanes cannot fly without horizontal and vertical tails, although flying wings have been designed to blend the main wings with the functions of the empennage. Such “flying wings” use special aft-cambered reflexed airfoil sections to help balance the pitching moments on the aircraft about its center of gravity.

Fuselage

The main body or fuselage of an aircraft is primarily designed to carry the payload, i.e., the weight being carried that pays for the flight, which for a commercial airliner is the passengers and their luggage as well as any other cargo, as shown in the figure below. While most of the fuel is carried in the wings, there may also be fuel tanks in the fuselage under the main cabin floor or (in some aircraft) in the vertical tail. It should be remembered, however, that fuel is not a payload.

A cross sectional illustration of an Airbus A380.
The anatomy an Airbus A380 commercial transport aircraft showing the internal structure and systems. A large fraction of the aircraft’s internal volume is devoted to payload (i.e., passengers and cargo) as well as to fuel tanks.

The shape of an aircraft’s fuselage (length, cross-section, etc.) depends on many factors, including the number of passengers to be carried. Structural strength and weight considerations are also essential design factors besides the shape and form for good aerodynamic behavior, i.e., low drag.

On a commercial airliner, the fuselage is pressurized to an effective internal pressure that is higher than the ambient outside air pressure when the aircraft is at altitude, allowing the passengers to breathe normally as if they were closer to sea level, which keeps them comfortable. This means that the fuselage must be designed as a large pressure vessel from an engineering perspective, which imposes additional challenges and constraints in the structural design, especially from a weight perspective.

Another important consideration in the aerodynamic design of the airframe is to keep any changes in cross-sectional shape relatively gradual, which reduces the tendency of the flow to separate from the surfaces, which will minimize drag. For an airliner, the shape of the airframe near the wing may also be subtly shaped in such a way as to reduce wave drag in cruise flight at transonic flow conditions (i.e., just below the speed of sound) where shock waves start to manifest.

As shown in the image below, the anatomy of a military fighter aircraft is quite different from an airliner. Fighter aircraft are often required to fly supersonically, so the wings are thin and more delta-shaped with relatively short spans. The overall size and weight of the aircraft must be minimized to give good airspeeds and maneuverability, which may impose extremely high loads on the structure. Instead of passengers, the aircraft will carry a payload of various types of offensive and/or defensive weapons, and perhaps also bombs and rockets.

The anatomy an F-15 military fighter aircraft showing some of the internal structure and systems. The design requirements for this type of aircraft are different to an airliner, including the need for supersonic flight, extreme maneuverability and agility, offensive and defensive weapon systems, and ballistic protection.

A large part of the internal structure of a fighter aircraft accommodates the engine (or engines) and the needed fuel. Jettisonable external fuel tanks are often used to extend the range and/or mission endurance because of the limited internal fuel tankage and the high fuel burn rates, especially when using the afterburners. Relatively large and aerodynamically powerful all-flying vertical and horizontal stabilizers are often used, which help give the aircraft good maneuverability. Ballistic protection may also be needed for the pilots and critical systems, which can add significantly to the aircraft’s empty weight.

Engines & Powerplants

The engines on an aircraft, often called powerplants, provide the propulsive force or the thrust to move the aircraft through the air and overcome aerodynamic drag. On a modern commercial airliner, as shown in the photograph below, the engines produce large amounts of thrust. They have high propulsive efficiency with low specific fuel consumption, i.e., the fuel used per unit thrust. The engines also provide power for essential systems on the aircraft, including hydraulic, pneumatic, and electrical, hence their more general designation as powerplants.

A photograph of a large turbofan engine. Three individuals are inspecting the engine.
A turbofan engine undergoing inspection on a twin-engine commercial airliner.

The principle of operation of a turbofan engine is shown in the figure below. The purpose of the fan is to accelerate a large mass of flow through the engine but with a relatively low flow velocity change and low exit or “jet” velocity. This particular approach increases the thrust-producing propulsive efficiency compared to, for example, a turbojet, where the thrust is produced by having a very high jet velocity out of the nozzle.

Schematic diagram illustrating the operation of a high-bypass turbofan engine.
Schematic illustrating the operation of a high-bypass turbofan engine.

Not all commercial aircraft are powered by turbofan engines, however, and turboshaft engines that drive propellers, i.e., turboprops, may power smaller commuter aircraft. Smaller aircraft (e.g., general aviation types) will usually be powered by piston engines, i.e., a reciprocating internal combustion engine driving a single propeller or perhaps in a twin-engine configuration. With few exceptions, helicopters will use one or more turboshaft engines.

An essential consideration in designing a multi-engine airplane is its ability to fly safely if one engine fails during its flight, which is called One Engine Inoperative (OEI) flight. For a twin-engine airplane, each engine by itself must be powerful enough to continue the flight, even though this gives the aircraft reduced performance levels. Therefore, OEI flight performance is an important design consideration for all multi-engine aircraft. This aspect is scrutinized during flight testing and certification to prove that the aircraft can still be safely operated OEI.

Forces of Flight & Flight Axes

Before describing the flight controls and their effects on an aircraft, it is important to understand the forces of flight and the definition of the axes about which the aircraft moves. Four forces act on an airplane, as shown in the figure below. First, the effects of “gravity” manifest as the aircraft’s weight, given the symbol W, which can be assumed to act downward at a center of gravity. The primary forces on the airplane are lift, given the symbol L, which is upward and in a direction that is opposite to the weight, and the drag D, which is parallel to the direction of flight.

A grey diagram of an aircraft with arrows indicating each of the 4 forces of flight.
The four forces on an airplane in steady, equilibrium flight are: Weight (i.e., mass times acceleration under gravity), lift, drag and thrust. For steady level flight equilibrium then lift equals weight and thrust equals drag.

The underpinning of flight is the lift on the wings, so understanding the aerodynamic characteristics of the wings is a vital part of airplane design. Lift generation results from the net pressure forces produced on the wing surfaces. As the airflow during flight approaches the wings, the flow is accelerated more over the upper surfaces, lowering the pressure relative to the pressure on the lower surfaces. Therefore, this pressure difference between the upper and lower wing surfaces is the source of the lift force that sustains flight. However, a consequence of lift generation is always drag.

In steady, unaccelerated, equilibrium flight, the lift on the aircraft will be equal to its weight, and the thrust required for flight will be equal to the aircraft’s drag, i.e.,

(1)   \begin{equation*} L = W \end{equation*}

and

(2)   \begin{equation*} T = D \end{equation*}

The airplane can also pitch, roll, and yaw. It will pitch about the lateral axis, roll about the longitudinal axis, and yaw about the vertical axis, as shown in the figure below. In general, moments can be produced about each of the three flight axes, i.e., a pitching moment, M_y, a rolling moment, M_x, and a yawing moment, M_z, from the application of the flight controls. The lift and drag can be assumed to act at a specific location on the wing called the center of pressure, where the pitching moment is zero; this position is often called the neutral point.

A gray diagram of an airplane with 3 axes running through the center, and arrows indicating the impact of roll, pitch, and yaw.
An airplane can pitch and roll and yaw about three axes. Pitch occurs about the lateral axis that runs from wingtip to wingtip., yaw occurs about a vertical axis, and roll occurs about the longitudinal axis.

In steady-level equilibrium flight, the airplane’s net moments about its center of gravity must be zero, i.e.,

(3)   \begin{equation*} M_x = M_y = M_z = 0 \end{equation*}

The need for force and moment equilibrium is not necessarily the case, however, in maneuvering or accelerated flight.

Notice that the origin of the coordinate system that might be used for analysis can be at any convenient point; in engineering practice, different origin points may be used depending on the type of analysis being conducted. The center of gravity of the airplane is often used as a reference, although it should be remembered that the center of gravity is not a fixed point and will move somewhat during flight as fuel is burned off and the airplane’s weight decreases.

Flight Controls

The wings and empennage have flight control surfaces, i.e., the ailerons, elevators, and rudder, as shown in the figure below. The pilot controls the airplane’s flight to give it the desired flight attitude by simultaneously using the elevator, ailerons, and rudder. The skill required in the coordination of the flight controls must be learned. Each type of aircraft can have somewhat different flight characteristics, but the basic functionality of the flight controls is the same.

A gray airplane with ailerons, flaps, rudder, and elevator labelled.
The basic flight controls include ailerons for roll control, elevators for pitch control, and rudder for yaw control.

The purpose of the ailerons is to give the airplane roll control about the longitudinal axis. As shown in the figure below, the ailerons are a set of differentially operated trailing edge wing flaps. When an aileron on one wing is deflected down, the other simultaneously deflects up, thereby producing a relative difference in the lift production on the two wings. The net result is a rolling moment in one direction or the other. Therefore, the ailerons control the angle of the airplane’s bank and help it turn.

A grey airplane diagram with arrows indicating the forces exerted by ailerons.
The ailerons are a pair of differential flaps, which when applied will cause the aircraft to roll.

On some airplanes, particularly larger ones, there may be multiple sets of ailerons (or segmented ailerons), with one set near the wing tips and another set further inboard. The inner and outer sets of ailerons are used together at low flight speeds, such as takeoff and landing, to give the airplane better roll control, but only the inner sets of ailerons are needed in cruise. On modern airplanes, the phasing in and out of the appropriate sets of flight controls is all done automatically by the flight control system, i.e., the activation of the outboard and inboard segmented ailerons are phased in and out as a function of airspeed. This latter approach also helps minimize the wing’s structural bending and torsion loads that might be associated with outboard control surface deflections at higher airspeeds.

The horizontal and vertical tails also have trailing edge flaps. On the horizontal tail, they are called the elevators, and on the vertical tail, it is called the rudder. Deflecting the elevators up and down (both sides of the elevator move together), as shown in the figure below, the relative lift on the horizontal tail is increased or decreased. The primary effect is changing the airplane’s pitching moment about its center of gravity. Therefore, the pilot’s use of the elevator controls the airplane’s pitch attitude. The function of the trimmable tail has already been discussed.

The elevators are a pair of differential flaps on the horizontal tail, which when applied will cause the aircraft to pitch.

Similarly, deflecting the rudder left or right produces a yawing moment, i.e., the rudder deflection produces a nose-left or nose-right response,depending on the direction of angular deflection, as shown in the figure below. Like the ailerons, the elevator and the rudder may have segmented sections, especially on larger airplanes, the activation of which is phased in or out as a function of airspeed.

A grey airplane diagram with arrows indicating the forces exerted by rudder.
The application of the rudder causes the aircraft to yaw.

Flaps & Slats

The flaps and slats on a wing allow the airplane to fly at lower airspeeds without stalling and losing lift, so they are primarily used during takeoff and landing. Flaps and slats are called high-lift devices. As airspeed is reduced, the wing must operate at an increasingly higher angle of attack, which the onset of stall will eventually limit lift production. Typically, wings operate successfully without stalling only at low angles of attack to the flow. However, the deflection of the flaps and possibly slats, as shown in the figure below, allows the airplane to fly at lower airspeeds without stalling, commensurately reducing the takeoff and landing distances.

The application of the flaps and slats allows the aircraft to fly at a lower airspeed such as for takeoff and landing.

The wing shape of a commercial airliner is primarily designed for efficient aerodynamic flight at higher (transonic) airspeeds and higher altitudes, so this relatively thin wing shape tends not to function as well at low airspeeds. For this reason, flaps and slats are used for takeoff and landing. For larger airplanes operating at high gross weights, high-lift devices are critical to keep takeoff and landing distances down to match the available runway lengths.

A view of the port wing of an airliner showing the aileron, flaps and spoilers.
A view of the port wing of an airliner showing the aileron, flaps and spoilers.

In many cases, the flaps are designed to deflect not only downward but rearward, as shown in the figure below, which increases the wing’s effective area as well as its curvature or camber. The net effect is that the wing can now operate at a lower airspeed without a tendency to stall, although the drag on the wing also increases with the flap deflection angles. The application of large flap deflections creates much drag and usually requires that the pilot apply extra thrust to maintain level flight at the same airspeed.

 

Diagrams of a side-view of a wing with flap closed versus flap open.
The deployment of trailing edge flaps increases both the lifting area of the wing as well as its camber, allowing the wing to operate at lower airspeeds before stalling.

Flap systems may have secondary elements (e.g., double- slotted or triple-slotted flaps), which are designed to deploy progressively in stages, as shown in the figure below. The flaps are partly extended and deflected to reduce the takeoff airspeed and the takeoff distance. The airplane quickly needs to build up airspeed and does not need the high drag associated with full flap deflection. After takeoff, the flaps on the airplane are progressively retracted as its airspeed builds.

 

Diagram of three different wing cross sections, showing the progression of slat and double slotted flaps from retracted through takeoff, and in landing.
High lift devices, such as trailing edge flaps and leading edge slats, may be employed in stages, with one configuration for takeoff and another for landing.

For landing, the flaps will eventually be fully deflected; the pilot extends them progressively in stages as the airplane descends from altitude and slows down to its final landing approach speed. The extra drag from full flap deflection helps to slow the airplane down to acceptable landing airspeeds and steepens its final approach angle to the runway, both of which help the pilot control the flight path and rate of descent of the aircraft during the landing approach.

The leading-edge slats also work in synchronization with the flaps, and like the trailing-edge flaps, their purpose is to delay the onset of stall, allowing the airplane to fly at lower airspeeds. The slats initially move forward and downward to increase the camber of the wing section and, when fully deployed, open up a small gap between the slat and the leading edge of the main wing. This gap between the flap segments is critical and helps to keep the flow attached over the flap surfaces at higher flap deflection angles.

Slats are highly effective in delaying the onset of a stall on a wing and creating more lift, although, like the flaps, they also increase drag somewhat, and may change the center of lift on the wing and create a pitching moment on the airplane. For this latter reason, the slats and flaps are usually activated simultaneously to minimize any changes in pitching moments, with partway deflections being used for takeoffs and full deflections for landings. With the slats and flaps both fully deployed, the airplane can typically fly at airspeeds that may be nearly half of what it would be without them, i.e., compared to flying in the “clean” condition with flaps and slats retracted.

Main Wing Designs

Airplanes come in many shapes and sizes, with various combinations of main wings, tails, and undercarriage configurations. Even a cursory look at the early history of aviation will show nearly as many different wing and tail configurations as airplane designs. As shown in the figure below, examples of main wings include high-mounted wings, low-mounted wings, mid-wings, gull wings, etc., and various types of swept wings.

Black and gray diagrams of various wing locations, dihedral angels, and planform shapes available for wings.
The are many different wing configurations, each one having certain advantages and disadvantages for the aircraft.

Wings can also have different planform shapes (i.e., the outline shape of the wing when viewed from above), such as rectangular, tapered, elliptical, or some other variation. Naturally, there are sound engineering reasons (in most cases) for preferring one wing shape over another. Swept wings are designed for high-speed flight, the sweepback helping to alleviate compressibility effects and so reducing drag, allowing the airplane to fly faster. However, sweeping back a wing does create other engineering concerns, so the sweepback is usually kept to a minimum. In addition, the use of forward-swept wings is unusual because of their susceptibility to undesirable aeroelastic effects.

Wings can also be cantilevered (i.e., no external bracing with all internal structure), braced with struts and/or wires, and also appear as monoplanes (single wings), biplanes, and even triplane configurations, as shown in the figure below. The cantilever monoplane design is the most common wing found on airplanes because of its low aerodynamic drag. However, lower-performance airplanes may use braced wings because of their comparative lightness compared to a full cantilevered wing.

Black and gray diagram showing different wing structures, number, thrust methods, and biplane wing positions possible.
Wings may also in monoplane, biplane (both without stagger and staggered) and triplane configurations.

In the early days of aviation, wings were built either as biplanes or triplanes, which gave the wood and fabric wing structures the needed bending and torsional strength and stiffness. However, the high aerodynamic drag of the struts and wire bracing between the wings significantly reduced the airplane’s performance, especially limiting its maximum achievable airspeeds.

The advent of aluminum alloys as a construction material soon allowed “stressed-skin” monoplanes to be built, which gave the wing much strength as well as significantly less drag. As a result, monoplane airplanes could soon fly at much higher airspeeds. However, a problem with early monoplane wings was flutter, an aeroelastic phenomenon that can lead to a catastrophic structural failure of the wing. Designers soon learned about the flutter problem and established design techniques to give wings the needed structural stiffness to avoid flutter.

At the tips of a wing, the flow leaks around the edges and develops into a swirling flow called a wing tip vortex, a source of drag called induced drag. The detailed shape of the wing tips influences the roll-up of the tip vortices. Over the decades there have been many different wing tip shapes designed to reduce the induced drag, as shown in the figure below. One of the most common designs today is the winglet, which has proven to reduce the drag on commercial airplanes and result in significant fuel savings over the airplane’s operational life.

Simple black and grey diagrams of different wing tip shapes, showing the overhead view and end view.
There have been many different wing tip shapes designed to reduce the drag on the wing.

Tail or Empennage Designs

The tail section or empennage of an airplane can also take on different configurations, including the conventional or “standard” tail with horizontal and vertical surfaces, but also “T,” “H,” and “V” or butterfly tails, as shown in the figure below. Remember, the purpose of the empennage is to give the airplane directional stability in pitch and yaw and allow for pitch and yaw control via the elevator and rudder, respectively.

Simple black and gray diagrams of various tail configurations.
Different types of tail or empennage design. The conventional configuration has historically been the most popular.

Each tail configuration has relative advantages and disadvantages, but in some cases, there may be a stronger design preference for using one type of tail over the other. For example, the V-tail or butterfly tail has the advantage of only two lifting surfaces versus three, potentially saving weight and manufacturing costs. However, the response of the flight controls (e.g., the separate application of elevator and rudder) can become aerodynamically coupled (i.e., pitch, roll, and yaw are not as clean and separate responses). To this end, an appropriately designed flight control system must be designed to decouple the responses.

A T-tail is out of the turbulent wake of the main wing, thereby improving its aerodynamic effectiveness for a given control deflection. However, this type of design is usually structurally heavier than the conventional tail. A T-tail design has also been shown to be more prone to flutter when attempts to reduce its weight have also led to reduced structural stiffness. The high torsional stiffness of the rear fuselage is also essential. Nevertheless, a T-tail configuration is common for modern airplane designs. Historically, the T-tail design also has some aerodynamic concerns, such as a susceptibility to “deep stall” at high angles of attack where the turbulent flow from the wing blankets the tail surfaces and reduces the elevator’s effectiveness, and the rudder issue must be thoroughly investigated during flight testing.

Engine Placement

All airplanes have thrust-producing devices (i.e., engines or so-called powerplants) to sustain flight, unless a glider. The powerplant consists of the engine (and propeller) and the related accessories such as electrical generators, hydraulic pumps, pneumatic pumps, oil pumps, fuel pumps, etc. The main engine types are the reciprocating (or piston type) and the reaction type engines such as the ramjet, pulsejet, turbojet, turboprop, and rocket engine.

The history of aircraft shows us that various engine placements have been used. The preference for using one type of engine and/or engine placement over another depends on many factors, including the airplane’s purpose. Examples of engine placement for propeller-driven aircraft are shown below.

 

Diagrams showing different engine placements. Airplanes are in gray and black, engines are red, and propellers are shown in blue.
Some of the many varied engine placements that have been used on propeller-driven airplanes.

There are also many different engine placements for jet aircraft, as shown in the figure below. Wing-mounted underslung engines are the most common configuration for airliners, although rear-mounted engines are standard for smaller regional jets and business jets. Military fighter aircraft generally have engines installed inside the fuselage, reducing drag and also giving better ballistic tolerance.

 

Simple diagrams showing the placement of jet engines on airplanes.
Some of the many varied engine placements that have been used on jet airplanes, the most common for airliners being underslung wing-mounted engines.

Undercarriage Designs

The undercarriage or landing gear supports the aircraft’s weight while on the ground and absorbs landing loads. However, the landing gear is also subjected to high forces during landing, including vertical and sideward loads. Therefore, besides the needed strength, the landing gear assembly must be as light as possible. To this end, the landing gear must be made from materials such as steel, aluminum castings, or in some cases, titanium. Titanium is lightweight and strong but is extremely expensive compared to steel and aluminum.

There are numerous undercarriage or landing gear designs. The most common is the tricycle gear, which has two main gear leg assemblies or “bogies” and a single steerable nose wheel. This design gives the airplane good directional stability on the ground. Larger airplanes may use two or more wheels on each landing gear leg, and the very largest airplanes may use three or four main gear legs, e.g., the Boeing 747, A380, etc.

Another typical design is the tailwheel undercarriage or “tail-dragger.” Tailwheel aircraft were used in the early days of aviation, and so it has become known as conventional landing gear. In this design, the two main wheels that are placed forward will carry most of the aircraft’s weight, and a smaller wheel is located at the tail. The conventional landing gear has the advantage of reduced weight, but this design is much less directionally stable when on the ground. Also, the pilot may have difficulty seeing ahead during taxiing on the ground. Nevertheless, the tailwheel undercarriage is relatively common, especially on smaller airplanes, because it is simple and lightweight. However, such a configuration would be unsuitable for larger commercial airplanes.

There can be many different types of undercarriage design, but the conventional tricycle (nosewheel) type is the most common.

The tricycle gear is the most prevalent landing gear configuration used in aviation. In addition to the main wheels, to aid with the potentially high impact of landing, most landing gear systems have a means of either absorbing shocks or accepting shocks thereby distributing the loads so that the structure is not damaged. This is usually done with an oleo strut containing air and oil. The air in the strut is compressed under load, giving progressive stiffness, and the oil provides the damping. Wheels and tires are designed specifically for aviation use, with characteristics that include the ability to absorb high-impact loads and some side loads.

Smaller aircraft generally have a fixed landing gear, i.e., one that does not retract in flight. This approach is a simple design with low weight but has higher aerodynamic drag. Sometimes spats are used to cover the wheels and streamline them. Larger and faster aircraft will inevitably have retractable landing gear, which is retracted into the fuselage and the wings after takeoff. While the retractable gear design significantly reduces drag, there is also a weight penalty as well as increased cost and maintenance requirements; usually, a hydraulic system is used to raise and lower the landing gear.

Two photographs showing the difference between fixed and retractable landing gear.
A fixed landing gear (left) and a retractable gear (right).

Not all aircraft have landing gear configured with wheels. Seaplanes are equipped with pontoons or floats for operation on water. A large amount of drag (shear force) is produced on this type of gear during water operations, but an aircraft that can operate from water can be very useful. Even skis are used on some aircraft for flight operations from snow and ice.

Three photographs showing different types of landing gear.
Other types of landing gear besides wheels include skids, (which are used on many helicopters), floats and skis.

Anatomy of a Helicopter

A helicopter’s advantage is that it can take off vertically from land or sea, hover motionless over a point, and fly in almost any direction. The main rotor provides the thrust to overcome the helicopter’s weight; changing together the pitch of all the blades modulates the rotor thrust. The main rotor is also used for control and forward propulsion, which is achieved by cyclically changing the pitch of the blades and tilting of the plane of rotation of the rotor disk. This approach alters the line of action of the rotor thrust vector, which provides aerodynamic forces and moments on the aircraft.

The tail rotor provides a side force and so a moment to compensate for the torque reaction produced by driving the main rotor through a vertical shaft. Furthermore, by modulating the thrust of the tail rotor, which is done by changing the pitch of the blades through the pilot’s control inputs, directional (yaw) control is achieved.

A diagram of a helicopter broken out to highlight different structures.
The basic anatomy of a helicopter. What makes the helicopter unique is its ability to take off and land vertically from almost any place and any surface, as well as to hover efficiently for extended periods of time.

However, despite all the helicopter’s advantages, it is a relatively low-speed aircraft, with maximum cruise speeds of only 160 knots. It is also unable to fly very far, with flight ranges of less than 500 miles, depending on the payload being carried. The limitations of the conventional helicopter have prompted the development of hybrid concepts such as the tiltrotor, e.g., the V-22 Osprey, which have some of the advantages of the helicopter (i.e., a vertical takeoff and landing capability) and the airplane (i.e., able to fly faster and further).

Anatomy of Spacecraft

It is much more difficult to define or classify a spacecraft in a manner that is done for aircraft because there are no officially designated categories and/or classes of spacecraft. The size, shape, and arrangement of the components of a spacecraft tend to be very mission-specific, so many different types of spacecraft have been built and flown over the years.

The name “spacecraft” can refer to launch vehicles or satellites, or anything else that is intended to leave the Earth’s atmosphere. Some types of launchers are reconfigurable in that they can be built with different stages or with optional solid rocket boosters, depending on the mission and/or the payload. For example, a payload launch to high Earth orbit may require a different booster stage rocket compared to one that has to go into low orbits. Spacecraft have one thing in common, however, in that they are all designed to operate at and beyond the limits of the Earth’s atmosphere in space and so must be powered by rocket engines or at the very least have used rocket engines to get into space.

With a multi-stage launcher, a satellite or other payload will be housed in a fairing at the top of the launch vehicle. The stages are basically three separate rocket-powered launchers stacked on top of each other; taken together these components are called the launch vehicle.

A diagram of the Saturn V rocket showing the different stages of the craft.
Example of a multi-stage launch vehicle, in the case the Saturn V, the spacecraft itself being located on the top of the vehicle.

The first stage is used for the initial part of the launch until its propellant is expended, and then this stage is jettisoned. The second stage then takes over and propels the vehicle to a much higher altitude, and this second stage is also jettisoned after its propellant is exhausted. Next, the third stage takes over and rapidly accelerates the payload to its orbital velocity. Finally, the third stage may be jettisoned as the payload reaches its required orbital altitude.

The advantage of launching the satellite with a multi-stage rocket in a staging process is that it reaches a higher final velocity because the mass (weight) of the rocket is substantially reduced after each stage is depleted of propellant and jettisoned, so less total propellant is needed. The ultimate engineering goal of staging is to maximize the payload ratio, meaning the largest amount of payload is carried up to the required burnout velocity using the least amount of non-payload weight (i.e., the empty weight of the rocket structure plus its propellant).

Rockets with two stages may launch payloads that need to go into low Earth orbit. However, three stages may be required for payloads that need to go to higher altitudes or reach deep space. For example, multi-stage boosters are always required to reach a geosynchronous orbit.

Very high propellant flow rates through a rocket engine are needed to create the necessary thrust from the nozzle exit. Upstream combustion chambers drive turbopumps to supply and regulate the propellant and oxidizer’s needed flow rates. The fuel is circulated in chambers around the nozzle to keep it cool and pre-heat it, which is called regenerative cooling. Pre-heating the fuel also increases the final combustion efficiency and the rocket’s net thrust.

Schematic diagram showing the flow of fuel and oxidizer through a rocket engine into the combustion chamber.
Schematic showing the flow of propellant (fuel and oxidizer) through a rocket engine into the combustion chamber. Notice that fuel is directed around chambers in the nozzle to keep it cool.

Another advantage of staging a launch booster is that each stage can use a different type of rocket engine, each being tuned for its particular operating conditions. For example, the first-stage engines are optimized for use in the atmosphere. In contrast, the last stage can use engines suited more to space conditions, i.e., for operations in a vacuum. Furthermore, different fuels may (and often will) be used for the different rocket engines.

A significant disadvantage of multi-stage launch vehicles is that the stages and their engines are lost because they burn up from kinetic heating as they re-enter the Earth’s atmosphere. Reusable launch vehicles are becoming more feasible, the initial stage (or stages) being candidates for recovery, refurbishment, and reuse. Solid rocket boosters are often lost, but sometimes if they are jettisoned at lower altitudes they can be recovered using parachutes and subsequently refurbished.

Additional solid rocket boosters can be used to increase payload capability or augment launch speeds to reach higher orbital velocities and altitudes. This approach allows a basic launch vehicle more flexibility to be configured to the specific mission. While attaching a cluster of solid rocket boosters is sometimes viewed as rather inelegant, it is convenient (for reconfigurability) and relatively low cost.

A Delta launch vehicle with a cluster of solid rocket boosters attached.
A Delta launch vehicle with a cluster of solid rocket boosters.

The commercial launch company SpaceX has been routinely recovering the first stage of their Falcon rocket, the stage being steered back to the launch pad (or to an offshore barge) to make a vertical landing. Over the past few years, SpaceX has made many successful recoveries of its Falcon 9 rockets.

Photograph of the first stage of a Falcon 9 rocket returning to the ground as the rest of the rocket continues to space in the background.
The SpaceX launch system returns the first stage of the Falcon 9 rocket back to Earth using only its own propulsion systems.

However, to accomplish this impressive feat, ample fuel must be carried on the first stage, over and above the fuel needed to launch the payload. Also, aerodynamic flight control surfaces and cold-jet attitude thrusters must be added to the first stage to help steer it along the required trajectory to the landing point. Finally, retro-braking is performed by reigniting one or more of the main rocket engines. There are obviously unique challenges in performing this amazing feat of engineering.

The ability to reuse rockets, such as the first stage booster and its engines, is key to making space launches more affordable. Launch vehicles can cost hundreds of millions of dollars, and reusing a first-stage booster, for example, can cut tens of millions of dollars off the cost of a launch to a customer. The Space Shuttle concept, now retired from service, was the best example of a mostly reusable spacecraft. The heart of the concept, the Orbiter, was partly a spacecraft and partly an aircraft designed to re-enter the atmosphere after the mission and glide to a landing on a runway.

Cutaway drawing of the Space Shuttle concept.
Anatomy of the NASA Space Shuttle concept, which was used from 1981 to 2011.

The Orbiter was powered by three extremely powerful rocket engines, which were only used during launch. The propellant for these rockets (liquid hydrogen and oxygen) was stored in the large external tank, which was jettisoned as the vehicle reached orbit, and the tank then burned up in the atmosphere. The two solid rocket boosters or SRBs, burned for just two minutes into the launch, and when their fuel was exhausted, they were jettisoned and parachuted into the ocean. The payload for the mission was contained entirely within the Orbiter, which allowed it to be deployed into space. Other things could also be captured and carried back to Earth, which was an extremely useful feature when servicing the space station.

The payloads launched by rockets may comprise satellites, deep-space probes, interplanetary probes, telescopes, provisions for a space station, crewed capsules, or almost anything else. For example, the image below shows some details of the Voyager deep space probe, which has several different instruments and communication antennas. Satellites and spacecraft also have solar panels to provide the electrical power needed for their systems. Remember that there is no air in space to cause any drag, so aerodynamics is not a consideration, and the spacecraft can be almost any shape. Nevertheless, its mass distribution is still crucial because the inertial characteristics of the spacecraft about all three axes are essential for guidance and control purposes.

A diagram of the Voyager deep space probe with various components labelled.
Anatomy of the Voyager deep space probe.

Since it was launched into low Earth orbit by the Space Shuttle, the Hubble Space Telescope (HST) has provided  astronomers with a much more detailed understanding of our solar system and beyond. A cutaway of the HST is shown in the figure below. The forward part of the HST houses the main optical assembly. The middle of the telescope houses the control electronics. Finally, the aft part houses all of the instruments and sensor electronics. The solar panels provide the needed power.

A NASA cutaway diagram of the Hubble Space Telescope.

Summary & Closure

Aircraft and spacecraft come in many shapes and sizes, each uniquely designed to meet specific functions and operational requirements. The design of these vehicles is driven by the specific operational requirements, and over the years, advancements in construction and materials have allowed for significant improvements in the capabilities of flight vehicles. The traditional stressed skin aluminum semi-monocoque construction remains popular, but composite materials offer advantages in terms of strength-to-weight ratio. Additive manufacturing and multi-functional materials have the potential to further improve aerospace structures, and there are opportunities here for further advancements. The ultimate goal of aerospace engineering is to create structures that are strong, light, durable, and repairable.

5-Question Self-Assessment Quickquiz

For Further Thought or Discussion

  • Do some research about unusual types of airplanes that perhaps do not conform to any of what might be called the “normal” airplane configurations discussed in this lesson.
  • Why do birds not have vertical tails?
  • Research the purpose of using “stagger” on a biplane configuration.
  • Discuss the relative advantages and disadvantages of a flying wing over a conventional airplane configuration.
  • Discuss the potential relative engineering risks associated with a reusable rocket booster stage.
  • Why is a helicopter a “low-speed” aircraft? Do some research to find out the factors that may limit the forward speed of a helicopter.
  • What might be the relative advantages of a tiltrotor aircraft compared to a helicopter and an airplane?

Other Useful Online Resources

To learn more about the anatomy or aircraft and spacecraft, try some of these online resources:

  • For interactive resources on how things fly, explore the National Air & Space Museum website here.
  • Airplane parts and functions tutorial by NASA – see here.
  • For more information about aircraft anatomy, including differences between early and modern airplanes, explore the National Air & Space Museum website here.
  • Video on building an Airbus A-350.
  • Test your understanding of the parts of a rocket here.
  • Ever wondered how to start a rocket motor?
  • Aircraft anatomy quizlet.
  • The worst looking rockets ever designed!
  • Why a rocket launch requires millions of gallons of water!
  • See here for some great details about the anatomy of the Orion crew module.