Monocoque (// or //) is a structural approach that supports loads through an object's external skin, similar to a pingpong ball or egg shell. The term is also used to indicate a form of vehicle construction in which the skin provides the main structural support, although this is rare and is usually confused with either semi-monocoque or a unibody. The word monocoque comes from the Greek for single (mono) and French for shell (coque). The technique may also be called structural skin or stressed skin.
Alternatives to the monocoque structure are the truss structure, geodetic structure, unibody, inflatable, and semi-monocoque. The semi-monocoque is a hybrid of a mutually reinforcing tensile shell and compressive structure. A common aircraft has longerons (ribs or frames) and stringers. Most car bodies are not true monocoques; instead modern cars use unitary construction which is also known as unit body, unibody, or Body Frame Integral construction. This uses a system of box sections, bulkheads and tubes to provide most of the strength of the vehicle, to which the stressed skin adds relatively little strength or stiffness.
Early aircraft were constructed using internal frames, typically of wood or steel tubing, which were then covered (or skinned) with fabric such as irish linen or cotton, notably Madapolam. Aircraft dope was then applied which tightened the fabric and provided a smooth sealed surface necessary to prevent excessive drag. The skin added nothing to the structural strength of the airframe and was essentially dead weight beyond providing a smooth sealed surface. By thinking of the airframe as a whole, and not just the sum of its parts, it made sense to adopt a monocoque structure and it did not take long for various companies to adopt practices from the boat industry such as laminating thin strips of wood.
Design and development
In 1912 Deperdussin introduced a monocoque racer using a fuselage made up of three layers of laminated strips of glued poplar veneer, which provided both the external skin and the main load-bearing structure. This reduced drag so effectively it was able to win most of the races it was entered into. This style of construction was copied, with some variations, in Germany by Albatros, LFG Roland, Hannover and others; however, it was prone to damage from moisture and delamination.
The 1922 Lancia Lambda was the first car to use monocoque or unibody construction instead of a separate body and frame that was used by most contemporary cars. On the Lancia the driveshaft tunnel and rear axle tunnel doubled as the backbone of the chassis, adding strength to the monocoque structure.
Tanks and other armored vehicles generally use a body or chassis which is built of armor rather than attaching armor to a body-on-frame design. Though this generally produces a fairly heavy vehicle, it can reduce weight for a given amount of armor compared to soft-skinned vehicles to which armor has been added either as a modification or a kit. For example, the German Fuchs 2  and RG-33 have monocoque hulls rather than a separate body and frame, while the truck-based M3 Half-track and up-armored humvee have separate bodies to which armor has been added.
A monocoque-framed motorcycle was developed by the Spanish motorcycle manufacturer, Ossa, for the 1967 Grand Prix motorcycle racing season. Although the single-cylinder Ossa had 20 horsepower (15 kW) less than its rivals, it was 45 pounds (20 kg) lighter and its monocoque frame was much stiffer than conventional motorcycle frames, giving it superior agility on the racetrack. Ossa would win four Grand Prix races with the monocoque bike before their rider was killed during the 1970 Isle of Man TT, causing the Ossa factory to withdraw from Grand Prix competition.
Notable designers such as Eric Offenstadt and Dan Hanebrink created unique monocoque designs in the early 1970s. The 1973 Isle of Man TT was won by Peter Williams on the monocoque-framed Norton John Player Special that he helped design. Honda also experimented with a monocoque Grand Prix racing motorcycle named the NR500 in 1979. However, the bike also featured other innovative features including an engine with oval shaped cylinders, and eventually succumbed to the problems associated with attempting to develop too many new technologies at once. In 1987 John Britten developed the Aero-D One, featuring a composite monocoque chassis that only weighed 12 kg. In 2009, Ducati introduced the Desmosedici GP9 with a carbon fibre semi-monocoque chassis.
Various rockets have used pressure-stabilized monocoque designs, including Atlas and Falcon I. Atlas was very light weight since its sole structural support was provided by its single-wall steel "balloon" fuel tanks, which held their shape by internal pressure.
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