In military aircraft or space exploration, the payload is the carrying capacity of an aircraft or space ship, including cargo

Cargo

Cargo is goods or produce transported, generally for commercial gain, by ship, aircraft, train, van or truck. In modern times, containers are used in most intermodal long-haul cargo transport.-Marine:...

, munitions, scientific instruments or experiments. External fuel, when optionally carried, is also considered part of the payload.

The fraction of payload to the total liftoff weight of the air or spacecraft is known as the "payload fraction

Payload fraction

In aerospace engineering, payload fraction is a common term used to characterize the efficiency of a particular design. Payload fraction is calculated by dividing the weight of the payload by the weight of the otherwise empty aircraft when fully fueled...

". When the weight of the payload and fuel are considered together, it is known as the "useful load fraction". In spacecraft, "mass fraction

Mass fraction

In aerospace engineering, the propellant mass fraction is a measure of a vehicle's performance, determined as the portion of the vehicle's mass which does not reach the destination...

" is normally used, which is the ratio of payload to everything else, including the rocket structure.

Aircraft

There is a natural trade-off between the payload and the range

Range (aircraft)

The maximal total range is the distance an aircraft can fly between takeoff and landing, as limited by fuel capacity in powered aircraft, or cross-country speed and environmental conditions in unpowered aircraft....

 of an aircraft. A payload range diagram (also known as the "elbow chart") illustrates the trade-off.

The top horizontal line represents the maximum payload. It is limited structurally by maximum zero fuel weight (MZFW) of the aircraft. Maximum payload is the difference between maximum zero-fuel Weight and operational empty weight (OEW). Moving left-to-right along the line shows the constant maximum payload as the range increases. More fuel needs to be added for more range.

Weight in the fuel tanks in the wings does not contribute as significantly to the bending moment in the wing as does weight in the fuselage.
So even when the airplane has been loaded with its maximum payload that the wings can support, it can still carry a significant amount of fuel.

The vertical line represents the range at which the combined weight of the aircraft, maximum payload and needed fuel reaches the maximum take-off weight

Maximum Take-Off Weight

The Maximum Takeoff Weight or Maximum Takeoff Mass of an aircraft is the maximum weight at which the pilot of the aircraft is allowed to attempt to take off, due to structural or other limits. The analogous term for rockets is Gross Lift-Off Mass, or GLOW...

 (MTOW) of the aircraft. If the range is increased beyond that point, payload has to be sacrificed for fuel.

The maximum take-off weight is limited by a combination of the maximum net power of the engines and the lift/drag ratio of the wings.
The diagonal line after the range-at-maximum-payload point shows how reducing the payload allows increasing the fuel (and range) when taking off with the maximum take-off weight.

The second kink in the curve represents the point at which the maximum fuel capacity is reached. Flying further than that point means that the payload has to be reduced further, for an even lesser increase in range. The absolute range is thus the range at which an aircraft can fly with maximum possible fuel without carrying any payload.

Space craft

For a rocket the payload can be a spacecraft launched with the rocket, or in the case of a ballistic missile, the warheads. Compare the throw-weight

Throw-weight

Throw-weight is a measure of the effective weight of ballistic missile payloads. It is measured in kilograms or metric tons. Throw-weight equals the total weight of a missile's warheads, reentry vehicles, self-contained dispensing mechanisms, penetration aids, and guidance systems — generally all...

, which includes more than the warheads.

Examples

Examples of payload capacity:

• Antonov An-225
  Antonov An-225
  The Antonov An-225 Mriya is a strategic airlift cargo aircraft, designed by the Antonov Design Bureau in the 1980s. It is the world's heaviest aircraft. The design, built in order to transport the Buran orbiter, was an enlargement of the successful An-124 Ruslan...
  
  : 250,000 kg
• Saturn V
  Saturn V
  The Saturn V was an American human-rated expendable rocket used by NASA's Apollo and Skylab programs from 1967 until 1973. A multistage liquid-fueled launch vehicle, NASA launched 13 Saturn Vs from the Kennedy Space Center, Florida with no loss of crew or payload...
  
  :
  • Payload to Low Earth Orbit
    Low Earth orbit
    A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
    
     118,000 kg
  • Payload to Lunar orbit 47,000 kg
• Space Shuttle
  Space Shuttle
  The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
  
  :
  • Payload to Low Earth Orbit 24,400 kg (53,700 lb)
  • Payload to geostationary transfer orbit
    Geostationary transfer orbit
    A geosynchronous transfer orbit or geostationary transfer orbit is a Hohmann transfer orbit used to reach geosynchronous or geostationary orbit....
    
     3,810 kg (8,390 lb)
• Trident missile
  Trident missile
  The Trident missile is a submarine-launched ballistic missile equipped with multiple independently-targetable reentry vehicles . The Fleet Ballistic Missile is armed with nuclear warheads and is launched from nuclear-powered ballistic missile submarines . Trident missiles are carried by fourteen...
  
  : 2800 kg
• Automated Transfer Vehicle
  Automated Transfer Vehicle
  The Automated Transfer Vehicle or ATV is an expendable, unmanned resupply spacecraft developed by the European Space Agency . ATVs are designed to supply the International Space Station with propellant, water, air, payload and experiments...
  
  

Payload: 16,900 lb 8 racks with 2 x 0.314 m3 and 2 x 0.414 m3

• • Envelope: each 1.146 m3 in front of 4 of these 8 racks
  • Cargo mass: Dry cargo: 1,500 - 5,500 kg
  • Water: 0 - 840 kg
  • Gas (Nitrogen, Oxygen, air, 2 gases/flight): 0 - 100 kg
  • ISS Refueling propellant: 0 - 860 kg (306 kg of fuel, 554 kg of oxidizer)
  • ISS re-boost and attitude control propellant: 0 - 4,700kg

Total cargo upload capacity: 7,667 kg

Payload constraints

Launch and transport system differ not only on the payload that can be carried but also in the stresses and other factors placed on the payload. The payload must not only be lifted to its target, it must also arrive safely, whether elsewhere on the surface of the Earth or a specific orbit. To ensure this the payload, such as a warhead or satellite, is designed to withstand certain amounts of various types of "punishment" on the way to its destination. The various constraints placed on the launch system can be roughly categorized into those that cause physical damage to the payload and those that can damage its electronic or chemical makeup.

Examples of physical damage include extreme accelerations over short time scales caused by atmospheric buffeting or oscillations, extreme accelerations over longer time scales caused by rocket thrust and gravity, and sudden changes in the magnitude or direction of the acceleration caused by how quick engines are throttled and shut down, etc. Damage to electrical or chemical/biological payloads can be sustained through things such as extreme temperatures (hot or cold), rapid changes in temperature, rapid pressure changes, contact with fast moving air air streams causing ionization, and radiation exposure from cosmic rays, the Van-Allen Belts, solar wind, etc.