In military aircraft or space exploration, the payload is the carrying capacity of an aircraft or space ship, including cargo, 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". When the weight of the payload and fuel are considered together, it is known as the "useful load fraction". In spacecraft, "mass fraction" is normally used, which is the ratio of payload to everything else, including the rocket structure.
There is a natural trade-off between the payload and the range 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 take off weight and maximum fuel 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 (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.
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, which includes more than the warheads.
Examples of payload capacity:
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