Payloads and Payload Processing

The machines, equipment, hardware, and even people that are carried into space atop rockets or inside space shuttles are often called payloads. The term originated in World War I (1914-1918) during efforts to determine the amount of cargoes and people that could be carried by land tanks. The term is also often applied to the amount of useful weight that can be lifted by airplanes and inside trucks. Without a useful amount of payload—the "pay" carrying load—any transportation system would be of minimum value since the objective of a transport is to carry cargoes from destination to destination. This is true whether the transport in question is a rocket or a car and the payload consists of satellites or groceries. Payloads can consist of nearly anything that researchers, government, or industry seek to place into space. Satellites, robotic probes, or instrument packages can act as payloads. In human spaceflight programs, payloads can be the astronauts themselves, along with their life-sustaining equipment and supplies.

In space transportation, payloads arrive in space with minimum activity involving people. If the transport is an expendable, throwaway rocket, there are no people present when the craft arrives in space. Even if the space shuttles are used for launching the payload, astronaut interaction with the payload during a flight is kept at a minimum except under unusual circumstances. Thus all of the payloads sent into space are carefully prepared before the launching and their checkouts and activation automated to the maximum extent possible. Because people will not be present when these payloads arrive in space, payload processing and prelaunching preparation is an important part of the flight itself.

Payload Design and Storage

Payload preparation actually begins when the payload is under design. Space engineers often design a satellite to absorb the types of effects that the launching system—a rocket or shuttle—places upon the machine. These can include the effects of the thrust of the rocket and the amount of gravity that its thrust into space generates on the payload and everything else aboard the rocket. Depending on the flight path chosen, the type of rocket, and the final destination planned for the payload, this can be many times the pull of gravity experienced on Earth's surface. Other effects, such as friction, heat, vibration, and vacuum , also affect the payloads as they rise through the atmosphere and move out into the space environment.

Once the craft reaches its planned destination in space, designers must factor in the final environmental conditions, such as radiation and the surface conditions of a planet if a landing is planned. If the planetary destination is far away, engineers must build the craft to sustain the long flight. If the spacecraft is flying toward the Sun, it must be shielded from the harsh and continuous heat streaming out from the Sun. If the craft is flying in the opposite direction, then the craft and its electronics must be heated to keep warm during its long cruise in the cold dark of space.

Once a payload has been designed and manufactured, it must be kept in storage until the time draws near for its launch. Usually the manufacturer prepares a storage container and location that maintains the payload in environmentally friendly conditions as the launch is awaited. This is a period that could last months or even years. For example, when the space shuttle Challenger exploded in 1986 all shuttle missions were placed on hold. Their payloads had to be stored for several years because of this unexpected delay. Such large satellites as the Hubble Space Telescope and other military spacecraft bound for a shuttle ride had to be specially stored during the delay.

Preparation for Launch

As the date of a planned launch draws nearer, payloads are shipped to the launching base where the flight will take place. Following its arrival from the manufacturer, the payload is rechecked to assure that it has not been damaged or affected in transit. Sometimes this includes partially dismantling the payload and conducting extensive recheckouts. More complicated payloads such as the Russian modules to the International Space Station are shipped only partially built, with construction completed at the launching site itself. Once engineers have assured themselves that the payload has arrived at the launch site without damage, the next phase of preparation usually consists of readying the payload for mating with its rocket transport.

Shuttle Launches.

If the launching vehicle is a space shuttle, much of the preparation process serves to ensure that the payload poses no risk to astronauts on the shuttle. Careful review of the payload's fuels, its electrical systems, and any rocket engines that might be part of its design are conducted. Once that step is completed, the craft is then checked for the method by which it is to be attached to the shuttle's cargo bay. Attachments, release mechanisms, and other devices that will act to deploy the payload away from the shuttle or allow it to be operated while still attached inside the bay are tested and verified ready for flight.

At a certain stage in the final launch preparations the payload is moved from its preparation facility to the launching pad and installed inside the shuttle. Once in place, many of the tests and verifications are repeated to assure workers that the payload and its shuttle interfaces are working together. Unlike cargoes that fly inside commercial airliners, cargoes that are launched aboard the space shuttles are partially integrated with the shuttle itself. This even includes the selection of the location where the payload is attached to the shuttle bay.

When all of these steps have been completed, a complete dress rehearsal of the final days of the countdown and liftoff is conducted. Called a Terminal Countdown Demonstration Test, this simulated launching even includes suiting up the astronaut crew and having them board the shuttle just as they will do on the day of the actual flight. The payload is activated at the same level it will be on launch day, and the test goes all the way up to the point where the rocket engines would be ignited to start the actual mission into space. If all goes well with this test, the payload and the shuttle are deemed ready for their space mission.

Expendable Rocket Launches.

If the launching vehicle is an expendable rocket, the process is somewhat less complex. Once at the launching site, the checkout and testing is conducted and the craft made ready for installation atop the rocket. In the United States, France, and China, the test and integration procedure is done with the rocket and payloads stacked vertically. Russian space launch vehicles use a horizontal integration technique. Whichever method is used, the payload is attached to the final propulsive stage of the rocket or to its own rocket stage, and the completed assembly is carried to the rocket pad or final assembly building and becomes part of the overall launch vehicle.

As is the case with the shuttle, tests are conducted to verify that the attachments have been correctly made and that the rocket's computers are "talking," or exchanging data, with the payload computers. A dress rehearsal of the launch is also conducted, although it is usually less extensive than that done for the shuttles. A successful completion of this test clears the way for the final countdown. Rocket fuels and explosive devices to separate the rocket's stages in flight or to destroy the craft if it veers off course are loaded into the rocket. Checks of the weather along the vehicle's flight path are also conducted.

When liftoff occurs, information on the health of the payload is sent by radio to tracking stations along the path that the rocket takes towards space. When the point in the flight is reached where the payload becomes active, it comes alive through radio commands, and begins its own role in achieving its space mission goal. If a malfunction occurs, radio data give mission controllers and engineers information on the cause, so that future versions of the rocket and payload can be redesigned to avoid the trouble.

Present-day launching rockets have an average of one chance in ninety-five or ninety-seven of experiencing an actual launching disaster. The most reliable rockets thus far designed have been the Apollo Saturn lunar boosters and the space shuttles. The Saturns had a perfect flight record in their missions from 1961 through 1973. The space shuttle has failed once in 100 missions.

see also Launch Services (volume 1); Launch Vehicles, Expandable (volume 1); Launch Vehicles, Reusable (volume 1); Payload Specialists (volume 3); Payloads (volume 3); Satellites, Types of (volume 1); Spaceports (volume 1); Space Shuttle (volume 3).

Frank Sietzen, Jr.