Efficiency of bridge construction

Design

History

A ROBOTIC SPACECRAFT

A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to lower cost and lower risk factors. In addition, some planetary destinations such as Venus or the vicinity of Jupiter are too hostile for human survival, given current technology. Outer planets such as Saturn, Uranus, and Neptune are too distant to reach with current crewed spaceflight technology, so telerobotic probes are the only way to explore them.

The first space mission, Sputnik 1, was an artificial satellite put into Earth orbit by the USSR on 4 October 1957. On 3 November 1957, the USSR orbited Sputnik 2, the first to carry a living animal into space – a dog.

The USA achieved its first successful space probe launch with the orbit of Explorer 1 on 31 January 1958. Explorer 1 weighed less than 14 kilograms compared to 83.6 kg and 508.3 kg for Sputniks 1 and 2 respectively. Nonetheless, Explorer 1 detected a narrow band of radiation surrounding the Earth, named the Van Allen belts after the scientist whose equipment detected it.

Only seven other countries have successfully launched orbital missions using their own vehicles: France (1965), Japan (1970), China (1970), the United Kingdom (1971), India (1981), Israel (1988).

Most American space probe missions have been coordinated by the Jet Propulsion Laboratory, and European missions by the European Space Operations Centre, part of the European Space Agency (ESA). ESA has launched many spacecraft to carry out astronomy, and is a collaborator with NASA on the Hubble Space Telescope. There have been many successful Russian space missions. There have also been a few Japanese, Chinese and Indian missions.

In spacecraft design, the United States Air Force considers a vehicle to consist of the mission payload and the bus (or platform). The bus provides physical structure, thermal control, electrical power, attitude control and telemetry, tracking and commanding. The "flight system" of a spacecraft into subsystems. These include:

- physical backbone structure (provides overall mechanical integrity of the spacecraft; ensures spacecraft components are supported and can withstand launch loads);

- command and data subsystem. (responsible for command sequence storage, maintaining the spacecraft clock, collecting and reporting spacecraft telemetry data (e.g. spacecraft health), collecting and reporting mission data (e.g. photographic images);

- attitude control subsystem (responsible for the spacecraft's orientation in space and the positioning of movable parts);

- telecommunication subsystem (includes radio antennas, transmitters and receivers which are used to communicate with ground stations on Earth, or with other spacecraft);

- electrical power subsystem (includes solar cells and a radioisotope thermoelectric generator, batteries for storing power and distribution circuitry that connects components to the power sources);

- temperature control and protection from the environment subsystem (includes mirrors and sunshades for additional protection from solar heating).

A bridge's structural efficiency may be considered to be the ratio of load carried to bridge mass, given a specific set of material types. In one common challenge students are divided into groups and given a quantity of wood sticks, a distance to span, and glue, and then asked to construct a bridge that will be tested to destruction by the progressive addition of load at the center of the span. The bridge taking the greatest load is by this test the most structurally efficient. A more refined measure for this exercise is to weigh the completed bridge rather than measure against a fixed quantity of materials provided and determine the multiple of this weight that the bridge can carry, a test that emphasizes economy of materials and efficient glue joints.

A bridge's economic efficiency will be site and traffic dependent, the ratio of savings by having a bridge (instead of, for example, a ferry, or a longer road route) compared to its cost. The lifetime cost is composed of materials, labor, machinery, engineering, cost of money, insurance, maintenance, refurbishment, and ultimately, demolition and associated disposal, recycling, and replacement, less the value of scrap and reuse of components. Bridges employing only compression are relatively inefficient structurally, but may be highly cost efficient where suitable materials are available near the site and the cost of labor is low. For medium spans, trusses or box beams are usually most economical, while in some cases, the appearance of the bridge may be more important than its cost efficiency. The longest spans usually require suspension bridges.

Дата добавления: 2014-12-27