SPACE SHUTTLE: OPEATIONS IN NEAR-EARTH ORBIT

by Robert Haynes

NASA's Space Shuttle is the first space-flight system that can be reused. Much more than a launch vehicle, it not only delivers cargoes to space, but can return them to Earth. Its versatile design lets it take off like a rocket, fly in Earth's atmosphere and gravity like an airplane, and behave in space like a spacecraft, guided by reaction thrusters.

The Shuttle has already proven itself a space-worthy-craft, turning missions that might have been failures into successes. For example, after the multi-million dollar Solar Maximum satellite failed to operate properly in space, Shuttle crew members repaired it in orbit and returned it to full operation. The Shuttle retrieved two other satellites (Westar and Palapa-B) and returned them to Earth for refurbishing.

Crew members have an array of sophisticated computers and machinery at their fingertips. A Remote Manipulator System (RMS) moves items in and out of the cargo bay and from place to place in the vicinity of the Shuttle. Spacelab, built and paid for by the European Space Agency (ESA), can be carried in the cargo bay and has expanded the Shuttle's capability to perform scientific operations in space. The modular form of Spacelab allows it to fly in many configurations to meet different needs. Rockets attached to space vehicles launched from the Shuttle's cargo bay propel payloads into higher orbits or toward their missions in deeper space.

The Space Shuttle will play a major role in shaping the U.S. space efforts for the rest of this century. Many lessons have been learned since that first 12-second flight at Kitty Hawk more than 80 years ago. The Shuttle routinely travels up to 320 kilometers (200 miles) above the Earth and can orbit-there for as long as 10 days. On a typical 7-day mission, the Shuttle lets scientists reach beyond the planet's surface to perform regular studies and observations, learning important lessons that will benefit their disciplines.

Designed for Hard Work

The Shuttle Orbiter is a reliable and reusable workhorse that carries payloads and crews routinely to and from space. On the launch pad, it is mated to two solid fuel rocket boosters and a large external fuel tank. The boosters help lift the assemblage away from Earth's gravitational pull and fall into a designated ocean area where they are retrieved for reuse. The external fuel tank holds propellants that power the three main engines.

The only part of the launch configuration that is not reused is the external tank. It holds a total of 720,000 kilograms (1,584,000 pounds) of propellants, consisting of liquid hydrogen (fuel) and liquid oxygen (oxidizer). Fuel from the external tank drives the Shuttle to a point just short of its final orbit, where the tank separates and drops back to Earth. The tank breaks up in the atmosphere and falls into a predetermined remote ocean area. The Shuttle coasts a few seconds and then fires its two orbital maneuvering engines to position itself in orbit.

The Orbiter is about the same length and weight as a commercial DC-9 airplane. It measures 37 meters (122 feet) long and 17 meters (57 feet) high, has a wingspan of 24 meters (78 feet), and weighs about 77,300 kilograms (170,000 pounds) without fuel or payload. Its appearance, however, is markedly different. High performance double-delta (or tnangular) wings and a large cargo bay (where passengers would sit on an airplane) give the Shuttle a chubby appearance.

Another difference when compared with a conventional airplane is the Shuttle's thermal protection system. Every part of the Shuttle's external shell is shielded by some type of thermal protection. Most notable are the rigid silica tiles that protect areas of intense heat when the Orbiter reenters Earth's atmosphere. Tiles covering the upper and forward fuselage sections and the tops of wings can absorb heat as high at 650 C (1200 F). Black tiles on the underside absorb higher temperatures, up to 1260C (2300 F). Areas that receive the most heat on reentry, such as the nose and leading edges of wings, are covered with black panels made of reinforced carboncarbon (RCC). These panels soak up heat in excess of 1260 C (2300 F).

The engines that push the Shuttle into an Earth orbit and steer it on its way have been tested in successive flights. They will lift continuously heavier payloads until their full lifting capacity is reached. Payload capacity is not only affected by cargo weight, but also by the weight of fuel necessary to lift it, the orbital azimuth and length of mission in space, and the size of the crew and consumables needed to support them. The three main engines are designed for reuse on more than 50 missions, and the two maneuvering engines, on either side, for about 100 missions.

From launch to landing. On the launch pad, the Shuttle Orbiter is mated to two solid fuel booster rockets and a large fuel tank. At about 24 nautical miles after lift off, the two booster rockets are jettisoned. They fall in an arc, slowed by parachutes, to a remote ocean area, where they are retrieved for later use. Shortly before orbital injection, the main engines are shut down, and the external tank is separated. The tank tumbles to Earth, falling in pieces over an untraveled ocean area. The Orbiter coasts a few seconds, then maneuvering rockets fire to position its orbit. Cargo bay doors open, and the crew performs its mission. When the mission is complete, the bay doors close and the Shuttle leaves orbit with thrust from its maneuvering rockets. It reenters the atmosphere and lands on a runway like an airplane.

While the Orbiter is in space, thrusters in its nose and aft sections control its attitude. The nose contains 14 thrusters, called primary reaction control engines, and 2 vernier engines for fine tuning. In the tail or aft section, each pod contains 12 primary engines and 2 vernier engines. All primary thrusters and vernier engines were designed for a 100-mission lifespan.

The Orbiter cabin, a 71.5-cubicmeter (2500 cu. ft.) area, normally houses a 4- to 7-member crew. It is pressurized to Earth's air pressure at sea level. Temperatures are kept between 18 and 26C (65-80F), and living quarters below the flight deck offer sleeping, hygiene, and food preparation facilities.

Heavy-duty hydraulic systems provide power for the Orbiter's ascent, reentry, and landing operations. The hydraulic system is powered by three independent auxiliary power units, fueled by monopropellant hydrazine. Hydraulic actuators move wing aerodynamic control surfaces, a body flap, and the rudder. They also operate valves, lower landing gear, control main engine thrust direction, and provide power for Orbiter brakes.

Electrical power operates everything else. Electricity is provided by three fuel cells. The cells provide 20-30 kilowatts during ascent, when most of the payload equipment is turned off, and 14-36 kilowatts while payload machinery is in use during orbit. A by-product of the fuel cells is potable water, used for drinking, hygiene, and on-board experiments.

All Shuttle subsystems are monitored by a unique five-computer network configured in a redundant operating group (four operate at all times and one is a backup). They simultaneously process data from virtually every area of the Shuttle, each "communicating" with the others and comparing data. If the computers disagree, they, in effect, vote between themselves, and the data from the outvoted computer are ignored.

Spacelab will be carried inside Orbiter's cargo bay on as many as three or more missions per year. Spacelab consists of two main elements, which can be flown separately or together: a pressurized habitable module, where experiments are conducted in a shirtsleeve environment; and pallets, on which experiments can be mounted for exposure to space. As many as five pallets can be flown at a time. When only pallets are flown, essential subsystems are normally carried in an 8-foot-long cannister called an igloo. The igloo houses subsystems for operating experiments, including command, data management, and electrical power subsystems, which on other missions are located in the habitable module.

Shuttle Is Tested

On April 12, 1981, John Young and Robert Crippen stepped aboard the Shuttle Columbia for the first manned flight of a reusable spacecraft. But long before their maiden voyage, this new vehicle had been tested and retested.

In the mid-1970s, after the basic aerodynamic delta design had been decided, wind-tunnel tests began. One by one, the Shuttle's components were tested and pronounced ready. In 1977, the first manned Orbiter, named Enterprise, was released from its aeronautical capabilities on approach and landing. Engines were ground tested, as were computers, the heat resistant tiles, and all linkages joining the assemblage.

But it was not until mid-April 1981 that the Shuttle proved to be an integrated space transportation system. It was the first of four test flights with manned crews, dubbed the Orbital Flight Test (OFT) program. During OFT, crews conducted more than 1,100 carefully outlined tests and data collections. OFT was declared a success and, after the fourth flight landed on July 4, 1982, the Shuttle was declared operational.

Airline To Space

The Shuttle can be thought of as the airline that will transport humankind to a future habitat in space. Acceleration is limited to less than 3 times the acceleration of gravity during ascent, and less than 2.5 times during reentry (with 1 g being the acceleration of gravity at sea level). This means men and women not specially trained as astronauts can travel in it, experiencing about the same acceleration as on some carnival rides. By comparison, crews of Apollo had to withstand as much as 8.1 g during reentry into Earth's atmosphere.

While riding the Shuttle, travelers live and work in a shirtsleeve environment, without cumbersome spacesuits and breathing apparatus. The main thing space travelers will have to get accustomed to is the near absence of gravity (microgravity), which can drastically change the way they work, sleep, and eat. However, microgravity (near weightlessness) itself is an essential element for certain experiments Shuttle crews will conduct while in orbit.

Looking To The Future

On one of its upcoming missions, the Shuttle will carry a space telescope in its cargo bay and place it in orbit. Unhindered by Earth's distorting atmosphere, this telescope will be able to study objects in the universe 50 times fainter and 7 times farther away than even the large 200-inch Earthbound telescope at Mt. Palomar, California.

The space telescope will reveal objects we have never before viewed. It will scan the very edge of the universe, looking back toward the beginning of time for possible clues about the universe's origin. By simply letting scientists view 350 times more space than they do now, it is expected to reveal extraordinary and unanticipated phenomena.

The Shuttle makes building a manned space station possible. As currently envisioned, modules and other components of the Station will be delivered into space on 6 to 8 Shuttle flights. Astronauts will assemble these elements in orbit using the Shuttle as a work platform. This first phase of the Space Station, called the "initial operational capability," is scheduled for the early 1990s. Once the Space Station is built, the Shuttle will periodically revisit it, replenishing expendable supplies, such as fluids, oxygen, food, clothing, and spare parts for experiments.

Also on board the Shuttle will be Spacelab, flying either as "mixed cargo" or on "dedicated" missions occupying the whole cargo bay. In October 1985, the German Spacelab mission D-l was a dedicated Spacelab flight, using a long habitable module to conduct experiments in life sciences and materials sciences. Germany developed the payload and was responsible for mission operations. The United States flew only one experiment on this October 1985 mission.

Spacelab specialists perform the kinds of tasks better done in space than on Earth. Scientists have already studied the Earth from space. They have checked on weather conditions, land uses, air pollution, and lake silting, and explored the depths of our solar system and beyond through orbiting telescopes (without Earth's atmosphere to obscure their view). Experiments on board Spacelab have included growing large near-perfect crystals, developing pure alloys, drugs, and lenses of a purity unattainable under the influence of Earth's gravity.

As inhabitants of Earth, we are no longer restricted to living on our native planet. We are well on our way to becoming permanent residents and workers in space. Already regular visitors to this formerly hostile envlronment, we will soon be able to perform in friendly surroundings, stretching the limits of our knowledge and understanding.