SAT; ACT; GRE
Test Prep Material
Click Here
xx
|
Launch Vehicles, Reusable
A reusable launch vehicle (RLV) is a craft designed to place payloads or crews into Earth orbit, and then return to Earth for subsequent launches. RLVs are designed to reduce launch costs by reusing the most expensive components of the vehicle rather than discarding them and building new
ones for each mission (as is the case with expendable launch vehicles, known as ELVs). The definition of RLVs does not include reusable craft launched from expendable launch vehicles. As of 2001, the only operational RLV was the U.S. space shuttle. A number of concepts were being developed or studied. Some were partially reusable. Most employed rockets, while others used jet engines, aircraft, or high-speed rail systems.
RLVs may be categorized by whether the vehicle takes off horizontally or vertically and whether it lands horizontally or vertically. An RLV may also be described as single-stage-to-orbit (SSTO) or two-stage-to-orbit (TSTO). Vehicles such as the space shuttle, which takes off vertically using a two-stage system and lands horizontally, have the easiest design because horizontal takeoff involves more demanding flight loads, vertical landing requires the craft to carry enough propellants to land, and SSTO requires a higher ratio of propellant to vehicle weight. Nevertheless, the economics of preparing a single stage, rather than two stages, have kept space engineers interested in SSTO designs. Future RLVs also are expected to employ more advanced, reliable systems, making them safer than expendable launch vehicles, and thus allow launches from inland sites (i.e., no stages to splash down into the ocean), perhaps even airports, where weather is less of a concern than at coastal spaceports.
Early Concepts
Because it was easier to adapt existing military missiles, which are designed for a single flight, most launchers have been expendable. Nevertheless, space visionaries have often focused on RLVs. One of the most significant early concepts was a three-stage vehicle designed by German-born American engineer Wernher von Braun in 1952 and popularized in his book Across the Space Frontier. The first two stages would parachute into the ocean for
recovery while the winged third stage, carrying crew and cargo, landed like an airplane. The 1951 movie When Worlds Collide depicted a rocket-powered sled that gives a vehicle its initial boost.
Several RLV concepts were advanced in the 1960s. Notable among these was a reusable design by a man named Philip Bono, then with Douglas Aircraft Company. His design comprised a core vehicle holding a payload bay, liquid oxygen tank, and a ring of small rocket engines around the base. Liquid hydrogen was carried in external tanks that could be hinged outward to enhance atmospheric control during entry. This unique engine arrangement followed the aerospike concept developed by Rocketdyne. In this approach, the pressure from the shock wave produced by the vehicle's high-speed ascent becomes the outer wall of the engine nozzle from which the exhaust streams. The resulting exhaust appears to be a spike of hot gas, thus leading to the nickname "aerospike."
In the late 1960s the U.S. aerospace industry offered a number of reusable designs as the National Aeronautics and Space Administration (NASA) sought ways to reduce the cost of space launches. Maxwell Hunter, then with Lockheed Missiles & Space Co., proposed a wedge-shaped reusable vehicle with main engines in its tail, and a large external tank that was shaped like an inverted V and was wrapped around the nose of the vehicle. The tank would be discarded after its propellants had been consumed, leaving the main body to return to Earth.
Space Shuttle
Following the Apollo 11 Moon landing in 1969, NASA proposed a space program that would provide the basic building blocks in support of a wide range of human space missions: a space shuttle, a space station, a space tug, and a nuclear interplanetary stage. In this plan, the space shuttle was a fully reusable vehicle. The booster would fly back to the launch site after launch, and the orbiter at the end of the mission; both would be quickly prepared for the next mission. NASA soon realized that such a massive craft would cost more than it could afford. A series of redesign efforts traded the high development cost and low per-flight cost of the original design for a lower development cost and a higher per-flight cost. Literally dozens of variations were studied before arriving at the final design. One interesting variation employed two piloted flyback boosters and a piloted orbiter outwardly identical to the boosters. The concept was to reduce costs by designing one airframe for two purposes. This design meant, however, that three vehicles had to be prepared for each launch. The Soviet Union largely copied the final space shuttle design for its Buran shuttle, which flew only once.
Advanced RLVs
Even before the shuttle started flying, designers continued to look at advanced reusable concepts, such as North American Rockwell's immense winged Star Raker, which was envisioned as taking off and landing like a jetliner. The SSX (Space Ship eXperimental) proposed by Hunter in 1984 was based on an earlier design of a passenger vehicle. Hunter's efforts helped lead to a U.S. Department of Defense (DoD) project that opened the current reusable era. The DoD's purpose was to design a single-stage vehicle that could orbit military replacement satellites during a national emergency.
McDonnell Douglas Corporation was contracted to build and test fly the DC-X, a one-third-scale suborbital model of the Delta Clipper, a larger version that would launch satellites on short notice. While not capable of spaceflight, the DC-X incorporated many of the technologies needed for an SSTO vehicle, including highly automated systems enabling a quick turnaround (just twenty-six hours) between launches. It made eight successful test flights between August 18, 1993, and July 7, 1995, and then was taken over by NASA and flown four times as the DC-XA between May 18 and July 31, 1996. It was destroyed on its last flight when one landing strut failed to deploy and the vehicle tipped over at landing.
The DC-X led NASA to a broader launch vehicle technology program to reduce the cost of putting a payload in space from $22,000 per kilogram ($10,000 per pound) to $2,200 per kilogram (__BODY__,000 per pound) or less. The principal programs as of the early twenty-first century were the X-33 and X-34. The X-33 was a one-third-scale test model of the Lockheed Martinconcept for Venture Star, an automated vehicle capable of launching up to 18,650 kilograms (50,000 pounds). In operation, VentureStar would launch, orbit, and land much as the shuttle does, but without discarding boosters or tanks. Other major differences include systems that can be readied for reflight with less maintenance (or no maintenance) than the shuttle requires. Significant structural and other problems raised the cost of the X-33 project and in 2001 NASA canceled the project. Also canceled was the X-34, a demonstration vehicle built largely from commercially available parts. It would have been launched from a jumbo jet, flown to an altitude of 76,200 meters (250,000 feet) and then glided to Earth for landing. It, too, encountered severe technical problems.
In place of the X-33 and X-34 programs, NASA initiated the Space Launch Initiative (SLI) program to study more conventional two-or three-
stage-to-orbit second-generation RLV, possibly using the aerospike engine concept, which looked promising in the X-33 project. The important underlying features would be new electronics and materials that would allow automated preparation and checkout of vehicles and more rapid launches, and highly automated manufacturing processes. Goals include reducing the risk of crew loss to once per 10,000 missions, and the cost of launches to less than __BODY__,000 per pound of payload in orbit. Beyond the second-generation RLV, NASA is looking at advanced space transportation concepts that could realize the earlier dreams of combining jet rocket combustion cycles in a single power plant, use electromagnetic railways as an Earthbound booster stage, or even laser-and microwave-powered craft.
In addition to NASA's efforts, several private ventures have initiated activities to develop RLVs for business, including space tourism. Most have stalled or failed for lack of financial backing. The Roton, conceived by Rotary Rocket, would employ high-speed helicopter blades to provide controlled flight following reentry (a concept studied by NASA in the 1960s). The vehicle would have a two-person crew, would launch vertically, and could place a 2,600-kilogram (7,000-pound) payload into orbit.
In 1996 the X PRIZE was announced. Like the Orteig prize, which stimulated aerial flight across the Atlantic Ocean (and was won by Charles Lindbergh with the first nonstop New York-Paris flight in 1927), the X PRIZE is intended to stimulate nongovernmental space travel, includingtourism. It will award $10 million to the first entrant that achieves a nongovernmental, suborbital flight reaching 100 kilometers (62 miles) in altitude with pilot and payload equivalent to three people total, and makes a repeat flight within two weeks. Burt Rutan, creator of the Voyager round-the-world aircraft, is designing the Proteus vehicle, which will air-launch one of the competing spacecraft.
Bibliography
Bono, Philip, and Kenneth Gatland. Frontiers of Space. New York: Macmillan, 1969.
Jenkins, Dennis R. The History of Developing the National Space Transportation System. Cape Canaveral, FL: Author, 1997.
Neal, Valerie, Cathleen S. Lewis, and Frank H. Winter. Spaceflight: A Smithsonian Guide. New York: Prentice Hall Macmillan, 1995.
Sparks, James C. Winged Rocketry. New York: Dodd Mead, 1968.
von Braun, Wernher, Frederick I. Ordway III, and Dave Dooling. Space Travel: A History. New York: Harper Collins, 1985.
Wilson, Andrew. Space Shuttle Story. London: Deans International Publishing, 1986.
Launch Vehicles, Reusable
Copyright © 2002 by Macmillan Reference USA, an imprint of the Gale Group
All rights reserved
|
Teacher Ratings: See what
others think
of your teachers
|
