Reusable Launch Vehicles

views updated Jun 11 2018

Reusable Launch Vehicles

The last decade of the second millennium saw the emergence of the idea of sending payloads into space with reusable launch vehicles (RLVs). It appeared to make economic sense to reuse a launch vehicle that cost as much as a small airliner, rather than throw that vehicle away after one use. Two prototypes—the McDonnell Douglas Delta Clipper and Rotary Rocket's Roton—were built and flown at low altitude. A number of small companies emerged, each seeking to build an RLV. Although this idea has gained broad acceptance, no RLV has flown in space in recent years and none is likely to for many years.

An Old Idea and a Proven Technology

It is a misconception that a number of technological breakthroughs are required before RLVs will be feasible. An American experimental RLV, the X-15, made its maiden flight on June 8, 1959. The X-15 was not called anRLV but a hypersonic airplane. It was incapable of reaching orbital speed (24,000 kilometers [15,000 miles] per hour) but flew fast enough (7,160 kilometers [4,475 miles] per hour) to reach an altitude above 100 kilometers (328,080 feet), the officially recognized boundary between Earth and space. In 199 flights the X-15 topped this altitude once, on August 22, 1963. With pilot Joe Walker at the controls, the X-15 reached 109,756 meters (360,000 feet) and became the first and only successful RLV.

The idea of the RLV can be traced back to 1928, and since that time a great many proposals have been made. The classic The Frontiers of Space (1969) vividly illustrates a number of RLV concepts, all of which were technically feasible at that time.

Fated by History

If technical feasibility is not an issue, why, then, has the RLV not replaced the expendable launch vehicle (ELV)? The reasons include a complex mix of economics, politics, historical accident, and human psychology. To understand them, it is necessary to appreciate how the ELV came into being, how the market for commercial ELVs emerged, and how the operational aspects of ELVs prevent new commercial space markets from developing, and consequently, why no RLVs have been or will soon be developed.

Arthur C. Clarke's The Promise of Space, Chapter 14, "The Birth of Apollo" (1968), gives the most concise summary of the historical events that made the ELV imperative. In essence, the space age was a child of the Cold War. If technology had evolved in a logical manner, RLVs would have been the product. In 1957, however, the Soviet Union shocked the world by using an intercontinental ballistic missile (ICBM) to launch the world's first satellite. A series of space firsts by the Soviet Union began to make the United States look technologically and economically inferior.

This perception was a threat to national security. On May 25, 1961, President John F. Kennedy responded by making a commitment to land a man on the Moon and return him safely to Earth by the end of the decade. Such a feat required decisions to be made early on, the first of which was how to get there. The difficulties of making aircraft that could carry a large payload and fly fast enough to reach orbit were well known, and no such airplane had ever been built. However, hundreds of satellites and spacecraft had been launched on ICBMs, and it was correctly thought that there was no limit to the size of the payload that could be launched with a scaled-up ICBM. Ultimately, President Kennedy's goal was achieved by using the Saturn V rocket, a 2,902,991-kilogram (3,200 tons), 111-meter-tall (365 feet) ELV designed specifically to send astronauts to the Moon. On July 20, 1969, Neil Armstrong and Edwin "Buzz" Aldrin landed on the lunar surface, signaling the beginning of the end of the Cold War.

Sadly, that triumph closed the gates to space for future generations. By establishing the ELV as the "existing launch vehicle," a mode of space transportation had been established that was too expensive to permit any normal economic development of the space frontier. In the forty-five years since the Soviet Union launched the first satellite, only one commercial use has been found for space: as a location for relay stations (geosynchronous communication satellites [GEOSATs]) to bounce radio and television signals around the world.

Even that market would not have emerged if it had not been initiated, as a matter of national security, by the U.S. government. In 1962 Congress passed the Communications Satellite Act, which led to the formation in 1963 of the Communications Satellite Corporation (Comsat). The financing of this "risky" venture was possible only because the government backed it. The communications satellite industry grew at an astonishing rate, and was eventually was "privatized" by the Space Act of 1984. It has proved phenomenally profitable but has welded closed the gate to space that Apollo locked.

The reason for this is psychological. According to management consultant W. Edward Deming, "If you always do what you always did, you'll always get what you always got." In the case of space, doing what you always did is a matter two things: Having the government underwrite the risk of any new space venture—be it a new launch vehicle or satellite system—and reaching space by means of "launch vehicles" of any kind are things of the past. Only space projects tied to national security should be backed by the government, and since the end of the Cold War, these projects have not included commercial ventures.

The private sector has seldom had the financial courage to undertake a new kind of space venture without government guarantees. The most notable exceptions have been the global cellular telephone projects Iridium and GlobalStar. Both have been spectacular failures because they have relied on "launch" by the means used for GEOSATs. Although each GEOSAT produces revenue and requires only one launch, Iridium and GlobalStar had to place large numbers of satellites in orbit before any revenue could flow. The cost and time required to do this on single-use launch vehicles were so great that corners had to be cut in terms of the size and power of the spacecraft. The result in each case was substandard service at a price no one could afford.

The Concept of "Launch" as the Barrier to RLVs and Space

The chief barrier to large-scale commercialization of space is the concept of the launch. The practice of making each satellite a complete, independent, stand-alone unit results in an upward cost spiral for both satellite and launch vehicle (the size of both continually increases to squeeze every ounce of revenue out of the increasingly expensive hardware) and a consequent reduction in the number of spacecraft launched each year.

An RLV must fly many times to recover its cost of development and construction. Further, it would take an extremely large and expensive RLV to carry stand-alone satellites of the GEOSAT class. An expensive RLV carrying a small number of complete stand-alone satellites each year is not economically viable.

However, there is no reason, other than those imposed by launch, that every payload cannot be a complete, stand-alone unit. On Earth one does not deliver an office building to its lot on a single truck. One brings in many trucks, each carrying small components of the building. Erecting an "office building" in space is done the same way. The International Space Station could not have been "launched" on a single rocket. It had to be taken up in modules and assembled in orbit. The significance of this is that on-orbit assembly has been demonstrated on a massive, complex scale. The assembly of smaller spacecraft on orbit should be no more difficult.

The Prescription: Change the Way We Operate in Space

No technological breakthroughs are required for RLVs to flourish. What is required is the discarding of the very concept of the launch and adopting the same approach to space operations that is used routinely on Earth: build spaceplanes and other space transport systems and use them to carry components of space factories into orbit. After the factories are built, they should be used to produce the only thing that can be built better in space than on Earth: spacecraft. The parts for the spacecraft, along with the people to put them together and the supplies needed to keep them alive, can be delivered by space transports on a regular basis.

In this system a space transport that delivers a smaller payload but does so economically (i.e., a single-stage-to-orbit vehicle) has a decided advantage. Because it can deliver only a small load, it must fly frequently. It will therefore spread its cost of development and construction over a large number of flights, just as an airliner does.

The first spacecraft to be assembled in orbiting factories would be communications satellites, since there is an established market for them. Having put in place an orbital infrastructure involving people living and working in space, one then can branch out into other areas. The same habitats used as factories could be replicated, with modifications, as orbiting hotels. Because a large number of people would have already flown into space to assemble the factories, the communications satellites, and the hotels, enough experience would have been accumulated to make passenger flights safe and easy.

Once passenger travel is established, the promise of space will be realized. There are 6 billion potential payloads in the form of human beings. This far exceeds the number of spacecraft that will ever be built and represents the real market for future space transportation systems.

Wanted: A Howard Hughes

Getting to this point will not happen soon. In light of the realities of finance and markets, the only hope for change is the emergence of an individual with the personal financial resources, technical know-how, business acumen, and vision to make it happen. What is needed is a Howard Hughes,^* who possessed all of these attributes and used them to advance aviation.

When such a person appears and brings about the needed changes, the opportunities will be endless. No one knows what new activities and industries will result when large numbers of people travel into space. One can be sure that things that we have never dreamed of will emerge. When people are placed in a completely new environment, they adapt both themselves and that environment in ways that cannot be predicted. This has been the history of humanity, and it will be the future of our expansion into space.

see also Accessing Space (volume 1); Business Failures (volume 1); Communications Satellite Industry (volume 1); Getting to Space Cheaply (volume 1); Hotels (volume 4); Launch Vehicles, Expendable (volume 1); Launch Vehicles, Reusable (volume 1); Satellite Industry (volume 1); Space Tourism, Evolution of (volume 4); Tourism (volume 1).

Michael S. Kelly

Bibliography

Bono, Philip, and Kenneth Gatland. The Frontiers of Space. New York: Macmillan, 1969.

Clarke, Arthur C. The Promise of Space, New York: Harper & Row, 1968.

McLucas, John L. Space Commerce. Cambridge, MA: Harvard University Press, 1991.

Thompson, Milton O. At the Edge of Space—The X-15 Flight Program. Washington, DC: Smithsonian Institution Press, 1991.

^*In 1966 Fortune magazine declared aviator and financier Howard Hughes the richest man in the United States.

Space Sciences Kelly, Michael S.

Launch Vehicles, Reusable

views updated May 29 2018

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 ($1,000 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 $1,000 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.

see also Launch Services (volume 1); Launch Vehicles, Expendable (volume 1); Reusable Launch Vehicles (volume 4); Rockets (volume 3); Spaceports (volume 1); von Braun, Wernher (volume 3); X Prize (volume 1).

Dave Dooling