To date, the only way to achieve the propulsive energy to successfully launch spacecraft has been by combustion of chemical propellants, although there are a few other approaches currently being researched. There are two groups of rocket propellants, liquids and solids. Many spacecraft launches involve the use of both types of rockets, for example the solid rocket boosters attached to liquid-propelled expendable rockets, or the space shuttle. Hybrid rockets, which use a combination of solid and liquid, are also being developed. Solid rockets are generally simpler than liquid, but they cannot be shut down once ignited. Liquid and hybrid engines may be shut down after ignition, and conceivably could be re- ignited. A sampling of commonly used launch vehicles follows.
Delta is a family of two- or three-stage liquid-propelled ELVs, produced by McDonnell Douglas, that use multiple strap-on solid boosters in several configurations. The liquid engines burn kerosene and liquid oxygen (LOX). A Delta II is capable of placing payloads of up to 2200 kg into low equatorial orbit (LEO). A Delta II placed the German X-Ray Observatory ROSAT into orbit in 1990, and launched the Japanese Geotail satellite in 1992.
Titan, produced by Martin Marietta Aerospace Group in Denver, Colorado, is a liquid-propelled, multiple stage expendable launch vehicle (ELV) that can accommodate solid propellant strap-on boosters. The liquid engines burn hydrazine and nitric acid. Depending on the upper stage used, the Titan IV can put payloads of up to 18,000 kg into LEO, over 14,000 kg into polar orbit, or 4,500 kg into a geostationary transfer orbit (GTO). A Titan III launched the Viking spacecraft to Mars in 1975. A Titan IV, equipped with two upgraded solid rocket boosters and a Centaur upper stage, will launch the Cassini spacecraft on its interplanetary trajectory in 1997. Titan III vehicles launched JPL's Voyager 1 and 2 in 1977, and the Mars Observer spacecraft from the Kennedy Space Center (KSC), Cape Canaveral in 1992. The smaller Titan II can place about 2,000 kg into LEO.
Atlas, produced by General Dynamics Corporation, is a liquid-propelled ELV which accommodates a variety of upper stages. Its engines burn kerosene and LOX. With a Centaur upper stage, Atlas is capable of placing 4000 kg into LEO. An Atlas/Centaur launched the Infrared Astronomical Satellite (IRAS) into Earth orbit in 1985, and an Atlas is planned to launch the Space Infrared Telescope Facility (SIRTF) into solar orbit in 1998.
Ariane is a system of highly reliable liquid-propelled ELVs combined with a selectable number of solid strap-on boosters or liquid boosters. They are launched from the Kourou Space Center in French Guiana by Arianespace, the first space transportation company in the world, composed of a consortium of 36 European aerospace companies, 13 European banks, and the Centre National d'Études Spatiales (CNES). Ariane 4 is capable of placing 4200 kg in GTO. Ariane 4 launched the Topex/Poseidon spacecraft into a high-altitude Earth orbit in 1992. An Ariane 5 launcher is under development, targeted to fly the manned Hermes mini-shuttle and 18,000 kg into LEO.
The Proton is a liquid-propellant ELV developed by the Soviet CIS Interkosmos. It is launched by Russia from the Baykonur Kosmodrome in Kasakhstan, and is capable of placing 20,000 kg into LEO. It has launched many Earth satellites and interplanetary spacecraft, and is scheduled to send an additional spacecraft to Mars in 1994, with cooperation from the U.S. and France. A western-built satellite for Inmarsat, the 67-country consortium, is planned to be launched by Proton in 1995.
America's space shuttle, as the Space Transportation System (STS) is commonly known, is a reusable launching system whose main engines burn liquid hydrogen and LOX. After each flight, its main components, except the external propellant tank, are refurbished to be used on future flights. The STS can put payloads of up to 30,000 kg in LEO. With the appropriate upper stage, spacecraft may be boosted to a geosynchronous orbit or injected into a planetary transfer orbit. Galileo, Magellan, and Ulysses were launched by the STS, using an Inertial Upper Stage (IUS), which is a two-stage solid-propellant vehicle. The STS may be operated to transport spacecraft to orbit, perform satellite rescue, and to carry out a wide variety of scientific missions ranging from the use of orbiting laboratories to small self-contained experiments.
Many NASA experiments, as well as commercial and military payloads, are becoming smaller and lower in mass, as the art of miniaturization advances. The range of payload mass broadly from 100 to 1300 kg is becoming increasingly significant as smaller spacecraft are designed to have more operational capability. The market for launch vehicles with capacities in this range is growing.
Pegasus is a small, winged solid-propellant ELV built by Orbital Sciences Corporation. It resembles a cruise missile, and is launched from under the wing of an aircraft in flight at high altitude, currently a B-52. It is planned to be able to lift 400 kg into LEO. The Scout was a ground-launched, reliable solid-propellant ELV capable of placing 200 kg into LEO.
The Conestoga space launch vehicle is a low-cost, solid-propellent launcher made by Space Services, Inc., SSI, in Houston, and is capable of placing payloads of up to 1360 kg into LEO, and 450 kg into GTO. Conestoga is a name aptly reminiscent of 19th-century broad-wheeled covered wagons, the expendable "launch" vehicles used by American pioneers to cross the prairie. They were named after the town where they were manufactured in Lancaster County, Pennsylvania.
If a spacecraft is launched from a site near Earth's equator, it can take optimum advantage of the Earth's substantial rotational speed. Sitting on the launch pad near the equator, it is already moving at a speed of over 1650 km per hour relative to Earth's center, a velocity which can be applied to the speed required to orbit the Earth (approximately 28,000 km per hour). This means that the launch vehicle needs less propellant for launch, or that a given vehicle can launch a more massive spacecraft into orbit. A spacecraft intended for a high-inclination Earth orbit has no such free ride, though. As mentioned in Chapter 5, the launch vehicle must provide a much larger part, or all, of the energy for the spacecraft's orbital speed.
For interplanetary launches, the vehicle must take advantage of Earth's orbital motion as well, to accommodate the limited energy available from today's launch vehicles. In the diagram below, the launch vehicle is, in addition to using Earth's rotational speed, accelerating generally in the direction of the Earth's orbital motion, which has an average velocity of approximately 100,000 km per hour along its orbital path.
Of course, the spacecraft must fly a specific direction for its particular trajectory, but it can utilize at least a major component of the Earth's pre-existing motion. In the case of a spacecraft embarking on a Hohmann interplanetary transfer orbit, recall the Earth's orbital speed represents the speed at aphelion or perihelion of the transfer orbit, and the spacecraft's velocity merely needs to be increased or decreased in the tangential direction to achieve the desired transfer orbit.
The launch site must also have a clear pathway downrange so the launch vehicle will not fly over populated areas, in case of accidents. The STS has the additional constraint of requiring a landing strip with acceptable wind, weather, and lighting conditions near the launch site as well as at landing sites across the Atlantic Ocean, in case an emergency landing must be attempted.
Launches from the east coast of the United States (the Kennedy Space Center at Cape Canaveral, Florida) are suitable only for low inclination orbits because major population centers underlie the trajectory required for high-inclination launches. The latter are accomplished from Vandenberg Air Force Base on the west coast, in California, because the trajectory for high-inclination Earth orbits is out over the Pacific Ocean. An equatorial site is not required for high-inclination orbital launches.
Complex ground facilities are required for heavy launch vehicles, but smaller vehicles such as the Conestoga require only trailer-mounted facilities, and the Pegasus requires none except its parent airplane.
A launch window is the span of time during which a launch may take place while satisfying the constraints imposed by safety and mission objectives. For an interplanetary launch, the window is constrained typically within a number of weeks by the location of Earth in its orbit around the sun, in order to permit the vehicle to use Earth's orbital motion for its trajectory, as well as timing it to arrive at its destination when the target planet is in position. The launch window is also constrained typically to a number of hours each day of the previously described window, in order to take advantage of Earth's rotational motion. In the illustration above, the vehicle is launching from a site near the Earth's terminator which is going into night time hours as the Earth's rotation takes it around away from the sun. If the example in the illustration were to launch in the early morning hours on the other side of the depicted Earth, it would be launching in a direction opposite Earth's orbital motion. These illustrations are over-simplified in that they do not differentiate between launch from Earth's surface and injection into interplanetary trajectory . It is actually the latter that must be timed to occur on the proper side of Earth. Actual launch times must also consider how long the spacecraft is to remain in low Earth orbit before its upper stage places it on the desired trajectory (this is not shown in the illustration).
The daily launch window may be further constrained by other factors, for example, the STS's emergency landing site constraints. Of course, a launch which is to rendezvous with another vehicle in Earth orbit must time its launch with the orbital motion of that object. This was the case with the Hubble Space Telescope repair mission executed in December 1993.
The spacecraft must be transported from the site where it was built and tested to the launch site. The spacecraft is sealed inside an environmentally controlled carrier for the trip, and internal conditions are carefully monitored throughout the journey. Once at the launch site, additional testing takes place, and propellants are loaded aboard. Then the spacecraft is mated to its upper stage, and the stack is mated to the launch vehicle.
Pre-launch and launch operations of a JPL spacecraft are typically carried out by personnel at the launch site while in direct communication with persons at the Space Flight Operations Facility at JPL. Additional controllers and engineers at a different location are typically involved with the particular upper stage vehicle, such as the Lockheed personnel at Sunnyvale, California, controlling the inertial upper stage (IUS). The spacecraft's telecommunications link is maintained through ground facilities close to the launch pad prior to launch and during launch, linking the spacecraft's telemetry to controllers and engineers at JPL. Command sequences must be loaded aboard the spacecraft, verified, and initiated at the proper time prior to launch. Spacecraft health must be monitored, and the launch process interrupted if any critical tolerances are exceeded.
Once the spacecraft is launched, the DSN begins tracking, acquiring the task from the launch-site tracking station, and the cruise phase is set to begin.