Horizontal launch: A Versatile Concept for Assured Space Access
Fully reusable horizontal takeoff and landing single-stage-to-orbit (SSTO) launch vehicle systems have long been viewed by many countries, organizations, and individuals as the ultimate answer for providing low-cost, flexible, and assured access to space. As early as 1952, Wernher von Braun envisioned a reusable shuttle-type logistics vehicle to supply a space station. In the 1960s, the Air Force Aerospaceplane design study proposed scramjet propulsion and liquid oxygen supplied by an in-flight air collection and enrichment system, setting a goal to develop and prove these and other technologies that would be required to make such a system a reality.
In the late 1960s, a two-stage reusable-airbreathing and rocket—horizontal launch system was a proposed design option for the Space Shuttle. During this period, there was an intense debate about which shuttle design would provide the best combination of lifecycle costs and capability. The technologies needed for a fully-reusable system were found to be immature and too expensive to develop, and in 1972, the Space Shuttle design was fixed as a vertically-launched rocketpowered system with only partial reusability.
In the early 1980s, while expendable vertical launch vehicles were in wide use for military and commercial payloads, various studies continued to investigate horizontal launch opportunities, including the Reusable Aerodynamic Space Vehicle (RASV), Trans-Atmospheric Vehicle (TAV), Advanced Manned Spaceflight Capability (AMSC), and Advanced Manned Launch Systems studies. These efforts looked at airbreathing and rocket propulsion, at SSTO and multistage-toorbit systems, and at sled-launch and air-launch.
Beginning in 1984, the $2 billion DoD-NASA National Aero-Space Plane (NASP) program was initiated to develop an airbreathing SSTO system similar to those studied in the 1960s. The program was cancelled in 1994, as the necessary technologies—while much more advanced than 20 years previous to this—were not sufficiently mature. The projected costs and cost uncertainties were too great.
Several new concepts for horizontal launch system were introduced in the 1980s and 1990s. A British program investigated the single-stage-to-orbit Horizontal Takeoff and Landing (HOTOL) concept using air-breathing rockets fed by pre-cooled air to reach Mach 5. A German program proposed the Sänger reusable two-stage system with a turboramjet-powered first stage to reach Mach 6 and a rocket-propelled orbiter stage. American efforts leveraged the NASA High-Speed Civil Transport (HSCT) program by adding high-efficiency turbojets to the
carrier aircraft. These programs were terminated because the amortized design, development, test, and evaluation (DDT&E) costs overcame any return on investment when compared to longrange
subsonic aircraft.
From the early 1990s through the mid-2000s, NASA investigated several next-generation space access candidates, including horizontal and vertical launch configurations, both airbreathing and rocket-powered. Payload classes of primary interest were initially comparable to the Space Shuttle—50,000 lb or less. By 2005, however, payload requirements to support the human space exploration program were increased to greater than 200,000 lb, with large volumes. This scenario overwhelmingly favored large vertical, rocket-powered launch systems.