The HL-42 was a proposed scaled-up version of the HL-20 re-usable crewed spaceplane design, which had been developed from 1983 to 1991 at NASA's Langley Research Center but never flown. Like the HL-20 ("Horizontal Lander 20"), the HL-42 would have been launched into low Earth orbit mounted on top of a two-stage expendable rocket. At the end of the mission, it would have re-entered and glided to a runway landing. The HL-42 was suggested as one possible successor to the Space Shuttle in the 1994 NASA Access to Space Study. However, another alternative, a Single-stage-to-orbit design, was chosen for further development, and work on the HL-42 was abandoned.
Background: NASA launch policy In the early 1980s, it had been NASA policy to promote the use of the Space Shuttle for almost all civilian and military launches; only then, with Shuttle missions taking off almost every week, would the Space Shuttle program make economic sense. However, the Challenger disaster in 1986 forced a reconsideration, and in the following years, many studies attempted to chart a way forward without reaching any consensus except for a growing feeling that "faster, better, cheaper" would be a good idea. There was also disagreement over the design (and indeed purpose) of the proposed Space Station Freedom.
The Access to Space Study of 1994 Finally, as President Clinton took office for the first time in January 1993, the new NASA Administrator Daniel Goldin commissioned a major study that would reduce the multitude of possibilities to three well-defined options for launch systems. In November 1993, while that study was still underway, an agreement was reached with Russia to develop the Freedom design into the International Space Station, so for Space Station operations the study authors were told to design for the 'worst case': Assume a 4-person Station like Freedom that would have been built and maintained solely by the US, but put it in a Mir orbit with an inclination of 51.6 degrees (a significant change, as this would be more difficult to reach from Cape Canaveral and would reduce the Shuttle's payload by one third). On the other hand, the new era of cooperation with Russia would make it easier to buy and use the promising Russian first-stage engines of the RD-170 / RD-180 family and the innovative tri-propellant RD-701. The NASA Office of Space Systems Development published this Access to Space Study in January 1994; it had a major influence on space policy for the rest of the decade. The study aimed for the chosen new launch hardware to be introduced starting from 2005 as the current Shuttles approached retirement and to remain in use until 2030. Three expert teams each mapped out a possible path ahead:
Option 1: Continue to be Shuttle-based until 2030, but with upgrades Option 1 was the most conservative, consisting of incremental upgrades to the existing Shuttle fleet and possibly constructing several more Shuttles using lighter, stronger materials. Vastly improved avionics could make uncrewed, autonomous operations practical and desirable for some missions.
Option 2: Conservative engineering, expendable launchers, and some new vehicles including the HL-42 Option 2, slightly more adventurous and focused on economy and efficiency, would involve new vehicles but only small advances on existing materials and methods. Team 2 planned to use only technology expected to be readily available in 1997 and to have the new hardware ready for service in 2005. All launch vehicles will be expendable in the recommended version of Option 2. Delta II would be retained; Atlas II would be upgraded with a Russian RD-180 engine; and the heavy Titan IV and Shuttle would be replaced by a new heavy launcher (using RD-180 engines and a J-2S-powered upper stage), which would support all Space Station operations using either a disposable uncrewed Automated Transfer Vehicle (for cargo) or the HL-42 re-usable spaceplane (for crew).
Option 3: Go straight for Single-stage-to-orbit (SSTO) Option 3 was ambitious, but Team 3 felt the time had come for a new generation of large, fully re-usable single-stage-to-orbit (SSTO) launchers. They had been impressed by the April 1993 roll-out of the McDonnell Douglas DC-X and its central philosophy of drastically simplified "airline-like operations". Indeed, they concluded their part of the study with this italicized credo: The bottom line is this: operability must not be simply a goal; it must be THE design driver. Team 3 investigated several air-breathing, horizontal take-off alternatives building on the experience of the Rockwell X-30 (NASP) project but concluded that the future lay with an all-rocket, vertical take-off, horizontal-landing SSTO design. Their reference design used the proposed but unbuilt Russian RD-704 tri-propellant engines, small wings, and a cylindrical fuselage with a central Shuttle-style payload bay. This could carry cargo or passengers, but the operation would be fully automated in both cases. The team calculated that the first operational vehicle could be delivered in 2007, with a fleet of four ready to take over all Delta, Atlas and Shuttle operations by 2011. This final reference design strongly resembled the X-2000 Advanced Technology Demonstrator proposed in August 1993 by a group at NASA's Marshall Space Flight Center.
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