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.

Final recommendation: Develop technology for SSTO After analysis, the study decided on Option 3: "Adopt the development of an advanced technology, fully reusable single-stage-to-orbit rocket vehicle as an Agency goal." "It has the greatest potential for reducing annual operations costs as well as life-cycle costs ... it would place the U.S. in an extremely advantageous position concerning international competition and would leapfrog the U.S. into a next-generation launch capability."

As for commercial satellite launches, it was felt that even Option 2 would have difficulty competing against Ariane 4 and Ariane 5, identified as being "the most efficient of the foreign systems". "Option 3, on the other hand, would lower launch costs so dramatically that the U.S. industry could underprice all competitors. The U.S. would likely capture, and once again dominate, the international satellite launch market for a considerable period, utilizing these unique advanced technology vehicles."

Option 3 was recognised as presenting "moderate to high technical risk", but this "was felt to be manageable due to the 4 to 5 year technology maturation phase which would develop and demonstrate the needed technologies to at least a level 6 technology readiness level (proven in their operating environment)."

Outcome: Failure of hopes for SSTO Once the decision had been made to concentrate on SSTO, NASA lost interest in the HL-42 and, indeed, in the whole concept of launching a crewed spaceplane on top of an expendable launcher. The last HL-20 studies based at NASA's Langley Research Center had been done in 1991, and they were not continued.

NASA now took over the DC-X from the Pentagon and renamed it the DC-XA Clipper Graham. Two new projects were also started as technology demonstrators: the Orbital Sciences X-34 and the Lockheed Martin X-33. Once the technology had been proved, the next step would have been VentureStar, a commercial SSTO spaceplane.

However, the "4 to 5 year technology maturation phase" failed. The DC-XA was cancelled in 1996 after the prototype was badly damaged in a landing accident, and the X-34 and X-33 were both cancelled in 2001 after disagreements and technical difficulties. Work on VentureStar also ceased in 2001, effectively ending the push towards SSTO.

The HL-42 in Option 2

Option 2D launch system in detail Team 2 arrived at their conclusions after an exhaustive examination of many possibilities, starting with 84 vehicle families, narrowing this to 28, then to four, of which three included the HL-42 crewed spaceplane. Their final detailed recommendation (Option 2D in the Study) was as follows:

For 5 tonne to low Earth orbit (LEO) missions, continue to use the Delta II (considered reliable and good value for money). For 10 tonne to LEO missions, replace the Atlas II with a new launcher using one "low-cost, low-risk" Russian RD-180 engine on the first stage, and a newly developed Centaur upper stage with a single RL10 engine instead of two.

For heavier Titan IV or Shuttle-class payloads, retire the expensive Titans and develop a new two-stage expendable heavy launcher, with three Russian RD-180 engines in the first stage and a single J-2S in the second (which would in effect be an upgraded S-IVB stage). This new launcher would be able to lift about 38 tonnes to LEO without an upper (third) stage.

The heavy launcher could then carry:

the new 23-tonne single-engine Centaur upper stage plus a geostationary satellite or an interplanetary spacecraft; 8 tonnes of disposable adapter and escape system plus the 21-tonne fully loaded HL-42 to LEO; the disposable 7-tonne ESA-built ATV plus up to 30 tonnes of cargo or structure to the Space Station in LEO.

At this stage the ATV would have been a short, squat orbital tug with no storage space, similar to the Service Module section of the later operational ESA Automated Transfer Vehicle. This tug could propel: a Pressurized Logistics Module (PLM) with a large CBM-sized docking port; an Unpressurized Logistics Carrier (ULC) with fuel and gas tanks; complete new modules and truss structures for the Space Station itself.

The teams had been told to assume that maintaining a 4-person Freedom-class Station would need 70 tonnes of up-going cargo each year. In Option 2, most of this would be carried out in five ATV missions (three PLM and two ULC). Waste for disposal would be loaded into the ATV/PLM or ATV/ULC, which would then de-orbit and burn up on re-entry. If necessary, the ATV could also de-orbit whole damaged or obsolete Space Station modules or structures.

The launch system aimed for economy and efficiency, with common components (the RD-180 engines, the Centaur upper stage) used as far as possible. The ATV would have been provided at European expense in exchange for some use of the Space Station; in this cost-effective way, ESA could preserve a foothold in space after the cancellation of the Columbus program in 1991. Even the ATV launch fairing was borrowed from the Titan IV.

Six new vehicles or "program elements" would be required:

Notably, many of the Option 2 recommendations were put into practice in later years despite this option not being selected. Three of the proposed new vehicles were built and widely used.

How big should the HL-42 be? The down mass problem Team 2 gave a great deal of thought to the question of downmass, cargo that would have to be returned safely to Earth (mainly equipment and completed experiments). This presented no problem for Options 1 and 3; the Shuttle could easily carry down mass, for example, in a Multi-Purpose Logistics Module (MPLM) in its payload bay, and VentureStar would have a similar system. In Option 2D, however, all down mass would have to be carried in the HL-42, which would need to be larger than the HL-20 to give some cargo capacity and crew seats. But how much capacity (volume and mass) would be needed in practice?

Since Freedom existed so far only on paper, it was hard to estimate this because there was no practical experience. The Russian space stations had sent down hardly any return mass since the disposable Progress cargo modules were designed to burn up on re-entry, and the cramped three-person Soyuz re-entry modules had little room to spare. But Freedom and the ISS program were much more ambitious and envisaged returning large biological and especially industrial-processing user experiments regularly.