HL-42 (spacecraft)

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. 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." 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. • 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. 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). The system was designed to be flexible. If experience revealed a need for extra down mass at some times, extra HL-42 flights could be scheduled, or the sequence could be changed. Since all three vehicles used the same new heavy launcher, this would cause minimum disruption to launch processing. Team 2 was very much aware that their minimal down mass provision might attract criticism: "The acceptability of this level of return (approximately 15 percent of delivered mass) represents an issue that should be addressed in the final Space Station Freedom logistics scenario." The actual downmass figures for its first four Commercial Resupply Services missions CRS-1 to CRS-4 were 0.9 tonnes, 1.4 tonnes, 1.6 tonnes, and 1.5 tonnes, over the three years from 2012 to 2014. This suggests that the HL-42 would indeed have had sufficient capacity, especially with three flights a year. It would also have given the down-cargo a gentler ride than the Dragon (1.5g deceleration compared to 3.5g for the Dragon) and a much more convenient landing on a runway (rather than the Pacific Ocean splashdown of the Dragon). Safety advantages of the HL-42 Since the Challenger disaster in 1986 it had been recognised that the Shuttle was not safe enough. The Study estimated its current "crew survivability" at 0.98. One of the aims of the Study was to increase this to 0.999. However, despite examining many possible Shuttle upgrades extending as far as 2030, Team 1 could not find a practical way of providing crew escape. The only effective way of making the whole system significantly safer would have been to upgrade the Shuttle's avionics and enable autonomous operation, to avoid risking lives on missions that were essentially just cargo delivery: "Providing additional crew escape capability was not recommended due to cost, weight, and center of gravity impacts, and technical risks. Several means to reduce costs further and increase flight safety were identified. One is an uncrewed orbiter, which would allow the flight rate to increase without impacting human safety ...". However, even if half the missions had been uncrewed, this would only have halved the risk and increased "survivability" to 0.99, still an order of magnitude worse than the target of 0.999. The general principle of improving safety by separating crew and cargo (not risking lives on cargo missions) had already been studied by NASA for several years before the study, and Option 2 was designed this way from the start. The HL-42 would only be flown when crew rotation was necessary. This also meant that the cargo-only hardware (ATV, PLM, ULC) did not need to be human-rated, making the whole system cheaper. Being much smaller than the Shuttle, the HL-42 could be mounted on top of its launcher, so on the pad and for the first minute of flight a simple launch escape system (LES) could "provide a high-thrust impulse to rapidly distance the HL-42 from a catastrophic booster event" and allow it to glide back to a runway near the launch-pad. To deal with a "catastrophic event" in the next few minutes of flight, there would be an option to fit the launch adapter (between the rear of the HL-42 and the top of the launcher second stage) with larger solid rocket motors, to provide a boost-back and a Return To Launch Site (RTLS) intact abort. If the accident happened beyond RTLS range, the LES would push the HL-42 clear and be jettisoned, and the HL-42 itself would glide back into the lower atmosphere. If no suitable long runways (possibly even at large commercial airports) were within reach, it would deploy parachutes and splash-down in the ocean. Since all the manoeuvring engines in the HL-42 itself used non-toxic methane fuel and liquid oxygen (rather than the toxic hypergolic fuels of the Shuttle) there would be no need for a runway "safing" procedure after any of these emergency landings, and less danger in a crash-landing. The HL-42 would have survived the circumstances that destroyed the Shuttles Challenger in 1986 and Columbia in 2003. In 1986 the LES would have pulled it clear of the booster explosion; and on top of its launcher it would have been safely out of the way of the falling debris that fatally damaged the thermal insulation on Columbia. These considerations led the study to conclude that Team 2 had met the safety target with the HL-42: "The improvement of crew safety (probability of crew survival) to at least 0.999 from the 0.98 of the Space Shuttle was met or exceeded by the new vehicles of Options 2 and 3." ==Design==

The HL-42 drew indirectly on three decades of experience with lifting body spaceplanes, but depended mainly on its immediate predecessor, the HL-20. Team 2 described the background of their design as follows: "The HL-42 design stems directly from the HL-20 lifting body vehicle concept under study since 1983 at Langley Research Center. It is a 42 percent dimensional scale-up of the HL-20, and retains key design and operational features of the HL-20 design. The applicable HL-20 design data base includes extensive NASA aerodynamic, flight simulation and abort, and human-factors research, as well as results of contracted studies with Rockwell, Lockheed, and Boeing in defining efficient manufacturing and operations design." However, there was one major difference between the Shuttle and the HL-42: the type of fuel. The Shuttle used the toxic and hypergolic monomethylhydrazine (MMH) and dinitrogen tetroxide (N2O4) for both OMS and RCS. In keeping with the new "faster, better, cheaper" airline-style operations principle, Team 2 decided to switch to methane (CH4) and liquid oxygen for both OMS and RCS. Finally, for both the Shuttle and the HL-42, as the airspeed dropped below Mach 3.5 with denser air and a lower angle of attack, the rudder would begin to meet the airflow and take over yaw control from the RCS. From here until landing the HL-42 would behave more like a normal aircraft, controlled mainly by elevons and rudder, with some roll control from the lower body flaps. Moving together at lower speeds, the upper body flaps could also assist the elevons in controlling pitch (at higher speeds, they would be out of the airflow and ineffective). After touchdown, the upper body flaps will be extended with the lower body flaps to serve as air brakes. However, in the following decades, some spacecraft engineers regarded hydraulic power as unnecessarily complex, unreliable, and hard to maintain. Even Team 1, in their list of possible Shuttle upgrades, had recommended replacing some or all of the hydraulic systems with electro-mechanical ones. One of their reasons was to simplify ground processing by removing the toxic hydrazine APU fuel, and this fitted in well with the new airline-style operations policy. However, the HL-42 electrical system would now have to cope with large and unpredictable surges in power demand during landing. It was therefore designed with two power sources. Normal baseline power was supplied by hydrogen–oxygen fuel cells as on the orbiting Shuttle; but the HL-42 also had rechargeable silver–zinc batteries to provide reserve power for short periods of very high demand. (This was the same combination of power sources used on the Apollo CSM.) Team 2 recognised that further development would have to be done on this two-source electrical system, particularly the procedures for switching reserve power in and out. They, therefore, made this the second of the five Advanced Development Tasks Required: "... actuators and their electrical power driving and switching systems must be matured, with emphasis on the power supply systems." Avionics Electronic hardware had advanced tremendously in the two decades since the Shuttle was first planned, and the HL-42 was designed to make maximum use of these advances. It could check itself out on the launch and then fly an entire mission autonomously, navigating by GPS and continuously monitoring its health. "Significant portions of mission operations will be accomplished through automatic systems. Launch, ascent, on-orbit operations, entry, and landing are automated and require no crew intervention, thus reducing cost by eliminating major requirements for facilities and crew training. ... Ground management of on-board systems will be reduced by automation and on-board vehicle health management. Trajectory and navigation management are decreased by using the Global Positioning Satellite system." Autonomous operation was a major departure from the philosophy of Apollo and the Shuttle, which had been designed as piloted vehicles. Flight control, guidance, and navigation had already been automated in other aircraft, but "on-board vehicle health management" would be more of a challenge; practical spaceflight experience had shown that a major part of the pilot's job was to decide which alarms or combinations of alarms could be safely or temporarily ignored, and which demanded urgent action given the overall situation. These decisions would now have to be written into the software. Testing and debugging this software would be a challenging task given that opportunities for actual flight-testing would be extremely limited (to hours or, for launch and re-entry, even minutes per year). Avionics hardware obsolescence would bring other problems. While it might seem logical to upgrade the electronics every five to ten years, all of the software would then have to be re-validated on the new faster hardware, causing operational delays and expense that might well outweigh the benefits of better hardware. Team 2 was well aware of all this and chose avionics and software as the first of the five Advanced Development Tasks Required: "avionics systems that can be upgraded, software that is automatically generated and validated, and the health management of in-flight functions." ==Development: Business-as-usual versus Skunk Works==

In the early 1990s a feeling began to develop among some engineers that NASA's culture had become too bureaucratic, with too much paperwork and too many middle managers, and that better results might come from a slimmed-down "Skunk Works" approach. Two of the Study teams shared this feeling: "The Options 2 and 3 teams recommended a streamlined management and contracting approach patterned after the Lockheed "Skunk Works," which features smaller, but dedicated and collocated government oversight, a more efficient contractor internal organization, rapid prototyping, and team continuity from design to flight." Team 2 in particular hoped for major cost savings from this approach. "The development of the HL-42 ... could use a "Skunk Works" type approach. This approach has been used successfully in major military programs such as the Hercules, U-2, and SR-71. In a study conducted on the HL-20 payload system by the Langley Research Center and Lockheed, it was determined that significant savings could be achieved using this approach. Based on those results, the new approach for the HL-42 ... could yield reductions as high as 40–45 percent in the total spacecraft development and production cost estimates, compared to the traditional "business-as-usual" estimates. Team 2 characterised "Skunk Works" development as including: "firm requirements, single management authority, small technical staff, customers on site, contractor inspections, limited outside access, timely funding, reports only important work, simple drawing release, rapid prototyping, etc." ==Routine operations==

In routine operations the HL-42 would be delivered to one of the three OPFs at Kennedy Space Center (KSC) to be prepared and loaded. It would arrive in flight-ready condition; all test and checkout procedures that would duplicate those already performed at the manufacturing facility would be eliminated. Once loaded with its flight cargo, it would move to the VAB to be rotated to the vertical and mated with the second stage at the top of the heavy launcher. After this it would check itself out using its autonomous systems "with minimum personnel time and in one to two shifts". The whole stack would then be moved to one of the two Launch Complex 39 pads as with the Shuttle. Exactly the same procedure would be used for cargo launches using the ATV. For Shuttle ground operations the ratio of support staff to those who actually worked on the vehicle (the "nontouch-to-touch" ratio) was six-to-one. For the HL-42, Team 2 hoped to reduce this to three-to-one, a ratio more typical of commercial airlines, thus halving not just salaries but also accommodation costs. KSC would handle all the launches, handing over to a small (10–12 consoles) mission control room at Johnson Space Center in Houston as soon as the HL-42 had separated from the launcher second stage. "Autonomous systems that had targeted the booster to the separation point would transfer control to the orbital vehicle's autonomous system. This system would calculate the orbital insertion and steer the vehicle to that position. The vehicle would than proceed to the next pre-defined phase of the mission. This sequence would continue until all the mission events had been completed. Ground monitors will have the capability to terminate any phase and re-initialize the autonomous flight system with new instructions." Since the 'crew' would now be just passengers, training could be greatly simplified and entirely simulation-based. "All training would be conducted in the central simulation facility. Training facilities should mirror flight control facilities for flight monitoring. The training facilities would be used to verify pre-flight analyses. The primary mode of training would be computer based. No motion based, fixed based, or flight aircraft facilities will be required." All these detailed plans, however, were shelved when the SSTO option was chosen in 1994. == Legacy ==

Situation in 2001–2004: Option 2 partially adopted By 2001 it had become evident that the SSTO Option 3 would be too difficult in practice (at least given the funding that Congress was willing to allocate) and the X-33, X-34 and VentureStar were cancelled in that year. Option 1, substantially upgrading the Shuttle system, had also been abandoned. The Study had shown convincingly that this could not be made cost-effective: "... it is clear that the major cost savings targeted as a goal for this study only accrue in architectures employing new vehicles." In practice, then, it was only Option 2 that was ultimately followed up, though not completely. Delta II was retained. Atlas II was upgraded with a Russian RD-180 engine and flew as the Atlas III in 2000. The expensive Titan IV would be retired in 2005 and replaced by a new heavy launcher introduced in 2004, although this new launcher would be the Delta IV Heavy (26 tonnes to Mir orbit), not the more powerful triple RD-180 version (38 tonnes to Mir orbit) proposed for the Option 2 system. With these upgrades the Atlas and Delta families would continue to launch American uncrewed spacecraft for some time to come; and the ESA ATV (launched on the European Ariane 5) would be ready to take over supplying cargo to the International Space Station three years before the Shuttle was retired. None of these vehicles, however, would be capable of ferrying crew to and from the ISS. Crewed spaceplanes not reconsidered Even though the problem of assuring post-Shuttle crew access to the ISS was now becoming more urgent, NASA did not revisit the Option 2 combination of a crewed spaceplane with an expendable launcher. The proposed X-38 Space Station 'lifeboat', while looking superficially similar to the HL-20, would have been ferried up as cargo in the Shuttle's payload bay, and used once or not at all; even this was cancelled in 2002. On the other hand, the military Boeing X-37, while operational from 2010, was much smaller (5 tonnes at launch), uncrewed, and never intended to support Space Station operations. NASA was able to reject all three of the options for post-Shuttle ISS crew access presented in the Study because a fourth option had recently become available: using the Russian Soyuz program infrastructure for all crew transport, a possibility that had not been considered in the Study. The fourth option: Soyuz–Progress In 1993, while the Access to Space Study was being created, several developments occurred in quick succession that would lead to greatly increased Russian cooperation with NASA. As a result, the status of Russian cooperation was still uncertain while the Study was being written between January 1993 and January 1994. The terms of reference allowed the authors to use Russian companies as equipment suppliers (notably for engines); but they were to plan for a 'worst case', and not rely on Roscosmos, the newly established Russian Federal Space Agency, for finance or services. Crew access was therefore assumed in the Study to be provided only by the US, Europe, Canada and Japan, the original Space Station Freedom consortium as it was in January 1993 when the Study was commissioned. Initially Soyuz–Progress was not considered reliable: "From the beginning, challenges arose with Russia’s participation. Many promises were made by high ranking Russian government officials .... Most were not kept. ... Russia’s ability to provide sufficient Soyuz 'lifeboat' spacecraft and Progress 'reboost' spacecraft also was questioned. Funding for Russia’s space program was under severe stress ..." The SLI was much less structured than the Access to Space Study with its three clearly defined alternatives. The SLI would start with "Hundreds of concepts"; then "In the program's first two years, a range of risk reduction activities and milestone reviews will gradually narrow viable reusable space transportation systems to two or three candidates." Hopes were high: "With new technologies and operations ... the cost of delivering a payload will drop dramatically from today's price of $10,000 per pound." HL-20 and HL-42 revival impossible under SLI These research priorities explain why the HL-20 and HL-42 programs were never revived by NASA. If even the SSTO X-33 (with its aerospike engine and innovative all-metal thermal protection system) was considered not cutting-edge enough without a composite tank, the HL-20 and HL-42 stood even less chance of being built with government money: • With their expendable launchers they were very far from bringing the desired tenfold reduction in launch costs; • They had been deliberately designed not to use any breakthrough technology; • Their job was already being done by Soyuz. In these circumstances there was no chance that they would be developed further by NASA. However, commercial space transportation companies would be quite free to develop the HL-20 and HL-42 designs if they wished; NASA now welcomed commercial participation. But companies doing so would risk facing competition from SLI itself. If NASA-funded research really did produce breakthrough technology with $1000 per pound launch costs (a tenfold reduction) then spaceplanes with expendable launchers could never be competitive. SLI discontinued in 2004 By 2004 it had become evident that NASA would never be given sufficient funds for the type of high-risk, high-return program advocated by Bekey: "well-funded parallel component developments", so that if some lines of advance failed, as they inevitably would, still one of them might succeed and bring immense rewards – perhaps even reducing costs to as little as $100 per pound. This is the combination that finally, in January 2016, won a six-launch Commercial Resupply Services contract with NASA. ==See also==