The HL-20 was a NASA Langley design for a manned spaceplane as a backup to the space shuttle (in case it was abandoned or grounded) and as a CERV (Crew Emergency Return Vehicle) for the Freedom space station. Also known as the ACRV (Assured Crew Return Vehicle) or PLS (Personnel Launch System). the HL-29 was a lifting body re-entry vehicle based on the Russian BOR-4 design. It was designed for two flight crew, eight passengers, and piloted landing at airfield on landing gear. The HL-20 was studied by contractors and a full size mock-up was built. However the design was not selected for further development. Soyuz was designated as the International Space Station CERV. When doubts about the availability of Soyuz developed in 1995, NASA proceeded with development of the X-38, a NASA Johnson concept - a smaller version of the X-24 lifting body with a parafoil.
During launch a fairing from the Titan IV booster to the spacecraft would have had solid rocket motors (154,000 kgf) for launch abort, with parachutes for a tail-down water landing.
In 1983, the Vehicle Analysis Branch began the investigation of the BOR-4 small spaceplane being orbited several times by the Soviets starting in 1982 and recovered in the Indian Ocean and Black Sea. During recovery operations of the space plane in the Indian Ocean, an Australian P-3 Orion aircraft obtained photographs of the vehicle both floating in the water and being hauled aboard the recovery ship. This provided valuable insights into the shape, weight and centre of gravity of the vehicle. Based on this information, small wind tunnels models were produced and tested in the NASA Langley wind tunnels. The results demonstrated the vehicle had good aerodynamic characteristics throughout the speed range from orbital entry interface to low supersonic speeds. Wind tunnel tests showed configuration directional stability at all speeds from subsonic to Mach 20, trimmed to maximum L/D with 10 degree elevon deflections in subsonic range, with no control deflection at Mach 0.6 to 0.9, at 3 degree angle of attack in transonic range, and then again with no deflection from Mach 2.0 to Mach 20. The Soviet design had a 2,040 km cross-range capability and an outstandingly benign thermal profile at peak heating conditions.
Langley Research Center (LaRC) continued to investigate the aerodynamic characteristics of this shape and examined some shape changes to improve the low speed aerodynamics from transonic down to subsonic speeds. LaRC personnel who had worked in the 1960's on lifting bodies, especially the HL-10, were available to conduct these aerodynamic and shape modification tests.
Crew Emergency Rescue Vehicle
As a result of the 1986 Space Shuttle Challenger accident, interest rapidly developed in developing a crew emergency rescue vehicle (CERV) for the proposed US/International space station. If the Shuttle was unavailable for use or station astronauts had to return to Earth in an emergency this would provide continued manned assured access to space. In late 1986, Langley began to study the use of the BOR-4 lifting body shape as a CERV. This involved internal layouts, weight estimations, and centre-of-gravity estimates for a vehicle of large enough scale to accommodate up to 8 space station crew members.
This concept, designated the HL-20, was designed for low operations cost, improved flight safety and conventional runway landings. The proposed Personnel Launch System (PLS), would utilise the HL-20 and an expendable launch system to provide manned access complementing the Space Shuttle. A full-size engineering research model of the HL-20 was constructed by the students and faculty of North Carolina State University and North Carolina A & T University for studying crew seating arrangements, habitability, equipment layout and crew ingress and egress. This engineering research model was 8.84 m long and provided the full-scale external and internal definition of the HL-20 for studies at the Langley Research Center.
The PLS mission was to transport people and small amounts of cargo to and from low-Earth orbit. Although not approved for development, the PLS was designed as a complement to the Space Shuttle and was considered for addition to the manned launch capability of the United States for three main reasons:
The two designs considered for PLS differed in their aerodynamic characteristics and mission capabilities. The Johnson Space Center's approach used a ballistic bi-conic design shape (L/D of .75 to 1.0) with a parafoil for a precision landing. The Langley Research Center's HL-20 design was a lifting body that could make a conventional runway landing on return from orbit.
History of Lifting-Body Research
Predating and influencing the design of the Space Shuttle, several lifting body craft including M2-F2, M2-F3, HL-10, X-24A, and X-24B were flown by test pilots during the period 1966-1975. The M2-F2 and the HL-10 were proposed in the 1960s to carry 12 people to a space station following launch on a Saturn IB. In the Soviet Union, the BOR-4 shape was developed and studied by Langley based on recovery photographs. The HL-20 PLS concept evolved from the BOR-4 with modifications based on experience with the early US shapes. The "HL" designation stands for horizontal lander, and "20" reflected Langley's long-term involvement with the lifting body concept, which included the Northrop HL-10.
Advantages of Lifting Bodies
A lifting-body spacecraft, such as the HL-20, would have several advantages over other shapes. With higher lift characteristics during flight through the atmosphere while returning from orbit, the spacecraft can reach more land area, and the number of available landing opportunities to specific sites would be increased. Loads during entry, in terms of g-forces, would be limited to about 1.5 G’s. This was important when returning sick, injured, or deconditioned Space Station crew members to Earth. Wheeled runway landings would be possible, permitting simple, precision recovery at many sites around the world, including the Kennedy Space Center launch site.
HL-20 PLS Missions
Delivery of passengers to Space Station Freedom would be the primary mission of a PLS. For the baseline space station mission, the crew size would be eight passengers (a space station crew) and two flight crew members.
A typical PLS mission operation scenario, using an HL-20, would commence at the Kennedy Space Center with the HL-20 being processed horizontally in a vehicle processing facility while an expendable launch vehicle is processed vertically in a separate facility. The launch vehicle and HL-20 would be mated at the launch pad and the launch sequence initiated as the space station passes over the launch site.
Following launch, the HL-20 would initially enter a low 185 km orbit to chase after the space station and then transfer up to the space station orbit altitude of 410 km. After rendezvous and docking at Space Station Freedom, crews would be exchanged, followed by a HL-20 return to Earth at the earliest opportunity.
The HL-20 would land horizontally on a runway in manner similar to the Space Shuttle. Total mission duration would not exceed 72 hours.
Other potential missions defined for a PLS included the orbital rescue of stranded astronauts, priority delivery and observation missions, and missions to perform satellite servicing. For these other missions, the basic HL-20 design would be unchanged, but interior subsystems and arrangements would be modified according to crew accommodations, duration, and equipment required for the particular mission.
HL-20 PLS Launch Vehicles
The HL-20 concept of the PLS was adaptable to several launch vehicle concepts. Titan III was an existing booster system which could be used for unmanned prototype launches but would require modification to be used as a manned system. A future launch system option at that time was the National Launch System under study by the Air Force and NASA. Choice of a launch system for the HL-20 PLS would depend both on the required date of initial PLS operations and the cost of booster development and launches.
The design philosophy of the HL-20 PLS concept was to complement the Space Shuttle with safe, reliable manned transportation at the lowest cost. Of utmost importance was crew safety with emphasis being given in the HL-20 design to launch abort situations and the protection of the crew during vehicle recovery. Other requirements focused on minimising life-cycle costs of the system by insuring simple operations, low-cost manufacturing, and high utilisation potential.
With an overall length of 8.84 m and span across the wing tips of 7.16 m, the HL-20 PLS concept would be a much smaller craft than the Space Shuttle Orbiter. In fact, the HL-20 could fit within the payload bay of the Shuttle with wings folded. Overall, the HL-20 would weigh 10,000 kg without crew compared to the Space Shuttle Orbiter's empty weight of 84,000 kg. The space available inside for the crew and passengers, although less than the Shuttle, would be more than found in today's small corporate business jets.
A very important aspect of the HL-20 PLS concept which would help insure low cost operations was its design for maintainability. Large exterior access panels permitted technicians easy access to subsystems which would be exposed and easily replaced if required. The vehicle would be processed in a horizontal position. Selection and design of subsystems would emphasise simplicity and reduce maintenance requirements. For example, hydraulic systems would be replaced by all-electric controls. Unlike the Space Shuttle, the HL-20 would not have a payload bay or main engine propulsion, thereby reducing the processing time. The thermal protection system would be similar to the Space Shuttle's, but the much smaller size of the HL-20 would result in major reductions in inspection and maintenance times. These design changes and subsystem simplifications, along with the adoption of aircraft maintenance philosophies, could reduce the HL-20 processing man-hours to less than 10 percent of that used for the Space Shuttle Orbiter.
The design of the HL-20 PLS concept had taken into account crew safety and survivability for various abort modes. The interior layout with a ladder and hatch arrangement was designed to permit rapid egress of passengers and crew for emergencies on the launch pad. For on-the-pad emergencies or during launch where time is a critical element (launch vehicle fire or explosion), the HL-20 would be equipped with emergency escape rockets that would rapidly thrust the PLS away from the booster. The method was similar to that used during the Apollo program. Once at a safe distance, a cluster of three emergency parachutes would open to lower the vehicle to a safe ocean landing. Inflatable flotation devices ensured that it rode high in the water, with at least one of two hatches available for crew emergency egress.
HL-20 Lifting-Body Research
A significant amount of research effort went into experimental and computational investigations of the baseline HL-20 shape. The goal was to amass a data base of information about the system to aid in management decisions for PLS development.
Using the extensive wind tunnel resources at Langley, researchers compiled a comprehensive aerodynamic and aerothermodynamic data base on the HL-20 concept spanning the entire speed range the PLS would fly through. Several models were built for testing in the various tunnels ranging from a 1.5 m model used for force and moment tests at low speeds to 15 cm models used in hypersonic tests. Results showed the shape possesses good flying qualities in all flight regimes. In addition to measurements of aerodynamic properties, experimental aerothermodynamic heating studies were performed. A new thermographic phosphor technique was used to study the heat transfer characteristics of a HL-20 model in high-speed wind tunnel tests. The model, coated with a phosphor, radiated at varying colour intensities as a function of temperature during test when illuminated by ultra-violet light.
Computational fluid dynamics (CFD) codes, which mathematically simulate the flow field in the vicinity of the HL-20, were also used at Langley. These advanced computational grid techniques were used in conjunction with wind tunnel tests to study patterns of flow field phenomena, shock waves, stability and control and heating on the windward and leeward surfaces of the vehicle. Such computational analyses become critical in regimes where wind tunnels cannot duplicate the entry environment. For example, heating in the flight environment on this concept was predicted to be within the limits of Space Shuttle-based high-temperature, reusable surface insulation (HRSI) everywhere except at the nose of the vehicle, where Shuttle-based carbon-carbon thermal protection was required.
Two of the major questions that had to be answered about the lifting-body concept were how to control the HL-20 during the high-heating portion of the re-entry from orbit, and what was the proper guidance scheme for use in the landing phase to enhance its flying qualities. Langley researchers used a six-degree-of-freedom trajectory analysis technique along with mass, inertia and aerodynamic properties of the vehicle to investigate the entry phase of flight.
Results showed that the concept could be controlled through the hypersonic entry using only 14 kg of reaction control thruster fuel in nominal cases, or less than 90 kg of fuel in cases where the vehicle centre of gravity was offset and the upper atmosphere density and wind profiles were off-nominal. The entry analysis also showed the effects of using the vehicle's aerodynamic surfaces in conjunction with thrusters for control purposes.
In addition to computer modelling of vehicle controllability during entry, a flight simulator was set up at Langley to permit pilots to study the final landing phase of flight. Starting at an altitude of 4,600 m, the simulation presented the pilot with a realistic view of the approach to a runway landing. Using a side-stick controller, pilots, including one who had flown the X-15 rocketplane and the lifting bodies, demonstrated this configuration to be controllable and capable of pinpoint landings.
In October 1989, Rockwell International (Space Systems Division) began a year-long contracted effort managed by Langley Research Center to perform an in-depth study of PLS design and operations with the HL-20 concept as a baseline for the study. Using a concurrent engineering approach, Rockwell factored supportable, efficient design and operations measures into a detailed, cost-effective design along with a manufacturing plan and operations assessment. A key finding of this study was the realisation that while design and technological factors can reduce costs of a new manned space transportation system, further significant savings are possible only if a new operations philosophy is adopted -- treating PLS in a manner similar to an operational airliner rather than a research and development space vehicle.
In October 1991, the Lockheed Advanced Development Company began a study to determine the feasibility of developing a prototype and operational system. The study objectives were to assess technical attributes, to determine flight qualification requirements, and to develop cost and schedule estimates.
A co-operative agreement between NASA, North Carolina State University and North Carolina A&T; University led to the construction of a full-scale model of the HL-20 PLS for further human factors research on the concept. Students at the universities, with requirements furnished by Langley and guidance from university instructors, designed the research model during their spring 1990 semester with construction following during the summer.
The human factors research objectives, using this model, were to assess crew ingress and egress operations, assess crew volume and habitability arrangements, and determine visibility requirements for the crew during critical docking and landing operations.
The testing, using Langley Research Center volunteers as subjects, was completed on the HL-20 model in December 1991. Langley volunteers, wearing non-pressurised flight suits and helmets, were put through a series of tests with the craft placed in both horizontal and vertical modes.
The horizontal study found, for example, that a 10-member crew had adequate volume to quickly and in an orderly manner get in and out of the spacecraft. The available volume and proximity to others was found to be more than reasonably acceptable for a 10-member crew. Recommendations for revision included: more side-head room for the last row of seats to accommodate those taller than 170 cm; a wider aisle; removable seats and more training could improve emergency personnel capabilities and performance; more downward viewing capability for the pilot; structural supports in the windows could reduce viewing; and the cockpit display and seat design must be integrated with window placement.
Testing the HL-20 in a vertical position as oriented for launch posed a new set of factors. Getting in and out of the spacecraft, for example, required climbing through a hatch and up or down a ladder. In the horizontal mode, crew members walked along an aisle leading through the tail, which would be the exit-entry path at a space station or on the ground after a runway landing.
Partial-pressure suits, borrowed from the Johnson Space Center in Houston, were used for part of the study. Participants noticed less head room and restricted movement with the bulkier and heavier suits.
The results of the human factors studies showed where improvements in the baseline HL-20 design were desirable. These improvements would have little impact on overall vehicle shape or aerodynamic performance.
In the end, space station Freedom became the International Space Station. As the initial crew emergency rescue vehicle, the Russian Soyuz spacecraft was selected. However NASA, looking for a higher-capacity alternative and concerned about reliable availability of the Soyuz in the future, did begin development of the X-38 CERV in 1997. The X-38 was however based on the Johnson concept of parachute-assisted landing, and used the pure-USA X-24 lifting body shape....
Crew Size: 10. Design Life: 3 days. Orbital Storage: 3.00 days. Length: 8.93 m (29.29 ft). Maximum Diameter: 7.16 m (23.49 ft). Span: 7.16 m (23.49 ft). Habitable Volume: 16.40 m3. Mass: 10,884 kg (23,995 lb). Crew mass: 1,270 kg (2,790 lb). Payload: 545 kg (1,201 lb). L/D Hypersonic: 1.80. Electrical System: Batteries. Associated Launch Vehicle: Titan 4.
Bibliography and Further Reading
© Mark Wade, 1997 - 2006 except where otherwise noted.
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