Design Features
The HL-20 concept of the PLS is adaptable to several launch vehicle concepts. Titan III was an existing booster system which could have been used for unmanned prototype launches or, with modification, as a manned system. A future launch system option would have been the National Launch System under study by the Air Force and NASA in the 1990s. 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 HL-20 PLS concept was to complement the Space Shuttle with safe, reliable manned transportation at the lowest cost. Crew safety was of utmost importance with emphasis being given in the HL-20 design to launch abort situations and the protection of the crew during vehicle recovery. Other requirements had focused on minimizing life-cycle costs of the system by insuring simple operations, low-cost manufacturing, and high utilization potential. When not including the time of the mission, turnaround time was expected to be 43 days.
With an overall length of about 29 feet (9 m) and wingspan of 23.5 feet (7.2 m),the HL-20 would be a much smaller craft than the Space Shuttle Orbiter; it could fit within the payload bay of the Shuttle with wings folded. Projected empty weight of the HL-20 was 22,000 pounds (10,000 kg) compared to the Space Shuttle Orbiter's empty weight of 185,000 pounds (84,000 kg). The space available inside for the crew and passengers, although less than in the Shuttle, would be more than found in today's small corporate business jets.
An important aspect of the HL-20 PLS concept which would help insure low cost operations is its design for maintainability. Large exterior access panels permit easy access to subsystems, allowing easy servicing or replacement. The vehicle would be prepared in a horizontal position. Selection and design of subsystems would emphasize 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. The thermal protection system would be similar to the Space Shuttle's combination of tiles and ablative coatings, but the HL-20's much smaller size 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 preparation manhours to less than 10 percent of the Space Shuttle Orbiter's requirement.
The HL-20 PLS concept considered 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 to push the PLS away from the booster. The method is similar to that used during the Apollo program. Once at a safe distance, three emergency parachutes would open to lower the vehicle to a safe ocean landing. Inflatable flotation devices ensure that it rides high in the water, with at least one of two hatches available for crew emergency egress.
Read more about this topic: HL-20 Personnel Launch System
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