The Shuttle II study began as an Agency activity in January 1985. Midway through the planned two-year effort, the Challenger accident occurred which reshaped the effort. Emphasis was placed on more immediate, near-term Shuttle replacements and a focus on safety and reliability issues. The National Aerospace Plane (NASP) and Advanced Launch System (ALS) studies were also underway. Thus, Shuttle II became viewed as one element of an overall architecture which included unmanned heavy-lift vehicles, a near-term expendable, and far-term replacement of Shuttle II by NASP-derived vehicles.
Shuttle II Orbiter
In 1988, the Shuttle II study was superceded by The Next Manned Transportation System Study (TNMTS).
Primary Requirements Determined During Study:
Requirements evolved during the course of the study. Meet civilian government, commercial needs, Mutlti-mission, flexibility and Low cost (Low investment, low dollars/lb, low dollars/flight).
20,000 lb (15’dia x 30′ volume) Space Station delivery (270 nmi, 28.5° inc) 12,000 lb (15′ dia x 30′ volume) Polar Platform servicing (150 nmi, 98° inc).
Initial Phase Vehicle Configurations:
In the initial phase of the Shuttle II study, a variety of configuration options were examined. These included single-stage and two-stage vertical takeoff rocket systems, an all-rocket horizontal launch SSTO, and single-stage and two-stage horizontal systems with mixed airbreathing and rocket stages. Preliminary analyses demonstrated the air-breathing systems to have higher dry weights than comparable all-rocket systems. The NASP studies were also expected to define far-term Shuttle replacements beyond the time-frame of the current study options. The focus of the study shifted to trades between near-term single vs. two-stage all-rocket systems.
The level of technology available at the time a new system begins developed has a pronounced effect on the vehicle designs. Specifical sets of Shuttle, near-term (1992), and far-term (NASP derived) technologies were compiled and used to define single- and two-stage vertical takeoff rocket systems. These system designs also reflected a particular set of design assumptions, e.g. the use of dual-fuel with separate LOX/LH2 and LOX/HC rocket engines, the use of externally-mounted payload canisters, and double-bubble propellant tank arrangements that provided a “flat-bed” area for canister mounting.
The SSTO core configuration was designed to satisfy the baseline space station mission (20 Klb). This SSTO had relatively little payload capability to a 98° inclination orbit. To capture the polar platform servicing mission would require a much larger SSTO vehicle. The Shuttle II study demonstrated, however, that relatively small levels of augmentation, in the form of expendable or reusable rocket strap-ons, significantly boosted payload performance. Small solids (approximately 11% by weight of the Shuttle Solid Rocket Boosters) permitted the required polar platform servicing mission.
To capture the polar platform deployment mission (28 Klb to polar orbit), a small reusable liquid (LOX/HC) booster was used to augment the SSTO. This low-technology glideback booster also provided the SSTO with a large space station payload delivery capability.
The augmentation approach used in the Shuttle II studies is not unlike the Ariane 4 expendable launch vehicle which can be launched without boosters or, for heavier payloads, can use 2 or 4 solid and/or liquid boosters, thus tailoring the launch system to meet the payload delivery requirements. For Shuttle II, a large percentage of the missions did not require strapon boosters. For the less frequent heavier payloads, the solid or liquid boosters could be utilized.
A brief look was made of the “Bimese” configuration, which essentially mated two nearly identical SSTO vehicles with one of the SSTO vehicles acting as a booster for the orbiter stage. This performance analysis indicated very large payloads could be orbited, well beyond the mission model requirements noted.
A two-stage, vertical takeoff rocket was sized to capture the polar platform servicing mission and baseline space station mission. The low-technology, reusable booster utilized LOX/hydrocarbon (HC) propulsion, crossfed propellants to the orbiter engines, and staged at Mach 3 to glide back to the launch site. The orbiter utilized LOX/LH2 and LOX/HC separate engine propulsion.
In early 1987, the two-stage, vertical takeoff rocket system was selected as the baseline option for further, in-depth Shuttle II studies. This selection was made for several reasons related to anticipated cost, cost risk, and availability.
The desire to examine a near-term Shuttle replacement dictated only those technologies that might reasonable be available by 1992, The significantly higher dry weight and gross weight of this design SSTO, The higher risk (greater sensitivity) of this design SSTO system to weight growth at this level of technology advancement, The desire to include significant operational features (e.g. full engine-out capability on each stage, full launch escape capability for passengers and crew, payload canister operations, switch from orbiter dual position nozzles to single position nozzles) that would have smaller weight growth impact on a two-stage system.
Shuttle II Booster
Shuttle II Overview – 1988
The reference manned, reusable Shuttle II booster-orbiter system was designed to perform priority, or sortie-class missions involving personnel transport, on-orbit servicing and repair, and transportation to and from orbit of high-valued payloads and supplies. Designing the booster-orbiter vehicle to carry 12,000-lb to polar orbit provided a capability of 37,000-lb to a space station in a 28.5° orbit.
As a result of various trade studies, this vehicle had a lift-off thrust-to-weight if 1.3, and a thrust split of 60 percent booster thrust and 40 percent orbiter thrust at liftoff. The system utilized parallel burn with crossfeed, which means all engines were firing at liftoff with the orbiter engines drawing its propellants from the booster. Crossfeed was shown to have large benefits in scale reduction over a non-crossfeed system. The booster staged at Mach 3 to glide back to the launch site. The orbiter, full of propellants at staging, then continued on to orbit. Staging at Mach 3 with glideback was not optimum from a dry weight point of view, but had the operational benefits of not requiring a thermal protection system or cruise back systems to return the vehicle to the launch site.
The rocket engines used in the Shuttle II study were based on the the results of the STME (Space Transportation Main Engine) and STBE (Space Transportation Booster Engine) studies performed at Marshall Space Flight Center. These studies examined operationally efficient reusable propulsion systems for next-generation space transportation systems. The Shuttle II booster used 6 methane fuel STBE-type engines. Methane was cited by the STBE engine study contractors as the fuel of choice since it was clean burning without the combustion instabilities associated with RP-type fuels. The orbiter used 5 hydrogen fuel STME-type engines.
Both the booster and orbiter had engine-out capability built in, which meant that a booster engine and an orbiter engine could both fail benignly at liftoff, and the vehicle could complete its mission.
The orbiter carried a full crew escape system in the form of a jettisonable crew cabin which would function as a recovery capsule complete with stabilization fins and parachutes. Incorporating this system into the design reduced the payload capability of the system by 12 percent.
Phased Approach Architecture:
During the later stages of the Shuttle II study, a scenario was developed that suggested how a Shuttle II development could be integrated within a space transportation architecture to satisfy national needs. The scenario was referred to as a “phased approach architecture” and included unmanned heavy-lift elements and an “assured access to space” element. The intent was to integrate systems into a common architecture and share launch sites, operational facilities, and workforce to reduce life-cycle costs.
A heavy-lift core vehicle element would be developed first, augmented by three solid rockets and providing up to 100 Klb to low-Earth orbit in the mid 1990’s.
The next step would be the Shuttle II glideback booster to replace the solid boosters for the heavy lift giving 150 Klb orbit capability by the late 1990’s. The core stage would also incorporate a recoverable propulsion/avionics module.
The Challenger accident in 1986 heightened awareness as to the reliability issue in space transportation. The challenge was to provide an assured human access to space if the Space Shuttle or Shuttle II were unavailable for whatever reason. This led to the inclusion of the Space Taxi and Recovery (STAR) vehicle launched by the heavy-lift core vehicle and available in the late 1990’s. The small STAR vehicle could be configured in a variety of mission roles including space station crew rotation and crew emergency return vehicle (CERV).
Finally, shortly after the turn of the century, the fully reusable booster-orbiter Shuttle II would have been introduced to gradually replace an aging Space Shuttle fleet.
A statement of Shuttle II operational groundrules and goals drove the system design and operational scenarios. Rocket system designs of the past have generally been performance driven because of restricted development budgets, the desire to maximize payload to orbit, or the exceptional mission needs. These usually penalize the operational characteristics of the systems with consequential increased operational costs. For this study, a design-for-operations approach was groundruled. Rather than designing the system for maximum performance and lowest dry weight, technology advantages were reinvested in designing the system for operations, reliability and safety . Often, these operational features necessitated a dry weight increase of the system – for the levels of technology assumed and particular system design, this forced the selection of a two-stage reusable system.
As an example, the orbiter stage was designed with tip fins and “double-bubble” propellant tanks which provided a “flat-bed” area for the mounting of a removable payload container system (PCS). The PCS concept was originally proposed in the FSTS study, but examined in detail in the Shuttle II study including a contractor study with Teledyne Brown Engineering to define system designs for a number of mission types — deployment, delivery, personnel tranport and servicing containers. A significant weight penalty was accepted for each of these container systems (aerodynamic fairings, structures, subsystems). The intent was to decouple the processing of vehicle and payload elements with assembly of the PCS to the orbiter late in the ground processing flow.
The ground processing concept for Shuttle II shows the horizontal processing of the booster, orbiter and payload containers in low-bay work facilities. The ground assembly procedure demonstrates the mating procedures envisioned for the processing flow. The low dry weights of the assembled vehicle allow it to be towed to the launch area eliminating the need for a mobile launch platform. At the launch pad a strongback system would raise the assembled vehicle to the vertical position before fueling begins. Minimal launch pad access and servicing are key factors in reducing ground turnaround times for the vehicle fleet. The ground processing timeline , based on an analysis of the turnaround workforce and time requirements for the system elements, shows how the 12-day turnaround goal is met. Following launch and mission completion both the booster and orbiter elements land at a runway near the processing facilities.
A number of technology needs were identified for the Shuttle II baseline configuration:
– Reusable aluminum cryogenic tanks Composite structures
– Advanced durable thermal protection system (TPS)
– Advanced carbon-carbon for high temperature areas
– STME and STBE Main Propulsion Rocket Engines
– Common cryogenic propellant
– OMS and RCS systems with no hypergolic propellants
– Electromechanical actuators
– Fault-tolerant subystems with built-in test equipment (BITE)
– Autonomous flight systems with adaptive flight control
– Advanced avionics Control-configured design
The primary structural technology assumptions for the booster and orbiter reflect the state-of-the-art of technologies that could meet the 1992 technology availability requirement to support an early 2000 initial operatioal capability for Shuttle II.
Transition to Advanced Manned Launch System Study:
In the spring of 1988, NASA began looking at Space Shuttle evolution as an alternative to a new, “clean-sheet” system such as Shuttle II. By the fall of 1988, the study was formalized as The Next Manned Transportation System (TNMTS) Study. The Shuttle II effort was renamed the Advanced Manned Launch System (AMLS). In addition to AMLS and Shuttle evolution, a third option – initially viewed as a “simple rugged people carrier” was included. It was subsequently named the Personnel Launch System (PLS). This three-pronged approach provided the opportunity to directly compare significantly different development options in satisfying future space transportation needs. During this study transition, the Shuttle II effort was also broadened to examine other “clean sheet” multi-stage design approaches. This included a smaller Shuttle II glider launched by a reusable booster and expendable core stage, the glider launched by a two-stage expendable, and a two-stage airbreather/rocket system that took off horizontally (a placeholder drawing of the vehicle was used until a system was defined). The AMLS section describes these latter Shuttle II studies as the study transition to AMLS.
Note: Although Langley through VAB was designated as lead for Shuttle II and AMLS and shared the lead for PLS efforts with Johnson Space Center, a multi-center team including Lewis, Kennedy, Ames and Marshall Space Flight Center provided major contributions in technology and operations aspects of these studies.
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