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.
In 1988, the Shuttle II study was superceded by The Next Manned Transportation System Study (TNMTS).
Version 1 of the Civil Needs Data Base (CNDB) was used to define a baseline set of missions which would be utilized to screen configuration options.
Although the CNDB contained heavier missions, including 28 Klb polar platform deployment, and 39 Klb space station logistics module delivery, the baseline missions became the study "driver missions" for point designs and comparisons. Sensivity trades/options for heavier payload classes were conducted.
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 availability), and far-term (NASP derived) technologies were compiled and used to define single- and two-stage vertical takeoff rocket systems. These system designs also reflect 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, double-bubble propellant tank arrangements that provided a "flat-bed" area for canister mounting, etc. System gross liftoff weights (GLOW) for the range of technology levels assumed demonstrate SSTO vs. two-stage comparisons for these design assumptions. The technology level is represented as a composite percentage of dry weight reduction of all structural, propulsive, and subsystems using Space Shuttle technologies as a reference. These dry weight reductions have a ripple effect in sizing as lower dry weight in one area scales down landing gear, propellant tanks, engine weight, etc requirements. Thus, removing one pound of dry weight in a subsystem scales down the gross weight or vehicle dry weight by several pounds, the degree being related to where one is along the curves shown. Both curves steepen in slope as one moves back towards Shuttle technologies. The SSTO vehicle, in particular, becomes very sensitive to weight growth and may not even present a design solution (infinite weight) if the technology level is too low. On the other hand, as one moves to advanced technology levels (NASP-derived technologies) both systems have low weights. At sufficiently advanced tehnology levels, the dry weight of the SSTO may actually be less than a comparable two-stage system which has duplication of systems for the two stages.
Comparison of such vehicles by overall size for different technology levels also demonstrates the role technology assumptions play in vehicle design.
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 :
Shuttle II Orbiter
Shuttle II Booster
Shuttle II Overview - 1988
The reference manned, reusable Shuttle II booster-orbiter system, shown above, 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.
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 be 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:
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.
The best single reference for the Shuttle II activities conducted at VAB is found in:
Parametric Trade Studies on a Shuttle II Launch System Architecture by D. O. Stanley, T. A. Talay, R. A. Lepsch, W. D. Morris, J. C. Naftel and C. I Cruz. NASA TP 3059, March 1991.
Other references include:
Ted Talay, VAB/LaRC