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An airbreathing spaceliner takes off to LEO with the assistance of a magnetically levitated and propelled sled. Routine, affordable access to space will be enabled by advances in propulsion and materials that increase operability and reusability toward the day of aircraft-like operations to space.
Defining the Parameters for Affordable Space Transportation
Edgar Zapata, NASA Kennedy Space Center and Walter F. Dankhoff, executive secretary, Space Propulsion Synergy Team
This paper was originally published in the November 1997 issue of Aerospace America, a publication of the American Institute of Aeronautics and Astronautics (AIAA), under the byline "MANAGEMENT".
The success of expanded human presence and commercial activity in space hinges on the affordability of future space transportation. Reducing space launch costs by factors of 20 to 50 over current systems is the key.
This requirement is well recognized. Current planning in space transportation development focuses on methods of reducing the cost of developing, building, buying and operating high-flight-rate launch systems, which is the primary goal of reusable launch vehicle (RLV) technology developments such as the X-33 and X-34.
But these systems strive for relatively modest gains in affordability. Looking beyond them to the day of truly cheap and routine access to space requires a different planning approach, to capitalize on what appear to be a broad range of abundant possibilities. This approach is embodied by bringing together diverse stakeholders in space transportation to construct a synergistic planning process.
A new planning approach
Conventional top-down planning involves multiple variables and interdependencies. Traditional variables may include functional requirements, long term needs, and short term constraints, for example: "Will it work?," "Will it be affordable?," "What resources are available?" Characteristically, short term constraints (sometimes called programmatics) become the planning drivers, and there is not enough focus and understanding of customers’ long term needs.
This planning approach was pioneered in the early 1990’s by an independent organization, the Space Propulsion Synergy Team. This team consists of members from the aerospace industry, government (NASA, the Dept’s of Defense and Transportation) and academia. Together they represent a broad cross section of expertise in space vehicles and propulsion, with diversified backgrounds in research and technology as well as transportation system development, production and operations. Having established this, the next step is to implement a consensus making process by developing criteria that are measurable a priori. Next, because systems having the best long term payoffs can lose support when constraints become tight, a dual prioritization approach must be implemented to balance long term strategic thrusts against current programmatic constraints. This process allows stakeholders to make decisions based on their individual project needs as well as their long term strategic goals.
The value of this dual prioritization approach is also visual; that is, there is an intuitive gain in first distinctly defining customer wants and then conforming them to meet resource constraints. This approach was successfully employed on the results of NASA’s 1994 Access to Space Study, and later was utilized in the planning and formation of the NASA Reusable Launch Vehicle (RLV) technology program. More recently it has provided a basis for the approach to the Highly Reusable Space Transportation System studies discussed below.
Answering the "what" and "how"
Defining customers needs in the launch business was recently implemented by the Commercial Space Transportation Study and is an ongoing function of the FAA Office of Commercial Space Transportation’s Commercial Space Transportation Advisory Committee. The attributes most wanted by the customers for tomorrow’s launch systems are low cost, a responsive spaceport, standard interfaces, and specific payload size and mass capabilities.
Knowing "what" is wanted, the next question becomes "how?" Having defined cheap access to space as a priority, the challenge then becomes one of achieving consensus on the strategy and tactics to get us there. Clearly, it is necessary to involve the "operator" as a stakeholder in the planning process. Coengineering between technology developers and operators is essential to future technology program planning and funding. This includes manufacturers and testers as well as the actual operators at the launch site.
Development of truly low-cost launch systems will require nontraditional planning approaches that place less emphasis on short-term needs
Defining those qualities that are desirable in future space transportation may seem a simple matter - low cost, higher flight rate, for starters. It is often incorrectly assumed that decision makers or designers have full understanding of what is meant by "low costs." It is often further assumed that there are ample methods available to prioritize technology and ideas for improvement.
Simple trades that compare a current system to a new one can easily be made to determine cost effectiveness. But while tactics and specific decisions may be made once a system is well defined, that is not the case in the early stages of technology planning, where system definition is not yet well established.
One approach to the lack of sufficient information in the early conceptual design phases is to determine the wants for future space transportation systems. U.S. launch industry wants cut across three basic program phases: research and development, acquisition, and operations. These wants are often referred to as "attributes." In operations, for example, attributes include affordability, dependability, responsiveness, safety, environmental compatibility, and public support.
But the task of defining wants does not end there. Priorities are next. Again, a structured process is available to provide the community of developers, designers, manufacturers, and operators with insights that are key to improvement. Prioritization involves not only the importance of the want but also requires the answers to two questions: Where are we today on this attribute? and Where do we want to go? The result is a weighted set of prioritized attributes.
The next task is to determine the measures that characterize the wants that have been defined. These measures constitute the path that connect where we want to go with how we can get there. The leap here is from the qualitative to the quantitative. Using relational matrices, groups of experts having relevant but diverse backgrounds establish a consensus among themselves as to the nature and quantitative scope of the defining characteristics for each of the prioritized attributes. The result is a prioritized understanding of the relationships between the design and the technology requirements of affordable space transports.
Traceable results are the goal of any planning process. Designs such as the X-33 advanced technology demonstrator already utilize features based on criteria that surfaced as key factors in this planning approach. For example, the growing trend in future systems to use more electric and fewer fluid systems was directly reflected as one of the top criteria of the planning process described herein-"to reduce the number of different fluids" in a system architecture. Basic demonstration becomes more focused and technology investment matures more rapidly when the correct generic drivers of cost in operational systems can be established early on.
The results of NASA’s two-year Highly Reusable Space Transportation (HRST) study will appear in these pages soon. The synergistic planning approach described here was used to provide independent strategic insight into the study.
The HRST study acknowledges the assumption that open ended growth in space activity can be enabled only by radically reducing the recurring costs of space transportation. RLV technology goals are to reduce recurring launch costs an order of magnitude below current levels, to $1000/lb of payload. The HRST study focuses on concepts and technologies that could achieve another order of magnitude decrease, to $100/lb of payload.
To reach this level of cost reduction, the concepts considered in the HRST study go beyond the well known single stage rocket principle to radically new concepts in propulsion, including magnetohydrodynamic and aerospike drag reduction techniques. Launch assist using maglev accelerators and other advanced concepts, is one of the variables given attention. While it is relatively easy to determine if investment in a particular technology will result in a positive yield, it is quite difficult to determine which investment combinations will result in the highest possible yield. Since resources are at a premium, this determination is what marks the difference between long-term success and mere marginal improvement.
"Aircraft-like operations" is the Holy Grail of space launch system technologists and operators alike. One of the synergistic concepts examined by the HRST study is the rocket-based combined cycle (RBCC). This propulsion concept represents a basic architectural departure from the rocket equation that governs conventional rocket powered launchers-in effect circumventing it.
In an RBCC-powered launch system, unlike a rocket propelled one, the propellant does not constitute the majority of the gross launch mass. In a single-stage-to-orbit rocket only about one-tenth of the vehicles gross liftoff mass is the vehicle itself (including the payload); the other nine-tenths is propellant. With airbreathing launch systems, on the other hand, much of the propellant is obtained from the atmosphere, allowing three times as much vehicle mass. That is, perhaps only seven-tenths of the gross liftoff mass would be required for propellant. Although this is not quite the "aircraft-like" attribute of, say, a 747 transport, it is in the right direction of improvement over nay single stage rocket.
Moving further toward the day of "aircraft-like" spaceliners, combined cycle airbreathers will render obsolete the nonpowered landing practice employed by current and near-term reusable launchers such as the Shuttle and X-33. The HRST study explored supercharged rocket-based combined-cycle concepts employing fans, sometimes called "synerjets," which allow fully powered landings.
As these new propulsion prospects become better defined, it is clear interactions are as important as the function being considered. Ground-to-flight vehicle interactions-the spaceliner and it’s spaceport-become critical. Technology and architecture interactions-propulsion to structure to thermal protection to Mach-number transition points-all become intertwined in complex relationships unique to airbreathing propulsion systems.
The question for the HRST study team then becomes "How can we understand the strategic connection between all these technology possibilities and goals such as $100/lb of payload recurring cost and what process can provide the proper insights?"
The strategic dual-axis approach just outlined was employed to identify the connection or path between the HRST goals and the multitude of technology options that could be pursued. One product of this work was a prioritized, consensus-built list of drivers oriented to the achievement of $100/lb of payload.
Among the top drivers identified by this planning process are a need for environmentally benign nontoxic systems, a need for greater design margins and the associated robustness they bring, and a need for more sophisticated health management systems. These benefits are measurable a priori as potentials in any launch-system concept.
Balanced against these drivers are the short-term constraints that may be used to assess, in effect, how close we are to having such systems available and how much effort is needed to make them so. Again, both long term user needs and short-term constraints are considered, but they are treated separately, which is a strength of this planning approach.
Any concept, of course, will have certain technologies associated with it. Airbreathers aiming for true aircraft-like operability will need characteristics that give it responsiveness, such as high operational reliability (high mean time between failures) and good maintainability (low mean time to repair).
In summary, the use of synergistic teaming by industry, government, and academia has played a significant role in planning and defining future space transportation concepts. Basic relationships between the qualitative "pie in the sky" goals we want and the quantitative understanding that will get us there are being addressed and understood. The "synergy" process described here brings together the diverse backgrounds required to see the whole picture: "where" we are in launch systems today and "what" we must do-and "how"- to improve them dramatically. Affordable, operable systems must go from being goals to being real. The "synergy" strategy, which teams industry, government, and academia in the planning and system definition process, can help move that process to its successful conclusion.