(Photo courtesy of NASA)

Small-Satellite Costs

The forces that drive the costs of today's small satellites are very different from the forces that drive the costs of all other satellites. NASA and DOD needed a new model to gauge small-satellite costs—and The Aerospace Corporation created one.

David A. Bearden

Highly capable small satellites are commonplace today, but this wasn't always the case. It wasn't until the late 1980s that modern small satellites came on the scene. This new breed of low-profile, low-cost space system was built by maximizing the use of existing components and off-the-shelf technology and minimizing developmental efforts. At the time, many thought that because of their functional and operational characteristics and their low acquisition costs, these small systems would become more prevalent than the larger systems built during the previous 30 years.

But exactly which spacecraft fell into the new category? A precise description of small satellites, or "lightsats," as they were also called, was lacking in the space literature of the day. The terms meant different things to different people. Some established a mass threshold (e.g., 500 kilograms) to indicate when a satellite was small; others used cost as a criterion; still others used size. Even scarcer than good descriptions of small satellites, however, were guidelines for cost estimation of small-satellite projects. Clearly, a more useful definition of small space systems was needed.

Dollars-per-kilogram comparison of DOD large satellites ($500 per kilogram), modern small satellites ($100 per kilogram), and traditional small satellites ($150 per kilogram). Data points for these three categories cluster differently, and regression analysis shows that each set of points determines a different cost-estimating relationship. This information confirms the need for a new model using contemporary small satellites as its basis.

By the 1990s, because of increased interest in small satellites for military, commercial, and academic research applications, the Air Force Space and Missile Systems Center (SMC) and the National Reconnaissance Office (NRO) asked The Aerospace Corporation for information about capabilities and costs of such systems. In response, Aerospace commissioned a study to compare cost and performance characteristics of small satellites with those of larger, traditional systems. Of specific interest was the ability to examine tradeoffs between cost and risk to allow assessment of how traditional risk-management philosophies might be affected by the adoption of small-satellite designs.

Estimating costs for small systems raised many questions. What parameters drove the cost of small satellites? Were traditional parameters known to drive the cost of large systems still applicable? How did small systems compare with large ones? Did small-satellite acquisition philosophies, which prompted reductions in levels of oversight, independent reviews, and paperwork, enable a reduction in cost-per-unit capability? What advantages might small satellites offer for rapid incorporation of new technologies? Could they help reduce the long development cycle for military space programs? Were small satellites really economical for operational applications, such as navigation and communication?

This graph compares the dollars-per-kilogram ratio for traditional NASA missions ($900 per kilogram) with the ratio as noted in NASA's faster-better-cheaper missions ($120 per kilogram). It's clear that the different sets of data points determine markedly different cost-estimating regimes.

These questions led to a series of studies on technical and economic issues involved in designing, manufacturing, and operating small satellites. The studies found that existing spacecraft cost models, developed during the previous 30 years to support the National Aeronautics and Space Administration (NASA) and the Department of Defense (DOD), were of limited utility because of fundamental differences in technical characteristics and acquisition and development philosophies between small-satellite and traditional-satellite programs.

This finding prompted NASA and DOD to seek cost-analysis methods and models specifically tailored to small-satellite programs. To meet this need, Aerospace eventually developed the Small Satellite Cost Model, a small-satellite trade-study software tool that captures cost, performance, and risk information within a single framework. Before looking at the development of Aerospace's trade-study tool, though, it will be valuable to backtrack to the late 1980s and review just exactly how small-spacecraft programs had been perceived.

Streamlined Development Activities

In the 1980s, the DOD Advanced Research Projects Agency and the United States Air Force Space Test Program served as the primary sources of funding for small satellites, which typically were used for technology experiments (see sidebar, The History Behind Small Satellites). The Space Test Program coordinated experimental payload flights for the Army, Navy, Air Force, and other government agencies. Reduced development complexity and required launch-vehicle size enabled affordable, frequent access to space for research applications. Relatively low acquisition costs and short development schedules also allowed university laboratories to participate, providing individual researchers access to space—a privilege previously reserved only for well-funded government organizations.

Small satellites were procured under a specifically defined "low cost" philosophy. They were smaller in size and were built with maximum use of existing hardware. A smaller business base (i.e., a reduced number of participating contractors) was involved in the development process, and it was perceived that small satellites represented a niche market relative to the more prevalent large systems. Mission timelines from approval to launch were on the order of 24 to 48 months, with an on-orbit life of 6 to 18 months. Launch costs, either for an existing dedicated small launcher or for a secondary payload on a large launcher, remained high, but developments such as the Pegasus air-launched vehicle and new small launchers (such as Taurus and Athena) offered promise of lowering these costs. Additionally, small-satellite flight and ground systems typically used the most mature hardware and software available to minimize technology-development and flight-certification costs.

A cost-percentage comparison that makes use of an older model and the updated dollars-per-kilogram relationships shown in previous graphs to estimate modern small-satellite costs. Each bar's height represents the percentage difference between a satellite's estimated cost and its actual cost. Thus for Clementine, with a percentage of 109 percent, the older model's estimate was twice the actual cost, and for RADCAL, with a percentage of 801 percent, the older model's estimated cost was nine times the actual cost. Because the estimates far outweighed the real cost in many cases, the chart illustrates the inadequacy of a traditional cost model for modern small satellites.

Emerging advances in microelectronics, software, and lightweight components enabled system-level downsizing. Spacecraft often cost more than $200 thousand per kilogram and could reach $1 million per kilogram with delivery-to-space costs included. With miniaturization, every kilogram saved in the spacecraft bus or instruments represented a possible saving of up to five kilograms in launch, onboard propulsion, and attitude-control systems mass. Reduced power demands from microelectronics, instruments, and sensors could produce similar payoffs. For interplanetary missions, reduced mass had the capability to produce indirect cost savings through shorter transit times and mission duration. All this downsizing eliminated the need for large facilities and costly equipment such as high bays, clean-room areas, test facilities, and special handling equipment and containers.

Engineering development units—prototypes built before the actual construction of flight hardware—were not built; instead a protoflight approach was favored, where a single unit served as both the engineering model and the actual flight item. Quality parts were used where possible, but strict adherence to rigid military specifications was avoided. Redundancy—the use of multiple components for backup in the event the primary component fails—was also avoided in favor of simpler designs. Designers relied on multifunctional subsystems and software to allow operational work-arounds or alternate performance modes that could provide functionality if something went wrong during a mission.

As a result of these unorthodox approaches that sought ways to save time and money, small-spacecraft programs came to be perceived as fast-paced, streamlined development activities. Dedicated project leaders with small teams were given full technical and budgetary responsibility with goals tailored around what could be done inexpensively on a short schedule. Fixed-price contracts became the norm, and requirement changes (and associated budgetary adjustments) were held to a minimum.

The Next Decade

With the advent of the 1990s came a movement toward realizing routine access to space. The development of a broad array of expendable launch vehicles provided increased access to orbit for many different kinds of payloads. Satellite programs attempted to incorporate advanced technology and demonstrate that fast development cycles, low acquisition costs, and small flexible project teams could produce highly useful smaller spacecraft. This different paradigm opened up new classes of space applications, notably in Earth science, commercial mobile-communications, and remote-sensing arenas.

System-level cost-estimating relationships that were developed for early versions of the Small Satellite Cost Model. The first cost-estimating relationships related total spacecraft bus cost to individual parameters such as mass, power, or pointing accuracy. These were the early predecessors of today's more sophisticated cost model that represents costs at the subsystem level utilizing a variety of cost drivers.

Small-spacecraft designers, in their quest to reduce costs through use of off-the-shelf technology, in many cases pioneered the use of microcircuitry and other miniaturized devices in space. Whereas small satellites had been unstabilized, battery-powered, single-frequency, store-and-forward spacecraft with limited applicability to operational space endeavors, the level of functionality achievable in small spacecraft took a dramatic leap forward in the early 1990s, mainly because of the availability of increased space-compatible computational power and memory. These advances led to the current rich suite of spacecraft bus capabilities and the large array of missions using small spacecraft (see Small-Satellite Database table).

The trend toward cost reduction and streamlined management continued to gain momentum with increased interest in small spacecraft from NASA and DOD. A shift in philosophy, where a greater tolerance for risk was assumed, was evident in programs like the NASA-sponsored Small and Medium Explorer Programs, the Ballistic Missile Defense Organization-sponsored Clementine, DOD-sponsored Space Test Experiment Platforms, and the Small Satellite Technology Initiative's Lewis and Clark, among others. The end of the Cold War (in 1991) and the drive toward reduced development and launch costs created a political and budgetary imperative where small satellites were viewed as one of the few vehicles available for science and technology missions.

In response to budget pressures and in the wake of several highly publicized lost or impaired billion-dollar missions, NASA's administrator Dan Goldin in 1992 embraced small spacecraft and promoted the notion of a "faster-better-cheaper" approach for many of NASA's missions. The programs implemented as a result of this tactic dictated faster and cheaper by specifying launch year and imposing a firm funding cap. These constraints laid the groundwork for what would become a decade of ongoing controversy about the definition and success of faster-better-cheaper.

The Need for a New Model

It was against this backdrop that Aerospace began collecting a large body of information concerning technologies and program-management techniques that affected small-satellite cost-effectiveness. The programmatic aspects of traditional satellite programs (e.g., long schedules, large amounts of documentation, rigorous procedures, and risk abatement) were known to dramatically affect cost. In particular, two distinct but interrelated factors drove the cost of the system: what was built and how it was procured. In many cases, how the system was procured appeared to be as important as what was procured because cost and schedule were dependent on the acquisition environment.

The cost modeling process. This is an ongoing iterative process that involves collecting data and performing regression analysis to arrive at cost-estimating relationships. The data are validated against actual program costs. The model is delivered to the users for trade analyses.

A study that compared spacecraft mass versus cost for traditional small spacecraft of the 1960s and 1970s, traditional large spacecraft of the 1970s and 1980s, and modern (post-1990) small spacecraft revealed two important messages. First, the modern small spacecraft differed dramatically from traditional large spacecraft as well as their similarly sized cousins of the past. It was postulated that the latter difference, as evidenced by cost reduction, was the result of a combination of new business approaches and advanced technology. Second, cost and spacecraft sizing models based on systems or technologies for traditional spacecraft were inappropriate for assessing modern small satellites.

This was an understandable departure from traditional-spacecraft cost trends. New developments in technology are often based on empirical models that characterize historical trends, with the assumption that future missions will to some degree reflect these trends. However, in cases where major technology advancements are realized or where fundamental paradigms shift, assumptions based on traditional approaches may not apply. It became clear that estimating small-system costs was one such case.

Early small-satellite studies showed that cost-reduction measures applied to small-satellite programs resulted in system costs substantially lower than those estimated by traditional (primarily mass-based) parametric cost-estimating relationships (equations that predict cost as a function of one or more drivers). The studies analyzed the applicability of available cost models such as the Air Force Unmanned Spacecraft Cost Model and the Aerospace Satellite Cost Model to predict costs of small satellites.

These cost models—based on historical costs and technical parameters of traditional large satellites developed primarily for military space programs—were found inappropriate for cost analyses of small-satellite programs. It became readily apparent in comparing actual costs against costs estimated by these models that a new model, dedicated to this new class of mission, was needed. Credible parametric cost estimates for small-satellite systems required new cost-estimating relationships derived from a cost and technical database of modern small satellites.

Three-dimensional cost-estimating relationships (CER). Later versions of the Small Satellite Cost Model used multiparameter cost-estimating relationships derived at the subsystem level. Emphasis was placed on a combination of mass- and performance-based cost drivers.

The Making of a Model

Developing a small-satellite cost model that related technical parameters and physical characteristics to cost soon became the primary objective of small-satellite studies. To accomplish this, a broad study of small satellites was performed, with emphasis on the following tasks:

• definition of small satellite and identification of small-satellite programs
• collection of small-satellite cost and technical data from the Air Force, NASA, and university, government laboratory, and industry sources
• examination of cost-reduction techniques used by small-satellite contractors and sponsors
• performance of parametric analysis to determine which factors should be used in the derivation of cost-estimating relationships by using best-fit regressions on data where cost correlation is evident
• development and validation of a cost model with parametrics and statistics; evaluation of the cost model by performance of cost and cost-sensitivity analyses on small-satellite systems under development
• creation of a corporate knowledge base of ongoing small-satellite activities and capabilities, technology-insertion opportunities, and project histories for lessons learned, systems studies, etc.
• maintenance of a corporate presence in the small-satellite community to advise customers about relevant developments
• development of a cadre of people with expertise and tools for continued studies of the applicability of small satellites to military, civil, and commercial missions

The cost-modeling process entailed aggressive data acquisition through collaboration with organizations responsible for developing small satellites. One unanticipated challenge was actually gaining access to cost data. Small-satellite contractors, in their quest to reduce costs, would often not be contractually bound to deliver detailed cost data, so in many cases costs were not available. Despite this difficulty, Aerospace collected data over a period of two to three years for about 20 small-satellite programs at the system level (i.e., total spacecraft or spacecraft bus costs only). From this initial database, analysts derived parametric costing relationships as a function of performance measures and physical characteristics. The model estimated protoflight development costs and cost sensitivities to individual parameters at the system level.

The model was of great value in instances where evaluations needed to be performed on varying proposals with differing degrees of detail or when limited information was available, as is often the case in an early concept-development phase.

The Second-Generation Cost Model

While initial system-level small-satellite studies were sponsored by DOD and internal Aerospace research and development, in 1995, the need to respond to increasingly frequent questions about NASA-sponsored small-satellite architectures and a need for refined small-satellite system analysis at the subsystem level prompted NASA to seek better cost-analysis methods and models specifically tailored to small-satellite programs. Consequently, NASA asked Aerospace to gather information regarding capabilities and costs of small satellites and to develop a set of subsystem-level small-satellite cost-estimating relationships.

To allow assessment of a complete spacecraft bus cost, Aerospace collected more data in order to be able to derive cost-estimating relationships for each of the spacecraft bus subsystems:

• attitude determination and control
• propulsion
• electrical power supply
• telemetry, tracking, and command
• command and data handling
• structure, adapter
• thermal control

Emphasis was placed on obtaining data on spacecraft bus subsystem characteristics. In addition to technical data, costs in the areas of spacecraft integration, assembly and test, program management and systems engineering, and launch and orbital operations were requested.

To gather information on the state of the industry as a whole, as well as specific data, analysts surveyed and interviewed contractors who build small satellites or provide small-satellite facilities (e.g., components, launchers). A cost and technical survey sheet was distributed to virtually every organization and contractor in the small-satellite industry. It was important to obtain information about mass, power, performance, and other technical characteristics because the development of credible subsystem-level cost analyses of small-satellite missions depends on the analyst's ability to relate cost to those characteristics. Programs either already completed or awaiting launch in the next year were targeted.

These selected screen shots from the Small Satellite Cost Model demonstrate the parametric cost-estimating model. The model is easy to work with and provides useful outputs such as cost probability distributions.

Because Aerospace operates a federally funded research and development center, it was in a unique position to receive proprietary data from private companies and enter it into a special-purpose database to support government space-system acquisition goals and provide value added to the industry as a whole. Proprietary information delivered to the corporation was treated in a restricted manner, used only for the purpose intended, and not released to organizations, agencies, or individuals not associated with the study team. The information was used exclusively for analysis purposes directly related to cost-model development. Only derived information depicted in a generalized manner was released, and the database itself has remained proprietary. In some cases, formal nondisclosure agreements between the companies and Aerospace were necessary to facilitate delivery of proprietary data.

After properly categorizing cost data, adjusting it for inflation, and breaking it out on a subsystem basis, analysts developed cost-estimating relationships for each of the subsystems, using a subset of the more than 70 technical parameters collected on each of the small satellites. The effort to develop a cost-estimating relationship for a small-satellite subsystem took full advantage of advanced developments in regression techniques. Choosing the proper drivers involved combining a knowledge of statistics, sound engineering judgment, and common sense. Graphics software tools assisted in the development of these cost-estimating relationships, enabling the analyst to view the shape of a function against its data points and to identify the function (whether linear, logarithmic, exponential, or some other form).

The end product was a set of subsystem-level bus-related cost-estimating relationships based entirely on actual cost, physical, and performance parameters of 15 modern small satellites. This was a major advancement over available tools for estimating small-satellite costs. Analysts also developed factors to use in estimating both recurring and nonrecurring costs of bus subsystems, to enable studies of multiple builds—such as the ones that are needed for constellations of small satellites. The cost-estimating relationships enabled the inclusion of cost as a variable in system design tools. They were also incorporated into a stand-alone, menu-driven computerized model that could be distributed to government organizations and private companies that contributed data.

Cost Model Leaves Earth Orbit

In 1996, NASA was moving to smaller platforms for planetary exploration. This movement afforded an important application for the Small Satellite Cost Model. Following well-publicized problems with the Galileo and Mars Observer spacecraft, there had emerged in the early 1990s a growing apprehension in the NASA planetary science community that opportunities for planetary science data return were dwindling. After Galileo was launched in 1989, the next planetary mission scheduled was Cassini, which would launch in October 1997 and begin returning data in 2003, a full six years after Galileo had stopped sending data. Since a steady stream of new data is important to maintaining a vigorous program of planetary and scientific investigation, the situation was naturally a cause for concern. Out of this concern emerged a new NASA small-spacecraft program called Discovery.

The Discovery program's primary goal was to conduct frequent, highly focused, cost-effective missions to answer critical questions in solar-system science. Formally started under NASA's fiscal-year 1994 budget, the Discovery program featured small planetary exploration spacecraft—with focused science goals—that could be built in 36 months or less and would cost less than $150 million (fiscal year 1992), not including the cost of the launch vehicle.

To apply its cost model to this new domain, Aerospace performed, in collaboration with Johns Hopkins University's Applied Physics Laboratory (JHU/APL), a cost-risk assessment of the Near Earth Asteroid Rendezvous (NEAR) mission. This mission, one of NASA's first two Discovery missions, was designed to leave Earth orbit on a trajectory to the near-Earth asteroid Eros. The study identified a number of limitations in applying the Small Satellite Cost Model to interplanetary missions. Out of this information came a concerted effort to gather data on small interplanetary missions to enhance the model. Analysts collected data on missions such as Mars Pathfinder, Lunar Prospector, Clementine, and Stardust, developing cost-estimating relationships appropriate to a Discovery-class mission. Less than a year later the model was again applied successfully to the Near Earth Asteroid Rendezvous spacecraft, demonstrating cost estimates within a few percent of the actual costs.

Once Aerospace demonstrated the ability to assess small interplanetary mission costs, NASA's Langley Research Center Office of Space Science asked the corporation to participate in the Discovery mission evaluation process. Aerospace evaluated 34 Discovery proposals submitted by government, industry, and university teams. These proposals included a wide variety of payloads (including rovers, probes, and penetrators)—more than 120 in all. The goals were to provide independent cost estimates for each proposal, identify cost-risk areas, determine cost-risk level (low, medium, or high) for each proposal, and evaluate proposals in an efficient and equitable manner. Five finalists were selected.

In 1997, as a follow-on to the successful Discovery mission evaluation, the NASA Office of Space Science asked Aerospace to assist in the selection of Small Explorer missions. This was a series of small, low-cost interplanetary and Earth-orbiting science missions designed to provide frequent investigative opportunities to the research community. Aerospace served on the Technical, Management, and Cost review panel. Fifty-two Small Explorer mission concepts were evaluated, from which two final missions were chosen.

NASA commended Aerospace for its work on Discovery and Small Explorer missions. Because of the work it had done on these programs, Aerospace was invited to participate in a National Research Council workshop, from which a report titled "Reducing the Costs of Space Science Research Missions" was generated. Aerospace was also invited to join the editorial board of a new international peer-reviewed technical journal (Reducing Space Mission Cost, published by Kluwer Academic Publishers) and to become a member of the Low Cost Planetary Missions Subcommittee of the International Academy of Astronautics Committee on Small Satellite Missions.

Examining the Faster-Better-Cheaper Experiment

Successful NASA programs such as the Mars Pathfinder and the Near Earth Asteroid Rendezvous mission effectively debunked the myth that interplanetary missions could only be accomplished with billion-dollar budgets. They set a new standard against which all later missions were not only forced to measure up but go beyond. Designers were asked to meet unrelenting mission objectives within rigid cost and schedule constraints in an environment characterized by rapid technological improvements, immense budgetary pressure, downsizing government, and distributed acquisition authority.

As a result of these constraints, NASA had greatly increased its utilization of small spacecraft to conduct low-cost scientific investigations and technology demonstration missions. The original tenets of the small-satellite paradigm, including low cost, maximum use of existing components and off-the-shelf technology, and reduced program-management oversight and developmental effort, had been applied to increasingly more ambitious endeavors with increasingly demanding requirements. This move had clearly benefited the scientific community by greatly diversifying the number and frequency of science opportunities.

A number of failed small scientific spacecraft, however, such as Small Satellite Technology Initiative's Lewis and Clark, and the Wide-field Infrared Experiment, fueled an ongoing debate on whether NASA's experiment with faster-better-cheaper missions was working. The loss of the Mars Climate Orbiter and the Mars Polar Lander within a few months of each other sent waves of anxiety throughout government and industry that the recipe for successful faster-better-cheaper missions had been lost. Impaired missions or "near misses," such as the Mars Global Surveyor, contributed to the debate as well, and many wondered whether programs currently on the books or late in development were too ambitious for the time and money they had been allotted.

At the heart of the matter was allocation of cost and schedule. Priorities had changed. During the last few years the traditional approach to spacecraft design, driven by performance characteristics and high reliability to meet mission objectives, had completely given way to developments dominated by cost- and schedule-related concerns. While it was readily apparent that the faster-better-cheaper strategy resulted in lower costs per mission and shorter absolute development times, these benefits may have been achieved at the expense of reduced probability of success. Some questions lingered. When was a mission too fast and too cheap with the result that it was prone to failure? Given a fixed amount of time and money, what level of performance and technology requirements would cause a mission to stop short of failure due to unforeseen events?

Risks often do not manifest ahead of time or in obvious ways. However, when examined after the fact, mission failure or impairment is often found to be the result of mismanagement or miscommunication in fatal combination with a series of low-probability events. These missteps, which often occur when a program is operating near the budget ceiling or under tremendous schedule pressure, result in failures caused by lack of sufficient resources to thoroughly test, simulate, or review work and processes.

Having maintained an extensive historical database of programmatic information on NASA faster-better-cheaper missions to support the Small Satellite Cost Model development, Aerospace was well positioned to examine the situation. With a decade of experience and more than 40 scientific and technology demonstration spacecraft flown, sufficient information existed for use in conducting an objective examination. To understand the relationship between risk, cost, and schedule, Aerospace analyzed data for missions launched between 1990 and 2000, using technical specifications, costs, development time, and operational status.

The study examined the faster-better-cheaper strategy in terms of complexity measured against development time and cost for successful and failed missions. The failures were categorized as partial, where the mission was impaired in some way; catastrophic, where the mission was lost completely; or programmatic- or launch-related, where the mission was never realized because of cancellation or failure during launch.

Cost and schedule plotted against a complexity index derived from performance, mass, power, and technology choices. The regression curves may be used to determine the level of complexity possible for a set budget or development time. Although the complexity index does not identify the manner or subsystem in which a failure is likely to occur, it does identify a regime by which an index calculated for a mission under consideration may be compared with missions of the recent past.

A complexity index was derived from performance, mass, power, and technology choices, as a top-level representation of the system for purposes of comparison. Complexity drivers (a total of 29) included subsystem technical parameters (such as mass, power, performance, pointing accuracy, downlink data rate, technology choices) and a few general programmatic factors such as heritage (reuse of a part that flew on a previous mission) and redundancy policy. The process used to estimate spacecraft complexity included the following steps.

• Identify parameters that drive or significantly influence spacecraft design.
• Quantify the parameters so that they can be measured.
• Combine the parameters into an average complexity index (expressed as a value between zero and one).

To determine whether the faster-better-cheaper experiment was successful, analysts plotted a comparison of complexity relative to development time and cost, noting failures. Some interesting trends emerged. Correlation between complexity, cost, and schedule was evident. A threshold, or "no-fly zone," was apparent where project resources (time, funds) were possibly insufficient relative to the complexity of the undertaking. While it is unknown whether allocation of additional resources would have increased the probability of success of a given mission, this much is clear: When a mission fails or becomes impaired, it appears that it is too complex relative to its allocated resources.

The observation of a correlation between cost and development time and complexity, based on actual program experience (i.e., actual costs incurred and development time required as opposed to numbers used during the planning phase), is encouraging because this model can be applied to future systems. The index may reveal a general risk of failure, but it won't necessarily specify which subsystem might fail or how it will fail. Nevertheless, it does identify when a new mission under consideration is in a regime occupied by failed or successful missions of the recent past. This process should allow for more informed overall decisions to be made for new systems being conceived.

Conclusion

In summary, early small-satellite studies showed that older cost-estimation models based on historical costs and technical parameters of large satellite systems could not be successfully applied to small systems. It was necessary to develop a model that would be tailored specifically to this new category of spacecraft. To this day, there remains no formally agreed-upon definition of "small spacecraft," although such spacecraft are typically considered to be Discovery-class in size or less (i.e., for interplanetary applications, they fit on a Delta II launch vehicle; for Earth-orbiting applications, they weigh less than 500 kilograms), and most are budgeted in the $50- to $250-million range.

Aerospace has been studying small satellites since 1991, and the main product of its ongoing work is the Small Satellite Cost Model (see sidebar, Missions Evaluated by the Small Satellite Cost Model). Based on actual physical and performance parameters of small Earth-orbiting and interplanetary spacecraft flown during the last decade, this software tool was developed to estimate the cost of a small spacecraft. It has addressed many of the questions that were originally raised about cost estimation for small systems. The model is used in assessment of small-satellite conceptual designs, technology needs, and capabilities, and it is continually updated to model state-of-the-art systems.

The Small Satellite Cost Model has developed through several generations, with additions to the database and improvements to the cost-estimating relationships serving as the primary drivers from version to version. Currently, the small-satellite database has evolved to include more than 40 programs. While initial small-satellite studies were funded by DOD and Aerospace internal funds in the early 1990s, the development of Small Satellite Cost Model version 1.0 was funded by NASA in 1995. The database and model

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