By Jason Thomas
What is old is new again, and Modular Open Systems Approach (MOSA) is no different. MOSA is an acquisition and design approach consisting of a technical and business architecture that supports using system interfaces compliant with widely supported and consensus-based standards, to the extent that standards are available and suitable.1 Despite being a concept and approach important for defense systems for decades, adoption and implementation have been sporadic at best. There are many proposed benefits of implementing a MOSA, but program teams are often left with more questions than answers, including starting counts, what to include, and does it apply to their program. There needs to be a better understanding of the history of MOSA, and some common program challenges deserve to be addressed in detail. Forthcoming guidance from the Department of the Navy can help show the path forward.
The first use of “open standards” in defense acquisition was when the Undersecretary of Defense for Acquisition and Technology formally established the Open Systems Joint Task Force in 1994 via a memorandum.2 The memorandum highlighted the commitment to use performance and commercial specifications and standards, and wanted to further emphasize that commitment by using “open systems” specifications and standards in acquisition to the greatest extent practical.
A decade later, Department of Defense (DOD) Directive 5000.1, which provides management policies and procedures for managing all acquisition programs, had language included for MOSA.3 The directive stated that “Acquisition programs shall be managed through the application of a systems engineering approach that optimizes total system performance and minimizes total ownership costs. A modular, open-systems approach shall be employed, where feasible.” The National Defense Authorization Act (NDAA) of 2017 is where MOSA was formally established in law as a requirement for defense weapons systems. The language defined which systems were affected and at which acquisition milestones, the purported benefits of including MOSA as part of program development, sustainment, and fielded operations, and the important technical and business considerations.
The law also made an important wording change, namely that MOSA would be applied “where practicable” instead of the earlier guidance of “practical.” Practicability is not about want, desire, or whether it applies, but more on the feasibility of implementation. In 2019, the Service Secretaries jointly published a memo directing their respective Service Acquisition Executives to “publish specific implementation guidance” for acquisition programs, because victory in future conflict may heavily depend on sharing information across systems using standard interfaces.4
A Recent Resurgence?
Despite the previously posited benefits of MOSA, defense leaders find themselves reaffirming MOSA as a priority across the services.5 Recently, MOSA or its elements have been called out in recently proposed legislation. The Fostering Reform and Government Efficiency in Defense (FoRGED) Act identifies key sections for removal and modifications to facilitate the department’s ability to meet the intent of MOSA.6 Similarly, the Streamlining Procurement for Effective Execution and Delivery (SPEED) Act discusses modularity and faster acquisitions to meet emerging threats and address mission needs.7
In SecWar Pete Hegseth’s confirmation hearing, he emphasized the importance and need for modular system designs to address evolving threats, and emphasized the importance of having modular systems and the advantages they provide in upgrading and fielding capabilities quickly.8 With the recent announcement of the Air Force’s newest fighter aircraft, the F-47, the President, Secretary of War, and Air Force Chief of Staff all made comments about the adaptability of the platform throughout its service life.9
The benefits of MOSA are also being realized through the evolutionary tactics by Ukrainian forces in their ongoing war with Russia, where modular, open designs are enabling rapid adaptation on the battlefield.10 Lastly, the acquisition community is observing contract protests favoring requirement to include MOSA in proposals and contracts.11 Clearly, MOSA is not going away and appears to be growing in importance and popular backing.
Misconceptions Against Adopting a MOSA
If something is objectively better, then ideally there is little to prevent it from becoming the new standard way of doing business. But with any ‘new’ concept, some level of pushback is to be expected, particularly when the change is not well understood. There are common arguments and concerns on the applicability of implementing MOSA that need to be settled.
“Lifecycle Phase”
A common argument is related to the lifecycle phase the program is in or the size of the program, therefore saying MOSA is not applicable or practicable. “I’m too late in the lifecycle to implement a MOSA” is a common argument for a program to raise when discussing MOSA, and it is a fair one in certain cases. How does a program manager, engineering team, or other stakeholder inject a MOSA once the system is completed, tested, and fielded? What about the contract structure or negotiated data rights? The program must consider the mission objectives of their system, the technology improvements relevant to their system, all missions the system contributes to joint operations, and the contractual and sustainment posture over the course of the remaining life of the system.
The discussion surrounding this argument can be summarized in the “practical” vs “practicable” distinction. If a weapon system is complete and fielded, it should have a system-level architecture. The architecture will highlight key interfaces internally or externally. Analysis will report on how system requirements were verified and satisfied via measures of performance from testing based on that architecture. These are all fundamental building blocks for MOSA adoption because they each describe the system, the behaviors of the systems, and what is expected from each component. The program team should know the system well enough to anticipate when the inevitable service life extension or obsolescence challenges will arise.
Historical experience suggests that weapon systems are usually required to remain fielded beyond their initially intended service lives. The decision to extend the service life of a weapon system is dependent on the material condition of the system, its effectiveness, supportability, and the external resource constraints such as budget or personnel. Traditionally, each program or program office has a technology or capability roadmap that details capability enhancements from science and technology initiatives that are being tracked or planned to support growth and modernization. Service life modernization or pre-planned program improvements are an opportune time for the original fielded system to take advantage of a MOSA during those requirement engineering efforts. The laws surrounding budgets and continuing resolutions make MOSA more attractive since the inability to initiate new start programs plays a key role in system acquisition and ultimately delivered capability. Even if the program is late in its lifecycle, MOSA is often still practicable.
“MOSA will add cost to the Program”
The total cost of a major acquisition program is measured as either Average Procurement Unit Cost (APUC), which is the total procurement cost of a program divided by the number of units procured, or the Program Acquisition Unit Cost (PAUC) which adds in the costs of the Research, Development, Test, and Evaluation (RDT&E) for the program. Adopting a MOSA will not only drive down costs from reuse of the non-recurring engineering (NRE) design and development costs, but also the laboratory, integration, and operational testing. This can allow a program to separate functions into specific modules, and if they are reusing the technical details or the technical version is unchanged, there is a significant reduction in costs. Furthermore, by leveraging commercial technologies through the use of open, common standards, programs would be postured to be more proactive rather than reactive in how they plan for upgrades. One needs to consider the full lifecycle costs of how a program is managed, including contract actions, development timelines, and testing, and the burden they can put on resources if operating in a more reactive approach.
“We don’t have the resources to design MOSA in”
During the design and development of any weapon system, requirements definition, decomposition, and architectural design work are required. As a design matures, test events are conducted, planning and product support crystalizes, and these same skills are used whether a program adopts a MOSA or not. The first key difference is the level of engagement across the program team in executing a MOSA. Those adopting a MOSA benefit from a stronger alignment between the various disciplines, resulting in a more robust solution. Another added benefit is the enhancement of the skillsets developed by the workforce due to more rigorous design work and sustainment planning. Teams are required to evaluate more options, think more deliberately and understand the impacts of the various courses of action where they may not have had to consider if they were not adopting a MOSA.
The second main difference comes from the level of resources required after initially fielding the system due to capability needs, mission performance, obsolescence, or as other pressures manifest. Fundamentally, the analysis and work are no different if MOSA is “designed in” or not, and therefore a MOSA does not require additional resources. If a MOSA is adopted, resources are postured for efficient and effective use when upgrades or enhancements are sought or present themselves due to an active demand signal to the industrial base to address mission needs.
“My system architecture isn’t based on modularity”
This argument typically follows a presumption of not using one of the MOSA “enabling standards” such as the Future Airborne Combat Environment (FACE), Open Mission Systems (OMS)/Universal Command and Control Interface (UCI), Sensor Open Systems Architecture (SOSA), or something similar. Every system is developed with some level of architecture that identifies and articulates the system components, how they are organized, and how they relate to one another. Architectural views of a system and their artifacts for weapon systems are mandated in law and required for various technical and acquisition reviews to assess the maturity of the system as it develops Modularity, or components of a system built on modules, is inherent in nearly all systems. Circuit cards, avionics boxes, sensor components, software algorithms, wiring bundles, and more are all modular components within a system. Modularity is critical when failure analysis or supportability analysis are conducted and have a direct impact on system safety and maintainability. If a given system is not designed with open standards, that is a different argument, and one that must be supported in a business case analysis.
Affordability and capability over the lifecycle of a program are assessment criteria in technical and acquisition reviews and should be documented and well-reasoned. If a system is not modular today, it does not mean that will remain the case into the future. MOSA practicability must be addressed throughout the lifecycle.
“MOSA will delay my program”
When asked about how he would accelerate program timelines and reduce costs during his confirmation hearing, then-nominee Hegseth said he would focus on investing in the industrial base, understand the threats we are going to face in the future, and use “off-the-shelf technologies or standard designs [and] modular designs.” This “acquisition churn” is the result of many factors, but it is inherently easier to address with the proactive posture MOSA offers. Having a MOSA incentivizes the program to more proactively focus resources on delivering relevant and timely outcomes for the force. That focus prevents delay, rather than adds to the risk of it. Absent modular, open designs a program is more likely to become subject to frequent scrutiny from oversight bodies, which in turn requires the program team to spend more time justifying the program’s initial requirements and planning for how the program will address oversight concerns.
“MOSA is another unfunded requirement”
This argument and its cousin, “we didn’t generate a Program Objective Memorandum (POM) for it,” are arguably red herrings. MOSA is in law as a requirement for defense acquisition programs for consideration and assessment for practicability. The requirements are further codified in technical and acquisition assessment criteria as a weapon system matures and advances through the acquisition process. The perception that MOSA is something that is added onto the system to make it more expensive, instead of a fundamental design approach that ensures the program remains relevant, is a misconception that needs to be clarified with better awareness and education.
The Navy’s Approach to MOSA
MOSA is a strategic imperative for the Navy. As such, it is taking a holistic and enterprise view in strategy, planning, and implementation. The Navy has several focus areas for implementation and execution, including Education and Training, a Collaborative Digital Environment, Updating Policy, Processes, and Guidance, and Outreach and Engagement. These areas will culminate in a Department of the Navy MOSA Guidebook v2, expected to release in the coming months.
Updating Policy, Processes, and Guidance
The Navy is reviewing and making changes to all policy and guidance where MOSA is either identified or ought to be included to ensure programs are supported in their MOSA journey. While MOSA is identified in multiple acquisition and technical documents, the Navy is taking additional steps to provide better traceability and consistency across policy, and also in technical disciplines across the entire lifecycle and acquisition pathways. The Navy is ensuring that regardless of where a program is in its lifecycle, the discipline (i.e. engineering, logistics, program management, testing, budgeting, etc.), or the acquisition pathway (Major Acquisition Program, Middle Tier Acquisition, Software Pathway, etc.) the available guidance and support is consistent and understandable. Furthermore, by aligning the Department’s MOSA strategy with acquisition policy, the Navy will align the enterprise and maximize efficiency and effectiveness.
Education and Training
People are our number one advantage. The top priority is ensuring our talented workforce has a firm understanding of what a MOSA is, the planning and implementation considerations, and how it informs the way they design and deliver capabilities. This requires a review of current MOSA education coursework, professional material, and other media that support the DON’s desired end-state for a MOSA-savvy workforce. The DON is developing new methodologies for business case analysis for a MOSA to facilitate conversations across disciplines supporting our programs, as well as resource sponsors, policymakers, industry, and academia. Furthermore, education will cover opportunities for overlapping topics such as Intellectual Property and Data Rights, Right to Repair, Interoperability, Digital Engineering, and Reference Architectures.
In his book Legacy, James Kerr wrote, “the best teams have a culture of accountability.”12 Accountability surrounding MOSA will be enabled when all the requisite disciplines are involved, share the same understanding and playbook, have defined expectations, and are supported throughout the process.
Collaborative Digital Environment
The Navy has established a Collaborative Digital Environment to serve as the authoritative source for the Department of the Navy approach to MOSA. The environment sources policy and guidance documents, training content, terminology, standards, a repository for modular system interfaces, and other references to support programs so they can achieve their objectives relative to MOSA. This site is accessible to anyone with a common access card to facilitate learning and dissemination. The site is hosted in the same environment used for the Naval Integrated Modeling Environment, an enterprise service to enable integrated modeling.
Conclusion
MOSA can increase combat capability, mission effectives, and mission readiness. The Department of the Navy is taking an enterprise-wide, systems engineering view of modular open systems, where each program is better positioned because of the strategic investments made from those who came before. By adopting a modular open systems approach, we can rapidly field emerging technologies, enhance our industrial base, interoperate with our allies and partners, and strengthen deterrence. When a program adopts a MOSA, our adversaries must consider a wider range of capabilities they need to address. When multiple programs adopt a MOSA, U.S. collective strength increases supporting our goal of peace through strength.
The Navy is making a concerted effort to communicate with the myriads of stakeholders regarding its MOSA strategy. Regardless of their footprint in the industrial base, there is a place for them at the table. The message is clear – the Navy is fully embracing modular open systems in its weapons systems as a warfighting imperative.
Jason Thomas serves as lead for Systems Engineering for the Department of the Navy in the Office of the Assistant Secretary of the Navy for Research, Development, Test and Engineering. He is the Department’s lead for MOSA and is leading efforts across the service, and partnering with other services, OSW, and other stakeholders.
These views are presented here in a personal capacity and do not necessarily represent the official views of the DOW, its components, or the DON.
References
1. Undersecretary of Defense for Research and Engineering, “Implementing a Modular Open Systems Approach in Department of Defense Programs,” February 2025.
2. Under Secretary of Defense Memorandum, “Acquisition of Weapon Systems Electronics Using Open Systems Specifications and Standards,” November 29, 1994.
3. Under Secretary of Defense for Acquisition, Technology and Logistics, “DODD 5000.1: The Defense Acquisition System,” May 12, 2003.
4. Spencer R., Esper, M., Wilson, H., “Modular Open Systems Approaches for our Weapons Systems is a Warfighting Imperative.” January 9, 2019.
5. Briggs, Connie, “Prominent Leaders Queue for MOSA Summit 2025.” August 21, 2025. https://navalaviationnews.navy.mil/Editorial-Staff-Tools/Article-Submission/Article/4281635/prominent-navy-leaders-queue-for-mosa-summit-2025/
6. Fostering Reform and Government Efficiency in Defense Act of 2025, 118th Congress (2025). https://www.congress.gov/bill/118th-congress/senate-bill/5618/text/is
7. Streamlining Procurement for Effective Execution and Delivery Act, 118th Congress (2025). https://armedservices.house.gov/uploadedfiles/speed_act_full_text.pdf
8. Confirmation hearing transcript of Peter Hegseth, Senate Armed Services Committee, January 14, 2025, https://www.armed-services.senate.gov/imo/media/doc/01-14-25_nom-transcript.pdf.
9. “Air Force Awards Contract for Next Generation Air Dominance (NGAD) Platform, F-47,” Secretary of the Air Force Public Affairs, March 21, 2025, https://www.af.mil/News/Article-Display/Article/4131345/air-force-awards-contract-for-next-generation-air-dominance-ngad-platform-f-47/.
10. Bondar, K. (2025). Ukraine’s future vision and current capabilities for waging AI-Enabled autonomous warfare. https://www.csis.org/analysis/ukraines-future-vision-and-current-capabilities-waging-ai-enabled-autonomous-warfare
11. U.S. Army. (2025). Future Long-Range Assault Aircraft (FLRAA). In CRS Reports. https://www.congress.gov/crs_external_products/IF/PDF/IF12771/IF12771.7.pdf
12. Kerr, J. (2013). Legacy. Constable & Robinson.
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