Weapon Systems Acquisition: DOD Needs Better Planning to Attain Benefits of Modular Open Systems
Fast Facts
A "modular open systems approach" allows DOD to easily add or replace weapon parts over time—similar to plug-and-play computer parts.
Deciding on this kind of approach early helps DOD save time and money on upgrades and maintenance later. DOD often cited short-term costs as a hurdle to this approach. But it didn't analyze whether those costs outweighed potential long-term benefits like faster upgrades.
Also, DOD doesn't consistently coordinate these types of design decisions across weapon programs—possibly missing opportunities to save money by sharing common parts.
We recommended that DOD improve how it uses modular open systems approaches.
The Army’s Future Long Range Assault Aircraft Is Using a Modular Open System Approach
Highlights
What GAO Found
A modular open systems approach (MOSA) is a strategy that can help the Department of Defense (DOD) design weapon systems that take less time and money to sustain and upgrade. Recent legislation requires acquisition programs to implement a MOSA to the maximum extent practicable. GAO found that 14 of the 20 programs it reviewed reported implementing a MOSA to at least some extent. Other programs cited barriers to doing so, such as added cost and time to conduct related design work. While a MOSA has potential benefits, it may also require programs to conduct additional planning, such as to ensure they address cybersecurity aspects related to a MOSA.
Potential Benefits of a Modular Open Systems Approach
However, none of the 20 programs GAO reviewed conducted a formal analysis of costs and benefits for a MOSA because DOD's policy does not explicitly require one. As GAO reported in March 2020, program officials often focus on reducing acquisition time and costs. Unless required to consider the costs and benefits of a MOSA, officials may overlook long-term MOSA benefits.
Further, most programs did not address all key MOSA planning elements in acquisition documents, in part, because the military departments did not take effective steps to ensure they did so. As a result, programs may not be well-positioned to integrate a MOSA into key investment decisions early in the life of the program. Also, DOD's process for coordinating MOSAs across portfolios does not ensure the level of collaboration needed to achieve potential benefits such as lower costs from using common components across programs.
The military departments are statutorily required to ensure availability of certain resources and expertise related to MOSA implementation. However, they have yet to assess their departments' MOSA needs or determine how resources should be aligned across their respective departments. Until they do this, programs risk having insufficient resources and expertise to achieve the potential benefits of a MOSA.
DOD has updated some acquisition and engineering policies and is drafting regulations and guidance to address MOSAs. But gaps remain that could hinder MOSA implementation. For example, DOD policy does not address how MOSA requirements apply to programs using the middle tier of acquisition pathway—those intending to complete rapid prototyping or fielding in 5 years or less.
Why GAO Did This Study
A MOSA enables weapon programs to better respond to changing threats by allowing them to replace components more easily. Further, a MOSA can help address concerns like high weapon system sustainment costs that GAO has reported on.
House and Senate reports include provisions for GAO to review DOD's use of MOSAs. This report assesses the extent to which (1) programs implemented MOSAs and why; (2) programs and portfolios planned for MOSAs; (3) the military departments invested in necessary resources for MOSAs; and (4) DOD developed policy, regulations, and guidance for MOSAs.
GAO reviewed planning documents for 20 acquisition programs that started after relevant laws were passed in 2016. GAO selected the programs based on their acquisition approach and military service. GAO also reviewed policy and guidance documents and interviewed DOD and military department officials.
Recommendations
GAO is making 14 recommendations to DOD, including that it develop a process to analyze MOSA costs and benefits; improve military department processes for ensuring quality MOSA planning documents and for coordinating MOSA implementation across programs; and address gaps in MOSA policy and guidance. DOD concurred with these recommendations.
Recommendations for Executive Action
| Agency Affected | Recommendation | Status |
|---|---|---|
| Department of Defense | The Secretary of Defense should ensure the Under Secretaries of Defense for Acquisition and Sustainment and Research and Engineering, in coordination with the Director, Cost Assessment and Program Evaluation, develop a method for program offices to use when assessing the costs and benefits of pursuing a MOSA on weapon systems. (Recommendation 1) |
DOD concurred with our recommendation. In March 2025, DOD stated that it had addressed the recommendation through its February 2025 MOSA Implementation Guidebook. The guidebook states that programs should employ a cost-benefit analysis that incorporates a thorough discussion of risks and potential impacts on program costs and schedules. However, the guidebook does not include a specific method for program offices to use to conduct such analysis, as GAO recommended. In June 2026, a DOD representative told GAO that the department is not currently pursuing MOSA cost estimation methods but rather is focusing on ensuring programs implement a MOSA as a core element of their contract solicitations. However, this approach does not ensure a program's MOSA is shaped by an understanding of the unique costs and benefits for the system in question. As GAO noted in its report, without developing a consistent method to compare MOSA costs and benefits over the life cycle to support early decision-making, DOD risks missing opportunities to field systems that are interoperable, adaptable to emerging threats, and less costly to sustain. GAO will continue to monitor DOD's efforts to develop such a method.
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| Department of the Air Force | The Secretary of the Air Force should ensure that the Assistant Secretary of the Air Force for Acquisition, Technology & Logistics and the Assistant Secretary of the Air Force for Space Acquisition and Integration identify steps needed during the review of relevant acquisition documentation to effectively determine whether programs have addressed key MOSA planning elements required by statute, policy, and guidance. (Recommendation 2) |
DOD concurred with our recommendation. In April 2026, the Air Force stated that it was partnering with offices within the Office of the Secretary of Defense on MOSA updates related to the 2025 Acquisition Transformation Strategy and that it planned to update the Air Force Guidebook for MOSA to align with these updates once completed. The Air Force estimated the updates to its guidebook would be completed by March 2027.
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| Department of the Army | The Secretary of the Army should ensure that the Assistant Secretary of the Army (Acquisition, Logistics and Technology) identifies steps needed during the review of relevant acquisition documentation to effectively determine whether programs have addressed key MOSA planning elements required by statute, policy, and guidance. (Recommendation 3) |
In October 2025, the Army released an update to its Modular Open Systems Approach implementation guide. The guide states it is for program managers and program executive officers responsible for providing oversight of MOSA implementation, among others. It includes an updated appendix with substantial additional detail about the acquisition documentation that is supposed to address MOSA. This information includes an explanation of what should be included in each document and hyperlinks to the relevant requirements for each document in statute, policy, and guidance. This updated guide will help enable Army leadership to ensure acquisition documentation addresses key MOSA planning elements.
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| Department of the Navy | The Secretary of the Navy should ensure that the Assistant Secretary of the Navy for Research, Development and Acquisition identifies steps needed during the review of relevant acquisition documentation to effectively determine whether programs have addressed key MOSA planning elements required by statute, policy, and guidance. (Recommendation 4) |
DOD concurred with our recommendation. In August 2026, a Navy official told us that the Navy has a process to ensure review of acquisition documentation for compliance with MOSA-related requirements. To formalize the process, the Navy is continuing to review all applicable policies and guiding documents and is working with Office of the Secretary of Defense teams on department-level documents that will affect the service-level ones.
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| Department of the Air Force | The Secretary of the Air Force should ensure PEOs within the department establish a formal process for coordinating MOSAs across programs to enable portfolio-wide benefits. (Recommendation 5) |
DOD concurred with our recommendation. In April 2026, the Air Force stated that it planned to develop guidance including establishing formal process(es) for coordinating MOSA across Department of the Air Force programs by March 2027.
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| Department of the Army | The Secretary of the Army should ensure PEOs within the department establish a formal process for coordinating MOSAs across programs to enable portfolio-wide benefits. (Recommendation 6) |
DOD concurred with our recommendation. In November 2025, the Department of Defense issued an acquisition transformation strategy. Among other things, the strategy called for the military departments to establish portfolio acquisition executives and transition all major acquisition activities to these individuals' portfolios within two years. These portfolio acquisition executives are directed by the strategy to maximize use of Modular Open System Architectures (MOSA) for development programs within their portfolios. In April 2026, the Army provided updated information stating that it planned to require portfolio acquisition executives to develop cross-portfolio MOSA governance processes and that it would update Army regulations accordingly by March 2027.
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| Department of the Navy | The Secretary of the Navy should ensure PEOs within the department establish a formal process for coordinating MOSAs across programs to enable portfolio-wide benefits. (Recommendation 7) |
DOD concurred with our recommendation. In November 2025, the Department of Defense issued an acquisition transformation strategy. Among other things, the strategy called for the military departments to establish portfolio acquisition executives and transition all major acquisition activities to these individuals' portfolios within two years. These portfolio acquisition executives are directed by the strategy to maximize use of Modular Open System Architectures (MOSA) for development programs within their portfolios. In August 2026, a Navy official told GAO that the Navy has created a working group across Navy portfolio acquisition executives and that the Navy is working to develop metrics and scorecards at the program office and portfolio acquisition executive level to better align efforts and measure progress. GAO will continue to monitor the Navy's efforts to implement this recommendation.
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| Department of the Air Force | The Secretary of the Air Force should identify the resources needed at the military department and program levels to implement a MOSA in all acquisition programs to the maximum extent practicable. (Recommendation 8) |
DOD concurred with our recommendation. In April 2026, the Air Force stated that it would continue to conduct assessments at the program level and across the Air Force to assess resourcing for MOSA to meet Air Force needs. The Air Force estimated that related actions would be complete by March 2027.
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| Department of the Army | The Secretary of the Army should identify the resources needed at the military department and program levels to implement a MOSA in all acquisition programs to the maximum extent practicable. (Recommendation 9) |
DOD concurred with our recommendation. In March 2025, the Army stated it would identify the resources needed at the department and program levels to implement MOSAs in all acquisition programs to the maximum extent practicable. Once resource requirements are identified, the Army will document the process in its MOSA implementation guides to ensure it has an enduring process in place. In April 2026, the Army provided an updated estimate of March 2027 for completion of actions needed to implement this recommendation.
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| Department of the Navy | The Secretary of the Navy should identify the resources needed at the military department and program levels to implement a MOSA in all acquisition programs to the maximum extent practicable. (Recommendation 10) |
DOD concurred with our recommendation. In August 2026, a Navy official told GAO that although the Navy has not received dedicated funding for department-wide MOSA implementation efforts, the Navy is continuing efforts to help identify the necessary funding and resources for MOSA implementation at the program and department level, including developing MOSA-related expertise to advise programs on implementation.
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| Department of Defense | The Secretary of Defense should ensure that OSD policy governing the MTA pathway provides direction for DOD acquisition programs to implement a MOSA to the maximum extent practicable and addresses related statutory requirements. (Recommendation 11) |
DOD concurred with our recommendation. In June 2026, a DOD representative told GAO that the department currently plans to transition away from setting MOSA requirements in policy. This representative stated that instead, the department's acquisition leadership will set a clear expectation that all programs implement a MOSA, and these MOSAs will be shaped by appropriate specialists without the need for additional policy requirements. GAO will continue to monitor DOD's efforts to address this recommendation.
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| Department of Defense | The Secretary of Defense should ensure that OSD guidance that fully addresses the statutory requirements to facilitate the implementation of a MOSA for any program prototyping, acquiring, or sustaining a new or existing weapon system is issued in an expedient manner. (Recommendation 12) |
DOD concurred with our recommendation. In February 2025, the Office of the Under Secretary of Defense for Research and Engineering issued a MOSA implementation guidebook that covers MOSA implementation across DOD's various weapon system acquisition pathways. This guidance will help ensure programs realize more of MOSA's benefits, like lower modernization and sustainment costs.
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| Department of the Navy | The Secretary of the Navy should ensure that department guidance for acquisition programs provides direction for Navy acquisition programs to implement a MOSA to the maximum extent practicable and addresses all statutory implementation requirements. (Recommendation 13) |
DOD concurred with our recommendation. In January 2025, the Office of the Assistant Secretary of the Navy (Research, Development and Acquisition) issued a MOSA implementation guidebook that covers MOSA implementation across the service's various weapon system acquisition pathways. This guidance will help ensure programs realize more of MOSA's benefits, like lower modernization and sustainment costs.
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| Department of the Air Force | The Secretary of the Air Force should ensure that department guidance related to MOSA covers all of the department's acquisition programs and provides appropriate information on MOSA considerations when developing capability documents and conducting analyses of alternatives. (Recommendation 14) |
DOD concurred with our recommendation. In April 2026, the Air Force stated that it was partnering with offices within the Office of the Secretary of Defense on MOSA updates related to the 2025 Acquisition Transformation Strategy and that it planned to update the Air Force Guidebook for MOSA to align with these updates once completed. The Air Force estimated the updates to its guidebook would be completed by March 2027.
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