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. 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 an approach.
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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 March 2025, the Air Force stated that it planned to develop additional guidance on assessing implementation of MOSA in accordance with policy and statute and that it was in initial planning for a study that would address how to provide guidance for MOSA at Space Force offices. The Air Force estimated the actions would be complete in April 2026.
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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) |
DOD concurred with our recommendation. In March 2025, the Army stated that it planned to update relevant MOSA guidance and establish a process to evaluate programs' compliance with the guidance. The Army estimated the actions would be complete in October 2025.
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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 March 2025, the Navy stated that it had addressed the recommendation through issuance of its January 2025 MOSA Implementation Guidebook. However, while the guidebook does address the consideration of MOSA in selected acquisition documents, it does not address the steps Navy leadership needs to take during the review of acquisition documents to ensure that programs have addressed key MOSA planning elements required by statute, policy, and guidance, as GAO recommended. GAO will continue to monitor the Navy's efforts to identify and implement such review steps.
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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 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. We will continue to monitor whether this direction in DOD's transformation strategy improves coordination of MOSA implementation across programs.
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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. We will continue to monitor whether this direction in DOD's transformation strategy improves coordination of MOSA implementation across programs.
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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 March 2025, the Navy stated it had established formal MOSA working groups with representation across its systems commands, who work closely with each respective program executive office. It added that the Navy is in advanced stages of establishing a formal process for sharing MOSAs across programs, program executive officers and services. GAO has requested documentation of the Navy's actions but has not received it as of January 2026. Additionally, 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. We will continue to monitor whether this direction in DOD's transformation strategy improves coordination of MOSA implementation across programs.
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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 March 2025, 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 April 2026.
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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 U.S. Army will document the process in its MOSA implementation guides to ensure the U.S. Army has an enduring process in place. The Army estimated that related actions would be complete by September 2025.
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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 March 2025, the Navy stated it was working to identify support needed by the program offices, including training, and planned to work with systems commands and resource sponsors to meet program needs. The Navy estimated that related actions would be complete by December 2025.
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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 March 2025, DOD stated that on its Adaptive Acquisition Framework Document Identification website, MOSA is listed as a statutory requirement under the acquisition strategy for major systems and as a regulatory requirement for non-major systems. However, this website does not provide pathway-specific policy for implementing MOSA, such as the specific program stages where MOSA should be considered and the specific types of documents that should address MOSA. DOD stated it plans to update its MTA acquisition policy in the next update cycle. 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 March 2025, The Air Force stated that the Secretary of the Air Force would 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. The Air Force estimated the actions would be complete in April 2026.
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