Model-Based Systems Engineering (MBSE) Explained

Model-based systems engineering (MBSE) is a methodology that focuses on creating and exploiting digital system and engineering domain models as the primary means of exchange of information, feedback, and requirements, as opposed to document-centric systems engineering. It involves the entire process of capturing, communicating, and making sure that all the digital models we use to represent a system are coordinated and maintained throughout the entire lifecycle of the system.

Prior to about 1990, a system engineering design was likely to be a bunch of related papers and documents containing drawings, diagrams, mathematical formulas, requirements, and other specifications for how the system would work. But around that time projects became too large to rely on disconnected documents. Problems included (among others):

• Upkeep of the design specification as the concept of the system evolved: If an engineer changed a single dimension on one document, they had to go through the entire stack of documents and make sure that same change was made in every other document that contained that dimension. If other copies existed, you could not be sure that the change was recorded in all copies.
• Human interpretation: It is entirely possible that two engineers could read the same sentence in the specification and come away with different meanings, given the ambiguities of human writing systems and understanding.
• Verifiability: How could you be sure that a calculation made on one page of the model was error-free before using it as input to another calculation further along in the process?

MBSE was developed to replace static documents with “intelligent” digital models that contain everything important about the system — the requirements, the architecture, and the interfaces between the pieces of the system. Instead of paper documents that were at best organized into folders, these digital models are connected by a “digital thread” that can be followed to understand the entire design.

The overarching systems architect model (SAM) serves as an “authoritative source of truth” for everyone working on the project. This digital model has a central location that can be accessed by all the engineers working on the project, but cannot be modified independently of others, preserving the single source of truth. Any changes made are automatically propagated throughout the model and checked for internal consistency and accuracy by the software.

The Core Components of MBSE

MBSE relies on three major components:

1. The SAM, in which the system being designed is represented by a series of connected block diagrams to describe the system’s physical and functional architecture. It also contains a comprehensive list of qualities that the system is required to have or functions that it is required to perform (the requirements). SAMs are created using specialized software programs and utilize purpose-built languages for describing system architectures.
2. Engineering simulation software. A SAM can be compared to a computer-aided design (CAD) drawing: It describes the system in detail, but there is no way to tell from the SAM alone if the system meets requirements. For this, the SAM must be coupled with engineering simulation. If the airplane that you are designing is required to withstand 6 Gs, then engineers need to run a simulation to tell them if the airplane that they are designing is up for the task. Because a complex system might require engineers to execute many different types of simulation solvers — structural, fluids, electromagnetic, embedded software, safety, cybersecurity, etc. — it is important to be able to use many different types of simulation tools.
3. A centralized computation center. Whether located somewhere on the company’s premises or in the cloud, this centralized computation center contains the SAM and the executable software. It performs all functions and stores all results of the MBSE process.

Achieving an MBSE Environment with Ansys Software Tools

In MBSE, the combination of the SAM, CAD, and computer-aided engineering (CAE) simulation tools create the “digital thread” that links all the models and engineering data together. This occurs early in the design cycle and continues to function through the entire operational life cycle of the product until it is retired from use. When changes are made, the digital thread ensures that updates to one model are automatically transferred to all the models in the system.

Ansys offers a cloud-based system architecture modeler (SAM) that is is built from the ground up to support real time collaboration and will feature close integration with other Ansys MBSE tools as well as a full featured API that allows it to be integrated with third-party tools such as Requirements Management tools and PLM tools. Ansys ModelCenter provides that connection between the engineering simulation software and the SAM, enabling engineers to virtually verify their designs. So, if you are working on a project that has digital models containing structural, fluids, electromagnetic, safety, and embedded software simulations, ModelCenter will coordinate the operation and data collection from Ansys Mechanical, Ansys Fluent, Ansys HFSS, Ansys medini analyze, Ansys SCADE, and any other simulation tool, and connect these simulations to the SAM to enable MBSE. Consistent with Ansys’ overall corporate strategy to provide an open ecosystem, ModelCenter is a vendor-neutral solution that can automate the execution of any simulation tool within a workflow, even those from other software vendors.

As the design cycle progresses and the product design is refined, engineers can use Ansys MBSE technology to assess whether the as-designed system meets the specified requirements, or whether changes have to be made. When changes are made to the requirements or the SAM, the entire procedure can repeat until results satisfy specifications, verifying that the design will work as intended throughout the product’s lifetime. Only after these requirements have been satisfied will the team build a physical prototype of the design to perform physical tests on it.

The value of MBSE is that it enables better decisions to be made throughout the design lifecycle, and as early in the design lifecycle as possible. The earlier a stakeholder can identify a problem, the easier and cheaper it is to correct.

MBSE provides value from requirements through to retirement by enabling a disciplined system engineering approach. The digital models can be inspected, verified, and validated against a single source of truth, ensuring the internal consistency of the models that improves delivery, product yield, and both the top and bottom line.

MBSE Examples: High-Performance Aircraft Design

As an example, imagine that the Air Force has put out a bid to design a new plane that can fly 3000 miles on a single load of fuel. You can design, build, and test the airplane the old way, or you can use a model-based approach.

If you decide to use an MBSE approach to design this complex new plane, you would first create a SAM  that documents all the important requirements for the airplane (including the requirement that it needs to be capable of flying 3000 miles on a single load of fuel). You would then use the SAM to describe the design that you intend to build, including the system architecture, the desired behavior, interfaces between system components, etc. At this point, you would use simulation to verify that the design that you described meets all the requirements (both physical and behavioral). Simulation would include fluids simulations for verifying the aerodynamics, structural simulations for verifying mechanical strength, electromagnetic simulations for verifying the functions of the communications devices, etc.

For more complex, mission-based applications, engineers can use digital mission engineering software to simulate the complex “system of systems” structure of modern aerospace and defense missions. Digital mission engineering combines digital modeling, simulation, testing, and analysis for aerospace, defense, telecommunications, and intelligence applications to evaluate mission outcomes at every phase of a system’s life cycle. These missions might involve hundreds of ground, air, water, or space-based weapon systems that all have to communicate and coordinate their actions. Digital mission engineering simulation enables engineers and military personnel to virtually execute a complicated mission using physics-based simulations, which gives them confidence in the mission.

In a real-world example, Lockheed Martin Space performed MBSE using Ansys ModelCenter to simulate the mission trajectory for the OSIRIS-Rex spacecraft, whose mission was to grab a sample of an asteroid in a “touch-and-go” operation. In October 2020, OSIRIS-REx successfully conducted this maneuver to collect at least 60 grams, a far larger sample size than any other previous sample retrieval mission. It is due to return the sample to Earth in 2023. 

“Automating and integrating the simulation into this system model allows the team to rapidly identify potential issues with changes to mission requirements, as well as perform continuous verification of requirements and mission design parameters throughout the lifecycle of the spacecraft ...” said Phathom Athena Donald, a Systems Engineer for Lockheed Martin Space.  “The overall improvement versus the original process was about a 7X speedup in turnaround time.”

U.S. Department of Defense Drives MBSE Adoption

By far the biggest boost to MBSE came from the U.S. Department of Defense (DoD), which issued DoDI 5000.02 on January 7, 2015. Enclosure 3, Section 9 of this document addresses modeling and simulation. The first line reads: “The Program Manager will integrate modeling and simulation activities into program planning and engineering efforts.” With this simple statement and the details that follow in DoDI 5000.02, the DoD essentially made MBSE a requirement for all proposals related to U.S. defense and weapon systems. This directive has made the A&D sector the leading proponent of MBSE, in large part as a way to avoid project delays and cost overruns.

Based on this DoD requirement, Northrop Grumman delivered an MBSE-based proposal in 2020 that won them the prime contract for the U.S. Air Force’s Ground Based Strategic Deterrent (GBSD) project. GBSD is designed to modernize the U.S. land-based nuclear infrastructure, replacing the aging LGM-30 Minuteman III intercontinental ballistic missile system (ICBM) with an integrated weapon system that will meet defense requirements through 2075.

But, while A&D is leading the charge, other sectors are quickly catching up. The automotive industry, in pursuit of solutions for complex autonomous driving challenges, is rapidly adopting MBSE also. As complexity increases in other sectors, such as semiconductors, medical devices, alternative energy, the smart grid, and 5G communications, among others, MBSE adoption will likely accelerate there, too.

Request the on-demand webinar: Digital Transformation: Connecting Your MBSE Models to Mission Outcomes to learn more.