In our journey through systems engineering, we’ve already explored what a system is and the fundamentals of systems engineering ( See ‘Systems Engineering – Introduction‘). Now, let’s dive deeper into some key system concepts that form the backbone of systems engineering: the System of Interest, System Boundaries, External Systems, System Context, System Behaviour and System Structure. Understanding these concepts is essential for any systems engineer.

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System Of Interest (SOI)

The system of interest (SOI) is the central focus of a systems engineering effort i.e the system under consideration. It’s the specific system that we’re designing, analysing, or improving.

A crucial aspect of effective systems engineering is the clear delineation of the SOI’s boundaries. These boundaries encompass the full scope of the SOI, including all its components, subsystems, and interfaces.

Example: Consider a smartphone as the system of interest. This SOI would include:

• Screen,
• Processor,
• Battery,
• Operating System,
• Built-in Apps.

Example: Consider a commercial airliner as the system of interest. This SOI would include:

• Airframe structure
• Propulsion systems (engines)
• Avionics and flight control systems
• Environmental control systems
• Landing gear
• Fuel systems

Example: Consider a modern fifth-generation fighter aircraft as the system of interest. This SOI would include:

• Airframe with stealth characteristics
• Advanced propulsion system (e.g., turbofan engines with thrust vectoring)
• Avionics and flight control systems
• Weapons systems and internal weapons bay
• Sensor suite (radar, infrared search and track, etc.)
• Electronic warfare systems

System Boundaries

System boundaries are the conceptual limits that separate the system of interest from its environment. Defining system boundaries is a critical task in systems engineering:

1. Scope Definition: Boundaries clarify what is included within the system and what is considered external.
2. Interface Identification: They help identify where and how the system interacts with external entities or systems.
3. Responsibility Delineation: Boundaries help determine which aspects are under the control of the system engineers and which are not.
4. Complexity Management: By clearly defining boundaries, engineers can manage the complexity of the system under consideration.
5. Analysis Framework: Well-defined boundaries provide a framework for analysing system behaviour and performance.

For a fighter aircraft, system boundaries typically include:

1. Physical boundaries: The aircraft’s airframe, encompassing all onboard systems and components.
2. Functional boundaries: The limits of the aircraft’s operational capabilities and designed mission profiles.
3. Informational boundaries: The extent of data processing and decision-making performed by the aircraft’s onboard systems.
4. Operational boundaries: The designed range of environmental and combat conditions within which the aircraft can function effectively.

These boundaries help systems engineers focus on the elements they can directly control and design, while also identifying critical interfaces where the fighter aircraft must interact with external systems and its operational environment. Understanding and clearly defining these boundaries is crucial for effective systems integration, performance optimisation, and overall mission success of the fighter aircraft.

External Systems

External systems are those that interact with or support the SOI, but are outside its boundaries.

These systems play a crucial role in shaping the context in which the SOI functions. The relationship between external systems and the SOI is characterised by mutual influence, where external systems can affect the SOI’s behaviour and performance, and conversely, the SOI can impact these external systems. It is essential to note that these interactions occur within a specific context, which may vary depending on the operational scenario or phase of the system’s lifecycle.

For systems engineers, a thorough understanding of these external systems is paramount. This knowledge enables the design of a system that not only meets its internal requirements but also operates effectively within its intended environment, adapting to and interacting appropriately with the various external systems it encounters.

Example: Continuing with our smartphone example, external systems would include:

• Cellular networks
• Wi-Fi networks
• Cloud services
• Other devices for data transfer (like computers or smartwatches)
• App stores

Example: For our commercial airliner, external systems would include:

• Air Traffic Control (ATC) systems
• Airport infrastructure (runways, terminals, ground support equipment)
• Weather systems
• Satellite navigation systems (GPS)
• Maintenance and logistics systems
• Passenger booking and management systems

Example: For our fifth-generation fighter aircraft, external systems would include:

• Command and Control (C2) systems
• Air Traffic Control (ATC) systems
• Aerial refuelling systems
• Enemy air defence systems
• Friendly and enemy aircraft
• Satellite communications and navigation systems
• Ground-based maintenance and logistics systems

Deep Dive into External System

External Systems can be categorised as:

• Interfacing Systems
• Interoperating Systems
• Enabling Systems

Interfacing Systems

Interfacing systems are external systems that directly connect or communicate with the SOI, exchanging data/information, energy, physical or materials.

Examples:

1. Ground-based air traffic control (ATC) systems
2. Airborne tanker aircraft for aerial refuelling
3. Aircraft carrier catapult and arresting gear systems
4. Ground-based mission planning systems
5. Satellite communication networks

These systems are all external to the fighter aircraft but interface directly with it, providing crucial support, information, or resources necessary for its operation and mission success. They represent the broader ecosystem of systems with which a fighter aircraft must interact during various phases.

Interoperating Systems

Interoperating systems are interfacing systems that interface with the SOI in the operational environment of the SOI.

Examples:

1. Other fighter aircraft in a combat formation
2. Airborne Early Warning and Control (AEW&C) aircraft providing battlefield surveillance
3. Air-to-air refuelling tanker aircraft
4. Ground-based air defence systems providing cover for the fighter
5. Electronic warfare aircraft supporting the mission

These systems operate independently of the fighter aircraft but work in conjunction with it to accomplish broader mission objectives. They share information, coordinate actions, and provide mutual support, enhancing overall combat effectiveness and mission success.

Enabling Systems

Enabling Systems are external systems that support the SOI throughout its lifecycle but are not directly part of its operational environment. These systems facilitate the development, production, deployment, maintenance, and retirement of the SOI. Some enabling system interface with the SOI, and some do not.

Examples:

1. Flight simulators for pilot training
2. Maintenance hangars and repair equipment
3. Fuel storage and refuelling trucks at airbases
4. Spare parts manufacturing and storage facilities
5. Transport aircraft for deploying fighters to distant locations

These enabling systems are crucial for supporting the fighter aircraft throughout its entire lifecycle, from initial concept and design through to eventual retirement. While they don’t directly participate in combat operations, they are essential for ensuring the fighter’s readiness, performance, and longevity.

System Context

In systems engineering, the system context refers to the broader environment in which a system exists and operates. It encompasses all external factors, conditions, and entities that interact with or influence the system of interest. Understanding the system context is crucial for several reasons:

1. Defining Scope: It helps in determining what should be included within the system and what should be considered external.
2. Identifying Interactions: It reveals how the system interacts with its environment, including inputs, outputs, and constraints.
3. Understanding Influences: It highlights external factors that may affect the system’s performance, behaviour, or effectiveness.
4. Risk Assessment: It aids in identifying potential risks or opportunities arising from the system’s environment.
5. Requirements Elicitation: It assists in deriving and validating system requirements based on contextual needs and constraints.

The system context extends beyond the operational environment. It encompasses all external systems and entities that interact with or support the system throughout its entire lifecycle. This includes development, testing, deployment, operation, maintenance, and decommissioning phases. By considering this broader context, systems engineers can develop more effective, integrated, and sustainable solutions.

For a fighter aircraft, the system context might include:

1. Operational Environment: The physical conditions in which the aircraft operates, such as altitude, weather, time of day, and geographical location.
2. Mission Profile: The specific objectives and requirements of the mission, whether it’s air superiority, ground attack, reconnaissance, or interception.
3. Threat Landscape: The nature and capabilities of potential adversaries, including enemy aircraft, air defence systems, and electronic warfare threats.
4. Technological Ecosystem: The network of supporting systems and technologies, such as satellite communications, air traffic control, and allied forces.
5. Regulatory Framework: The rules of engagement, international laws, and operational guidelines that govern the aircraft’s use.
6. Logistical Support: The infrastructure and resources available for maintenance, refuelling, and rearming.
7. Human Factors: The capabilities, limitations, and interactions of pilots, ground crew, and command personnel.
8. Interoperability Requirements: The need to work in conjunction with other military assets, both air and ground-based.
9. Political Climate: The geopolitical situation that may influence the aircraft’s deployment and operational constraints.
10. Disposal considerations: Recycling facilities, environmental regulations.

Understanding the system context is crucial for systems engineers as it helps define the boundaries of the system of interest, identify relevant external systems, and anticipate the range of conditions and scenarios the fighter aircraft must be designed to handle. This comprehensive view ensures that the aircraft is not only technically advanced but also practically effective in its intended operational environment.

System Behaviour

System behaviour describes how a system responds to various inputs, conditions, and stimuli. It’s the observable actions and reactions of the system.

Example: For our smartphone:

• Input: User taps the screen to open an app
• Behaviour: The system processes the touch input, loads the app from memory, and displays it on the screen
• Output: The app is opened and ready for use

This behaviour involves multiple subsystems working together (touch sensor, processor, memory, display) to produce the desired result.

Example: For our commercial airliner:

• Input: Pilot adjusts throttle for takeoff
• Behaviour: The engine control systems increase fuel flow, the engines produce more thrust, the flight control surfaces adjust, and the aircraft accelerates down the runway and lifts off
• Output: The aircraft transitions from ground to air, climbing to its initial assigned altitude

This behaviour involves multiple subsystems working in concert (engines, flight controls, hydraulics, avionics) to produce the desired result of safe takeoff.

Example: For our fifth-generation fighter:

• Input: Pilot receives radar warning of incoming enemy missiles and initiates evasive action
• Behaviour: The threat detection system classifies the threat, the electronic warfare suite begins jamming, chaff and flares are automatically dispensed, and the flight control system executes a high-G evasive manoeuvre
• Output: The fighter successfully evades the incoming missiles, maintaining its combat effectiveness

This behaviour involves multiple subsystems working in concert (threat detection, electronic warfare, countermeasures, flight controls) to produce the desired result of threat evasion and aircraft survival.

Deeper Dive into System Behaviour

Attributes & Processes

Attributes are inherent characteristics or properties of a system, while processes are sequences of actions or operations that transform inputs into outputs within the system.

Attributes describe what a system is or has. They can be physical (like size, weight, or colour) or abstract (like reliability, scalability, or efficiency). Attributes often determine how well a system can perform its intended functions (Airspeed).

Processes, on the other hand, describe what a system does. They involve the transformation of inputs into outputs, often utilising the system’s attributes.

In systems engineering, understanding both attributes and processes is crucial for system design, analysis, and optimisation.

For example, in a manufacturing system, attributes might include production capacity and equipment specifications, while processes would involve the steps from raw material input to finished product output.

Dynamic Behaviour

Dynamic behaviour refers to how a system changes and responds over time, often in reaction to various inputs, stimuli, or changing conditions.

Dynamic behaviour is a fundamental aspect of many systems, especially complex ones. It involves understanding how a system evolves, adapts, and responds to both internal and external factors over time.

In systems engineering, analysing dynamic behaviour is crucial for predicting system performance under various conditions, designing control systems, and ensuring system stability. It often involves mathematical modelling, simulation, and real-time monitoring.

For instance, the dynamic behaviour of an economic system might include how it responds to policy changes, market fluctuations, or external shocks over time.

Emergent Behaviour

Emergent behaviour refers to properties, behaviours, or patterns that arise from the interactions between system components but are not predictable from the properties of individual components alone.

Emergence is a key concept in systems thinking. It highlights that a system can exhibit behaviours or properties that are more than just the sum of its parts. These emergent behaviours often arise from complex interactions and can be both beneficial and problematic.

In systems engineering, understanding and anticipating emergent behaviour is crucial, especially in complex systems. It requires a holistic approach to system design and analysis, considering not just individual components but also their interactions and the system as a whole.

An example of emergent behaviour is the formation of traffic jams in highway systems. While individual drivers follow simple rules, the collective behaviour of many vehicles can lead to complex traffic patterns that weren’t explicitly designed or easily predictable from individual behaviours.

System States & Modes

System states refer to the condition or configuration of a system at a particular point in time, while modes are predetermined sets of states or configurations that a system can operate in.

System states represent the values of all relevant variables that describe a system at a given moment. These could include things like on/off status, current temperature, or processing load. States can be discrete (like on/off) or continuous (like temperature).

Modes, on the other hand, are predefined operational configurations that a system can enter. Each mode typically corresponds to a set of system states and behaviours designed for specific purposes or conditions. Modes help manage system complexity and adapt to different operational requirements.

In systems engineering, understanding states and modes is crucial for system design, control, and fault management. It involves defining allowable states, designing transitions between states and modes, and ensuring the system behaves correctly in each mode.

For example, a spacecraft might have different modes such as launch mode, cruise mode, and landing mode, each with its own set of system states and behaviours optimised for that phase of the mission.

Systems Structure

Systems Structure refers to the organisation, arrangement, and interrelationships of the components that make up a system.
In systems engineering, understanding the structure of a system is crucial for its design, analysis, and management. The structure can be viewed from different perspectives, including hierarchical (vertical) and horizontal views.

Hierarchical (Vertical) View

The hierarchical view, exemplified by the PBS, shows the system’s structure in terms of levels of abstraction or detail. It helps in understanding:

• System composition
• Levels of integration
• Scope of different system elements

Product Breakdown Structure (PBS)

PBS is a hierarchical decomposition of a system into its constituent parts, subsystems, and components.
The PBS provides a tree-like structure that shows how a system is broken down into progressively smaller elements. It typically starts with the system at the top level and branches down to subsystems, assemblies, subassemblies, and individual components.

Key aspects of PBS:

• Hierarchical nature: Each level represents a more detailed breakdown of the level above.
• Completeness: All elements of the system should be accounted for.
• Non-overlapping: Each element should appear only once in the structure.

Example: For an automobile, the top level might be the car itself, followed by major systems (engine, transmission, body), then subsystems (fuel injection, cooling system), and so on down to individual parts.

Horizontal View

The horizontal view focuses on the relationships and interactions between elements at the same level of the hierarchy. It helps in understanding:

• Functional flows between system elements
• Interfaces and dependencies
• System integration challenges

Black Box and White Box Concepts

A black box view considers a system or component in terms of its inputs, outputs, and overall function, without detailing its internal workings.

The black box:

• Focuses on what the system does, not how it does it
• Useful for understanding system boundaries and interfaces
• Often used in early stages of design or when internal details are not necessary or available

A white box view (also known as clear box or glass box) considers the internal workings, processes, and structures of a system or component.

The white box:

• Provides insight into how the system achieves its functions
• Necessary for detailed design, troubleshooting, and optimisation
• Allows for analysis of internal efficiencies and potential improvements

In systems engineering, both black box and white box views are important:

• Black box views help in understanding overall system behaviour and interfaces
• White box views are crucial for detailed design, integration, and problem-solving

The choice between black box and white box approaches often depends on the stage of the system lifecycle, the level of detail required, and the specific engineering task at hand.

Round-Up

Understanding these various aspects of Systems Structure – hierarchical and horizontal views, black box and white box concepts – provides systems engineers with powerful tools for conceptualising, designing, and analysing complex systems. These perspectives allow for a comprehensive understanding of both the composition and behaviour of systems, facilitating effective system development and management throughout the lifecycle.

Conclusion

Understanding these key system concepts – is crucial for effective systems engineering. By clearly defining the SOI, identifying relevant external systems, and analysing system behaviour, engineers can design and manage systems more effectively. Whether you’re working on smartphones, aerospace systems, or any other complex project, these concepts provide a solid foundation for systems thinking and problem-solving.