In Part 1 of this blog we introduced resource margins and reserves during development and operations.
In Part 2 this blog we addressed the demand side of the resource balance equation by going into more detail concerning the terms associated with resource margins and reserves for mass and consumables.
In this part of the blog we address the supply side of the resource balance equation by going into more detail concerning the terms associated with resource margins and reserves for resource production.
Unlike what has been defined for mass, power and other consumables, NASA’s Expanded Guidance for NASA Systems Engineering, Volume 2 does not address resource production – the supply side of the resource balance equation.
Figure 2: Resource production margins and reserves
While the terminology is similar to the terms used for mass and consumables in the previous blog, the approach for supply has to be looked at from several different perspectives. As shown in Figure 2, there are two approaches to defining the Resource Production Requirement. The first approach is from a “capability” perspective, the second approach is from a “need” perspective – “what can be provided” vs. “what is needed”.
– From the “capability” perspective, the question is: “How much of a given resource can the system of interest produce given the constraints imposed on it (see below)?” This is the perspective a developer of a new system would take when maturing a technology, but has not yet been incorporated in to an overall mission program architecture.
– From the on a “need” perspective, the question is: “How much production of a resource is needed to meet mission objectives?” This is the perspective of the program that is including the system of interest in their mission program architecture.
Both perspectives are valid and need to be considered. Hopefully, the “gap” shown in Figure 2 will be positive and Resource Production Requirement (1) is greater than or equal to Resource Production Requirement (2). If not, there will be more risk during development if you have to compromise on Management Reserve or accept more risk during operations if you have to compromise on Operational Margin.
Supply from a Capability Perspective:
New systems being developed independent of a funded mission program, whose primary purpose is proof of concept and the maturation of the TRL level of the technology being used, will take the capability perspective approach shown in Figure 2.
- “Ceiling Value”, also known as Current Best Estimate (CBE), represents an assessment of the quantity of the resource the system should produce – theoretically. The ability to achieve the Ceiling Value is dependent on a variety of constraints: the most recent design concepts, estimated conversion efficiency (% output versus inputs), estimated quantity and quality of the inputs, the maturity of the technology (TRL) for the specific resource being addressed, the mass allocated to the system, and the power available to the system. From the development team’s point of view, the Ceiling Value is their best estimate of what the output of the system should be given the bounding design constraints. Experience has shown that, for resource production, the ability of the system to produce at the Ceiling Value defined at the beginning of the project will decrease as the design matures, depending on the maturity of the technologies, whether this complex task is being done for the first time (low TRL) in a specific but only partially understood operational environment, and how much the estimate was driven by developer optimism. This decrease could result in an inability to meet the planned or needed usage for the resource.
Note: Conversion efficiency is an important topic. For low TRL systems, the actual conversion efficiency is often lower than the theoretical conversion efficiency and the early units will often have outputs that are less than early estimates. Solar panels are a good example. Because of the importance of increasing the conversion efficiency, further research and operational lessons learned will allow future systems to increase their conversion efficiency. However, for the first units put into operation, we have to meet the program’s production requirements with these lower conversion efficiencies.
- “Production Shortfall Margin”, also known as “Contingency Margin”, is a margin that accounts for the expected shortfalls in production of the resource. The predicted shortfall is applied to the Ceiling Value of the resource (power, water, air, food, fuel production,) resulting in a value lower than the Ceiling Value. The size of this margin is based on an assessment of the design maturity, fabrication status of the item, and an estimate of the in-scope design changes that may occur. Guidelines for assigning and managing % Production Shortfall Margin need to be defined in the program’s SEMP or MMP for each project phase, and are typically established in Phase A.
- “Predicted Value” is the Ceiling Value minus the Production Shortfall Margin. The Predicted Value is an estimate of the final resource production value based on the current requirements, design and technology maturity.
- “Development Margin” is a margin to account for the unexpected shortfalls in a resource over the project lifecycle. The Development Margin is in addition to the Production Shortfall Margin to address change that cannot be predicted. At the subsystem level, this is similar to the Management Reserve defined for the overall system (see below.) The Development Margin is often included with the Production Shortfall Margin. Guidelines for assigning and managing %Development Margin need to be defined in the program’s SEMP or MMP for each project phase, and are typically established in Phase A.
- “Resource Production Requirement (1)”. The Resource Production Requirement on the system of interest includes the Predicted Value minus the Development Margin. Once defined, the Resource Production Requirement (1) is intended to remain constant until there is a change to the requirements. The Resource Production Requirement is a constraint on the system-of-interest design.
Resource Production Requirement (1) = (Predicted Value – Development Margin)
A template for a Resource Production Requirement (1) is as follows:
The [System-of-Interest] shall produce a quantity of [resource] of at least [Resource Demand Requirement (1) value per time value].
Note (1): Production Shortfall Margin and Development Margin are defined and managed at the system level by whomever has the responsibility for resource management. For systems responsible for supply, these resource margins are subtracted from the resource Ceiling Value and then allocated to the subsystems in the form of an allocated Resource Production Requirement (1). If the Production Shortfall Margin and Development Margin are exceeded, the system of interest team will have to work with the systems engineer to change Resource Production Requirement (1) value.
Note (2): The capability perspective and resulting Resource Production Requirement (1) will be used mainly during new system development and technology maturation activities. As the TRL level increases, the uncertainties in capability will decrease and the confidence in the system of interest being able to meet the Resource Production Requirement (1) will increase and the Production Shortfall Margin and Development Margin will be reduced.
Supply from a Need Perspective:
Once a program has been funded, feasible system concept completed, and corresponding system architecture defined, the program will adopt the need perspective “How much production of a resource is needed to meet mission objectives?”. For funded programs, per GAO and NASA guidelines, the TRL of a system must be at least at TRL 3 at program start and an achievable plan to get to TRL 6 by PDR must be included in the program plan and SEMP.
The supply side of the equation includes the “store” plus production. Key decisions that have to be made include deciding how much of a resource will be initially included in the store and how much of the remaining supply needs will be produced using in-situ resources. These decisions are both a risk as well as a cost and schedule decisions. The variables defined below address the production part of the supply equation from the need perspective as shown in Figure 2.
- Resource Minimum Limit: For production systems, the overall design will be constrained with a “minimum not to go under” production value of a resource (mass, power, fuel, oxygen, food, etc.). This “Resource Minimum Limit” is imposed on a system due to contractual, performance, control, transport, or other requirements. For example, the ascent vehicle that will be used to return the crew to Mars Orbit and mate with the Earth Return Vehicle requires a minimum amount of propellant, The quantity of propellant needed would be a hard “Resource Minimum Limit”. If you go under this limit, you may not be able to return the crew to the Earth Return Vehicle! If you need a certain minimum quantity of a consumable (oxygen, water, food) produced for survival, a minimum production rate, quantity, or storage capability represents a Resource Minimum Limit that you cannot go under without placing the crew at risk.
The Resource Minimum Limit value is computed as part of multi-variate optimization model that requires statistical permutation analysis (Monte Carlo).
- Management Reserve: Management will frequently include a “Management Reserve” (defined previously) controlled by the project manager and lead systems engineer in addition to the Resource Minimum Limit.
- As discussed in Part 1 of this blog, an “Operational Margin” (defined previously) needs to be included in order to reduce the risk of unexpected things that may (and probably will) happen during operations. Guidelines for assigning and managing %Operational Margin need to be defined in the program’s SEMP or MMP for each project phase, and are typically established in Phase A. Note: Operational Margin can be included in both the store and the production part of the supply equation. How much to include in each is another key decision the program manager and lead systems engineer will have to make. Again, due to the complexity of the problem, a high-fidelity model with simulation capability will have to be developed.
- “Resource Production Requirement (2)”. As shown in Figure 2, the Resource Production Requirement (2) on the system of interest includes the Resource Minimum Limit plus the Operations Margin and the Management Reserve. Once defined, the Resource Production Requirement (2) is intended to remain constant until there is a change to the requirements. The Resource Production Requirement (2) is a constraint on the system of interest design.
Resource Production Requirement (2) = (Resource Minimum Limit + Operational Margin + Management Reserve)
A template for a Resource Production Requirement (2) is as follows:
The [System of Interest] shall produce a quantity of [resource] of at least [Resource Demand Requirement (2) value per time value].
Applying resource production margins and reserves to your project
Similar to the Production Shortfall Margin and Development Margin, Operational Margin and Management Reserve are defined and managed at the system level as defined in the SEMP or MMP. For systems responsible for supply, these values are allocated to the subsystems in the form of an allocated resource requirement. The team responsible for the system of interest is responsible for managing development to this requirement. This activity is very dynamic in the early stages of system design, with the actual production values maturing as the design matures. As stated previously in the capability perspective discussion, the numbers will begin to stabilize as the parts are procured and integrated.
A funded program will want to include in their system architecture production systems that are able to achieve a Resource Production Requirement (1) (capability) that is greater than or equal to the mission program’s Resource Production Requirement (2) (need) – a positive gap as shown in Figure 2. If there are no systems where this is true (Resource Production Requirement (1) (capability) is less than Resource Production Requirement (2) (need) – a negative gap); then the system of interest team will have to work with the systems engineer to resolve the issue. This issue can be addressed on both sides of the resource balance equation.
– On the supply side, maybe the Resource Production Requirement (2) (need) can be lowered by eating into the Management Reserve or Operational Margin. If the Operational Margin is reduced, additional risk is being added to the mission and any decreases in the Operational Margin will need to be approved by the lead systems engineer and program manager to accept this additional risk.
– Also, on the supply side, any shortfalls due to a lower than needed production value could also be made up in the “store” – the part of the resource supply brought with the astronauts to Mars from Earth. However, as stated earlier, increased store size means added mass and volume!
– Alternately, if management cannot or does not want to reduce the Operational Margin and there is not sufficient Management Reserve on the supply side to resolve the issue, the demand side of the resource balance equation will have to be addressed to keep the resource equation balanced. One way of doing this would be to reduce the scope and mission objectives such that the reduced demand results in a reduced Resource Production Requirement (2) such that the gap becomes positive.
As can be seen, computing and managing resource margins and reserves can be very complex especially when the resources are dependent on each other. As stated in Part 1 of this blog, consumption, production, mass, and power are all dependent variables and the Resource Demand/Supply balance equation must be kept in balance for each resource. If the required supply requirements cannot be met, the Resource Demand Maximum Limit (described in Part 2 of this blog) for that resource will have to be decreased. If the decrease in resource demand cannot be accommodated within the resource margins or the management reserve and the resource balance goes negative (supply cannot meet demand or demand exceeds supply depending on our perspective), a means to get back in balance will have to be developed. This may mean system level resource allocations will need to be changed to give the problem system of interest more and take away from the other systems that may be coming under their allocations. When this is done, a change to the Resource Demand Requirement must be approved. If this cannot be done, then the Management Reserve will have to be used, assuming there is adequate Management Reserve.
In Part 4 of this blog, the focus is “how” to manage resource growth margins and reserves over the development life of your system.
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