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Module Purpose: Margins and Contingency • Describe the need for and use of resource margins and contingency in system development. • Define and distinguish between margins and contingency. • Demonstrate that, historically, resource estimates grow as designs mature. • Provide a representative margin depletion table showing prudent resource contingency as a function of project phase.
What Are Margins and Contingency? • For any system at any point in its development life there is a maximum possible, maximum expected and current best estimate for every technical resources. In general terms, the current best estimate of a resource changes as the development team improves the design; i.e., as the design matures. • A margin is the difference between the maximum possible value and the maximum expected value. • Contingency is the difference between the current best estimate and the maximum expected value. • For a system in development, most technical resources carry both margin and contingency. Typical spacecraft resources include: mass, end-of-life power, average and peak data rate, propellant, and data storage.
Resource Margin and Contingency Definitions Maximum Possible Value Margin Maximum Expected Value Contingency Current Best Estimate Resource
Why Projects Need Margin and Contingency As designs mature, the estimate of any technical resource usually grows. This is true historically and, independent of exactly why, developing projects must plan for it to occur. Expected growth - contingency accounts for expected growth • Recognize mass growth is historically inevitable. • As systems mature through their development life cycle • Better understand design => from conceptual to actual • Make-play changes - fixes to a test failure; change of a vendor • Requirements changes often increase resource use Unplanned growth - margins account for unexpected growth • Recognize space system development is challenging • Projects encounter “unknown unknowns” • Use of new technology difficult to gauge • Uncertainties in design execution • Manufacturing variations
contingency % contingency = max expected value - contingency x 100 Calculating Percent Contingency • Contingency (or Reserve): When added to a resource, results in the maximum expected value for that resource. Percent contingency is the proposed value of the contingency divided by the maximum expected value of the resource minus the contingency. • Takes into account expected development threats. • Contingency use is usually managed by the subsystem lead as part of the design process.
Calculating Percent Margin • Margin: The difference between the maximum possible value of a resource (the physical limit or the agreed-to limit) and the maximum expected value for a resource. Percent margin for a resource is the margin divided by the maximum possible value minus the margin. • Used to cover “unknown unknowns” • Margin is usually managed by the systems engineering lead as part of the project level design process. margin % margin = x 100 max possible value - margin
Typical Technical and Programmatic Contingencies For Robotic Spacecraft by Project Phase Project Phase Technical Prog.
Considerations For Contingency Use • While there are commonly accepted NASA definitions for margin and contingency, the use of these two terms is frequently confused which is complicated by the fact that the terms are frequently used interchangeably. For each project make sure you understand how these terms are defined and used. • All contingency guidelines assume an average level of uncertainty. • Adjust upward for items with higher uncertainty. • Adjust downward for items with lower uncertainty. • In order not to over-budget, contingency may be applied individually to portions of the system and then summed to define the system contingency. • Increased dollar contingency may be used to offset lower contingency in other areas, e.g., technical performance or unknown development schedules. • Each project should generate a list of contingencies and highlight critical parameters that must be tracked (as discussed in the technical performance measures module).
Additional Types of Contingencies • In addition to design contingency at the system and subsystem level • Consumables contingency • May take into account mission duration variability; space environment • Qualification contingency • May take into account load criteria and safety factors • Other resources that use contingency • Power • Delta-V • Safety • Cost • Schedule
Pause and Learn Opportunity Have the students read the NASA ASK magazine article: The Cassini Resource Exchange (Cassini_resource-margin_trade.pdf) Discuss the effectiveness of the Cassini project’s novel approach to margin management.
Module Summary: Margins and Contingency • Contingency is the difference between the current best estimate of a resource and its maximum expected value. • A margin is the difference between the maximum possible value of a resource and its maximum expected value. • Estimated resource use for a system in development grows as the design matures. Contingency is used to account for this growth, so the project can predict maximum expected values for each resource. • The amount of recommended contingency for a resource is based on historically demonstrated trends and decreases as the design matures.
Example Tracking of Mass Performance: Ares I (Lunar) Mass Delivered PREDICTED 99.86% NET Threats Min Perf. Reference Trajectory Delta Payload (lbm) Structure Loads LC3 FS internal threats (4 & 5 likelihood) (675) US internal threats (4 & 5 likelihood) (1,106) US external threats (4 & 5 likelihood) (1,664) Interstage internal threats (4 & 5 likelihood) (63) USE internal threats (4 & 5 likelihood) (97) 57,190 lbm Opportunities Delta Payload (lbm) FS internal insulation change 512 US meets mass requirement 541 Interstage meets mass requirement 45 55,881 lbm 53,948 lbm 52,070 lbm* 51,290 lbm* (incorporating liens) External Liens (requires CARD change*) LAS Control mass from 13,290 to 14,000 lbm -90 lbm New Orbit & Insertion Alt. from 55 nmi to 70 nmi -690 lbm Delta Payload *Note: CARD requirement still at 52,250 lbm – needs to be adjusted per Cx SRR Pre-Board Decision (52,070 lbm) and External Liens (~780 lbm) Rev 3 Ref Traj ADAC-2 Start Predicted 99.7% Net = Predicted Mean Gross LESS: Launch Window Allowance (500) lbm 3s knockdowns (to get 99.7%) (1,741) lbm Total Margin = 99.7% Net - CARD Req’t 2,658 lbm • Design MaturityCLV HardwareNo Heritage • Estimated 112,884 lbm 41.5 % 93.9% • Calculated 13,095 lbm 2.7 % 6.1% • Actual 145,412 lbm 55.8 % 0% • Trajectory Assumptions: • Estimates based on Element predicted masses • J-2x Isp at minimum (448 s)
The Concept of Margin as Explained by Gentry Lee Graphic from the G. Lee DVD: “So You Want to be a Systems Engineer? Personal Behaviors of a Systems Engineer.” Capability Requirements