Margins and Contingency Module Space Systems Engineering version

Margins and Contingency Module Space Systems Engineering, version 1. 0 Space Systems Engineering: Margins Module

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. Space Systems Engineering: Margins Module 2

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. Space Systems Engineering: Margins Module 3

Resource Margin and Contingency Definitions Maximum Possible Value Margin Maximum Expected Value Contingency Current Best Estimate Resource Space Systems Engineering: Margins Module 4

Historical Spacecraft Mass Growth (1/2) Space Systems Engineering: Margins Module 5

Historical Spacecraft Mass Growth (2/2) Space Systems Engineering: Margins Module 6

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 “unknowns” • Use of new technology difficult to gauge • Uncertainties in design execution • Manufacturing variations Space Systems Engineering: Margins Module 7

Calculating Percent 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. % contingency = Space Systems Engineering: Margins Module contingency max expected value - contingency x 100 8

Calculating Percent 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 “unknowns” Margin is usually managed by the systems engineering lead as part of the project level design process. % margin = Space Systems Engineering: Margins Module margin max possible value - margin x 100 9

Typical Technical and Programmatic Contingencies For Robotic Spacecraft by Project Phase Prog. Technical Project Phase Space Systems Engineering: Margins Module 10

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). Space Systems Engineering: Margins Module 11

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 Space Systems Engineering: Margins Module 12

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. Space Systems Engineering: Margins Module

Module Summary: Margins and 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. Space Systems Engineering: Margins Module 14

Back-up Slides Space Systems Engineering: Margins Module

Example Tracking of Mass Performance: Ares I (Lunar) Mass Delivered Min Perf. Reference Trajectory PREDICTED 99. 86% NET Threats Structure Loads FS internal threats (4 & 5 likelihood) US external threats (4 & 5 likelihood) Interstage internal threats (4 & 5 likelihood) USE internal threats (4 & 5 likelihood) 57, 190 lbm Opportunities 55, 881 lbm 53, 948 lbm 52, 070 lbm* Delta Payload (lbm) 512 541 45 51, 290 lbm* (incorporating liens) *Note: CARD requirement still at 52, 250 lbm – needs to be adjusted per Cx SRR Pre-Board Rev 3 Decision (52, 070 lbm) and Ref Traj External Liens (~780 lbm) FS internal insulation change US meets mass requirement Interstage meets mass requirement Delta Payload (lbm) LC 3 (675) (1, 106) (1, 664) (63) (97) ADAC-2 Start External Liens (requires CARD change*) LAS Control mass from 13, 290 to 14, 000 lbm New Orbit & Insertion Alt. from 55 nmi to 70 nmi Delta Payload -90 lbm -690 lbm Predicted 99. 7% Net = Predicted Mean Gross LESS: Launch Window Allowance (500) lbm 3 s knockdowns (to get 99. 7%) (1, 741) lbm Total Margin = 99. 7% Net - CARD Req’t 2, 658 lbm - Design Maturity Estimated Calculated Actual CLV Hardware 112, 884 lbm 41. 5 % 13, 095 lbm 2. 7 % 145, 412 lbm 55. 8 % No Heritage 93. 9% 6. 1% 0% Trajectory Assumptions: • Estimates based on Element predicted masses • J-2 x Isp at minimum (448 s) Space Systems Engineering: Margins Module 16

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 Space Systems Engineering: Margins Module 17

Mass Properties Control Space Systems Engineering: Margins Module 18