Margins and Contingency Module Space Systems Engineering, version 1.0

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Margins and Contingency Module Space Systems
Engineering, version 1.0

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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.

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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.

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Resource Margin and Contingency Definitions
Maximum Possible Value
Margin
Maximum Expected Value
Contingency
Current Best Estimate
Resource
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Historical Spacecraft Mass Growth (1/2)
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Historical Spacecraft Mass Growth (2/2)
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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 gt 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

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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.

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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
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Typical Technical and Programmatic Contingencies
For Robotic Spacecraft by Project Phase
Project Phase
Technical
Prog.
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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).

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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

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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
  projects novel approach to margin management.

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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.

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Back-up Slides
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Example Tracking of Mass Performance Ares I
(Lunar) Mass Delivered
Threats
PREDICTED 99.86 NET
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 Reqt
2,658 lbm

• Design Maturity CLV Hardware
  No 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)

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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
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Mass Properties Control