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.
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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 => 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.
% contingency =
contingency
x 100
max expected value - contingency
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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
max possible value - margin
Space Systems Engineering: Margins Module
x 100
9
Typical Technical and Programmatic Contingencies
For Robotic Spacecraft by Project Phase
Project Phase
Pre-Phase A
Phase A
Phase B
Phase C
Technical
Parameter
Prog.
Te c hn i c
al
Pro grammat i
c
Weight
Power EOL
Pointing Accuracy
Pointing Knowledge
Pointing Jitter
Propellant
Data Throughput
Data Storage
RF Link Margin
Torque Factor
Strength Factor (Ultimate)
Cost (Including De-Scope
Options)
Schedule
Space Systems Engineering: Margins Module
25-35%
25-35%
X2
X2
X3
30-35%
30-40%
40-50%
6 dB
X6
2.1
25-35%
15%
25-35%
25-35%
X2
X2
X3
30-35%
30-40%
40-50%
6 dB
X6
2.1
20-30%
15-20%
X1.5
X1.5
X2
20-25%
20-30%
40-50%
6 dB
X4
2.1
15-25%
15-20%
X1.5
X1.5
X2
10-15%
15-25%
30-40%
4 dB
X4
1.75
25-35%
20-30%
15-20%
15%
10%
10%
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).
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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 project’s novel
approach to margin management.
Space Systems Engineering: Margins Module
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
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 internal threats (4 & 5 likelihood)
US external threats (4 & 5 likelihood)
Interstage internal threats (4 & 5 likelihood)
USE internal threats (4 & 5 likelihood)
60.0
57,190 lbm
Opportunities
55.0
FS internal insulation change
US meets mass requirement
55,881 lbm
53,948 lbm
52,070 lbm*
50.0
45.0
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)
Interstage meets mass requirement
Delta Payload (lbm)
LC3
(675)
(1,106)
(1,664)
(63)
(97)
ADAC-2
Start
Performance Interval
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 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-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
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