Title: Margins and Contingency Module Space Systems Engineering, version 1.0
1Margins and Contingency Module Space Systems
Engineering, version 1.0
2Module 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.
3What 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.
4Resource Margin and Contingency Definitions
Maximum Possible Value
Margin
Maximum Expected Value
Contingency
Current Best Estimate
Resource
5Historical Spacecraft Mass Growth (1/2)
6Historical Spacecraft Mass Growth (2/2)
7Why 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
8Calculating 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.
9Calculating 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
10Typical Technical and Programmatic Contingencies
For Robotic Spacecraft by Project Phase
Project Phase
Technical
Prog.
11Considerations 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).
12Additional 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
13Pause 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.
14Module 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.
15Back-up Slides
16Example 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)
17The 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
18Mass Properties Control