Margins and Contingency Module - Arizona State …origins.sese.asu.edu/ses405/Class...
Transcript of Margins and Contingency Module - Arizona State …origins.sese.asu.edu/ses405/Class...
Exploration Systems Engineering: Margins Module
Margins and Contingency Module
Exploration Systems Engineering, version 1.0
Exploration Systems Engineering: Margins Module 2
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.
Exploration Systems Engineering: Margins Module 3
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. In some (or many?) cases the maximum possible value is defined as maximum expected value plus margin.!
♦ 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.
There may be instrument held margin and project held margin!
Exploration Systems Engineering: Margins Module 4
Resource Margin and Contingency Definitions
Contingency
Margin
Current Best Estimate
Maximum Expected Value
Maximum Possible Value
Resource
Contingency is barometer of development risk!Expended margin is a measure of development trouble!
True MPV is unknowable in some cases!
Exploration Systems Engineering: Margins Module 5
Historical Spacecraft Mass Growth (1/2)
Gray area =!above margin?!
Exploration Systems Engineering: Margins Module 6
Historical Spacecraft Mass Growth (2/2)
Exploration Systems Engineering: Margins Module 7
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
Exploration Systems Engineering: Margins Module 8
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
max expected value - contingency x 100
Exploration Systems Engineering: Margins Module 9
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 x 100
Exploration Systems Engineering: Margins Module 10
Typical Technical and Programmatic Contingencies For Robotic Spacecraft by Project Phase
Parameter
Pre-Phase A Phase A Phase B Phase C
Weight 25-35% 25-35% 20-30% 15-25%Power EOL 25-35% 25-35% 15-20% 15-20%Pointing Accuracy X2 X2 X1.5 X1.5Pointing Knowledge X2 X2 X1.5 X1.5Pointing Jitter X3 X3 X2 X2Propellant 30-35% 30-35% 20-25% 10-15%Data Throughput 30-40% 30-40% 20-30% 15-25%Data Storage 40-50% 40-50% 40-50% 30-40%RF Link Margin 6 dB 6 dB 6 dB 4 dBTorque Factor X6 X6 X4 X4
Technical
Strength Factor (Ultimate) 2.1 2.1 2.1 1.75Cost (Including De-Scope Options) 25-35% 25-35% 20-30% 15-20%
Programmatic
Schedule 15% 15% 10% 10%
Project Phase
Tech
nica
l Pr
og.
Exploration Systems Engineering: Margins Module 11
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).
Exploration Systems Engineering: Margins Module 12
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
Exploration Systems Engineering: Margins Module
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.
Exploration Systems Engineering: Margins Module 14
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.
Exploration Systems Engineering: Margins Module
Back-up Slides
Exploration Systems Engineering: Margins Module 16
45.0
50.0
55.0
60.0
Performance Interval
Pa
ylo
ad
Ma
ss
(K
lb
m)
Example Tracking of Mass Performance: Ares I (Lunar) Mass Delivered
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
Rev 3 Ref Traj
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)
Delta Payload (lbm) FS internal insulation change 512 US meets mass requirement 541 Interstage meets mass requirement 45
52,070 lbm*
57,190 lbm
ADAC-2 Start
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)
*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)
Delta Payload
Threats
Opportunities
Min Perf. Reference Trajectory PREDICTED 99.86% NET
55,881 lbm 53,948 lbm
51,290 lbm* (incorporating liens)
Predicted 99.7% Net = Predicted Mean Gross LESS: Launch Window Allowance (500) lbm 3σ knockdowns (to get 99.7%) (1,741) lbm Total Margin = 99.7% Net - CARD Req’t 2,658 lbm
Exploration Systems Engineering: Margins Module 17
The Concept of Margin as Explained by Gentry Lee
Requirements
Capability
Graphic from the G. Lee DVD: “So You Want to be a Systems Engineer? Personal Behaviors of a Systems Engineer.”
Exploration Systems Engineering: Margins Module 18
Mass Properties Control