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.


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!


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!


Historical Spacecraft Mass Growth (1/2)

Gray area =!above margin?!


Historical Spacecraft Mass Growth (2/2)


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


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


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


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.


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).


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


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.


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.


Back-up Slides


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


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.”


Mass Properties Control

margins and contingency module - arizona state …origins.sese.asu.edu/ses405/class...

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