Space Systems Engineering: Technical Performance Measures Module Technical Performance Measures...

Space Systems Engineering: Technical Performance Measures Module

Technical Performance Measures Module

Space Systems Engineering, version 1.0

Technical Performance Measures Module

Space Systems Engineering, version 1.0

2Space Systems Engineering: Technical Performance Measures Module

Module Purpose: Technical Performance Measures

To define Technical Performance Measure (TPM).

To show how TPM trends are used to predict delivered system performance.

To describe how TPMs are used to monitor project progress and, when compared with standard resource contingency values, highlight when corrective action should be considered.

To provide example TPMs from current NASA development projects.


Technical Performance Measures

TPMs are measures of the system technical performance that have been chosen because they are indicators of system success. They are based on the driving requirements or technical parameters of high risk or significance - e.g., mass, power or data rate.

TPMs are analogous to the programmatic measures of expected total cost or estimated time-to-completion. There is a required performance, a current best estimate, and a trend line.

Actual versus planned progress of TPMs are tracked so the systems engineer or project manager can assess progress and the risk associated with each TPM.

The final, delivered system value can be estimated by extending the TPM trend line and using the recommended contingency values for each project phase.

The project life trend-to-date, current value, and forecast of all TPMs are reviewed periodically (typically monthly) and at all major milestone reviews.


Tracking Technical Performance Measures

Tracking TPMs and comparing them against typical resource growth provides an early warning system designed to detect deficiencies or excesses.

Contingency allocations narrow as the design matures. TPMs that violate their contingency allocations or have trends that do not

meet the final performance should trigger action by the systems engineer.

Mass Allocation

Ma

ss

Concept CDRPDR Test

Time

Launch

35

% 20

% 15

% 5%

Mass Contingency Plan

2%

Contingency violated, decisions are needed!

Is the trend dependable and no action is needed?Act now to avoid more drastic action in the future?

Current Best Estimate

Trend

Today


Chandra Mass TPMSystem Requirements Review to Launch


Design Contingencies

Design contingencies are largest during concept exploration and uniformly shrink as the project matures. For example, mass contingencies are typically 35% at SRR, 20% at PDR, 15% at CDR and 2% at the launch readiness review.

Why? Contingencies are used to account for development risks, interface uncertainties, and less than perfect design fidelity. As the design becomes more established and the team has greater confidence in their estimates for resource use or system performance, less contingency is needed.

The trends of past, successful projects have been used to create guidelines for new projects.

Why not carry even more contingency? Say 50% mass contingency at PDR to cover an even greater range of possible risks against system mass. With greater contingencies there is less allocation for the design - greater contingencies make the design problem harder. So there is a balance between contingency for risk management and allocation for design flexibility.


Contingency Guidelines for Common TPMs For Different Project Phases


JWST Key Technical Performance Measures

Observatory Mass Margin

Observatory Power Margin

Observing Efficiency

OTE Wave-front Error

Wave-front Error Stability

Strehl Ratio

Sensitivity

Image Motion

Stray Light Levels

Cryogenic Thermal Margins

Commissioning Duration

Data Volume / Link Margin

Momentum Acceleration

James Webb Space Telescope (JWST)


JWST TPM - Mass


JWST TPM – Mass Reserve


JWST TPM – Power

Notes: 5/05: ISIM allocation changed to 740 W 12/06: Power Margin being carried as Load Margin not Solar Array Margin (Golden Rules Compliance) 4/07: Solar Array Capability decrease due to 1 wing baseline 8/07: Cryocooler separated from ISIM, Solar Array Capability increased

6 year Power System Capability = 1826 WattsSpacecraft + OTE Allocation (882 + 50) = 932 WattsISIM + Cryocooler Allocation (310+430) = 740 WattsPower Margin (Estimate vs. Allocated) = 25 %


Module Summary: Technical Performance Measures

TPMs are measures of the system technical performance that have been chosen because they are indicators of system success.

The trends of past, successful projects have been used to create contingency guidelines for new projects.

Tracking TPMs and comparing them against typical resource growth provides an early warning system designed to detect deficiencies or excesses.

TPMs that violate their contingency allocations or have trends that do not meet the final performance should trigger action by the systems engineer.

The final, delivered system value can be estimate by extending the TPM trend line and using the recommended contingency values for each project phase.

There is a balance between contingency for risk management and allocation for design flexibility. This balance is apparent since contingency allocations shrink as designs mature.

Space Systems Engineering: Technical Performance Measures Module

Backup Slidesfor Technical Performance Measures Module


Technical Performance Measures

TPM Basics

• Parameter for meeting key requirements and constraints.

• Sound engineering parameter that is always tracked regardless of mission, such as mass margin or milestone achievements.

• TPMs are usually tracked over the development life cycle of a project.

• TPM trends over time usually compare a planned profile with the actual profile…planning is very important in order to meet specified targets.

• TPMs are usually reported monthly or quarterly in management/engineering status meetings.

TPM Sources

• Responsible NASA Center guidance (e.g., GSFC STD-1000 “The Golden Rules”)

• Industry Practices

• Mission-specific risk assessments


JWST TPM - Mass


JWST TPM – Strehl Ratio

Science Requirement:L1-14 The Observatory, over the field of view (FOV) of the Near-Infrared Camera (NIRCam) shall be diffraction limited at 2 micrometers defined as having a Strehl Ratio greater than or equal to 0.8.

Definition: The modern definition of the Strehl ratio is the ratio of the observed peak intensity at the detection plane of a telescope or other imaging system from a point source compared to the theoretical maximum peak intensity of a perfect imaging system working at the diffraction limit. This is closely related to the sharpness criteria for optics defined by Karl Streh. Unless stated otherwise, the Strehl Ratio is usually defined at the best focus of the imaging system under study.


JWST TPM – Wavefront Error

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