Draft NASA Constellation Program Management Directive Regarding Use of English Units of Measure

Status Report From: Jet Propulsion Laboratory
Posted: Friday, May 29, 2009

image From: Muirhead, Brian K. (JPL-1900)[Caltech-JPL]

To: Geyer, Mark S. (JSC-ZV111); Kirasich, Mark A. (JSC-ZV111); Curry, John M. (JSC-ZV311); Cook, Stephen A. (MSFC-JP01); Woods, Daryl (MSFC-JP10); Phillips, Pepper (KSC-LX000); Horvath, Gregory T. (KSC-LXS00); Spivey, Jimmy L. (JSC-DA411); Hirshorn, Steven R. (JSC-DA811); Kearney, Lara E. (JSC-XA111); Davis, Jeffrey A. (JSC-XA111); Laurini, Kathy (JSC-ZA111); Dorris, Clinton (JSC-ZL111); Culbert, Christopher J. (JSC-ER111); Ivins, Marsha S. (JSC-CB611); Hansen, Lauri N. (JSC-ZL111); Laws, Burt A. (JSC-ZF111); Caram, Jose M. (JSC-EA311); Goforth, Montgomery B. (JSC-ZF311); Otterson, Timothy M. (JSC-ZF511)

Sent: Tue May 26 15:55:04 2009

Subject: New Management Directive on Units


You've all heard the news that we're going back to English as the primary unit of measure. Attached is the draft Management Directive. We're planning on bring this MD to the CxCB on 6/5/09. Please let me know if you have any issues.


Brian K. Muirhead
CxP Chief Architect
Chief, SE&I Design Integration Office
818 687 7003




DATE X/XX/2009

TO: Distribution

FROM: Jeff Hanley, Manager, Constellation Program

SUBJECT: Use of a hybrid system of units, and English as the primary system of measure for the Constellation Program, Projects, Systems and Missions.


1. PURPOSE: This directive establishes a hybrid system of English and International System of Units (commonly known as SI, Systeme Internationale, and Metric) with English being the primary and default unit of measure of and throughout the Constellation Program (CxP). Cases of mixed use of English and SI units will be continued in specific cases as defined in this directive.

2. SCOPE: This directive applies to all constituents and aspects of the CxP, including organizations, personnel, missions, systems, products, processes, plans, operations, communications, data, documents, hardware, software, contracts, contractors, vendors, and deliverables.

3. RATIONALE: This directive defines and communicates a consistent approach to the use of engineering units throughout CxP. The program conducted an extensive and detailed effort to implement a primary SI units based system for design, analysis, test and operations while allowing English units for most of the hardware. This effort looked at various implementation strategies and worked the implementation details to the point of having workable implementation plans in all the projects. However, the cost estimates to achieve these plans significantly exceeded the resources that could be made available in the critical years. Therefore, given budget constraints and the need for consistent practice of units throughout the CxP lifecycle to minimize risks and to achieve mission success, the program is revising its previous management directive to a primarily English units based program with limited usage of SI as defined in the MD. This MD constitutes the basis for the waiver to NASA policy directing the usage of SI.


4.1. NPD 8010.2E, Use of the SI (Metric) System of Measurement in NASA Programs.

4.2. ANSI/ASME Y14.5M, American Society of Mechanical Engineers Standard for Dimensioning and Tolerancing.

5. POLICY: The specifics of the CxP program engineering units policy are enumerated below.

5.1. English is the CxP standard and default system of measure.

5.2. Limited exceptions are allowed only as identified in this directive, and as defined and approved through program and project implementation plans.

5.3. Direction over the CxP lifecycle:

5.3.1. Flight Operations - The CxP will "fly" in English.

5.3.2. Flight and Mission/Ground System (those elements that interact with the flight system) test and verification - The CxP will test in English in order to "test as we fly".

5.3.3. Flight performance analysis and test data (including ground systems supporting flight systems) Analysis and test results data that may be needed to support flight should be in English Tests in which data may be used to support flight conditions should be taken in English and recorded/documented in English.

5.3.4. The Orion/Ares I stack for ISS and lunar missions will be managed solely in English. Some engineering, subsystems and parts internal to Level 4 may be SI and dual dimensioned for purposes of reusing legacy hardware, and sake of cost and expediency. Details are to be clarified in the Project Implementation Plans.

5.3.5. The Ares V vehicle will be managed solely in English. Some engineering, subsystems and parts internal to Level 4 may be SI and dual dimensioned for purposes of reusing legacy hardware, and sake of cost and expediency. Details are to be clarified in the Project Implementation Plans.

5.3.6. For the Altair vehicle the units of development and operations will be decided by the project and concurred by the program no later than PDR or initiation of procurement contracts, whichever comes first.

5.3.7. For lunar surface systems provided by the U.S. the units of development and operations will be decided by the project and concurred by the program no later than PDR or initiation of procurement contracts, whichever comes first.

5.3.8. For lunar surface systems provided by international partners the expected and accepted set of units will be SI. It will be incumbent on the NASA organizations responsible for integration to manage the engineering and operations interfaces in a hybrid set of units to be defined as part of any international partnership agreement.

5.3.9. For lunar surface operations the environment is expected to be mixed units (like ISS). The crew training and operations will need to accommodate the actual mixed units systems to achieve safe and efficient operations.

5.4. Facilities and non-flight structures and related systems:

5.4.1. It is recognized that, in general, the design and fabrication of facilities and non-flight structures and related systems will be done in English units.

5.4.2. For those facilities and non-flight structures and related systems build in English units that must interface with flight hardware not build in English special attention, practices and procedures will need to be identified and defined in the Program and Project Implementation Plans (e.g. cranes, ground support equipment, to assure safe and proper physical/mechanical handling).

5.5. Specific directions by organization and product breakdown structure (except facilities):

5.5.1. Level 2 - All Level 2 Offices will exclusively use English, with existing documentation maintained in dual units. (TBR, should order of units be reversed)

5.5.2. Level 2/3 - data exchanged and products delivered between Levels 2 and 3 is English or dual units.

5.5.3. Level 3 - All Level 3 Projects and systems are English (except as noted in section 5.7).

5.5.4. Level 3/3 Interface - All Projects will interface with each other in English (e.g CM to LL).

5.5.5. Level 3/4 - data exchange and product delivered between Levels 3 and 4 is English.

5.5.6. Level 4 - All Level 4 modules/elements are English at their interfaces to other modules/elements (for transitional projects may be mixed).

5.5.7. Orion and Ares I Subsystems and below - Internal subsystems and components of CM, SM, LAS and SA modules of CEV are English. Some engineering, subsystems and parts internal to Level 4 may be SI and dual dimensioned for purposes of reusing legacy hardware, and sake of cost and expediency. Details are to be clarified in the Project Implementation Plans.

5.5.8. All other subsystems and components, unless specifically exempted are English.

5.5.9. Public information (i.e. PAO products) will be in English with SI in parenthesis. (TBR)

5.6. Direction by selected flight system disciplines and products:

5.6.1. Mass tracking will solely be reported in: lbm

5.6.2. Flight software: English (i.e. specifications, design, test, etc. any legacy subroutines/objects in SI may remain in SI with conversions carefully managed)

5.6.3. Mission design, planning and analysis: English

5.6.4. Flight simulation software: English

5.6.5. Operational procedures and constraints (e.g. flight rules) : English

5.6.6. System-level analysis, trades, performance calculations: English

5.6.7. In-flight Maintenance and Repairs. Crew accessible fasteners, couplings, connectors, and other interfaces should be English but it is recognized that commonality and minimizing the number of tools may drive to a hybrid implementation for international elements/ payloads.

5.7. Procurements and component selection (non-facilities) should be specified in English units or dual dimensioned.

5.8. New test rigs/facilities/gauges for testing should output in English or dual units; existing rigs that can take data via automated data acquisition systems (e.g. Labview) should output in English or dual units.

5.9. Use of dual dimensioning (dimensions shown in two units, one in parentheses)

5.10. In general, dual dimensioning will be avoided. However, to assure clear, unambiguous definition of information dual dimensions may be used at interfaces between English and mixed subsystems, assemblies or parts. The order will be English with SI in parentheses e.g 122 F (50 C). No special automated method for documenting the native unit has been identified therefore care must be exercised when converting between native units in SI into English.

5.11. Timeline for implementation of this directive

5.11.1. Program implementation plan is due 45 days after the issuance of this directive

5.11.2. Project implementation plans are due 90 days after the issuance of this directive


6.1. SE&I Office:

6.1.1. Is the CxP authority for measurements and units use.

6.1.2. Reviews, approves project implementation plans and ensures program-wide compliance to this directive.

6.1.3. Assigns units lead engineer/manager

6.1.4. Identifies and is knowledgable of relevant technical standards and practices.

6.1.5. Identifies standard units usage and conversions.

6.1.6. Resolves technical questions of use and practice.

6.1.7. Adjudicates issues and waivers.

6.1.8. Interfaces with NASA/HQ OCE.

6.1.9. Assures consistency across documents.

6.2. Other Level 2 Offices:

6.2.1. Compliance with this directive.

6.3. Project Offices:

6.3.1. Development and implementation of project-level units implementation plan.

6.3.2. Project-wide and internal compliance with this directive.

6.4. Level 4 Offices, Functional Managers, Subsystem Leads:

6.4.1. Compliance with this directive and project implementation plan.


7.1. No program level implementation plan is required. .

7.2. Units List and Conversions. The list and conversions for the official set of units used by the program is contained in TBD.

8. PROJECT IMPLEMENTATION PLANS: Projects are responsible for the generation of a plan for implementing this directive per the schedule of paragraph 5.12. This plan will be reviewed and approved by the designated units lead engineer/manager in SE&I.

8.1. The projects are responsible for defining their implementation plans at level 3 and below per the guidance of this Directive.

8.2. The plan will define where (e.g. by subsystem), when, and by whom transitions from non-English are made and verified.

// end //

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