Figure 1
– Starboard Pump Module Location on the International Space Station (ISS) (photo
courtesy of NASA)[i]
In mid-December 2013, “the pump module on one of the space station’s two external cooling
loops automatically shut down when it reached pre-set temperature limits. These
loops circulate ammonia outside the station to keep both internal and external
equipment cool.”[ii] During a pre-Christmas news interview, astronaut
Doug Wheelock said that on “Christmas morning we [hope
to] find a new pump module under the Christmas tree on the Space Station.”[iii] Finally, on Christmas Eve, NASA reported “Following two spacewalks to replace a degraded pump
module on the truss … of the International Space Station, flight controllers in
the Mission Control Center at NASA's Johnson Space Center in Houston
successfully restarted the new pump….”[iv]
Clearly, the need for a spare part can happen to any system, anywhere, at any
time!
Portraying physical items in technical data
So, how did ISS engineers know what spare
parts might be required to perform an emergency on-orbit cooling system repair?
In a
previous posting, reparable
items were
defined as “high-cost items that are not consumed
in use. It is often mechanically and economically feasible to repair these
items … [which] retain their identity when in use….” The ammonia pump module that the ISS astronauts needed certainly qualifies
as a reparable item.
A reparable item exists and operates in a 3-dimensional
environment; it has a length, width and height. Further, you can disassemble a
reparable item into its constituent components which can also be characterized
dimensionally.
The technical data for complex systems (such
as the ISS cooling system) are traditionally stored and maintained in some
combination of paper and digital formats. An illustrated parts breakdown (IPB) is
typically used to portray an item and its components, and “contains information required for ordering parts,
including stock numbers, and for identification of parts and arrangements of
parts in assemblies.”[v]
Figure 2 is an example of a reparable item and its components as they
would be represented in an IPB.[vi]
Figure 2 – Example
of a Reparable Item as Portrayed in an Illustrated Parts Breakdown
What is a bill of material?
The IPB essentially portrays an item’s 3-dimensional physical form as a technical drawing. The
challenge, of course, is in translating an item’s technical drawing into
an analytical data structure that is better suited to readiness-based sparing
(RBS) computations. Figure 3 is a very simple example of the beginning of this
process where a 3-dimensional cube is translated (essentially “unfolded”) into
a 2-dimensional perspective.
Figure 3 – Example
of translating a 3-dimensional object into a 2-dimensional perspective
A bill of material (BOM) is used to represent
a physical item in an analytical database which lists “all
[of] the subassemblies, intermediates, parts, and raw materials that go into a
parent assembly showing the quantity of each required to make an assembly.”[vii] Figure 4 is an example of a BOM for the cube
in Figure 3, listing the end-item (indenture 0), sub-assemblies (indenture 1),
and components (indenture 2); along with any multi-unit quantities (in parens).
Figure 4 – Example
of a Bill of Material
So why is knowing the indenture important?
Let’s consider the term indenture a little
more carefully. An indenture is “used to indicate an order of dependence when items are
broken down into assemblies, subassemblies, components, and parts. A lower
indenture item is a part of the next higher assembly.”[viii]
In the logistics vernacular, indenture level 0 is the end-item (or
system), level 1 items are referred to as line replaceable units (LRUs), and
level 2 items are referred to as shop replaceable units (SRUs). The terms LRU
and SRU derive from where items at these indenture levels are used. For
example, failed LRUs are removed and replaced by flight line maintenance
activities (working directly on the end-item) while failed SRUs are removed and
replaced by back shop maintenance activities (conducting repairs of LRUs).[ix]
In readiness-based sparing, the term
indenture is also used to describe how many BOM levels are explicitly
considered in an inventory optimization. Muckstadt [x] is
recognized as developing an early, if not the first, reparable inventory model
that accounted for the differential system impacts between LRU and SRU
shortages while optimizing spares levels.
If
an RBS model has a multi-indenture capability, its selection process considers
the position of the item in the system or end item's configuration. A first
indentured item goes directly on the end item, while a second-indentured item
goes on a first-indentured item. If an item is at the top of the hierarchy
(i.e., is a first-indentured item), its impact on operational availability is
more direct and consequently greater than if it is lower in the hierarchy
(i.e., second-, third-, etc., indentured items). For example, a spare for a
second-indentured item will reduce the repair time for its associated
first-indentured item and that will, in turn, reduce the time the system or end
item is not available when the first-indentured item fails. On the other hand,
a spare for the first-indentured item will directly reduce that time.[xi]
When a readiness-based sparing model
optimizes spares stockage at just the LRU-level, it is referred to as a
single-indenture optimization. However,
if the optimization extends beyond the LRU to include the SRU-level (or even
indentures below that), the model is referred to as a multi-indenture
optimization.
This brings us back to NASA, the pump, and the
ISS ammonia pump module. For NASA, its
LRU-equivalent is called an orbital replacement unit (ORU). “Orbital
replacement units are parts of the main systems and subsystems of the external
elements of the ISS…. ORUs, can be hardware such as radiators, or simply
batteries or communication antennas. Essentially any element [on the ISS] that
can readily be removed and replaced when required.”[xii]
As noted in a U.S. Government Accountability
Office report, the “ISS is composed of about
1,000,000 pounds of hardware brought to orbit over the course of more than a
decade.” [xiii]
This mass can be divided
into two general types of hardware:
(1)
primary structures, i.e., the
external trusses which serve as the backbone of the station and the pressurized
modules that are occupied by the ISS crew, and (2) functional systems composed of orbital replacement units (ORUs), i.e., system
components modularized to support simple on-orbit replacement.[xiv]
Figure 5 shows a replacement pump module in
storage prior to its retrieval and installation during an August 2010
spacewalk.
Figure 5 – Spare
pump module in storage. (photo courtesy of NASA) [xv]
Conclusion
The tendency of systems is to evolve towards
ever greater complexity (and specialization) over time—be it aircraft, automobiles,
ships, computers, or the like. RBS needs to make this complexity
analytically-tractable; that is, translating complex, 3-dimensional physical items
into a 2-dimensional representation called the BOM.
The accuracy of a BOM is dependent not only
on its completeness in terms of capturing the many components and
sub-components of a system, but also on how well it identifies the quantities
of each item as well as their position with regard to the system’s indentured
relationships. As we’ll see in later
postings, each of these dimensions of BOM accuracy is critical to RBS
computations and their resulting stockage recommendations.
In future postings, we’ll take a comparable
look at another important RBS model characteristic – how to represent reparable
supply chains and account for their echelon support structures.
Postscript
On 7 May 2014, I attended the Charles A.
Lindbergh Memorial Lecture at the Smithsonian Institution’s National Air and
Space Museum in Washington, DC.[xvi]
Prior to Mr. Norman Augustine’s formal lecture, the IMAX® film Hubble 3D was shown. This is an
excellent film which documents the space shuttle’s Hubble Space Telescope
servicing mission STS-125 in 2009. The film shows astronauts going on several
spacewalks to replace ORUs on the Hubble Space Telescope and gives viewers a
good appreciation for the challenges of maintaining complex equipment in space.
The movie trailer provides a nice summary of the show and can be found at http://www.youtube.com/watch?v=WIgdxNo8n0k.
[i] NASA, “Starboard Pump
Module Location.” Downloaded from http://www.nasa.gov/sites/default/files/stbd_pump_module_location_ov.jpg,
on 12 December 2013.
[ii] NASA, “Space Station - Update
on Space Station Cooling System,” 11 December 2013. Downloaded from http://www.nasa.gov/content/update-on-space-station-cooling-system/index.html,
on 11 December 2013.
[iii] Doug Wheelock Interview,
ABC World News Tonight, 22 Dec 2013
[iv] NASA, “Station's
Replacement Pump Successfully Restarted,” 24 December 2013. Downloaded from http://www.nasa.gov/content/stations-replacement-pump-successfully-restarted/index.html,
on 24 December 2013.
[v] McCann, Colonel John A.,
USAF (Ret.), ed. Compendium of Authenticated Systems and Logistics Terms,
Definitions, and Acronyms.
AU-AFIT-LS-3-71. School of
Systems and Logistics, AFIT, Wright-Patterson AFB OH, 1981. (p. 336)
[vi] MIL-DTL-38807D, “Detail Specification
Technical Manuals - Illustrated Parts Breakdown,” Department of the Air Force, 17
September 2012. Downloaded from https://acc.dau.mil/adl/en-US/521383/file/72599/MIL%20PRF%2038807C%20Rev%20D.pdf on
24 February 2014. (p. 22)
[vii] APICS, APICS Dictionary,
definition downloaded from http://www.apics.org/gsa-main-search#bill%20of%20material|dictionary
on 24 February 2014.
[viii] McCann (1981), p. 341.
[ix] In the U.S. Navy,
indenture level 1 and 2 items are referred to as weapon replaceable assemblies
(WRAs) and shop replaceable assemblies (SRAs), respectively. See Department of
the Navy. Operational Availability of
Equipments and Weapons Systems and Operational Availability Handbook: A
Practical Guide for Military Systems, Sub-Systems, and Equipment . OPNAV
INSTRUCTION 3000.12A, Office of the Chief of Naval Operations N40, 2 September
2003. (pp. 71-2) Downloaded from https://acc.dau.mil/CommunityBrowser.aspx?id=32668.
[x] Muckstadt, John A., “A
Model for a Multi-Item, Multi-Echelon, Multi-Indenture Inventory System,” Management Science, Vol. 20, No. 4, Part
I, December 1973.
[xi] Slyman, George L. and
Dennis L. Zimmerman. Improved Inventory
Models for the United States Coast Guard Requirements Determination
Process. LMI Report CG201RD6. Bethesda, MD: Logistics Management Institute,
October 1993. (Page A-4) Downloaded from
http://www.dtic.mil/docs/citations/ADA284294 on 8 March 2012.
[xii] Wikipedia, “Orbital
Replacement Unit.” http://en.wikipedia.org/wiki/Orbital_replacement_unit,
5 March 2014.
[xiii] United States
Government Accountability Office, “International Space Station (ISS) – Ongoing
Assessments for Life Extension Appear to be Supported,” Washington DC, April 11, 2011, (p. 4)
[xv] NASA, “Spare Pump Module.” Downloaded from http://www.nasa.gov/images/content/472097main_04_mcmillan_080210.jpg
on 5 March 2014.
[xvi] The video of this
lecture is available at http://airandspace.si.edu/events/detail.cfm?id=9746.
