Life Cycle Considerations
Carnegie Mellon University
18-849b Dependable Embedded Systems
Author: Philip Koopman
The life cycle of an embedded system varies dramatically, from processors
embedded in disposable consumer goods to applications requiring maintenance and
support for decades. Designing an embedded system often requires taking into
account the complete product life cycle, from initial product concept, through
its operational period, and into replacement with newer equipment. While the
design phase is covered by other topics, areas of specific concern to a life
cycle perspective are: an accurate life cycle economic model to guide
engineering tradeoffs, taking into account requirements for logistics and
support over the product operational period, and issues specific to
refurbishing/retiring/discarding the system at end-of-life. While the term
"life cycle" has different meanings to different technical
communities, the central idea is to expand the traditional engineering emphasis
on the "design cycle" to encompass optimizing utility, profits, and
tradeoffs across the entire lifetime of the embedded system being designed.
The idea of a product life cycle acknowledges the fact that designing and
selling a product is only part of the story. In fact, every product goes
through a series of steps between the time it is first conceived and the time
the manufactured product is retired or discarded. Figure 1 shows one view of
the various phases of a produce lifecycle [ref to edrc material].
The product starts with a need or opportunity in the marketplace. In
embedded applications, this need has historically been a goal of reducing cost
or increasing the functionality of an existing electromechanically controlled
system by inserting computer technology. An example is digital engine
controllers for automobiles, which were virtually required to meet tougher
emission controls that could not be handled by mechanical engine controllers.
At other times, completely new opportunities exist because of the progress of
technology. As a somewhat frivolous example, greeting cards that play music or
recorded messages require fairly sophisticated, inexpensive electronics for
Once a need or opportunity is defined, a concept for a product is created.
As an example, a product concept might be something like a digital cell phone
instead of an analog one, in order to reduce audio noise heard by the user.
With a life-cycle approach this stage involves not only defining the product,
but also addressing the business model of how the company will make money from
the product, how users will employ the product, how the manufacturer and/or
user will benefit from any potential upgrades, and how the product will be
retired, disposed of, or refurbished.
After concept development, both the product and manufacturing process are
designed. In some cases an existing manufacturing process is taken as a fixed
parameter, but often it is more efficient to permit changes to the product to
accommodate manufacturing needs as well as changes in the manufacturing process
to accommodate product needs. In many embedded systems the manufacturing
volumes are quite high, making it worthwhile to modify or even create new
manufacturing processes. Additionally, in smaller companies that create
embedded products, much of the manufacturing process may actually be created
via contract manufacturing, meaning that creating the manufacturing process
consists largely of selecting vendors for various process steps and then
arranging them in the appropriate order rather than arranging manufacturing
equipment at the company's own plant.
Within product design, it is common enough to take the approach of designing
something that functions correctly and can be manufactured for a profit.
However, with a life cycle approach it is often better to maximize profit
across the entire lifecycle, even if that means reaping less product on the
initial equipment sale. For example, a system that is to be upgraded might be
designed with higher-priced component sockets or connectors that support
multiple installations and removals, rather than less expensive sockets that
can only be used a single time. (There is, obviously, a tie-in to the business
model at this point, and there may be structural impediments within a company
that prevent such global optimization.)
Once the design process is completed, the equipment is produced and
deployed. Considering for these life cycle phases in product design can reap
significant profits. For example, automatic component placement machines can
only handle a fixed number of different components, so designing to stay within
that limit either speeds up production throughput or avoids the cost of buying
a second placement machine. Similarly, considering deployment requirements
during the design phase can be quite beneficial. For example, a user interface
on a new piece of equipment might be made to mimic the user interface on an
older, non-computerized piece of equipment to reduce the need for customer
training even though a radically new interface might be slightly more efficient
or cheaper to manufacture.
After the product is deployed, it must be supported and maintained. Support
and maintenance for desktop software is dominated by technical support phone
calls with problems of installation, bug reports, and difficulty using advanced
features. Desktop hardware maintenance far less frequent, and involves
situations such as disk crashes, dirt, and power supply failures. Embedded
systems seem to have a somewhat different focus for support and maintenance.
Since the embedded software is often more focussed on controlling some other
system, the amount of software functionality exposed may be significantly less
than in a software product such as a spreadsheet or database. However, user
interfaces are often minimal, making it difficult for the user to access the
available functionality (the classical example is setting the time on a VCR).
Thus, technical support is more likely to be help due to user interface
obstacles or mismatches between the consumer's mental model of operation and
the actual software design. From a maintenance point of view, interactions with
other products are less likely to be an issue since the embedded system comes
with its own dedicated computer. However, embedded maintenance is complicated
by the fact that problems may be due not only to a computer failure, but in
fact are more likely to be due to failures in sensors, actuators, or mechanical
components. Thus, in support and maintenance it is important to provide
extensive diagnostic and monitoring in complex embedded systems to facilitate
The need for upgrades varies dramatically depending on the life expectancy
and usage of the embedded system. At the low end, some embedded systems are
completely disposable, such as the electronic greeting card mentioned above,
many toys, and consumer products in which repair labor is more expensive than
the product, or in which products are evolving so quickly that products are
obsolete before they have a chance to break (this is probably the case with
digital cameras at the moment). On a larger time scale, however, enduring
embedded systems must be upgraded once or more during their product life. For
example, elevators can remain operation for up to a hundred years or more, but
are upgraded every decade or so. The same is true of many expensive pieces of
equipment such as military weapon systems or aircraft, which must be operated
for many years to amortize purchase costs. With equipment that is likely to be
upgraded, it is important to design the system in such a way that upgrades can
be done cost-effectively, and that system downtime is minimized during upgrade.
Perhaps the most extreme example of design-for-upgrade is in telephone
switches, which are required to operate continuously through a system upgrade.
Finally, all embedded systems are eventually retired, discarded, or
replaced. Designing the system to be retired gracefully can significantly
reduce costs to the manufacturer, the user, or society as a whole, as is
discussed in a later section.
While the notion of a life cycle encompasses a very broad range of topics,
there are three particular life cycle areas that are not covered in other topic
discussions: life cycle costing, logistics, and design-for-disposal (also known
as life cycle assessment, or "green design").
The life cycle cost of a product includes not simply the cost of materials
and labor to manufacture it, but in fact all costs associated with the product
from inception to retirement. The idea of a life cycle approach to cost is not
specific to embedded systems, but rather is more generally applied to very
expensive capital purchases such as buildings, factory machinery, and military
systems (ships, planes, tanks, etc.). However, since embedded system
designers are often embedding computers in such expensive systems, it behooves
them to understand the financial model for life cycle costing so that they can
take this into account in their work.
Kirk & Dell'Isola's book [Kirk95] provides a
comprehensive look at life cycle costing from the perspective of operating a
commercial building, such as an office building (the following discussion is
based on material from that book with some augmentation). However, the concepts
they discuss apply to many embedded systems in general. (As an aside, an office
building is in fact an embedded system. There are computers controlling the
climate, operating the elevator system, and in many cases controlling the
lighting. In some buildings these three systems are beginning to be coordinated
for efficient operation as well as lower cost maintenance.) In general, the
factors included in the life cycle are those discussed in the previous section
with respect to design and retirement of the product. In general, the idea is
to minimize total cost for owning and operating an embedded system over the
complete life of that system. This means that in addition to technical design
factors, specific economic factors must be considered.
The life of a product is the shortest of three different aspects of system
- Useful Life (utility). This is the obvious notion of equipment
lifetime, in which eventually equipment wears out to the point it is beyond
- Technological Life (obsolescence). A system may become expensive or
impractical to maintain even though it still is theoretically repairable or
operable in general. For example, it may be impossible to find technicians
trained in repairing vacuum-tube operated computers, or it may be impossible to
find replacement parts for 16 Kb DRAM chips. Or the system may simply not
incorporate the latest technology that in and of itself is seen desirable by
users (for example, a rotary dial telephone system).
- Economic Life (cost of operation). A system may still be functional,
but become too expensive to be worth continuing to use. One example is because
of a high cost of repair using obsolete components (this is a typical problem
in long-lived embedded systems). Another reason may be that newer versions can
be purchased and have lower operating costs so that the "payback"
period of making that purchase is brief. This has, for example, happened
recently with fuel-efficient furnaces and air conditioners.
Although it may not be possible to completely predict the lifetime of a
system in advance, it is estimated taking these three factors into account.
Then, the direct costs of ownership are considered, including:
- Initial purchase cost. Clearly purchase cost is part of total cost.
The usual issue is optimizing whether one should pay a higher up-front purchase
cost in hopes of reaping lower operating costs. In some cases there is a limit
on the amount of money that can be spent, such as a credit line limit, which
may cap the allowable purchase cost.
- Energy costs. Operating equipment usually requires energy, and can
be a significant portion of total costs. In many cases embedded computers are
used to increase energy consumption efficiency, and thus reducing energy cost
is a primary goal. As an example, high-end home furnaces perform energy
management to maximize heat delivered to the house and minimize heat that
escapes into a basement from hot water "stranded" in the basement
heating elements when the thermostat reaches its set point.
- Maintenance/Repair/Custodial costs. A low initial purchase cost may
be indicative of a system which will need frequent maintenance, repairs, or
upkeep. Presumably a higher purchase cost indicates a system that contains more
durable components. For an embedded system, it is more likely that components
other than computers will break. However, installing sensors, data logging, and
diagnostic capabilities for the system can substantially reduce these costs. As
an example, elevators may come with minimal diagnostic sensors, but a contract
maintenance company may well add sensors in an up-front investment to reduce
the cost of later service calls.
- Alteration/replacement costs. In a long-lived system that will be
upgraded, it is important to take into account eventually removing or upgrading
the equipment. As an example, a component or housing may be glued into place to
save on installation costs, but be very difficult to remove, whereas a
bolted-in system is more expensive to install, but cheaper to replace. A more
specifically electronic example is the use of flash memory to permit field
software upgrades without replacing read-only-memory chips.
Additionally, there are many indirect costs that must be taken into account
in a complete financial model. These indirect costs of ownership include:
- Interest (debt service). In some cases the most important indirect
cost is the cost of borrowing money to pay the initial purchase cost in order
to reduce later operating costs (or, alternately, the opportunity cost of not
investing the purchase cost in some other way). Thus, any life cycle savings
must be higher than extra initial cost savings to take into account the fact
that extra money may need to be borrowed early in the system life cycle, but
the savings are reaped later in the life cycle.
- Administrative costs. These can vary considerably, but might include
such factors as periodic safety inspections, the cost of arranging for and
administering service agreements, the cost of tracking capital equipment via
property tags, and the like.
- Staffing of equipment to operate it. In some embedded systems
computers are used to automate tasks previously performed by people. Perhaps
the best everyday example of this is in elevators, which used to be operated by
a trained person, but now are completely automatic. Clearly, saving operator
wages is worth a significant increase in initial purchase cost in
industrialized countries (but not necessarily in countries with low standards
- Opportunity cost of down time. A system that is frequently
unavailable for service may not be as cost-effective as a more dependable
system because of reduced productivity, the cost of stockpiling against
potential service outages, or the cost of paying operators while their
equipment is broken. Minimizing down time is extremely important most embedded
system industries, including manufacturing and transportation. Embedded
computers often contribute significantly to dependability by permitting
operating in degraded modes while awaiting repair, by alerting maintenance
personnel to impending failures, or by helping diagnose problems for quick
- Other non-quantitative factors. Some factors are important, but hard
to set a fixed price on. These include potential productivity increases from
comfortable employees and comfort taken from dealing with a company with a
track record for standing behind their products.
The above life cycle costing approach works well for many large-scale
embedded systems, where consumers are very sophisticated and equipped to invest
for the long term. However, with consumer products it may be much more
difficult to create a design that is optimized both to achieve profitable sales
as well as maximum utility for the purchaser.
Antonides [Antonides90] discusses a combination
of economics and psychology for durable goods purchases. While they come to the
conclusion that frequency of use is generally related to reliability, and the
cost of the good is generally related to its lifetime (when comparing different
similar goods of different prices). While this is probably no surprise, the
finding that makes life difficult for embedded system designers is that the
decision of when to scrap a product is made on a potentially distorted view of
life cycle economics. It should be noted that this is a European study and thus
not necessarily representative of North American consumers. However, the
specific points mentioned seem to ring true in general (the following is an
interpretation and amplification, not a quotation):
- The opinion of consumers with respect to how their equipment works does not
agree with objective technical measurements. In particular, there is a bias to
replacing rather than repairing, even when repair would be more economical in a
life cycle cost sense.
- Consumers, and especially low-income consumers, tend to underestimate the
benefit of paying a higher purchase cost and reaping lower operating costs.
This is presumably because they have little cash, and they have little credit
or only have access to expensive credit such as credit cards rather than home
equity loans. (Thus, many such systems are extremely cost-sensitive, squeezing
the budget for embedded system components.)
- The age of the consumer may affect the degree of patience and tendency to
perform long-term planning (thus, younger or very old people may not value
features such as diagnostics or upgradability; additionally they may not be
inclined to wait and save for buying a high-end model).
When taken as a whole, the point of life cycle costing is to take into
account all the direct and indirect costs of a product, and optimize for the
lowest total cost given the constraints of customer preferences/behaviors.
Logistics is the process of managing acquisition, movement, and storage of
parts, materials, and products [Christopher92]. It
has been a specialized discipline for a very long time in the military, which
is of course vitally connected with keeping troops properly supplied. In the
embedded systems life cycle, logistics affects the life cycle phases from
production through retirement.
Logistics can be viewed along two axes: by type of item, and by activity. In
either case the objective is to assure that materials are available where and
when needed at minimum cost. This often involves tradeoffs of risks that demand
or supplies will fluctuate vs. the cost of maintaining inventory (these
costs include not only the cost to make space and staffing available, but also
the debt burden of the money tied up in inventory as well). The particular
types of items affecting logistics are:
- Manufacturing components. Component flow must be assured to keep
manufacturing lines working, but maintaining excessive inventory costs money.
Many manufacturing plants in general have switched to "just in time
delivery", in which they dramatically reduce inventory while depending on
assurances from suppliers that component flow will be uninterrupted.
Additionally, many products have undergone a process known as "design
rationalization," in which inexpensive components may be replace with more
expensive components or materials if doing so results in a lower global cost.
An embedded system example might be using two different components at a
quantity of 50,000 per year, where one component is $.01 cheaper than the other
because of reduced performance, but otherwise they are identical. Using 100,000
per year of the more expensive component might yield a quantity price break of
$.02 per component, lowering total cost.
In addition to these general techniques, embedded systems can exploit design
features such as programmable memory instead of mask-programmed memory so that
they use a lesser number of different components. These individual components
may be expensive, but those costs may be offset by reduced inventory cost as
well as flexibility to respond to sudden changes in product mix demands or
changes in designs (for example, even high-volume auto component manufacturers
are considering using flash memory instead of masked ROM because they receive
frequent change requests from customers, resulting in costs for new masks and a
need to potentially discard obsolete components that have not yet been used).
An additional technique that can be used is incorporating field-programmable
logic into designs to displace many different standard components with fewer
types of generic components.
- Finished products. Finish products must be stored and transported to
customers. Changing designs to make completed systems simpler to store and
transport might reduce logistics costs. For example, having a single generic
system model that can be warehoused and customized at shipment time or by a
product dealer may permit a single system to be manufactured and sold as
several different products (this might be local language customization, or in
some cases has been known to be two products with different feature sets and
different prices that are in reality a single design).
- Spare parts. There is a potentially very large opportunity in
embedded systems to reduce costs for spare parts, but this is a difficult
problem. Because technology becomes obsolete far more quickly than long-lived
embedded systems are retired, it is common for support companies to be forced
to make "end of life buys" for semiconductor components. They must
therefore have money tied up for years or decades in components that may or may
not ever be needed for eventual repairs. As an example, the auto industry is
required by law to provide spare parts for a decade after a car is
manufactured. The high cost of doing this in part accounts for the fact that
spare parts are very expensive (so expensive, in fact, that building a car from
spare parts is about 7 times more expensive in materials than the purchase
price of a new car). Embedded system designers can help reduce this cost by
attempting to design generic systems in which new-technology components can be
plugged in to replace old ones; however this is still an open research area.
The types of activities that most affect logistics are below. They are to a
large degree a different view on the same issues as the types of items, with
the exception of disposal concerns:
- Delivery. Embedded systems and components must be transported to the
factory, to the customer, and to disposal.
- Inventory. Components, systems, and spares must be kept in stock.
- Disposal. Obsolete or broken systems must be disposed of. Systems
may be refurbished, in which case they must be collected and shipped to the
incoming inventory of a rework factory. They may be disassembled into
constituent materials and put into the recycling waste flow, or they may simply
be discarded. The discussion on Green Design below discusses embedded system
strategies for this activity.
The main concern about logistics for embedded system designers is to ensure
that design decisions are made on a more global scope than simply minimizing
per-unit component costs. In particular, it is important to take into account
the cost of sinking a large amount of money into spares that must be stored
over many years for long-lived systems.
A third general area of life cycle considerations is specifically that of
designing products for ecologically friendly retirement and disposal. This is a
relatively new, but fairly active area of academic research, with roots back to
the ecology movement of the 1960s. The basic idea is that products should be
designed with disposal, operational consumption of resources, and operating
pollution specifically in mind, just as manufacturing efficiency is considered
in many designs. This area has now matured to the point that there are
international standards for approaching the problem, including the ISO 14000
series, and in particular
14040 "Life Cycle Assessment".
As an idea of the magnitude of the impact a system designer can have on
recycling effectiveness, [Goldberg98] states that
there is an IBM estimate that discarded computers will occupy 2 million tons of
US landfill space by the year 2000. Although embedded systems are more
prevalent, it is in some cases more difficult to understand their impact on
landfill space because they are so often hidden inside other equipment.
However, even so, there are many techniques that embedded system designers can
employ to reduce the environmental impact and societal costs of their designs,
- Design for repair. Some electronic assemblies are not designed for
repair or replacement. While we live in a "throwaway" society, the
question of whether something will actually be repaired is one that should be
considered seriously rather than taken for granted.
- Design for upgradability. In some cases embedded systems can be
designed in a way that permits upgrading with new capabilities via exchange of
electronic components. While this may add to initial cost, the company might in
fact be able to make higher overall products by selling proprietary upgrade
electronics rather than potentially losing the sale of a next-generation system
to a competitor. Obvious upgrades for embedded systems include faster
processors and larger memories to support added functionality. Of course, in
some cases a simple capability for updating programming is sufficient to
- Design to minimize power consumption. For battery-powered equipment
this reduces the number of batteries that enters the landfill. (Remember that
even rechargeable batteries have finite life, and may contain substances such
as Cadmium that are quite toxic when disposed of.) Additionally, minimizing
power reduces the consumption of fossil fuels, although in some larger embedded
systems the power consumption of the rest of the equipment dwarfs electronic
- Design for recycling/clean disposal. This can include designing a
system so that different material types are readily separated for recycling. It
can also encompass using less toxic materials, such as non-lead bearing solder
in circuit board manufacturing.
- Design for clean manufacturing. Manufacturing uses resources and
produces waste materials. A recent example of manufacturing becoming cleaner
for embedded systems is the switch from Freon as a circuit board cleaner to
aqueous cleaners (even rubbing alcohol and a scrub brush will work, but static
discharge must be avoided).
In general, the Green Design approach to the life cycle is to consider the
end of the product life when doing the design. A tricky way in which this ties
in to the life cycle costing model is that society as a whole (and our
descendents) pay a hidden price for pollution and resource exhaustion that are
difficult to take into account in a single-product life cycle model. The only
proven wide-scale techniques for accounting for these costs is government
taxation/rationing (such as with Freon to discourage manufacture and
consumption) and recent European initiatives to make manufacturers accountable
for disposal of waste streams their products generate.
The state of the art in tools, techniques, and metrics varies widely
depending on the particular area in life cycle concerns. Life cycle costing is
generally an accounting approach that is not encountered by every-day design
engineers, and measures results in dollars. Logisticians have a variety of
mathematical optimization tools available, and in fact are intimately tied to
the field of Operations Research (e.g., the Simplex method), and
measures results in terms of achieving maximum efficiency for a given flow rate
objective. Green design currently employs tools of a sort of
"spreadsheet" nature that link raw materials and components with the
waste streams they generate at end-of-life, and typically measures results in
terms of tons of landfill space consumed.
Life cycle concerns are not a particular technical expertise area the way
that many of the other topics are. Instead, it is an overlay concept that
suggests that each technical area consider the impact of decisions on the rest
of the areas.
- Design Process/Market Forces. In a mature
business environment, life cycle considerations shape the design process.
Additionally, life-cycle based market forces such as environmentalism (or,
conversely, a "disposable society") affect how products are designed.
- Verification/Validation/Certification. Life
cycle costs include certification and related expenses, making small design
changes more difficult to justify.
- Shoddy Spares & Customer
Circumvention. Part of the life cycle for most embedded systems involves
the opportunity for system owners to perform proper or improper maintenance.
- Profits & Business Model. Not all
businesses make their profit on the design/manufacture portion of the life
- Manufacturing/Quality. Design for
manufacturing and designing a system which is readily made to high quality
standards is usually just as important as the more traditional design metrics
of raw performance and features check-lists.
- Maintenance and Dependability. To paraphrase
Murphy: any system that can be maintained wrong will be maintained wrong. And,
the maintenance/operation phase is often the longest part of an embedded system
- Ultra-Dependability. Making a system
that is ultra-dependable is the ultimate life-cycle challenge, because
- End-of-Life Wearout & Retirement. In
addition to green design considerations discussed here, another factor in the
life cycle is whether the system remains dependable enough when it is operated
beyond end-of-life for components or the system as a whole.
The following are the key ideas for this topic:
- The term "life cycle" has several meanings in various
communities, but they include a cost model for total cost of ownership (and/or
total profit); dealing with the logistics of supplying, transporting, and
maintaining equipment; and end-of life disposal/retirement issues.
- A sufficiently rich economic model can help designers and users make
informed decisions to minimize total cost of ownership. However, the real
economic constraints and potentially suboptimal behavior of customers may lead
to making choices for purchases and operation that are less than what would be
optimal without these constraints.
- A true life cycle perspective includes designing not only for manufacturing
cost, but also includes the costs (monetary and otherwise) for all the phases
of the lifecycle, including manufacturing, deployment, maintenance, operation,
and eventual retirement/disposal.
- [Antonides90] Antonides, Gerrit, The lifetime
of a durable good : an economic psychological approach. Boston: Kluwer
This book combines economics and psychology to study the effect of people's
attitudes and equipment characteristics on decisions to dispose of durable
goods (in other words, the equipment lifetime based on people's behavior rather
than on when the goods are "really" worn out). The study was done in
- [Christopher92] Christopher, Martin,
Logistics: the strategic issues, London: Chapman & Hall, 1992.
A book with twenty papers spanning the range of how logistics affects
manufacturing industries. Of particular interest for embedded systems are
Chapter 16 about General Motors and Chapter 18 about BMW.
- [Goldberg98] Goldberg, L., "The advent of
'green' computer design." IEEE Computer, Sept. 1998, vol.31, no.9,
This is a general introduction to design-for-disposal, using an example of
personal computers to illustrate the issues.
- [Kirk95] Kirk, Stephen J.; Dell'Isola, Alphonse J.,
Life Cycle Costing for Design Professionals. New York: McGraw-Hill,
This is a thorough text on the economics/accounting/finances of equipment
purchasing with respect to life cycle cost optimization.
- AT&T, Design's Impact on Logistics, McGraw-Hill, 1993.
General coverage of logistics from a company that is used to long-range
planning. Has a case study on the F-16 about how logistics was given short
shrift and how it caused problems.
- Bodsberg, L.; Hokstad, "A system approach to reliability and
life-cycle cost of process safety-systems." IEEE Transactions on
Reliability, June 1995, vol.44, no.2, p. 179-86.
This paper is about using a tool to quantify reliability, safety, and life
cycle cost, driven by the needs of off-shore oil drilling. The part of
particular interest to a wide audience is a taxonomy of failure modes, means to
achieving reliability, and failure causes used within the model. A key
conclusion is: "According to offshore field data, system environment is as
important as nominal reliability with respect to hardware reliability."
has a pointer to work reported on in this paper.
- Burall, Paul, Green Design, London: The Design Council, 1991.
A relatively accessible discussion of the issues with designing for
environmental friendly products.
- Evans, Joel, (ed.) Product Life Cycles in the Automobile, Food, and
Proprietary Drug Industries: evolution, description, and analysis, Hofstra
University Yearbook of Business, 18(1), 1983
This book actually has nothing to do with real "life cycles." The
cycles they refer to are business cycles in terms of units sold (depression,
recession, economic boom, market shifts). But, it is included in this list to
illustrate that "life cycle" is in the eye of the beholder.
- Hendrickson, C. et al., Product disposal and re-use for portable
computer design, Technical report EDRC-12-64-94, Pittsburgh, Pa. : Carnegie
Mellon University, Engineering Design Research Center, 1994.
Brief discussion of the green aspects of portable/wearable computers,
especially battery considerations
- Warwick (ed.), Aircraft Engine Life Cycle Cost, SAE SP-721,
This is about life cycle cost models (financial) more than about engineering
Index of other topics