Constraints

Cost

Chap. 20

Manned flight, number of spacecraft, size and complexity, orbit

< $20M/yr + R&D

Schedule

Sees. 1.3,19.1, Chaps. 2,12

Technical readiness, program size

Initial operating capability within 5 yrs, final operating capability within 6 yrs

Regulations

Sec. 21.1

Law and policy

NASA mission

Political

Sec. 21.1

Sponsor, whether international program

Responsive to public demand for action

Environment

Sees. 8.1, 212.

Orbit, lifetime

Natural

Interfaces

Chaps. 14,15

Level of user and operator infrastructure

Comm. relay and Interoperable through NOAA ground stations

Development Constraints

Chap. 2

Sponsoring organization

Launch on STS or expendable; No unique operations people at data distribution nodes

develop and promote national engineering resources. The technical community often sets aside nontechnical considerations and regards them as less important or less real than technical constraints. But a successful mission design must include all requirements and constraints placed on the system.

Finally, we reiterate that preliminary mission requirements should be established subject to later trades. Mission designers often simply try to meet the procuring group's requirements and constraints, because not meeting them appears to be a strong competitive disadvantage. Consequently, designers may not modify them, even if changes could make the system cost less or perform better for a given cost Section 3.3 and Chap. 4 detail this process of trading on system requirements to maximize performance vs. cost

As an example, we consider the requirement for mission duration or spacecraft lifetime, which may or may not be the same. This parameter exemplifies the difficulty of establishing requirements. The length of the mission is often indefinite. We want to detect, identify, and monitor forest fires continuously at a reasonable cost per year. In practice, however, we must develop a system that meets this need and then deploy it with an established design life and, perhaps, a replenishment philosophy. The design life of the individual FireSat spacecraft will strongly affect cost and will determine the level of redundancy, propellant budgets, and other key system parameters. In principle, we would like to obtain a graph of spacecraft cost vs. design life as shown in Fig. 1-5. We could then easily compute the total expected cost per year for different design lives, as shown by thé dashed line, and the minimum spacecraft cost per year. We could also assess technological obsolescence, or the point at which we wish to replace the spacecraft because of better or cheaper technology.

Best Spacecraft Design Ule '

Cost

Launch Cost

Spacecraft Design Life

Fig. 1-5. Hypothetical Curve of Cost vs. Spacecraft Design Ufe. The cost per year Is the total cost divided by the design life. In principle, we should use such curves to set the Spacecraft Design Life requirement In practice, they rarely exist See Sec. 20.5 for a Cost vs. Design Life curve for FireSat

In practice, figures such as 1-5 are almost never done or, at best, are done qualitatively. The mission duration is normally assigned rather arbitrarily with a general perception of cost per year. Thus, there may be a push to produce spacecraft lasting 5 or 10 years because people believe these will be more economical than ones lasting only a few years. No matter how we choose the design life, we would like to go through the process described above for decisions about mission lifetime. If at all possible, it would be desirable to create a chart similar to Fig. 1-5 based on even crude estimates of spacecraft cost Doing so provides a much stronger basis for establishing mission requirements and, e.g., determining whether we should push harder for a longer spacecraft lifetime or back off on this requirement to reduce spacecraft cost

Having made a preliminary estimate of mission requirements and constraints, we proceed in Chap. 2 to define and characterize one or more baseline mission concepts. The issue of refining requirements and assessing how well objectives can be met is discussed in Chaps. 3 and 4.

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