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as 15% of the dry weight. Spacecraft structure weight generally foils in the range of 15% to 25% of spacecraft dry weight (see Appendix A). Spacecraft structural weight may also be estimated at 8% to 12% of injected weight (dry weight + propellant + injection stage). Spacecraft thermal subsystem weight is between 2% and 5% of spacecraft dry weight. Weight percentages for other subsystems vary widely and require more detailed investigation. (See Sec. 10.43, Chaps. 11 and 17.) To account for uncertainties during preliminary design we add 25% to these weights for new equipment and 5% or less for known hardware. We should hold a small allowance (1% to 2%) at the system level to account for integration hardware, such as brackets and mounting hardware, which are often overlooked.

TABLE 10-11. Preparing a Reliability BudgeL

Step

Comments

Reference

1. Establish mission success criteria

The criteria should be numerical and may have multiple elements. For instance, a communication spacecraft having multiple channels for several types of service might define success as one channel of each type of service or as a total number of channels and total radiated power.

Chaps. 1,2,3

2. Assign numerical success probability to ' each element of the mission success criteria and define the method for computing success probability

This might be stated as a probability of 0.5 of operating service "A" for 5 years and a probability of 0.7 of operating service "B^ for 2 years and 0.4 of operating service for 7 years. For each element of the success criteria, numerical values and associated lifetimes are assigned. Several methods of evaluating success probability are available—see Chap. 19.

Sec. 19.2

3. Create the reliability budget by allocating the success probability (reliability) to each item of hardware & software

If, for instance, a system reliability of 0.6 is required, it might be allocated as:

Propulsion 0.95 Comm 0.93 Structure 0.99 C&DH 0.93 Thermal 0.99 Power 0.93 ADCS 0.9 Payload 0.89 .

Sec. 19.2

4. Evaluate the system reliability and iterate the design to maximize reGability and identify and eliminate failure modes

Assuming independent, serial operation, hardware failure rate is generally evaluated by summing piece part failure rates. (See Chap. 19 and MIL-HDBK-17 [1991].) Failure mode analysis and elimination are discussed in Sec. 10.4. Effect of failures can be reduced and reliability raised by changing the design, selecting more reliable hardware, or adding redundant hardware and software.

Sec. 19.2

From the start of the spacecraft design we must design our hardware and software to achieve reliable operation. The process of design-for-reliability starts in the conceptual design phase with the determination of system reliability requirements and allocation of these requirements to the spacecraft subsystems. This is a four-part process as shown in Table 10-11. Hist, we establish the mission success criteria, which is a list of events and operations that together constitute success. Second, we assign a numerical probability to meeting each element of the mission success criteria and select a set of ground rules for computing the probability of success. Third, we allocate reliability requirements to all spacecraft hardware and software. Fourth, we evaluate system reliability and iterate the design to maximize the reliability assessment, and identify and eliminate failure modes. (See Sees. 10.5.2 and 19.2 for further discussions of reliability.)

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