Spacecraft Power System Reliability

The computation of spacecraft power system reliability is strictly based on the rehability data of the components involved. The rehability theory indicates that component reliability is defined as the probability that a specific component will perform as per its performance specifications during an interval of time under specific operating conditions. The determination of component reliabihty involves computing the component failure rate from the part failure rate data, applying this 

SOC(%) Open circuit voltage for the cell (V) Internal resistance (Cl) 

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The data summarized in Tables 2.4 and 2.5 indicate that spacecraft power system reliability improves with an increase in redundant units, regardless of the type of regulator integrated in the power system. Furthermore, it is evident from the reliability data that the number of redundant units (2, 2) for the dissipative regulator offers higher reliability compared with the PWM regulator. 

Reliability Improvement of the Spacecraft Power System Using CC and PWM Regulator Techniques... 

Table 2.4 Reliability Improvement due to Redundant Unit Allocation for the Dissipative Regulator in the Spacecraft Power System as a Function of Mission Duration...

Table 2.6 Reliability Improvement of tbe Spacecraft Power System Using All Redundant Units Sucb as Direct Energy Transfer, Charge Controller, and Battery Booster...

Rough estimates [1] for the increase in total system cost and weight as a function of mission duration are summarized in Tables 2.7 and 2.8, respectively. Based on the data presented in these tables, it can be concluded that the constant reliability of the spacecraft power system can be estimated by means of hnear interpolation among the reliability figures that correspond to systems containing whole redundant components. 

Even though the reliability of the spacecraft power system decreases with the increase in mission duration, the power system will not experience catastrophic failure unless the spacecraft crashes or sustains serious structural damage. In the case of (CC, DSR) redundant system option (2, 1), the reliability improves to 95.3% from 91.8% for redundant system option (1, 1) regardless of mission length or duration. The power system reliability, however, will continue to degrade with the... 

Table 2.9 Degradation of Reliability in the Spacecraft Power System due to Redundant System Components and Longer Mission Durations...

On the basis of the tabulated data it can be stated that the increase in cost factor is minimum with the DSR redundant system. These conclusions are valid for this spacecraft power system, and these conclusions indicate a trend in the increase in weight and cost factor as a function of mission duration using various redundant systems. In summary, performance data available from the power systems deployed by various spacecraft and satellites do indicate that redundant systems tend to yield higher reliability as a function of mission duration or length. Furthermore, the power system reliability does decrease with the increase in mission length regardless of redundant system deployed. 

The physical form of the thermocouples varies significandy according to applications. Most spacecraft power supplies utilize separate thermocouples that can be checked for performance at successive stages of manufacturing and be replaced if necessary. This approach fits in very well with the extremely high reliability requirements imposed on such systems. In terrestrial systems where such individualized attention is not economically feasible, modular assemblies are generally used, which can contain tens to hundreds of couples in a single unit. 

When all three redundant units such as DET, CC, and battery booster (BB) are integrated in the spacecraft power, the data presented in Table 2.6 indicate that the reliability improvement is not impressive. The DET redundant option, however, offers the lowest system cost and system weight as a function of mission duration. The major reliability improvement is due to the shunt regulator circuit used in the DET system as illustrated in Figure 2.5. 

Preliminary studies performed by the author indicate that implementation of redundant components in the spacecraft or satellite power systems does improve the system reliability, but it does so at the expense of higher weight and component costs as a function of mission length or duration. An estimated increase in total power system weight and cost as a function of mission duration is evident from the published data summarized in Tables 2.10 and 2.11, respectively. 

Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations, and in certain military applications. A fuel cell system running on hydrogen can be compact, lightweight and has no major moving parts. Because fuel cells have no moving parts, and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability. This equates to less than one minute of down time in a six year period. 

Error correcting codes have been applied to a variety of communication systems. Digital data are commonly transmitted between computer terminals, between aircraft, and from spacecraft. Codes can be used to achieve reliable communication even when the received signal power is close to the thermal noise power. As the radio waves spectrum becomes ever more crowded, error-correction coding becomes an even more important subject because it allows communication finks to function reliably in the presence of interference. This is particularly true in military applications, where it is often essential to employ an error correcting code to protect against intentional enemy interference. 

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