ncnxrnox Cbtstaluxes Plans

When the Mission Definition Cycle is followed to its completion, the output obtained is the Mission Plan (7) consisting of selected missions and their requirements. The inputs to the System Definition Cycle that follows, are the requirements for one of the missions chosen for advanced planning (8).

The system and its elements are defined in Cycles <H) and (III), but only as far as necessary to determine the advanced development requirements for technologies personnel or facilities. In Cycle (II) the system is defined by determining its elements; whereas in Cycle < HI) the system elements are defined by determining their design approaches. The output of Cycle (III) is Advanced Development Requirements (21) for the design approaches of the system elements elements for which development programs must be initiated in other words. Advanced Development Requirements are "the needs** which exceed present capabilities.

It is essential to stress that in the Definition Cycles (I), (II) and (III), judgment must be exercised to avoid unnecessary detail that can obscure the overall vision essential to this type of planning.

The Systems Approach was not conjured overnight nor in a fortnight; it was born of necessity after a gestation period of almost ten years. It is a complicated tool, still being refined, and is used as a plan of attack to complex problems. Therefore, if the reader who is new to this concept has stayed on the tortuous trail thus far, he is to be congratulated for having reached the "first plateau."

So far, we have discussed the Systems Approach in general terms, as applicable to a "product area." To better understand the application of the Systems Approach as an aid to planning let's take a specific but compressed example, and assume retaining the interested reader's attention to the very end.

The example to be cited is a planetary exploration system that will be treated in the light of Systems Approach principles already described. Since the planning procedure for technologies, personnel and facilities (Cycle IV) are quite similar, only the technologies planning cycle will be discussed.

PABTICrUUUXATIOX OF PBOCEDUBE

A "road-map" for planning cycles was provided in general terms through Fig. 4; substitution of specific terms applicable to the planetary entry mission is then our first task. In fully-expanded form this substitution generates a detailed plan for each of the four definition cycles shown in Fig. 4.

MISSION DEFtSmOS CYCLE

To begin this cycle, we translate the objectives into terms suitable for analysis, identify quantitatively the constraints, and determine the selection criteria. A partial example of this is given in Fig. 5. It should be noted that the objectives for this cycle are the output of a higher level policy cycle which is the responsibility of the cognizant top management.

In identifying the constraints quantitatively it is not always possible to specify a numerical value with a high degree of confidence because of the many external contributing factors. Nevertheless, as John Stuart Mill noted in the last century. "Knowledge insufficient for prediction may yet be valuable for guidance." For instance, the total funds available for planetary programs over the next five to ten years may not be known with certainty. Hence, the proper approach is to specify the most likely available range of funds, in order to make it possible to understand the effect of such a variable on the conclusions, and thus to develop an approach (if possible) that is valid for the entire range.

Another consideration is whether to treat a constraint as "physical" or '"financial." In our example, launch vehicle availability is treated as a physical constraint, while it is obviously dependent to a certain extent on funds available. This is resolved by excluding launch vehicle development funds from funds available, and including only flight article procurement funds. However, it should be noted that placing of constraints in the financial area tends to broaden the scope of the analysis: this should be minimized if effective results are to be achieved.

The selection criteria are a means of measuring how well the possible ai>proaches meet the objectives. There are two problems associated with characterization of selection criteria the dtermination of appropriate criteria and the establishment of their relative importance. For instance, criteria are derived from the objectives and constraints by asking such questions as: How well does the proposed set of missions match the scientific objectives? Is the emphasis on the proper place? Since the scientific objectives cover a wide range—extending from the exploration of Mars to exploration of comets and major planets—is the proposed set of missions broad enough and yet of sufficient depth to really advance our state of knowledge of the solar system? Are they consistent with NASA objectives, e.g., to maintain program continuity?

The establishment of the relative importance of the criteria can only be made after a detailed study of the objectives and constraints. One example of this judgment is given in Fig. 5.

Here the analysis step Involves development of possible approaches, within the constraints, in attaining the objectives. The procedure is to analyze the possible approaches in successive stages of increasing detail. For instance, in Fig. 5, the classes of possible planetary programs are first identified, then the classes of spacecraft which fit within each of these programs are developed. The classes of possible spacecraft must be identified to the extent that they can be compared with the overall objectives and determination made as to whether or not they are within the constraints. Therefore, the definition must include such items as weight, scientific capability, cost, and manpower requirements. It is then possible to combine these into any number of credible combinations. For the planetary exploration systems, for example, a total of 24 combinations could be identified. For brevity, only three combinations have been included in Fig. 5.

Having identified the possible mission combinations, there must be evaluated in terms of how well they meet the selection criteria. Every mission combination must be compared with each of the others to assess relative values. This is repeated for each of the criteria and a total evaluation is obtained by combining the results for each.

It is always wise to assess the validity of the conclusions by conducting a sensitivity analysis. This helps to determine the effect of slight changes in each of the value assessments, on the overall conclusions. If the conclusion is sensitive to a small change in the value assessment, then further effort should be made to insure the exactness of the value assessments; or, the plans should be constructed so as to be as compatible as possible with the more likely mission combinations.

The output of the Mission Definition Cycle is the identification of the missions for each year that best meet the objectives within the identified constrains. This is shown in Fig. 5 where the missions are described in terms of target body, type of mission, spacecraft class, launch vehicle and the broad scientific objectives. In this example, the first opportunity for a sophisticated planetary entry and landing system is judged to be the Mars 1975 Voyager. Therefore, the next three planning cycles concentrate on the requirements for this Mars entry lander system.

System definition cycle

As in the Mission Definition Cycle, the first step in the System Definition Cycle (Fig. 6) is to translate the objectives into terms suitable for analysis, identify quantitatively the constraints, and determine the selection criteria. More specifically, he objectives are translated into top-level functional requirements needed to implement the system for the mission. We also need to determine for each of the functional requirements the duration, the external environment during performance, and the reliability goals for each in order to meet overall reliability objectives. At this point, however, we come face-to-face with some constraints: external factors that influence the solution of our problem. For instance, the two chief physical constraints within which the planetary entry lander must be defined are: a) the uncertainties in our knowledge of the planetary environment; and b) the requirement that the entry-lander must be functionally compatible with the spacecraft that will deliver it to the vicinity of Mars. In addition, we must take into account the influence of financial constraints on the system definition. If the total cost of the lander exceeds the cost allocation from the previous cycle, the possibility of performing such a mission in 1975 is greatly jeopardized. Likewise, the selection criteria also are chosen to provide a measure of how well the possible systems meet the overall objectives. At the system definition level we see that the selection criteria range from reliability to growth capability for future missions.