The objective of Step 2 is to compile a comprehensive list of feasible decision alternatives through a discussion of a reasonable range of alternatives. The result is a set of alternatives that can potentially achieve objectives and warrant the investment of resources required to analyze them further.
3.1.2.1 Compiling an Initial Set of Alternatives
Decision alternatives developed under the design solution definition process [2] are the starting point. These may be revised, and unacceptable alternatives removed after deliberation by stakeholders based upon criteria such as violation of flight rules, violation of safety standards, etc. Any listing of alternatives will by its nature produce both practical and impractical alternatives. It would be of little use to seriously consider an alternative that cannot be adopted; nevertheless, the initial set of proposed alternatives should be conservatively broad in order to reduce the possibility of excluding potentially attractive alternatives from the outset. Keep in mind that novel solutions may provide a basis for the granting of exceptions and/or waivers from deterministic standards, if it can be shown that the intents of the standards are met, with confidence, by other means. In general, it is important to avoid limiting the range of proposed alternatives based on prejudgments or biases.
Defining feasible alternatives requires an understanding of the technologies available, or potentially available, at the time the system is needed. Each alternative should be documented qualitatively in a description sheet. The format of the description sheet should, at a minimum, clarify the allocation of required functions to that alternative's lower-level components. The discussion should also include alternatives which are capable of avoiding or substantially lessening any significant risks, even if these alternatives would be more costly. If an alternative would cause one or more significant risk(s) in addition to those already identified, the significant effects of the alternative should be discussed as part of the identification process.
Stakeholder involvement is necessary when compiling decision alternatives, to assure that legitimate ideas are considered and that no stakeholder feels unduly disenfranchised from the decision process. It is expected that interested parties will have their own ideas about what constitutes an optimal solution, so care should be taken to actively solicit input. However, the initial set of alternatives need not consider those that are purely speculative. The alternatives should be limited to those that are potentially fruitful.
3.1.2.2 Structuring Possible Alternatives (e.g., Trade Trees)
One way to represent decision alternatives under consideration is by a trade tree. Initially, the trade tree contains a number of high-level decision alternatives representing high-level differences in the strategies used to address objectives. The tree is then developed in greater detail by determining a general category of options that are applicable to each strategy. Trade tree development continues iteratively until the leaves of the tree contain alternatives that are well enough defined to allow quantitative evaluation via risk analysis (see Section 3.2).
Along the way, branches of the trade tree containing unattractive categories can be pruned, as it becomes evident that the alternatives contained therein are either infeasible (i.e., they are incapable of satisfying the imposed constraints) or categorically inferior to alternatives on other branches. An alternative that is inferior to some other alternative with respect to every performance measure is said to be dominated by the superior alternative. At this point in the RIDM process, assessment of performance is high-level, depending on simplified analysis and/or expert opinion, etc. When performance measure values are quantified, they are done so as point estimates, using a conservative approach to estimation in order to err on the side of inclusion rather than elimination.
Figure 19 presents an example launch vehicle trade tree from the Exploration Systems Architecture Study (ESAS) [23]. At each node of the tree the alternatives were evaluated for feasibility within the cost and schedule constraints of the study’s ground rules and assumptions. Infeasible options were pruned (shown in red), focusing further analytical attention on the retained branches (shown in green). The key output of this step is a set of alternatives deemed to be worth the effort of analyzing with care. Alternatives in this set have two key properties:
They do not violate imposed constraints
They are not known to be dominated by other alternatives (i.e., there is no other alternative in the set that is superior in every way).
Alternatives found to violate either of these properties can be screened out.
Planetary Science Mission Example: Compile Feasible Alternatives
A trade tree approach was used to develop potential alternatives for the mission to Planet “X.” As shown in the trade tree below, the three attributes that were traded were the orbital insertion method (propulsive deceleration vs. aerocapture), the science package (lighter, low-fidelity instrumentation vs. heavier, high-fidelity instrumentation), and the launch vehicle (small, medium, and large). However, initial estimates of payload mass indicated that there was only one appropriately matched launch vehicle option to each combination of insertion method and science package. Thus, eight of the twelve initial options were screened out as being “infeasible” (as indicated by the red X’s), leaving four alternatives to be forwarded to risk analysis (alternatives 1 – 4).
Trade Tree of Planetary Science Mission Alternatives Feasible Alternatives forwarded to Risk Analysis Scientific Orbiter for
Collection of Atmospheric Data at Planet X
Propulsive Deceleration into Planetary Orbit
Aero-capture Maneuver to Decelerate into Planetary
Orbit
High Fidelity Science Instrumentation Low Fidelity Science
Instrumentation
Small Lau Vehicle
Medium Launch Vehicle
Large Lau h Vehicle
Small Lau Vehicle
Medium La ch Vehicle
Large Launch Vehicle
High Fidelity Science Instrumentation Low Fidelity Science
Instrumentation
Small Launch Vehicle
Medium L ch Vehicle
Large Lau h Vehicle
Small Lau Vehicle
Medium Launch Vehicle
Large Lau h Vehicle
Medium High Fidelity Aerocapture 4 Small Low Fidelity Aerocapture 3 Large High Fidelity Propulsive Insertion 2 Medium Low Fidelity Propulsive Insertion 1
Launch Vehicle Size Science Package Orbital Insertion Technology Alt # Medium High Fidelity Aerocapture 4 Small Low Fidelity Aerocapture 3 Large High Fidelity Propulsive Insertion 2 Medium Low Fidelity Propulsive Insertion 1
Launch Vehicle Size Science Package
Orbital Insertion Technology Alt #