This article is an overview of the NASA document “Apollo Experience Report – Consumables Budgeting”. The report was published in 1972.
The consumables budgeting and predicting experience gained in the Apollo development missions and on the first lunar-landing mission are presented. The predicting experiences are limited to the propulsion, environmental, and electrical subsystems of the command and service module and the lunar module. Basic considerations pertaining to subsystem modeling and consumables prediction are presented with recommendations for advanced systems.
The development of simplified computer models that simulate the consumables systems has been proven to be an invaluable tool for use by the system engineer in the evaluation of the consumables budget. All the system requirements and constraints affecting the consumables predictions as well as the system procedures, the environment in which the subsystem will operate, the system-performance data, contingency and redline philosophy, crew procedures and modes of operation have to be identified for accurate predictions. Design considerations for advance systems should include better systems gaging, redundant-system performance modes that are similar to primary modes, the capability to transfer consumables that are mutually similar to different systems, and more conscientious documentation of performance data during system tests.
The importance and need of subsystem consumable budgeting and analysis for mission planning were recognized early in the Apollo Program. For example, the earth orbital spacecraft development flights required a great deal of mission/system compatibility evaluation because systems designed for the lunar environment and operations had to be used and verified in the near-earth environment. Trajectory and attitude-maneuver techniques, sequences, procedures, and timelines affecting the consumables for all the Apollo missions had to be selected that met the test objectives within continually evolving system constraints. Adequate consumable margins had to exist for the nominal flight plan as well as for the alternate and contingency plans. In some cases, the consumables for the nominal plan were dependent on whether the mode of operation would be manual or automated or if the primary or the backup subsystem would be used. Also, the fact had to be considered that a particular subsystem consumable (for example, the reaction control system (RCS) propellant) had to be budgeted for three different modules, each of which had different requirements and constraints.
Because of these and other factors, the task of predicting the consumable budgets for a lunar-landing mission was very complex.
The purpose of the report is to document how the consumables- budgeting task was accomplished for the Apollo electrical, environmental, and propulsion subsystems. The basic considerations for subsystem modeling and consumables predictions are presented together with the consumables-prediction experiences gained and significant problems experienced during the development flights and the first two lunar- landing missions. Recommendations for advanced systems are presented also.
DEFINITION OF TERMS
A consumables analysis is complete when every aspect of the usage is defined.
Definition of this usage should be consistent for all consumables subsystems to allow the spacecraft manager to communicate in a common language with the mission-planning engineer and the spacecraft designer. The following items, which make up the budget, are used in this report in the context shown.
Loaded or capacity: The capacity is that quantity of consumables that is the nominal expected load. For batteries, it will represent the nominal expected ampere-hour rating.
Unavailable: That quantity of consumables that nominally cannot be considered for use in the mission plan is termed “unavailable.” Unavailable consumables consist of the sum of defined nominal unusables, which are as follows: trapped and otherwise unavailable consumables; outage resulting from a mean mixture-ratio imbalance; and gaging, telemetry (TM), and real-time computational errors
Available for mission: That quantity of nominal-usage consumables remaining after the unavailable consumables are accounted for is termed “available for the mission.”
Required for mission: That quantity of consumables that is needed in order to perform the nominal design or operational mission is termed “required for mission.”
Nominal remaining: That quantity of consumables remaining after consideration of the unavailable and nominal mission performance requirements is termed ” nominal remaining.”
Dispersions: That quantity of consumables that involves consideration of the dependent and independent variables is called “dispersions.” Some sources to be considered are variations in loading, flow rate, inert weight, system performance, to perform the nominal design or operational mission is termed “required for mission.”
Nominal remaining: That quantity of consumables remaining after consideration of the unavailable and nominal mission performance requirements is termed ” nominal remaining.
Dispersions: That quantity of consumables that involves consideration of the dependent and independent variables is called “dispersions.” Some sources to be considered are variations in loading, flow rate, inert weight, system performance, mixture ratio, flight parameters, and crew effect. Usually, sources are root sum squared; however, in some cases they may be considered as a separate bias.
Pad: The pad is that quantity of consumables remaining after all the previous considerations have been accounted for.
Contingencies: The contingencies are that quantity of consumables allotted for an operational philosophy with low probability of occurrence, or those events that must be budgeted to ensure successful recovery, or both types of events. Contingencies can be related to partial system failure or to flight-plan changes that will necessitate the use of additional consumables. All candidates for contingency allowance should be identified by subsystem and mission designers.
Outage: Propellant outage is that amount of fuel or oxidizer that remains when the other component is depleted. This amount of either fuel or oxidizer then is not available for maneuvers.
Margin: That quantity of consumables remaining after the highest probability of consumables usage has been accounted for is called the “margin.”
From the consumables-analysis experience gained in the Apollo development and early lunar-landing missions, the following conclusions are stated.
1. The consumables analyst must define the consumable available for usage and must know when the system is to be used, in what mode of operation it will be used, and what constraints must be observed.
2. The development of simplified computer models that simulate the consumables systems has proven to be an invaluable tool for use by the system engineer in the evaluation of the consumables budgets. Also, these models have been extremely useful for conducting analyses for management-proposed system redesign. Proper evaluation of the consumables subsystem schematic usually resulted in methods for further reducing the modeling requirements and computation time.
3. Usually, performance data on the system were obtained as a second-order priority in verifying the operational capability of the system.
4. System procedures and the environment in which the subsystems will operate have to be established and identified.
5. All the system requirements, constraints, and assumptions affecting the consumable predictions need to be identified.
6. Monitoring flight-crew techniques during simulations resulted in major improvements in propellant-usage predictions.
7. For some consumables, a system-prediction accuracy of 10 percent is acceptable. Other systems, such as main propulsion systems, necessitate a prediction accuracy of 1 percent to prevent excessive weight problems.