7.6 Systems Engineering on Multi-Level/Multi-Phase Programs 56
7.6.5 Multi-Phase Design and Assembly Considerations 60
Designing a system that grows by adding elements over time introduces unique SE considerations relative to those typical of more traditional SL/SP systems. A central
characteristic of ML/MP development is that the system needs to be able to operate acceptably not only in the assembly complete configuration, but also in interim configurations during the assembly phase. Operation during interim configurations enables the system to survive and potentially to be productive (e.g., enables limited research) as it is being built.
Figure 7.6-3 illustrates such a scenario for a notional space station. Flight elements (FEs) are designed and tested on the ground, placed into a launch vehicle as launch packages (LPs), and assembled onorbit to existing, incrementally larger stages over time. The system is shown as it exists at stages 1 and 2 (interim configurations) and at stage 10 (assembly complete
configuration). Stage Launch Package (LP) Flight Element (FE) On Orbit Assembly LP 2 FE 2 LP 1 FE 1 Stage 1 FE 2 Stage 1 Stage 2 FE 10 LP 10 FE 10 Stage 10 Stage 9
Figure 7.6-3 Flight Elements of a Notional Space Station being Designed, Launched and Assembled Onorbit into Incrementally Larger Stages over Time Three key SE considerations in designing an ML/MP system (utility sizing, launch and assembly sequence, and on-orbit maintenance) are discussed below.
7.6.5.1 Utility Sizing
The need to function during multiple, incrementally growing stages of assembly can drive utilities on ML/MP systems to operate under an unusually wide range of conditions. Utilities might include power, thermal control, environmental control and life support, propulsion,
attitude control, command and data handling, communication, etc. Utilities that provide bus-type services at interfaces to multiple elements across a space station-like facility typically are sized for the most demanding condition, usually that associated with the assembly-complete
configuration with a full crew and with full mission and payload operations. However,
performance needs to be verified not only for the most demanding condition, but also for a wide range of lower-load conditions at interim stages of assembly. For example, thermal trunk lines and pumps that transport thermal control system fluid from element (e.g., pressurized module) interfaces to radiators are sized to reject the required heat load at assembly complete, even though those trunk lines and pumps are operated under much lower-load conditions in prior stage configurations. Doing this avoids the need to retrofit trunk lines and pumps each time heat load from a new element is added. As a consequence, however, these trunk lines and pumps may be significantly oversized for operation at lower loads during early assembly stages.
Performance and operation of the utilities may also be affected by changes in external geometry as well as by varying attitudes required during assembly. For example, geometry changes and/or attitude changes can influence aspects such as power generation, thermal radiation, propulsion plume impingement, attitude control, flight mechanics, communication lines of sight, and approach corridors for launch vehicle rendezvous and docking.
7.6.5.2 Launch and Assembly Sequence
Choreographing the assembly sequence such that the required functional capability, fault tolerance, and maintainability is present at each stage while also effectively utilizing launch vehicle payload is an ever-present consideration in ML/MP systems. The system, e.g., flight elements, utilities, etc., needs to be incrementally manifested on the launch vehicle in a useful sequence to provide the required on-orbit capability during assembly. For example, if a habitable human presence is required at stage 3 to conduct selected experiments, stage 3 needs to have both a pressurized module and the required utilities. If that required human presence is
permanent, the stage 3 utility capability needs to be sufficient to sustain the crew permanently while conducting selected experiments and to provide full fault tolerance for crew survival. If, however, that human presence is only temporary, i.e., the crew is aboard when a launch vehicle with full crew return capability is present, stage 3 may need neither the capability to support the crew permanently nor the capability to provide full fault tolerance for crew survival as the launch/return vehicle may be able to supplement the required stage 3 capability and fault tolerance.
Assembly sequence and SE complexities greatly increase with the number of stages. For example, at each stage, requirements that meet stakeholder needs have to be defined and controlled, the design has to be analyzed and verified, a concept of operations has to be defined to validate operations, etc. As the number of stages typically is driven by available launch
capacity, the launch vehicle becomes a central consideration in ML/MP systems. Included in this consideration is launch vehicle reliability, noting that larger launch systems enable reduced on- orbit assembly time but also risk larger portions of system assets to a single launch failure.
7.6.5.3 On-Orbit Maintenance
As ML/MP systems are likely to be associated with programs that have relatively long on-orbit lives, they typically will need to be designed for on-orbit maintenance. Depending on the fault tolerance philosophy employed, components needed for time critical repair may need to be stored on board, whereas other components may be able to be provided by scheduled resupply flights. On-orbit maintenance during assembly also needs to be planned for, particularly for programs that have extended assembly phases.