Because the International Space Station is being assembled in orbit, there was a need to verify in advance that it could, indeed, be assembled there and that the various assembled parts would function as intended. A digital preassembly process was devised to satisfy this need for verification, without having to perform assembly on Earth. The process enables designers to simulate the assembly of major elements of large structures by use of a computer-aided design (CAD) system. The process could also be applied in any type of manufacturing and in many types of construction.

Computational Models of mating end items (EIs) are refined by use of photogrammetry and used to simulate mating, in order to detect potential obstacles to final assembly.
The verification problem arises because Space Station components are being produced by subcontractors scattered across the United States and by international partners. Costs and schedule restrictions prohibit either building a full scale, high-fidelity mockup or shipping Station components to different locations for design verification. Components being built in Russia, Japan, Europe, and Canada will not be available for pre-launch interface tests with those components built in the United States.

In the digital preassembly process, two-dimensional hardware-production drawings are used to create three-dimensional computational models of the structural elements or other end items (EIs) that mate at a given interface (see figure). The process includes modeling of the mating surfaces and hardware as well as all external components installed near a stay-out zone (a region in the vicinity of the interface that must be kept clear of any potential obstruction). The process includes identification of any off-nominal variances of mating surfaces, alignment and latching components, externally mounted components, fluid lines, or cables that could potentially encroach on the stay-out zone and interfere with mating.

Once the EIs on both sides of the interface have been modeled as designed, mating is simulated in the CAD environment. Portable digital photogrammetric equipment is then used to measure the real EIs. These measurements are mapped back into the computational model, creating an as-built computational model of the mating EIs. The revised computational model of the first-measured EI (say, EI1) is loaded into the computer of the portable digital photogrammetric equipment, which is moved to the location of the other EI (EI2) when that EI becomes available. The revised computational model is then used to perform a digital mating, which helps to identify variances between mating elements and potential interferences that could create problems during mating. Thus, the digital preassembly process provides early indications of potential problems in mating and assembly. The data gathered in the digital preassembly process could also be mapped into flight-element parametric models (when available) to extend the assessment to more dynamic thermal and pressure conditions.

This work was done by Vincent E. Heyworth and William F. McGilton of Boeing for Johnson Space Center . For further information, contact the Johnson Space Center Commercial Technology Office at (281) 483-0474. MSC-22756