A modular payload carrier for use in a launch vehicle includes at least one module configured to fit in a payload bay of the vehicle and attachable to at least one other module configured to fit in the bay. The track edges bounding each channel are longer than the opposite edges of the respective tracks in that they follow parallel meandered paths, with the result that each channel deviates from a mean helical path and is longer than the corresponding portion of the mean helical path. There is a total tool seat assembling length in between the first datum mark and the tool seat.