Two ports and a supply line

Salyut 6 had two docking ports. A resident crew could retain its Soyuz while a second spacecraft visited. That arrangement changed what the station could do: resupply and visiting crews no longer necessarily required removing the residents’ return vehicle from the only port.

A port is not just a hole in the structure. It belongs to a system of approach geometry, attachment, seals and services. Its value depends on compatible vehicles and an operating plan that can use it.

Progress 1 (Salyut 6) reached Salyut 6 in January 1978. The uncrewed freighter brought supplies and supported refueling. Instead of relying only on what launched with the station, the program could replenish resources during operations.

The distinction between stock and flow is useful here. A larger initial stock extends a stay once; a dependable supply flow can support successive expeditions. Neither is unlimited: transport capacity, launch opportunities, storage and consumption still constrain the schedule.

Consider a fictional station that uses supplies faster than its available transports can deliver them. Adding another docking port might reduce traffic conflicts, but it would not by itself increase the cargo carried on each flight or the available launch rate.

Conversely, increasing cargo capacity helps little if there is no usable place to attach. Salyut’s lesson is to evaluate the station and its transport together. Duration emerges from connected capabilities rather than from any one impressive component.