enter and clear. The display could show minutes and seconds which would count down to orbital benchmarks, or it could display keystrokes when using the interface. The interface could be used to change the software program. The user interface was called the Digital Address System (DAS) and could send commands to the computer's command system. The command system could also get commands from the ground. For personal computing needs Skylab crews were equipped with models of the then new hand-held electronic scientific calculator, which was used in place of slide-rules used on prior space missions as the primary personal computer. The model used was the Hewlett Packard HP 35. Some slide rules continued in use aboard Skylab, and a circular slide rule was at the workstation. Plans for re-use after the last mission The three crewed Skylab missions used only about 16.8 of the 24 man-months of oxygen, food, water, and other supplies stored aboard Skylab. A fourth crewed mission was under consideration, which would have used the launch vehicle kept on standby for the Skylab Rescue mission. This would have been a 20-day mission to boost Skylab to a higher altitude and do more scientific experiments. Another plan was to use a Teleoperator Retrieval System (TRS) launched aboard the Space Shuttle (then under development), to robotically re-boost the orbit. When Skylab 5 was cancelled, it was expected Skylab would stay in orbit until the 1980s, which was enough time to overlap with the beginning of Shuttle launches. Other options for launching TRS included the Titan III and Atlas-Agena. No option received the level of effort and funding needed for execution before Skylab's sooner-than-expected re-entry. The Skylab 4 crew left a bag filled with supplies to welcome visitors, and left the hatch unlocked. Skylab's internal systems were evaluated and tested from the ground, and effort was put into plans for re-using it, as late as 1978. NASA discouraged any discussion of additional visits due to the station's age, but in 1977 and 1978, when the agency still believed the Space Shuttle would be ready by 1979, it completed two studies on reusing the station. By September 1978, the agency believed Skylab was safe for crews, with all major systems intact and operational. It still had 180 man-days of water and 420-man-days of oxygen, and astronauts could refill both; the station could hold up to about 600 to 700 man-days of