# Flight-critical calculator

“In addition to helping the crew organize its time, the second HP-41 computer was kept ready for flight-critical, deorbit-burn calculations. Once during each orbit around the Earth, the shuttle has an opportunity to land at one of six contingency locations. During a routine flight, Mission Control supplies the shuttle crew with deorbit-burn information. Should the shuttle encounter an emergency, however, the astronauts would rely on the HP-41 for these calculations.”

―”HP-41’s Again Aboard Columbia.” HP Key Notes, March-May 1982.

To prepare an orbiting Space Shuttle for re-entry through the Earth’s atmosphere it is critical that the spacecraft be “balanced” by taking into account the mass of the fuel left in the tanks at the end of a mission. An astronaut would use a handheld computer or programmable calculator to determine how many minutes and seconds of fuel to burn to get the center of gravity correct for a smooth descent and landing. The “personal computing system” used for this was made by Hewlett Packard in the 1980s. NASA donated one of these, a model HP-41CV, to Ladd Observatory after the retirement of the Shuttle program.

It contains a clock and calender with alarm functions. The orbit of the vehicle was used to compute the future position and it would sound an audible tone when within range of a communications ground station. It was also used to compute the time of “sunset” when the Shuttle moved into the shadow of the Earth. The memory is large enough to hold programs with a few thousand instruction steps.

This one was used at the SMS, the NASA Shuttle Mission Simulator. It was manufactured in 1983. There were no programs in memory when it arrived as the batteries had been removed before it was shipped to us. It is in good working condition and can run programs such as solving Kepler’s Equation of orbital motion.

There are custom labels on the keypad such as O2 FLOW, N2 FLOW, and CABVOL indicating that it was used to calculate the oxygen and nitrogen mixture of the atmosphere in the crew compartment. The program was started by pressing a button labeled LEAK. The results would then be used to set the switches on the Pressure Control System panel.