HP-65 in Space-how powerful?

[Gene Dorr]

The HP-65 was only used on the 1975 Apollo-Soyuz (ASTP) flight. It was used to cross-check rendezvous solutions computed by the onboard Apollo Guidance Computer (AGC), and those computed by the RTCC (Real-Time Computer Complex) in Mission Control and radioed up to the crew. As for how complex the calculations were, the programs occupied 12 magnetic cards (capable of holding up to 100 steps each). The crew also had a program (which occupied 4 cards) that provided solutions for pointing the spacecraft high-gain antenna at a relay satellite.

The "usefulness" was in providing confidence in the rendezvous solutions -- if any one of the 3 computed solutions diverged from the others, the two closest solutions would have been given more weight. As it was, according to the crew (during the post-flight debriefing) the 3 solutions agreed pretty closely.

As for how "powerful" it was: compared to the onboard computer it was probably computationally in the same league. After all, the AGC had roots stretching back to the early 1960s. But you need to understand that the AGC was tightly integrated into the Apollo spacecraft, and maintained the state vector (orientation, location and direction and speed of travel) of the spacecraft at all times -- that's what it was designed for. The only thing the crew needed to tell the AGC was "where" the Soyuz was, which they did with the navigation optics (which were also integrated with the AGC). On the other hand, the HP-65 operated in complete isolation, and all the pertinent data needed to be keyed in by the crew. And while the AGC may have had very limited memory compared to today's computers (or even contemporary minis or mainframes), it was a LOT more than what was in the HP-65.

As for the HP-41, that's a completely different story. Certainly not in the same league as the AP-101 computers used on Shuttle, but extraordinary useful. It was flown on the very first Shuttle flight, and on every subsequent one for many years (basically until it was obsoleted by the HP-48). Multiple HP-41s were flown on each mission, and they provided the crew with information that hadn't been programmed into the Shuttle's computers. On the first few flights they provided contingency calculations (like, if something goes really wrong and the flight has to be cut short, where is the nearest available landing site?); but after the Time Module became available, the HP-41 was the primary means the crew used to know when the next communications pass would happen. They were also used to remind the crew when to perform certain routine tasks. There was an excellent article by Keith Jarett on all this in the short-lived magazine "Professional Computing" volume 1, number 4 (October/November 1984). (available at https://groups.io/g/hpseries80/files/Doc...20Magazine)