HP-65 in Space-how powerful?

[Matt Agajanian]

Hi all.

Wondering.…

How useful was the 65 on the space missions?

On that note, how about the 41?

[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)

[Archilog]

Many thanks, Gene Dorr, for this very documented answer.

[Felix Gross]

The usefulness of the HP-65 is e.g. documented in the NASA publication "CSM Rendevouz Book" available on the HP Museum USB Stick (always highly recommended).

More relevant to the initial question might be a webpage dedicated to HP calculators in space. E.g. https://hpinspace.wordpress.com/category/hp-65/

There is plenty of documentation about the Apollo onboard computer available. I found this http://web.mit.edu/digitalapollo/ very interesting reading as it also deals with the general human - spacecraft interface (e.g. total automatic control vs. pilot control).

Given this, comparisons between e.g. the computational power of the different units can be made. But a mere "floating point operation per second" (or something similar) comparison might be not a useful measure as the questions to include something in a mission were complex. Certainly the HP-65 beat the Pickett Slide Rule N600-ES e.g. used on Apollo 13 and now on display in the National Air and Space Museum. But that needed no recharger which might have been a plus in particular on that mission.

Feiix

[Archilog]

Many thanks, Felix Gross, for this very very documented answer.

[Eddie W. Shore]

Thank you for posting, Felix! It must have been a blast for the astronauts to use the HP 65 in space.

[Gene Dorr]

I include a link to the Rendezvous book Felix cites, as well as the HP-65 Checklist (kind of a "User Manual" for the ASTP crew) on my "HP-65 on ASTP" page - http://hp65.genedorr.com/.