I just found this very interesting video on Youtube from Mr. Gerry Ouellet (sorry about the spelling, I couldn’t quite catch his last name) reminiscing about his use of various HP calculators while he was a “rocket scientist” as he puts it.
The book “Homesteading Space” recounts how the HP-35 was used in one of the Apollo flights to Skylab. However, if you read carefully, you will notice that the writer describes a calculator which can read magnetic cards. This of course can not be the HP-35 but more likely the HP-65. Apollo flights to Skylab occured in 1973 and 1974 and the HP-65 was released in 1974.
Here are some passages about calculator use in Skylab:
“I had to make his backup calculations on the closure rate,” he said. “I was sitting there with this little HP calculator and punching all those numbers in, going through this formula and backing up what the ground saw and what we saw in the spacecraft. There had to be a third vote and that was me. I never enjoyed making that calculation. You had to get it right. If you missed one keystroke, you had to start all over again and it was a long one. But that kept me busy. It kept me from bothering everyone else and being worried.”
“We had no general-purpose computers available in Apollo, only the special-purpose computers for navigation and other calculations,” he said. “So before flight I obtained a HP-35 hand-held calculator to assist me in tracking our motion around the Skylab. We still had to estimate our range and range rate by eye, but we measured angles with the Apollo ‘attitude ball,’ and I entered the numbers into the calculator. The HP-35 was quite helpful with a small program I had written manually and entered into the calculator on a small magnetic strip.
“When I resigned from NASA some thirteen years later, I still had this now ancient calculator in my possession. Technology was now leaps and bounds ahead of this old ‘antique.’ But I listed all the government property in my possession at that time, including the HP-35, with a request to pay for and retain it personnally. Naturally, this was more than government bureaucracy could manage, so I had to turn it in, after which it was probably junked some years later and lost to posterity as a potentially interesting artifact.”
But at last, I found a credible source stating that HP-35 calculators were indeed used in Skylab missions. The Smithsonian National Air and Space Museum states that “HP-35 calculators, however, were used in the Skylab missions replacing the slide rules carried on previous Apollo spacecrafts“.