This is the only reference I found so far about the presence of the HP-48 series in shuttle flights:
“and that, children, is why they take up a pile of HP handheld calculators with them. Used to be the HP41C/CV/CX series, today it’s the HP48SX/GX series. Without going into extended/expanded memory, the older 48SX can hold 288 K of memory. With extended/expanded memory, the new 48GX can hold over 1 M of memory.“
I am indebted to Daniel Weed, whom I made the acquaintance of at http://www.hpmuseum.org’s online forum, for the following testimonial. Daniel worked at NASA and here is his recollection of how the HP-42S was instrumental in his work:
I worked at the Johnson Space Center as an engineer from about 1986 to 1995. I originally worked on the Space Shuttle program and much of that work after the Challenger disaster was to develop “Contingency Abort” options. These were scenarios involving 2 or 3 Space Shuttle Main Engine (SSME) failures during launch.
I used the 42s while designing a new shuttle ascent abort guidance algorithm. In those days, getting time on the mainframe simulation was slow. We had to be very sure of the changes made to the simulation. Then we could run it in batch mode, then analyze the results. I could only get time every few days, so it was not a good tool for iterative design work. What I needed was a tool to validate the equations of motion before going to the mainframe. This is where the HP-42S comes in. I found it a lot easier to do all the prototype work on the 42S. Only after I verified the equations of motion, did I code it into the main simulation.
The abort mode I designed was called “TAL Droop”. TAL is the acronym for “Trans-Atlantic Landing”, an abort mode where the Shuttle flies across the Atlantic and lands in Europe or North Africa. TAL Droop was a scenario where 2 SSMEs fail sometime soon after SRB sep (Solid Rocket Booster separation). The T/W (Thrust to Weight ratio) was initially so low, below one, that the vehicle would begin to loose altitude. Literally falling back to earth. After a while it would burn enough fuel that the thrust to weight ratio became greater than one and the Shuttle began rising again.
The minimum altitude it drooped to was critical. If too low, thermal stress on the ET (External Tank) was expected to cause the tank to explode. Marshall told us the critical altitude, if I recall correctly, was 270,000 feet. I had to predict what this minimum droop altitude would be well ahead of time and display it in the cockpit so the crew would know to either ride it out hoping for a TAL landing, or get off the tank before it was too late. This meant a ditch in the Atlantic. The algorithm also had to balance the need to stay above this altitude, by pitching the nose up, and the need to gain as much forward velocity as possible by thrusting more to the horizontal.
I started with a set of differential equations of motion under a single engine scenario and solved these for position and velocity so that the vehicle state could be integrated to the minimum droop altitude. I didn’t have computer tools to help with the integration, so I borrowed a slab of 17 inch fan-fold printer paper and started the task by hand. This is where the HP-42S came in. After integrating, I developed a simulation on the HP-42S containing these equations. I programmed it to print out the altitude over time on the thermal printer. With the 42S I was able to rapidly validate the equations and prove the overall algorithm. I could also assess the effect of different burn attitudes to find the optimal thrust vector to maximize forward velocity while staying above the droop altitude limit.
I used the thermal printer to create graphs of the shuttle altitude profile under different initial conditions. I’d end up with graphs about 6 inches long showing the altitude going up, then drooping down, then up again. It was a beautiful thing to see the first time it worked and this perfect altitude profile printed out.
I kept the program on the 42s for over a year, through at least one breathless battery change. Then I printed the program, cut it to fit a sheet of paper, taped it to the paper and photocopied it. I did that because the thermal printing fades over time. I probably have it in my records somewhere.
The TAL Droop algorithm was the largest and most important program I had built to support my work at NASA. I continued to use it for years of course. Later when I worked on a robotic lunar lander I used the 42S similarly to prototype trans-lunar injection burns, lunar orbit insertion burns, Hohman transfers, and lunar landing guidance. By that time though we had good simulations and tools hosted on desktop Unix workstations. This obviated much of the need to use the calculator for developing algorithms.
The calculator I used was purchased from EduCALC in February 1989 for $89.95. The same month I also purchased the HP leather case ($17.95), and the thermal printer (105.95).
In November of 1989 I discovered that when I removed the batteries the calculator immediately lost all program and memory content. That was a bummer. HP replaced the unit. The serial number of the original was 2840A-xxxx, and of the replaced unit is 2919S-xxxxx.
Written by Daniel Weed.
Daniel graduated from the University of Texas at Austin with an Aerospace Engineering degree, and worked at the Johnson Space Center from 1985 to 1995. Daniel worked mainly on the Space Shuttle program, but later worked on the Space Station and a variety of advanced Lunar and Mars projects.
Excerpt from article “HP 41 in Orbit“:
By the time of the Space Shuttle’s first flight in 1981, microelectronics technology had advanced rapidly, driven by the economic forces of the consumer market. (To give a sense of this rapid pace, only ten years earlier, Apollo astronauts carried 5-inch slide rules with them on missions to the Moon.) One could thus purchase a pocket calculator, at a very low cost, which gave its user more computing power than a custom-made device produced specifically for a given space mission. For the Shuttle, NASA engineers did just that: they purchased an HP-41C programmable pocket calculator and loaded it with a variety of software for use by the crew. Only minor modifications were made: adding Velcro strips to the case, and removing a few parts that might “outgas,” or give off gases that might contaminate the cabin’s air.
Because of their low cost, NASA was able to buy several calculators and provide them to members of the crew for their personal use. The HP-41C in the National Air and Space Museum’s collections was used by Astronaut Sally Ride and several other astronauts on a total of nine Shuttle missions. The photograph of Ms. Ride shows her on the Challenger flight of June 1983. The calculator later donated to the Smithsonian is the one on her left. NASA eventually replaced these calculators with more sophisticated personal computing devices, including laptop computers, for Shuttle crews.
The Shuttle’s primary functions were still controlled by an on-board, custom-designed system, built by IBM and subject to a rigorous program of testing and validation. But the HP-41C took a lot of the computing load off that machine for mundane but necessary calculations, such as calculating when a given ground station was available to send data to or receive a communication from. The HP-41C also contained a clock, which allowed astronauts to set alarms and schedule experiments. (The clock was not available to consumers at the time of the first Shuttle launches, but NASA obtained pre-production modules that plugged into the calculators. Hewlett-Packard later offered these as an option.) It also served as an emergency backup for re-entry calculations in event of a main computer failure, although this has never happened. One program that was especially useful identified and calculated distance and direction to the nearest emergency runway, if the Shuttle developed a problem and had to de-orbit quickly. Fortunately, this program also was never needed.
Although the calculators themselves were identical to what anyone could purchase at the time, the software developed for these missions was mission-specific and far beyond what most calculator users might encounter. NASA wrote custom software for each mission and loaded it onto read-only-memory (ROM) modules that plugged into the machines. The crew carried spare modules, so that if there were a hardware failure, one could run the programs on one of the other calculators. The calculators weighed about 200 g (seven ounces), measured 14cm x 7.5cm x 3cm, and of course operated independently of the Shuttle’s main power supply.
The Air and Space Museum sought to obtain this artifact for two reasons. The first was its status as a object carried into space and used as personal equipment by a well-known astronaut, Sally Ride. The second was to illustrate how the pace of innovation in microelectronics-a technology whose origins were in the space programs of an earlier day-have given consumers access to computing power that matches what NASA provides its crew.
Press release describing the proposed use of the HP-65 in flight:
HP-65 IN SPACE
“65 Notes,” July 1975
Volume 2, Number 6, Page 7
PALO ALTO, Calif., July 8 — An 11-oz, $795 pocket calculator that can be programmed like a computer will play an important role in the historic Apollo/Soyuz rendezvous in space July 17.
The Hewlett-Packard HP-65 fully programmable pocket calculator will be used to calculate two critical mid-course correction maneuvers just prior to the linkup of the U.S. Apollo and the Russian Soyuz spacecraft. These maneuvers will take place 12 and 24 minutes after terminal phase initiation (the beginning of the last part of the flight before rendezvous).
The calculator also will be used as a backup for Apollo’s on-board computer for the final maneuvers prior to rendezvous and docking. The first use will be for the coelliptic maneuver (putting both spacecraft into the same orbit) when the vehicles are within approximately 100 miles of each other. The second will be for the terminal phase initiation calculations when Apollo is 22 miles from Soyuz. In both instances, the HP-65 will be used to solve the problems, and its answers will be compared with those of the on-board computer.
In the event of an on-board computer failure, however, the HP-65 will provide the only available solution for the mid-course maneuvers, since the spacecraft will not be in communication with ground stations at that phase of the mission.
A third set of calculations to be performed by the battery-powered HP-65 will allow the astronauts to precisely point Apollo’s high gain antenna at an orbiting satellite to assure proper communications with Earth.
NASA scientists have written programs of up to 1,000 steps and recorded them on tiny magnetic cards (100 steps per card). The astronauts will feed these cards into the HP-65 to automatically perform the critical calculations. In previous space flights, backup maneuver calculations were made manually, using charts. The HP-65 will substantially reduce the time needed to make the complex calculations and improve the quality, accuracy and confidence in resulting solutions.
Two HP-65s will be taken on the space flight, along with four sets of program cards and six spare battery packs.
The HP-65 is not the first HP pocket calculator to venture into space; an earlier model, the HP-35, went along on the Skylab missions.
The HP-65 is a general purpose calculator that can be programmed to go through a step-by-step routine at the touch of a few keys, solving extremely complex, lengthy or repetitive calculations quickly, easily and accurately. Users can write their own programs for the calculator or buy prerecorded program cards from Hewlett-Packard in the fields of finance, mathematics, statistics, electrical engineering, thermodynamics, stress analysis, surveying, medicine, aviation and marine navigation.
Hewlett-Packard pocket calculators are no strangers to adventure. They have served high upon the rugged slopes of Mt. Everest; at the LeMans, France, professional auto races; at the navigator’s station of the sailboat “Courageous” the successful America’s Cup defender; and in the cockpits of Powder Puff Derby aircraft race contestants.
The U. S. spaceship will begin its mission July 15 from Cape Canaveral.
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“.
Here’s an excerpt from a HP fan’s webpage about the HP-9100:
A standing ovation
Chance returned from Palo Alto to the Loveland Division and continued work on the HP 9100A. Shortly before its announcement in September, 1968, Chance and Jack Dunn, HP Loveland’s Marketing Manager, took a pre-production HP 9100A and an HP 9125A plotter to the Jet Propulsion Laboratory in Pasadena, California. As Chance explains, “It was a significant historic moment.” The System 9100 pumped out complex Bessel-function antenna patterns on the plotter and the JPL audience stood up and cheered. This demonstration of the HP 9100’s ability to excite the scientific and engineering community was an early indicator that HP really had developed a significant new product category.
Many people first learned about computers from their HP 9100A experiences. At least three famous people have interesting HP 9100A stories. The first is the story of Apple’s Steve Jobs. The HP 9100A was the first small computer Jobs saw and he “fell in love with it.” He realized that personal computing had the power to change people’s lives. Then he built a company to produce machines to do just that.
Author Arthur C. Clarke, who crafted one of the most infamous fictitious computers ever, the HAL 9000 in 2001: A Space Odyssey, was extremely impressed with the HP 9100A and mentioned that he’d like to have one. Barney Oliver presented an HP 9100A to Clarke in April, 1970. Clarke dubbed the machine HAL, Jr.
Finally, Dr. James van Allen, space scientist and professor emeritus at the University of Iowa, used an HP 9100A and the HP 9125A plotter to study the feasibility of using a gravity slingshot around Jupiter to allow Pioneer 11 to intercept Saturn. Pioneer 11 was retasked en route and arrived at Saturn before Voyager 1. Pioneer 11 is now leaving the solar system, heading towards the stars.