Introduction

Those of you old enough to remember the Apollo Moon missions no doubt also remember watching the splashdowns on TV. For those of you not old enough to remember, the predicted splashdown points were so accurate that, in most cases, the carriers in the recovery fleet had to back off about a mile so that the Apollo Command Module (CM) would not land on the deck. It just so happens that I was the computer programmer who wrote the software that predicted the splashdown points for all of the Apollo missions. You may find some of my Apollo experiences interesting.

Technical Description

The software in question was the Apollo Reentry Runge-Kutta Numerical Integrator (Reentry Integrator) which was part of the Reentry Subsystem of the Apollo ground control system. The Reentry Integrator was written in the FORTRAN programming language and ran on an IBM 360-75 computer, one of the fastest computers available at the time. The Apollo ground control system was developed by IBM using requirements developed by Rockwell International. The work was performed under contract with the NASA Manned Space Center (MSC) (now known as the Lyndon B. Johnson Space Center (JSC)), Houston, Texas.

For those of you who are not familiar with integration in mathematics, if one integrates acceleration, one gets velocity, and if one integrates velocity, one gets position. That means that if you know the current position, velocity, and time, the acceleration can be measured or calculated and integrated to get a new position and velocity at some small increment of time in the future. Generally, analytical integration is not possible on a computer, so a numerical iterative method is used. The Runge-Kutta numerical integration method was selected for the Apollo System.

Of course, the most critical part of the Apollo reentry was the Service Propulsion System (SPS) burn to decrease the velocity for reentry. The CM had to hit the entry interface (400,000 ft.) at the correct velocity and angle of attack for a safe reentry. The nominal velocity was 36,200 feet per second (about 24,700 MPH) at an angle of attack of -6.5 degrees with respect to the horizontal with a tolerance of about 0.5 degrees. At a steeper angle, the CM would burn up and kill the astronauts. If the actual entry interface conditions were not perfect, some adjustments could still be made to the reentry trajectory using the CM roll thrusters to control the lift. At a shallower angle, the CM would skip off the atmosphere and into orbit. Since the CM had only 45 minutes of life support, skipping into orbit would also kill the astronauts.

After the SPS burn, the CM separated from the Service Module. The Reentry Integrator picked up the state vector (position vector, velocity vector, and time) for the Apollo CM just after Command/Service Module Separation. The Reentry Integrator used the Earth's gravitational attraction, the Moon's gravitational attraction, a model of the Earth's atmosphere, the aerodynamic characteristics of the CM, and other factors to calculate the current acceleration vector at one-second intervals. That acceleration vector was then integrated to determine a new velocity vector which in turn was integrated to determine a new position vector. This whole process was continued until the chutes were deployed.

The Reentry Integrator used the same guided reentry algorithm that was used in the Apollo Guidance Computer onboard the Command Module. The Apollo Guidance Computer and the guided reentry algorithm were developed by the Charles Stark Draper Laboratory at Massachusetts Institute of Technology.

Throughout reentry, updates were made to the state vector, as needed, based on ground radar tracking data. For example, during the approximately 3-minute communications blackout period, the calculated state vector may deviate from the actual state vector necessitating an update. The blackout period was of significant concern because it coincided with part of the reentry trajectory where the CM skips off the atmosphere. While in blackout, the CM skipped from about 180,000 ft. to about 300,000 ft. before continuing downward to chute deployment and splashdown.

Software Development Environment Circa 1966

The computers used for software development and testing, as well as controlling the Apollo missions, were located at NASA MSC in the Mission Control Center (MCC) located in Building 30. The computer complex itself was known as the Real Time Computation Center (RTCC) and was located in a room that must have been between one-quarter and one-half acre in size. There were 5 IBM 360-75 computers in the RTCC, including IBM 360-75 serial number 0001. The IBM 360-75 had the equivalent of 256 kilobytes of memory, compared to the 128 megabytes on the 450MHz Pentium II computer on my desk. My desktop computer is at least 350 times as fast as the IBM 360-75.

When we programmers developed software we first wrote out all of the code on coding sheets, in this case, FORTRAN coding sheets. We then sent the coding sheets to the keypunch operators who punched the FORTRAN statements onto computer cards. Then the cardpunches were verified by other operators and were returned to us. This deck was known as the source deck. We then attached the appropriate Job Control Language (JCL) cards to the deck and sent the entire deck to the computer room for a FORTRAN compilation. We also attached additional JCL cards and data cards to the end of the deck so that if we got a successful compilation, we could execute the program. If we got a successful compile, the computer also punched out an object deck in binary format. For further executions we could use the object deck with attached JCL and data cards. If the program failed to compile, we got a printout of the errors. We then corrected the errors and resubmitted the deck.

I worked in the IBM building about a half mile from the MSC gate, but the computers we used were in the RTCC at MSC so we had to send our card decks there by courier. In the case of the Reentry Integrator, the source deck was about 4,000 cards (2 card boxes) that compiled into an object deck of about 200 binary cards. Because of the size of the deck and the amount of handling they received, it was inevitable that accidents would happen where the deck was dropped and the cards ended up out of order. We very quickly learned to punch sequence numbers in all of the cards so that we could put them back in order with an IBM sorting machine.

When it was necessary for us to take the cards to the RTCC, we had a card case to carry the cards. This case was made of steel with a handle on one end and was long enough to hold about 4,000 cards. It looked like a long, thin, steel suitcase painted gray.

Testing

Since lives were at stake, the Reentry Integrator was probably one of the most thoroughly tested software programs ever produced. I probably ran at least 1,000 test cases through the program and every branch was executed many times. After the Reentry Integrator was incorporated into the Reentry Subsystem and into the whole Apollo Control system, it was executed thousands of times more before it was ever used in a live mission.

In the early days of Apollo software development and testing, the IBM 360-75s were new, having just been announced in April 1964. Consequently, the operating system (OS) and the FORTRAN compiler were very unstable. We often had to schedule two hours of computer time just to attempt one FORTRAN compilation only to have the compiler or the OS fail. We quickly learned to arrange our lives around scheduled time at any time, day or night.

Eventually, the hardware, the OS, the compiler, and everything else started to work together in harmony. We were then able to send our jobs to the computers by courier and get the results by courier a day or two later. In addition, we still had to support scheduled time, but then it was for sub-system and system tests.

We used an Apollo simulator developed by TRW and we were able to exercise our software with simulated telemetry and tracking data. We sat at the consoles and viewed the results of the simulations and if we found a problem, we would get a dump of all of the data in the computer. Then we would go back to our desks and read the dumps to find out what happened. The dumps were printed in the hexadecimal (base 16) number system so we had to convert the data to decimal (base 10) before we could analyze it.

We all took turns supporting the simulations by manning the consoles and acting as flight directors. Occasionally we performed live simulations with astronauts sitting in the Apollo Command Module (CM) at the Kennedy Space Center (KSC). It was during one of those live simulations on January 27, 1967, that the Apollo 1 CM caught fire and killed Gus Grissom, Edward White, and Roger Chaffee. It was not my turn to man the console that night, but it was one of my friend's turn. He said that they just shut down the simulation and went home. When he came to work the next day, he was visibly shaken over the tragedy. (Historical note: It was exactly 19 years and one day later that the Challenger exploded.)

That fire set the Apollo Program back about 18 months to change the interior materials in the CM to something that would not burn. Apollo 7, launched October 11, 1968, was the first manned orbital flight. As you probably know, Apollo 11 was launched on July 16, 1969, and at 4:17 PM ET on July 20, 1969, Neil Armstrong announced: "Houston. Tranquillity Base here. The Eagle has landed."

History

You may ask, "Why was the Manned Space Center located in Houston?" There were three main reasons for locating the MSC in Houston, TX: 1) Lyndon B. Johnson (D, TX), as Vice President, was chairman of the Space Council for President Kennedy, so the MSC had to be in Texas; 2) Rep. Albert Thomas (D, TX) was Chairman of the House Space Committee, so the MSC had to be in his district; and last, but certainly not least, 3) Rice University donated the land for the MSC. The land was part of what was known as the Old West Ranch, a cattle ranch owned by "Silver Dollar Jim" West (he received that nickname because he tipped everyone with silver dollars). West was already a wealthy cattle rancher when oil was discovered on his ranch early in this century and he became even richer from the royalties from Humble Oil Company (now Exxon). West bequeathed part of his ranch to Rice University with the stipulation that it be used for scientific research. The trustees ruled that donating the land to NASA fulfilled that stipulation. The West Mansion still stands just to the east of NASA JSC on NASA Road #1.

Personal Note

Before the end of the Apollo Project I was transferred to New Jersey to work on another space project. During the Apollo 15 flight in July 1972, we were on a camping trip through upstate New York, Ontario, Upper Michigan, Wisconsin and back to NJ. The whole time we were gone I was lamenting the fact that we had no TV and I wouldn't be able to watch the splashdown. We camped overnight near Manitowoc, WI so that we could take the Chesapeake & Ohio ferry across Lake Michigan to Luddington, MI. Lo and behold, there was a TV on the ferry and I got to watch the splashdown as we crossed Lake Michigan. To this day, my wife claims that I scheduled the whole trip so that I would be on that ferry to watch the splashdown on TV.

In my whole career with IBM as a computer programmer and software systems engineer, I never worked so hard and enjoyed myself more than the years that I worked on the Apollo Program. I later worked on the Space Shuttle Program and I was working on the Space Station Program when I retired in 1991.