Mike Slack’s NASA Career Background

Mike Slack’s Aerospace Engineering Education

After obtaining his B.S. in Aerospace Engineering in May 1973 from Texas A&M University, Mr. Slack entered graduate school in June 1973 in pursuit of an M.S. in Aerospace Engineering. During graduate school, he worked with Dr. James Rand in Texas A&M’s Hypervelocity Impact Laboratory. He completed his graduate academic course and his thesis defense in August 1974. His thesis focused on the accuracy of numerical methods to predict satellite orbital positions over a long period.

Mike Slack’s Initial Career at NASA

In August 1974, Mr. Slack went to work at the NASA Johnson Space Center, Houston, Texas in the Vehicle Dynamics Section of the Structures and Mechanics Division. His initial responsibilities involved supporting the analytic work of the dynamics of docking scenarios between the Apollo and Soyuz vehicles for the Apollo-Soyuz Test Project (“ASTP”) mission scheduled for July 1975.

After the ASTP mission, Mr. Slack’s section leader asked him to investigate concepts for mitigating Space Shuttle structural interface loads during ascent. The Space Shuttle consisted of three primary elements: an Orbiter, an External Tank, and two Solid Rocket Boosters (“SRBs”).

The interface loads of primary concern involved structural elements that connected the Space Shuttle Orbiter to the External Tank.

With the Orbiter, a winged vehicle, connected to the External Tank by struts, the structure was susceptible to excessive loads as the Shuttle flew through and reacted to certain upper-level wind conditions in the jet stream. The Orbiter was influenced by thrust loads from the three main engines, aerodynamic and inertial loads. The External Tank contributed inertial loads which changed as the fuel and oxidizer volumes were depleted during ascent. The Solid Rocket Boosters imparted thrust and inertial loads. Most importantly, as the upper-level winds changed in speed and direction, the loads on the Shuttle interface elements changed for a given ascent trajectory profile.

Mr. Slack started by building a 6-degree-of-freedom computer simulation that would take wind data and ascent trajectory data and compute rigid body loads in the various structural elements that were critical during ascent. The trajectory data used in Mr. Slack’s program was supplied by another NASA group doing OFT-1 trajectory development work in a “made to order” format specified by Mr. Slack.

Once the computer program had been developed and the accuracy of the computations verified, Mr. Slack started examining many different wind profiles for the ascent trajectory planned for the first Space Shuttle mission designated Orbital Flight Test-1 (“OFT-1”). The computer program not only identified the load conditions for each of the interface elements, but it computed the contributions of aerodynamic, inertia, and thrust loads to the total load on a given interface element. That information was provided to the engineers doing the OFT-1 ascent trajectory development.

Mr. Slack confirmed that the upper-level wind profiles from the launch complex in Florida which caused the most concern for ascent loads occurred in the spring and fall months. As he did more and more analyses of wind profiles, he corroborated his ascent load data with that of the primary contractor, North American Rockwell.

The wind data Mr. Slack used in his initial studies were gathered by weather balloons carrying equipment that measured wind speed, direction, and altitude as the balloons ascended. The data would be compiled and written on computer tapes. The wind data for a particular date and time was referred to as a wind profile. Wind data collected near the launch facility at the Kennedy Space Center would be saved to computer tapes and the reels would be sent to Houston where the data would be used in Mr. Slack’s computer program. Later, as technology improved, the wind data could be sent between the Kennedy Space Center and computers in Houston on 2400 Baud modems.

The initial version of the program that Mr. Slack wrote predicted structural loads for a given wind profile and trajectory. Eventually, Mr. Slack started tailoring the trajectory to a specific wind profile so that the aerodynamic, inertia, and thrust loads acting on the vehicle were minimized. The program was modified and refined to yield the guidance inputs to the Shuttle main engines and ailerons that would produce the minimum loads possible for the wind profile. In other words, the Shuttle could be told what to anticipate from the winds and provide the appropriate vehicle reaction to eliminate or mitigate as many adverse load effects as possible. Mr. Slack describes the process as “snaking the Shuttle through the friendliest load path to orbit.”

Eventually, Mr. Slack determined that the closer the winds could be measured before launch and the ascent trajectory tailored to the wind profile, the less risk of excessive loads in the interface structural elements. The risk of a launch delay due to unfavorable winds that were predictive of excessive loads could be greatly reduced using loads-tailored guidance inputs resulting in cost savings and enhanced safety.

Mr. Slack traveled to San Diego to study a system being used on Atlas boosters where upper-level wind data collected just a few hours before launch was being used to modify the baseline trajectory so that the ascent loads on the booster were minimized. The question was whether this concept could be applied to the Shuttle which had more complexity in terms of the influences of upper-level winds than the symmetrical Atlas booster.

Mike Slack’s Work with “Day of Launch I-Load Update” or DOLIU

On his return, Mr. Slack’s supervisors cleared him to begin developing a computer program that would do, for the Shuttle, what was being done with the Atlas. Unfortunately, the notion of uploading instructions to the guidance system on an unproven Shuttle just a few hours before launch to accommodate the prevailing wind profile was unsettling to managers. That concern was certainly justifiable since the Shuttle had never flown and guidance uploads shortly before launch were not a risk managers could accept short of further testing, assessment, and experience gained with Shuttle launches.

Regardless, Mr. Slack pressed ahead with his studies and his belief in what was initially called the “Day of Launch Wind Update.” As the development of the system progressed the concept was later renamed the “Day of Launch I-Load Update” or DOLIU in NASA-speak.

As the first test flight of the Shuttle approached, Mr. Slack worked with flight controllers to establish a reporting position in a room next to Mission Control for an engineer to report on the predicted structural ascent loads before launch. This would be done using the loads prediction program Mr. Slack originally developed. Wind data would be collected at the launch site starting several days ahead of launch. The wind data would be analyzed for trends. At some point, well in advance of launch, a final wind profile would be “frozen” and used to compute guidance inputs for the launch. Mr. Slack recalls that when he left NASA in August 1980, the most recent wind profile contemplated for updates to the guidance system was several days old relative to the time of launch. That was to afford ample time to check the wind data and verify the guidance system updates before they were uploaded.

Despite Mr. Slack’s advocacy for DOLIU until he left NASA in August 1980, Shuttle development priorities pushed it back to later missions. Mr. Slack handed off his programs and work product to a successor in his former Section at NASA. Work continued with pre-launch structural load predictions. As far as Mr. Slack knew, DOLIU went on the shelf with an uncertain future as he left for law school.

However, DOLIU eventually flew on mission STS-48 in 1991. By the time DOLIU flew, Mr. Slack was a lawyer. Shortly after Slack & Davis was formed in 1993, Mr. Slack’s NASA colleagues sent him a framed photograph of the STS-48 liftoff signed by the astronauts and fellow engineers who had followed his work on ascent flight loads.

In service on several Shuttle missions, DOLIU greatly increased the certainty that a launch would occur, and made the ascent safer. It opened the door to “on-demand” Shuttle launches such that winds and load exceedances were never a factor in launches.