TACSAT–Dick Brandes

After the early successes of commercial communications satellites in the 1960’s with the formation of Intelsat and the deployments of Intelsats I & II, the US Air Force saw the need to rapidly explore the utility of communications satellites for tactical military purposes. To that end, the TACSAT program was initiated. It called for the expedited construction of a satellite carrying UHF and SHF repeaters using proven technology to the maximum extent possible, and to deliver the flight model within 18 months on a fixed price basis.

The Hughes Space Division management saw this program has a high priority target. It would provide for an early orbital demonstration of our patented gyrostat concept, that was planned to be proposed for future Intelsat and government programs. And it would undoubtedly evolve into follow-on operational satellites. Thus, we accepted the considerable schedule and financial risks, and put forth a major proposal effort that was ultimately successful. The contract was awarded in January 1967.

The resultant design, designated as HS-308, is shown in Figure 1 with Dick Johnson, systems analysis manager, Dick Brandes, Systems Engineering and Analysis Manager, and Louie Fermelia, systems engineer for telemetry and command. The spacecraft was huge by contemporary satellite standards, about 25 feet tall and 9 1/4 feet in diameter. It weighed about 1600 lbs in orbit. The spinning solar arrays generated almost a kilowatt of power. The size of TACSAT required the construction of a 3-story Hi-Bay assembly and test building. This iconic building (still in use) became the system level assembly/test area for virtually all future Hughes (and Boeing) spacecraft.

Figure 1. TACSAT flight model with Dick Johnson, Dick Brandes, and Louie Fermelia.

TACSAT was to be launched by the new Titan IIIC booster that would directly insert it into synchronous orbit. With no apogee motor requirement, the simplest interface to the booster was the lower solar panel that became part of the primary load structure. The so-called bellyband separating the upper and lower solar panels was the base for a structural cone that rose upward nearly to the top of the upper solar panel At its apex was the bearing and power transfer assembly (BAPTA), that despun a platform mounting the communications equipment, ancillary equipment, and the nutation damper necessary for spacecraft stability. A slip ring assembly transferred power and signals across the rotating interface. As proposed, the platform itself was an annular ring about midway between the BAPTA and the bellyband, supported by a conical structure attached to the BAPTA.

Mounted on the spinning cone were the spinup and station-keeping fuel systems, the power control and battery systems, and the despin control electronics that achieved a pointing accuracy of 0.1 degree using earth and sun sensors. Attitude control and station-keeping were accomplished through ground commands similar to previous Hughes comsats.

The communications system was enormously powerful and complex.  The UHF peak output was over 40 db EIRP including an antenna gain of about 17 db. The corresponding SHF values were 33-db EIRP and 19-db antenna gain. UHF power was provided by 16 solid state amplifiers inputting a unique power summer that allowed any number to be operated depending on available spacecraft power. Normally, that meant 13 amplifiers with the others providing redundancy. SHF power came from three 20-watt traveling wave tubes (TWTs) paralleled in a three for two redundancy scheme. However challenging these transmitters were, the rest of the system was not far behind. The repeaters were operated in both straight through, e.g. UHF TO UHF, and in crossover modes, e.g. UHF to SHF. There were 8 different modes, variously having bandwidths of 50 KHz, 100 KHz, 425 KHz, 1 MHz, and 10 MHz. The UHF diplexer required over 175-db isolation between transmit and receive frequencies. This design would never have come together without the inspired leadership of Clint Lew, the Communications Labs system engineer.

Our proposal had, of course, touted the extent of proven technology in the design. And we had some. The spinup and propulsion systems were space proven technologies, as were the sun sensors, earth sensors, nutation damper, solar cells, and telemetry and command (T&C) antenna. After that, the list got very short. Besides the basically all new communications system, we had to develop solar panel substrates doubling as spacecraft structure, a machined beryllium BAPTA, extruded beryllium support tubes for the helical UHF antenna, and a slip ring assembly for transferring T&C signals and power across the spin/despin interface. The T&C system itself was a new all PCM design, which had to be compatible with customer furnished encryption/decryption units. The thermal design was novel, an open rear end cavity to radiate excess thermal energy to space. The spacecraft structural integrity would be demonstrated not by the traditional vibration test, but by low force tests to verify that the structure’s modes and frequencies agreed with the analytic values used to determine loads. We were also guinea pigs for Martin-Marietta’s first integration contract on the Titan IIIC. (Thank you Lee Groner for running interference for two years). Oh yes, then there was that stabilization system.

LET THE FUN BEGIN

Most development programs encounter development problems, and we had our share. The initial fabrication of the lower load-bearing substrate resulted in ”mechanical property deficiencies” according to an Aerospace report. In other words, it failed its loads test. A significant research project ensued, which ultimately solved the problem. The beryllium BAPTA development went well, but the first extruded beryllium antenna support tubes had unacceptable surface qualities and excessive bow, triggering another successful research project. The slip ring assembly failed its life test; the brush slip ring interface created excessive debris thath shorted out electrical signals. More research!

The spacecraft thermal design was compromised by growth in platform equipment footprint. A few months into development, it became clear that the annular platform didn’t have enough area for all the units. So the ring level was raised closer to the BAPTA, enlarging it. Within a short time even more area was needed. The proposal to crank the platform up a little more was vetoed by Program Manager Dick Bentley. He said to take it to the top, i.e. the BAPTA level. “That’s all the room there is. We’ll have to make it work.” Making it work required a major redesign of the thermal control system. But thermal system manager, George Wolodkin, and his troops were up to the task. Gone was the open rear cavity, replaced by a thermal barrier to maintain proper temperature in the spinning compartment. Excess payload heat was radiated forward by a high emissivity/ high reflectivity sun shield. The new design aced the thermal model test.

Many developments went relatively smoothly. The T&C and power subsystems produced few headaches. The structural analysis team under Paul Bernstein produced a coupled loads analysis that was thoroughly confirmed in the modal testing. The complex communications system had a variety of hiccups, but no crises until we began system test at the spacecraft level. There, we found the UHF receiver flooded with spurious inputs. I joined Brian Rose, the UHF project leader, on the test floor to examine the data. I couldn’t contribute much. But I’ll never forget Brian staring at the data displays like Rodin’s statue of the Thinker. After what seemed like forever, he opined “It has to be the diplexer.” The unit was removed and sent to the vendor where an internal mechanical connection was identified as a suspect. After a welding rework, the unit was returned to spacecraft test and the problem disappeared. Whew!

Probably the most disturbing development for me occurred the day Dick Johnson and one of his analysts, John Velman, appeared in my office many months into the program. John had been creating a complex and complete digital simulation of the spacecraft dynamics including the damper and the despin control system. He ruefully announced that he had gotten his first complete runs back, and unfortunately the spacecraft was unstable; it would tumble in orbit. Oops!

John had traced the problem to the fact that TACSAT’s despun platform was not balanced; it had cross- products of inertia. When the despin control system applied rotational torque to the platform, a tipping torque was induced by the asymmetrical mass properties. The phasing of this torque was such that it would augment any nutation. We had a positive feedback loop producing instability.

Balancing the platform meant starting over. And we quickly determined we didn’t have to. Bob McElvain’s despin control system engineers suggested they could increase the servo lead to change the phase of the torque and eliminate positive feedback. A meeting was convened a few days later to hear their results. It was well attended by customer and Aerospace personnel. Bob’s design engineer gave a nice presentation for over half an hour, describing the design changes and their effect on stabilizing the spacecraft. Then he observed that while the spacecraft would be stable, the despin servo would not. The fix he’d described at length wouldn’t work. How embarrassing!

Fortunately, another of Bob’s guys had a plan B; to wit, increase the lag in the system to accomplish the same end. This was a more difficult implementation, but they worked it out, and Velman confirmed its success. In fact, the control system now acted as an active nutation damper. We had turned a vice into a virtue. We momentarily thought of chucking the damper, but wisely thought better of it.

DELIVERY AND LAUNCH

TACSAT was delivered to the customer in December 1968, 23 months after start. We hadn’t achieved the 18-month objective, but still considered it a major accomplishment. Launch was scheduled for February 8. The system engineering team deployed to the Air Force Sunnyvale spacecraft control center anxious but confident. A one-day postponement of the launch resulted in an enlightening experience. The team decided to kill the day by visiting San Francisco for a little sightseeing. The sightseeing included a tour of Haight-Asbury where, like country bumpkins visiting a big city for the first time, we gawked in amazement at the surrounding scene. Our astonishment drove home how out of touch we were after spending two years holed up in our cubicles 50 to 60 hours a week.

The next day the launch was a go. The booster performance was spot on, and the spacecraft was deposited in synchronous orbit. After spinup, unlocking the despun platform, and activating the despin control system, we focused on the small nutation angle that had been induced by these maneuvers. To feelings somewhere between consternation and panic, we watched the angle slowly grow instead of diminish. Then it stopped and stayed at .8 degrees. After several hours of monitoring with no change, we believed we had a stable situation, but we were baffled.

We returned home to participate in a tiger team to revue every possible energy-dissipating element on the rotor to see if we had overlooked or miscalculated anything. Meanwhile the nutation angle displayed erratic behavior. It would spontaneously disappear and reappear, stabilizing between .6 deg. and 1.2 deg. After a few weeks we seemed at a dead end. All our calculations continued to show the damper alone had two orders of magnitude of stabilizing margin. Bob Telle, Controls Laboratory Manager, then posited that the problem had to be in the BAPTA. A sophisticated test was arranged on the qualification model BAPTA and did in fact show dissipative forces on the spinning side larger than the damper for small angles. The source was believed to be an oil filled gap between the spinning shaft and the inner bearing race. Modification of the unit to eliminate this feature and retest verified that we had found the culprit, to the relief of our other spacecraft programs utilizing gyrostat stabilization.

IN-ORBIT PERFORMANCE

In-orbit testing showed all other spacecraft systems performing as expected. The small nutation had no significant effect on the earth coverage communications system. Then, some five months after launch, another mystery arose. The UHF EIRP dropped about 3db and started to vary erratically over a 2-db range. Within the time span this occurred, the spacecraft experienced a change in spin speed of .3 RPM and an attitude change of .09 degrees. Testing of the antenna pattern showed significant deviation from initial orbital tests. One first sidelobe had been subsumed into the main beam and the boresight had shifted. Ground testing on a scale model antenna found possibilities for the behavior involving damage to one of the outer helices. Some postulated that a meteorite had struck the antenna, accounting for the dynamic changes noted. While this notion was decried as highly improbable, I have never heard of another explanation.

TACSAT was used extensively for R&D and operational testing by many military communications agencies. It served as a test bed for its advanced design technologies. It supported recovery operations for Apollo. Most importantly for Hughes, it validated the gyrostat concept, which was used in our spacecraft designs for decades. As Fred Adler, Space Division Manager, noted maybe the little stabilization “wrinkle” was a not a bad thing. Unfortunately, there was no direct follow-on spacecraft. Hughes would have to wait for the LEASAT program to once again provide tactical communications.

KUDOS

TACSAT would not have happened without the dedication and personal sacrifice of scores of individuals, many of whom were collocated in one of our airport buildings. I’ve mentioned some of them, and time has fogged over the names of others who were equally critical. But certainly the leadership of Program Manager Dick Bentley and his assistants Roger Clapp and Tom Mattis bear noting. I was privileged to be part of that team.