Telecommunications capacities have changed dramatically from the one-channel circuit carried by Syncom in 1963 to the 33,000-circuit capacity of the new Intelsat VI series.
Yet, no matter how complex or how large Hughes satellite systems have become, the basic spin-stabilized design has remained the same.
Developed by Hughes engineers, the concept for spin stabilization broke a technological barrier, providing continuous communications on a global basis live via synchronous satellite.
“Spin-stabilization is a simple, elegant, brilliant system on which Hughes has built year after year,” said Lynn Grasshoff, associate manager of Space and Communications Group’s Systems Design Laboratory.
“The best solutions are always the simplest.”
Mr. Grasshoff has been working in attitude dynamics and orbital control of spin-stabilized satellites since joining the company in 1961, shortly after the development of Syncom.
“The system is so reliable and easy to implement that nothing has ever come close to it,” he continued. “Every aspect of it has been ideally realized, making Hughes the leader in spin-stabilized satellites for synchronous orbits.”
Thirty-nine of 41 spin-stabilized commercial satellites in synchronous orbit operating today were designed and built at SCG.
Early satellites of the late 1950s had a tendency to tumble uncontrollably, compounding an existing problem of positioning them in particular orientations to Earth.
Two basic methods were subsequently developed to hold spacecraft steady and pointing towards Earth: spin stabilization and three – or multi-axis stabilization.
Three – or multi-axis stabilization requires the use of flywheels and little rocket or gas jets to control the spacecraft through its roll, pitch, and yaw axes.
Twelve to 18 thrusters are required for orbit and attitude control of the three – or multi-axis spacecraft, which, during the first full-scale effort in 1958 to design a synchronous communications satellite, provide too complex to implement.
Meanwhile, work was progressing on an in-house study conducted at Hughes that featured a spinning control system in which all orbit attitude requirements were accomplished by two thrusters, or four for full redundancy.
“Spin-stabilization is the method that nature prefers, even assists,” said Mr. Grassfhoff. He explained that once spinning, the spacecraft has angular momentum like a gyroscope, and like a gyroscope, is reluctant to change its attitude and resists such change.
The orbit and attitude control system for synchronous satellites was developed by the late Don Williams. A patent was filed in his name for the idea in August 1964 with the U.S. Patent Office.
“The Williams patent is still used today for all of our spinners,” said Mr. Grasshoff. With the exception of Surveyor, all Hughes spacecraft are “spinners.”
The Hughes Leadership in the spin-stabilized market was later strengthened by the Iorillo patent, issued in 1969.
Anthony Iorillo, Group vice president and manager of SCG’s Defense Systems Division, showed the industry that gyroscopic dynamics do not restrict the size of despun platforms on which communications electronics are mounted.
Mr. Iorillo discovered that stabilization of a large despun platform was not only possible, but also practical, even with destabilizing elements on the spinning portion.
The Hughes system developed from this concept was called the Gyrostat, a motorized technique for dualspun spacecraft that uses a passive damper to dissipate energy and overcome all destabilizing forces, such as propellant sloshing and structural flexing.
By using the Gyrostat approach, a satellite can be virtually unlimited in platform size.
The concept was confirmed in February 1969 with the launch of Tacsat, a 1600-pound, two-story experimental communications satellite that ushered in the era of large, complex satellite systems.
“Tacsat also introduced a new technique for the damping of nutation, coning motion, by utilizing to advantage the natural dynamic coupling between the despin torque and the nutational motion of the spacecraft,” said Mr. Grasshoff.
“Those two patents, the Williams and the Iorillo, form the basis of spin-stabilized technology and are the reason that Hughes has advanced so far in the field.”