Boeing 601
The Boeing 601 (sometimes referred to as the BSS-601, and previously as the HS-601) is a communications satellite bus designed in 1985 and introduced in 1987 by Hughes Space and Communications Company. The series was extremely popular in the 1990s, with more than 84 purchased by customers globally. The more advanced 601HP derivative (for "high power") was introduced in 1995. Hughes, and the 601 platforms, were acquired by Boeing in 2000. The last commercial 601 satellite was ordered in 2001 and launched in 2004. The NASA Space Communications and Navigation (SCaN) Program Office in December 2007 selected the  BSS-601HP for its third generation TDRS spacecraft, adding the two 15-foot (4.5m) diameter steerable antennas. The TDRS-M satellite, launched on August 18, 2017, became the last 601 satellite to reach orbit. Background. The Boeing-601 model was Hughes’ first major design and development for a communications satellite with three-axis, or body stabilization. All previous Hughes satellite models (HS-376) had been cylindrical spacecraft that were spin-stabilized at 50 revolutions per minute. Design of the Boeing-601 began in 1985, with full-scale development following two years later. The new satellite's first official public presentation took place at  the Telecom 87 conference in Geneva, Switzerland. Design. Structure. The 601 bus is divided into two modules. The first module houses the propulsion system, batteries, and electronics for the bus, and bears launch vehicle loads. The second module contains shelves carrying the communications equipment, payload electronics, and heat pipes. Solar arrays, reflectors, and antenna feeds are mounted to the payload module. Payload. The standard 601 platform supports up to 48 transponders and provides up to 4,800 watts of power. The 601HP supports up to 60 transponders and provides up to 10,000 watts. Failures. A significant number of Boeing 601s have  experienced failures in orbit, some resulting in complete failure of the satellite. Spacecraft Control Processor (SCP). An unconfirmed number of 601s launched prior to August 1997 have a design flaw in their SCPs, where a tin-plated relay forms crystalline "whiskers" under certain specific conditions. These whiskers eventually caused an electrical short. Each satellite contains two SCPs and the backup unit will take control in the event of a failure of the primary unit. In some cases, both SCPs have failed, rendering the spacecraft unusable. A notable example was the Galaxy IV satellite. At least eight 601s have experienced SCP  failures; four of which were double failures resulting in total loss of the satellite. Hughes switched to nickel plating on later 601s to resolve this problem, at the expense of payload weight. Batteries. Some 601HPs have experienced problems with their batteries, resulting in a reduction of eclipse protection. This would require some transponders to be shut down during eclipse periods. Xenon Ion Propulsion System (XIPS). Some Boeing-601 satellites featured the optional electronic propulsion system, called Xenon Ion Propulsion System or XIPS, for station keeping. Compared with traditional chemical propulsion systems, this xenon-based system is more efficient. Boeing 601 The