Vehicle Upgrades: Station-Shuttle Power Transfer System (SSPTS)
A new Space Shuttle upgrade that connects into the electrical power system of the International Space Station (ISS) will enable the orbiter to stay on-orbit longer, provide additional crew time for science activities and Extra Vehicular Activities (EVA), and permit additional cargo to be unloaded by astronauts.
The Station-Shuttle Power Transfer System (SSPTS) allows the ISS to supplement the orbiter's electrical power with the ISS solar arrays resulting in a lower consumption rate of liquid hydrogen and oxygen used for making electricity by the orbiter's fuel cells. The SSPTS upgrade will also allow the orbiter to increase its time docked to the station from 6-8 to 9-12 days depending on the mission configuration.
The space shuttle currently has the capability to transfer power from its 28 volt system to the ISS 120 volt system through a device called the Assembly Power Converter Unit (APCU); however, it cannot transfer power from the ISS to the orbiter. The SSPTS upgrade will replace the APCU with a new device called the Power Transfer Unit (PTU). The PTU will include the capabilities of the existing APCU with the additional capability to convert from the ISS 120 volt system to the orbiter's 28 volt system. With this upgrade, the ISS will be able to transfer up to eight kilowatts of power to the orbiter in a package that fits into the same footprint as the existing APCU.
In addition to the PTU, new power cables and displays will be integrated into each of the orbiters. The ISS will also be outfitted with additional cables routed along the outside of the Boeing-built U.S. Destiny lab. The power will pass along the Pressurized Mating Adapter 2, where the orbiter docks to the ISS through the existing electrical ports used by the APCU.
All of the ISS and orbiter cables have been delivered to Kennedy Space Center (KSC). The ISS cables are currently planned to be launched to the ISS on Assembly Flight 12A.1, scheduled for STS-116 in December 2006 or February 2007. Orbiter cables are currently being installed in Endeavour (OV-105) at KSC. Engineering Model (EM) testing of the PTUs was recently completed at the manufacturer, Pratt and Whitney Rocketdyne with very favorable results. The EM units will now be used for system level testing at Johnson Space Center and the orbiter fuel cell vendor, United Technologies Corporation. Delivery of the flight PTUs are expected in December 2006 in preparation for launch on ISS flight 13A.1, scheduled for flight on STS-118 in 2007.
The SSPTS will allow the shuttle to remain docked to the ISS for a longer period and provides the ISS and visiting crew members with more time for logistics supply transfer, additional experiments and detailed orbiter inspections. "Now the orbiter can stay docked for quite a bit longer and that longer period allows the crew more time to transfer cargo from the MPLM (Multi-Purpose Logistics Module)," said Eric Gietl, Boeing SSPTS integration manager. The Space Shuttle supplies 120 volt power for two purposes: "First, it provided the startup capability to initially jump start the space station power system. Initially, it was used to deploy the solar arrays and charge the batteries. This feature is not required any more, since the station is already started," Gietl said. "The other use is to power the MPLM when in the orbiter cargo bay and provide heater power to pressurized modules being delivered to the station."
The SSPTS project is unique because it marks the first major development project that the Boeing ISS and space shuttle programs have worked on together. The ISS program is providing the funding under its existing contract. "Our customer has been very supportive of the things we have done, and in some cases, we've had to come up with new processes and break some new ground," he said.
Boeing began work on the project in September 2003, but had a slight head start due to some internal development work that was done at the Huntington Beach and Canoga Park, California facilities. Rocketdyne Propulsion and Power (Canoga Park) is manufacturing the PTU, while the Houston Product Support Center manufactured the cables for the ISS. Boeing Huntington Beach has developed the orbiter upgrades, such as the new cables for each orbiter, cockpit control switches and crew displays.