July 2005 
Volume 04, Issue 3 
Integrated Defense Systems

Share the power

Space Shuttle upgrade lets astronauts at ISS stay in space longer


Share the powerFuture Space Shuttle crews will enjoy extended visits to the International Space Station thanks to an upgrade that will connect docked shuttles to the ISS electrical power system. The modification will allow the shuttles to stay on orbit longer, provide additional time for science and extravehicular activities, and permit astronauts to unload additional cargo. More work will get done on each mission.

The Station-Shuttle Power Transfer System (SSPTS) allows the ISS to supplement the shuttle's electrical power using electricity generated by ISS solar arrays. This results in a lower consumption of liquid hydrogen and oxygen, components used by the shuttle's fuel cells for making electricity. The power transfer system upgrade also will result in a 50 percent increase in the amount of time the orbiter can dock to the station: from about seven days to between nine and 12 days, depending on the mission configuration.

"This much-needed upgrade will give NASA greater flexibility in bringing up supplies," said John Elbon, vice president and ISS program manager, "while positioning the orbital spares to allow us to operate the station until at least 2017."

The Space Shuttle currently has the capability to transfer power from its 28-volt system to the ISS's 120-volt system through a device called the Assembly Power Converter Unit (APCU). However, at this time the ISS cannot transfer power in the other direction—from its electrical system to the shuttle.

The upgrade will replace the APCU with a new device called a Power Transfer Unit. It will afford the ability to convert power from the ISS 120-volt system to the shuttle's 28-volt system. With this upgrade, the ISS will be able to transfer up to eight kilowatts of power to the orbiter using an equipment package that fits into the same space as the existing APCU.

Share the powerNew power cables and displays will be integrated into each of the shuttles as well. The ISS will be outfitted with additional cables routed outside the Boeing-built U.S. Destiny lab. The power will pass through existing APCU electrical ports.

The Space Shuttle needed the ability, given by the original converter, to transform its 28-volt power into a 120-volt supply for two purposes, said Eric Gietl, SSPTS integration manager. "Initially, it was used to deploy the solar arrays and charge the batteries," Gietl said. "This feature is not required any more. The other use is to power the Multi-Purpose Logistics Module when in the shuttle cargo bay and provide heater power to pressurized modules being delivered to the station."

The SSPTS project 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.

"We are rapidly developing the Power Transfer Unit hardware since it is currently targeted to fly on ISS mission 15A in the fall of 2006," Gietl said.

Full capability of the SSPTS will be realized when all four ISS power modules are on orbit, and the ISS has plenty of reserve power to transfer to the orbiter.

Gietl said the development schedules are accelerated about one year in comparison to a typical development program. Testing with the engineering model hardware is scheduled for this winter in the shuttle and ISS power labs at Johnson Space Center in Houston and at United Technologies Corporation, which makes the Space Shuttle fuel cells. Boeing will install SSPTS flight hardware in a shuttle for the first time in the summer 2006 time frame.

Rocketdyne Propulsion and Power in Canoga Park, Calif., is manufacturing the Power Transfer Unit, while the Houston Product Support Center is manufacturing the cables for ISS. Boeing in Huntington Beach, Calif., is developing the shuttle upgrades, including new cables for each orbiter, cockpit control switches and crew displays.

The SSPTS will have a critical design review in late summer. "The engineering model designs have been manufactured and are in testing," Gietl said. "In addition, we are conducting early thermal vacuum testing, and so far results from the tests have been extremely good."

Some minor changes in the flight design are being incorporated as well.

"The Boeing Space Shuttle and ISS engineering teams have done a great job working together on this project," Elbon said. "Since we are responsible for sustaining engineering on both programs, we are able to deliver a well-integrated product at a reduced cost. This, of course, makes our customers very happy."



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