ISS Partners Set Docking Interface Standards

Aviation Week: ISS Partners Set Docking Interface Standards
Designers of future spacecraft that need to dock with each other for crew and cargo transfers are likely to use a new set of interface standards just published by the International Space Station partners in the hope they will simplify human exploration beyond low Earth orbit.

Available to anyone with an Internet connection—including Chinese, Indian and commercial companies—the standards give engineers the information they need to build docking systems for the space station as well as possible lunar excursions, rescue missions and any future “international cooperative demonstration” that brings together spacecraft from different nations.

An industry day next month at Johnson Space Center (JSC) to explain the new standards will be open to international and commercial participants, apparently without restriction. NASA expects the companies vying for a shot at providing commercial crew transportation to the ISS to be present, and it is not ruling out Indian and Chinese participation.

“We’ll have the hardware set up and teams out in place to talk to whomever decides to come,” says Stephen Gaylor, the space shuttle program flight manager who was chairman of the team that drafted the international docking standards.

Chinese participation at the Houston workshop will not be out of the question. Senior Chinese space officials have expressed readiness in discussing docking-interface parameters for human spacecraft (AW&ST April 19, p. 32), and a delegation from the China Manned Space Engineering Office (CMSEO) is due to visit the U.S. next month as part of an exchange set up last year by Presidents Barack Obama and Hu Jintao.

NASA Administrator Charles Bolden visited the Chinese human launch facility at Jiuquan last week as a guest of the CMSEO. Bolden, accompanied in China by astronaut Peggy Whitson and other NASA officials, toured space facilities and met with officials in Beijing, as well.

The interface standards do not cover the actual technology that would operate a docking system. Instead, they give measurements and force loads for engineers to match as they design their own docking system. For example, the diameter of the opening in the mating plane listed in the documents posted at http://www.internationaldockingstandard.com/ is 1,045 mm., but it is up to individual designers to design the mechanisms that will use that opening to guide and lock the two halves together.

The new standard is based on the old Russian androgynous peripheral assembly system (APAS) used to dock the space shuttle to the ISS, according to Caris A. “Skip” Hatfield, the senior NASA engineer assigned to implement the new standard on the station.

The mechanical APAS has been modified with elements of the electromechanical low-impact docking system (LIDS) under development at JSC to create a standard for systems that work like APAS but require less force to drive the two spacecraft together.

“They have a table that extends, that has the soft-capture system that makes first contact,” Hatfield says. “That soft-capture system is what takes out the initial forces and moments from the contact. And then that table is withdrawn, and there’s a hard-capture system, a set of hooks that reach up and bring it together and achieve the pressure seal and the physical interface.”

To date, NASA is closest to having a system based on the new standard. In its active mode, the NASA docking system uses a closed-loop computer control system that senses the forces and moments, and commands electromechanical actuators to dampen them out before drawing the two halves together.

The European Space Agency is at work on a similar system for its proposed Advanced Reentry Vehicle (ARV), a spin-off of its Automated Transfer Vehicle (ATV) that would be able to return cargo from the ISS (AW&ST Oct. 4, p. 47). Known as the International Berthing Docking Mechanism, it would also be made compatible with the new standard and would allow the ARV to dock at the U.S. end of the ISS, according to Simonetta di Pippo, ESA director of human spaceflight.

The NASA system is likely to be the first to be used in space. Plans call for the two APAS-based systems on the Harmony node to be moved to the Tranquility node in 2014 and replaced with common docking adaptors using the new NASA docking system.

That is where the commercial cargo vehicles being developed with NASA seed money at Space Exploration Technologies (SpaceX) and Orbital Sciences Corp. will dock, as would any commercial or government crew vehicles developed under NASA’s emerging new space policy (AW&ST Oct. 11, p. 32). Russia will continue to use the probe-and-cone docking system in place on its end of the ISS for docking Soyuz and Progress modules and the ATV.