After a 2012 course correction, efforts by ’s International Space Station program to develop a new universal docking system standard for use aboard the 15-nation orbital science lab and future deep-space exploration vessels is on track for an operational debut by 2017.
Rivals in’s efforts to develop a U.S. commercial crew transportation capability — ’s CST-100, Sierra Nevada’s Dream Chaser and the Dragon — are in line to initiate and wring out the new universal NASA Docking System (NDS). NASA is targeting 2017 for the first ISS commercial crew missions and planning two U.S. segment docking ports equipped to accept the new, non-proprietary system.
Several years of station operations with the Boeing-inspired Soft Impact Mating Attenuation Concept (Simac), which has replaced NASA’s in-house International Low Impact Docking System (Ilids) design, are envisioned to help qualify the NDS international standard for the rigors of deep space.
“That is the driving force, a more simplified design that is lighter overall, less costly,” says Mike Suffredini, NASA’s ISS program manager. “We want to fly beyond low earth orbit one day, and one of the tenants of the space station is to wring out critical systems at station before we use them for deep space.”
The NDS goal is to accommodate dockings between spacecraft with masses ranging from 5 to 350 metric tons.
The project began with an international docking system standards discussion by ISS Multilateral Control Board representatives in 2009 to encourage greater global cooperation in space, while establishing a more robust rescue capability.
In taking the project lead, NASA’sturned to Ilids prototyping underway within the center’s engineering directorate since the mid-1980s. Ilids, intended to eliminate the need for the jarring post-contact thrusting that accompanied shuttle dockings, was adopted in the mid-1990s as part of NASA’s ultimately cancelled X-38 ISS crew rescue vehicle, then NASA’s Orion capsule under the also cancelled Constellation program. One Ilids unit was installed at the base of the Hubble Space Telescope by astronauts in 2009 during a final shuttle servicing mission to the observatory.
However, Ilids was dropped in favor of Simac as the new international standard for the ISS and the post-Constellation Orion Multipurpose Crew Vehicle in 2012 to address several concerns.
Those included the width of the outer soft contact ring in the Ilids system that constricted the post docking passageway, or tunnel connecting the two joined spacecraft and through which astronauts and cargo pass; as well as weight and cost considerations, Suffredini says.
The Russians pushed to retain the current 800-mm (31.5-in.) width afforded by the probe-and-drogue system used by the Soyuz and Progress capsules as well as the Russian-derived androgynous peripheral attach system (APAS) used by the shuttle to dock with the ISS. Engineers with NASA, the agency’s Jacobs Engineering Group support team and Boeing assessed options for narrowing the width of the Ilids soft contact ring to meet the Russians’ request.
Unable to do so, ISS managers turned to an alternative, Simac, proposed by Boeing.
“It was clear we were struggling with our Russian colleagues to agree on the inner diameter,” Suffredini says. “So, we went back and looked at our requirements again and realized we could build a different system that meets our requirement without building a light impact docking system.”
Ilids relied on a network of magnets and software controls as part of the soft capture ring to lower the initial impact loads that drive the latches and actuators of the older APAS hardware.
Spacecraft dockings unfold in two stages — soft capture and hard capture, which typically take 20 min. The first joins the two vehicles. The second draws them together to form an airtight seal.
Simac is an actuator-driven latching system that meets the international system’s low impact requirements. One of those was to join a pair of space vehicles as light as five metric tons each. Simac features two rather than six electronics boxes. The lower weight opens up the center of gravity, another concern for multiple vehicle docking operations.
However, Simac, which has reached the pre-preliminary design phase, will not eliminate all post-contact thrusting.
“We prefer not to do them [post-contact thrusting], but the spec does not prevent them. The early data says for nominal docking you will not have a need for that,” Suffredini says. “But as we explore the outer envelope of all the different conditions and modes, with the angles and contact conditions, we will have to see if some thrusting is required for certain occasions. That is certainly okay.”
Simac’s lower complexity helps to address the larger goal of creating a non-proprietary spec that will support production by multiple U.S. suppliers, while enabling it to be copied globally.
The NASA Docking System, as currently conceived, will require changes to the station’s two U.S. segment APAS-equipped docking ports. The ISS docking adapter (IDA), which is in production, will modify the APAS inner ring soft capture mechanism to accommodate commercial crew vehicles with Simac, Suffredini says.
The IDAs should be ready for station delivery in 2015, according to his estimates. The Simac docking hardware should be ready a year later, and NASA will likely provide the early production units to the winning commercial crew transportation initiative participants.
As far as Ilids, the development effort is being archived so that it could be available to the commercial sector, perhaps through a Space Act Agreement (SAA) with NASA.
“If someone would like it for whatever reason, I’m sure we could work out a SAA,” Suffredini says. “We have no conversations like that going on right now.”