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.