A Grapple Fixture is a connection fixture used on capsules, satellites and other objects handled by the Space Shuttle and International Space Station (ISS). The fixture allows Canadarm robotic arms aboard the Space Shuttle orbiters and Canadarm2 aboard the ISS to safely grapple large objects. Grapple fixtures are flat in appearance, with a central grapple pin topped with sphere which the snares of Canadarm2 latch on to. They also feature three curved "ramps" that help to lock the end effector on the robotic arm to the fixture.
The Grapple Fixture was developed at Spar Aerospace in the 1970s. Its invention is credited to Frank Mee, who also invented the Canadarm end effector. The Grapple Fixture design was further refined by Barrie Teb.
Flight-Releasable Grapple Fixture
A Flight-Releasable Grapple Fixture (FRGF) is a passive fixture which is identical to the PDGF but lacks power and data ports. The SpaceX Dragon includes a standard FRGF which is used by the ISS to grapple the capsule on approach to the station for berthing.
Power Data Grapple Fixture
A Power Data Grapple Fixture (PDGF) is a connection fixture used on the International Space Station (ISS). PDGFs can be "grappled" by the Canadarm2 robotic arm, in order to allow Canadarm2 to manipulate a grappled object, or be controlled by operators based inside the ISS.
The Canadarm2 is self-relocatable and can move end-over-end to reach many parts of the Space Station in an inchworm-like movement. In this movement, it is limited only by the number of Power Data Grapple Fixtures (PDGFs) on the station. PDGFs located around the station provide power, data and video to the arm. In addition, they feature four rectangular-shaped electrical connectors, that either transfer power and data to Canadarm2 from the ISS, or to a grappled payload from Canadarm2.
FRGF visible (dark circles half-way up) on the Hubble Space Telescope prior to its launch
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