Watching Dragon/Endeavour dock with the ISS, I was curious as to how they used thrusters to orient the spacecraft and “soft dock” with the ISS. The narrators didn’t mention what happened after the soft dock, but I imagine the “hard dock” was something like a convertible closing the roof where pins and hooks are used to lock both spacecraft together.

I was thinking it might be a simpler procedure for either Dragon or the ISS to extend a pole/boom like they use during aircraft in-flight-refueling where a boom is extended to catch a port. Once the boom is locked in, it could orient Dragon and pull it in for the soft dock. Has that ever been considered?

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    $\begingroup$ That sounds like the previously used berthing procedure where a ship (or "space barge" logistics module carried by the Shuttle in the past) parks near the ISS and the robotic arm grabs it and sticks it on a berthing port. $\endgroup$
    – ikrase
    Jun 1, 2020 at 6:47
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    $\begingroup$ Or a bit like the probe and drogue docking mechanisms (albeit without booms) used by Apollo and Soyuz $\endgroup$
    – user20636
    Jun 1, 2020 at 8:51

2 Answers 2


This is how Spacex's previous dragon spacecraft got docked to the ISS. Canadarm2 would grab dragon and pull it to dock to the ISS

See the Dragon Wikipedia page:

For the ISS Dragon cargo flights, the ISS's Canadarm2 grapples its Flight-Releasable Grapple Fixture and berths Dragon to the station's US Orbital Segment using a Common Berthing Mechanism.

Dragon grappled by Canadarm2

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    $\begingroup$ I thought I heard on the broadcast that the ISS went in to a different attitude adjustment mode for Dragon 2. Can you source your statement that it can remain in the same attitude? $\endgroup$
    – PearsonArtPhoto
    Jun 1, 2020 at 13:21
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    $\begingroup$ @PearsonArtPhoto My statement was wrong so I removed it. Thank you for bringing it to my attention. Looking at space.stackexchange.com/questions/13425 it seems the ISS keeps rotating when docking, although it might change its attitude a bit. I got the idea that the ISS stopped its rotation for the Soyuz from an old article or answer about Dragon 2 (that said Dragon 2 would be able to match the ISS's rotation) but can't seem to find it anymore $\endgroup$ Jun 1, 2020 at 13:39

Bit late, but:

Yes, they did.

Until it was dropped for the ISS, it was NASA's design for docking the Space Shuttle Orbiter to Space Station Freedom.

OP asks:

the ISS to extend a pole/boom like they use during aircraft in-flight-refueling where a boom is extended to catch a port. Once the boom is locked in, it could orient Dragon and pull it in for the soft dock. Has that ever been considered?

The design was abruptly dropped in light of Russian docking hardware availability and docking experience in the Shuttle-Mir project.



Further info:

For the last few years of its life, from 1988-1992/3, Space Station Freedom had one docking mechanism design, attached to two nodes, a Primary berthing docking adapter and a Secondary berthing docking adapter.

The Orbiter berthed, using a combination of its own Canadarm to grapple and the boom-arm, called a retractable vehicle and station coupling system, in a nose up attitude (relative to the station) to the Primary berthing adapter; the Orbiter berthed in a nose-down attitude to the Secondary berthing adapter.

(For a number of years, docking was dropped in favor of berthing only, to reduce impact stress to the station structure and lessen the impact of engine plume impacting other parts of the station, such as the solar arrays).

The structure was offset to one side to allow for the retractable vehicle and station coupling system to extend and retract.

A lattice framework, similar to the work platform concept allowing astronauts to work on the SFF structure from within the Payload Bay, surrounded the pressurized docking tunnel. It had grapple fixtures for the Orbiter to use its arm to berth itself with the SSF and the lower half had trunnions and keel pins that lined up with the forward section of the Orbiters Payload Bay.


The retractable vehicle and station coupling system:

  • This was a boom-like arm that extended to meet and attach to the corresponding coupler that was attached to the right side of the Orbiter Payload Bay.

  • Once attached, the linear actuator devices attenuated and limited impact loading to an acceptable level, and if required, could realign the Orbiter.

  • the extended system would then retract, bringing the station and the Orbiter together for a hard dock.

At this time the docking hatch adapter would have been brought to meet the Orbiter airlock hatch and they would mate, forming a pressurized docking tunnel through to the space station. The design required the Orbiters to have modified airlocks to mate with the pressurized docking tunnel housed within the berthing adapter.

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They got as far with this concept by having a lattice framework installed on the mock up Orbiter Payload Bay in a facility that has since been demolished.

(This may have also have been for the work platform which was used when looking at early truss extension designs, if anyone remembers seeing it there)

During the early unmanned phases of construction, Space Station Freedom would have these lattice structures to interface with the Orbiters Payload Bay and were called unpressurized docking adapters.

The End:

In the late 1980s and early 1990s, NASA had a docking mechanism design, but in the early 1990s, it was decided that NASA and the Russians would cooperate to build the ISS.

To do this, NASA began procuring Russian hardware and abandoned its formal docking mechanism design work.



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