Why does the Dragon berth?

[lextacy]

Why not just dock like everybody else? The ATV can do it, the HTV can do it , the Progress. Why spend the extra 5 hours berthing?

[General Rarity]

I'm pretty sure they berth because it is much safer than docking with RCS. Docking uses RCS, which usually is very toxic and can damage the station. You 'save' time but add risks. Progress docks obviously because it was designed waaaay before berthing was actually used, I'm not sure why the ATV only docks though, maybe it's too heavy or large for the arm.

[Bill Phil]

Probably not too heavy for the arm... It could have to do with it's mass and thus higher inertia...

[Kryten]

ATV docks because a large part of it's function is fuel transfer, and only the Russian docking ports are plumbed up for that, whereas HTV does actually berth.

[Pipcard]

Berthing ports have a larger diameter than docking ports, so larger equipment such as these racks can be taken through.

The connections between berthing ports are stronger and more secure than docking ports, and it takes longer for two of them to lock into position. So a robotic arm has to hold it in place. I think that's how it works.

Edited January 10, 2015 by Pipcard

[Dispatcher]

One might say its a labor of love.

[problemecium]

Perhaps after playing KSP for a while, the SpaceX guys realized that docking is way easier when you have a big robotic arm to just grab the ship and pull it in.

[lextacy]

I once had to berth a soyuz to my station, and only this time the arm got used to dock a ship. the reason was a docking bug prevented the ship from docking. So I used the arm. So moral of the story, in KSP always bring an arm to the station just in case

[megatiger78]

what is berthing though?

[astropapi1]

what is berthing though?

Using the arm instead of going for a direct docking, me thinks.

[Nibb31]

what is berthing though?

Berthing is when the vehicle is captured by the arm and attached to a CBM port. CBM ports do not support docking. They were primarily designed for orbital assembly of the USOS modules with the robotic arm.

Docking is when the vehicle actively docks with a docking port. The only ports that support docking are the Russian hybrid ports, the US APAS ports (which are unused and obsolete) and the future LIDS/NDS (which will be fitted to the APAS ports but are not available yet). Only the Russian hybrid ports support fuel transfer and are in a location suitable for reboosting the station.

Why not just dock like everybody else? The ATV can do it, the HTV can do it , the Progress. Why spend the extra 5 hours berthing?

One of the requirements for cargo vehicles was to use the CBM ports, because these are the only ports large enough to transfer ISPR racks. You can't fit those interchangeable rack modules through any of the available docking ports.

HTV, Dragon, and Cygnus don't dock. They berth to CBM ports, just like Dragon.

ATV and Progress dock to the Russian hybrid ports because they are used for station reboost and fluid transfer to the Russian segment.

CST-100 and Dragon V2 will dock to the new NDS ports. Docking is a requirement for crew vehicles because they need to be able to undock autonomously in case of an evacuation, which is impossible with the CBM ports.

Edited January 12, 2015 by Nibb31

[Bill Phil]

Wouldn't they just use smaller racks rather than waist so many resources just to get a proper size material transfer rack? I guess that even in the space age the ultimate problem is getting things to where they need to go.

[Kinglet]

Wouldn't they just use smaller racks rather than waist so many resources just to get a proper size material transfer rack? I guess that even in the space age the ultimate problem is getting things to where they need to go.

The racks are standardized, and there is no waste of resources. For a cargo craft, berthing is safer, and much cheaper than docking. Plus, these berthing ports are quite old and have been there for a while.

[Bill Phil]

But they designed the dragon around it, meaning it's only compatable with berths, of course there would be adapters, but they screw up CoM. Why have two different types of connection at all? Just use the one and only.

[KerbMav]

http://en.wikipedia.org/wiki/Common_berthing_mechanism

So the arm is needed because thrusters are not precise enough and CBMs are not designed to assist in the maneuver?

[Nibb31]

Wouldn't they just use smaller racks rather than waist so many resources just to get a proper size material transfer rack? I guess that even in the space age the ultimate problem is getting things to where they need to go.

ISPRs are standardized:

https://en.wikipedia.org/wiki/International_Standard_Payload_Rack

The ISS lab modules and are designed around the standard ISPR racks. The ISPR racks are designed to fit through CBM ports. Changing the ISPR standard would require refitting the USOS lab modules. The ISS is a complex machine. You can't change one system without affecting other parts of it.

But they designed the dragon around it, meaning it's only compatable with berths, of course there would be adapters, but they screw up CoM. Why have two different types of connection at all? Just use the one and only.

Because they serve different purposes.

The CBMs are wide and designed to bear structural loads. They are the large "doors" you see on the inside of the ISS. Because they are large, they do not have any mechanism to mate when they are not perfectly aligned. Modules must be guided very precisely into position with an arm.

CBM berthing requires an arm, which means that it was not possible before the Shuttle brought up the SSRMS. The Shuttle itself could only dock to the PMAs, but it carried cargo (ISPR racks) inside MPLM modules. They used the arm to remove the MPLMs from the Shuttle payload bay and to attach them to CBMs for cargo transfer.

Docking ports are not designed to support as much load but they provide autonomous docking without assistance from the outside (which is a safety requirement). They are narrower and more complex. A docking port that provided as much clearance as a CBM, while providing some sort of retractable guiding mechanism would be heavier, larger, and more expensive than a CBM.

So CBMs are a requirement for cargo vehicles, because of their clearance, and docking ports are a requirement for crew vehicles, because they allow emergency evacuation.

http://en.wikipedia.org/wiki/Common_berthing_mechanism

So the arm is needed because thrusters are not precise enough and CBMs are not designed to assist in the maneuver?

Exactly. Docking ports are designed to tolerate some degree of axial misalignment. They have soft-dock and hard-dock mechanisms that retract to pull the two vehicles together. CBM ports require near-perfect manual alignment.

Edited January 13, 2015 by Nibb31

[Pipcard]

That's a better explanation. Thank you!