|Japanese HTV Spacecraft Approaches ISS for Berthing|
|Russian Progress Vehicle Post-Docking|
The vehicles of the ISS resupply fleet are an interesting bag, and later this month, members of every type may be present on-station at the same time (which might make an interesting photo-op if either STS-133 or STS-134 happen to be in place to take it). Below is an image comparing the vehicles I'm talking about.
|ISS Resupply Vehicle Comparison Chart|
To explain that a bit, the US end of the station is largely built around the use of modular racks, which can be easily mounted, transferred, or swapped around. These racks may contain life support equipment (such as the new water purifications systems that went up within the last few years), experimental setups, or even crew amenities like exercise equipment (though in 0-g, that's almost under life support). This modularity is a great capability, since it allows entire experiments and systems to be transferred to the station fully configured and ready to be plugged in and powered up, but the racks are big. They can be up to 540 kg each and require a 40 inch square to pass through. Inside the US end of the station, this is allowed by the Common Berthing Mechanism (CBM) hatch, which is a 50 inch square inside a circular ring. For an idea of just how much bigger this is than the hatches used on the Russian end of the Station (where Progress and ATV dock) and the Shuttle (which uses a Russian-heritage docking mechanism, the APAS), take a look at the image below.
|Comparison of Hatch Sizes (Left--Progress/ATV, Right--CBM/HTV)|
However, CBM does have a drawback: unlike the docking mechanisms the Russians and Shuttle use (variously: probe and drogue, APAS, hybrid, and transformed hybrid), it requires the use of the station's robotic arm to move the vehicle the last bit to the station. The vehicle takes up a position just within arm's reach, shuts down its thrusters, and then the arm grabs the ship and does the precision work of aligning the berthing mechanisms for capture. Progress, Soyuz, Shuttle, and ATV don't need to go through this. Instead, they just maneuver in and dock with the station directly. This arm requirement is why no CBM vehicles (like the cargo Dragon that's now being qualified) is considered for crew transport and particularly no "life-boats" filling the role Soyuz has now would ever use CBM. In an emergency where the station loses power, the arm couldn't go through to motions to release the vehicle.
|ISS Arm Preparing to Grapple SpaceX Dragon for Berthing|
In my mind, I've been calling this the Common Androgynous Docking System (CADS). The system could be common in that it could be used almost everywhere--the same hatch system to link together station elements could also be used to dock ships to the station, or to allow two ships to dock in situations where crew needed to transfer from one to the other where no station could provide a middle man (like the Apollo CSM and LEM or the Apollo and Soyuz in the Apollo-Soyuz Test Project Flight). I already talked about why I think it should be androgynous, and it's a docking system rather than a berthing mechanism because no arms or other outside aids are required. Now, I'm only a student, and I know there's a lot more to designing something like a new docking standard then identifying a possible role, basic parameters, and giving it a snappy acronym. Doing the detail design and testing work for CADS and implementing it on vehicles could be a lot of work (and money), but if done right, it could make a big difference in the operations of future stations and ships.
My link to close today is this video of Saturday's Progress docking. From approach to capture was actually about 40 minutes, so the 3 minutes of the video is a lot of trimming, but even so the whole operation is incredibly fast compared to the HTV-2 grapple and berthing sequence, which took something like 5 hours. A time-lapse and computer animation of the HTV-1 docking can be seen here (Notice the clouds zipping past in the time-lapse starting at 1:10!). Food for thought.