Wednesday, February 2, 2011

Linking Together

All the thinking about connections for Aerodesign Team (which is going well, thank you very much) has put a spin in my mind on some things going on in spaceflight right now. Last Thursday (Jan. 27, 2011), the second Japanese H-II Transfer Vehicle (Kounotori-2) was captured and berthed to the International Space Station. 
Japanese HTV Spacecraft Approaches ISS for Berthing
Only two days later (Jan. 29, 2011), a Russian Progress supply ship docked to the Russian side of the station. In only another few weeks, the second European Automated Transfer Vehicle (Johannes Kepler) will dock to the ISS, also to the Russian side. This extraordinary pace of international vehicles has a lot to do with getting the ISS to the point where the retirement of the Shuttle won't have a catastrophic impact on the station's supply situation. However, something occurred to me about all this looking at some documentation about these various missions, and in talking to some people about it, I thought I might as well toss it up here on Engineer in Progress.
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
Progress vehicles are the workhorses of the ISS resupply fleet, and are capable of transferring food, water, fuel to adjust the ISS orbit, and other supplies to the station. ATV is designed to perform a similar role, and in fact is in a couple of way more capable, due mainly to the larger size and mass of the vehicle (more cargo and fuel can be transferred). However, though the HTV is smaller than the ATV and flies far less often than Progress, it can transfer some cargo that ATV and Progress can't handle. Why? In a situation familiar to many college students on move-in day, the doors are too small, and it has a lot to do with how the station is put together.

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)
This image compares a hatch similar to those in use on the Russian end of the station (and thus on Progress and ATV, as well as the Shuttle crew section) and the CBM mechanism (used between modules on the US end of the station, and on the HTV). The male figure is my height, just about 6 feet even (I was so annoyed when I grew the last half inch and spoiled my calibration). The extra size of the CBM hatch makes a big difference in what can fit through and I think even to how the ends of the station feel. In the pictures and videos I've seen, the big CBM hatches give the US section a feeling of spaciousness and connectedness where the Russian end looks cramped and cut off.

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 
So CBM has a lot of advantages, but also some serious drawbacks. It's big, and that allows the US section's dependence on modular racks for experiments and equipment. However, the fact that it is a berthing mechanism and it dependent on the station's arm for final approach and capture restricts where it can be used. Also, if a future vehicle or station were to use CBM, it too would have to have an arm for the same role. Thus, I've been thinking for a while about the idea of a new docking standard, something like the Russian APAS or the new American LIDS (Low Impact Docking System) in that it's a docking system that doesn't require an arm, but big, preferably as big as CBM. This would allow the same modularity of systems that the US orbital section has to be applied to future stations (like Bigelow commercial stations or other research outposts to succeed ISS) or vehicles (like a reusable Mars transit vehicle) but still allow the the standard to be used for independent crewed vehicles. Such a docking standard would ideally also be androgynous, meaning that any hatch could mate to any other hatch, unlike something like the Soyuz probe-and-drogue where the ends of the connection are not interchangeable.

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.


  1. Bob, I forgot to ask...Could I use the term CADS in Black Desert? I would, of course, site you as the source.

  2. ...I also forgot to ask if you mind being called Bob, Mr. Davidoff ;)

  3. Sure, you can borrow the CADS term. I prefer going by Rob, though. :)