And just an FYI, Dragon will not have the capability to launch or return racks to Earth - so rack return capability goes away with the Shuttle (although that capability is not needed anymore anyway).
Sorry Pete, I have to provide references: I know this is a news thread but...
The whole reason to have a CBM on the dragon cargo version was to enable up to a 4 rack capacity with additional small item and bag storage. It was intended to enable, albeit limited to only 4, ISPR transfer after the shuttle retirement. While the specs on this are yet to be published by Space X, the COTS contracts call for "a vehicle" to meet this requirement, and the Dragon was designed to meet the specs.
See article: search for ISPR
http://www.spaceref.com/news/viewnews.html?id=1095
See article: search for rack
http://www.spacex.com/dragon.php
The modular racks are called out by COTS to meet the same specs as for the shuttle mid deck items.
The capability to move racks(ISPRs) to and from station will be needed over the liftime of the station. Out with the old, in with the new. Science experiments in ISPRs will be the primary need. Other replacement actions may be needed like the WRS, Galley, Refridgerators, or Hygene stations.
http://www.nasa.gov/pdf/508318main_ISS_ref_guide_nov2010.pdf
Cygnus CBM is the right diameter, but the current hatch(37") is too small.
If the hatch size were increased, than the PM would need to be long enough (about 12' internaly) currently it's planned PM is only 10' long internaly. The diameter of a Cygnus is about the same as a MPLM.
http://www.orbital.com/CargoResupplyServices/index.shtml
HTV could deliver but the ISPRs are again too long. The HTV PLC is not long enough at 3.14 meters, the total PM version(6.5 meters) would need to be used. It's still in design. The diameter is about the same as an MPLM.
http://esamultimedia.esa.int/docs/hsf_research/ISS_User_Guide/31_HTV_web.pdf
I have to check references to design the Gateway Staion, which is currently a fictional(but highly tech compliant) ISS replacement, and uses many features of ISS, including ISPRs and transfer vehicles. Why re-invent the wheel?
Again sorry Pete, I'll use another thread for further conversation on this topic.
Keep up the great work...
Kindly, Darren.
---------- Post added at 09:13 PM ---------- Previous post was at 09:00 PM ----------
The problem with putting a "PMM-2" on ISS is that there just aren't enough (usable) berthing ports to do it, since in the future we'll need two open ports for cargo vehicles and two open ports for crewed vehicles.
My bad, Apologies...
After further review at NASAs site I'm forced to agree, It looks like power consumption is the primary concern, not docking/berthing ports. But not possible as NASA illustrated in the graphs and charts.
I still think it's possible to move the node to the forward axial position to free up another CBM currently in use by PMA 3. the Common Docking Adapters will replace the PMAs sometime on or about 2014.
http://dockingstandard.nasa.gov/Meetings/TIM_(Nov-17-2010)/NDS_TIM_presentation.pdf
It would seem NASA didn't like the CG changes by my proposed configuration. The study showed slightly unfavorable results from added permanent length. Tumble issues...
Again sorry, I had to look it up.
Kindly, Darren