There has been much said of the recent budget proposal, that zeros out funding for the International Space Station in the year 2025. What never seems to be mentioned is that the ISS was due to be retired in 2024. This is already a four year extension over the original retirement date of 2020. The station costs a sizable portion of NASA's budget to operate, and may not be sustainable from a structural standpoint after the year 2028. The ISS has a pressurized volume of 388 cubic meters, and cost $100 billion to construct.
Bigelow Areospace originally planned to have two of its BA330 modules ready to launch by 2017. These modules have 330 cubic meters of habitable volume - almost equal to the entire ISS. Note that the power input from the BA330 solar panels is much lower, so such a new station would not have the same capacity to run experiments. The ISS has solar arrays the size of an American football field, and generate 84-120 kilowatts. The BA330 power levels are not yet published, but the solar arrays are far smaller.
Bigelow already has a small prototype, called BEAM, attached to the ISS. This is giving NASA experience with the technology and allows the crews on ISS to experiment with this new inflatable habitat system. Bigelow is now proposing a new module, a full BA330, to be attached to ISS. This module is called XBASE. This could be ready by 2021, although it may be delayed due to the requirement to launch on the as-yet-undeveloped ULA Vulcan launch vehicle.
ISS was built with parts that are no longer in production. Much like the Space Shuttle before it, the cost of maintaining a system with limited spare parts becomes prohibitive with time. Also, without the Space Shuttle, large modules cannot be returned to earth to be rebuilt and relaunched. Similarly, the spacesuits on the US side needed to maintain ISS are in many cases leftover from the Shuttle era. They are far beyond their designed lifespan and are having age-related and design flaw-related issues. Extending the life of the ISS beyond 2024 becomes more of a risk with each passing year, and the costs will increase as parts become harder to come by. A key strength of the ISS, the vast solar power system, must run on batteries 45 percent of the time as the station goes into the shadow of the earth on each orbit. Those batteries also have a finite lifespan.
We are in a race to build a new system before the old system is retired. We failed to do this with the Space Shuttle, as several proposed replacements were cancelled prior to retirement of the shuttle. We also need new spacesuits sooner rather than later. The XBASE proposal could be launched as an adjunct to the old space station. This would give the new station access to power and more extensive facilities, while still being a brand new facility. Not everything on board the ISS is completely worn out. Some facilities are a decade newer than others. It may be possible to move some systems over to the Bigelow module to extend their operational lives. It may also be possible to use the XBASE as a primary facility while maintaining some experiments on the old ISS indefinitely. ISS already does this to some degree - the original Russian module was the habitat, propulsion, and power system for the ISS in the early stages of construction. Now, it is primarily just a hallway connecting newer Russian modules on one side with newer US/EU/Japan modules on the other.
A system with ISS and XBASE combined, with the later addition of one or two more Bigelow habitats, would give a graceful exit to the ISS facility and offer a new platform for life support, habitation, and more modern experiments. As systems on ISS wore out or were no longer maintainable, they could be shut down and bypassed. When the overall cost to benefit ratio of the system exceeds practicality, we can simply separate the facilities and send ISS to the bottom of the Pacific as originally intended.
Having BA330 dependent on Vulcan appears to be a mistake. Perhaps they can launch something on the Falcon Heavy on a shorter time frame if Vulcan is delayed. Since all new rocket projects are delayed, that seems a solid bet. The default Falcon fairing is far too small for BA330, but the payload capacity in terms of mass is far better than Vulcan. It should be possible to equip a Falcon Heavy with an oversized fairing for the mission, provided the aerodynamics and weight/balance issues can be settled.