Before becoming a Guardian, Sen had long been working on solidifying the physics of Capsule technology in SL-Space. There were huge successes, but still gaps. Many capsules sent from N-Space were extremely difficult to refit, their existing mechanical components being all but impossible to make work here.
While we still hadn't figured out a way to grow new Titans here in SL-Space, two young new additions to our growing capital city named Zaros and Kea were finally successful in creating a uniform mechanical tech protocol that seemed effective for every old TTC that was brought to SL-Space by The Abbot. We called it the Type Theta. We blew through the first few Marks in testing and field trials, by the time I write this every TTC in SL-Space is now fitted with Type Theta, Mk VI mechanical components. In order for any of this to make sense, we need some TTC back story. I'll dig into Sen's ancient memories here, and do you up a nice lecture on Gallifreyan Time Travel Capsules.
A Time Travel Capsule is actually a careful melding of technology and a living being. Titans are extra-dimensional beings of supreme age and wisdom, who have been linked to Gallifrey in some unknown way since the dawn of the metaverse. Supremely powerful and wild, these beings are so much more advanced than us that even basic communication is beyond most Timelords.
The ancient Gallifreyans harnessed these Titans by “saddling” then with mechanical technology we know as a machine of Time And Relative Dimensions In Space. At this point, the saddled Titan was refereed to as the “Heart” of this constructed TARDIS.
The first full production run of TARDISes were referred to as Type 10's. After some refinements, the Type 11 was produced. Note that these are simply enhancements to the mechanical portions of the Capsule that are melded with the living Heart.
As Type 11's entered into service, Type 10's were returned for refit – their mechanical portions replaced and upgraded. These crafts were then referred to as Type 10, Mark I. This made them equal in specifications to the new Type 11's. This process continued. Type 12's were issued, meaning Type 11 Mk. 1's amd Type 10 Mk. II's still performed along side them.
At some point, the technology simply became too outdated, and the mechanical portions were scraped completely and replaced all new. This is when the Type designation increased to the next decade. The introduction of the Type 20 meant that all TTC's between Type 10 and Type 19 were decommissioned. The Heart stripped of all its mechanical components and completely refitted. Often, given the age of the Titan at that point, it was “retired” and return to the wild for breeding stock.
When I fled Gallifrey with Sen, the Type 65's had just been issued. Timelords who have come to SL-Space during or after the Time War speak of Type 90 War TARDISes, and extremely advanced Agency crafts in the Type 80 series. All of this is now ancient history. The mechanical portions of the Capsule are rendered as little more than pretty life-support chambers upon arriving to SL-Space, no longer able to connect and function with the Titan now injured and severed from its saddle.