Shuttle
Very very original ik
I. In the announcement of Orion program it was stated that it would go through 3 phases before attempting mun landing.
Build a kerbal rated vehicle that can put astronaut in space reliably, cheaply and is reusable
Build a space station that can keep kerbals there for months allowing research into long-term exposure of kerbals to space
Develop a new launch system that can carry kerbals to the moon and transport materials there that can allow permit stay
-The launch system program must cost less than $8 billion dollars
Many concept ideas were purposed to meet the first phase, from traditional capsules that would carry crew compartment having to be docked in orbit to SSTO however the designs that peaked interests at management and the air force was the shuttle design. It was designed to release, return and fix satellites in orbit. Had huge payload bay capacity and could put 6 kerbals into orbit for 2 weeks. These designs hit goals or exceed what KHA wanted making it easy sale pitch for government, KHA management and air force. You also had the fact that only 1 piece of the launch vehicle was not reusable, something that the SSTO concepts were the only ones was capable of competing with meaning it would win the competition in 2008.
The shuttle Reliant being used for aerodynamic testing on Shuttle carrier vehicle
II. Testing the design of shuttle would begin in 2011 but building of the first shuttle named Endurance would start in 2016, costing $6 billion per orbiter due to their complexity. You also had cost of building facility's that could support shuttle (i.e new runway) which was $800 million dollars. You also fact that KHA would have to dedicate 17% of its entire budget initially and after 8 years it would have be 20%. These high cost and budget requirements would be death nail for most programs if wasn't for the fact that the contractor promised it would "pay itself off eventually*" and carry military payloads if required. The contractor also projected that could build and support 6 shuttles and if the demand was there up to 10 with expansions of their facilities, but the program would only get funding for 3 due to how experimental it was so the 2 other shuttles built named Discovery and Pathfinder was in 2018.
The first shuttle to fly was obviously Endurance in late 2023 being flown by two female kerbals and they would stay in space for 4 days before landing successfully back at KSC.
Future?
III. The shuttle program so far has had 10 flights since it was announced operational including polar orbit. The builder of shuttle has started work on new shuttle named Adventure that will have new engines that will have twice the performance of their predecessor by using liquid hydrogen and oxygen. Also included in these upgrades are new SRBS, updated avionics, and Endurance would get these upgrades as well.
Space station & Successor. Now that its 2025 the first modules for the space station are now in development and the shuttle will be key part in building plus maintaining the station for a time before replacement is made which wont be long. See a private company just recently demonstrated heavy launch that's fully reusable, able put payloads beyond mun orbit (Octavia Space Telescope) and return it successfully. This has for obvious reasons raise concerns at shuttle's management so secretly development of vehicle that can compete with starship has started even though current shuttle doesn't even have retirement date.