SSTO - or Single Step To Orbit - are suborbital cargo spacecrafts that are used to transport goods across the "Fleet of Fog" controlled territories, primarily Pacific and Atlantic oceans. SSTOs are launched vertically, like traditional rockets, however they land horizontally, like airplanes or space shuttles - technically, these spacecrafts can be considered to be the space shuttle successors.
SSTOs are pretty large and are estimated to be able to carry up to a hundred tons of cargo. However, launching even one of these is very expensive, thus the SSTO can't really affect the blockade much. Probably, it is because of that, the "Fleet of Fog" usually ignores these, even if the Fleet does have means to shoot them down. However, should an SSTO carry goods or information that may pose a possible danger to "Fleet of Fog", it will be destroyed.
In Japan, the only confirmed launch site for SSTO is located in Saga Prefecture (decentralized capital Nagasaki jurisdiction territory). At some point, I-401 was summoned to protect an SSTO carrying the blueprints and one of the prototypes of the Vibration Warhead Torpedo during its launch phase. While I-401 was successful here and had sunk light cruiser Nagara, which was attacking the launch site, the SSTO didn't reach the recipient in USA - it was shot down over Hawaii.
It appears that SSTOs are based off of canceled completely-reusable spacecraft Lockheed Martin X-33. However, while X-33 was meant to be unmanned, it looks like SSTOs do have human pilots on-board.