In an atmosphere, the TRTs can use an Air-Ram mode, which draws air through it, superheating it, and ejects it out the nozzles. Like a Jet. However, unlike a jet, fuel isn't expended. Instead, it remains in the reactor, realizing its full reactability.
When there's not enough, they switch to Thermonuclear Thrust, where the superheated plasma from the reactor is expended (This is the fuel that isn't fully used up. Like an A/B). You can fly for HOURS without refueling in Air-Ram, but only for short periods on TT.
The tanks don't carry enough fuel to reach LEO. Like I said, it's my guess that the 1S has specially tuned engines that can air-ram at higher altitudes.
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