- "The TV-21 was Dad's baby, the first ever super-Mach 20 ship. It was the prototype to Thunderbird 1."
— Scott Tracy
The TV-21, also known as the first Thunderbird, was International Rescue's first prototype craft. It was a prototype to Thunderbird 1. It was designed by Jeff Tracy after investing everything into the construction of the ship, it was constructed by Brains and it was piloted by Jeff himself. The ship became the world's first super-Mach 20 as it contains the most powerful thruster Brains ever built, even more powerful than the ones equipped in all the later Thunderbird aircrafts, despite being a dated craft. The ship was powered by fusion generators.
During its test fight, the ship was sabotaged by the Hood. Jeff was forced to abandon his ship and ditch it somewhere over the Mariana Trench instead of letting the Hood get his hands on it. Even though Jeff survived, he was heartbroken, believing the craft to have been destroyed after ditching in the ocean.
It was rediscovered intact years later after Jeff Tracy when MIA in the Mariana Trench by a pair of deep sea explorers, in Up from the Depths Part 1 and Part 2, piloting a survey craft ostensibly made to map out the rest of the Mariana Trench. The survey craft was however, built and supplied to the team, and ultimately controlled, by The Mechanic, specifically to find TV-21, with the survey team remained in the dark until the last minute. As a meticulously well-built aircraft that was built to last by Brains, not only did TV-21 survive being crashed and sunken in the Mariana Trench in one piece, all of the internal systems remained intact and fully functional despite nearly two decades of disuse and lack of maintenance, as well as prolonged stay in an environment that it was not built to sit in.
Everyone in the Tracy household but Alan, who was too young to remember anything about the craft when it went missing, was excited about the discovery, and went with extra zeal about the rescue of the survey team, whose craft mysteriously shut down as soon as the rediscovery was made, not knowing that it was a trap set up by The Mechanic all along.
Having successfully baited the International Rescue to help dig out the craft and confirm its status, The Mechanic then wrestled with the Tracy brothers for possession of the craft that he had stolen shortly after it was dug out, severely damaging two Thunderbirds and destroying one in the process. Fortunately, the survey team was able to be rescued before International Rescue ran out of options. Eventually it became clear that International Rescue had the upper hand and The Mechanic, having no way to turn the tables around, overloaded the fusion generators of TV-21, resulting in a massive explosion that destroyed TV-21 alongside the survey craft, to cover his tracks and to rob International Rescue of their victory.
All that was salvaged from the craft was a hat that Jeff Tracy used to wear on missions, which he left on the seat of TV-21 when he was forced to ditch the craft.
- The TV-21's command bridge has a resemblance to the original Thunderbird 3's pilot seat.