The second Earth Starfleet warp test vehicle, taken without permission by Jonathan Archer and A.G. Robinson in 2143 to prove that Henry Archer's engine could hold warp 2.5.
The NX-Beta was the twin of the NX-Alpha, built for the trials that came before the NX class. It features in the Enterprise episode First Flight.
When the Alpha was lost and Starfleet Command, under Vulcan pressure, grounded the programme, Jonathan Archer became convinced that the fault lay in the intermix ratio and not in his father's design. He and A.G. Robinson, with the young engineer Charles Tucker helping from the ground, launched the Beta without authorisation.
The flight held together and reached warp 2.5. The two pilots were reprimanded, but the data kept the programme alive. A later prototype went on to break warp 3, and Enterprise NX-01 was launched in 2151 with Archer in command.