What is the procedure for a rejected Jet/Rocket Assisted Takeoff (J/RATO)?
JATO (more correctly: RATO) typically uses simple solid-fuel rockets for their ease of maintenance, simple construction, long-term storability, robustness, and reliability. However, a simple solid-fuel rocket has to burn out once ignited, there is no way to shut it down. Techniques for extinguishing and even throttling solid-fuel rockets do exist, but they were developed rather recently, have not yet been commercialized, and clash with the goal of simplicity.
Currently, the &quot;usual&quot; way to shut down a solid-fuel rocket is to rapidly decrease the pressure in the combustion chamber, aka an explosive Flight Termination System, aka blowing up the rocket – not something you want to happen with a rocket strapped to your wings (IOW directly below the fuel tanks). Alternatively, you could get rid of that extra thrust by jettisoning the rockets. Having a dozen or so flaming projectiles uncontrollably flying around the airplane is also not a good idea.
So, you either reject before igniting the rockets or you don't reject at all. V₁ is when you hit the button, which may be as low as 0 kt, depending on the specific application.