Seeing the image of a Lockhead LC-130 doing a jet assisted takeoff in snow mentioned in this answer, I wonder:
What is the procedure for a rejected takeoff when using JATO? Can the JATO rockets be turned off once ignited?
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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 "usual" 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. As per @ratchet freak's comment, $V_1$ is when you hit the button, which may be as low as $0\,kt$, depending on the specific application.
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1$\begingroup$ I'm curious about extinguishing a solid-rocket motor. You say one way is to rapidly depressurize the combustion chamber, "aka blowing up the rocket". Isn't that the opposite of decreasing the pressure? $\endgroup$ – Steve Apr 15 '16 at 14:01
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8$\begingroup$ The space shuttle had an explosive charge on the side of the SRBs that would cut them open to release the pressure. These were used by range safety, after the SRBs were separated during the Challenger accident. $\endgroup$ – Adam Apr 15 '16 at 14:55
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2$\begingroup$ Actually, extinguishable solid-fuel rockets have been around since the early 1960s, having been developed to allow solid fuels to be used in an ICBM without their inherent burn variability completely ruining the missile's accuracy. $\endgroup$ – Sean Jun 13 '18 at 17:34
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Once the rockets are ignited, it is basically too late to abort the takeoff, so the decision would have to be made before doing that. This is one reason why RATO is not used today as much as it used to be, the increased power of modern turboprop engines reduces the need for it in any case.
