there could be 2 separate engine dedicated cylinders.

and both don't have to work. only 1 does out of the two.

 

and something happens, it fails,

then, the cylinder holding those engines can swivel down at an angle,

and drop those engines,

while the second one at the back can activate.

 

whichever way, if to design it for maximum safety.

backed up.

 

then there would hardly be any casualties.

 

it doesn't have to be electric, at all.