Pistons aren't the only things started by those things. The Pratt & Whitney J-57-P-59W's mounted on the old KC-135A's had what amounted to an Estes rocket engine - an Estes engine that was 6" long and 8" wide! They burned in about 5 seconds and in that time the pilots had to do all the lever and switch flipping they normally did in 15-20 seconds. Eng's 1 and 4 were started that way, then those were run up and the inboards were started on bleed air. Less than one minute from butts in the seats to taxi-ready ain't nuthin' to sneeze at!
The only bad part was cleaning the cartridge receivers afterwards!