So where is the inlet for the 'booster' engine - does it get the air from the conventional center engine inlet? Of is that an inlet just above the center engine inlet?
Boeing did something different for the 727 for Mexico City - they mounted some JATO bottles in the wheel wells - if they lost an engine during takeoff the JATO bottles would fire to give that extra thrust needed. Testing resulted in some pretty spectacular videos...
Eventually they were able to uprate the JT8D engines enough that they could get rid of the JATO assist.