Firetrucks have the world fire written two ways, as seen here, on the front of the truck. why is that ?

While the image this question refers to is missing, I am assuming it shows the word FIRE (1) written in a normally readable way, (2) written in a mirrored way. The display (2) is used in order for other vehicle drivers being able to read the word FIRE in their mirrors (rearview and side).  The word printed in a mirrored way will appear normal via a mirror (i.e., the mirror "undoes" the mirroring originally introduced).

the device is called “ rocket engine”

see earlier answer