Just some off the cuff thoughts on this...
1. your sprite object would ideally have bullet behavior
2. when the player touches somewhere on screen, set the bullet behavior angle to angle(sprite.x,sprite.y,touch.x,touch.y) (I don't know if those are the correct selectors, but you get the point)
2b. set the sprites bullet behavior acceleration to, say, 100
3. if no touch input is detected, set the acceleration to 0
have you tried that?