By placing a trapdoor in the vertical position at the end of your desired trench (probably the block after the end of the trench), you can stop the propellant from traveling to the projectile rest and still have it travel far enough to avoid blowing up the cannon. Vertical seems to be necessary because TNT seems to slide on top of horizontal trapdoors.
I think I deleted the previous post. Basically there used to be a way to change the trajectory of a cannon by altering where the propellant TNT goes instead of altering what the projectile lands onto. It involved putting a block between where the propellant TNT goes to and where the projectile lands. However, in 1.8 it seems like this doesn't work anymore due to the way TNT works. The explosive force of TNT is blocked by blocks.
By placing a trapdoor in the vertical position at the end of your desired trench (probably the block after the end of the trench), you can stop the propellant from traveling to the projectile rest and still have it travel far enough to avoid blowing up the cannon. Vertical seems to be necessary because TNT seems to slide on top of horizontal trapdoors.
I think I deleted the previous post. Basically there used to be a way to change the trajectory of a cannon by altering where the propellant TNT goes instead of altering what the projectile lands onto. It involved putting a block between where the propellant TNT goes to and where the projectile lands. However, in 1.8 it seems like this doesn't work anymore due to the way TNT works. The explosive force of TNT is blocked by blocks.