This is how to make a simple mortar that I created.

1. Make a 3X3 base of obsidian

2. Put another layer of obsidian on top of that, leaveing out the center and one block from the middle of any of the sides

3. Put yet another layer on top of that, this time leaving out just the center piece

4. Repeat step 3 untill the tower is 6 blocks high off the ground

5. Load the center of the tower with TNT untill it is level with the top (there should be 5 blocks of TNT)

6. Going to base, place a redstone torch at the opening that leads to the TNT

7. Watch the TNT fly

Thanks for reading. If there are any questions about the mortar or some confusion in what I said, feel free to post