Fighter
Air Traffic Control Developmental
Confusion; Situational Awareness
ATC Issue All Types; Conflict NMAC
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action
Environment - Non Weather Related; Human Factors; Procedure
Both planes were staying in the pattern. Aircraft Y requested Runway X and Aircraft X requested Y. Aircraft Y departed first and I put him in left traffic to avoid the departure of the Aircraft X; when Aircraft X departed I put him in right traffic at the time initially thinking if I put him in left traffic that he would quickly catch up to Aircraft Y and become an issue. I called traffic to the Aircraft X about Aircraft Y and he did not have him in sight and asked the order to the field when I told him he would be number 1 to the field with both aircraft about midfield downwind at this point and Aircraft X; 500 feet above Aircraft Y. I then called traffic to Aircraft Y about Aircraft X and he had him in sight. I gave Aircraft X clearance for the option at this point on Runway Y and then to give a little more spacing I told Aircraft Y to extend his downwind and that I would call his base for Runway X. As Aircraft X was descending and turning base he flew towards Aircraft Y who was extending downwind and Aircraft Y maneuvered to the left to avoid him and Aircraft X told me he was going around.  After the incident I talked to a fellow CPC and a Supervisor and found a few things I could have done better to have avoided that situation and to have dealt with the conflict as it was arising.For starters putting Aircraft X in left traffic would have been the better option rather than right which caused the planes to be going nose to nose towards each other. Also when Aircraft Y had traffic in sight I should have told him to maintain visual separation. I also should have changed the sequence and extended Aircraft X downwind while I let Aircraft Y make a short approach and come in #1 to keep Aircraft X on the outside of Aircraft Y's flight path. I also learned a little more about the characteristics of Aircraft X such as its speed in the pattern and how it descends and makes a much more rounded turn from downwind to final rather than squared turns to base and then again to final.
Tower Controller in training reported an NMAC between two aircraft in the traffic pattern.