That is a simple question but one that requires some explanation. Both Quest and Estes make standard rocket motors of the same size (18mm x 70mm), and they are interchangeable. These motors are designated 1/2A6, A6, A8, B4, B6, and C6. Also of note, the Quest A6-4 and the Estes A8-3 are for all purposes considered the same motor. Both companies also make mini and/or large motors. Those generally are not interchangeable. Igniters are interchangeable in regards to motors, but are not interchangeable in regards to launch controllers. Estes igniters will work with either Estes or Quest launch controllers. Quest Q2G2 igniters should only be used with Quest launch controllers. Using a Quest Q2G2 igniter with an Estes launch controller can cause an uncontrolled launch of the rocket. All Quest rockets use a standard launch rod of 1/8 inch. Estes rockets using mini or standard motors also use the 1/8 inch launch rod. Estes rockets using “D” or “E” motors require a thicker launch rod of 3/16″ (maxi-rod). Quest launch pads will only handle the 1/8 inch rod. The Estes standard launch pad will handle either the 1/8 inch or 3/16 inch launch rod.