Your UKROC rocket must contain an electronic altimeter approved for use in the UKROC regional finals. Should you reach the final then a Perfectflite APRA altimeter is supplied to you. As such your altimeter bay needs to be able to accommodate the APRA. Suitable altimeters for use in your UKROC rockets whilst testing and at the regional stages include the Estes altimeter, and the Jolly Logic one and two. We sell all of these listed here. The APRA is no longer available to buy. For reference the APRA size is 2.75"L x 0.55"W x 0.62"H (70mm x 14mm x 16mm) in size to ensure your rocket can accommodate one at the final.