Single stage. Launched 29 June 1959 at 21:03. Apogee 275 miles.
BK05, a re-entry experiment using a double cone eroding head, was designed for greater penetration at high speed into the atmosphere with the object of obtaining much greater heating, particularly in the nose cone which was made of doorstops. A complicated parachute recovery system was built into the head in an attempt to prevent damage to the nose cone on impact.
Overheating in the propulsion bay, as in BK03 (unfortunately not confirmed until after BK05) again caused premature engine cut-out resulting in a reduced re-entry velocity.
A hitherto unsuspected long decay time of thrust at engine shut-down resulted in collision of body and head at separation. The body telemetry continued to function to re-entry but head telemetry ceased just after head separation. The head aerial was probably broken by the impact with the body. The head was recovered and it was found that the parachute had torn out the inner core of the head and the base dome had been pulled off. Early deployment of the parachute would have resulted in excessive drag loads and it can only be assumed that this happened.
Some supporting evidence is that the barometric switch used to deploy the parachute was found on recovery to operate at a pressure equivalent to 22,000 ft instead of the expected height of 10,000 ft. However, the trial was not a complete failure since recovery of the head yielded data on erosion, albeit at a lower reentry speed than intended.