Final answer:
Without a specific altitude given for '30 minutes after takeoff,' we can't provide an exact number of feet. However, Joe Kittinger is known for his high-altitude jump where he ascended to around 102,800 feet, which is at the upper edge of the stratosphere. The middle of the stratosphere ranges from approximately 33,000 to 82,000 feet.
Step-by-step explanation:
30 minutes after takeoff, Joe Kittinger reached the middle of the stratosphere. Kittinger was an Air Force Captain who, in 1960, took part in Project Excelsior, jumping from a high-altitude balloon at 102,800 feet. In your question's context, however, an exact altitude is not provided, so we do not have a specific number of feet to fill in the blank. Typically, the middle of the stratosphere is considered to be around 10,000 to 25,000 meters (about 33,000 to 82,000 feet), which is way above the highest altitudes where commercial airliners typically cruise, but Kittinger reached the upper edge of the stratosphere.
It's also worth noting that the phrase "30 minutes after take off" relates to a specific event involving Joe Kittinger or another person associated with a high-altitude jump or flight. We would need additional context to provide an exact figure here.
Given the information provided about pilot deceleration and mentions of altitudes typically associated with skydiving or high-altitude jumps, the reference is most likely to a historical figure and event related to aviation history and high-altitude survival or experiments.