Return to Human Space Flight home page

Mission Control
Answers Your Questions

From: Bryan McComb, of London
To: Bob Castle, flight director

Question: I was lucky enough to be staying in Titusville, FL, when STS-106 Atlantis landed and was glued to the NASA broadcast in my hotel room. I noticed after the landing that there were jets of what appeared to be steam spurting intermittently at the base of the tail after the landing. Is this normal and what is it? Sincerely Bryan McComb

Answer: The exhaust plumes from the auxiliary power units (APUs) exit the orbiter at the base of the tail (APU 1 and APU 2 on the port side, and APU 3 on the starboard). Occasionally the exhaust plumes can ignite (the plume is composed of hydrogen, ammonia, and nitrogen), as happened post-landing on STS-106. This poses no danger to the crew or the orbiter, but the flame is very impressive! The pulsing is caused by the method used to control the APU speed - the fuel is sent to the APU until it reaches a certain speed, then the fuel flow is cut off until a lower speed is reached, then repeats. The water spray boilers (WSBs), used to cool APU lube oil and the hydraulic fluid, also exhaust at the base of the tail. All three WSBs exhaust steam continuously (vs. pulsing) out the starboard side.

Planning team MMACS/Kevin McCluney

View list of answered questions or ask MCC your own Question.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
Web Accessibility and Policy Notices