Why was the nose cone on the Space Shuttle's external tank pointy?

Question

Inspired by comments under this answer to this question Just how pointy does a rocket's nosecone need to be? (see also Why are the nose cones of current spacecraft less pointy?)

Why was the External Tank's nose cone pointy?

(Image source: https://steemitimages.com/DQmXscrjPZRpFwkAdvTvUHkL1Bb9csesUpqzBz2JA8Q8JZk/image.png)

Answer 1

The original External Tank nose cone design was indeed blunt - almost like a fireplug, as seen in this 1975 concept art.

[Image source - lost in the mists of time to me, but NASA somewhere]

However, wind tunnel testing at the Arnold Engineering Development Center revealed that this configuration caused unsteady aerodynamic buffeting at some conditions. A bi-conic spike design was conceived and further wind tunnel testing showed that it eliminated the problem.

The nose cone served several functions besides its aerodynamic one.

• The "flared" portion of the nose cone covered the area where the GO2 pressurization line and the cable tray entered the tip of the O2 tank.

[Cropped from https://www.nasa.gov/centers/marshallimages/content/149235main_IMG_2558_3072x2048.jpg ]

• It also covered the LO2 vent/relief valve and provided vent exits for gases boiled off from the LO2 tank, which were subsequently carried off by the "beanie cap" umbilical arm.

[Image Source https://contactsam.files.wordpress.com/2014/11/vent_arm_141113_2.jpg ]

• Finally, the aluminum spike at the tip acted as a lightning rod. The large lower cone was made of composite material.