What Went Wrong
Why was Roger Boisjoly so concerned about O-Rings? These
seemingly insignificant pieces of rubber played a critical role
in the joints between segments of a solid rocket boster
(SRB ).
The two SRBs attached to a space shuttle orbiter provided
eighty percent of the thrust necessary to propel the shuttle
into space. About two minutes after a normal launch, the SRBs
would detach and parachute back to the ground to be reused in
subsequent missions. Several cylindrical segments make up the
149.1-foot- (45.4-meter-) tall SRB. Each joint between these
segments contains two O-rings, positioned concentric with the
SRB. The O-rings must be in perfect condition to prevent hot
gasses from leaking through the joints of the SRB.
The Challenger: What Went Wrong
Within a second of the launch of Challenger on January 28,
1986, the first signs of failure of a joint in the right SRB
were visible. Puffs of black smoke, whose color suggested that
5800-degree gases were eroding the O-rings, spewed out of that
joint three to four times each second. At the end of the first
minute, a small but steady flame was evident.
Atmospheric and aerodynamic conditions directed the flame
plume onto the surface of the External Tank, used to supply liquid
hydrogen and liquid oxygen fuel to the shuttle's engines during
the launch. The flame eventually breached the tank, and a
massive amount of hydrogen and oxygen burst into flame. At 73
seconds, a nearly explosive burn of the hydrogen and oxygen
quickly resulted and claimed the Challenger with its crew.
Above: A Solid Rocket Booster and its segments. The aft
field joint (arrow) failed in the Challenger's right SRB.
A Solid Rocket Motor Joint. Its parts are colorized in this
diagram for clarity. In pink is the tang,
which joins the clevis, colored orange. 177
huge steel pins (yellow) hold the joint in place. The O-rings
shield the joint from 5800-degree gases inside the booster.
On the left scenario, hot gases (red arrows) are shielded
from the joint by the zinc-chromate putty. On the right,
immense pressure creates a blowhole in the putty, allowing the
O-rings to move into the positions needed to seal the joint as
the gap between tang and clevis expands. Through the blowhole,
gases penetrate and wear away the O-rings.
Boisjoly had noticed that O-rings eroded, to an extent, in
this fashion previously. NASA and Thiokol, however, decided
that, since the O-rings were not completely eroded, there was
minimal risk. Boisjoly's concern was that the low launch
temperature would cause the O-rings to contract and further
compromise their sealing value.
This diagram is a cross section. In actuality, the joint
itself, tang, clevis, and O-rings have the circular shape of
the SRB.
Sources:Report to the President by
the Presidential Commission on the Space Shuttle Challenger
Accident.
Photographs adapted from pages 112, 52, and 57 of the
Presidential Commission Report.