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The Challenger Accident (1986): Cold O-Rings That Recovered Too Slowly

On January 28, 1986, the Space Shuttle Challenger broke apart about 73 seconds after launch and all seven crew members died. The Presidential Commission found that the cause was the failure of the pressure seal in a joint of the right solid rocket booster. The launch morning was the coldest to date, and the cold rubber O-rings were slow to return to their original shape.

What happened

Date
January 28, 1986, 11:38 a.m. (EST)
Vehicle
Space Shuttle Challenger (mission 51-L)
Casualties
The vehicle broke apart about 73 seconds after launch at about 46,000 ft (about 14 km); all seven crew members died
Temperature
36 °F (about 2 °C) at launch, 15 °F colder than the next coldest previous launch
Joint
Aft field joint of the right solid rocket booster; estimated at 28 ± 5 °F (about −2 °C) at the joint’s 300-degree location
Investigation
Report of the Presidential Commission on the Space Shuttle Challenger Accident (1986)

Sequence of events and the investigation’s findings

At 0.678 seconds after liftoff, a strong puff of gray smoke was already coming from near the aft field joint of the right solid rocket booster, and the puffs that followed grew blacker. A flicker of flame appeared on the same booster at 58.788 seconds and became a continuous plume at 59.262 seconds. At 73.124 seconds a circumferential white vapor pattern appeared on the bottom of the external tank, regarded as the beginning of the structural failure of the hydrogen tank. Structural breakup of the vehicle began at about 73 seconds.

The Commission concluded that the cause of the accident was the failure of the pressure seal in the aft field joint of the right solid rocket motor, due to a faulty design that was unacceptably sensitive to a number of factors, including temperature, physical dimensions, materials, the effects of reusability, processing and the joint’s reaction to dynamic loading.

The chain of causes from the materials side

Challenger: the chain of causes from the materials sideA cold morning (about 2 °C)Joint estimated at about −2 °CThe rubber O-rings get coldSlow to recover after being squeezedIgnition pressure opens the jointThe gap in the joint widensThe O-rings cannot follow the gapDelayed pressure from putty also a factorHot gas leaks through the jointSmoke at 0.678 s, flame at 58.788 sBreakup at about 73 seconds
Challenger: the chain of causes from the materials side

The booster joint was sealed by two rubber O-rings with a cross-section diameter of 0.280 in (about 7.1 mm). The primary sealing mechanism is gas pressure pressing on the upstream side of the O-ring. The putty in the joint was meant as a thermal barrier to keep combustion gas from touching the O-rings directly, and its performance as a pressure seal is highly variable.

Right after ignition the joint gap opens (joint rotation), and dynamic effects increase the opening. The Commission found that O-ring resiliency is directly related to temperature, that a warm compressed O-ring returns to its original shape much more quickly than a cold one, and that at the cold launch temperature the O-ring would be very slow to return to its rounded shape and probably did not follow the opening gap. It also found that the putty’s temperature, humidity and other variables could delay the application of pressure to the O-ring and could have been a factor in the initial joint failure, and that water in the joint may have frozen, impeding the secondary seal.

In earlier flights, only four launches at 61 °F or above showed thermal distress of the O-rings, whereas every launch below 61 °F showed thermal distress in one or more O-rings.

On interpretations of the cause: some university teaching material (the University of Michigan) explains that the launch-day temperature was below the rubber’s glass transition temperature, so the O-rings became brittle. The Presidential Commission’s report states that the O-rings’ resilience slowed with temperature and that they probably did not follow the opening of the gap.

The clues that revealed the cause

Open the guide with this case’s clues

Opening the guide with these clues ranks loss of elasticity and embrittlement of polymers at low temperature at the top.

Related failure modes: Loss of elasticity and embrittlement of polymers at low temperature

What changed afterwards: design, rules and inspection

Lessons

Check your understanding

Which statement about the glass transition temperature Tg and melting point Tm of a crystalline (semicrystalline) polymer is correct?

  1. ATg is higher than Tm
  2. BTg is the temperature at which chain motion in the amorphous regions freezes or is released, and it is lower than Tm
  3. CTg is the temperature at which the crystalline regions melt, and amorphous polymers, which have no crystals, have no Tg
Show answer and explanation

Answer: B. Tg is the temperature at which chain motion in the amorphous regions freezes or is released, and it is lower than Tm

A crystalline polymer consists of crystalline and amorphous regions. Tg is the temperature at which segmental motion of the chains in the amorphous regions freezes or is released; below Tg the polymer is hard and glassy, above it rubbery. Tm is the temperature at which the crystalline regions melt; Tg is always lower than Tm, and for many polymers Tg/Tm (as a ratio of absolute temperatures) is about 0.5–0.8. PET, for example, has a Tg of about 70–80 °C and a Tm of about 260 °C. It is Tm that represents melting of the crystalline regions; amorphous polymers such as polystyrene have no Tm, but they do have a Tg. Polymers such as polyethylene, whose Tg is below room temperature, are flexible because their amorphous regions are rubbery at room temperature.

When a plasticizer is added to poly(vinyl chloride) (PVC), what happens to the glass transition temperature Tg and the properties of the material?

  1. ATg rises and the material becomes hard and brittle at room temperature
  2. BTg falls and the material becomes soft at room temperature
  3. CTg is unchanged and only the melting point of the crystals falls
Show answer and explanation

Answer: B. Tg falls and the material becomes soft at room temperature

A plasticizer works its way between the chains, weakening interchain interactions and increasing free volume, so segmental motion becomes easier and Tg falls. Rigid PVC has a Tg of about 80 °C, but adding plasticizer brings Tg below room temperature, giving the flexible PVC used for hoses and wire insulation. Raising Tg to make a polymer harder is done by introducing crosslinks or stiff aromatic rings, or by increasing the molecular weight. PVC has low crystallinity, and the plasticizer acts mainly by lowering the Tg of the amorphous regions, so it is not that only the melting point falls. When the plasticizer migrates to the surface over time, the material hardens or stains its surroundings.

Which statement about mechanical models of viscoelasticity is correct?

  1. AThe Maxwell model, a spring and dashpot in series, can represent stress relaxation under constant strain
  2. BThe Voigt model, a spring and dashpot in parallel, can describe stress relaxation under constant strain
  3. CThe Maxwell model can represent creep in which strain approaches a constant value under constant stress
Show answer and explanation

Answer: A. The Maxwell model, a spring and dashpot in series, can represent stress relaxation under constant strain

When a constant strain is applied to the Maxwell model (series), the spring stretches first and stress develops; then the dashpot slowly extends while the spring contracts, so the stress relaxes as σ = σ₀exp(−t/τ) (τ = η/E). In the Voigt (Kelvin–Voigt) model (parallel), the spring and dashpot always have the same strain, so the stress does not relax under constant strain. What the Voigt model represents is delayed elasticity (creep), in which strain approaches a constant value over time under constant stress. Under constant stress, the strain of the Maxwell model does not approach a constant value but keeps increasing in proportion to time. Real polymers are approximated by combinations such as the four-element model.

Which correctly describes the idea of time–temperature superposition in polymer viscoelasticity?

  1. ARaising the temperature has the same effect as measuring over longer times (lower frequencies)
  2. BRaising the temperature has the same effect as measuring faster, over shorter times (higher frequencies)
  3. CViscoelastic properties depend only on temperature, so changing the time scale or frequency of the measurement leaves the modulus unchanged
Show answer and explanation

Answer: A. Raising the temperature has the same effect as measuring over longer times (lower frequencies)

Molecular motion in polymers is faster at higher temperatures, so relaxations that occur in a short time at high temperature take a long time at low temperature. Relaxation modulus curves measured at different temperatures can therefore be shifted along a logarithmic time axis to overlap into a single composite curve (the master curve). In other words, raising the temperature has the same effect as measuring at longer times (lower frequencies). The amount of shift (the shift factor) is described by the WLF equation near Tg and by an Arrhenius-type expression well below Tg. Using this principle, long-term creep and stress relaxation at room temperature can be predicted from short tests at high temperature.

Related terms

Glass transition temperature Viscoelasticity

Main sources

  1. Report of the Presidential Commission on the Space Shuttle Challenger Accident, Volume 1, Chapter III: The Accident (NASA History)
  2. Same report, Chapter IV: The Cause of the Accident
  3. Same report, Recommendations
  4. NASA, Implementation of the Recommendations of the Presidential Commission (1987), Chapter 1
  5. University of Michigan, Materials Science and Engineering: The Glass Transition of Polymers (demonstration notes)

Dates, numbers of people, dimensions, temperatures and other values were checked against the official reports above and sources that cite them.

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