The loss of Challenger

On 27-28 January 1986 the overnight chill threatened the water pipes on Pad 39, so the valves were opened to allow the water to flow to prevent the pipes freezing. It was so cold that soon the gantry walkways were laced with sheets of ice, and icicles adorned the structure. The next morning, the launch of STS-51L was held for two hours to allow the ice to melt. Nevertheless, when Challenger lifted off at 11:38 EST the temperature was 15oC below that of any previous launch.

At Thiokol, a group of engineers led by Roger Boisjoly had expressed doubts about the resilience of the rubber O-rings that would have to seal the joints of the segments of the SRBs, but Joseph Kilminster, the company's vice president for boosters, was very aware of the imperative to build up the flight rate. Lawrence Mulloy, who managed the Thiokol contract at the Marshall Space Flight Center, asked incredulously: ''When do you want me to launch? Next April?" Kilminster overrode his engineers and recommended a launch. In the NASA way, a waiver was issued to recertify the SRBs for this colder temperature. This was regarded as yet another step in the continuing process of stretching the Shuttle's operating envelope. However, although no one was aware of it, the O-ring in the lowest field joint of the righthand SRB was so cold that it failed to seat properly in its groove when the motor was ignited, and let a blast of hot gas pass through the joint. The puff of dense black smoke that issued from the side of the casing was spotted only by the highspeed cameras that filmed each launch from close range for subsequent analysis. When the joint flexed as the casing accommodated the longitudinal stress of acceleration, a rapid series of smaller puffs followed, and a succession of blasts of gas seared further into the O-rings before the joint finally sealed. As the vehicle went supersonic, the thrust was throttled back in order not to over-stress the stack. After the aerodynamic loads had peaked, the SSMEs were increased to 104 per cent, and the geometry of the propellant in the SRBs increased their thrust. A few seconds later, there was a catastrophic explosion. CNN, the only major network to show the launch live, relayed NASA's video feed. As the flight controllers in Houston stared in disbelief at their telemetry displays, which had ceased updating, Stephen Nesbitt, the Public Affairs Officer, told the television audience that it was ''obviously a major malfunction". The loss was all the more shocking because there seemed to have been no indication of a problem.

In fact, as the SRBs increased thrust at T+59 seconds, the hot gas had reopened the breach in the right motor, which emitted a continuous plume similar to a blow torch. The first indication in the telemetry was at T+60 seconds, when the internal pressure of this motor began to depart from nominal. The issue for the flight controllers was whether this was a temporary decrease in pressure (in which case the focus of attention would shift to how well the Shuttle was able to correct its trajectory by swivelling the nominal engines) or whether it marked the onset of a divergence. The pressure could vary within a narrow band and remain acceptable, but at T + 63 seconds it left this band. The reason for this under-performance was not evident. At T+64 seconds, the plume breached the ET and ignited the resulting hydrogen leak. The SSMEs gimballed to counter the unwanted vehicle motions. At T + 72.2 seconds the plume severed the strut connecting the base of the right SRB to the ET, and as the booster pivoted on its upper strut its rear skirt struck Challenger's wing. The SSME gimballing increased to 5 degrees per second in an effort to counter the disturbance resulting from the divergent pitch and yaw rates between the two SRBs. At T + 73.124 seconds the bottom cap of the ET failed, and dumped the pressurised hydrogen into its wake. At T + 73.137 seconds the nose of the loose SRB struck the ET's intertank and fractured the base of the oxygen tank. Although Challenger survived the resulting detonation of the propellants, it was ripped apart by the extreme aerodynamic stress.

The Shuttle fleet was grounded during an investigation that interviewed 160

O B 16 24 32 40 49 56Ü S S64J 5 g72 Elapsed lime {seconds)

A plot of the pressure in the righthand SRB of STS-51L, and (inset) the plume that spewed from the field joint that was responsible for the anomaly.

people, examined 6,000 documents, and spun off 35 panels to analyse specific issues in depth. Attention focused on the O-rings. Over the years, all but two of the SRBs had been recovered (those from STS-4 had been lost due to identical parachute failures), and evidence of hot gas flowing past the main O-ring had been detected in nine casings. A broad section of the 75-millimetre-thick insulation that lined the aft nozzle of one of the boosters on STS-8 had been almost completely eroded by the hot efflux. This was attributed to a faulty batch of material. In fact, this had been the first use of an uprated booster. The erosion of the SRBs was to become a matter of some concern, not least because there was so much variation in the damage. When gas burned completely through a narrow arc of the inner O-ring ring and severely eroded the outer ring on one of the SRBs on STS-51B, Boisjoly had 'red flagged' the issue. Thiokol had established a formal study in October 1985, but it was judged that more data was required to characterise the problem, which entailed the inspection of more spent boosters. After the Report of the Presidential Commission on the Space Shuttle Challenger Accident was issued on 6 June, NASA redesigned the tang-and-clevis joint to reduce flexure, but in doing so it minimised the modification in order to enable it to upgrade the segments that it had in stock. In addition, a third O-ring was added to each joint, putty was applied to the interior of the joint, an electrical heater was installed to protect the O-rings from chill, and a 'weather strip' was wrapped around the joint to prevent the seepage of rainwater.

The Shuttle had carried 20 commercial communications satellites, which, in view of its operational complexity, was a fair achievement, but after the loss of

Table - Commercial satellites launched by the Shuttle

Satellite

Flight

Supplier

Bus

Operator

SBS 3

STS-5

Hughes

HS-376

SBS, USA

Anik C3

STS-5

Hughes

HS-376

Telesat, Canada

Anik C2

STS-7

Hughes

HS-376

Telesat, Canada

Palapa B1

STS-7

Hughes

HS-376

Perumtel, Indonesia

Insat 1B

STS-8

Ford

-

ISRO, India

Westar 6

STS-41B

Hughes

HS-376

WU, USA

Palapa B2

STS-41B

Hughes

HS-376

Perumtel, Indonesia

SBS 4

STS-41D

Hughes

HS-376

SBS, USA

Telstar 3C

STS-41D

Hughes

HS-376

AT&T, USA

Anik D2

STS-51A

Hughes

HS-376

Telesat, Canada

Anik C1

STS-51D

Hughes

HS-376

Telesat, Canada

Morelos 1

STS-51G

Hughes

HS-376

SCT, Mexico

Arabsat 1B

STS-51G

Aerospatiale

-

ASCO, Arab League

Telstar 3D

STS-51G

Hughes

HS-376

AT&T, USA

Aussat A1

STS-51I

Hughes

HS-376

Aussat, Australia

ASC 1

STS-51I

RCA

S-3000

ASC, USA

Morelos 2

STS-61B

Hughes

HS-376

SCT, Mexico

Aussat A2

STS-61B

Hughes

HS-376

Aussat, Australia

Satcom K2

STS-61B

RCA

S-4000

RCA, USA

Satcom K1

STS-61C

RCA

S-4000

RCA, USA

Challenger, the White House ordered the National Space Transportation System to cease commercial operations. Furthermore, safety concerns prompted the cancellation of the wide-bodied Centaur that was to have enabled the Shuttle to dispatch the new generation of heavyweight planetary probes.22

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