The Space Shuttle Challenger was an Engineering Disaster that involved, Nasa, Morton Thiokol and the United States Government. Describing the incident, the space shuttle Challenger launched at 11:40am EST on January 28^th 1986 carrying 7 crew members, the shuttle exploded 73.137 into the flight. All seven members perished. The cause of the incident is not singular, but a combination of human error, design flaw and system breakdown. Total system failure occurred due to the o-rings separating the fuel section from the combustion section failing and allowing combustion gas into the fuel section, causing additional thrust and stress from the side of the solid rocket booster. This additional shear stress further increased the compromised section of o-rings allowing more combustion gas into the fuel section increasing the unintended burns intensity.

      Design flaws play a large role in the failure of the solid rocket booster or (SRB), first the rockets housing containing the o-rings was out of round, second housing itself was already out of spec for the o-ring gap of .030 was now .008 from previous burns. Third, the o-rings used became extremely brittle at temps below 60 degrees F. There were previous launches with o-ring failures, from launching on cold days temp below 60 degrees, with soot showing on both primary and secondary o-rings. Fourth, the insulation putty after the secondary o-ring was determined to be a partial sealant. But given all of these “flaws” the main reason for failure was the decision to launch outside the designed environmental parameters, the engineers at Morton Thiokol were adamantly against the launch on cold weather days, because of the known fact of o-ring failure. The launch was pushed due to political pressure, from the Regan administration, the Shuttle program was over budget and behind the intended launch schedule. Nasa officials ignored the obvious safety concerns and pushed for launch, that decision terminated the lives of 7 individuals.

      A solid rocket booster will produce 14.7MN of thrust or 3.3 million pounds. The temperature at which the outer casing of the rocket operates at is around 300 degrees F. Wind shear was withing the designed limits of the craft, the external hydrogen tank on the shuttle carries 384,071 gallons of fuel or 227,800 lbs. 6-1 ratio of oxygen to hydrogen. The composition of the SRB fuel is Aluminum oxide and Ammonium Percolate which burns at a temp dependent on the ratio of the two but roughly around 4000F.

F = mdot * Ve → mdot = F / Ve

Each SRB produces 11,520 kN of thrust and has 237 sec Isp (sea level), 2325 m/sec exhaust speed. x2

Each SSME produces 2,278 kN of thrust and 362 sec Isp (sea level), 3551 m/sec exhaust speed. x3

Lbs to Kg= 1lb=.45359237kg, therefore there was 103328.34 Kg of hydrogen on board at launch,

      Given that it takes 8.5 min to reach orbit and it will consume all of the fuel, in the external tank I will assume it has a fuel rate use of 446.67 Lbs/sec for Oxygen Hydrogen mix. At the 72 second mark that will leave 195640 lbs of fuel still in the external tank given 227800lbs to start- 446.67lb/sec*72sec.= 195640 lbs. Igniting that mixture resulted in a massive explosion that tore the craft from the SRBs and the external fuel tank. The craft hit the Atlantic Ocean 17 miles from the launch pad at 207 Mph.  

      Walking through the accident from start to finish, first there is a major design flaw within the SRB sections regarding out of roundness and o-ring choice. Second there is the launch decision on the cold weather day, leading to brittle o-rings. So, starting with launch .678 seconds into the launch a gray smoke puff is observed from the aft field joint on the right SRB (this is the section the o-rings are housed and failed booster), eight more distinctive puffs were observed with increasingly blacker smoke between .836 to 2.500 seconds, the puffs were occurring roughly 4 times per second correlating with the structural flexing and load. As the shuttle increased its velocity and altitude the last smoke puff was visible above the field joint at 2.733 seconds, this indicates that both layers of o-rings had failed and combustion gases were now burning the grease, joint insulation and remaining or-rings. At 37 seconds the shuttle experienced high altitude wind shears which lasted until about 64 seconds. These winds were counter acted by the guidance system firing the thrusters causing additional shear forces on the rockets. These wind shear forces were the greatest of any encountered on previous shuttle flights, but were still within calculated limits. However, it is believed that the aluminum oxide formed a temporary seal for the SRB o-rings and when the craft encountered the wind shear forces this caused a large increase in deflection, which now unsealed the aft section of the SRB. This is noticed at 58.788 seconds into the flight when a small flame is present at the field joint (Object C in Diagram 1)of the SRB, within a second it grew into a continuous plume, this was confirmed on the flight telemetry as the right SRB was operating at a lower drive pressure than the left unit. As the flame grew in intensity it was directed on to the external fuel tank of the shuttle, due to the aerodynamic slipstream created by the nose cone. The first visual indicator that the external tank was breached was at 64.660 secs when the flame shape and color changed abruptly. Within 45 milliseconds there was a bright orange glow on the heat tiles on the belly of the shuttle. At 72.20 seconds the lower strut connecting the SRB and external tank was pulled away from the hydrogen tank and caused the SRB to rotate around the upper attachment strut. At 73.124 Seconds the structural integrity of the external tank was compromised releasing massive amounts of liquid hydrogen creating a sudden thrust of 2.8 million lbs. Around the same time the right SRB impacted the lower interlinked oxygen tank causing a massive explosion. Travelling at Mach 1.92 and an altitude of 46,000 ft the shuttle broke into several pieces before crashing into the Atlantic Ocean.

      Engineering Lessons to be learned would include not letting non engineers dictate safety, if they want to operate in an unsafe manor. Design with the utmost safety in mind with multiple fail safes especially if for critical systems and components. Design with a large range of working conditions in mind, given that ideal conditions may not exist. I believe that NASA has been overhauled since this event, but maybe not enough given the 2003 Shuttle Columbia Explosion, granted that was not due to SRBs. The engineering mindset for aerospace has been altered forever given that the new acceptable failure rate is zero or rather should be. The tolerances for SRB have been dramatically increased with a project engineer, who was working on the faulty SRBs and gave well documented reports regarding the faults to department heads, spearheading the new and improved safety standards. To date, there have been no failures within the SRB rockets. Personally I will always strive to make the best product possible and when designing a vehicle where a single component failure can lead to death, the component must have a zero failure rate.

Resources:

https://www.biography.com/news/challenger-explosion-crew-astronauts-names-list

https://www.history.nasa.gov/Biographies/challenger.html

https://www.nasa.gov/centers/kennedy/pdf/167433main_Propellants08.pdf

https://blog.thinkreliability.com/o-ring-failure-breaking-one-cause-into-causes

https://www.simscale.com/blog/2019/01/space-shuttle-challenger-disaster/