STS-51L: Definition of "Criticality 1"

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Author	Topic: STS-51L: Definition of "Criticality 1"
ASCAN1984 Member Posts: 1049 From: County Down, Nothern Ireland Registered: Feb 2002	posted 06-25-2010 03:42 PM I have been watching the excellent Challenger DVD set from Spacecraft Films and during a conversation at the Rogers Commission between a member and Larry Malloy the definition is challenged of "Criticality 1" by the board member. He says by definition of Criticality 1 you cannot rely upon the secondary seal. How much truth is there to this?
Jay Chladek Member Posts: 2272 From: Bellevue, NE, USA Registered: Aug 2007	posted 06-26-2010 11:46 AM I'll have to check Allan McDonald's book "Truth, Lies and O-Rings" to see what he says about the issue from the Thiokol contractor side. I believe the definition as it was used in the testimony is essentially correct, meaning it is the first thing to worry about (I asked an engineer friend about how they use it). There is no backup if it fails and the result can be loss of vehicle and crew. Although the original SRB field joint design had two O-rings, the design's weakness is when the SRB (as I understand it) pressurized at ignition like a balloon, the joint would rotate outward, pulling the second o-ring out of guide groove (in a sense, the groove expands rather then contracts and the o-ring doesn't expand with it). Granted the putty used ahead of the joint is the first line of defense, but if there is an air bubble void in the putty, then the joint is more likely to see hot gases.
moorouge Member Posts: 2454 From: U.K. Registered: Jul 2009	posted 06-26-2010 12:49 PM Never heard of 'Critically 1'. Heard of 'Critical 1'. Always thought that a 'Critical 1' system was where a system failure would result in the loss of both spacecraft and crew. For example, on Apollo the SPS engine was a 'critical 1' system. On the Shuttle the landing gear is 'critical 1' as is the fuel connection from the external tank to the shuttle main engines.
Jay Chladek Member Posts: 2272 From: Bellevue, NE, USA Registered: Aug 2007	posted 06-26-2010 01:50 PM Well, I did manage to find this PDF from Iowa State Online and it does discuss critical engineering items as related to some space projects. So "criticality-1" is a term used in NASA circles these days.
moorouge Member Posts: 2454 From: U.K. Registered: Jul 2009	posted 07-03-2010 04:00 PM Document Practice # PD-ED-1240 , 'Identification, Control and Management of Critical Item Lists' issued by NASA in October '95 uses the word 'critical'. The definition given is - The FMEA results in the identification of single failure points (SFPs) and critical redundant items. A typical SFP is defined as a single item of hardware (usually at the component level) the failure of which could result in the loss of life, vehicle, mission, or damage to a vehicle system. It is the general policy of NASA not to permit the retention of single failure points in design unless special conditions prohibit designing it out, such as technology, operations or cost. Retention of a single failure point requires that a justification or rationale be prepared which describes actions taken, safety margins, failure prevention measures, tests, or inspections that will ensure that the critical item of hardware will not fail in the mode indicated in the FMEA.
Robert Pearlman Editor Posts: 42988 From: Houston, TX Registered: Nov 1999	posted 07-03-2010 04:40 PM As you can see from the original Critical Items List, the entry for the Solid Rocket Motor case joint seals establishes them as "Criticality 1R" (redundant). See upper right: "Criticality Category: 1R." And as defined in SP-4219: Criticality 1 items were those where a failure could cause loss of life or vehicle; Criticality 1R, where a failure of all redundant hardware items could have the same effect; Criticality 2, where failure could cause loss of mission; Criticality 2R, where failure of all redundant hardware items could have the same effect.

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