Re: The RCS: Voice Transcript

From: rk (stellare_at_NOSPAMPLEASE.erols.com)
Date: 06/18/04 

Date: 18 Jun 2004 00:19:45 GMT

Ooops, bad form to respond to your own post but I left a few questions off.

rk20: Who was the manufacturer and what was the part number for the
      failed "thermo switch?" Was it bought to a military specification,
      source control drawing (SCD), or through normal manufacturer's
      processes?

rk21: Were there other failures of this "thermo switch" on Apollo before
      the fire? After the fire? On other programs? Were there any
      alerts or advisories put out on this switch? NASA started issuing
      such alerts in 1966.

rk22: Have you gone through the circuit analysis for the circuit that
      failed. If not, why not? If so, was it correct? If it was not
      correct, please show the calculations, assumptions, or methods that
      were not correct. Was it a worst-case circuit analysis? RSS?
      Typical values only? Other?

rk23: Was an FMEA or FMECA performed on the circuits that failed? If so,
      was it correct and what did it show? If it was not correct, please
      show the calculations, assumptions, or methods that were not correct. 

-- 
rk
rk wrote:
> LaDonna Wyss wrote:
> 
>                      [ snip ]
> 
>> Yes, sustained power to the heater is what did it.  That thermo switch
>> fused closed (a la Apollo 13) due to the amount of current that was
>> flowing through that circuit which SHOULD NOT HAVE BEEN.  By the
>> switch fusing closed, it allowed the heater to remain on, and it
>> ignited the surrounding materials.
>> Keep this up, and I might have to change my opinion of you, Daniel! 
>>:-)
> 
> Let's discuss this, please, your claim that the
> 
>    thermo switch fused closed (a la Apollo 13) due to the amount of
>    current that was flowing through that circuit which SHOULD NOT HAVE
>    BEEN. 
> 
> rk9:  Please state the amount of current that was flowing through that
>       circuit and how did you determine that value?  
> 
> rk10: What was the anamolous level of current flowing through the "thermo
>       switch" and how did you determine that value?
> 
> rk11: What was the source of this anamolous current?  Please show the 
>       fault that existed on the schematic.  If it was a design error,
>       please show the calculations proving that the circuit had
>       inadequate margin. 
> 
> rk12: Please show the engineering change order, waiver, or other such 
>       document that corrected this deficiency or allowed it to remain?
>       If there was no engineering change order (or whatever they called
>       them) then please see rk13.
> 
> rk13: If there were no changes, then please explain why there were no
>       problems on the rest of the spacecraft in the Apollo program.
> 
> rk14: Please provide the nominal amount of current on that circuit, at
>       the closest point upstream that was monitoring the current.
> 
> rk15: Please show current telemetry data support your assertions.
>       
> rk16: Please provide a schematic of the circuit in question.  This should
>       include wire types, gauge, insulation type, and what circuit
>       protection devices that were in place (e.g., fuses, breakers,
>       resistors, relays, etc.).  If no protection was in place please
>       state that with reference to the complete circuit on the schematic.
> 
> rk13: Please state how much power was dissipated in the heater.
> 
> rk14: Please provide the locations of temperature sensors near the
>       affected area of this heater.
> 
> rk15: Please state what type of temperature sensors were used for
>       spacecraft instrumentation?
> 
> rk16: Please show on your timeline the temperatures indicated by the
>       sensors?
> 
> rk17: Please show physical pictures or records describing the
> post-accident 
>       condition of the "thermo switch" and the heater that support your
>       theory.
> 
> rk18: Please describe the results of your examination of the "thermo
> switch" 
>       and heater.  If you have no examined it, why not?  If the material
>       was not destroyed, when do you plan on examining it?  And if you 
>       do plan on such an examination, would you object to my witnessing
>       your examination?
> 
> rk18: Please show the results of your testing that replicated the
> conditions 
>       that you hypothesize initiated the fire.
> 
> rk19: Please explain why no witnesses that I am aware of reported any 
>       indications of this anamoly.  You state that you have gone through 
>       witness statements.
> 
> I trust that you are aware of the findings of Cortright Commission on
> this matter.  Since you made use of the similarity for support, you must
> show such similarity.  Answering the above questions will get you
> started.  For reference, here are some of the key findings.  If you find
> fault with any of these findings, then please state so and technically
> show why they are incorrect.
> 
> b. It is now known, however, that the tank contained two protective 
> thermostatic switches on the heater assembly, which were inadequate and
> would subsequently fail during ground test operations at Kennedy Space
> Center (KSC). 
> 
> h. A number of factors contributed to the presence of inadequate
> thermostatic switches in the heater assembly. The original 1962 specifi-
> cations from NR to Beech Aircraft Corporation for the tank and heater
> assembly specified the use of 28 V dc power, which is used in the space-
> craft. In 1965, NB issued a revised specification which stated that the
> heaters should use a 65 V dc power supply for tank pressurization; this
> was the power supply used at KSC to reduce pressurization time. Beech
> ordered switches for the Block II tanks b-tit did not change the switch
> specifications to be compatible with 65 V dc.
> 
> 1. The thermostatic switch discrepancy was not detected by NASA, NR,
> or Beech in their review of documentation, nor did tests identify the in-
> compatibility of the switches with the ground support equipment (GSE) at
> KSC, since neither qualification nor acceptance testing required switch
> cycling under load as should have been done. It was a serious oversight
> in which all parties shared.
> 
> j. The thermostatic switches could accommodate the 65 V dc during
> tank pressurization because they normally remained cool and closed. How-
> ever, they could not open without damage with 65 V dc power applied. They
> were never required to do so until the special detanking. During this
> procedure, as the switches started to open when they reached their upper
> temperature limit, they were welded permanently closed by the resulting
> arc and were rendered inoperative as protective thermostats.
> 
> and we have this determination:
> 
> e. During the special detanking of oxygen tank no. 2 following
> the countdown demonstration test (CDDT) at KSC, the thermo-
> static switches on the heaters were required to open while
> powered by 65 V dc in order to protect the heaters from over-
> heating. The switches were only rated at 30 V dc and have
> been shown to weld closed at the higher voltage.
> 
> Lastly, note how specific numbers are used in the report along with the
> use of telemetry and identification of wire insulation type.  This is
> similar to how questions are asked here by some along with some rather
> interesting common issues.
> 
> The electrical circuit for the quantity probe would generate
> only about 7 millijoules in the event of a short circuit and
> the temperature sensor wires less than 3 millijoules per
> second.
> 
> Telemetry data immediately prior to the accident indicate
> electrical disturbances of a character which would be caused
> by short circuits accompanied by electrical arcs in the fan
> motor or its leads in oxygen tank no. 2.
> 
> The pressure and temperature within oxygen tank no. 2 rose
> abnormally during the l-1/2 minutes immediately prior to the
> accident.
> 
> The telemetered data indicated electrical arcs of sufficient
> energy to ignite the Teflon insulation, as verified by sub-
> sequent tests. These tests also verified that the l-ampere
> fuses on the fan motors would pass sufficient energy to ig-
> nite the insulation by the mechanism of an electric arc.
> 
> The combustion of Teflon wire insulation alone could release
> sufficient heat to account for the observed increases in
> tank pressure and local temperature, and could locally over-
> heat and fail the tank or its associated tubing. The possi-
> bility of such failure at the top of the tank was demon-
> strated by subsequent tests.
-- 
rk, Just an OldEngineer
"Dealing properly with very rare events is one of the attributes that 
distinguishes a design that is fit for safety-critical systems from one that 
is not."  -- John Rushby in "A Comparison of Bus Architectures for Safety-
Critical Embedded Systems," March 2003

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