Members of a Special PRCB (Program Requirements Control Board) meeting on Wednesday have once again worked through a vast set of documentation, which concluded with positive results for STS-119’s Flow Control Valve (FCV) flight rationale, ahead of the ultimate launch date decision at Friday’s Flight Readiness Review (FRR).
The presentations presented to the PRCB (all available on L2) concentrated on associated risk assessments, a key hurdle to overcome to achieve the required flight rationale for a NET (No Earlier Than) March 11 launch date.
At present, confidence is high that the three replacement valves – now installed into Discovery – are free of any cracks, thanks to recent “eddy current screening” testing, which has proved capable of finding small cracks that can form on the valves’ poppets.
In turn, this mitigates the risk of a failure that could threaten an over-pressure of the tank, which would result in the ET venting – a fire hazard during early first stage flight – or under-pressure of the tank, which risks the orbiter’s SSMEs (Space Shuttle Main Engines) shutting down.
Systems Engineering and Integration Office (SE&I), one of the main bodies tasked with approving flight rationale, overviewed the main risks that are threatened by such scenarios.
“Risk due to Over-pressure/Under-pressure: ET LH2 tank over-pressure from increased flow is protected by relief valve. ET Intertank over-pressure not credible based on venting capability,” noted one of their presentations from Wednesday’s Special PRCB.
The comment of the event not being credible is based on the assessment that such an event is “Infrequent” on the risk matrix, which is very small for what is flight rationale for just one launch.
“Risk due to Flammability of GH2 leak (Classified as Infrequent Catastrophic due to Uncertainty),” added the presentation. “Risk from flammability present from engine start to T+120 sec. GH2 venting through ET vent valve risk due to FCV poppet failure (1:41000-5th, 1:4500, 1:1400-95th).”
Following positive news on ability to gauge how much of the FCV’s poppet diameter is at risk of liberating during a failure, SE&I were also able to class an under-pressure scenario as “non credible”.
Click here for NASASpaceflight.com articles on the FCV issue since STS-126.
It has been evaluated that it would take two valves to liberate pieces greater than a diameter of 125 degrees within the first 60 seconds of flight to cause venting. An under-pressure scenario would be caused by the liberation from the valve punching a hole in the 90 elbow immediately downstream of the FCVs.
“ET LH2 tank under-pressure is a non credible scenario based on size of FCV particle relative to size of hole required for under-pressure condition,” added SE&I. “Orbiter aft compartment overpressure not credible for size of FCV particle and the hole size required to create delta pressure that exceeds certification.”
Ending on a positive note, “SE&I Recommends proceeding to Launch of STS-119 based on Orbiter Project mitigations with hardware inspections and acceptable risk of the effects.”
The Orbiter Project, which has been involved with the FCV issue since it was observed during STS-126’s ascent, also gave a positive overtone on the status via their 85 page presentation to the PRCB.
“FCV poppets installed in OV-103 (Discovery) are crack-free to limit of NDE (Non Destructive Evaluation) detection capability. Cracks at NDE detection limit require multiple flights to grow to failure.
“Fractography and analysis indicate that cracks require some number of flights to grow in high-cycle fatigue plus additional time in a static loading environment. Bounding analysis predicts a max particle size of 125 degrees.
“Venting analysis shows no ET venting for 160 degree poppet liberated at T-0. ET Venting prior to 120 seconds requires two 125 degree poppet failures in the first 60 seconds. ET under press requires a leak size much larger than demonstrated in impact testing
“Monte Carlo analysis of particle impact consequences shows a low risk of damage causing leakage. Risk of orbiter aft over pressurization requires a significant line rupture and is unlikely.”
Only one item appears to be outstanding, approval for the installation of the doubler plate, which would be used to add protection to the 90 degree elbow, should a liberation from the valve occur during the ride uphill.
“The use of the doubler assemblies are assessed in four categories: 1) Capability to contain poppet fragment from entering the aft compartment. 2) Capability to prevent or minimize a GH2 leak into the aft compartment. 3) No adverse affect to the MPS GH2 manifold. 4) Maintain structural integrity,” outlined the Orbiter Project.
“A 0.25 gram poppet fragment impacting the elbow inner wall at a knife edge orientation, 90 degrees at 850 fps punctures a hole in through the elbow wall.
“The doubler assembly successfully prevented a 0.25 gram fragment knife edge orientation, 90 degrees at 1025 fps from breaching the MPS (Main Propulsion System) system. The particle pierced the elbow wall but contained within the system by the doubler assembly.”
However, despite the doubler’s ability to add protection to the elbow, more data is required on the loads assessment, which is being carried out by Boeing at Huntington Beach. This testing is believed to be taking place on Wednesday evening, with a schedule to approve and begin installation into Discovery’s MPS this week, should the results prove to be favorable.
“Orbiter Project Elbow Reinforcement Recommendation: Pending Resolution of Boeing Huntington Beach loads assessment and other concerns expressed with fracture criticality etc, all OPO members were GO to fly doubler. (Numerous departments) require vibration testing prior to flight. (Several Departments) do not require vibration testing.”
Interestingly, with the latest data showing that it would likely take a double valve failure – over the expected margins of liberation from the FCV poppet – the threat of a breach causing an under-pressure scenario in the External Tank are low enough to potentially fly without the doubler. This is likely to be the focus of attention over the next few days.
“OPO (Orbiter Project Office) Recommendation: Acceptable to fly doubler on STS-119 mission but not required for flight.Poppet liberation risk low based on eddy current inspection. Must resolve certification loads issue before installation.”
The positive news from the PRCB has come at the end of a long process, that ultimately is only likely to delay STS-119 by a total of one month, a major achievement that earned praise from shuttle manager John Shannon.
“The team needs to be proud. After finding additional cracks in the 1301 poppets, it developed the eddy current screening technique and did a great deal of work on the Monte Carlo modeling to combat this problem,” noted Mr Shannon to the teams. It is an amazing job.
“By performing this work, the team has preserved the option to launch in this period – a critical issue to NASA. I cannot tell you how proud I am of the team. It has been a great effort.”
L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.