Michael T. Kezirian

Composite Overwrapped Pressure Vessels (COPV): Flight Rationale for the Space Shuttle Program

3Cornell University, Ithaca, NY, 14850, USA Each Orbiter Vehicle (Space Shuttle Program) contains up to 24 Kevlar49/Epoxy Composite Overwrapped Pressure Vessels (COPV) for storage of pressurized gasses. In the wake of the Columbia accident and the ensuing Return To Flight (RTF) activities, Orbiter engineers reexamined COPV flight certification. The original COPV design calculations were updated to include recently declassified Kevla r COPV test data from Lawrence Livermore National Laboratory (LLNL) and to incorporate changes in how the Space Shuttle was operated as opposed to originally envis ioned. 2005 estimates for the probability of a catastrophic failure over the life of the prog ram (from STS-1 through STS-107) were one-in-five. To address this unacceptable risk, th e Orbiter Project Office (OPO) initiated a comprehensive investigation to understand and mitigate this risk. First, the team considered and eventually deemed unfeasible procuring and replacing all existing flight COPVs. OPO replaced the two vessels with the highest risk with existing flight spare units. Second, OPO instituted operational improvements in ground procedures to significantly reduce risk, without adversely affecting Shuttle capability. Th ird, OPO developed a comprehensive model to quantify the likelihood of occurrence. A fully-instrumented burst test (recording a lower burst pressure than expected) on a flight-cer tified vessel provided critical understanding of the behavior of Orbiter COPVs. A more accurate model was based on a newly-compiled comprehensive database of Kevlar data from LLNL and elsewhere. Considering hardware changes, operational improvements and reliability model refinements, the mean reliability was determined to be 0.998 for the remainder of the Shuttle Program (from 2007, for STS-118 thru STS-135). Since limited hardware resources precluded full model validation through multiple te sts, additional model confidence was sought through the first-ever Accelerated Stress Ru pture Test (ASRT) of a flown flight article. A Bayesian statistical approach was devel oped to interpret potential ASRT results. Since the lifetime observed in the ASRT exceeded initial estimates by one to two orders of magnitude, the Space Shuttle Program deemed there was significant conservatism in the model and accepted continued operation with existing flight hardware. Given the variability in tank-to-tank original proof-test response, a non -destructive evaluation (NDE) technique utilizing Raman Spectroscopy was developed to directly measure COPV residual stress state. Preliminary results showed that patterns of low fib er elastic strains over the outside vessel surface, together with measured permanent volume growth during proof, could be directly correlated to increased fiber stress ratios on the inside fibers adjacent to the liner, and thus reduced reliability. Associated with this volumetr ic response, thought tied to void compaction, was the discovery, though laser profilo metry inspection of the interior of several

A Novel Method for Prediction and Warning for Uncontrolled Re-Entry Object Impact

ABSTRACT The growing Micrometeoroid and Orbital Debris (MMOD) environment in Earth orbit poses an increasing risk not only to active satellites but also to the general public. Of particular concern are the objects in low earth orbit (LEO) which have the potential for catastrophic consequence upon entry and collision with vulnerable terrestrial targets including airborne planes. There are limitations to the ability to predict uncontrolled object re-entry with sufficient precision to give timely or actionable warnings to threatened air traffic zones or surface locations. A system called R-DBAS (Re-entry Direct Broadcasting Alert System) invented by the co-author of this paper, T. Sgobba former head of the European Space Agency Independent Safety Office, has the potential to avoid catastrophic impacts by generating timely surface or air traffic impact warnings. The system includes a self-contained housing with on-board geolocation receiver, processing, memory with debris breakup models, and direct warning broadcast capability.

Bayes Analysis and Reliability Implications of Stress-Rupture Testing a Kevlar/Epoxy COPV using Temperature and Pressure Acceleration

Composite Overwrapped Pressure Vessel (COPVs) that have survived a long service time under pressure generally must be recertified before service is extended. Flight certification is dependent on the reliability analysis to quantify the risk of stress rupture failure in existing flight vessels. Full certification of this reliability model would require a statistically significant number of lifetime tests to be performed and is impractical given the cost and limited flight hardware for certification testing purposes. One approach to confirm the reliability model is to perform a stress rupture test on a flight COPV. Currently, testing of such a Kevlar49 ® /epoxy COPV is nearing completion. The present paper focuses on a Bayesian statistical approach to analyze the possible failure time results of this test and to assess the implications in choosing between possible model parameter values that in the past have had significant uncertainty. The key uncertain parameters in this case are the actual fiber stress ratio at operating pressure, and the Weibull shape parameter for lifetime; the former has been uncertain due to ambiguities in interpreting the original and a duplicate burst test. The latter has been uncertain due to major differences between COPVs in the database and the actual COPVs in service. Any information obtained that clarifies and eliminates uncertainty in these parameters will have a major effect on the predicted reliability of the service COPVs going forward. The key result is that the longer the vessel survives, the more likely the more optimistic stress ratio model is correct. At the time of writing, the resulting effect on predicted future reliability is dramatic, increasing it by about one “nine”, that is, reducing the predicted probability of failure by an order of magnitude. However, testing one vessel does not change the uncertainty on the Weibull shape parameter for lifetime since testing several vessels would be necessary.

Probabilistic risk assessment of manned balloon programs

The Red Bull Stratos and Paragon StratEx were ground breaking projects in terms of pushing the human envelope for survivability. The data collected is of great value for future development of manned space programs. They showed among other things that it is possible for a human to survive transonic and supersonic speeds without a vehicle protection. One of the ways the two projects will be used is by evaluating the reliability of the systems and defining the parameters that are of high criticality during a crew escape. The PRA on mission architectures also helps in determining the safest mission hardware and human operations, leading to a more successful crew escape outcome. An after the fact PRA was performed by Teri Hamlin on the Space Shuttle program for all flights. The intent was to study the change in risk with changes in flight hardware and procedures [1]. This could be used to establish safety recommendations and flight procedures on future human spaceflight missions. A similar study is being carried out on the recent stratospheric balloon jumps, the Red Bull Stratos and StratEx in conjunction with Paragon Space Development Corporation. The two missions are being studied in detail for failure modes inherent in design and procedures and a risk analysis will be performed. The safety of high altitude bailout systems and procedures will be validated and the risk drivers hence found will be addressed for such a bailout. Due to different mission architectures used in the two jumps, a direct comparison between systems will be made which would help in a trade studies of such systems. Furthermore, the results found will be a design driver in stratospheric balloon programs for scientific and commercial applications.

Modeling and Design of a Communication and Navigation Satellite Constellation for the Lunar South Pole

Exploration architectures for lunar surface and lunar orbit exploration have been widely considered in the past with increased interest and attention since the Constellation program of the previous decade. Communication requirements for those architectures have figured prominently in analyses of both mission performance and mission safety. In this study, a comparison is made of architectures proposed as potential alternatives to that of NASA’s Space Communication Architecture Working Group (SCAWG) with a focus on safety/redundancy requirements for future space exploration. The approach of this study focuses on satellites in ‘frozen-orbits’ as proposed by previous researchers. These orbits are selected to optimize (reduce) propellant consumption to maintain the orbit and associated coverage. The constellation of satellites provides an opportunity to minimize the number of satellites while maintaining redundancy in order to achieve fault tolerance to future exploration missions which depend on the continued operation of the satellites. To consider coverage, this investigation selects Shackleton Crater, near the Lunar South Pole, as the primary destination. Secondary targets for analysis are Orientale 1 and Peary crater. This study shows that nearly continuous single coverage is achieved for the South Pole with majority time single fault tolerance (double coverage) for a highly elliptical and inclined three satellite constellation. By comparison, a near-circular and less inclined three satellite constellation gives geographically broader but less concentrated (redundant) coverage for the site of primary interest.

USC Rocket Propulsion Laboratory: Experiences Modeling a Composite Combustion Chamber

The University of Southern California Rocket Propulsion Laboratory is developing a single stage sounding rocket capable of delivering a small electronics payload past the Karman line at 100 km altitude. In order to launch the rocket, the FAA requires extensive ight quali cation testing and analysis of the vehicle. This development and quali cation process includes a full scale static ring of the solid fuel booster to verify propulsion performance and design integrity. The single stage solid motor design incorporates an entirely carbon composite combustion chamber. This unique design is necessary to meet weight requirements on the rocket, but introduces complexities in the structural analysis. The combustion chamber structure is a very long carbon composite tube, the ends of which are sealed with aluminum pressure bulkheads retained with steel dowel pins. Though successfully demonstrated in subscale static and ight testing, this motor case design experienced catastrophic failures during consecutive static rings of ight-scale hardware. Post ight accident investigation was performed in order to identify the root cause failure of these static rings. A high delity model of the thrust chamber accounting for the continuously varying directional properties was built using the Abaqus Uni ed Finite Element Analysis (FEA). The cause of the failure of the rst live ring was determined to be the direct result of loads applied by the thrust stand retention methods. The second livering failed as a result of a buckling failure of the thrust stand. The numerical simulation exonerated the ight motor case design and provided insight into how to x the thrust stand for future testing. Additionally, it a orded the student team the rationale to move forward with the development of ight hardware and the seeking of ight opportunities despite testing anomalies.