Zero Fault Tolerant (ZFT) frangible joints could be made single-fault tolerant If they were able to contain 2 core loads and not rupture the XTA. However, the inefficient designs of conventional frangible joints do not allow for multiple core loads to be actuated without rupturing the XTA. This leaves designers with too little operating space.
All frangible joints are bounded by many design constraints that determine functional margin (operational window). First, the device must be strong enough to stay assembled while subjected to launch forces. Second, a reasonable margin above structural strength must be maintained to ensure reliability and avoid structural failure. Third, the combustible load must be adequate to completely separate the stages at the proper time with a margin to ensure reliability. Finally, the XTA must contain all of the explosive byproducts without rupturing.
Frangible joint reliability can be predicted by comparing the overlapping areas of the energy it takes to fracture a frangible joint with the energy provided by the combustible load though the XTA, minus losses. The trailing edges of these curves reflect real-world manufacturing variations. It is obvious that a narrow operating range forces trailing edges to overlap so much that 99.95% fracture reliability without XTA rupture is not possible. The area of overlap between these two curves is known as the failure-to-fracture region. Until recently, narrow functional margins have allowed for failure rates that have been expensive but acceptable for un-manned flight. For example, the GLORY and OCO satellite launches each experienced frangible joint failures which resulted in the loss of mission. Together those two failures cost approximately one billion U.S. dollars. Despite these failures and the NASA requirements for human rated space systems, commercial partners have specified ZFT frangible joint as their baseline for vehicle staging and fairing separation on manned space flights. This forces NASA to apply a waiver to rule NASA-STD-8719.29-section-4.3.1. Variances are required to allow ZFT frangible joints to fly on manned space flights. (See (RFP) No. NNK14467515R, for NASA’s Commercial Crew Transportation Capability Contract (CCtCap), and Sierra Nevada Protest: B-410485; B-410485.2; B-410485.3
AEI TM has developed a two-fault tolerant frangible joint that exceeds NASA requirements for human rated space systems that do not rely on the highly dynamic emphasis of conventional frangible joints. Instead, a quasi-static mechanical approach is applied. While XTA strength is mostly invariant, the apparent strength of the frangible joint is reduced with a more efficient mechanism. This efficiency has allowed for the functional margin to be widened, which provides the opportunity for multiple fault tolerance. A major design goal of AEI has been to produce a robust frangible joint that is insensitive to uncontrollable factors such as wind loads, temperature gradients, corrosion, or shock wave interactions. Intrinsic noise factors such as aging and product related factors such as supplier crystal grain orientation or material extrusion thickness are now easily overcome by AEI's efficient FTX2 TM frangible joint.
AEI's FTX2 TM efficient frangible joints can contain three of the minimum loads required for complete separation without producing space debris. No time skew is necessary. A single explosive chain failure does not prevent total separation. Even if two explosive chains fail to fire, the remaining explosive chain can function the frangible joint with 100% separation. FTX2 TM weights are equivalent to conventional ZFT frangible joints. FTX2 TM style frangible joints have been scaled and actuated with ligament thicknesses and alloys equivalent to MLAS, ICPS, and ascent covers at temperatures below -200F. Slower actuation times of a few milliseconds results in much lower shock to the spacecraft's fuselage, systems, and payloads.
