This paper discusses the defects observed in the Cartridge Ejector Release Unit (CERU) used in aerial bomb deployment systems. The study provides valuable insights for preventing the recurrence of issues such as cracks in aluminum cartridge cases and blow holes that were encountered during batch production. The scientist or designer has overcome these defects by improving the process and eliminates these defects based on experimental trials. In particular, user approached to the designer to improve the present design as it affects the functionality of the system.

A systematic approach to defect investigation enables the identification of root causes behind CERU failures, thereby enhancing safety measures and minimizing the likelihood of critical breakdowns. Conducting failure analysis on CERU is particularly useful in detecting issues that may jeopardize safety or lead to malfunctions within complex systems.

This research is important as it underscores the role of design improvements in enhancing product reliability and performance. A review of existing literature reveals several investigations focused on defect analysis in ammunitions used for defense field. Sharma et al. [1] studied a failure analysis of cartridge cases, where microstructural studies at different locations were correlated with the material hardness. Their findings indicated that stress corrosion cracking (SCC) was the root cause of cartridge case failure, though the research gap on SCC remained. Song et al. [2] contributed numerical and theoretical studies for cartridge case headspace, accounting for their movements and axial deformations. Merlina Fitri et al. [3] reported the Taguchi method for quality analysis of defects in 5.56 mm ammunition. Similarly, Gamage et al. [4] examined adoption of Taguchi’s quality philosophy and its application among lean practitioners in apparel manufacturing. Mohammad and Roozbeh [5] presented key concepts for addressing quality-related issues in research information systems through a case study. Edwin [6] examined understanding of rubber seal failures in primary package using ozone chamber analysis and suggested corrective measures. Rafa? and Tomasz [7] identified malfunction causes in CERU units filled with double-base propellants. Parate [8–11] carried out defect investigations on various power cartridges, including those used in seat ejection systems and allied applications, under different operating conditions. Notably, new contributions such as the application of electroless nickel (EN) coating on aluminum cartridge cases have been implemented. CERUs are critical components in the emergency release of bombs, mostly in aerospace and military applications, where cartridges operate by generating a meticulous explosive force to actuate the ERU pylon.

CERUs are vital for the controlled release of aerial bombs, where precise, dependable actuation is mandatory. They function by converting the chemical energy of energetic materials (EM) into thermal and mechanical energy through combustion. Both geometric design and material selection strongly affect their operational performance and reliability. Although generally dependable, CERUs can fail under adverse conditions, potentially causing catastrophic results. In emergency aerial-bomb deployment applications, a modified CERU has been developed that contains an initiator cap, single-base propellant, and a pyrotechnic composition i.e. military explosive (ME-422). The redesigned case is electroless-nickel (EN) coated and configured to house the propellant and booster. Key cartridge components include a rubber ring, grid, celluloid cups that enclose the propellant, disc, Teflon insert and Aluminium cartridge case. Aluminium disc is positioned between two grids, with the propellant placed between the celluloid cups. An initiator cap is fitted at the case bottom and a single flash hole provides the combustion path. At the opposite end, the rubber ring and grid–disc assembly placed on a stepped case. When actuated by an electrical pulse, the initiator triggers the explosive train (cap → pyrotechnic booster → propellant burn). The earlier CERU design lacked the Teflon insert and EN coating; these are the primary changes in the modified configuration while other components remain unchanged.

The various components used in modified CERU used for aerial bomb deployment applications are shown in Fig. 1. The functions of important components of Al case, rubber ring and initiator cap are given below :

• Aluminium case: The main enclosure that houses the booster, propellant charge, and initiation components.
• Rubber ring: A neoprene rubber gasket that provides environmental sealing and prevents moisture ingress.
• Initiator cap: An electrically-activated device that starts the explosive train of CERU.

Figure 1: Components used in Modified CERU.

Function of CERU and its components

Function of the CERU is to deploy the externally carried out aerial bomb in an emergency from the parent aircraft so as achieve the positive separation between bomb and parent aircraft. This will ensure that there will be no damage to the the parent aircraft. The engineering sketch of CERU with internal components and image of CERU assembly are illustrated at Figures 2 and 3 respectively. Figure 3 depicts cartridge with EN plating and without EN plating.

Figure 2: Engineering sketch of CERU.

Importance of CERU

CERU is installed inside ERU pylon to operate ejector unit and used to release externally carried aerial bomb from the aircraft in war to achieve positive separation. This will avoid the potential damage to the aircraft. The photo ERU pylon is shown in Fig. 4. It has piston-cylinder arrangement wherein piston is expanded due to hot combustion products generated due burning of the propellant. This gas pushes the piston and provides the jerk to external store i.e. aerial bomb where it is attached inside the system [13].

Figure 4: ERU pylon to operate ejector unit.

The salient features of ERU system are:

• High time sensitivity: In an emergency, the actuation operation should occur with event of
• Safety: Safe during handling and transportation
• Reliability: Highly reliable as a one shot device
• Resistance: No change is in the resistance of bridge wire cap
• Extreme environments: The CERU must function reliably across wide climatic temperature ranges, varying altitudes, and harsh operational conditions.

Given the critical role of CERUs in release systems, a thorough understanding of their failure modes is essential to improve the reliability and safety of the entire aerial-bomb deployment system.

In this section, the main research method or scientific bases for the research was presented. CERU in this study is made up of Aluminium material without EN coating in the original design. CERU cases were manufactured from rod by machining process. The photo of propellant and Scanning Electron Microscope (SEM) of pyrotechnic composition at 100X magnification is shown in Figs. 5 and 6 respectively. Accepted materials as per specifications were used in the lot fillings of CERU. A propellant of well-characterized properties was selected to ensure compatibility with the aluminum case and to provide progressive burn behaviour. Based on these requirements, a single-base propellant with a multi-perforation grain geometry was adopted.

Figure 5: Propellant Photo.

Figure 6: Photo of pyrotechnic.

Figure 7: composition and its SEM.

A pyrotechnic composition was selected to serve as the booster in the explosive train. Specific chemical formulation of single-base propellant employed during Closed Vessel (CV) firings is presented at Table 1. Chemical constituents of the pyrotechnic booster are provided in Table 2

Table 1: Chemical composition of single-base propellant [14].

The CV trials performed on cartridges did not have the electroless-nickel (EN) coating nor the PTFE (Teflon) insert. Following the firings, Aluminium cases showed cracks and blow-holes as illustrated at Fig. 10. These failures was attributed due to direct impact of the flame from the pyrotechnic composition on the aluminium case.

Extensive study was undertaken to eliminate such defects. EN coating and Teflon insert was introduced for the cartridges during the proof trials. The images of CERU after firing trials without EN coating and Teflon insert with defects are shown in Figure 10. Images revealed that there are no blow holes and cracks on CERU cases after the firing trials. This is due to the flame generated by pyrotechnic composition is not directly coming in contact with the Al case. Teflon insert is bad conductor of heat and EN coating increases thickness of Al case [17] with few microns. This gives the cumulative effect for protection against these defects of CERU. This was experimentally proved by conducting the trials on 100 cartridges during investigation and proved for more than ten production lots. After conduct of trials, none of the cartridges exhibits the defects such as cracks and blow holes. Many lots after design modifications were accepted and supplied to the users till date. This process was successfully established and implemented for all the future production batches of CERU. The images of CERU after firing trials with EN coating and Teflon insert without any defects are shown in Figure 11.

Figure 10: Images showing CERU without EN coating and Teflon Insert with defects.

Figure 11: Images showing CERU with EN coating and teflon insert without any defects.

The output from DAS in the form of P-t curve at hot and cold conditions is shown in Figure 12.

Figure 12: Pressure-time (P-t) curve.

Table 4: Performance parameters of CERU in CV trials.

A comprehensive study was carried out to identify the defects for CERU. Factors such as high temperature generated by pyrotechnic composition that causes blow holes and cracks on Aluminum cartridge casings. Design improvements, such as introduction of Teflon insert and EN coating were recommended to mitigate these defects. Defects in CERU has significantly an impact on the mission reliability and operational effectiveness. Through systematic failure analysis and corrective actions, the defects of CERUs were entirely eliminated and improved. Implementing such design modifications, and stringent quality control measures ensured consistent and effective bomb release performance. The effective operation of bomb release mechanisms is paramount in ERU applications, as malfunctions can compromise mission success and safety. The CERU plays a pivotal role in ensuring effective bomb deployment. This study aims to identify and analyze defects in CERUs, proposing preventive and corrective actions to enhance performance and durability. From this research study, the key findings of failure analysis highlighted that with EN coating and use of Teflon insert provides defect free ERU manufactured.

Recommendations of Research Study

The committee proposed several recommendations to prevent the recurrence of such types of defects (cracks and blow-holes). These measures were subsequently incorporated into future production batches

• Introduction of Teflon insert between Aluminium case and propellant
• Application of EN coating on Aluminum cartridge case

This study emphasize application of EN coating on Al cartridge case with introducing Teflon insert that increases the durability to withstand the pressure generated inside the Auminium cartridge case. This research work differentiates with other studies available in open access.

The significance of the research results and the next development direction is that implementation of EN coating with Teflon insert for all production lots. Cartridges with these recommendations were produced and meeting requirements without any problems.

Acknowledgement

Authors are grateful to Director ARDE, Pashan, Pune - 411 021 for his kind permission to publish this research study.

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