1 Introduction

Since the 1950s, the development of several separation and release mechanisms in various aeronautical and astronautical systems such as launch vehicles, spacecrafts, missiles, satellites, and fighter aircraft has become necessary. For example, stage separation, fairing release, payload or satellite deployment, escape tower jettison, booster separation, and parachute release are typical applications that employ separation and release devices in spacecrafts and launch vehicles. Separation and release devices are widely used in missiles for stage separation, payload release, booster separation, and fairing release. Aircraft crew ejection systems, external fuel tank release, and weapon release are typical examples of separation and release devices used in fighter aircraft. Separation and release devices are classified into two types, namely a one-point separation device and a linear separation device, according to the location where the separation occurs.

Traditionally, pyrotechnics have been mainly employed in numerous separation and release devices for aeronautical and astronautical applications. Until the 1950s, pyrotechnics had been limited to military ordnance, fireworks, and rock blasting. In recent years, pyrotechnics have become prevalent for diverse applications owing to their numerous merits. Pyrotechnics possess competitive advantages, including a high power-to-weight ratio, instantaneous operation with simultaneity, long-term storage capability, high reliability, low costs, limited input energy requirements, and high-energy output production [1, 2]. They are extensively applied in launch vehicles; spacecraft; missiles; fighter aircraft; and production methods such as metal forming, cladding, and riveting. Pyrotechnics are more favorable in aeronautical and astronautical systems where allowable volume and weight are limited. In particular, pyrotechnics technology is intensively employed in many separation and release devices for aircraft, spacecraft, and missile applications.

Despite the intensive utilization of pyrotechnics, their application is often accompanied by reluctance due to inherent limitations, such as the lack of pre-use functional verification. In addition, safety concerns arise because of the use of explosive materials, leading to the production of contaminants [2, 3]. Specifically, high levels of pyroshock can induce severe problems, becoming a significant hurdle for certain applications. Pyroshock, sometimes referred to as pyrotechnic shock, is characterized as the structural reaction to stress waves of high frequency and significant magnitude produced by an explosive incident. Pyroshock is commonly classified according to its magnitude and spectral characteristics into three categories: near-field, mid-field, and far-field [4]. Although pyroshock seldom impairs structures directly, it has the potential to induce failures or disruptions in electrical components [5]. Typical examples of electrical component failures attributed to pyroshock include relay chatter, substantial breakdowns of minor circuit components, and short circuits due to contaminants dislodgment.

Extensive research has been conducted to address the issue of pyroshock. Initially, studies focusing on the propagation characteristics of pyroshock within structures were carried out using actual pyrotechnic devices. These studies aimed to understand the propagation of pyroshock through various structures and identify methods for its suppression [6, 7]. However, the repeated use of pyrotechnic devices presents challenges in terms of cost and safety. Consequently, devices that simulate pyroshock using mechanical impacts have been developed [8,9,10]. Research utilizing these simulators to investigate shock propagation characteristics has also been performed [11]. In addition, research has been directed toward reducing the generation of pyroshock by separation devices themselves. This notably includes the development of nonexplosive separation and release devices, also known as nonexplosive actuators (NEAs). NEA devices are typically reusable, resettable, and cost-effective [2, 12]. However, the separation reliability of NEA devices represents a notable drawback when compared with pyrotechnic release devices. Therefore, the incorporation of a redundant separation mechanism is essential for NEA devices [13].

This paper provides a comprehensive review of the current methodologies and innovations in separation and release devices used in aeronautical and astronautical systems. These devices are categorized as per separation location and method into one-point separation and linear separation and release devices. In addition, they are classified into traditional pyrotechnics-based mechanisms and newer nonexplosive methods according to the extent of high explosives used. Utilizing these classifications, this paper details the primary separation devices and mechanisms for each type, analyzing the corresponding recent research and development trends. This analysis aims to project the evolution of separation device technology to improve the safety and reliability of missions involving aeronautical and astronautical systems.

2 One-Point Separation and Release Devices

2.1 Explosive Bolts

Among the various types of pyrotechnic separation and release devices, explosive bolts stand out as reliable and efficient options for numerous applications, including launcher operation, stage separation, rocket sled release, thrust termination, and external tank releases [1, 3]. Explosive bolts typically comprise a bolt structure incorporating a cavity filled with explosive material or a detachable explosive cartridge. Their straightforward design ensures a high level of reliability and robustness, making them less susceptible to environmental factors compared to other release mechanisms. Generally, there are two main categories of explosive bolts—high explosive and pressure based—differentiated by their operational mechanisms. High-explosive-type bolts employ detonation shock waves to fracture the bolt, whereas pressure-type bolts use explosion-induced internal cavity pressure for separation. Figure 1 illustrates various concepts of explosive bolts [3].

Fig. 1
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Cross-sectional views of various explosive bolts [3]

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Ridge-cut explosive bolts, a subtype of high-explosive type bolts, are recognized for their exceptional reliability. These bolts generate few fragments, display limited expansion, and ensure neat separation [14]. Furthermore, they effectively reduce the “banana peel” effect, which can significantly affect the dependability of the separation process. Because of these advantages, recent studies have predominantly focused on ridge-cut-type explosive bolts. In a study, a numerical analysis technique based on hydrocodes was employed to identify the separation mechanism [14]. In addition, it was used to analyze the influence of contact conditions and the shape of the bolt ridge on the separation characteristics [15]. The pyroshock generated by the ridge-cut-type explosive bolt was predicted [16], and the amount of explosives was optimized using numerical analysis techniques during the actual design process [17].

Figure 2a illustrates the shape before and after separation of ridge-cut explosive bolts. Distinct from other types of explosive bolts, the ridge-cut explosive bolts feature a body shape facilitating shock wave interaction at the cutting plane. Consequently, it demands a lower amount of explosives compared to alternative types. The mechanism of separation shown in Fig. 2b operates via a splitting action in which the ridge separates into halves when explosives are detonated on the opposite side of the metallic structure containing the ridge. This detonation sends shock waves through the metal, which then turn into expansion waves when they hit the free surface of the metal. These expansion waves meet at the central line of the ridge, where their overlap produces a significant tensile force. This force causes the ridge to fracture along its centerline, effectively dividing it into two distinct segments. Numerical analysis affords a clear explanation of this separation mechanism, demonstrating phenomena such as shock wave propagation, reflection, and concentration, as depicted in Fig. 2c. In addition, numerical analysis facilitates the estimation of pyroshock, as shown in Fig. 2d, with calculated values closely matching measured ones.

Fig. 2
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Ridge-cut explosive bolt. a Separation test, b separation mechanism, c numerical results, and d pyroshock estimation [16, 17]

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Furthermore, a simulation of the separation process for a piston-type explosive bolt was conducted using a hydrocode [18]. The numerical analysis focused on examining the impact of varying charge amounts on separation time, separation speed, and the magnitude of the separation shock. This type of explosive bolt comprises a pyrotechnic initiator (or ignitor), high explosives, a piston, and a bolt body. As depicted in Fig. 3, on a side wall of the body, there is a pre-made circumferential groove, which possesses significantly reduced strength compared to other sections. When a release separation is needed, the pyrotechnic initiator is activated to initiate the detonation of the charge. The resulting shock wave from the explosion, along with the expansion of gas products, collectively propels the piston to fracture the bolt at the groove, accomplishing the desired separation.

Fig. 3
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Piston-type explosive bolt. a Schematic, b before and after separation, and c numerical results [18]

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Before the utilization of numerical analysis techniques as described above, the design heavily relied on the experience of designers, posing challenges in development. Therefore, the development of explosive bolts required significant effort, time, and resources. However, currently, numerical analysis techniques are actively being employed to develop relevant systems. Consequently, the development of new explosive bolts is progressing rapidly. Recently, several companies have been supplying explosive bolts. Pacific Scientific Energetic Materials Company manufactures both standard-type (pressure type) and ridge-cut-type bolts, with sizes and designs customizable to meet specific requirements for structural load, space constraints, and environmental conditions [19]. Cartridge Actuated Devices, Inc. supplies explosive bolts in a variety of sizes and materials, including fragmenting and nonfragmenting types [20]. The nonfragmenting type does not produce fragmentation, and the products of combustion are confined within the designated containment system. In addition, they are designed for underwater operation, particularly in marine environments. Pyroalliance, part of the Ariane Group, offers explosive bolts with high reliability (0.999955 at 90% confidence level) and is notable for not being subject to any International Traffic in Arms Regulations (ITAR) constraints [21]. Ensign-Bickford Aerospace & Defense Company supplies pressure-type explosive bolts, as shown in Fig. 4, ranging in size from 1/2″ to 2.25″, with preload capabilities ranging from 4000 to 86,000 lbf [22]. Redundant initiators can be employed as required to further enhance reliability.

Fig. 4
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Explosive bolts manufactured by Ensign-Bickford Aerospace & Defense [22]

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2.2 Separation and Release Devices with Pyrotechnic Initiator

When high explosives are used in separation and release devices, they can generate significant energy even with a small volume and weight, making separation easy and ensuring high separation reliability. However, there are drawbacks, including the generation of fragments and significant pyroshock. With the increasing complexity of aeronautical and astronautical systems and the growing use of electronic equipment, there has been a rising demand for separation devices that do not utilize high explosives. Except for explosive bolts that use high explosives, most one-point separation and release devices rely on mechanical movement triggered by a pyrotechnic initiator (or pressure cartridge) to perform separation operations. Various pyrotechnic initiators have been developed and are in use, with the National Aeronautics and Space Administration (NASA) Standard Initiator (NSI) depicted in Fig. 5 being a prominent example. The NSI functions by applying electrical heat to a thin metal bridgewire situated at the bottom of a compact alumina cup. Encasing this bridgewire is a pyrotechnic substance composed of zirconium potassium perchlorate (ZPP), which occupies the entirety of the cup. When energy is transferred from the heated bridgewire to ZPP, it ignites, triggering a chain of chemical reactions that culminate in a minor explosion. The pressures produced by this explosion can either directly operate smaller mechanisms or trigger chemical reactions within larger explosive materials [23]. The NSI shows production variations, particularly in the particle size and shape of the KClO4 oxidizer, which are due to different manufacturing processes [24]. The NSI is currently available for sale from companies such as Ensign-Bickford Aerospace & Defense Company [25] (whose product is shown in Fig. 5b), Pacific Scientific Energetic Materials Company [26], and AETC [27]. In addition to NSI, various pyrotechnic initiators, such as the European standard initiator, are also being supplied, each offering diverse performance characteristics [28, 29].

Fig. 5
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NASA standard initiator (NSI). a Sectional view. b Product currently being manufactured by Ensign-Bickford Aerospace & Defense Company [23, 25]

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Pyrotechnic separation nuts serve as dual-purpose components, functioning as both fasteners and unlocking devices [30]. These nuts establish a secure connection between the structures or systems, ensuring stability until the reception of a separation signal. Upon receiving the separation signal, the separation nut initiates an unlocking process, facilitating the release of the structures or systems. Unlike devices relying on high-pressure parts to cut fastening structures, separation nuts achieve release via a quasistatic pressure segment [31]. This design choice results in an undamaged separation mechanism, requiring a minimal amount of explosive. The objective is to maintain high load-carrying capacity while minimizing shock responses.

The structural configuration of a separation nut typically involves a pair of nut segments held together, locking with a corresponding bolt. The functionality is induced by the explosive material within a confined chamber, allowing the separation nut to achieve its intended purpose of unlocking and releasing the connected components as shown in Fig. 6 [32]. Figure 7 shows the separation nut series offered by Ensign-Bickford Aerospace & Defense Company, available in various sizes ranging from 1/4″ to 1″ with preload capabilities ranging from 11 to 240.2 kN [33]. The series is characterized using two NSIs to ensure redundancy and high reliability. In addition, Pyroalliance supplies separation nuts in various sizes, which similarly utilize two NSIs [34]. They have been used in applications such as satellite release from a dispenser and the release of antennas or solar panels. The Korea Aerospace Research Institute has developed three types of separation nuts [35]. These devices offer flexibility in initiation—using either pneumatics or pyrotechnic initiators—and are complemented by the capability of ground-based pneumatic test equipment for pre-performance verification, significantly enhancing reliability. The product range includes three specific devices: one designed for stage separation in the Korea three-stage launch vehicle (also known as the Nuri rocket) and two intended for satellite separation.

Fig. 6
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Stages of separation nuts mechanism [32]

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Fig. 7
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Separation nuts currently being manufactured by Ensign-Bickford Aerospace & Defense Company [33]

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A pin puller is also commonly used for the locking and releasing of aeronautical and astronautical systems structures, e.g., solar arrays, antennas, stage separation, wing deployment, and jettisoning of payload fairings [36, 37]. The pin shaft of the pin puller secures an object for release, and retracting the pin shaft to a predefined position facilitates the release of the object [38]. Generally, these devices are triggered by the ignition of a pyrotechnic initiator. The resulting combustion gases propel the pin shaft to break the shear pin and move it to the predetermined position. Then, the moved pin shaft is secured by a locking mechanism to prevent further movement. Various concepts of pin pullers are illustrated in Fig. 8.

Fig. 8
figure 8

Pyrotechnically actuated pin pullers [38, 44]

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As depicted in Fig. 9, a finite-element model for a pin puller powered by a pressure cartridge is used to simulate the activation process and estimate the resulting actuation shock, also known as pyroshock, by applying the deflagration equation of state [36]. The accuracy of this numerical model is confirmed by comparing simulated results with actual experimental data regarding both pressure levels and pyroshock effects. A detailed quantitative analysis achieved with this validated finite-element model compares the intensity of four primary sources of shock: the combustion process, cutting of the shear pin by the piston, release of the payload, and the impact of the piston. Pin pullers with various shapes, sizes, load conditions, and piston strokes are commercially available and sold by companies such as Pacific Scientific Energetic Materials Company and Pyroalliance [39, 40]. Pin pushers, which extend the pin instead of retracting it, have also undergone significant development; they can be used depending on the situation.

Fig. 9
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Numerical analysis of a pin puller. a Actuation process. b Pyroshock estimation [36]

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Meanwhile, low-shock separation bolts have been developed to perform the same function as explosive bolts but without using high explosives. Instead, they utilize only pyrotechnic initiators for separation. There exist two variants of low-shock separation bolts distinguished by the configuration of their locking mechanism [41]. The initial variety, termed ball-type separation bolt, is characterized by its minimal weight and swift operation. Nonetheless, these bolts are optimized for scenarios involving lighter loads, owing to the elevated stress concentration localized at the contact points of the balls. Conversely, the alternative variant, identified as a split-type separation bolt, can handle significantly greater loads. This capability is attributed to its expanded contact surface area, derived from its circular configuration. The internal configurations of ball-type and split-type separation bolts are illustrated in Fig. 10. The ball-type separation bolt features a ball-locking mechanism with three balls and a piston. It comprises a housing, pressure-generating initiator, shear pin, piston for converting pressure into mechanical motion, bolt, and three superalloy balls [42]. The split-type separation bolt comprises an initiator, a housing, and a cap containing internal components. The body, which integrates with the initiator, prevents four ring splits from expanding outward. These ring splits, divided into four pieces, secure the head groove of the bolt, ensuring it remains fixed before operation. In addition, the ring tube stabilizes the body to prevent movement and ultimately buckles when the body retracts owing to combustion pressure [41].

Fig. 10
figure 10

Low-shock separation bolts. a Ball-type. b Split-type [41, 45]

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A mathematical model was established for the abovementioned separation bolts, enabling prediction and analysis of their behavior, shape design, parametric study, sensitivity analysis, and optimization [41,42,43]. The mathematical framework encompasses a combustion simulation, a model for buckling resistance, analyses of split behavior concerning both static and dynamic friction, an O-ring friction calculation, a contact force evaluation, and slip angle estimation. These are combined into simultaneous differential equations to describe interactions among the different components. Furthermore, research has been conducted on the separation behavior and characteristics of separation bolts using numerical analysis based on hydrocodes. With this approach, as depicted in Fig. 11, the separation mechanism can be clearly identified. Here, the ball-type separation bolt functions in the following sequence: the initiator generates pressure, pushing the piston outward, releasing the restraints on the three balls, causing them to move into the piston, and ultimately separating the bolts owing to pressure and preload. The separation bolt generates mechanical shocks via an interaction between components during separation and pyroshock from the initiator, markedly less intense than those produced by explosive bolts. Therefore, the overall magnitude of these shocks is notably smaller compared to those of explosive bolts, making them particularly advantageous in systems where pyroshock is a concern.

Fig. 11
figure 11

Numerical results of ball-type separation bolts [45]

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2.3 Nonexplosive Separation and Release Devices

Separation and release devices with a pyrotechnic initiator have been widely utilized owing to the reduced pyroshock and excellent operational performance. However, the development of nonexplosive separation and release devices has been demanded because they do not involve the handling of explosives. One-point nonexplosive separation and release devices or NEAs are classified into four categories depending on their mechanism and actuator: (1) electromechanical spool and separation nut, (2) paraffin actuator, (3) thermal knife, and (4) shape memory alloy (SMA) device [12]. An illustration of the electromechanical spool is provided in Fig. 12a. The linear motion of the plunger is constrained by spool halves that are wound with a restraining wire [2]. The application of an electric current causes the linkwire, a component of the restraining wire, to fail. This failure leads to the release of the restraining wire, allowing for the separation of the spool halves. This device is characterized by reliability and ensures rapid actuation. However, the main drawback of the electromechanical spool is mechanical shock. To address this issue, a low-shock separation nut with a spool (Fig. 12b) is proposed. Ensign-Bickford Aerospace & Defense Company has developed the NEA Hold Down & Release Mechanisms (HDRMs) based on this concept [46]. Various HDRMs are illustrated in Fig. 13. An HDRM is an electrically activated, single-use release mechanism designed to maintain a high tensile preload until a release command is issued. The preload is held by two separable spool halves, tightly bound together by a restraining wire that is secured with backup electrical fuse wires. Upon application of sufficient electrical current, the wire unwinds, allowing the spool halves to separate and thereby release the rod along with its associated preload.

Fig. 12
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Several types of NEA devices: a electromechanical spool, b low-shock separation nut with the spool, c paraffin actuator, and d thermal knife [2]

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Fig. 13
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Assorted NEA Hold Down & Release Mechanisms (HDRMs) by Ensign-Bickford Aerospace & Defense Company [46]

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One type of paraffin actuator is depicted in Fig. 12c. The actuation temperature is determined by the type of paraffin used. As the paraffin expands, pressure is generated and transmitted through a hermetic squeeze boot seal [2]. This device is reliable and reversible and generates minimal shock. Nonetheless, its prolonged operating time and high power-input requirement pose significant challenges to its utilization. Therefore, it is utilized in a limited capacity. For example, the Gamma Ray Spectrometer of the Mars Observer Mission employs a specialized resettable launch mechanism designed for operating the sunshade. This mechanism, incorporating a “pin pusher” that functions similarly to a pin puller by the extension of a paraffin actuator, can unlatch and relatch the sunshade up to 100 times during the mission [47]. Sierra Space Corporation offers a variety of paraffin actuators, including pin pullers and two-position rotary latching actuators, which offer benefits such as being resettable over 1000 times, allowing for verification of flight hardware before launch, providing up to 4000 N of force, and requiring low power [48].

Figure 12d illustrates a variation of the thermal knife. Its release mechanism relies on the thermal degradation of a Kevlar and Aramid cable under pre-tension. The ceramic knife is electrically heated and gradually melts the cable, leading to its degradation and failure of fibers. This device generates minimal functional shock and offers low weight and cost. However, it necessitates a prolonged operating duration. The ARA Mk3 Hold Down and Release System, developed by Airbus Defence and Space Netherlands B.V., employs a thermal knife technique to precisely sever an aramid hold-down cable, significantly reducing deployment shock compared to conventional pyrotechnic systems [49]. This system is designed to be immune to electromagnetic interference, eliminating the risk of unintended releases.

Although the electromechanical spool, paraffin actuator, and thermal knife technologies were predominantly developed and successfully utilized in past flights, recent developments have increasingly focused on NEA devices utilizing SMA technology. Two types of NEA devices employing SMA exist [12, 13]. First, SMA actuators are directly employed to fracture a pre-notched bolt under tension. This bolt, commonly referred to as a frangibolt, is stretched by the SMA actuator until it fails at the notch, as depicted in Fig. 14a. The drawback of the frangibolt includes the potential generation of particles that may pose a threat to surrounding equipment, significant shock, and moderate separation time [12, 13]. Frangibolts, which range from #8 size screws to ¾” bolts in terms of fastener sizes and offer nominal tensile strengths from 1400 to 15,000 lbf, are currently being sold by Ensign-Bickford Aerospace & Defense Company [50].

Fig. 14
figure 14

Several types of NEA devices using SMA: a Frangibolt [59], b NEA device based on the separation of segmented nuts with a trigger of SMA wire [12], and c SMA wire-actuated resettable locking device [52]

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Second, SMA actuators are utilized as trigger mechanisms for release devices, leveraging the large stroke of the SMA wire. In recent years, several studies [12, 13, 51,52,53,54,55,56] have aimed to develop NEA devices utilizing SMA-based trigger mechanisms. Significant research includes the development of a nonexplosive release actuator based on the separation of segmented nuts activated by an SMA wire [12], as shown in Fig. 14b. This device is easily resettable and has undergone rigorous performance testing, including static load tests, separation tests, and environmental tests (thermal vacuum cycling, random vibration). It can withstand 15 kN of tension and release within 50 ms. A resettable latch system [53] has been designed using spooled SMA wire. The rotation of the rod is driven by the stroke of the spooled SMA wire, capable of supporting large structural loads with zero power consumption and releasing within 20 ms. Extensive separation experiments and theoretical and mechanical approaches were employed. Another development involved an SMA wire-actuated resettable locking device [52] utilizing two SMA wires for locking and unlocking during missions, as shown in Fig. 14c. The locking and unlocking processes require 0.9 and 5.6 s, respectively. Theoretical and mechanical design methods were proposed for this device. Recently, Ensign-Bickford Aerospace & Defense Company has commercialized the EBAD TiNi™ Pin Puller, based on SMA actuator technology, available in various sizes as shown in Fig. 15. These devices have been developed to provide a retracting force on the output pin ranging from 5 to 100 lb and operate in a matter of milliseconds. In addition, they are characterized by their ability to be reset in test environments [57]. A similar technology is being used to develop the TiNi™ Pinpusher [58].

Fig. 15
figure 15

EBAD TiNi™ pin puller featuring SMA actuator technology [57]

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Despite the invention of various NEA devices, it remains challenging to simultaneously satisfy all requirements, including minimal shock, short release time, and high reliability. Therefore, the selection of an NEA device must be made according to the specific requirements of the system.

3 Linear Separation and Release Devices

3.1 Pyrotechnic Linear Separation Devices Using Linear Charges

In aeronautical and astronautical systems, several types of disconnects (or cuts) are performed during a mission, such as stage separations, fairing separations, and external airframe cutting. For these separations, the connecting structures must be cut linearly rather than at a single point. This requires a linear separation device; traditionally, flexible linear shaped charges (FLSCs) and mild-detonating fuses (MDFs) have been widely used to cut through metal structures. FLSCs and MDFs are types of linear charges that are explosives arranged in a linear configuration. For example, the Saturn V, an American 1960s rocket, used FLSC for first-stage separation and MDF for second-stage separation, as shown in Fig. 16 [60].

Fig. 16
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Separation systems of the Saturn V launch vehicle: linear shaped charge and mild-detonating fuse [60]

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FLSC is a metal tube, such as aluminum or copper, filled with high explosives, such as PentaErythritol TetraNitrate (PETN), Research Department eXplosive (RDX), and Hexanitrostilbene (HNS), and then formed into a tube with an inverted chevron-shaped (or inverted V-shaped) cross section through a drawing process. Linear shaped charges (LSCs) that are thin and flexible enough to be slightly bent with a jig are called FLSCs. Most of the LSCs used in aeronautical and astronautical systems are FLSCs. Such FLSCs are currently being sold as a commercial product by multiple companies and have been developed for various explosives, metals, and amount of charge per unit length [61,62,63]. They generate a hypervelocity jet to cut the structures underneath [64]. The amount of charge per unit length can be varied, and the cutting performance of the FLSC can be adjusted. In addition, the FLSC must maintain an optimum stand-off distance for optimal cutting (or penetration) performance. The stand-off distance can be adjusted by applying temper to maximize the cutting performance. The conceptual diagram and fabrication process of an FLSC are shown in Fig. 17a.

Fig. 17
figure 17

Conceptual diagram and fabrication process: a FLSC and b PLSC [67]

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FLSCs are widely used in various systems; however, issues with poor reproducibility of cutting performance can occur because of improper manufacturing, such as asymmetry of the metal liner shape and uneven explosive charge. To compensate for this drawback, more explosives are charged than the optimal amount of explosive. In addition, owing to the nature of the manufacturing process, it is difficult to freely customize the shape of the liner and the explosives; therefore, it is difficult to achieve optimal cutting performance. To address the shortcomings of FLSCs, precision linear shaped charges (PLSCs) have been proposed [65]. In a PLSC, a liner is precisely manufactured separately, and the explosives are filled separately after the liner is manufactured so that the explosives can be filled with a uniform density. Therefore, compared to a conventional FLSC, a PLSC shows superior cutting performance with a relatively small amount of explosive charge. In addition, it is easy to change the geometry of the liner to optimize the cutting performance [66, 67]. Figure 17b shows the conceptual diagram and fabrication process of the PLSC. To identify the optimal penetration performance of FLSCs and PLSCs, the penetration performance according to the stand-off distance is verified via experiments by varying the distance from the liner to the target. On the basis of this, a numerical analysis method based on hydrocodes was established to predict the penetration performance and analyze the cutting mechanism [67]. In addition, it is possible to predict pyroshock by numerical analysis [66].

Figure 18 illustrates the jet formation process due to the detonation of a PLSC and the penetration process into the target. Initially, the high explosive inside the metal liner detonates, and owing to the inverted V-shape of the high explosive and liner, both liners converge toward the center. It can be observed that the horizontal component of the liner velocity cancels out, while the vertical component overlaps, resulting in the formation of a very fast jet in the vertical direction. An analysis of the development process of the jet reveals that if the stand-off distance is not sufficient, the jet collides with the target structure before it is fully formed, leading to reduced penetration efficiency. Conversely, if the stand-off distance exceeds the required distance, the jet elongates excessively and begins to break apart, resulting in a decrease in mass and velocity, thereby reducing the penetration efficiency. Therefore, after determining the optimal stand-off distance by analysis and testing, it should be considered when attaching the PLSC to the structure.

Fig. 18
figure 18

Numerical simulation of PLSC detonation-induced jet formation and target penetration process: a insufficient stand-off distance and b excessive stand-off distance [67]

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MDFs, sometimes referred to as mild-detonating cores (MDC), comprise a simple metal tube with a small diameter filled with explosives. The metal can be aluminum, lead, or zinc, and the explosives are typically RDX or HNS. MDF is mainly used to deliver explosions or synchronize multiple explosions; however, it can also be utilized by directly attaching it to the surface of thin plate structures for cutting or by developing MDF-based linear separation devices as shown in Fig. 19 [68, 69]. Unlike FLSCs, which focus the energy of the explosives in a specific direction to efficiently cut the structure by forming a jet, MDFs disperse explosive energy in all directions, causing the structure to be destroyed by high pressure. Consequently, a relatively large amount of explosive is required to achieve the same cutting effect. Unlike FLSCs, which require a significant stand-off distance from the structure for optimal performance, MDFs offer the advantage of compactness and ease of production, resulting in lower costs. This makes them competitive in small-to-medium-sized systems where space is limited and mass production is necessary at a low price [70]. Although an MDF or MDC is not typically introduced as a separation device on its own, it is readily available from many pyrotechnics-related companies, including Pacific Scientific Energetic Materials Company [71].

Fig. 19
figure 19

Cross-sectional views of MDF-based cutting mechanisms [68, 69]

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For MDF, a hydrocode-based numerical analysis environment was established on the basis of the results of the flat plate cutting test. Test and numerical analysis results were used to analyze the mechanism and characteristics of plate cutting using MDF; Fig. 20 depicts the experimental setup and key stages of the cutting process. First, the detonation products generated by the MDF explosion fill the U-shaped groove of the lower plate and transmit shock waves to both the upper and lower plates. The transmitted shock waves are reflected at the free boundary of the upper and lower plates. This reflection results in localized damage due to spalling on both plates, indicating the weakening of the structure. Thereafter, deformation of the weakened upper plate occurs because of the expansion of the detonation products, leading to cutting.

Fig. 20
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Aluminum plate cutting with MDF. a Experimental setup, b numerical model, and c numerical results [70]

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FLSCs and MDFs are widely used as linear separation devices because of their superior cutting performance; nevertheless, they have the disadvantage of generating fragments and detonation products. To overcome this issue, an expanding tube-based linear separation device has been developed, which utilizes MDF sealed in a metal (primarily steel) tube. This concept was initially proposed by Lockheed Corporation under the product name Super*Zip Separation Joint [72]. MDF-based expanding tubes are configured as shown in Fig. 21. When an MDF is initiated, it generates high pressure, expanding the elliptical steel tube and fracturing the structure's notch, shear pin, or frangible bolt [73, 74]. Consequently, the expanded tube can effectively separate structures as intended, without any risk of contamination. When the amount of explosive in the expanding tubes is insufficient, there may be insufficient force to expand the tube and induce fracturing. Conversely, an excessive amount of explosive can cause the expanding tubes to rupture, leading to the leakage of detonation products. Therefore, it is necessary to utilize numerical analysis techniques to determine the optimal amount of explosives for each linear separation device. Commercial expanding tube assemblies are being supplied by Pacific Scientific Energetic Materials Company, Ensign-Bickford Aerospace & Defense Company, and others [75, 76].

Fig. 21
figure 21

Configuration of expanding tubes with fracturing: a structure notch, b shear pin, and c frangible bolt. d Frangible joint assembly (FJA) by Ensign-Bickford Aerospace & Defense Company [72,73,74,75]

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Exploding bridgewire (EBW) detonators are commonly employed to initiate linear charges. EBW detonators, which utilize a fine gold wire vaporized by a high-voltage discharge to create a shock wave, are depicted in Fig. 22 [77]. These devices can initiate secondary explosives such as PETN, RDX, or HMX with exceptional precision, ensuring consistent and rapid detonations with minimal delay, typically less than 1 μs. Originally developed during World War II at Los Alamos National Laboratory, EBW technology has seen various advancements, including diverse wire materials, although gold remains preferred for its efficient vaporization energy and resistance characteristics. EBW detonators are commercially available from companies such as Pacific Scientific Energetic Materials Company, Teledyne Technologies Incorporated, and AETC [78,79,80].

Fig. 22
figure 22

Drawing of an EBW detonator [77]

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3.2 Clamp-Band Joint System with One-Point Pyrotechnic Devices

Linear separation devices such as FLSC, MDF, and expanding tubes are commonly utilized; however, owing to the substantial amount of explosives involved, there are concerns regarding fragments and pyroshock. As an alternative to using these linear charges, a method has been proposed to implement linear separation devices using one-point separation and release devices. Particularly in satellite separation scenarios, where damage caused by pyroshock to the satellite can be critical, linear charges may not be feasible.

The clamp-band joint is a prevalent satellite separation system in current usage. This mechanism uses clamp bands to firmly connect the satellite to the payload adapter, ensuring a dependable detachment from the launch vehicle upon separation. The tightening of the clamp bands is achieved using at least two pyrotechnic bolts [81]. The structure of this joint, as illustrated in Fig. 23, showcases the clamp band divided into two segments, along with explosive bolts, V-segments, and both lateral and longitudinal restraining springs [82]. Before separation, this joint tightly secures the satellite interface ring to the payload adapter. Upon separation, cutting the explosive bolts allows the clamp band and V-segments to displace from their original positions, with lateral springs managing the clamp band movement and longitudinal springs preventing any contact with the satellite by drawing the clamp band back toward the launch vehicle. Thus, separation springs aid in propelling the satellite away from the launch vehicle.

Fig. 23
figure 23

Schematic of a clamp-band joint system [82]

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Explosive bolts are being increasingly replaced by clamp-band opening devices (CBOD) owing to concerns regarding debris and pyroshock. The CBOD, featuring patented fast-acting shockless separation nut (FASSN) technology, effectively reduces shock by releasing the stored strain energy, allowing the band to expand outward with minimal shock impact [83]. As illustrated in Fig. 24, the CBOD consists of two bolts with opposing threads that connect to the same nut, functioning as a single bolt in a stowed state and facilitating radial expansion upon activation. This high-reliability release mechanism ensures effective separation with a single release point and incorporates redundancy through a dual-initiator pin-pulling device. The design of the CBOD facilitates reuse, requiring only initiator replacement for subsequent operations.

Fig. 24
figure 24

Clamp-band system with clamp-band opening device (CBOD) [83]

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This clamp-band joint system, also referred to as the Marman clamp system, has been extensively utilized by NASA for many years, leveraging substantial development experience to establish comprehensive design guidelines [84]. Subsequent advancements in numerical analysis techniques have led to further research on the clamp-band joint system. Initially, the clamp band joint was modeled in three dimensions using the ANSYS software for finite-element analysis (FEA) [85]. This step was followed by an investigation into the ability of the joint to withstand axial loads, its stiffness along the same axis, and its damping properties. The final phase involved conducting parametric analyses to understand how different parameters—like the initial tension applied, angle of the wedge, coefficient of friction, and both the number and dimensions of V-segments—affect the mechanical behavior of the joint.

Many studies have utilized multibody dynamics analysis techniques to analyze the separation behavior mechanism of clamp-band joint systems. Although these systems use explosive bolts for separation, the focus is on the overall behavior of clamp-band joint systems rather than the specific separation behavior of the explosive bolts. Therefore, assuming that the separation occurs normally because of the explosive bolts, the study analyzes the separation mechanism. Consequently, instead of hydrocodes suitable for analyzing blast loads caused by explosives, multibody dynamics software optimized for mechanical behavior analysis has been utilized.

In clamp-band systems, it is essential to comprehend the dynamic envelope of the clamp band and the shock experienced during satellite separation. The dynamic envelope delineates the outer boundary of the movement of all components during separation. As depicted in Fig. 25, a simulation was performed to demonstrate the dynamic properties of satellite separation and establish the criteria for evaluation [82]. Following this, a response surface methodology was employed to analyze parameter sensitivity in satellite separation dynamics. Subsequent sensitivity analyses aimed to determine the most influential parameters affecting satellite separation dynamics, particularly focusing on the dynamic envelope of the clamp band and the shock responses during separation. In a distinct analysis, the dynamics of separation were comparatively examined to assess the impact of flexible interface rings. This examination led to specific dynamic studies on satellite separation, factoring in the flexibility of interface rings to investigate the attitude dynamics of separating satellites, the dynamic envelope of the clamp band, and responses to separation shock [86]. In addition, a separate study was initiated to develop a modeling and simulation approach aimed at predicting the dynamic envelope of the clamp-band joint. This research delved into various aspects, such as the preload applied to the clamp band, the stiffness of lateral restraining springs, the pyroshock effects caused by explosive bolts, and the friction coefficient at the interface with the V-segments, to understand their influence on the dynamic envelope of the clamp band [81]. Meanwhile, there exists research that has concentrated on the separation shock produced by the Marman clamp system [87]. Experimental investigations have verified that the radial and axial components of separation shock result from structural vibrations caused by the release of strain energy. On the basis of these findings, the paper proposes a streamlined model to illustrate the impact of the Marman band on separation shock and introduces a predictive methodology that employs FEA.

Fig. 25
figure 25

Multibody dynamics simulation of a clamp-band joint system [82]

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Monte Carlo simulation is used to estimate the probability of failure and the reliability of the clamp-band joint [88]. To achieve this, a mathematical model is constructed to predict the specifications of the joint while considering the uncertainties associated with its parameters. The model, aimed at cost-effective accuracy, divides the joint into sectors replaced by equivalent springs, from which mathematical expressions for stiffness and load-bearing capacity are derived, factoring in diverse load combinations.

Sierra Nevada Corporation developed a specialized Marman band separation system called the Qwksep, designed specifically for small satellite missions [89]. This system is noteworthy for its low-shock release mechanism, compact design with only a 2-inch height envelope, and ability to provide bolt circle interfaces for both the launch vehicle and the payload. The system is tailored to small satellites ranging from 600 to 1100 lb, demonstrating the capabilities of the technology rather than focusing on a specific product. An engineering unit was built to showcase its functionality and load capacity, with FEA predicting its stiffness and service load capabilities effectively. Beyond Gravity has also commercialized clamp-band separation systems that utilize CBOD, drawing on decades of development experience [90]. These systems are designed to accommodate diameters ranging from 360 to 3100 mm and have been successfully implemented in numerous launch vehicles. Notably, they are recognized for having no known failures.

3.3 Nonexplosive Linear Separation Devices

The various linear separation and release devices introduced in previous sections have been in use for a long time owing to their high reliability. However, the need for nonexplosive linear separation devices has been continuously raised because of the difficulty in handling pyrotechnic devices and issues with pyroshock. Recently, emerging commercial aerospace companies, driven by the limitations and pyroshock issues associated with pyrotechnic devices, have actively pursued the development of nonexplosive linear separation devices.

The explosive bolts in the clamp-band system can be replaced with one-point nonexplosive separation and release devices. For example, as shown in Fig. 26, a reusable nonexplosive release actuator using SMA wire can be employed to release the restraint of the clamp-band system [91]. The device comprises three main components: the SMA trigger assembly, unlocking transmission assembly, and connecting/separating assembly. The connecting/separating assembly, including screws and flywheel nuts, supports the device structurally and facilitates axial locking. The unlocking transmission assembly, with swing arms and a stopper, ensures reliable locking and quick unlocking. The SMA trigger assembly, with a rotating ring and SMA wire, pre-tightens the unlocking transmission assembly during locking and triggers unlocking by providing force and displacement. Furthermore, efforts are underway to replace the pyrotechnic initiators of the pin puller used in CBOD with nonexplosive methods such as SMA.

Fig. 26
figure 26

Clamp-band system with a nonexplosive release actuator [91]

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Recently, there has been considerable development in pneumatic linear separation devices. NASA has explored the application of pneumatic separation systems for the separation mechanism between the Orion spacecraft and the Spacecraft Adapter in the Orion/Ares launch vehicle [92]. This evaluation considered pneumatic systems in comparison with mechanical springs and gas thrusters. Pneumatic actuators are noted for their significant specific force capacity (N/kg), although concerns have been raised about their potentially lower reliability due to a higher number of parts and pressurized components. As depicted in Fig. 27, the system appears to employ two pressurized gas-storage systems for pneumatic actuation, with actuators also duplicated to ensure redundancy, thereby aiming to mitigate reliability concerns.

Fig. 27
figure 27

Orion to spacecraft adaptor (SA) separation: a Orion/Ares configuration and b pneumatic actuator system [92]

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Rocket Lab’s Electron rocket is specifically designed to launch small satellites into low Earth orbit (LEO) [93]. The payload fairing of Electron uses pneumatic unlocking springs for separation, which protect the satellite during launch and ensure it is exposed to the external environment at the appropriate time. In addition, pneumatic pushers are used in the interstage (the section between rocket stages) separation system. These systems have been utilized to deploy over 50 satellites into orbit.

SpaceX’s Falcon launch vehicles are multipurpose space launch systems designed to transport a wide range of payloads, from small to medium and large satellites, into LEO, geostationary transfer orbit, and beyond. SpaceX’s Falcon launch vehicles extensively incorporate pneumatic systems for critical separation events, enhancing safety and reliability [94]. The first and second stages are connected via mechanical latches, which are released using a high-pressure helium system that also activates pneumatic pushers to ensure a clean separation. The fairing separation employs a similar pneumatic mechanism, utilizing high-pressure helium to release latches and pneumatic pushers for deployment, thereby minimizing shock and debris. For Falcon Heavy, pneumatic pusher mechanisms are also utilized to separate the side cores from the center core, ensuring precise timing and force application for optimal mission performance.

The low-shock payload fairing jettison system developed by RUAG (now known as Beyond Gravity) is used in SpaceX launch vehicles, among others [95, 96]. These systems comprise pneumatic actuators and preloaded hinges. The actuators provide the kinetic energy necessary for fairing rotation, while the hinges define the rotation axis and offer the kick-off energy for jettison. In addition, the mechanical latches are a part of the vertical separation system and are actuated using controlled forces, such as those from pneumatic actuators. This gradual release results in a low-shock environment, reducing the risk of damage to the payload and improving overall system reliability. The latches are mechanically engaged to hold the two halves of the payload fairing together during the ascent phase of the launch. This release mechanism initiates the separation and jettison of the payload fairing, exposing the satellite to space.

The clamp-band joint system has been widely used for satellite separation or deployment. However, as the number of microsatellites, nanosatellites, and CubeSats continues to increase, there has been a growing demand for nonexplosive satellite separation systems that generate minimal shock. EXOLAUNCH GmbH has developed satellite separation systems that are fully mechanically actuated and do not use pyrotechnics, catering specifically to various small satellites. CarboNIX is a separation system tailored for small satellites up to 500 kg (Fig. 28) [97]. This system features a shock-free technology that minimizes the risk to sensitive satellite components during separation. The locking mechanism uses a magnetic system that can be released. Once the S-Ring is released, the spring pusher mechanism uses three or four pusher arms spaced evenly around the exterior of the rings to create the necessary separation velocity between the launch vehicle and the satellite. In addition, CarboNIX ensures a consistent tip-off rate of less than 2°/s, regardless of the center of gravity of the satellite. This system is available in multiple sizes and can be custom-sized to meet specific needs, ensuring broad compatibility with various satellites and launch vehicles. Exolaunch’s Quadro is a four-point satellite separation system tailored for microsatellites, supporting payloads up to 300 kg on platforms such as the Falcon 9 Rideshare, as shown in Fig. 29 [98]. This system is renowned for its minimal mass and nonpyrotechnic, shock-free deployment, which are crucial for preserving the integrity of sensitive payloads. It also offers low tumbling with a tip-off rate of approximately 0.6°/s across all axes, vital for maintaining the orientation of the satellite post-deployment. In addition, the Quadro is fully mechanically actuated, featuring simple yet robust magnetic locks that provide resettability. These systems are not subject to ITAR restrictions, making them globally accessible.

Fig. 28
figure 28

CarboNIX separation system overview: a system schematic, b availability in various sizes, and c deployment on the SpaceX Transporter-2 mission in July 2021 [97]

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Fig. 29
figure 29figure 29

Quadro separation system overview: a system schematic, b deployment, c conceptual diagram of multiple satellite-secured pre-separation [98]

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The Mark II Motorized Lightband (MLB) is designed by Rocket Lab to facilitate the separation of space vehicles such as satellites from their launch vehicles or to separate different elements within the launch vehicles themselves [99]. This system is particularly noted for its range of sizes, accommodating diameters from 8.0 to 38.81 inches. The MLB utilizes a nonpyrotechnic separation mechanism that ensures a debris-free and low-shock separation, which is critical for the integrity and operational success of space missions. The separation process is facilitated by a motorized mechanism that releases the stored mechanical energy upon activation, allowing for a controlled and precise separation. This system can withstanding significant preloads and can be reset quickly by users, promoting efficiency in operations and testing. In addition, the use of two DC brush motors not only enhances the precision of the separation event but also provides redundancy. Once the motors are activated, the entire separation sequence is completed in just 0.06 s. The Advanced Lightband (ALB) is also a space vehicle separation system designed by Rocket Lab [100]. This system is engineered to operate without high-energy, hazardous materials and offers a range of diameters from 8.0 to 24.0 inches. The ALB system stands out for its ability to provide a low-shock, nonpyrotechnic separation, making it ideal for delicate payloads. Key benefits include easy integration, significant stiffness and strength increase, minimal reset time, and light weight compared to traditional systems. Furthermore, it boasts precise initiation timing critical for satellite swarms and is backward compatible with previous systems such as MLB. Notably, the operations of the ALB are straightforward, involving minimal wiring and no consumables, which simplifies the testing and operational processes. The separation mechanism in the ALB system utilizes separation springs, which create the necessary relative velocity for separation. These springs are engineered to achieve a precise force balance, ensuring minimal shock during the separation process.

The utilization of nonexplosive separation devices ensures the absence of debris and a substantial reduction in pyroshock while allowing for repeated pre-flight testing during the developing process. These nonexplosive separation systems are essential technologies for reusable launch vehicles, though details on the mechanisms are largely proprietary and not publicly disclosed.

4 Conclusion

This review systematically explores the evolution and current state of separation and release devices within the realms of aeronautical and astronautical systems. It delineates two main categories based on operation: one-point separation and release devices, and linear separation and release devices. In addition, it distinguishes between devices depending on the usage of high explosives: traditional pyrotechnic mechanisms and emerging nonexplosive methods.

The exploration of one-point separation and release devices underscores the transition from traditional explosive bolts, which utilize a cavity filled with explosive material for operation, to innovative nonexplosive options such as the SMA-based devices that offer resettable and low-shock alternatives. In addition, devices that rely solely on pyrotechnic initiators for separation, functioning without high explosives, represent a critical development in reducing pyroshock and enhancing safety. The discussion on linear separation devices elaborated on the use of FLSCs and MDFs for achieving linear cuts essential for mission-critical separations such as stage and fairing separations and introduced the concept of expanding tubes as a novel method to mitigate debris concerns associated with conventional pyrotechnics. Furthermore, the clamp-band joint system has been highlighted as a prevalent method for satellite separation, where clamp bands secure the satellite to the payload adapter, and separation is achieved by pyrotechnic bolts or dual-initiator pin-pulling devices. Recently, there has been significant development in pneumatic linear separation devices that do not use any explosives at all. Furthermore, the development of fully mechanical, nonexplosive satellite separation systems marks a significant transition toward safer and more flexible technologies, which are essential for achieving shock-free separations that protect the integrity of sensitive payloads. Table 1 summarizes the diverse separation and release devices used in aeronautical and astronautical applications by providing a concise comparison of device types, operational characteristics, typical applications, and availability.

Table 1 Comparison of separation and release devices for aeronautical and astronautical systems
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Given the extensive development and commercialization of a variety of separation and release devices, they are now readily available for acquisition and application. To ensure the optimal selection of these devices for specific applications, it is necessary to first conduct a comprehensive evaluation of the separation and release devices used in systems with similar separation requirements. Once this evaluation is completed, the most appropriate separation and release devices can be chosen and implemented. However, because pyrotechnic devices require additional facilities for handling explosives, many emerging companies with limited experience in this area often opt for nonexplosive approaches. This shift toward nonexplosive solutions not only addresses immediate operational challenges but also sets the stage for future advancements in separation technology.

The continued development and refinement of nonexplosive separation technologies are expected to revolutionize aeronautical and astronautical system design and mission planning. With ongoing innovation, the focus will likely shift toward enhancing the reliability and broadening the applicability of nonexplosive methods. This advancement will facilitate the creation of more versatile, robust, and mission-specific separation solutions, underscoring their critical role in the future of space exploration.