Exploding into Space: Explosive Ordnance for Space Systems
Selma Goldstein
Controlled explosions are a necessary part of every space mission. Aerospace, through its esoteric dedicated explosive ordnance expertise, has a part in ensuring that the explosively activated devices outfitted on space systems will perform reliably.
Explosive ordnance in the space systems industry is defined as any component or assembly on a spacecraft or launch vehicle that contains or is operated by explosive materials. The ability to rapidly release directed energy to perform multiple operations nearly simultaneously is the major attribute of explosive ordnance.
The use of explosive ordnance is far more prevalent than many people might assume. For instance, the NASA space shuttle might require more than 400 explosive initiations from launchpad release through payload deployment to parachute severance at landing. A typical military launch vehicle with payload can have more than 150 explosive initiations, while a satellite might employ between 200 and 400 explosive events. Explosively actuated devices are generally lighter and smaller than equivalent mechanical or electromechanical devices, which makes them attractive for space applications. Typical tasks include launch vehicle hold-down release, engine ignition, stage separation, fairing separation, appendage deployment. Explosive devices are also found in reentry systems and systems for destroying an errant launch vehicle. They are particularly valuable when an application requires that events at different locations occur simultaneously.
Because the functions they perform are almost always mission critical, these explosive systems carry some level of redundancy. Thus, few missions have been lost because of the failure of explosive ordnance, although some missions, such as the first Pegasus missions, have been degraded because of ordnance problems. Failures most commonly occur in prelaunch testing, so faults are detected before flight. On the other hand, recent failures attributed to the application of certain pyrovalve designs in liquid propulsion systems have resulted in mission losses.
Most failures stem from a lack of understanding of the performance envelopes of the explosive components used. Too often, a component qualified for one system is not requalified when applied to another. The space industry continues to enhance its understanding of the physics of explosive ordnance applications, and new approaches continually replace older designs; nevertheless, the primary cause of failures continues to be a lack of knowledge of performance characteristics and inappropriate corrective actions based on misunderstandings about the physics underlying the operation of these devices.
Explosive transfer lines that move the explosive reaction from one location to another. |
Aerospace has developed uncommon expertise in the use of explosive ordnance for launch vehicles and spacecraft. Through its dedicated explosives ordnance laboratory, Aerospace conducts independent research geared toward understanding the design and functionality of explosive devices and developing requirements for their use (see sidebar, Aerospace and Law Enforcement). Aerospace is active in ensuring proper testing and exploring ways to apply numerical analysis techniques to resolve anomalies. Aerospace also led a recent effort to write the first new explosive ordnance standard for space programs in almost 20 years, taking advantage of lessons learned from the history of spaceflight (see sidebar, Standards and Specifications).
Explosive Devices
An explosion is the sudden release of chemical energy by exothermic decomposition. This event is an inherently nonlinear process, not fully understood. It is initiated when a metastable chemical compound is subjected to a stimulus such as heat, impact, friction, or shock, causing rapid changes in its state. These materials may be either inorganic salts, mixtures of fuels and oxidizers, or more complex organic compounds. The explosive reaction is a rapid burning that consumes the materials essentially instantaneously.
To perform work, explosive materials must be configured so that outputs are controlled. This is accomplished through a series of aligned events called an explosive train. The first element in any explosive train—the initiator—is its most sensitive and smallest. Ignition is achieved by external forces such as electrical impulse, impact, heat, or laser energy, which can be imparted by low- and high-voltage hot-wire systems, percussion systems, and laser systems. Initiator designs include electro-explosive devices, such as the widely used NASA Standard Initiator, as well as laser-initiated devices, exploding foil initiators, percussion primers, and lanyard pull initiators.
The output of these first elements is either a detonation wave or a pressure impulse. This output can be used to operate a mechanism if its energy requirement is small, or to ignite a booster to produce more energy. Alternatively, the initiator may be used to ignite linear explosive assemblies, mild detonating fuses, primacords, shielded mild detonating cords, or thin-layered explosives to transfer the energy to an item at the end of the explosive train.
Numerous devices can be used to close the explosive train. These include shaped charges for the destruct system, separation devices for staging or payload fairing jettison, engine igniters, and pyrovalves for opening and closing fuel and oxidizer lines in the engines. On satellites, explosively actuated devices may operate the satellite propulsion system, or deploy antennas and solar panels when these components are ready for use.
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Flexible linear shaped charges of various sizes and materials used to cut or puncture structures for separation or pressure release. A ductile metal sheath formed into a chevron surrounds the explosive core. |
After initiation of the explosive, the chevron expands until the concave side (bottom side in the graphic) turns inside out to form a metal plume (jet) that is extruded outward toward the target plate, penetrating and severing it. The chevron shape concentrates the explosive energy into the jet, making it a more effective cutting tool than a charge with a circular cross section, such as mild detonating cord. |
Explosively actuated devices include separation nuts, ordnance thrusters, pin pullers and pushers, cable cutters, bolt cutters, valves, separation bolts, and safe-arm devices. Separation nuts and bolts use the shock pressure generated by an explosion to fracture a bolt or release a nut to allow the separation of two different parts of the launch vehicle. In pyrovalves, an explosive force drives a ram or piston to open or close the valve as needed. Ordnance thrusters rely more on the generation of gas by energetic materials than on shock pressure; these devices act as a piston or cylinder system where the expansion of gases produced in the explosion drives the piston to clear the separating component from the vehicle core. Cutters behave similarly to an open pyrovalve, wherein an explosive stimulus drives a knife blade into a target; these devices are used to sever and deactivate energy-transfer lines and destruct-system lanyards to deactivate rocket motor destruct systems upon motor separation.
Testing
Because of their one-shot nature, explosively actuated devices cannot be completely tested before use. Devising analyses and experiments to probe the basic physics of detonation is challenging, because explosives release thousands of joules of energy in time spans measured in microseconds. A three-stage procedure for testing samples has been developed over the years to compensate for these analytical difficulties.
The first stage, qualification testing, happens during the design phase and involves subjecting a device to its harshest anticipated environment plus a margin of uncertainty surrounding these expectations. The component is expended through the test firing.
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An explosive bolt before and after function. Shock waves from explosive charges at either end of the bolt meet in the middle and fracture the shank. |
Pin puller cutaway. The mechanism retracts the pin to release an arm or panel that had been held in a compacted or stowed configuration for flight. |
The second stage, lot acceptance testing, is performed as each lot of devices is manufactured. This stage is a repeat of qualification testing, but it's performed on a smaller number of units; it's designed to ensure that the production items are replicating the function of the qualified design. Lot acceptance testing verifies the integrity of a lot sample and offers the best measure of certainty that the devices being tested are identical to those not being tested. It is also imperative that the functionality of lot samples is robust, because actual flight units will only be tested by means of a nondestructive evaluation. Inherent and unavoidable manufacturing variabilities also contribute to the necessity of testing these one-shot devices in each production lot.
The final stage, age surveillance, involves periodically extracting devices from the program inventory and testing them. This step is essentially a repeat of the major steps involved in lot acceptance testing, along with a high-temperature storage to simulate aging. The purpose is to demonstrate that the device has maintained its performance characteristics in storage. If the lot passes this test, it will remain in inventory and continue to be used. This process may be repeated several times to ensure the service life of the devices being used on each program.
Numerical Analysis
Until the 1950s, the only means of calculating the parameters of explosives or blast effects was through the use of empirical equations that were of limited applicability. These equations used simplified assumptions that allowed the calculations to be completed without extensive computer resources.
Now, complex computer codes running on modern supercomputers are commonly used to analyze loads on mechanical systems through linear approximations of Newtonian equations, but these are not useful in calculating the motion of structures subjected to explosive loads. The speed and intensity of explosive loads generates high stresses that can exceed material strength, high strains that place the materials in the plastic regime, and shock waves that propagate through the materials. The large deformations that result cannot be predicted using conventional mathematical models that assume all motion can be broken down into small increments.
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Three frames of a computer-generated simulation of the Super*Zip during its function. Frame times shown, from left to right, are at approximately 0, 40, and 85 microseconds after initiation of the explosive. |
Severed segment of a separation ring with a sample of the Super*Zip expanding tube separation device, as it would have looked before separation, between the two halves. |
In explosive systems, nothing is ever small. Strains will exceed 10 percent and strain rates will exceed 105. To include these effects in a mathematical scheme, ordnance researchers have developed a novel analytical approach. Its basic premise is that if the material is experiencing stress several orders of magnitude above its ultimate strength, then a first approximation might assume it has no strength at all. Materials that have no strength are perfect fluids, so instead of using the equations of solid mechanics to describe materials under an explosive load, the equations for incompressible fluids could be used. Thus arose the term "hydrocode" for the computational methods used to analyze explosive systems.
The first hydrocodes were developed for nuclear weapons. They were then used on conventional weapons, and eventually on blast effects of space structures. Aerospace is among the few institutions that have successfully applied hydrocodes to the analysis of explosively actuated devices used in space. For example, Aerospace used hydrocode analysis to support the successful resolution of a failure investigation on the Inertial Upper Stage (IUS-16) separation anomaly in 2001. In this incident, an unusually long delay was noted during the separation of the payload from the Inertial Upper Stage. The separation device that may have malfunctioned was the Super*Zip expanding tube separation joint. A model was created using hydrocodes, and a series of parametric studies were performed to investigate the sensitivity of the joint and variables in its manufacture and assembly.
It is irrelevant to a hydrocode whether the source of shock is an explosion or a mechanical impact; thus, hydrocodes can be applied to various problems. For example, the hypervelocity impact simulation capabilities of hydrocodes have been used to evaluate the effectiveness of thermal blankets and other barriers as potential space-debris shields. Similarly, a hydrocode was used for a hypervelocity impact analysis of the Leonid meteor showers. Hydrocodes are applicable in such cases because meteor showers generate high-amplitude, transient impulsive loads. A hydrocode was also used to assist NASA in its return-to-flight studies for the space shuttle. The analyses included modeling low-velocity impacts of foam and ice debris on the orbiter's thermal protection system. Aerospace's hypervelocity impact expertise has also been requested in the analysis of the James Webb Space Telescope's sun-shield vulnerability.
Daniel Gunter aligns a laser in preparation for a test using the VISAR system to measure explosive initiator output. |
VISAR
Another form of analysis that has proven beneficial to the study of explosive ordnance is the Velocity Interferometer System for Any Reflector (VISAR). This technique has been helpful in developing an understanding of the characteristics, responses, and interactions of explosive mixtures and materials. Simply stated, VISAR measures the velocity of a moving target by determining the Doppler shift imparted to a laser beam reflected from its surface. It was developed in the 1970s at Sandia National Laboratories and has since undergone numerous technological developments. Examples of VISAR's versatility include detonation velocity measurements of explosive mixtures, ram velocity measurements for pin pullers and pyrovalves, and velocity output characterization of nonlethal weapons.
Aerospace has been using VISAR to begin characterizing the NASA Standard Initiator, which has not been used in analysis because of a lack of a mathematical description of its output energy. Aerospace has effectively measured the motion of metal shims accelerated by a NASA Standard Initiator with VISAR. When the resulting equation was then used in the hydrocode to model a pyrovalve that uses the initiator for energy, the predicted motion of the ram in the valve agreed with the experimental data from the valve. This agreement indicates that the VISAR data can be used accurately in other NASA Standard Initiator simulations. It also represents the first-ever accurate characterization of the NASA Standard Initiator for use in computational analyses.
Conclusion
Advancements in understanding the physics of explosive ordnance occurs daily. New approaches continually replace older designs, but the primary cause of failures continues to be lack of knowledge of performance characteristics. Independent research at Aerospace helps advance scientific understanding of explosive reactions while ensuring the performance and reliability of the diverse components that rely on explosive ordnance.
Acknowledgments
The author would like to thank Shmuel Ben-Shmuel, Daniel Gunter, and Bounmy Chhouk for their assistance in preparing this article.
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