• Changing the launch azimuth to preclude or reduce the overflight of populated areas,
• Modification of the mission trajectory shaping (such as altitude as a function of downrange location) to mitigate the potential spread of the hazard,
• Adjusting the timing of planned debris events, such as jettisons,
• Placing limits on the allowable dispersions of the vehicle from the intended trajectory for which continued flight will be allowed (even if the vehicle has the potential to achieve a usable trajectory or provide the operator with useful data), or
• Restricting launch during adverse wind conditions.
8 HAZARDOUS DEBRIS CHARACTERIZATION.
In accordance with § 450.121(a), a flight safety analysis must include a debris analysis that characterizes the hazardous debris generated from normal and malfunctioning vehicle flight as a function of vehicle flight sequence. Normal flight-related hazardous debris events are due to planned jettisons such as spent stages, fairings, nozzle covers, and similar items. Vehicle breakup during on-trajectory normal flight can also occur due to aerodynamic forces, inertial forces, structural vibrations, thermal loads, and other effects that exceed the structural design limits of the vehicle. Malfunctioning vehicle flights that do not become orbital will result in vehicle breakup or intact impact. In this chapter, guidelines to develop hazardous debris lists or ‘hazardous debris catalogues’ for these vehicle breakup events are presented. The methods described here comply with §§ 450.121(b) and 450.121(d)(1), (2), and (5). In accordance with § 450.121(b), a debris analysis must account for:
• Each reasonably foreseeable cause of vehicle breakup and intact impact;
• Vehicle structural characteristics and materials; and
• Energetic effects during break-up or at impact.
8.1 Developing Hazardous Debris Lists for Range Safety Analyses.
Development of vehicle fragmentation characteristics given command destruct action, self-induced failure, or aerodynamic or aerothermal breakup is a statistically uncertain and semi-empirical process due to the myriad of potential outcomes. Vehicle designers understandably focus the majority of their engineering design time and expertise on optimizing nominal flight performance and vehicle loading within a normal range of thrust and angle of attack variations. When flight deviations become extreme to the point of vehicle mechanical failure, the mission is lost as far as the manufacturer and operator are concerned. However, the consequences of these potential failures are a primary concern for the protection of the public and for the assessment of associated risks. Hence, a quantitative description of the physical, aerodynamic, and harmful characteristics of hazardous debris are required by § 450.121(d)(5).
8.1.1 Characterizing Attributes of Hazardous Debris.
Due to the uncertainties in break-up, manufacturers and operators are inclined to produce conservative debris lists rather than develop thousands of sets of high-fidelity predictions. Explosive breakup and fragment accelerations are complex processes involving release of compressed gas energy, possible detonation of confined propellants, degree of propellant mixing, propellant deflagration, combustion gas expansion, potential cryogenic liquid flashing, and impulse and drag effects on fragments. Accurate results from higher fidelity models pose the challenge of combining both flow dynamics and structural breakup integrated models. Higher fidelity models are inherently focused on narrow subsets of the solution space.
Empirical data from test programs or observations from real-world failure events are essential data sets needed to validate or tune higher fidelity models or provide empirically defined initial conditions that are not explicitly modeled. Deterministic model simulations, field tests, and real-world failures constitute discrete samples of highly variable and uncertain processes. Ideally, a statistical modeling approach is desired to characterize the attributes of hazardous debris. Therefore, in accordance with § 450.101(g), the method must produce results consistent with or more conservative than the results available from previous mishaps, tests, or other valid benchmarks, such as higher-fidelity methods.
8.1.2 Debris Analysis Requirements.
In accordance with § 450.121(b), a debris analysis must account for (1) each reasonably foreseeable cause of vehicle breakup and intact impact, (2) vehicle structural characteristics and materials, and (3) energetic effects during break-up or at impact. All models should be based on considerations of the loads and structural response that could be expected during flight, including the combined effects of:
• Aerodynamic loads,
• Thrust loads,
• Effects of flight termination system action, such as ordnance,
• Explosions or deflagrations of solid rocket propellant such as those that occur in a propellant burn-through,
• Secondary liquid propellant mixing and fireball expansion loads,
• Fracture mechanics, for example, from a failure of the pressurized liquid propellant tanks, overpressure of an SRM or losing a nozzle in an SRM,
• Breakup of structural elements, such as interstages, avionics wafers, payload attachments and fairings that do not act as a confinement surface for an energetic propellant, and
• Fragmentation impact among structural elements during breakup.
Note: These processes generally occur as the mechanical failure progresses through high strain rates and non-linear plastic deformation of vehicle components. Vehicle manufactures possess the greatest amount of detailed information about the construction of their own launch vehicles. The analysis should bound the 20
uncertainties involved. Debris catalogs are often a development that matures as knowledge of the vehicle matures. Initial assessments should err on the side of conservatism towards public safety, and higher fidelity models should be applied if necessary to demonstrate compliance with the quantitative risk criteria in § 450.101.
8.1.3 Comparing to Hazardous Debris Data for Vehicle Failures.
Actual failure events generally occur over broad ocean areas and most of the vehicle hazardous debris is seldom observed or recovered. When launch failures occur early in flight and the hazardous debris field is largely over land around the launch facility, (e.g., Titan 34D-9 at Vandenberg AFB), the ensuing accident investigation board generally focuses attention on investigation of the components that caused the launch failure (e.g., one SRM segment). The final accident report is often silent on the amount of total vehicle hazardous debris recovered. However, data does exist from some historic accidents and from some test campaigns. Debris catalog models must use results available from previous mishaps, tests – both for model accuracy and for assigning uncertainties around piece counts, sizes and imparted velocities in accordance with § 450.101(g). Small debris pieces generated during tests or accidents that may be hazardous to aircraft are of interest to debris list development but this class of fragment size may not have been collected or catalogued due to the difficulty in identifying and locating them or excessive time consumed attempting to collect them. Absence of such pieces in reported debris collections does not necessarily infer that these size classes were not generated, and, therefore the analysis should include uncertainties to account for ground rules applied in debris data collection campaigns.
8.1.4 Debris Characterization for Consequence Evaluation.
Range safety analysts are also charged in accordance with § 450.133(a) through § 450.133(d) with attempting to evaluate post-vehicle failure hazardous debris impact hazards and risks to people and structures on the ground, and to ships or aircraft operating in the launch area or under the launch flight path. To perform such analyses, the analyst needs to give reasonable consideration, in accordance with § 450.121(a) through § 450.121(c), to the total amount of launch vehicle hazardous debris generated and to estimate the numbers, sizes, shapes, masses, demise characteristics, drag characteristics, lift characteristics, and explosive potential for each fragment category.
8.1.5 Required Vehicle Information to Develop Breakup Hazardous Debris Lists.
Preparation of hazardous debris fragment lists resulting from failure-initiated vehicle destruction, planned jettison events, and intact impact events, should rely on several types of information. Ideally, empirical data defined from analysis of recovered hazardous debris is the most desirable. Such empirical data by itself is insufficient alone for the development of hazardous debris fragment lists because of the methods used to recover fragments after malfunctions usually only focus on the large, easily found pieces. The operator can use other information that could provide guidelines to assist in the development of hazardous debris fragment lists. Generally, this includes vehicle materials and methods of construction, and defining potential structural weaknesses.
The following are typical sources of data that may be available:
8.1.5.1 Vehicle Structural Description.
Descriptions of the vehicle and payload, including scaled diagrams that show the general arrangement and dimensions of components.
Three-dimensional Computer Aided Design (CAD) drawings are typically prepared during the design and fabrication of space vehicles. These digital 3-dimensional drawings could provide most of the geometric and mass information needed for generating a hazardous debris list.
8.1.5.2 Propulsion System Description.
Specifications of the propulsion system including case material (outer case, lining, insulation, thickness, density, strengths) should be provided, including descriptions of nozzles, steering mechanisms, propellant types and ingredients, propellant density, and propellant weights versus time in flight.
1. For solid propellants, the core radius (to outer edge of propellant), grain design, density, and internal pressure and web thickness versus time should be specified. For SRM cases, lining and insulation, the thickness, density and material strengths, as well as the expected burst pressure should be specified.
2. For a liquid fueled engine and associated tankage, the pumping and pressurization systems and associated stored energy, materials (thicknesses, densities, strengths), and pressurization, including expected operating pressures and burst pressures, should be specified.
3. For all propulsion system components, expected uncertainties around mass, dimensions, and material strengths should be specified.
8.1.5.3 Flight Termination and Other Destruct Systems.
Descriptions of destruct systems (command, automatic, separation), which includes descriptions of components and activation mechanisms, exact locations of all charges (beginning point, length, gap, ending point), descriptions of delays in activation of charges, and a discussion of whether, and under what circumstances, a destruct might ignite a nonthrusting motor.
8.1.5.4 Trajectory Data.
Trajectory data for a typical mission, which includes normal trajectories, malfunction trajectories, and event times (ignitions, steering programs, burnouts, jettisons). Trajectory data are used to obtain vehicle velocity, attitude or angle of attack, and altitude from which to calculate aerodynamic and inertial loads for use in estimating vehicle breakup.
Event times are used to indicate vehicle configuration at each breakup time. Some vehicle breakup simulation models apply both internal and external pressure conditions to calculate fragment acceleration and maximum velocities. The external pressure is a function of vehicle altitude, which can be obtained from the vehicle trajectory data.
Additional detail regarding trajectory data is available in AC 450.117-1, Normal and Malfunction Trajectory Analysis.
8.1.5.5 Flight Abort Rules.
Flight abort rules that define the allowable conditions for launch and activation of the flight safety system. These rules are detailed in AC 450.108-1, Using Flight Abort Rule as a Hazard Control Strategy.
8.1.5.6 Vehicle Material Properties and Design Limitations.
Knowledge of the vehicle’s material properties, anticipated operating envelope, design limitations, structural weak points (attachment points and points of transition between component geometries), and test results including the static and dynamic failure strengths of load-bearing components. Material properties should include, with associated uncertainties: density, yield and ultimate strengths, specific heat, thermal conductivity, and heat of fusion. Design limitations should include, with associated uncertainties: thicknesses, weld strengths, attachment types and materials, and margins of safety.
8.2 Effect of Type of Vehicle Breakup on Hazardous Debris List.
In accordance with § 450.121(b)(1), a debris analysis must account for each reasonably foreseeable cause of vehicle breakup and intact impact. The type of breakup affects the hazardous debris list and imparted velocities of those hazardous debris. There can be many failure modes for a vehicle and there can be multiple potential breakup modes for a given failure mode. The hazardous debris list depends on the breakup mode and not the failure mode. Vehicles can breakup due to many factors. In accordance with § 450.121(d)(1), an applicant must provide a description of all scenarios that can lead to hazardous debris. Some of the common breakup categories observed for vehicles are given below. It should be noted that newer vehicles have many different types of failures and breakup modes. It is the responsibility of the applicant to consider all the relevant failure and breakup modes for their vehicle whether that mode is listed here or not.
8.2.1 Catastrophic Explosion.
An increase in chamber pressure inside an SRM that exceeds the ultimate strength of the motor casing can cause catastrophic explosions of that malfunctioning motor and generally lead to breakup of the vehicle. Liquid fueled rockets may experience overpressure in the combustion chamber, turbopump failures, propellant leaks, or overheating that can result in an explosive event. The cause of the uncontrolled increase in internal pressures could be an internal anomaly or a malfunction in other parts of the vehicle. The main parameters that affect the hazardous debris list are (a) the location of the explosion, and (b) high net internal pressure (internal pressure minus atmospheric pressure at that altitude) at the time of explosion. Typically, vehicle parts near the center of explosion break up into smaller pieces and have higher imparted speeds than hazardous debris away from the point of explosion. SRM failures that result when the motor case burst pressure is exceeded can have significantly higher chamber pressures 23
than when the same motor is destroyed by flight termination system (FTS) ordnance.
Therefore, imparted hazardous debris speeds in these cases are typically higher than that for FTS induced breakup hazardous debris. Debris generation computer models should distinguish between vehicle components that comprise a containment structure for an explosive or energetic source (e.g., common bulkhead propellant tank, pressure vessel, ignited SRM) and those that do not because different assumptions apply to breakup and imparted algorithms for those cases.
8.2.2 Flight Termination System (FTS) Initiated Breakups.
When a vehicle malfunctions, the vehicle will either breakup inflight, impact the ground intact, or achieve orbital insertion. If unconstrained malfunction flight results in an unacceptable level of risk, then the vehicle may employ a flight abort system as a hazard control strategy and terminate powered flight in accordance with § 450.108(f)(2) to control risk. Thrust termination options include shutting down liquid fueled engines or inducing vehicle breakup using explosive charges. The breakup mechanism of the vehicle due to FTS destruct systems are different for different propellant types. The breakup hazardous debris characteristics also depend on the design and placement of the particular FTS system. FTS charges may be initiated automatically due to flight rules or manually by flight safety operators. It is also possible for them to be initiated during vehicle breakup due to other breakups such as aero breakup, aerothermal breakups, inertial and other structural breakups, and inadvertent stage separations. The breakup hazardous debris list and imparted speeds depend on the breakup mechanism and the time of flight. The aerodynamic forces, the amount of liquid propellant, the shape and thickness of the solid propellant casting, and the chamber pressure in currently burning SRMs all affect the imparted speeds of the hazardous debris.
8.2.3 Aerodynamic Breakup.
Launch and reentry vehicles are designed to withstand nominal acceleration loads along the axis of the vehicle and some small angles of attack. Higher angles of attack exert large bending moments, and they can break, typically near weaker parts of the vehicle, like mid-body construction joints, and inter-stages. Many vehicles have mechanisms to initiate destruct systems when a stage gets inadvertently separated. Therefore, many of these aerodynamic breakups may initiate FTS destruct charges and produce hazardous debris effectively the same as FTS type breakups. However, if there is no FSS, or FSS just cuts off thrust without inducing breakup, then the vehicle may break due to further structural loadings and the hazardous debris list would be much different from that due to an explosive FTS.
8.2.4 Structural Breakups.
Structural breakups can occur during normal trajectories as well as malfunction trajectories. Breakups on a normal trajectory are typically due to design or fabrication flaws. In this case, the vehicle breaks up due to excessive loads or vibrations experienced while traveling along the normal flight path. If the vehicle goes into a turn that results in a large angle of attack or results in a high rate of rotation during a malfunction in the lower atmosphere, it is likely to break up due to aerodynamic forces that exceed the vehicle Q-Alpha load limit. At high altitudes, with low atmospheric 24
pressures or in a vacuum, a high rotation rate can break up the vehicle if inertial forces exceed the structural limit of the vehicle. The breakup hazardous debris list from these events can be unique to the vehicle design and nature of the forces that drive the breakup sequence. Many breakup events that are initiated by a structural failure can be quickly followed by a catastrophic explosion can lead to initiation of an FTS destruct system. If that is the case, then the hazardous debris list may be substantially similar to that developed for the FTS event.
8.2.5 Aerothermal Breakup.
Aerodynamic heating can induce breakup of launch vehicles, stages, or reentry bodies, by virtue of both melting parts of the structure and weakening structural elements that are brought to high temperatures. Aerodynamic heating is proportional to velocity cubed, and therefore the heating rates are highly non-linear with respect to velocity.
When objects reenter the Earth’s atmosphere either from an initial orbital condition, or from a loss of thrust or guidance failure during upper stage phases of flight approaching orbital velocity, reentry velocities are high enough to produce significant aerodynamic heating that may result in structural breakup and potential demise of fragments resulting from the breakup process. Atmospheric density and shape of the vehicle (i.e., stagnation radius) also affect aerodynamic heating rates and are factors that should be considered in an aerothermal breakup model. An important and complicating factor that has significant effect on aerodynamic heating induced breakup is the design and integrity of thermal protective layers or systems used on a reentry body. A controlled reentry of a system with a properly designed thermal protection system will result in an intact vehicle surviving the reentry heating regime. A damaged thermal protection system can result in breakup of the vehicle even under controlled flight, as was the case for the Space Shuttle Columbia, which shed approximately 90,000 hazardous debris pieces during a several minute reentry breakup phase (see reference [6] in section 3.4 of this AC). Alternatively, there are aerothermal loads that can compromise a vehicle or a vehicle component that can result in a very different debris scenario – an example of which would be the jettisoned Space Shuttle External Tanks, which broke apart due to aerothermal loads and burst abruptly into substantially fewer pieces than the Orbiter reentry. Reentry vehicles that contain residual liquid propellants may also experience an explosive event that further breaks up the reentry body.
8.2.6 Reentry Breakup Fragments by Material Type.
Fragments released from an aerodynamic heating breakup process can be used to define hazardous debris classes and conservatively applied to ground risk calculations by ignoring any further reentry demise as the fragments fall to the ground from the release altitude. When risks from reentry aerothermal breakup are high, demise of fragments during free fall should be evaluated. Aluminum fragments are most likely to reach melt temperature and demise due to the low melting point of aluminum. Titanium, stainless steel, and carbon-carbon materials are more likely to survive reentry heating. Material properties of reentry fragments are needed to perform reentry demise calculations.
Reentry demise of fragments released from the breakup of a complex or large reentry body are often modeled as lumped mass objects with simple geometric shapes to support drag, stagnation radius, and heated area parameter allocations. Uncertainties 25
and conditions under which lumped mass assumptions break down (larger objects, complex shapes, and multiple material types in one piece) should be evaluated. A hazardous debris risk analysis using aerothermal breakup defined fragments should recognize that demise during free fall can change the ballistic coefficient of the fragments, and therefore shift the ground impact point relative to an approach that ignores demise.
8.2.7 Partial Breakups and Shedding.
It is possible for a vehicle to break up partially resulting in separated intact stages and boosters still thrusting or capable of residual thrusting. These large pieces should be propagated using proper trajectory propagation methods as described in AC 450.119-1, High-Fidelity Malfunction Trajectory Analysis, until reaching ground or secondary breakup. This progressive breakup should continue until all the hazardous debris can be considered as lumped masses that can be propagated using methods described in chapter 6 of this AC. It is also possible that a vehicle could breakup incrementally and shed hazardous debris along the way. For this case, main vehicle trajectory should be computed using the methods similar to vehicle propagation algorithms used for the full vehicle with proper consideration for change in thrust, mass, and aerodynamic characteristics of the malfunctioning shedding vehicle. Unlike hazardous debris lists for instant breakups, in addition to other required information, the shedding location of each hazardous debris also should be defined in the hazardous debris list for this case.
8.2.8 Other Vehicle Breakup Modes.
There are many types of space vehicles being designed and deployed and there will be new types of failure modes and breakup modes for these new vehicles. However, the principles used in developing hazardous debris lists for the above types of breakup modes are still valid for new vehicles.
8.3 Effect of Vehicle Structural Characteristics and Materials on Breakup Hazardous Debris List.
In accordance with § 450.121(b)(2), a debris analysis must account for vehicle structural characteristics and materials. Vehicle structural characteristics and materials can have significant effect on the breakup hazardous debris characteristics. Two main factors that affect vehicle breakup are: (a) type of loads applied to the vehicle, and (b) strength of the vehicle subassemblies. This information should be properly accounted for in the method and techniques used for generating a hazardous debris list.
Specifically, in a high fidelity debris modeling approach these factors should be modeled directly. When applying an empirical or statistical model, a structural expert should evaluate if the new vehicle design data falls within the design range from which the empirical model is derived.
8.3.1 Breakup Hazardous Debris List by Design Features.
Some examples of structural characteristics that can affect breakup include connection between different stages of the vehicle. If a vehicle starts tumbling during a malfunction turn, then the vehicle will start experiencing high bending moments that may lead to breakup at weak cross sections. Often, interstages are less robust than SRMs. Therefore, 26
the strength and design of the load bearing elements at interstages will influence how a vehicle breaks up. Similarly, the strength and design of the connections between a main body and strap on boosters will affect how easily the strap-ons become separated during a mishap. Therefore, an applicant should consider all unique design features of the vehicle when the hazardous debris list is generated.
8.3.2 Breakup Hazardous Debris List by Material Types.
The type of materials used also affects the hazardous debris list. For example, composite materials used in modern space vehicles fracture differently from older metal structure designs. They also have different strengths, densities, and melting points.
Therefore, it is important that the actual materials used to fabricate the vehicle be considered when developing the hazardous debris list.
8.4 Effect of Energetics in the Vehicle on Breakup on Hazardous Debris List.
In accordance with § 450.121(b)(3), a debris analysis must account for energetic effects during break-up or at impact. Energetic effects are additional velocities resulting from explosions or sudden release of internal compressed gas.
8.4.1 Effect of Propellant Type.
The characterization of an SRM breakup depends on the propellant types. Multiple types of propellants can be used on a single vehicle.
8.4.1.1 Solid Propellants.
Hazardous debris lists from the breakup of SRMs will be affected greatly based on whether the motor is burning and pressurized at the time of breakup or not. If it is burning at the time, then the amount of internal pressure and the thickness of the remaining propellant grain affect the hazardous debris size as well as imparted speed at breakup.
8.4.1.2 Liquid Propellants.
The mechanisms for mixing of liquid propellants is important. If the liquid propellants are stored in common bulkhead tanks with feed pipes that pass oxidizer through the interior of the fuel tank, or vice-versa, then this leads to greater initial mixing and greater explosive potential for the liquid propellants during breakup. If any of the liquid propellants are hypergolic, then estimating the explosive yield has more uncertainty. Mixing of hypergols is somewhat self-limiting because a reaction develops spontaneously on contact between the fuel and oxidizer liquid phases and that forces separation of fuel and oxidizer to limit the reaction. In addition, hypergols are toxic. Toxic fragments that contain these chemicals must be accounted for in accordance with § 450.139(c) since they can have an extended hazard area associated with release and atmospheric dispersion of the toxic chemical.
8.4.2 Intact Impact.
If a jettison body such as a spent stage or a malfunctioning space vehicle does not break up due to environmental forces or an FTS destruct system, then it will land intact on the ground. A hazardous debris list for an intact impact case has only one fragment.
However, it can pose higher risk at the impact location due to secondary breakup and explosion upon impact. Due to its size, an intact component could cause a collapse of a complete building and mass casualties. The debris analysis should provide for how the intact vehicle could break up upon impact taking into consideration the explosive effects from any propellants in the vehicle. How a vehicle breaks up upon impact causes a bigger or smaller explosion depending on the ground hardness and mixing of propellant and oxidizer, or if a solid propellant is used, how it breaks up and explodes.
The explosive effects may be defined as a TNT equivalent in the hazardous debris list.
See paragraph 12.10.2. Intact impact of large parts of a vehicle can lead to release of toxic propellants, and the hazardous debris list should provide sufficient information to perform toxic release hazard analysis in accordance with § 450.139.
8.4.3 Effect of Phase of Flight at Breakup.
The configuration of a vehicle changes with the phase of a flight. For example, consider a space vehicle having a main SRM with attached strap-on solid rocket boosters as the stage 1, an SRM stage 2, and a liquid fueled stage 3. Initially, only the main rocket and the boosters may be thrusting. Breakup at this stage should reflect that fact (i.e., hazardous debris list for burning SRMs should reflect they are burning and hazardous debris list from stages 2 and 3 should reflect they are not burning). In the next phase of flight after strap-on boosters are spent and separated, breakup will reflect only hazardous debris from stage 1, stage 2, and stage 3 and other parts of the vehicle. In current practice, flight safety analysts prepare hazardous debris models for each part of the vehicle, (i.e., stage 1, strap-on boosters, stage 2, stage 3, and payload) and combine them to form the full vehicle hazardous debris list at a given phase of flight. When hazardous debris lists from different stages are combined, those lists also should be correspond to the correct burning status of those stages at that phase of flight.
8.4.4 Effect of Altitude of Breakup.
Multiple analyses at various altitudes are typically required, ranging from the launch point, to apogee, and back to ground (if sub-orbital). Traditionally, higher altitudes cause higher explosion velocities due to the expansion of pressurized vehicle gases into the lower pressures of the ambient atmosphere. There is a greater pressure potential through which to accelerate hazardous debris until these pressures equalize.
8.4.4.1 Solid Rocket Motors.
This velocity increase is particularly the case with solid rocket motors, where, the later flight time results in both lower ambient pressure and higher internal gas volume, as the solid propellant is depleted.
8.4.4.2 Liquid Fueled Rockets.
Liquid fueled rockets experience balancing effects where higher altitudes are offset by depleted propellant, but the unique trajectories, structures, 28
and propellant loadings ultimately drive the expected effects of higher altitudes on the hazardous debris catalog.
Note: The subsequent FSA (i.e., using these hazardous debris catalogs) should be capable of capturing how equally-jettisoned hazardous debris will travel exponentially farther near exo-atmospheric conditions, due to both the relief from atmospheric drag and the curvature of the Earth (ultimately to include some hazardous debris approaching orbital trajectories).
8.4.5 Effect of Propellant Loading at Time of Breakup.
As propellant depletes during ascent, gases that may either mitigate or exacerbate the hazardous debris catalog (particularly the explosion velocity) occupy the remaining volume of the motor. A solid propellant motor burns its propellant and replaces that volume with hot gas that is at the full pressure of the combustion chamber (typically about 1,000 psi). This phenomenon exacerbates the explosion velocity, producing higher-speed hazardous debris as the propellant is depleted. For example, in a space shuttle solid rocket booster (SRB) late in burn, rupture of that motor case could expand thousands of cubic feet of gas from 1000 psi to a low ambient pressure, accelerating fragments to near Mach-1 speeds on its way.
8.4.5.1 Accounting for Volume, Pressure of Compressed Gases.
Any hazardous debris generating model that is applied should account for the volume and pressure of compressed gases inside an SRM chamber when computing fragment explosion velocities as the propellant loading factor decreases (i.e., propellant is consumed) during burn.
8.4.5.2 Accounting for Fracture Mechanics.
An SRM breakup model should consider the fracture mechanics that apply to a sudden release of chamber pressure and the affect that has on the motor case and the remaining propellant web thickness. Several modeling approaches have been developed that estimate fragment velocities based on impulse and drag effects that evolve as internal compressed gases flow outward through gaps between an SRM case and propellant fragments resulting from fragmentation of the motor. Examples are provided in references [7] and [9] in paragraph 3.4 of this AC.
8.4.5.3 Accounting for Acceleration of Fragments following Combustion.
Liquid fueled rockets generally replace their propellant (kerosene, hydrogen, methane, oxygen, or similar) with ullage (helium, nitrogen, or similar) as the propellant burns. Even though this ullage is often inert, it is still a pressurized gas—and a pressurized gas, in a rupturing tank, propels fragments. Combustion during a rupture (which first requires adequate mixing of the fuel and oxygen) arguably occurs a fraction of a second after the fragments have departed (accelerated by merely the rupturing of the pressurized tanks), but some circumstances (particularly intact impact, and maybe other mid-air failure mechanisms) may mandate accounting for 29
some further acceleration of fragments by a combustion or blast event. See references [3], [4], and [19] in paragraph 3.4 of this AC.
8.5 Steps for Developing Hazardous Debris Fragment Lists.
8.5.1 Explosive Breakups.
The following is a step-by-step procedure that should be used for developing a hazardous debris fragment list for an explosive breakup mode. Structural breakups and other malfunctions frequently lead to explosive breakups.
1. Acquire dimensioned drawings (3-dimensional CAD drawings with mass and dimensional data) of the total vehicle and major subsystems such as propellant tanks, interstages, nozzles, engine assemblies, strap-on SRMs, avionics bay, avionics components, propulsion and attitude control system components (small nozzles, pressurized tanks, propellant tanks and plumbing components), payload adapter assembly, payload, and payload fairing.
2. Acquire photographs of actual hardware assemblies and subassemblies. Photographs give a general feel for the extensiveness of items such as wiring harnesses, plumbing lines, mounting brackets, and miscellaneous small hardware.
3. Acquire flight hardware mass properties. Mass properties data should include masses of both subsystem components and the higher assemblies. Placement of subsystem components in terms of a vehicle reference system helps determine distance of individual components from the center of the explosion. Some mass properties may include masses of components that are distributed across the assembly such as paint, screws, washers, and adhesives.
4. Estimate the center of explosion realistically to match the design of the system.
5. Partition the vehicle construction into the following types of fragments or fragment sources:
a. Liquid propellant tanks
b. SRMs including case and propellant
c. Skin and panel pieces (not part of liquid or solid propellant tanks)
d. Struts and frame pieces
e. Discrete components
f. Piping and wiring harness pieces
g. Miscellaneous small irregular shaped pieces
h. Distributed material (adds to mass in items 3 and 4)
6. For liquid propellant tanks that break up with the vehicle, apply a semi-empirical engineering model. The model should be verified using either first principle high fidelity numerical methods, such as a coupled finite element code or advanced multi-phase multi-physics code, or information from test or accident history to generate a list of tank wall fragments. For most common liquid fueled rocket 30