3 AP PLICABLE REGULATIONS AND RELATED DOCUMENTS.
3.1 Applicable United States Code (U.S.C.) Statute.
Title 51 U.S.C. Subtitle V, Chapter 509.
3.2 Related FAA Commercial Space Transportation Regulations.
The following 14 CFR regulations should be accounted for when showing compliance with 14 CFR 450.137. The full text of these regulations can be downloaded from the U.S. Government Printing Office e-CFR. A paper copy can be ordered from the Government Printing Office, Superintendent of Documents, Attn: New Orders, P.O. Box 371954, Pittsburgh, PA, 15250-7954.
• Section 413.15, Review period.
• Section 450.31(a)(6), General requirements to obtain a vehicle operator license.
• Section 450.101, Public safety criteria.
• Section 450.115, Flight safety analysis methods.
• Section 450.107, Hazard control strategies.
• Section 450.123, Population exposure analysis.
• Section 450.135, Debris risk analysis.
• Section 450.137, Far-field overpressure blast effects analysis.
• Section 450.145, Highly reliable flight safety system.
• Title 49, CFR, Subpart B, Chapter 1, Subchapter C, Part 173.
3.3 Related Industry Documents.
1. Allahdadi, Firooz A., Isabelle Rongier, Tommaso Sgobba, and Paul D. Wilde, Safety Design for Space Operations, Sponsored by The International Association for the Advancement of Space Safety (IAASS), Elsevier, Watham, MA, dated 2013.
2. American National Standard ANSI S2.20-1983, Estimating Airblast Characteristics for Single Point Explosions in Air, with a Guide to Evaluation of Atmospheric Propagation and Effects, Standards Secretariat, Acoustical Society of America, New York, NY, 1983.
3. Blackwood, James M., Troy Skinner, Erin H. Richardson, and Michal E. Bangham, An Empirical Non-TNT Approach to Launch Vehicle Explosion Modeling, American Institute of Aeronautics and Astronautics, dated January 5, 2015.
4. Devoid, W. and C. Wass, Final PIRAT Yield Model for Impacting Solid Propellant, Revision 2, Report No. CSR3-00300-R2, A-P-T Research, Inc. Cocoa Beach, FL, dated September 16, 2011.
5. Elwell, R.B., Irwin, O.R., Vail, R.W., Jr. Project Sophy, Solid Propellant Hazards, Aerojet-General Corporation, dated September 30, 1966.
N ote: The industry and government documents referenced in this chapter refer to the current revisions or regulatory authorities’ accepted revisions.
3.4 Related U.S. Government Documents.
1. Department of Defense DOD Defense Explosives Safety Regulation (DESR), DoD 6055.09, Edition 1, Change 1, Department of Defense Explosives Safety Board (DDESB), dated February 23, 2024.
2. Min, I. A., and R. Walterscheid. “Overpressure calculation for BLASTC,” memorandum, dated 2001.
3. Needham, Charles E., and Joseph E. Crepeau. “The DNA Nuclear Blast Standard (1KT)” DNA 5648T, Defense Nuclear Agency, Alexandria, VA, dated 1981.
4. Swisdak, Michael M., “Simplified Kingery Airblast Calculations,” Minutes of the 26th DoD Explosives Safety Seminar, Naval Surface Warfare Center. August 1994.
3.5 Related FAA Advisory Circulars.
FAA Advisory Circulars (are available through the FAA website, http://www.faa.gov).
Some of these are not yet published.
• AC 450.115-1A, High Fidelity Flight Safety Analysis, dated June 24, 2021.
• AC 450.123-1, Population Exposure Analysis, dated October 12, 2022.
4 D EFINITION OF TERMS.
For this AC, the definitions from § 401.7 and the following apply.
4.1 Caustic A sonic velocity profile with a sonic velocity decreasing initially at altitudes above ground before increasing at greater altitudes. This results in an increased intensity of the blast wave as sensed at the ground level, potentially posing greater blast risks in some distant locations than other sonic velocity profiles. As the altitude increases, the slope of the sonic velocity profile may change multiple times.
4.2 Far-Field overpressure (Distant focusing overpressure) Air shock overpressure amplified by atmospheric conditions in the far-field. This typically applies to overpressures of less than 1 psi or 6894 Pascal (Pa).
4.3 Far-field Distances from an explosion with overpressures of typically less than 1 psi or 6894 Pascal (Pa).
4.4 Impulse Area under the curve of an airblast wave pressure time history in the positive phase.
4.5 Maximum range Here this refers to the maximum range of the FFBO hazard and is defined as the distance between the maximum credible explosive yield event location, for each phase of flight, and the point where the peak incident overpressure would not exceed 200 Pa under focusing conditions with a focus factor equal to 2.0.
4.6 Overpressure Peak value of an airblast wave pressure time history above ambient pressure.
4.7 Project PYRO A large-scale liquid propellant test program conducted in the 1960’s, which provided a basis for much of the liquid propellant blast yield models historically used in flight safety analysis for intact impacts following its completion.
4.8 Yield Trinitrotoluene (TNT) equivalent blast potential, based on either peak overpressure or impulse.
5 ACRO NYMS.
AC – Advisory Circular AST – FAA Office of Commercial Space Transportation A50 – Aerozine 50 ANSI – American National Standards Institute AST – FAA Office of Commercial Space Transportation CAS – Cylindrical Annulus Sector CFR – Code of Federal Regulations DDESB – Department of Defense Explosive Safety Board DESR – Defense Explosives Safety Regulation DFO – Distance Focusing Overpressure FAA – Federal Aviation Administration FFBO – Far-Field Blast Overpressure FSA ‒ Flight Safety Analysis FSS – Flight Safety System HD – Hazard Division IAASS – International Association for the Advancement of Space Safety IRFNA – Inhibited Red Fuming Nitric Acid LH2 – Liquid Hydrogen LOX – Liquid Oxygen MMH – Monomethyl Hydrazine PERMS – Propellant Impact Response to Mechanical Stimulus PIRAT – Propellant Impact Response Assessment Team PYRO – Greek word for fire RP-1 – Rocket Propellant-1 TNT – Trinitrotoluene UDMH – Unsymmetrical Dimethyl Hydrazine U.S. – United States U.S.C. – United States Code 6
6 B ACKGROUND.
6.1 Part of the safety analysis is concerned with risks to populations from broken window glass shards resulting from the airblast effects of large explosions that may be focused by certain conditions in the atmosphere through which the resulting blast waves propagate. Section 5.2 of the International Association for the Advancement of Space Safety (IAASS) publication, Safety Design for Space Operations1 provides supplemental information relevant to FFBO analyses.
1 Allahdadi, Firooz A., Isabelle Rongier, Tommaso Sgobba, and Paul D. Wilde, Safety Design for Space Operations, Sponsored by The International Association for the Advancement of Space Safety (IAASS), Elsevier, Watham, MA, dated 2013 7
7 A CCEPTABLE METHODOLOGY.
7.1 Topics Covered by this AC.
This AC provides a method for applicants to demonstrate that there are negligible risks from FFBO blast effects from the potential explosions caused by impact during a launch or reentry, and to accordingly tailor their FFBO analysis in a manner that meets §§ 450.137(a)(1) and (b). If the maximum credible yield input (developed in accordance with paragraph 7.5 of this AC) to the deterministic no damage yield analysis (shown in Figure 5 of this AC) indicates that no public population centers are vulnerable to window breakage, then the hazards from FFBO are negligible. The method described here is referred to as a screening method because it is appropriate to demonstrate when a FFBO risk analysis method is unnecessary. The FAA finds that use of the screening method outlined in this AC to demonstrate that there are negligible risks from FFBO blast effects, and to accordingly tailor a FFBO to meet §§ 450.137(a)(1) and (b), provides a level of fidelity sufficient to satisfy § 450.115(b).
As such, an applicant utilizing the means of compliance outlined in this AC need not submit further evidence of compliance with § 450.115 for the FFBO methodology used to meet § 450.137.
7.2 Explosive Capability of the Vehicle.
7.2.1 In accordance with § 450.137(b)(1), an analysis must account for the explosive capability of the vehicle and hazardous debris at impact and at altitude. The preamble to the final rule made it clear that § 450.115 applies to all flight safety analysis (FSA) methodologies, including FFBO analyses. In accordance with § 450.115(a) an operator’s flight safety analysis must account for all reasonably foreseeable events and failures of safety-critical systems during nominal and non-nominal launch or reentry.
Thus, for each phase of flight within the scope of the FSA, the screening analysis should compute a maximum credible yield that accounts for any foreseeable scenario during flight that would be expected to generate an explosion, including ground impacts and on-trajectory explosions involving liquid or solid propellants.2 In addition to foreseeable ground impact scenarios involving the vehicle and vehicle stages, the maximum credible yield should account for a potential explosion following a collision with the launch tower (if any) or any other structures. The yield from an intact impact is generally larger than the explosive yield from a breaking up in the air.
The analysis of the explosive capability of the vehicle should also account for:
• The vehicle mass, geometry, propulsion, and performance characteristics.
• The energetic material types onboard the vehicle whether solid, liquid, or gas.
• The effects of the impact velocity and impact surface hardness on the explosive response of the impacting energetic materials.
2 Failure modes that result in an uncontrolled disposal are a potential exception.
• T he potential for an intact impact given the time delay, including uncertainties, between the violation of a flight abort rule and the time when the flight safety system (FSS) is expected to activate; and
• The potential for the FSS to fail, unless the FSS meets the requirements of a highly reliable FSS in § 450.145.
7.2.2 The impact velocity should assume no drag account for aerodynamic drag or use conservate estimates of drag forces prior to impact.
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LO2 / LH2
1.6
1.4
1.2 1
0.8
0.6
0.4
0.2 0 Impact Speed (ft/s) Yield Factor 0 100 200 300 400 500 600 700 800 900 1000
09/10/2024 AC 450.137-1
7.3 L iquid Propellant Yield.
7.3.1 A screening analysis should use the Project PYRO results shown in Fig. 1 to compute an upper-bound yield factor as a function of impact velocity for three common combinations of oxidizer and fuel:
1. Cryogenic liquid oxygen and liquid hydrogen (LOX/LH2).
2. Cryogenic liquid oxygen and Rocket Propellant–1 (LOX/RP-1).
3. Hypergolic propellants, consisting of a liquid oxidizer such as nitrogen tetroxide (N2O4) or inhibited red fuming nitric acid (IRFNA) combined with a liquid fuel such as monomethyl hydrazine (MMH), unsymmetrical dimethyl hydrazine (UDMH) or Aerozine-50 (A50).
7.3.2 The yield factor is defined as the ratio of the equivalent weight of TNT to the weight of the propellant involved in the explosion, based on the air shock overpressure generated in surface level (hemispherical) explosions. The yield factors are expressed as functions of the impact velocity magnitude, i.e., the speed at which the propellant tanks impact the surface causing breakup. The terms ‘impact velocity’ and ‘impact speed’ are used interchangeably in this context.
Figure 1 – Yield Curve for Impacts of Li quid Oxygen and Liquid Hydrogen Propellant
7.3.3 Data from actual launch vehicle impacts show that for public safety purposes, impacts on soft surfaces, including sand, soft soil, and water, may be treated as hard surface impacts. Consequently, no distinction between the two types of surfaces is required.
Thus, Table 1 of this AC shows the data points that should be used to construct piecewise linear functions for the yield factor from any impact of these three liquid propellants.
T able 1 ‒ Liquid Propellant Yield Model Tabular Values LOX/LH2 Impact Yield Speed Factor fps lb. TNT/lb.
LOX/RP1 Impact Yield Speed Factor fps lb. TNT/lb.
Hypergols Impact Yield Speed Factor fps lb. TNT/lb.
0 0.16 0 0 0 0.020 85 0.16 170 0.215 100 0.085 745 1.50 800 0.215 200 0.135 800 1.50 300 0.180 400 0.220 500 0.245 600 0.260 700 0.275 800 0.280
7.3.4 Although some liquid monopropellants are used in rocket propulsion, liquid propellants for which yield factor models have been developed for use in FFBO risk analysis are most commonly bipropellants. These consist of separate fuel and oxidizer components that are combined in liquid propellant engines to propel the vehicle. For other propellant types, the explosive yield factor curve should be based on the best available explosive yield data for the corresponding type or class of solid or liquid propellant based on empirical data or computational modeling. The TNT equivalence for explosive events addressed in defense explosives safety regulation (DESR) 6055.93 are considered valid by the FAA; applicants should consult with the FAA to determine when these scenarios are deemed applicable by the FAA.
7.4 Solid Propellant Yield
7.4.1 Solid propellants relevant for flight safety analysis are typically represented in one of two categories: Hazard Division (HD) 1.1 detonable propellants, or HD 1.3 deflagration propellants4. For other types of solid propellant, a conservative option is to apply the HD 1.1 model. The yield factors for solid propellant should account for the size and shape of the propellant, type of impact surface, total impact speed, and orientation at impact if applicable.
7.4.2 For HD 1.1 impacts, yield factor FTNT values for various surface types should be based on Figure 2 of this AC. This plot indicates that the factor will either be 0 or 1.25 depending on the impact velocity. These curves make no distinction of whether the propellant is in a contained motor, its impact orientation, or uncontained hazardous debris created at vehicle breakup.
3 Department of Defense DOD Defense Explosives Safety Regulation (DESR), DoD 6055.09, Edition 1, Change 1, Department of Defense Explosives Safety Board (DDESB), dated February 23, 2024.
4 Title 49, Code of Federal Regulations, Subpart B, Chapter 1, Subchapter C, Part 173.
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,::, 60 Q) - >= 40 - 20 - 140 21C 250 370 0 I I I I I I I 0 100 200 300 400 500 700 80 Pro act Veloci 09/10/2024 AC 450.137-1 Figure 2 – Hazard Division 1.1 Propellant Yield Model
7.4.3 For HD 1.3 propellant impacts5, the screening analysis should assume that all solid rocket motors remain intact and impact in a side-on orientation. Yield factors for intact motor segments that impact on sand in a side-on orientation may be obtained from the curves in Figure 3 of this AC, where the segment sizes are measured by the lateral diameter of the motors. If the yield factor associated with the desired motors is not those presented in the figure or are not available, then the curves in Figure 3 of this AC should still be used. For other motor diameters between 41” and 146,” linearly interpolate between the nearest bounding curves. For diameters outside the range, use the closest bounding curve and do not extrapolate.
5 Title 49, Code of Federal Regulations, Subpart B, Chapter 1, Subchapter C, Part 173.
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0.15
0.10
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0.00 0 100 200 300 400 500 600 700 800 900 Impact Speed (ft/s)
- 4111 Segment
- 6011 Segment
- 9211 Segment
- 12411 Segment
- 14611 Segment 09/10/2024 AC 450.137-1 Figure 3 ‒ Yield Curves for HD 1.3 Propellant The following equation should be used to compute the yield factors as a function of impact speeds for a variety of motor sizes that use HD 1.3 propellants.
𝑇𝑇𝑇𝑇𝑇𝑇 𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 = 𝐹𝐹𝑆𝑆4 + 𝑏𝑏𝑆𝑆3 + 𝐹𝐹𝑆𝑆2 + 𝑌𝑌𝑆𝑆+e where S = Impact speed (feet per second) a-e = Equation coefficients (pounds TNT per pound propellant)
7.4.4 Tables 3, 4, and 5 in appendix A of this AC list the coefficients that should be used with this equation to compute the yield factor for side-on motor impacts on sand.
7.4.5 For surface types other than sand or soft soil impact, adjust the impact speed by:
𝑣𝑣𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑣𝑣 = 𝐹𝐹𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑖𝑖𝑖𝑖𝑠𝑠 where is 𝐹𝐹𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑖𝑖𝑖𝑖𝑠𝑠 0.55 for steel, 0.78 for concrete, 1.00 for sand, and 1.61 for water.
7.5 Sy mpathetic Yields.
7.5.1 The maximum credible yield should account for the potential for sympathetic yields when multiple motors and/or engines can impact near one another during the same event. If the motors or engines have different propellant compositions, then the yields should be computed separately, and not combined to estimate the maximum credible yield. If a vehicle that uses multiple motors or engines with the same propellant composition is expected to impact intact, then the propellant involved in the explosion should account for the total of all the propellants on the vehicle unless the applicant can demonstrate that a vehicle impact would result in explosions that are sufficiently separated by time or distance to be treated as separate explosions for the purpose of an FFBO analysis. For example, explosions that are coincident in terms of location, but are separated by at least 0.3 seconds may be treated as separate explosions. If insufficient information exists for such an evaluation, then the screening analysis should combine the propellant weights and use this as a single yield.
7.5.2 In accordance with § 450.137(c)(2), an application must include, at a minimum, a description of the methods used to compute the foreseeable explosive yield probability pairs, and the complete set of yield-probability pairs, used as input to the far-field overpressure analysis. A deterministic screening method should assume a probability of one for the maximum credible yield.
7.6 Exposed Windows and Populations Susceptibility to Injury.
7.6.1 In accordance with § 450.137(b)(3), an analysis must account for the characteristics of exposed windows and the population’s susceptibility to injury. Thus, a critical input to any valid screening method is a description of the population centers and potentially inhabited structures that could be subject to an FFBO hazard.
7.6.2 In general, an FFBO analysis should use population data that complies with the requirements of § 450.123. AC 450.123-1 provides guidance on population analyses that could be used to establish the population input data for FFBO screening analysis.
7.6.3 In accordance with § 450.123(a), an FSA must account for the distribution of people for the entire region where there is a significant probability of impact of hazardous debris.
The extent of the region should consider all types of hazardous debris (as defined in § 401.7), which accounts for hazards from explosive and toxic substances, as well as potential for consequences due to either planned operations (e.g., jettisons) or reasonably foreseeable failures.