C Advisory U.S. Department of Transportation Federal Aviation Circular Administration Subject: De Minimis Far-Field Overpressure AC No: 450.137-1 Blast Effects Analysis Date: 09/10/2024 Initiated By: AST-1 This Advisory Circular (AC) provides guidance on a simplified method for performing a Far-Field Blast Overpressure (FFBO) blast effects analysis as part of a flight safety analysis in accordance with Title 14 of the Code of Federal Regulations (14 CFR) 450.137. It is intended to assist prospective applicants in obtaining commercial space authorizations and operating in compliance with commercial space regulations for operations where FFBO hazards are de minimis (negligible). Specifically, this AC provides a method for applicants to screen the FFBO hazards associated with their flight and, where those hazards are negligible, to tailor their FFBO analysis in a manner that meets §§ 450.137(a)(1) and (b). In accordance with § 450.137(a), an applicant who elects to perform their FFBO analysis prior to the day of the operation does not need to analyze FFBO risk during the countdown as well.
The Federal Aviation Administration (FAA) considers this AC an accepted means of compliance for complying with the regulatory requirements of § 450.137(a)(1). This guidance is not legally binding in its own right and will not be relied upon by the FAA as a separate basis for affirmative enforcement action or other administrative penalty. Conformity with the guidance is voluntary only and nonconformity will not affect rights and obligations under existing statutes and regulations.
If you have suggestions for improving this AC, you may use the Advisory Circular feedback form at the end of this AC.
Daniel Murray, Executive Director, Office of Operational Safety Commercial Space Transportation
Table of Contents Paragraph Page
1 Purpose1
2 Applicability1
3 Applicable Regulations and Related Documents3
4 Definition of Terms5
5 Acronyms6
6 Background7
7 Acceptable Methodology8
7.1 Topics Covered by this Advisory Circular8
7.2 Explosive Capability of the Vehicle8
7.3 Liquid Propellant Yield10
7.4 Solid Propellant Yield11
7.5 Sympathetic Yields14
7.6 Exposed Windows and Populations Susceptibility to Injury14
7.7 Deterministic Screening Method18
8 Satisfying Submission Requirements20
8.1 Terrain and Population Data20
8.2 Explosive Yield-Probability Pairs21
8.3 Overpressure Computations21
8.4 Window Breakage22
8.5 Probability of Casualty Computations22
8.6 Probability of Casualty Threshold Locations23
8.7 Maximum Expected Casualties23
8.8 Meteorological Measurements23
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Figures Number Page
Figure 1 – Yield Curves for Impacts of Three Liquid Propellants10
Figure 2 – Hazard Division 1.1 Propellant Yield Model12
Figure 3 ‒ Yield Curves for Hazard Division 1.3 Propellant13
Figure 4 – Probability of Breakage for Average Windowpanes15
Figure 5 ‒ No-damage Limits for Surface Explosions17
Tables Number Page
Table 1 ‒ Liquid Propellant Yield Model Tabular Values11
Table 3 – TNT Yield Equations for Solid Propellant Geometries with Constant Web Thickness to Table 4 – TNT Yield Equations for Solid Propellant Geometries with Variable Web Thickness, Table 5 – TNT Yield Equations for Solid Propellant Geometries with Variable Lengths, Side-on
Table 2 ‒ Sample Description of Results from the Deterministic Screening Method19
Length Ratios, Side-on Impact in Sand24
Side-on Impact in Sand25
Impact in Sand26
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1 PURP OSE.
1.1 This advisory circular (AC) provides guidance on a simplified method for performing a Far-Field Blast Overpressure (FFBO) effects analysis as part of a flight safety analysis in accordance with 14 CFR § 450.137. T he FAA considers this AC an accepted means of compliance with the regulatory requirements of § 450.137.
1.2 In this context, “far-field” blast effect analysis refers to an assessment that accounts for peak incident overpressures below 1 pound per square inch (psi) or 6894 Pascal (Pa), which is the estimated threshold for which meteorological conditions can significantly influence the attenuation of explosive overpressures. Specifically, this AC addresses acceptable methods for assessing the potential for hazards 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 blast waves propagate. T his phenomenon is sometimes referred to as distance focusing overpressure (DFO). Under certain conditions explained below, launch or reentry vehicles may be shown to present a de minimis or negligible risk from DFO, considering vehicle and mission profile characteristics.
1.3 Level of Imperatives.
This AC presents one, but not the only, acceptable means of compliance with the associated regulatory requirements. The FAA will consider other means of compliance that an applicant may elect to present. In addition, an operator may tailor the provisions of this AC to meet its unique needs, provided the changes are accepted as a means of compliance by the FAA. Throughout this document, the word “must” characterizes statements that directly follow from regulatory text and therefore reflect regulatory mandates. The word “should” describes a requirement if electing to use this means of compliance; variation from the provisions of this AC is possible but must satisfy the regulation to constitute an alternative means of compliance. The word “may” describes variations or alternatives allowed within the accepted means of compliance set forth in this AC.
2 APPLICABILITY.
2.1 The guidance in this AC is for launch and reentry vehicle applicants and operators required to comply with 14 CFR part 450, Launch and Reentry License Requirements.
The guidance in this AC is for those seeking a launch or reentry vehicle operator license and a licensed operator seeking to renew or modify an existing vehicle operator license.
2.2 This AC provides a method for applicants to demonstrate that there are negligible risks from FFBO effects from the potential explosions caused by an intact vehicle (or stage) impact during a launch or reentry, and to accordingly tailor their FFBO analysis in a manner that meets §§ 450.137(a)(1) and (b). Explosions at altitude tend to have lesser ground amplification effects than surface explosions due to an intact impact. However, 1
t he burden of proof remains with the applicant to demonstrate to the FAA’s satisfaction that at-altitude explosions are negligible.
2.3 The material in this AC is advisory in nature and does not constitute a regulation. This guidance is not legally binding in its own right, and the FAA will not rely upon this guidance as a separate basis for affirmative enforcement action or other administrative penalty. Conformity with this guidance document (as distinct from existing statutes and regulations) is voluntary only, and nonconformity will not affect rights and obligations under existing statutes and regulations.
2.4 The material in this AC does not change or create any additional regulatory requirements, nor does it authorize changes to, or deviations from, existing regulatory requirements.
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.