Guide to Verifying 

Safety-Critical Structures 

for Reusable Launch and 

Reentry Vehicles 

Version 1.0 

November 2005 

 HQ-033805 

Guide to Verifying 

Safety-Critical Structures 

for Reusable Launch and 

Reentry Vehicles 

Version 1.0 

November 2005 

Federal Aviation Administration 

Commercial Space Transportation 

800 Independence Avenue, SW, Room 331 

Washington, DC 2059 
NOTICE 

Use of trade names or the names manufacturers or professional associations in this document does not 

constitute an official endorsement of such products, manufacturers, or associations, either expressed or 

implied, by the Federal Aviation Administration. 
iii

TABLE OF CONTENTS 

1.0 INTRODUCTION ...........................................................................................................1

1.1 Purpose ..........................................................................................................................1 

1.2 Scope .............................................................................................................................1 

1.3 Acronyms.......................................................................................................................1 

1.4 Definitions .....................................................................................................................2 

2.0 GENERAL RECOMMENDATIONS: STRUCTURAL AND MECHANICAL 

VERIFICATION.......................................................................................................................4

2.1 General Structural Systems Recommendations...............................................................4 

2.2 Structural Verification Approaches for Design and Test Factors of Safety......................5 

2.2.1 Prototype Testing: Design and Test Factors of Safety..............................................5 

2.2.2 Protoflight Testing: Design and Test Factors of Safety............................................5 

2.2.3 Verification by Analysis ..........................................................................................5 

2.2.4 Probabilistic Methods ..............................................................................................6 

2.3 Reliability and System Safety.........................................................................................6 

3.0 DETAILED RECOMMENDATIONS: STRUCTURAL AND MECHANICAL 

VERIFICATION.......................................................................................................................6

3.1 Applicability ..................................................................................................................6 

3.2 Design and Test Factors of Safety ..................................................................................7 

3.2.1 Metallic Structures...................................................................................................7 

3.2.2 Fasteners and Preloaded Joints.................................................................................7 

3.2.3 Composite and Bonded Structures............................................................................8 

3.2.4 Structural Glass........................................................................................................8

3.2.5 Bonds for Structural Glass .......................................................................................9 

3.2.6 Pressure Vessels, Lines, Fittings, and Components ..................................................9 

3.3 Fatigue and Creep Life Recommendations ...................................................................10 

3.4 Facture Control ............................................................................................................10 

3.4.1 Damage Tolerance Analysis...................................................................................11 

3.4.2 Damage Tolerance Testing.....................................................................................11 

4.0 ALTERNATE STRUCTURAL AND MECHANICAL VERIFICATION 

METHODOLOGIES ..............................................................................................................11

5.0 SUMMARY ...................................................................................................................11

6.0 BIBLIOGRAPHY..........................................................................................................12
iv

TABLES 

Table 1. Recommended Minimum Design and Test Factors for Metallic Structures....................7 

Table 2. Recommended Minimum Design and Test Factors for Fasteners and Preloaded Joints.8 

Table 3. Recommended Minimum Design and Test Factors for Composite and Bonded 

Structures .............................................................................................................................8 

Table 4. Recommended Minimum Design and Test Factors for Structural Glass ........................9 

Table 5. Recommended Minimum Design and Test Factors for Structural Glass Bonds ............9 

Table 6. Recommended Minimum Design and Test Factors for Pressurized Lines, Fittings, and 

Components........................................................................................................................10 

Table 7. Recommended Minimum Design and Test Factors for Pressurized Vessels ................10 
1.0 INTRODUCTION 

1.1 Purpose 

This guide provides acceptable methods of verifying safety-critical structures for reusable launch 

vehicles (RLV’s). It defines structural safety factors and methodologies for demonstrating 

compliance with the Federal Aviation Administration (FAA), Office of Commercial Space 

Transportation (AST), regulations and for the safe operation of the applicant’s proposed vehicle. 

This guide serves as a companion document to the three FAA/AST publications listed below. It 

provides very specific guidance on verifying safety-critical structures per the guidelines referenced in the following documents. 

• Guide to Reusable Launch Vehicle Safety Validation and Verification Planning, Version 

1.0, September 2003 (see http://ast.faa.gov). 

• Advisory Circular 431.35-2A – Reusable Launch and Reentry Vehicle System Safety 

Process, July 2005 (see http://ast.faa.gov). 

• Guide to the Identification of Safety-critical Hardware Items for Reusable Launch Vehicle (RLV) Developers (see http://ast.faa.gov). 

An applicant may use this document or an acceptable equivalent process to demonstrate compliance with 14 CFR 431 – Launch and Reentry of a Reusable Launch Vehicle and 14 CFR 437 – 

Experimental Permits. This guide makes no distinction between inhabited or uninhabited vehicles; however, structures of inhabited vehicles may be subjected to additional verification and 

safety requirements that are consistent with 14 CFR 460 – Suborbital Reusable Launch Vehicle 

Operations with Space Flight Participants and Crew. Potential RLV operators are strongly encouraged to start a pre-application dialogue with FAA/AST in the development phase of a proposed RLV program, so that any potential public safety concerns can be addressed as early as 

possible. 

1.2 Scope 

This guide provides a compilation of recommended structural design and acceptable methods of 

verification for RLV developers to use in demonstrating that their proposed vehicle’s design and 

operations satisfy regulatory licensing requirements. This list of vehicle structural analysis and 

design criteria provides an acceptable basis for demonstrating that the structural systems can survive and perform to an adequate level of safety in all operating environments, including launch, 

flight, on-orbit, reentry, and recovery. 

1.3 Acronyms 

AIAA – American Institute of Aeronautics and Astronautics 

AST – Office of Commercial Space Transportation 

COPV – Composite Overwrapped Pressure Vessels 
2

FAA – Federal Aviation Administration 

MEOP – Maximum Expected Operating Pressure 

MS – Margin of Safety 

NDE – Nondestructive Evaluation 

NASA – National Aeronautics and Space Administration 

RLV – Reusable Launch Vehicle 

1.4 Definitions 

Acceptance test – A test performed on each article of the flight hardware to verify workmanship, 

material quality, and structural integrity of the design. In the protoflight structural verification 

approach, acceptance, proof, and protoflight tests are synonymous. 

Acceptance test factor – A multiplying factor applied to the limit load or maximum expected 

operating pressure (MEOP) to define the acceptance test load or pressure (i.e. Acceptance test 

load = Acceptance test factor  Limit load or MEOP). 

Allowable load – The maximum load or combination of loads which a structure or a component 

of a structural assembly can sustain during its service life under all expected conditions of 

operation or use without potential rupture, collapse, or detrimental deformation. 

Creep – Time-dependent permanent deformation under sustained load and environmental conditions. 

Detrimental yielding – Yielding that adversely affects the fit, form, function, or integrity of the 

structure. 

Factors of safety (safety factors) – Multiplying factors to be applied to limit loads or stresses for 

purposes of analytical assessment (design factors) or test verification (test factors) of design adequacy in strength or stability. 

Fatigue – Cumulative irreversible damage incurred in materials caused by cyclic application of 

stresses and environments resulting in degradation of load-carrying capability. 

Fracture (safe-life) control – Rigorous application of engineering processes and methodologies, 

such as assurance management, manufacturing, and operations technology, dealing with the 

analysis and prevention of crack propagation. 

Limit load – The highest anticipated load or combination of loads which a structure may experience during its service life under all expected conditions of operation or use. 

Margin of safety1 – The parameter used by the structural discipline to express structural capability in terms of structural requirements which include factors of safety. Margins of safety are 

 

1 Note – Load may mean stress or strain. Mathematical formulation applies to a single loading condition (mechanical, thermal, or 

pressure). 
3

expressed for both yield and ultimate criteria. Margin of safety is mathematically expressed as 

the 

Limit load Factor of safety

Allowableload



 - 1 

Maximum expected operating pressure – The maximum pressure that a pressure vessel may 

experience during its service life under all expected conditions of operation or use. It is the highest possible operating pressure taking into consideration maximum temperature, relief pressure, 

regulator pressure, and, where applicable, transient pressure excursions. 

Pressure vessel – A container designed primarily for storing pressurized gases or liquids. Also 

defined as a container storing a pressurized gas or liquid which contains stored energy of 14,240 

ft-lb [0.01 lb of trinitrotoluene (TNT) equivalent] or greater, based on adiabatic expansion of a 

perfect gas. 

Pressurized component – A line, fitting, valve, or other part designed to contain pressure that is 

not (1) made of glass, (2) a pressure vessel, (3) a propellant tank, or (4) a solid rocket motor case. 

Primary structures – The structural elements that transfer load along adjacent interfaces in the 

primary load path. 

Proof test – A test performed on hardware to verify workmanship, material quality, and structural 

integrity of the design. In the protoflight structural verification approach, proof, acceptance, and 

protoflight tests are synonymous. 

Proof test factor – A multiplying factor applied to the limit load or MEOP to define the proof test 

load or pressure (i.e. Proof test load = Proof test factor  Limit load or MEOP). 

Protoflight test – A test performed on the flight hardware to verify workmanship, material quality, and structural integrity of the design. In the protoflight structural verification approach, 

protoflight, acceptance, and proof tests are synonymous. 

Prototype test – An assessment of a scaled model or other test article to verify the structural integrity of the design. Prototype tests and qualification tests are synonymous. 

Qualification test – An assessment of a scaled model or other structural article to verify the 

structural integrity of the design. Qualification and prototype tests are synonymous. 

Qualification test factor – A multiplying factor to be applied to the limit load or MEOP to define 

the qualification test load or pressure (i.e. Qualification test load = Qualification test factor 

Limit load or MEOP). 

Safety-critical structure – A component whose performance or tolerance is essential for a system 

to operate in a manner that does not jeopardize public safety. 

Secondary structures – Ancillary or auxiliary parts, including brackets to support individual 

components and attachments between primary structural elements, typically used to increase 

system robustness, provide redundancy, or both.

Service life – All significant loading cycles or events during the period beginning with manufacture of a component and ending with completion of its specified use. Testing, transportation, 
4

liftoff, ascent, on-orbit operations, descent, landing, and post-landing events should be 

considered. 

Service life factor (life factor) – A multiplying factor to be applied to the maximum expected 

number of load cycles in the service life to determine the design adequacy in fatigue or fracture. 

Structural failure – Rupture, collapse, excessive deformation, or any other phenomenon resulting 

in the inability of a structure to sustain specified loads, pressures, and environments or to function as designed. 

Ultimate design load – Product of the ultimate factor of safety multiplied by the limit load (i.e. 

Ultimate design load = Ultimate factor of safety  Limit load). 

Ultimate strength – Maximum load or stress that a structure or material can withstand without 

incurring a structural failure. 

Verification – For the purpose of this guide, verification is the evaluation by test, demonstration, 

analysis, or inspection to determine that applicable safety-critical requirements for an RLV and 

its operations have been properly implemented. 

Yield design load – Product of the yield factor of safety multiplied by the limit load (i.e. Yield 

design load = Yield factor of safety  Limit load). 

Yield strength – Maximum load or stress that a structure or material is designed to withstand 

without incurring detrimental deformation. 

2.0 GENERAL RECOMMENDATIONS: STRUCTURAL AND MECHANICAL 

VERIFICATION 

2.1 General Structural Systems Recommendations 

The structural system provides a basic framework for distributing external and internal loads resulting from all design flight and ground loads as well as from associated operational environments. These systems exist primarily to provide adequate strength and stiffness, such that the 

vehicle remains intact when exposed to all operating environments, including launch, flight, onorbit, reentry, and recovery. 

Structural verification of an RLV requires methodologies that accurately predict structural 

responses to applied loads and temperatures. Primary and secondary structures, including pressure vessels and mechanical systems, must be designed with adequate factors of safety and margins. Acceptable methods of determining whether a structure has met FAA safety requirements 

include verification by test, analysis, similarity, demonstration, and inspection. Acceptability of 

one method over another is predicated on the design, maturity, and proposed operations of the 

RLV system under consideration. 

Applicants should conduct tests and analyses to demonstrate that the proposed vehicle structure 

qualifies for the expected mission environments. These tests and analyses must also demonstrate 

that the design of the hardware complies with the specified verification requirements, such as 

factors of safety, interface compatibility, structural integrity, workmanship, and associated 

elements of system safety. In addition, the appropriate mass properties and mechanical functionality must also be verified. 
5

FAA/AST will evaluate the acceptability of a proposed set of verification methods during the 

pre-application consultation process and during the evaluation of the license or permit application. Components of the FAA/AST licensing or permit process include a pre-application 

consultation period and an application evaluation period made up of policy review, payload 

review, safety evaluation, financial responsibility determination, and environmental review. A 

detailed description of this process is available on-line at http://ast.faa.gov/lrra/. 

2.2 Structural Verification Approaches for Design and Test Factors of Safety 

The appropriate design and test factors for a given mechanical or structural flight hardware element depend on such parameters as the materials used, attachment methods (e.g., bonding), and 

verification approach. In applying the recommended minimum factors of safety specified in this 

guide, applicants must recognize that some safety-critical systems for inhabited vehicles may 

need to meet more stringent structural verification requirements that are consistent with 14 CFR 

460 – Suborbital Reusable Launch Vehicle Operations with Space Flight Participants and Crew. 

2.2.1 Prototype Testing: Design and Test Factors of Safety 

The prototype approach is the standard accepted practice for verification of launch vehicles. A 

separate, dedicated test structure (a scaled model or test article), identical to the flight structure, 

is tested to demonstrate that the design meets the factor of safety requirements. 

When using the prototype structural verification approach, the minimum ultimate design factors 

can be the same as the required qualification test factors for metallic structures as well as composite and bonded structures. Metallic structures should be verified to have no detrimental 

yielding at yield design load before testing to full qualification load levels. 

2.2.2 Protoflight Testing: Design and Test Factors of Safety 

The protoflight approach is a widely used acceptable alternative to prototype testing. Here the 

flight structure is tested to levels somewhat above limit stress (or load) but below yield strength. 

To preclude detrimental yielding during protoflight strength verification testing, the yield factor 

of safety for protoflight structural design must be greater than the test factor. Inspections and 

functionality assessments follow protoflight testing. When using the protoflight structural verification approach, applicants should use design factors that exceed the required acceptance or 

proof test factors to prevent detrimental yielding of the metallic structure or damage to the 

composite and bonded flight structure during testing. 

2.2.3 Verification by Analysis 

Verification by analysis involves mathematical modeling, computer simulation, or both, of the 

structural element. In the majority of cases, standard criteria cannot be specified for general use 

in designing structures for which no verification tests are planned. Structural designs generally 

should be verified by analysis and by either prototype or protoflight strength testing. Projects 

which propose to use the “no-test” (i.e. analysis only) approach are acceptable when verification 

by similarity of the structural element is possible, large factors of safety are used, or other methods prove infeasible, inadequate, or both. Wherever possible, this analysis only approach should 

be replaced with test or demonstration as the program evolves. 
6

Examples of criteria on which to base “a verification by analysis only” approach are listed 

below. 

• The structural design is simple (e.g., statically determinate) with easily determined load 

paths. It has been thoroughly analyzed for all critical load conditions. A high degree of 

confidence in the magnitude of all significant loading events exists. 

• The structure is similar in overall configuration, design detail, and critical load conditions 

to a previous structure that was successfully test verified. Good correlation of test results 

to analytical predictions has been demonstrated. 

• Critical, difficult to analyze structural elements have successfully completed development 

testing. Good analytical model correlation to test results has been demonstrated. 

2.2.4 Probabilistic Methods 

Design factors of safety and test factors are intended to conservatively compensate for uncertainties in the strength analysis. Current standard structural verification criteria are deterministic, 

and experience has shown these deterministic criteria to be adequate. The probabilistic method 

uses knowledge (or assumptions) of the statistical variability of the design variables to select design criteria for achieving an overall success confidence level. Any proposed use of probabilistic 

criteria to supplement deterministic factors of safety, within the context of an application for an 

RLV license or permit, must be approved by the FAA. The FAA approves such uses on a caseby-case basis. 

2.3 Reliability and System Safety 

The structural system must possess adequate strength and stiffness to withstand the maximum 

expected loads and pressures throughout its service life. It is expected that RLV developers will 

incorporate advanced technologies in designing and constructing future launch vehicles. The 

significance of the structural systems of these vehicles will always warrant a comprehensive 

analysis and test program in verifying that these new technologies are qualified for safe operation 

of the vehicle. Results of the structural verification effort provide input to the applicant’s subsystems and system reliability efforts as part of the system safety process. For a detailed 

description, refer to Advisory Circular 431.35-2A – Reusable Launch and Reentry Vehicle System Safety Process. See http://ast.faa.gov/rep_study/license_safe_report.htm.

3.0 DETAILED RECOMMENDATIONS: STRUCTURAL AND MECHANICAL 

VERIFICATION 

3.1 Applicability 

The following criteria apply to RLV’s, including their propellant tanks and solid rocket motor 

cases. These requirements present acceptable minimum factors of safety for use in analytical 

assessment and test verification of structural adequacy of safety-critical hardware. Designs must 

generally be verified by both structural strength analyses and tests. The factors are to be multiplied by the limit stresses, including additive thermal stresses. In addition, the structure must be 

verified not to exceed material allowable stresses (yield and ultimate) under the expected temperatures and other operating conditions. A complete verification approach includes frequent 

post-flight inspections to ensure structural integrity and durability. 
7

3.2 Design and Test Factors of Safety 

The test and design factors of safety stated in this guide are the recommended minimum required 

values for RLV safety-critical structures and apply to both mechanical and additive thermal 

stresses. Higher factors than those listed here may be required for proof testing if the proof test 

is to be used for fracture control flaw screening. If pressure or temperature has a relieving or 

stabilizing affect on the mode of failure, then for analysis or test of that failure mode, the unfactored stresses induced by temperature or the minimum expected pressure must be used in conjunction with the ultimate (factored) stresses from all other loads. 

Tables 1 - 7 specify the recommended minimum test and design factors of safety for metallic 

structures; fasteners and preloaded joints; composite and bonded structures; glass; bonds for 

structural glass; pressurized vessels; and pressurized lines, fittings, and components. The design 

for these hardware items must preclude any detrimental permanent deformation or functional 

degradation of the system under the limit loads and acceptance or proof test loads for programs 

employing the protoflight verification approach. 

3.2.1 Metallic Structures 

Metallic structures may be assessed using either the prototype or the protoflight approach. Table 

1 lists minimum design and test factors of safety for metallic structures, excluding fasteners. 

Table 1. Recommended Minimum Design and Test Factors for Metallic Structures 

Design Factors Test Factors 

Verification 

Approach Ultimate Strength Yield Qualification Acceptance or 

Proof 

Prototype 1.5 1.0* 1.5 N/A or 1.05**

Protoflight 1.5 1.25 N/A 1.2 

________ 

* Structure must be assessed to prevent detrimental yielding during flight, acceptance, or proof testing. 

** Propellant tanks and solid rocket motor cases only.

3.2.2 Fasteners and Preloaded Joints 

Table 2 lists minimum design and test factors for fasteners. The strength of fasteners used in 

preloaded joints must be assessed at zero and maximum preloads. For the zero preload case, the 

factor of safety must be applied to the induced fastener load. For the maximum preload case, the 

factor of safety need only be applied to the additional fastener load induced beyond the preload. 

In both cases, the preload plus induced fastener loads times the factor of safety must be less than 

the fastener ultimate strength (i.e., Ultimate strength > Preload + Induced loads  Factor of 

safety). Unless specifically designed to separate, all joints must maintain a factor of safety 

against separation. Minimum preload must be used in the separation assessment. 
8

Table 2. Recommended Minimum Design and Test Factors 

for Fasteners and Preloaded Joints 

Design Factors Test Factors 

Verification

Approach Ultimate 

Strength 

Joint 

Separation Qualification Acceptance or Proof 

Prototype 1.5 1.5 1.5 N/A 

Protoflight 1.5 1.5 N/A 1.2 

3.2.3 Composite and Bonded Structures 

At a minimum, composite and bonded structures, excluding glass, should adhere to the design 

and test factors specified in table 3. 

Table 3. Recommended Minimum Design and Test Factors 

for Composite and Bonded Structures 

Design Factors Test Factors 

Verification 

Approach

Geometry 

of 

Structure Ultimate 

Strength Qualification Acceptance or Proof 

Discontinuities 2.0* 1.4 1.05 

Prototype Uniform 

Material 1.4 1.4 1.05 

Discontinuities 2.0* N/A 1.2 

Protoflight Uniform 

Material 1.5 N/A 1.2 

________ 

* Factor applies to concentrated stresses. 

3.2.4 Structural Glass 

Table 4 lists recommended minimum design and test factors for pressurized and unpressurized 

glass. Structural integrity of all pressurized glass should be verified by both analysis and testing. 

Unpressurized glass may be verified by analysis only with an ultimate minimum design safety 

factor of 5.0. The prototype verification option is not available for glass. Protoflight tests of 

glass should be configured to simulate flight boundary conditions and loading. For glass 

protoflight testing, the total time during load, dwell, and unload should be as short as possible. 
9

This testing should occur in an inert environment to minimize flaw growth. Care should also be 

taken to configure protoflight hardware to prevent overloading any bonded joints during test. 

Table 4. Recommended Minimum Design and Test Factors for Structural Glass 

Design 

Factors Test Factors 

Verification 

Approach 

Loading 

Condition Ultimate 

Strength Qualification Acceptance or Proof 

Unpressurized 3.0 N/A 1.2 

Protoflight 

Pressurized 3.0 N/A 2.0 

Analysis Only Unpressurized 5.0 N/A N/A 

3.2.5 Bonds for Structural Glass 

Bonds for structural glass should be qualification tested on a separate article. Each flight article 

should be proof tested. Table 5 specifies the design and test factors. 

Table 5. Recommended Minimum Design and Test Factors 

for Structural Glass Bonds 

Design Factors Test Factors 

Ultimate 

Strength Qualification Acceptance or Proof 

2.0 1.4 1.2 

3.2.6 Pressure Vessels, Lines, Fittings, and Components 

The maximum design pressure of a pressurized system is the highest pressure defined by 

maximum relief pressure, regulator pressure, or temperature—including transient pressures. The 

design and test factors should be applied to the maximum expected operating pressure or MEOP. 

Where any combination of pressure regulators, relief devices, or thermal control systems (e.g., 

heaters) is used to control pressure, they should be two-fault tolerant to avoid exceeding the 

maximum expected operating pressure of the system. Pressure integrity should be verified at the 

system level by performing a leak check. Acceptable pressure vessels should demonstrate leakbefore-burst. At the component level, fracture-critical pressurized systems may require additional tests, inspections, or both. Tables 6 and 7 list recommended minimum design and test 

factors for pressurized vessels, lines, fittings, and components. 
10

Table 6. Recommended Minimum Design and Test Factors 

for Pressurized Lines, Fittings, and Components 

Design Factors Test Factors 

Component

Ultimate Strength Proof 

Lines and fittings with outside diameter 

< 1.5 inches 4.0 1.5 

Lines and fittings with outside diameter 

 1.5 inches 2.0 1.5 

Flexible hard lines 4.0 1.5 

Line-installed bellows and heat pipes 2.5 1.5 

Other components (e.g., valves, filters, 

regulators, or sensors) and their internal 

parts (e.g., bellows and diaphragms) 

2.5 1.5 

Table 7. Recommended Minimum Design and Test Factors for Pressurized Vessels 

Design Factors Test Factors

Burst Proof Leak 

1.5 1.25 1.0 

3.3 Fatigue and Creep Life Recommendations 

All non-fracture structural items should have adequate fatigue and creep life to achieve mission 

success. The recommended minimum service-life factor for fatigue and creep life assessments is 

4.0. This recommendation applies in the following situations: 

• Structures are made of well-characterized materials. 

• Sufficient load-cycle data exists to account for all in-service environments. 

3.4 Facture Control 

For a reusable launch vehicle, all fracture-critical structural items should be damage tolerant or 

have adequate safe-life (fracture control). The minimum service-life factor for damage tolerant 

assessments should be 4.0. For metallic structural elements, the largest undetected crack that 
11

could exist in the fracture-critical element will not grow to failure when subjected to cyclic and 

sustained loads in the specified number of service lifetimes. In the case of composite structural 

elements, a broader range of flaw sizes should be considered. Metallic pressure vessels, as well 

as composite overwrapped pressure vessels, should demonstrate leak-before-burst during damage 

tolerant testing. 

3.4.1 Damage Tolerance Analysis 

For all metallic, glass, and ceramic fracture-critical parts, safe-life (damage tolerant) fracture 

mechanics analysis should be performed to verify that the parts meet the safe-life verification 

requirements. Undetected cracks should be assumed to be in critical locations and in the most 

unfavorable orientations with respect to the applied stress and material properties. Crack size 

should be based on crack-screening proof test limits or the detection capability of appropriate 

NDE technique used in the acceptance tests. Nominal values of facture toughness and fatigue 

crack growth rate data should be used in the analysis. 

3.4.2 Damage Tolerance Testing 

Damage tolerant (i.e. safe-life) testing for fracture-critical metallic, glass, and ceramic parts may 

be performed using the prototype, protoflight, or both approaches. For protoflight testing, a prefabricated crack of controlled size is induced in the flight-quality item and test in a representative 

environment. Prototype testing of the item is allowed in lieu of full-scale, flight-quality articles 

only for metallic parts when the stress field is well defined, and the material properties are representative of the flight parts. The size and shape of the crack should correspond to the detection 

capability of the NDE to be imposed on the full-scale flight item. 

Safe-life verification for fracture-critical composite structures should be performed by test only. 

The test for composite parts should be performed using full-scale, flight-quality (i.e. protoflight 

techniques), with prefabricated flaws. The size of the flaws should be based on the detection 

capability of the NDE to be imposed on the flight element. For components where neither safelife analysis nor testing is appropriate, such as for some composite material failure modes, proof 

testing of each flight hardware item may be used to establish confidence in the damage tolerance 

or safe-life of a part. 

4.0 ALTERNATE STRUCTURAL AND MECHANICAL VERIFICATION 

METHODOLOGIES 

Situations may arise that are not covered in this guide or where an applicant either cannot or prefers not to meet a factor of safety requirement for a specific flight structure or hardware component. For example, future breakthroughs resulting from evolving technologies may eliminate the 

need for an applicant to meet a factor of safety requirement. In such cases, the applicant should 

propose an alternative or modified approach to verifying the strength adequacy of the proposed 

design. The organization with primary responsibility for the development of the structure or 

component should prepare a written risk assessment that justifies the use of the alternate 

approach. 

5.0 SUMMARY 

This guide contains a compilation of recommended structural verification methods for reentry 

and reusable launch vehicle (RLV) developers to use in demonstrating that the proposed design 
12

and operations of their vehicle satisfy Federal Aviation Administration, Office of Commercial 

Space Transportation, requirements. Applicants should use these methods or an acceptable 

equivalent process to demonstrate that the structural systems of an inhabited or uninhabited vehicle can survive and perform to an adequate level of safety in all operating environments. The 

FAA strongly encourages potential RLV and reentry vehicle operators to start a pre-application 

dialogue with the Office of Commercial Space Transportation, preferably in the development 

phase of a proposed RLV program, so that potential safety concerns can be addressed as early as 

possible. 

6.0 BIBLIOGRAPHY 

14 CFR 25. Airworthiness Standards: Transport Category Airplanes, Subpart C. Washington, 

D.C.: Government Printing Office, Revised as of January 1, 2001. (See 

http://www.access.gpo.gov/nara/cfr/waisidx_01/14cfr25_01.html.) (See also http://ast.faa.gov/

licensing/regulations/stat_reg.htm.) 

14 CFR 431. Launch and Reentry of a Reusable Launch Vehicle, Final Rule. Washington, D.C.: 

Government Printing Office, 2000. (See http://ast.faa.gov/licensing/regulations/stat_reg.htm.) 

American National Standard Institute/American Institute of Aeronautics and Astronautics. 

Space Systems – Metallic Pressure Vessels, Pressurized Structures, and Pressure Components. 

ANSI/AIAA S-080-1998, September 13, 1999. 

American Institute of Aeronautics and Astronautics. Space Systems – Structures, Structural 

components, and Structural Assemblies. AIAA S-110-2005, DRAFT. 

National Aeronautics and Space Administration, Goddard Space Flight Center. General Environmental Verification Specification for STS & ELV: Payloads, Subsystems, and Components. 

GEVS-SE Rev A, Greenbelt, Maryland, NASA, June 1996. 

____. Payload Flight Equipment Requirements and Guidelines for Safety-Critical Structures. 

SSP 52005 Revision C, NASA, December 18, 2002. 

____. Structural Design and Test Factors of Safety for Spaceflight Hardware. NASA-STD-5001, 

NASA, June 21, 1996. (See http://jsc-web-pub.jsc.nasa.gov/psrp/docs/5001.pdf.) 

____. Structural Design and Verification Requirements International Space Station, Revision C. 

SSP 30559, NASA, September 29, 2000. 

U.S. Department of Transportation, Federal Aviation Administration. Guide to Reusable Launch 

Vehicle Safety Validation and Verification Planning, Version 1.0. Washington, D.C.: FAA, 

2003. (See http://ast.faa.gov/files/pdf/VV_Guide_9-30-03.pdf.) 

 ____. Reusable Launch and Reentry Vehicle System Safety Process. Advisory Circular 431.35-

2A, Washington, D.C.: FAA, July 2005. (See http://ast.faa.gov/licensing/regulations/stat_

reg.htm)