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12.8.1. Flight Hardware Hypergolic Propellant System General Design Requirements
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12.8.1.1. Propellant systems shall have off-load capability through service valves that are dual failure tolerant.
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12.8.1.2. Off-load service valves and connections shall be accessible and located in the system to provide the capability of removing propellant from the tanks, piping, lines, and components at all times after loading. Estimated residual and locations shall be identified.
Note: The design goal is the ability to depressurize and/or offload the entire quantity of propellant, if necessary, to safe the system for transport to a payload processing facility. The maximum residual quantity of propellant remaining after contingency offloading operations should be identified in contingency plans and procedures that reflect the required actions necessary for subsequent safing, transportation, decontamination and processing activities.
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12.8.1.3. Propellant systems shall be designed to be flushed with compatible fluids and purged with inert gas.
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12.8.1.4. For prelaunch failure modes that could result in a time-critical emergency, provision shall be made for automatic switching to a safe mode of operation. Caution and warning signals shall be provided for these time-critical functions.
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12.8.1.5. Propellant systems shall also comply with the pneumatic system requirements of 12.6.
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12.8.1.6. Items used in any fuel or oxidizer system shall not be interchanged after exposure to the respective media.
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12.8.1.7. Bi-propellant systems shall have the capability of loading and/or unloading the fuel and oxidizer one at a time.
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12.8.1.8. Propellant (liquid or gas) migration into an associated pneumatic system shall be controlled.
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The pneumatic system should be compatible with all of the propellants served by the pneumatic supply.
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12.8.2. Flight Hardware Hypergolic Propellant System Piping and Tubing
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12.8.2.1. All flight hardware hypergolic propellant system piping and tubing connectors and fittings shall be welded in accordance with the design, performance and quality requirements prescribed in SAE Aerospace Recommended Practices (ARP) 899, Tube Fittings, Fluid Systems, Permanent Type, General Requirements for.
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12.8.2.2. Fittings and connectors with NPT or socket weld flanges shall not be used in hypergolic propellant systems.
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Certain mechanically attached tube connections have been allowed in previous payload hypergolic propellant systems via the NASA waiver process. Nonwelded fittings and connectors in hypergolic propellant systems may be considered on a case-by-case basis but shall be used only in applications where additional hazard mitigations are included (i.e. upstream isolation valves, toxic vapor detection, restrictions on personnel access during ground processing, etc.). The payload project must provide sufficient details to allow for evaluation by the PSWG and Range Safety. All proposed applications of nonwelded fittings and connectors in hypergolic propellant systems must be approved by the PSWG, Range Safety, and the NASA ELV Payload Safety Agency Team. The level of system details and the required hazard mitigations will be determined by the PSWG and Range Safety based on fitting design, heritage, reliability, application, quantity of propellant, response plans, etc.
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12.8.3. Flight Hardware Hypergolic Propellant System Valves
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12.8.3.1. Valve actuators shall be operable under maximum design flow and pressure.
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12.8.3.2. Flow control valves shall be designed to be fail-safe if pneumatic or electric control power is lost during prelaunch operations and shall be located as close as practical to tanks to allow for isolating the tank(s) from the rest of the system when necessary.
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12.8.3.3. Check valves shall be provided where back flow of fluids would create a hazard.
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12.8.3.4. Valve connectors and connections shall be designed, selected, or located, or, as a last resort, marked to prevent connection to an incompatible system.
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12.8.3.5. Remotely controlled valves shall provide for remote monitoring of open and closed positions during prelaunch operations. Monitoring of remotely controlled, pyrotechnically operated valve open and closed positions shall not be required if the function power is deenergized (in other words, an additional fourth inhibit is in place between the power source and the three required inhibits) and the control circuits for the three required inhibits are disabled (in other words, no single failure in the control circuitry will result in the removal of an inhibit) until the hazard potential no longer exists.
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12.8.3.6. All electrical control circuits for remotely actuated valves shall be shielded or otherwise protected from hazardous stray energy.
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12.8.3.7. Designs using uncontained seats are prohibited.
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12.8.3.8. Valves that are not intended to be reversible shall be designed or marked so that they cannot be connected in a reverse mode.
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12.8.3.9. Manually operated valves shall be designed so that overtorquing the valve stem cannot damage soft seats to the extent that seat failure occurs.
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12.8.3.10. Valve stem travel on manual valves shall be limited by a positive stop at each extreme position.
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12.8.3.11. The application or removal of force to the stem positioning device shall not cause disassembly of the pressure containing structure of the valve.
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12.8.3.12. All electromechanical actuator electric wiring shall be sealed to prevent fluid ignition.
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12.8.4. Flight Hardware Hypergolic Propellant System Pressure Indicating Devices
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12.8.4.1. A pressure indicating device shall be located on any storage vessel and on any section of the system where pressurized fluid can be trapped.
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12.8.4.2. These pressure indicating devices shall be designed to be remotely monitored during prelaunch operations.
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12.8.5. Flight Hardware Hypergolic Propellant System Flexible Hoses. Flexible hose requirements are specified in 12.1.10.4 in addition to the following:
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12.8.5.1. Flexible hoses shall consist of a flexible inner pressure carrier tube (compatible with the service fluid). This tube shall be constructed of elastomeric [typically poly-tetrafluoroethylene (PTFE)] or corrugated metal (typically 300-series stainless steel) material reinforced by one or more layers of 300-series stainless steel wire and/or fabric braid.
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In applications where stringent permeability and leakage requirements apply, hoses with a metal inner pressure carrier tube should be used. Where these hoses are used in a highly corrosive environment, consideration should be given to the use of Hastalloy C-22 in accordance with ASTM B575 for the inner pressure carrier tube and C-276 material for the reinforcing braid.
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12.8.5.2. Hose shall be dedicated to a service media. Interchanging of flexible hoses used in incompatible service media, such as hypergolic propellants, is not permitted. Permeation is not totally negated by the cleaning process.
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12.8.6. Flight Hardware Hypergolic Propellant System Pressure Relief Devices
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12.8.6.1. Pressure relief devices shall be installed on all systems having an on-board pressure source that can exceed the MAWP or MEOP of any component downstream of that source unless the system is single failure tolerant against overpressurization during prelaunch operation.
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12.8.8. Testing Flight Hardware Hypergolic Propellant System Components Before Assembly
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12.8.8.1. All systems elements shall be qualification tested in accordance with 12.2.2.6 and acceptance tested in accordance with 12.2.2.7 and 12.5.1.17.1.
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12.8.8.2. Pneumatic proof testing to a proof pressure of 1.25 times MAWP or MEOP is permissible only if hydrostatic proof testing is impractical, impossible, or jeopardizes the integrity of the system or system element. Prior approval for pneumatic proof testing at the payload processing facility and launch site area shall be obtained from the local safety authority.
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12.8.8.3. All hypergolic propellant valves shall be tested for both internal and external leakage at their MAWP.
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12.8.8.3.1. No external leakage is allowed. Valves shall be visually bubble tight, using approved soap solution and techniques. Internal leakage of valves shall not exceed limits specified in the valve performance specification.
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12.8.8.3.2. Certain critical system components may require helium leak checks using a mass spectrometer to verify leak rates not to exceed 1 x 10-6 cc/sec of helium gas at standard temperature and pressure (STP).
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12.8.9. Testing Flight Hardware Hypergolic Propellant Systems After Assembly. All newly assembled propellant pressure systems shall meet the test requirements of 12.5.1.17.2 after assembly.
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12.8.9.1. Flight Hardware Hypergolic Propellant System leak Tests
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12.8.9.1.1. Pneumatic leak testing at system MOP/MEOP of all completely assembled and cleaned vessel pipe and tubing sections, with components installed, shall be completed before introduction of propellant.
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12.8.9.1.2. Minimum test requirements are as follows:
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12.8.9.1.2.1. Test gas should use a minimum volume of 10 percent helium.
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12.8.9.1.2.2. All mechanical joints such as gasket joints, seals, and threaded joints and weld seams shall be visually bubble tight, using approved soap solution and techniques.
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12.8.9.1.2.3. The functional validity of installed block valves should be checked by incrementally venting downstream sections and pin hole leak checking. This test shall be conducted as a preparation to propellant loading operations.
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12.8.9.1.3. When required, a more sensitive method of leak detection (e.g. mass spectrometers) may be specified on a case-by-case basis.
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12.8.9.2. Flight Hardware Hypergolic Propellant System Validation and Functional Tests. All newly assembled pressure systems shall meet the system validation and functional testing requirements of 12.5.1.17.4.
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12.8.9.3. Flight Hardware Hypergolic Propellant Systems Bonding and Grounding. All newly assembled pressure systems shall meet the bonding and grounding requirements of 12.5.1.17.5.
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12.8.10. Testing Modified and Repaired Flight Hardware Hypergolic Propellant Systems. Modified and repaired flight hardware propellant systems shall meet the test requirements of 12.5.1.17.6.
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