Three Orbital ATK Cygnus spacecraft are to play a major role in understanding the threat of a fire onboard a vehicle in space. The Saffire (Spacecraft Fire Safety Demonstration Project) objectives will be conducted at the conclusion to the ORB-5, 6 and 7 mission, ahead of the spacecraft ending their flights in their own fire of a destructive re-entry.
An onboard fire is one of the obvious threats to spacecraft, with the International Space Station (ISS) well-versed in procedures to protect its crew in the event of such a scenario occurring.
The worst-case scenario would result in the crew evacuating to the two docked Soyuz spacecraft, which also serve the role of lifeboats during their docked phase, prior to returning safely to Earth.
Recent studies have confirmed the fire safety requirements for long-term exploration missions require additional investigation.
Spacecraft fire safety technologies have been identified as “enabling” for some exploration missions, “enhancing” for others, according to NASA scientists.
Preventing a fire occurring in the first place is the first step, but understanding one’s foe is also a primary aim of the Cygnus vehicles that will host the Saffire experiment.
Cygnus spacecraft are expendable, designed to conduct their resupply runs prior to unberthing and ending their missions with a destructive re-entry. The period between their departure from the Station and the death plunge back into Earth’s atmosphere provides a window of opportunity for science.
The Russian Progress resupply vehicle – also designed to be expendable – has been utilized for science opportunities on numerous occasions, sometimes spending weeks on orbit prior to the fiery farewell. Similar objectives have been conducted on other expendable resupply vehicles.
Three Cygnus spacecraft will host the Saffire experiment package, ready to be initiated during the final hours of the spacecraft’s lifetime.
Cygnus will return to flight following the loss of the CRS-3/ORB-3 spacecraft when her Antares rocket failed seconds after launch from the Wallops pad. The first mission will involve the hitching of a ride on the ULA Atlas V on December 3, known as the OA-4 (CRS-4) flight.
This mission will debut the enhanced Cygnus with a longer Pressurized Cargo Module (PCM) and lightweight solar arrays.
Cygnus will enjoy a second ride on the Atlas V early next year, ORB-5/CRS-5, prior to returning to Antares in her upgraded variant for ORB-6. The ORB-5, 6 and 7 Cygnus spacecraft will all host the Saffire experiment package.
The box of tricks, embedded into the pressurized module of the Cygnus, will investigate the spread of fires in microgravity, while also developing technologies for fire suppression, combustion products monitoring, and post fire clean-up.
The data will aid the understanding of the development and growth of a realistic fire for future exploration vehicles and determine low-g flammability limits for spacecraft materials.
The three missions will provide a collection of data points. Saffire-I, riding on ORB-5, will assess the flame spread of large-scale microgravity fire.
Saffire-II, flying on ORB-6, will verify oxygen flammability limits in low gravity, while Saffire-III, flying on ORB-7, will provide similar simulation to Saffire-I, but with a different air flow employed.
Data obtained from the experiment will be used to validate modeling of spacecraft fire response scenarios and evaluate NASA’s normal-gravity material flammability screening test for low-gravity conditions.
The data will also help address both the “no ignition” and “no flame spread” criteria involved in passing standard material flammability testing. This will further flesh out data on the materials that can pass the “NASA-STD-6001 Test 1” – where the ignition energy is not sufficient to start the flame spread process.
The question to be answered is if a material passes NASA Test 1 on the ground, will it pass the test in microgravity. How fire reacts in micro-g, on certain materials, is required information.
The test – initiated by ground controllers and with Cygnus in free drift – will run for a maximum of six minutes, within a window of 2.5 hours, monitored by thermocouples (six total shared by all nine samples), a Radiometer (two sides) and a Camera (front view). The primary test to assess material flammability will be to monitor “Upward Flame Propagation.”
Following a downselect on the materials to be tested, engineers opted for the use of Nomex (HT90-40) with PMMA promoter (1 sample), SIBAL cloth (2 samples at the same flow rates as Saffire 1 & 3), Silicone (3 thicknesses for concurrent spread and 1 thickness for opposed spread) and PMMA – which will be 10mm thick.
They will be inside an experiment structure that will provide protection against accidental ignition while Cygnus is doing her business in supplying the Station. It will remain inside the Cygnus at all times.
The experiment results will be downlinked to NASA Glenn engineers embedded at Orbital ATK’s control center and passed on to the participating scientists during a final pass over ground stations, in good time ahead of Cygnus conducting her destructive re-entry.
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