Design and Flight-Qualification of an Oxygen Resupply System to Support the Transport of Live Rodents to the ISS
Tozer, Stuart D.
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The transport of live rodents to the ISS aboard a pressurized commercial cargo resupply carrier, such as SpaceX Dragon or Orbital Cygnus, requires safe and reliable delivery of gaseous oxygen. This paper will present the oxygen system design for the Animal Enclosure Module – Environmental Control (AEM-E) payload, which has undergone flight certification and safety assessment and is planned to be flight-ready by early 2014. To support an animal load of 20 mice or 6 rats and a maximum mission duration of 10 days, up to 875 liters (1.26 kg) of oxygen are required. The first design challenge was to safely store this amount of oxygen within the limited volume of a middeck-locker-sized payload. The minimum risk design uses four small composite cylinders, each containing 183 liters oxygen (at ambient pressure), compressed to 20 MPa (3,000 psig). The four independent tanks provide redundancy while limiting risk from any single tank failure. Each of the four oxygen tanks is passively flow-restricted using a precision micro-orifice (25 micron diameter), reducing the risk of cabin overpressurization or exceeding safe oxygen levels – even under catastrophic failures. For a low-power and robust oxygen release mechanism, a novel oxygen-compatible wax-actuated valve was developed and certified. Using less than 4 watts each, the 20 MPa-rated valves can be opened and closed reliably. Individual electrolytic partial pressure oxygen sensors provide the control inputs to independent analog band gap controllers for each wax-actuated valve. Oxygen concentrations are controlled with a high setpoint at nominal oxygen concentrations (20.9%) and a low setpoint at 19.6%, as required for docking to the ISS. A unique analog circuit prevents activation of more than one oxygen tank at any one time, further reducing risk from system malfunctions. System qualification of the ‘Design for Minimum Risk’ oxygen resupply system will be presented, together with system validation and integrated performance test results.