breathing on MARS
Tiny Orifice to Help Sustain Human Life

-- Stacey David
edited by Richard Mandel

As we go to print, the Mars Global Surveyor spacecraft is taking high-resolution images of the Red Planet in search of the missing Mars Polar Lander. Flight controllers are continuing their attempts to communicate with the lander so that they can be certain they have exhausted all possibilities before they conclude their search. While recovery is still a possibility, the likelihood of hearing from the lander is considered remote at this point.

Relative size of the off-the-shelf 0.0004 in. Bird Precision orifice

Coupled with the loss of the Mars Climate Orbiter, questions abound as to the future of the Mars Exploration Program. With hopes of continued Martian exploration, engineers are working on a unit that can eventually support unmanned and manned missions, as well as help humans eat, breathe, farm and --among other things -- return home. With the help of a tiny orifice supplied by Bird Precision, Waltham, MA, aerospace engineers at the University of Arizona are creating a system to provide enough oxygen for rocket propulsion and to sustain life. Small, completely enclosed "farms" can be maintained, supplementing a rocket crew's diet with fresh vegetables on a planet of barren rock.

Let's back up

One of the greatest limiting factors to life on Mars is the lack of atmospheric oxygen. With an atmosphere that's almost 95% carbon dioxide, how can life survive? And yet, scientists found evidence of primitive life on Mars through photographic and space probe data returned from Mariner and Viking missions, as well as microscopic traces of life discovered on a small meteorite that traveled from Mars to Antarctica. These findings led to an increased desire to send a manned mission to Mars for further investigation. Obviously, such a mission not only has to get to the planet, but it has to get home as well, not to mention surviving a period of time there. An outbound voyage to Mars takes roughly six months, the return voyage about the same, with a very limited launch window. Stay for more than a few days, and you have to wait for the next window -- three years later.

So how do you cost-effectively transport enough fuel for a round-trip voyage to a location over 48 million miles away? And how do you sustain life for three years on a planet with no oxygen? The answer to the first question is: you don't. The answer to the second is: you make it.

The system

According to engineers at the Mars In Situ Propulsion Production (ISPP) at the Space Technologies Lab (University of Arizona), astronauts will create O₂ on the Martian surface by using an In Situ Resource Utilization (ISRU) unit called an Oxygen Generator System. The OGS works with any oxygen-bearing gas which, in this case, will come from Mars' 95% CO₂-bearing atmosphere. The central part of the OGS is an SOEC (Solid Oxide Electrolysis Cell) which consists of an oxygen ion-conducting solid electrolyte, Yttria Stabilized Zirconia (YSZ), sandwiched between platinum-based electrodes. The electrolyte disc allows conduction of oxygen ions through the membrane at around 750°C. At this temperature, oxygen dissociates from the oxygen-bearing gas (CO₂) and is conducted through the YSZ electrolyte from the cathode to the anode due to an externally applied d.c. potential. The carbon monoxide is then vented back into the atmosphere, and the O₂ passes through another platinum electrode layer into a containment area, where it is liquefied and stored in pressurized tanks.

Thermal expansion-matched metal manifolds are provided on both sides of the SOEC for gas transport. The manifolds and the electrolyte are integrated as a gas-tight package by a thermally cyclable precious metal seal. The Centigrade is reached by using fabricated ceramic heating elements. OGS utilizes fabricated miniature ceramic heaters with a platinum heating element to achieve the operating temperature of 750 degrees. The heating element is sandwiched between two ceramic substrates.

The flow

To make such a system work efficiently, the rate of flow input and output, as well as the system flow, must be strictly regulated. The ISSP team chose a Bird Precision fixed valve system using a ruby orifice to help with this regulation. By knowing Mars' ambient pressure and the desired system pressure, the flow rate can be calibrated with an orifice and fixed valve to predict oxygen generation and utilization for optimum efficiency.

The OGS system was designed around Bird's orifices' standard-sized dimensions to ensure the most cost effective and efficient solution for the system. The orifices are used in three different locations throughout the OGS system. One 0.001 in. orifice is used between the upstream compressor and generator that regulates the inlet flow of CO₂. This orifice drops the flow from 15 psi to 10 psi. A second 0.001 in. orifice is found at the exhaust line, dropping the system pressure from 10 psi to 6 torr, Mars' ambient pressure. The third orifice, Bird's smallest off-the-shelf model at 0.0004 in., is currently found on the oxygen line, and is calibrated to drop the O₂ system pressure from 12 psi to 6 torr as well. The ISPP team is one of the first to use the new 0.0004 in. orifice, which is now available as a standard size, complete with orifice housings. The ruby orifice was chosen not only for its size, but also for its composition. Synthetic ruby is almost completely wear resistant and heat resistant, and the Bird manufacturing procedure ensures a burr free hole for extremely regulated and consistent flow. The 0.0004 in. orifice is necessary for the initial, smaller test model that will be sent to Mars on the 2001 mission. On later missions, when a full sized OGS unit will be used, this orifice may be placed elsewhere, as the O₂ generated will be flowed at higher pressure directly into the liquefaction system. The O₂ generated will be used for rocket propulsion for several unmanned Mars-to-Earth return flights through 2007, and for both propulsion and life support for manned flights beginning about 2010.

For More Information:

Circle 521 - Bird Precision (800-208-6840)

Circle 522 - Space Technologies Lab

Thermal Cycling Phenomenon

The efficiency of the OGS is paramount since the unit currently has a short life span. The test model under construction is built for a 30-day working period. The next version will be designed for a 100-300 day life span, and the final working model will aim for operational capabilities of several years. The unit's life span is limited by a phenomenon known as thermal cycling.

Since Mars' surface temperature fluctuates from -110° C at night to a high of -30° C during the day, solar power is used to create the energy needed to, amongst other things, provide the 15 watts needed to run the OGS cell heaters. (This also means that the OGS can only be run during daylight hours.). Though this is less than the average light bulb needs, it is the most draining energy demand on the entire space mission. The ISPP team currently has no effective means of capturing and storing enough energy during daylight to keep the unit warm overnight. This results in severe temperature fluctuations of the OGS unit, known as thermal cycling, which eventually cracks the unit's seals, rendering it inoperable. Part of the problem will be solved by a new seal being developed. The other solution is to make the unit efficient enough during daylight hours that the volume of oxygen needed can be generated well before the seal fails.