Fueling a Trip from Mars
As researchers venture deeper into space, the need for a propellant refueling station millions of miles away from Earth is increasing.
It has been more than 26 years since man has set foot on a surface other than that of his own planet. The quest to land a man on the moon was achieved by the Apollo 11 mission in 1969. Five more successful Apollo lunar landing missions followed, with Apollo 17 making the final trip in December 1972. Since then, planetary surface exploration has been delegated to a host of sophisticated probes and robots. But that could change.
The August 1996 discovery on Earth of a martian meteorite, and the fascinating images of the red planet beamed back to Earth from the Mars Rover following its July 1997 landing, have kindled renewed interest in manned planetary exploration. While flying to the moon was certainly no easy task, getting astronauts to and from Mars is a challenge of a higher order.
Mars is the fourth planet in the solar system with a mean distance of approximately 141 million miles from the sun. The surface of Mars is a harsh environment in which to operate. Severe dust storms occasionally cover much of the planet, temperatures can be as low as -130°F even at the equator, and the atmosphere is primarily composed of carbon dioxide, with an air pressure one percent that of Earth's.
A round trip to Mars is far more complex than just launching into Earth orbit, such as is the case with the space shuttle, or traveling to the moon, which is 240,000 miles from Earth. Depending on the positions of the two planets, Mars is between 50 and 230 million miles from Earth.
One attractive option for a manned mission to Mars calls for the outbound and inbound trips to be 180 days each, with a 500-day stay on the planet. The long surface stay on Mars is required because of the wait for Earth and Mars to reach the proper alignment for a return trip, but this also allows for extensive scientific exploration of the planet.
Fueling a round-trip mission to Mars
One of the most significant challenges to overcome for a manned mission to Mars is that of fuel. But should all the fuel required for the trip be transported to Mars and back, or is there an alternative?
One option currently being explored by NASA is an in situ propellant production (ISPP) plant, essentially a refueling station on Mars, launched prior to the manned mission. NASA is looking at several ways to produce fuel on Mars, and called on engineers in the Southwest Research Institute (SwRI) Mechanical and Fluids Engineering Department to help evaluate existing concepts.
The advantage of producing fuel for the return trip can be determined with simple assumptions and calculations, by working backwards from the return-trip requirements for two mission scenarios -- one with an ISPP plant, and another in which all fuel is sent with the crew. An additional benefit for ISPP is realized if fuel is also required for surface exploration activities.
Simple rocket science
In each case, the Mars-Earth transit vehicle will have a mass of around 10 tonnes (1 tonne equals approximately 2,200 pounds) and require a 5-tonne ascent engine for Mars liftoff, to be jettisoned in Mars orbit. Add 53.2 tonnes of methane/oxygen propellant, and the resulting total mass that lifts off from Mars is 68.2 tonnes. Methane is only one of the proposed ISPP fuels for the Mars-Earth return rocket engine. Other candidate fuels being considered include methanol, carbon monoxide, and dimethyl ether.
In the ISPP option, the 53.2 tonnes of propellant would be produced on Mars, so only the "dry" mass of 15 tonnes needs to be brought from Earth. There will be another 20 tonnes of materials and equipment (such as power plant, communications, and lab equipment) for the Earth-Mars transit, as well as exploration equipment, which will be left on Mars. Thus, the mass that must be transported from Earth orbit to Mars increases to 35 tonnes for the ISPP option and 88.2 tonnes for the non-ISPP option.
The propellant required to inject the spacecraft from Earth orbit to the Mars transit trajectory is then 51.7 tonnes for the ISPP case and 130.3 tonnes for the non-ISPP option. In terms of total payload mass launched from Earth, the ISPP option would result in a 60-percent lighter vehicle -- just 86.8 tonnes, compared to 218.5 tonnes for a vehicle carrying its own fuel for the return trip.
With current launch costs as high as $4.5 million per tonne ($2,000 per pound), the use of ISPP can save nearly $600 million.
ISPP science and technology
For the purposes of this study, NASA asked SwRI to evaluate three principal candidate processes -- zirconia cell, Sabatier reactor, and reverse water gas shift -- for the production on Mars of both a hydrocarbon fuel and an oxidizer. The production of only oxygen using these processes was studied by SwRI in an earlier phase of the project. The Institute was also asked to evaluate existing fuel filtration technologies, because any ISPP plant placed on the planet would have to withstand Mars' violent dust storms.
These three methods yielded five options for producing fuel and an oxidizer: zirconia cell with methanol reactor, reverse water gas shift with methanol reactor, Sabatier reactor, Sabatier reactor with methane pyrolysis, and Sabatier reactor with reverse water gas shift. In all of these fuel production plants, carbon dioxide is taken from the martian atmosphere by a sorption compressor and fed to the plant. The carbon dioxide is then combined with hydrogen that has been transported from Earth and chemically converted into a fuel and oxygen.
All of the systems being considered by mission planners use well-established chemical processes, but the technology of an entire ISPP system is unproven for use in the martian environment.
The Sabatier reactor is a catalytic process and has a long history of nonspace applications. It is expected to be used on the International Space Station for air revitalization. The developmental technology was demonstrated in a testbed, but an engineering model for Mars applications has not yet been qualified.
The zirconia cell method, which is an electrochemical process, has been under development for more than 10 years, and all major components have been tested. However, a prototype system has not been tested. The zirconia cell method has been chosen for a demonstration test on NASA's Mars 2001 mission, which will further geological and climatic exploration of the martian surface using stationary sensors and a rover. As a result, zirconia cell technology will probably undergo significant advancement.
The reverse water gas shift process, also a catalytic process, is a common industrial process that has been widely used in chemical plants for many years, but an ISPP system using this process has only been tested in a laboratory configuration. Critical functions have been evaluated, but hardware applicable to the martian mission has not been tested.
Each of the five candidate ISPP plants was analyzed using a commercial chemical plant software package. This approach provided a uniform method for computing plant thermal and chemical performance and allowed the different options to be objectively compared on a power and mass basis.
Using the results from the chemical and thermal analysis, a trade study for the chemical process for producing both fuel and oxidizer was performed for each of the candidate plants. Eight criteria were selected to evaluate the choices: reliability, cost, mass and volume, power, production rate, balance of system impact, propellant fuel option, and scalability. Each criterion was given a weight of high, medium, or low.
Reliability, cost, mass and volume, and power were weighted as high, with reliability being the most critical of the four. The ISPP plant must operate autonomously for 500 days, must produce propellant reliably with no maintenance, and must survive the trip from Earth to Mars.
Cost is always an important factor, and mass and volume affect cost. The larger the mass, the higher the cost to put a payload into orbit and, eventually, onto the surface of Mars. Additionally, the system with the lowest power requirement would also receive the highest rating. Since the ISPP would be solar-powered, a system with high power demands would require large solar arrays, thus increasing launch mass and volume.
Production rate was rated medium, while propellant fuel option and scalability were rated of low importance. All candidate processes produce liquid oxygen and a hydrocarbon, and all candidates have the potential to move from laboratory scale to manned-mission scale. Balance of system impact, which encompasses system mass, power, and complexity, was also rated of low importance. ISPP plants capable of producing fuels that do not have to be stored at cryogenic temperatures have a more positive effect on other system components.
The five ISPP plants were rated in each of the eight categories. A grade of excellent, good, fair, or poor was possible according to the system's combined merit in each category. The grades were based on the objective thermochemical performance, or on the more qualitative assessment of such features as reliability and complexity.
Combining all factors, the Sabatier reactor with methane pyrolysis plant and the Sabatier reactor plant were rated "good." The other three were rated "fair." None received an overall "excellent" score, primarily because none has been tested for reliability of service on Mars.
So, what is the next phase in the development of ISPP technology? Clearly, the most pressing need is to test prototype hardware in the martian environment (or at least a simulated one). The most critical aspect of using ISPP on Mars is the lack of repair or maintenance staff. Reliability, therefore, is the most important technological challenge facing ISPP and was the least quantifiable of any of the selection criteria in this study.
The plant must function in the harsh martian environment for 500 days with no preventive maintenance or repair work. This would be comparable to driving a car 100,000 miles without replacing engine oil, air filters, or lubricants or making any mechanical repairs. The plant must function with 100-percent reliability. With this requirement, many of the standard chemical process methods proved unattractive.
A second critical issue is accommodating the dust environment on the martian surface. One of the biggest unknowns in the filtration analysis conducted at SwRI was the level of contaminants that could be tolerated by the compression and chemical processes. Specifically, how contaminant-tolerant is the sorption compressor, and how much of this would be passed on to the chemical process by the compressor? These chemical processes must also be examined to determine their sensitivity to solid and trace gas contaminants.
Other issues to be addressed are smart autonomous controls, thermal management, and power systems (both solar and non-solar). Since the ISPP plant must be totally autonomous, smart controls that identify system health and make the necessary adjustments are vital. These controls must be extremely reliable and dependable. The thermal management system must provide the proper environment for the health of the chemical processes. The power system, probably based on solar energy, should be optimized to provide continuous steady-state operation, or have a process that can undergo daily thermal cycles.
Future technology development programs to support NASA's manned mission to Mars must address practical engineering issues. No new technological breakthroughs are required, but the engineering challenges are significant. If successful, these solutions could be extremely useful here on Earth for, as an example, the chemical process industry, which currently depends on armies of maintenance staff to keep conventional process plants functional.
Dr. Michael Miller and Martin Treuhaft, both of SwRI, contributed to this project in the areas of gas phase separators and filtration. Jerry Sanders and Rob Moreland of the NASA Johnson Space Center in Houston provided technical guidance and insight into overall Mars mission planning.
Published in the Spring 1999 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Stothoff.