"ARCHITECTURE FOR RAPID
ARCHITECTURE FOR RAPID EXO-PALEONTOLOGICAL SURVEY OF MARS
At the recent 2005 Mars Society Conference, Chris McKay of NASA stated that the first priority on his list of tasks was "Determining if life had existed on Mars in the past" That implies detecting life signatures in the form of, most likely, stromatolitic fossils.(1) Stromatolites are layered fossil remains of primitive one-celled organisms that formed several billion years ago at about the same time that oceans may have existed on Mars.
Obviously, slow rover exploration would take many generations to adequately cover the surface area of Mars. Thus, this proposal for a rapid robotic aerial survey of the Martian landscape with streaming of the data back to Earth and massive public participation in its analysis.
One would start with a high-resolution radar data base of the Martian topography. The data base would then be loaded into a flying platform capable of long duration Mars flight. This would be pre-programmed to systematically fly remotely over all areas of geological interest, such as up and down all canyon walls and around all crater walls, etc. This flying platform should be capable of sustained flight for up to one year, which obviously goes beyond our current or proposed capabilities in Mars flight.
This flying platform would take a series of overlapping very-high resolution pictures with the best state-of-the-art spy camera available. These would be streamed back to Earth in near real time. This would likely require an upgrade of our Mars satellite communications up-link capabilities which could be provided on the delivery vehicle. NASA is also developing the NUGGET, an imaging device using beamed neutrons that can create 3-dimensional images of underground rock structures.(2) These images could perhaps also be streamed back to Earth.
The data from the Mars Global Surveyor's MOLA-2 (Mars Orbital Laser Altimeter) may be adequate. It has a vertical accuracy of <10 m., a surface spot size of 130 m., and an along-track shot spacing of 330 m. (3) If this is not adequate, perhaps it could be re-programmed for higher resolutions. If not, then a new surveyor would have to be put up.
A number of Mars UAVs are in development. Ames Research Center is developing the Matador, (4) which would fly for 45 min. before crashing. The Mars Ares flight vehicle being developed by Langley (5) has a mass of 149 kg, a payload of 20 lb, and an anticipated flight life of 1-2 hours until it crashes.(6)
A study conducted by NASA and others seemed to conclude that the only way to create a very long duration Mars airplane flight system was to beam power up from a ground based system. (7) Of course, this would not be very feasible for long distance flights, and flights in canyons.
This proposal would envision a plane with a somewhat larger payload, but with the capabilities for very long term sustained flight. One way to approach this problem is to mount a nuclear thermal rocket to an airplane. The propellant would be atmospheric CO2, which could be continuously replenished in flight. While CO2 is less efficient than hydrogen, it has the advantage of being free and available.
Dr. Robert Zubrin has demonstrated flight in a test plane using CO2 as a propellant and hot mass as a heat source. (8) This was simply a box of heated dense material. CO2 was fed through this hot mass and emerged through a rocket nozzle. The wing span of the test plane was a few meters wide. Film of test flight of this plane was shown at the 2005 Mars Society convention. (9) In its most fundamental sense, this current proposal simply replaces Zubrin's test plane's box of hot mass, which of course has to be periodically reheated, with a box of nuclear material which will remain hot for a very long time.
The Peewee and others in the KIWI and NERVA series of nuclear rockets demonstrate the feasibility of using solid core nuclear heat sources to accelerate gas to obtain good flight characteristics.(10) Dr. Stanley Borowski from the Glenn Research Center has been developing designs for a bimodal nuclear rocket for Mars space missions.(11) Currently the Pratt & Whitney Co. is developing the Triton, a trimodal nuclear rocket with a thrust of about 15,000lb or 66.7 kNewtons. (12) The proposed nuclear rocket for the Mars airplane would be .5% of this output. There was also a design called the Escort, which was a design based on the military requirements for a long-life, high-Isp propulsion system that could maneuver in orbit and defend and attack satellites. Its power was to be in the 500-1000 ft.-lb. range, so this design would be in roughly the size range needed. (referenced in the above P & W site)
The nuclear heat source for a Mars rocket-powered airplane could be a very simple straight-through design. There should be no requirements for complex electric power production, and controls could be very simple or even eliminated, as this plane would fly continuously at a set speed for its mission lifetime. The wings would provide ample area to mount enough solar cells to power the compressor, pump, and electronics, thus obviating the need for complex hybrid designs.
Since this would be designed to run continuously without stopping once started, and with no throttling, the heat unit could perhaps be designed to give off just enough power to accelerate the gas thruster, and no more, obviating the need for radiators to radiate excess heat.
The development of a thermal nuclear rocket running continuously for a year would be several orders of magnitude longer than test runs of any past nuclear rockets, which have only been run for periods of under 1 hour. The main problem is with the erosion of the nuclear fuel assemblies by the hot gasses. However, Russell Joyner, Discipline Chief, Propulsion Systems Analysis of Pratt & Whitney (see above) said in an interview "..running at low temperatures where the reactor surface temperatures are running at 1600oK or lower versus with the propulsion mode running at 2600oK-to-2700oK there is significant (engine) 'life', we're talking years"
Thus, if these lower temperatures can be used for continuous aircraft propulsion in the Martian atmosphere, then engine life should be adequate. Another potential problem is nozzle embrittlement at these time lengths. However, embrittlement is normally thought of as a problem with H2 entrapment at grain boundaries. If CO2 does not exhibit this property, then this may not be a problem.
Shielding requirements could be kept extraordinarily low, as there would be no human contact with the robotic plane. A temporary shield could be fitted for launch, and jettisoned in orbit.
The published specific impulse (Isp) of nuclear rockets may be somewhat misleading in the case of this design. Isp is a measure of how many seconds of thrust are produced per unit of fuel, both expressed in similar units. Thus, the best oxygen/hydrogen chemical rockets produce about 450 ft./lbs of thrust per pound of fuel, for an Isp of 450 sec. (one lb. of thrust for 450 seconds per lb. of fuel)
In the case of nuclear rockets, the published Isp's are roughly double the best chemical rockets, in the area of 900-1000 sec. However, this is figured on the stored gaseous fuel that is accelerated by the nuclear heat source, rather than the pounds of fissionable material.
So in the case of flying on Mars, where the gaseous fuel expelled is continuously available, the Isp for this gaseous fuel is essentially infinite.
POWER DENSITY OF NUCLEAR FUEL
The real Isp to be considered is how many pounds of thrust are produced per pound of fissionable material. A common rule of thumb is that nuclear fissionable fuel contains a million times as much energy per unit mass as chemical fuel. In theory the Isp of a thermal nuclear rocket with continuously available gaseous fuel would be one million times the 450 sec. Isp of the best chemical rocket ( assuming 100% efficiency). Thus 450 million sec of Isp per lb of fissionable material equals 5,208 days of 1 ft./lbs. of thrust.
The Ares Mars Airplane has a thrust/weight ratio of 1 to 24.6 . Thus for a 2000 lb Mars Airplane, this might require 81.3 lbs, or 361.6 Newtons of thrust.
For this 2000 lb Mars airplane, one pound of fissionable material would equal 64 days of flight. For one year of flight this would equal approximately 5.7 lb. of fissionable material, assuming 100% efficiency.
ALTERNATE POWER DENSITY CALCULATION
The measure of consumption of nuclear fuel in reactors is called burnup-rate. It is expressed in MWd/T (megawatt-days/metric ton). Common values in reactors are in the area of 50,000 MWd/T. (13) This equates to 1.2 MW Hours/gram of fuel, or 4.32 billion joules (kg-m.^2/sec^2).
Energy = Force X Distance
Thus for the proposed 2000 lb plane requiring 361.6 Newtons of thrust, each gram of fuel would propel the plane the distance determined by the following formula:
Energy [4.32 Billion Joules (kg-m.^2/sec^2)] = Force [361.6 Newtons (kg-m./sec^2)] X Distance (m.).
Distance (m.) = 11.9 million meters or 11,900 km. per gram of nuclear fuel.
The Ares plane flies at 145 m/sec. In 1 day (Earth) it would fly 12,528 km.
Thus in theory 1 gram of nuclear fuel could propel the plane for about one day. However, CO2 is much less efficient as a thermal propellant than hydrogen. Isp of a gas is proportional to 1/sq. root of molecular wt. (14) 1/sq. root wt. of Hydrogen (molecular) is .709, and in nuclear rockets the Isp of hydrogen is ~ 1000. Thus if 1/sq. root wt. of CO2 is .152, then the Isp of CO2 in nuclear rockets should be on the order of 21% of Hydrogen.
If CO2 is 20% as efficient as Hydrogen, then 5 times as much nuclear fuel will be needed, or 5 grams per day. In one year this would be 1.8 kg.
The main advantage of such a program is that it would involve many thousands of people in a direct and personal way. All of the images produced could be put on the web. The public would then be encouraged to survey them, as there would not be enough scientists to adequately analyze all the millions of images. This would bring tremendous interest, enthusiasm, and ownership of the space program to millions of people all around the world. People could sign up for their own square km. of terrain to search. Anyone could literally be the first person to discover fossil evidence of life on Mars.
Of course there would be stromatolite identification training sites and other support provided. I have developed such a Mars stromatolite identification training site (15) and would be glad to make any of my extensive stromatolitic research available at any time.
It is contemplated to submit this proposal at the next call for proposals for phase 1 studies from NIAC (NASA Institute for Advanced Concepts). Phase 1 funding could be used for things such as investigation of the adequacy of the MOLA data base, investigations of the ability to utilize other Mars plane designs in this proposal, investigation of suitable camera and uplink capabilities, investigation of the programming requirements for pre-programming the Mars topographic itinerary, and investigation of the feasibility of construction a very small scale nuclear thermal rocket.
Any people who would be in a position to be a Principal Investigator on this application, or to be involved in any other capacity should get in touch with the author at the e-mail address listed.
This proposal would make very long-term flights on Mars feasible using nuclear heat sources.
Another approach could be to set up a remote fuel-making station on Mars to manufacture fuel and oxygen from Mars atmosphere and periodically refuel a conventional bi-fuel rocket. This would have the disadvantages of requiring periodic landings and take-offs, as well as limiting the range of exploration. It would also most likely delay deployment until the development of the Heavy Lift Launch Vehicle.
Once areas of high interest are discovered by this process, then flying platforms with rovers can be dispatched to them, maximizing the effectiveness of the most precious resource, scientists' time on Mars.
I realize this is a very ambitious proposal, but I believe it maximizes the efficiency of data returned per time and money spent, as well as the very necessary factor of energizing public interest, enthusiasm and participation in future Mars and space exploration.