Richard,

You have an interesting idea. I took a quick scan of your paper that you posted the link to. Now if I understand your idea correctly, you envision a winged nuclear thermal rocket--but you do not specify how the propellant, martian air, is actually used. Do you envision something like a Martian version of a Turbofan engine, or is it more akin to a ramjet? A true nuclear rocket utilizing CO2 is possible (even practical) but the propellant through put would be such that such a vehicle will almost certainly have to land from time to time and replenish its propellant stores using a compressor.

Some proposals such as this have been made, such as MINRVa (Martian Indigenoius-fueled Nuclear Rocket Vehicle) [or something like that.] It was a two vehicle proposal--one for a long range ballistic "hopper" and the other for a hypersonic lifting body. Both used a nuclear rocket engine to heat liquid CO2 and expand it out a deLevaal nozzle. Both used the heat of the reactor to generate onboard power to run compressors to suck in Martian atmosphere (which is 98% CO2) and compress it to liquid form--but this was only done after landing. Several days on the ground would be enough time to accumulate enough propellant for another hop--which gives explorers time for local reconnoiter. The both vehicles would be capable of achieving Mars orbit, which allows the vehicles to be conveniently used either as primary or backup transport for docking with the mother ship for return to earth. Of course this was for a manned mission to Mars...

Your vehicle sounds doable in a Turbofan configuration...slightly less Isp, but Isp isn't necessarily everything, especially when you have an infinite source of propellant in the atmosphere.

I too have looked at a Martian ROV or airial robotic vehicle. I hadn't thought about nuclear rocket power, though. The concept I explored briefly on paper used Stirling cycle engines powered by the decay heat from an isotope source. I came to the conclusion that Pu-238 did not give enough heat per kilogram to be workable. Pu-238, which has a half-life of 87.7 years, has a specific decay heat of about 0.57 W/g, or about 570W/Kg. Even a respectable ultra light Martian aircraft would require something like about 2-3 hp of mechanical power to fly, throwing a big slow propellar, so say about 5Kw of mechanical power. A high performance, high pressure helium filled Stirling cycle engine, perhaps in the configuration of a small 4 cylinder Rotax engine, could achieve about 25-35% overall efficiency. Now assuming that the weight of the radiator isn't a problem (it is a big problem!) then the isotope source would have to put out about 5kw/(.3)=15-20kw of thermal power. If Pu-238 is used, then an isotope source massing 20Kw*1kg/(0.57Kw)= 35Kg (74lb) is needed. This is quite a lot of Pu-238! Pu-241 with a 14.7 year half life may reduce this to about 10kg, which seems doable. I'm not sure how massive the Stirling Rotax would be--probably anywhere from 10-20kg. Now factoring in a radiator, add maybe another 5-10 kg. We're now up to 25 Kg for the low end, to a heftier 65kg at the higher end.

Without cracking open any texts on aerodynamics and ultralight aircraft, any aircraft designed to fly in Martian atmosphere will face many difficult challenges. First and foremost will be lift (and equally important weight, which will be 1/3 less on Mars than on Earth.) The atmosphere of Mars at ground level has approximately the same density as Earth's atmosphere--at 110,000 feet! So any vehicle intending to cruise even at ground level will require substantial lift. Such lift can be achieved two ways: large gossamer wings, like a glider; or speed. Traveling at very high speed will create more lift from a smaller body, but will require far more power (and energy consumption) than a larger, but much slower vehicle. Landing a high speed vehicle will also present many challenges, as the approach speed will still be very high on Mars. Utilizing variable geometry wings is a way around this, but this greatly adds to the complexity of vehicle design. And control will be very difficult, as the vehicle will have to be autonomous. Even smart control systems have not prevented the loss of several Predator ROV's systems due to bad weather in Afghanistan and Iraq, even though the feedback control loop time is measured in seconds (from sattelite.) A fully autonomous vehicle will need to make decisions very fast, and consistantly make the right ones. The Spirit and Opportunity Rovers have done well, but their 'inches per minute' speed is pretty slow (and the Earth Bound operators admit that Luck has probably been a very big help to them!)

From this point of view I tend to think that a larger, more gossamer vehicle propelled by a large, slow turning propellar is probably the most efficient and practical Martian ROV design. If such a vehicle were to cruise at an altitude of say 500 m, and were to move at say 30 Kilometers per hour (8.3 m/s or 27 ft/s or about 19 miles per hour) then such a vehicle will still cover about 600+ kilometers per day of surface. A circumnavigation of Mars might take just under a month. Areas of interest could be thoroughly mapped by utilizing a circular loiter to see a feature from almost every angle. A nearby landing could release a small rover to further invesitgate by performing closeup analysis of features, just as Spirit and Opportunity are doing. However, having studied many of the photos returned by the MER mission, I tend to think that any landing by any aircraft on Mars will undoubtably be a tricky affair!


-- Edited by GoogleNaut at 09:20, 2005-10-26

-- Edited by GoogleNaut at 09:22, 2005-10-26

-- Edited by GoogleNaut at 09:25, 2005-10-26