Well, honestly I never progressed far enough in a design to nail down definate parameters, but I definately wanted a vehicle able to return several thousand tons of payload. Originally I was trying to explore the idea of financing a program by returning platinum group metals, specifically rhodium and platinum in about an 80% Rh/ 20 % Pt cargo mix. At current market prices ($4800 per troy ounce for Rhodium, $1200 per troy ounce for Platinum, KITCO prices as of 11/6/06 1:42 am) a 1000 metric ton cargo should gross about $131 billion.

800t * 32150.75 ozt/t*$4800/ozt + 200t * 32150.75 ozt/t * $1200/ozt = $1.31*10^11

If missions could sortie once every two years this is a gross of about $65.6 billion/yr (assuming of course that the market does not saturate--but demand for PGM's for vehicle catalytic converters, fuel cells, etc. will only increase, so demand should remain fairly high...) which is pretty impressive when one considers that this is one vehicle. A small fleet of vehicles returning PGM's may do much better, but the market prices may fall as the result of saturation...Anyways, I have yet to completely analyze the chemistry involved in breaking down asteroidal nickel-iron using carbon monoxide in the Mond Process to create carbonyls. So I can't estimate the amount of volatiles needed extract a given amount of PGM's. The reason for the LEO Clarke Station Depot and the High Earth Orbit Depot was to service a small fleet of vessels: some going out to comets and carbonaceous asteroids to extract volatiles. Others to take some of the volatiles in the form of propellant and process chemicals to PGM rich nickel iron bodies. This will require extensive orbital industrial infrastructure, probably something akin to an oil refinery and tank farm coupled with a busy shipping port. This will be the necessity to supply say four or five vessels with propellants and process chemicals. The whole thing get's really expensive quickly!

Anyways, back to the concept vehicle: what I had in mind was to use propellant tanks from SDV's (shuttle derived vehicles) such as the Ares V or something like that
as tankage building 'blocks' for transhipment of volatiles. For instance a single Shuttle ET could in principle carry 530,000 gallons of water which would mass oh about 2000 metric tons. This much water if electrolyzed would create the liquid hydrogen and liquid oxygen equivalent of about 2.8 ET's worth of propellant. That's potentially a very valuable resource! I wanted to create a single keel hexcell (six sided) truss structure that runs the entire length of the ship from the forward crew/cargo areas to the rear nuclear thruster areas. For radiation shielding purposes, the boom was long because distance and mass tend to attenuate radiation, so keeping the reactors far away from the people (the long boom) and then place stuff in between (propellant and cargo modules.) I had also looked at double keel variants which may have advantages, but I haven't detailed anything out yet.

Also, the idea with the truss was that it could be delivered collapsed to the Clarke Station in one piece. Expansion of the truss and then attachment of the various modules (Service Module with reactors, engine bells, control system;) Cargo Modules which would actually just be fixtures to attach to the truss things like propellant tanks, storage tanks, shipping containers for equipment and solid return cargo. of course you must also run electrical power, control, and propellant service lines that run along the truss connecting all the various modules. At the opposite end from the Service Module would be the Command Modules--the crew areas.

Initial guestimates on the size of this beast? I'd put that central hextruss at just about 120 m, so that with the forward and aft modules attached the ship might be just tickling 200 m in length. Pretty darn impressive--it should be quite easy to see on a clear night just after dusk from the ground if the ship were in Low Earth Orbit!
The trouble is, I don't have a clue as to what equipment it needs to carry to an asteroid or comet to do the job. How do you mine a comet or comet reminant for volatiles? Do you quarry slabs and then melt/evaporate the ices in a large oven (retort) and condense volatiles by using cold traps? Do you use a rotary drill bit? Do you use a saw? Mining nickel iron asteroids are even more challenging--how do you cut through a solid slab of what amounts to tool steel (nickel-iron can be very tough stuff?) Again do you quarry slabs and then melt the metal for spin casting into a powder for extraction using the Mond Process? Can you use a portable solar furnace to supply the heat for this? How much electrical power do you need to operate the various tools that you need to use? Can this power be generated as auxilary power from the ship--this was another reason for looking at VASIMR--it required a large nuclear electric power supply--a really nifty thing to have if you want to power all of your mining and processing equipment on a comet or an asteroid!

I don't even know how many crew are needed or how long they will need to stay--and this will steeply drive the living space, shielding requirements, and of course mass of consumables. I figure for safety reasons a crew of no fewer than six, and probably not more than twenty would be needed, but this is a complete WAG.

Yeah, this is where it get's dicey right off. It was my intention to design a ship with modularity in mind, and also to include a fair amount of processing or refining on the spot for seperation of volatiles. The type and degree of processing will scale the demand for power as well as--my best guess is that if comet ice is extracted by melting and then processed by 'flashing' follwed by controlled condensation in a newtwork of cold traps designed to extract higher condensing fractions first, starting with water and working our way down to liquification of carbon monoxide and methane, then we'll almost certainly be looking at electrical power demand of several megawatts. A big cryogenic plant for liquification of hydrogen or oxygen (maybe a multiuse liquid helium plant) could all by itself use 500KW of power, and maybe more than that!

MITEE, and Triton may be able to supply multi kilowatt loads, but a megawatt or more of electrical power may be a bit of a stretch. This is another reason why I looked at NEP systems--because when the thing was parked, the reactor could still supply plenty of electricity and heat to run everything...

Still, it may make more sense to just mechanically extract the ice in large blocks, stow them whole in a refrigerated cargo hold made out of a spent ET, and process only enough on site to refuel the space craft. This is where NTR would probably make sense...

But my intended use of bulk carbon monoxide for the Mond Process may preclude a trip to a carbonaceous asteroid should comet material not contain enough carbon in the form of methane or CO2 ice.

MITEE engine is interesting in many ways. However, I anticipate trouble with clustering a large number of nuclear engines: 1) reliability--all other things being equal, the probability of a failure increases with the number (complexity) of engines. However the probability of surviving a single point failure increases with increasing numbers of engines...
2) A tight cluster of small engines will couple neutron fluxes--the engines in the center of the cluster will recieve far more neutron flux leaking from surrounding engines than will engines on the perifary. What this means is that a large cluster of close set engines will behave to a certain extent as one large nuclear reactor. As such, then each engine will likely require a tailored fuel loading, a different degree of control rod actuation, and will generate differing levels of power. In effect, they will be different kinds of engines--this will be necessary for the cluster to function as a whole. So it makes more sense to slim down the design to two or three larger engines, and space them apart and/or add shielding between them to decrease this coupling effect. I have not run any simulations on this, but my limited understanding of nuclear physics indicates to me that reducing the number of engines and the spacing them out will be a safer, more controllable, and will as a result be more cost effective.

-- Edited by GoogleNaut at 10:29, 2006-11-07