Air-car energy sources and a little Radioactive decay

Robert J. Bradbury (
Sat, 23 Oct 1999 07:53:03 -0700 (PDT)

On Sat, 23 Oct 1999, Spike Jones wrote:

> Ja, but I fear that figuring out how to weigh an atom would be
> profoundly difficult. It is a mystery to me how any nanobot
> could tell a 14C from a 12C.

Spike, you really have to read Nanomedicine! Section 4.4.3, pg 103, "Single-Proton Massometer". Its basically a very long beam. Stick a molecule on the end and measure the change in the resonance period of the beam. Its the same principle now used in quartz microbalances.

> OK, but natural diamond is pure 12C, is it not?

Nope, Handbook of Chemistry & Physics says natural C is 1% 13C.
> Objection Your Honor! The presence of 13C would not cause
> distortions in the crystal lattice. 12C, 13C, 14C are all the
> same size. Can anyone here provide counterevidence? Website?

Not "macro-scale" distortions, but "nano-scale" distortions. This is at the edge of my knowledge of materials science properties. The question is what are the fundamental causes of both heat and electrical conduction. Most of this is a property of the electrons, but there has to be some coupling of the electrons to the nucleus. When you heat something up it isn't just the electrons vibrating, its the nuclei as well. 13C is going to vibrate at a rate that is different from 12C so I would expect it to cause distortions in the propagation of thermal "currents".

> > I seem to recall that there is a company
> > providing isotopically pure Si for use in semiconductors.
> 32Si has a shortish halflife of 108 yrs, so if one makes anything
> really small with it, one would like to not have any 32 present.

I think the main reason has more to do with better heat conduction and perhaps electrical/insulating properties and perfect crystal lattices (which is starting to become fairly important).

> I suspect a more reasonable approach is using small gas turbines.
> The first practical personal VTOL will surely be done with 2-stroke
> gasoline powered motors, like that two-ducted-fans arrangement
> those guys from Santa Clara are building. Whaddya think?

Quite probably. Our engineering is at a much higher level with these devices at this point (100+ years worth). I do seem to recall however that in the mid-50's there was some research done by the air-force into nuclear powered aircraft (bombers forever in flight, etc.). If our nuclear engineering gets to the point where our gas-engine/turbine engineering is now we might see nuclear air-cars or rockets. Dyson worked on Project Orion involving atomic bomb propulsion for interstellar travel. See: "Death of a Project", Freeman Dyson, Science 149(3680):141-144 (9 July 1965)

Of course, you can apply some "counter-intuitive" thinking here. If you can't engineer the car, engineer the driver and passengers. We probably want much better shielding and certainly better DNA repair in our cells anyway. Shielding the nuclei of each cell or simply letting the radiation hit the DNA and repairing it may be better solutions than weighing the air-car down with a lead shield around the engine.

Nuclear power is worth consideration because of the higher power output to fuel weight ratio. Now that I'm thinking about this it certainly seems like you could apply the same principles for nuclear nanobot power cells in your body (148Gd) to the air car and eliminate the shielding hassles. Nanomedicine discusses using higher energy power sources 210Po or 238Pu with some additional shielding. Robert says a 0.2 kg (~1 inch^3) block of 148Gd yields ~120 W for 100 years (NM pg 157). Lets say I need 500hp for the air-car. That works out to ~372000W, so it looks like I need a 15 inch^3 block of 148Gd. This weighs 675 kg, so it isn't light and doesn't take into account the shielding requirements. So you probably have to go to a "hotter" material or have some buffering mechanisms. [The car sitting overnight in your garage uses the energy being produced to charge up an array of nano-flywheels]. Since the flywheels have an energy storage density of 5x10^10 J/m^3 the question is how big do you want your "gas" tank? Say I want 10 hours of flight time, that means I need 1.3x10^10 W. So something around the size of a gas tank or engine bay worth of flywheels will store the energy required. The flywheels weigh a lot less than the nuclear power sources as well (carbon vs. 148Gd or something even heavier). For comparison purposes gasoline works out to 2.8x10^10 J/m^3, so the flywheels are about twice as dense.

NM (pg 141) discusses energy densities and power quite a bit. Since for flight we want the greatest power-to-weight ratio Robert says that is H2 @ 1000atm pressure. That gives you 4.9x10^9 J/m^3 density and 1.2x10^8 J/kg weight. If you want to fix the density problem (since this makes for a big fuel tank that drives up drag), you need to go to either solid-H2 (at 57,000 atm pressure or frozen) for 7.2x10^10 J/m^3 or burn diamond at 1.2x10^11 J/m^3.

It is interesting though that we can't get too much better than gasoline with either figure of merit (less than an order of magnitude). The only way around that would seem to be NASA's atomic H fuel.