Robert J. Bradbury wrote:
> The lack of a requirement for oxidizer in air-breathing rotors/turbines
> would appear to be a big part of the problem. I was thinking primarily
> of the thermodynamic efficiency and assumed that rockets had to be
> much better, apparently, not good enough to offset the additional
> weight of the oxidizer.
The other issue is that rockets use their fuel as reaction mass, while prop engines do not.
> The question becomes, can an Air-Nanoturbine-Rocket launch without
> oxidizer and turn itself into an Air-Nanoturbine-Ramjet-Rocket
> in the atmosphere on the way up (so you gather the oxidizer
> necessary for the final push to orbit above 40,000 ft)?
You are probably better off using a sail of fancloth to take off. Use electric fans to get above much of the atmosphere and accelerate to near the speed of sound, then switch over to jet engines. Once you've gathered a full load of fuel and oxygen you can switch to jets (I recommend one of those fancy new hybrid ramjet gadgets) until you're too high for them, and only then do you switch to rockets.
> And does this get me personal ground-to-orbit transport
> for less energy cost than a cross-country trip?
With nanotech, neither energy expense is significant. If every artificial surface on the planet does double duty as a solar power collector, we've got power to burn.
> But we can *build* a car that runs on water. Put the water in the
> tank, let the solar cells turn it into H2, run the H2 into a fuel
> cell and then drive off. We just can't build one at the same
> "cost" as current automobiles. This is in large part a problem
> of production volumes. If we could produce the fuel-cell cars
> in the same volumes as production model cars, the costs would
> be much lower. Historically there may have been problems with
> fuel cell materials (e.g. platinum), but these are either solved
> now or could be solved by recycling.
Well, it really runs on sunlight in that case. The result would be a car that has to spend several hours recharging for every hour of use, which would make it impractical as a consumer device. Not everyone needs to be able to run the car 8 hours per day, but we all want to know we can do so when we need to.
> The problem isn't with the science, it is with the unflexible
> manufacturing infrastructure.
That seems a bit unfair. Economical production of internal combustion engines was possible quite a while ago. Economical production of solar power collectors and fuel cells is a bit iffy even now, and certainly was not possible as recently as the 70s. High-speed recharging of electric vehicles is still problematic, as is storing hydrogen.
I'm not sure how one would calculate the hypothetical 'volume production' cost of such a vehicle with any precision. However, it seems clear that there are still technical hurdles that need to be overcome before such production will be practical, and it is not at all clear that the overall production cost would be lower.
Billy Brown, MCSE+I