> > hal@finney.org writes: ...But jetpacks only last 30 seconds. ...
>
> Robert J. Bradbury wrote: I'm not sure that I buy this. As I recall, the
> fuel in a JetPack was fairly low energy (perhaps ammonia based???)...
I did some calculations on air cars after reading Engines of Creation several years ago. Suppose nanotech gives us materials that have enormous strength. What then could be done?
As it turns out, the sci-fi version of aircars with four horizontal rotors is pretty hopeless even assuming superstrong and light materials. It takes enormous amounts of energy to spin small diameter rotors fast enough to create even moderate lift, even if one is given freedom from normal strength of materials considerations.
In fact, we already have air cars in a sense: helicopters. Of course these are inconvenient, since their rotor diameter is so large, but it helps illustrate the fact that as rotor diameter decreases, it takes relatively more energy to maintain hover. The human powered helicopter has an enormous rotor.
The amount of wind and noise involved in lifting an aircar is considerable. Our example in nature would be tornadoes, which sometimes lift cars. Of course a highly directed rotor would be far more efficient than a tornado, however one is still likely to wreck ones neighbor's flower bed on takeoff, and their cat may *never* be seen again. {8^D
There is a version of fighter jet now in the engineering development stage which achieves vertical takeoff and landing via a horizontal rotor in each wing. kewwwalllll. {8^D I seriously doubt we ordinary proles will have anything like that until nanotech changes everything. spike