From: Billy Brown (ewbrownv@mindspring.com)
Date: Sun Aug 22 1999 - 11:34:11 MDT
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???). But since they are
> essentially rockets, they chould be quite efficient at providing
> thrust.
Rockets are terribly inneficient at producing thrust. Of hand, I can't
think of a worse method that is actually in use. Jet packs also have some
hefty engineering constraints: you can't have a high-temperature exhause
(because the pilot's legs are exposed to it), the pilot introduces a lot of
excess drag, you have to keep the speed low, and you have big problems
maintaining stability if the pilot moves any of his limbs.
> Now a human with a JetPack is going to weigh ~100-150 Kg
> (200-300 lbs), a car must weigh ~1000-1500 kg (2000-3000 lbs).
> An AirCar is going to have to carry about an order of magnitude more
> fuel than a JetPack. So with regard to fuel-to-weight ratios, the JetPack
> is probably better.
Aircar designs don't use rockets. They use jet engines, ducted fans, and
all sorts of other gadgets, all of which are far more fuel efficient than
rocket engines. They also benifit from better streamlining, and the
constraints on the size and shape of the engines are not nearly as severe.
Even ignoing all of that, the jet pack needs to carry several times its own
mass in payload, while the aircar does not. That jet pack is 200 lb of
payload, 30 lb of engines and 70 lb of fuel. The aircare can be 600 lb of
payload, 600 lb of vehicle and 1800 lb of fuel. Thus, the aircar is a far
less demanding problem even if you want to be silly and insist on using
rockets.
> I believe the estimates for fuel efficiency on the
> aircar are something like 12-14 mpg, so it isn't much worse than
> an old gas guzzler automobile.
That sounds like a number based on ducted fans, or maybe an efficient jet
engine. Rockets would consume fuel a lot faster (maybe one of our rocket
plumbers has some numbers handy?)
> I believe the statement has been made that rocket-to-orbit
> requires ~ the fuel of a transcontinential trip.
No, it takes a lot more than that. An airliner can cross a continent
burning fuel equal to maybe 10-15% of its mass. A rocket burns fuel equal
to ~90% of its mass to reach orbit.
> Conclusion (1): JetPaks seem doable if you want to go to orbit.
> Cross-country or other flights may be iffy without wings.
No way. Not without a fusion-powered thruster - and the pilot won't survive
the trip in any event.
In essense, the reason why jet packs are a useless curiosity is that rockets
are terribly fuel-hungry beasts, and a jet pack has no room for huge fuel
tanks. Air cars are doable in theory becasue you can use less fuel-hungry
engines, and because you have a lot more design freedom.
Billy Brown, MCSE+I
ewbrownv@mindspring.com
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