From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Fri Aug 20 1999 - 21:01:12 MDT
> hal@finney.org writes:
>
> But jetpacks only last 30 seconds. The fuel is not energetic enough.
> And adding more fuel adds weight. It is questionable whether these can
> ever work.
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. 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. 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. I'm pretty sure this is because the
air-car isn't designed to provide much lift.
I believe the statement has been made that rocket-to-orbit
requires ~ the fuel of a transcontinential trip. Lets see,
a Boeing 767-300 in "tour" configuration carries 350 passengers with
91,370 L (24,140 gal) of fuel, so that works out to ~261 L (68 gal)
per person for a 11000 km (7000 mi) trip (~3x cross country). The
767 also has a max takeoff weight of ~500 kg/person (much greater than
a JetPack). The fuel efficiency of a 767 is ~43 person-km/L
(102 person-mi/g). [Kudos to the Boeing WWW site for making
available the 767 parameters so I could do the calculations!]
It seems JetPacks are a big win in terms of both the fuel-to-weight
and theoretical thermodynamic efficiency of the power plant.
I can't see why 10-30 minute "commutes" should not be possible
with JetPacks. Since you can eventually go to LH2/LO2 it only
gets better. The 767 will get better than air-car mileage
due to the greater thermodynamic efficiency of the jet engines
over internal combustion engines but the rocket on the JetPack
is theoretically better than either of these). I would presume that
most of the 767's fuel efficiency is due to the "lift" provided by the
wings. I'll leave it to one of the rocket/plane folks to comment on
the fuel efficiency of helicopters (which are still getting some "lift")
and the tradeoff between wing "lift" and increased frictional losses.
Conclusion (1): JetPaks seem doable if you want to go to orbit.
Cross-country or other flights may be iffy without wings.
Conclusion (2): I think I have to stop reading books by Eric
& Robert because I'm beginning to sound like them.
I think the two biggest problem I have with JetPacks is that
(a) you are strapping a bomb on your back and (b) there is
nobody to serve you drinks.
I think that there may be an additional problem in the JetPaks
that to avoid wasting fuel, you want a parabolic trajectory
at relatively high speed from point-to-point (to minimize
the fuel spent resisting gravity, balanced against the
losses from friction at higher velocities). The takeoff may be fun
but the landing is going to be pretty scary --
"... using nanotechnology we have produced a NanoRocketPack and
solved the problem of safe, reliable, inexpensive high-speed
personal air-transport... unfortunately we haven't determined how
to prevent the heart-attacks people have when they activate the
fuel-conservation/environmental-friendly trip option."
On Fri, 20 Aug 1999, Eugene Leitl wrote:
> An array of MEMS microturbines should be able to go a lot longer than
> just 30 s. Difficult to predict how silent (or loud) such a thing
> would be.
At the 1998 Foresight Senior Associates meeting, JoSH discussed
the work he was doing with NASA on theoretical aircar designs.
I believe he mentioned that noise scales with air-speed to the
*8th* power. So the real question becomes how fast do you have
to move the air for vertical takeoff. If you can move a lot of
air "slowly", then it may be quite quiet. He did comment however
that noise was going to be a significant limitation in these
designs.
>
> And, of course, things look even better with nano. (However, if you
> have nano good enough to do a jetpack, there are no monkeys left to
> lift which somewhat defies the purpose).
>
Unfortunately true. Virtual JetPacks seem much easier than the
real thing.
Robert
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