Re: JetPacks vs. AirCars [was Re: Brain Implants & Technologies]

Doug Jones (
Sun, 22 Aug 1999 13:04:51 -0700

Billy Brown wrote:

> Rockets are terribly inefficient at producing thrust.

Yep- and the H2O2 rockets in jetpacks are particularly poor, with an Isp of about 120. That is, to get 120 lbf of thrust, you have to burn 1 lbm of propellant *per second*. Even a poor bipropellant is over 250, LOX-kerosene can reach 310 at sea level, or 350 in vacuum.

> Jet packs also have some hefty engineering constraints: you can't
> have a high-temperature exhaust (because the pilot's legs are
> exposed to it),

That's actually not all that bad- the nozzles are canted outward a few degrees, and the cosine loss isn't much. Of course, even 85% peroxide exhaust ain't very hot anyway.

> the pilot introduces a lot of excess drag,

100% parasitic drag!

> you have to keep the speed low, and you have big problems
> maintaining stability if the pilot moves any of his limbs.

These two are coupled- rocket packs have only two controls, throttle and a *very* weak yaw control; pitch and roll are controlled by body english, very intuitive (you lean in the direction you want to go) but with limited control authority. Since the center of pressure is below the center of mass, if your forward speed gets too high, you just tuck under and fly into the ground. Bad juju. The recovery method is to bend the knees backward 90 degrees, giving reduced drag on the lower body and max offset cg for pitch up. I wouldn't want to explore the high speed end of the flight envelope myself, even if I do build and fly a high performance rocket pack.

> 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 benefit from better
> streamlining, and the constraints on the size and shape of the
> engines are not nearly as severe.

Yep. Making the entire system man-portable is a real bear. Ducted fans can achieve an effective Isp of over 10,000 thanks to using air for combustion and as abundant reaction mass.

BTW, there is an outfit working on a dual ducted fan flyer, but it has a pedestal which the system rests on when not flying. See Their system seems to have large control authority for the fans, swinging through twenty degrees or so. It seems that since the pedestal is also a footrest for the pilot, the intuitive weighshift method cannot be used and manual controls are needed- pity.

> Robert J. Bradbury wrote:
> > 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?)

Rockets are inherently best for acceleration missions, not cruise missions. The rocket equation,

dV = ln(MR)*Isp*g

gives the velocity change that a given mass ratio and exhaust velocity can produce. Since hovering requires 9.8 m/s delta-V per second, hover time is

t = ln(MR)*Isp

with a mass ratio of 1.5 and Isp of 120, that comes to just 48 seconds.

> > 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.

Actually, an SSTO would need about as much jet-A as an airplane of comparable dry mass. It's the oxygen the rocket carries that masses 2.6 times as much as the fuel... For a mass ratio of 17, the propellant fraction is to be 94%.

> > 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 a conversation with Eric Drexler, he suggested a launcher carrying a person in a spacesuit, launch mass about 2000 kg. As the rocket climbs, the tanks are disassembled and fed into the fuel pump, and the engine gradually shrinks. The passenger arrives in orbit with just a leetle tiny rocket pushing along... the last of the propulsion system masses maybe a kilogram. Add ten kg to the payload for a reentry shield.

> 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.

Good conclusion, I agree.

Doug Jones, Freelance Rocket Plumber