Re: JetPacks vs. AirCars

Philip Witham (
Sun, 22 Aug 1999 15:08:45 -0700

Massive mis-information on this thread, I'm afraid. I'm very surprised. Billy Brown and Spike got it right. Here's a few comments:

Rocket "efficiency" is rated is Specific Impulse, or ISP. For the sort of propellants in JetPacks, this is in the range of 100-200 seconds. This means that one KG of propellant will produce one KG of thrust for about 150 seconds. To provide one minute of sufficient lift, say, 150KG, would require about 60KG of propellant.

For a comparison to helicopters, the Ultrasport 254 is a one-seat ultralight machine that uses a 55HP two-stroke engine. It weighs 115KG, so with 85KG of pilot and fuel, it's developing 200KG of lift. On 5 gallons of gas it goes for 1.25 hours at 63 MPH. This works out to a specific impulse of 66000! 200KG * 1.25hours * 3600seconds/hr / 13.6KG (of gas) = 66000 Seconds.

Even if the jetpack gets you flying an average of 277MPH (about max for a jet pack, as a guess, based on the terminal velocity of a falling human), the helicopter is still 100 times as efficient. Little wings on the jetpack would improve things by only about 5x. A ballistic trajectory with the "JetPack", with oxygen for the spam - I mean pilot - would help by taking you through something like 20,000 feet apogee where the air is a bit thinner.

(See for info about these helicopters, by the way, very well designed machines by the reports I've read). A future-tech vertical takeoff machine could be at least three times as efficient, with only modest improvements in the engine and structure. A number of two-seat airplanes get 30-50MPG at 150-200MPH, as a side note.

As for noise, nothing would be noisier than a rocket pack, with it's supersonic exhaust jet. This is what makes rockets so much fun to test! The more noise the better! Earth-shaking, chest-pounding, ear-damaging NOISE! Yes! 8-0 ...Er, uh, must calm down... Ahem. Most of the noise from a helicopter is from the tail-rotor and the engine. If you generate the lift from a large area rotor (low "disc loading", as in a lightweight helicopter), the noise due to making lift is quite low. This is because of the low velocity in the rotor downwash.

And, yes, Spike, I think a nanotech Aircar can have a low rotor disc loading like a helicopter, even a non-nanotech aircar. By the way, a ducted fan like the Moller designs use has an effective area of the inlet area of the duct, if it's done just right, not just the fan area. Not that I like the Moller designs. I want 200,000 rotors about 7mm in diameter, spinning at >10,000 rotations per second (two blades each), to get the fundamental rotor noise frequency above human hearing range. This works out to be the area of a Cadillac or standard parking space, and a disc loading like a helicopter.

One of these days I'll write up an aircar paper for this group - I've been running simulations of automated air-traffic control - self piloting aircars - fun stuff.

Mr Osborne: the Army's flying platform you mention was probably the Hiller flying platform. Unfortunately the Flying Contraptions Page seems to have been converted into photos of a baby girl. Yech.