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it seems to me that the only way to produce a jetpack with a reasonable flight time and no heavy machinery is to use a complex compound which violently and exothermically dissociates into a range of very simple gaseous compounds or elements which can then be fed out through a nozzle. that would minimize the amount of fuel required for a reasonable flight time, maximise thrust from minimal fuel, and if set off by heat or catalysis, there is no complex machinery.

 

unfortunately, the only thing that i can think of is some kind of heat-sensitive liquid-form primary explosive, which could be fed into the (titanium-ceramic? :P) combustion chamber droplet by droplet past a small gas burner (may be extinguished by detonation) or electric element (probably fragile).

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for forward flight, certainly. but then you'd need a runway/undercarriage for takeoff- unless you mean that you'd simply dial back the thrust once airborne and at sufficient velocity, in order to conserve fuel. which could work, and still preserve VTOL/hovering capability. however, there's still the question of where these wings would deploy from, since it's a jetpack, not a turbofan hang glider. minimal size and portability must be maintained.

 

i have heard, actually, that it's possible to make petrol detonate, which could perhaps be a solution to the tanks-of-high-explosive problem. although no doubt there's a critical mass involved, so it couldn't be done on a small but continuous scale.

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The FJ22 and the GEN H-4

There are a few different types of jet packs out there, but Ive noticed there main problem is fly time, like the fly time on the basic Hydrogen peroxide jet pack is a few min at best. They are also working on a petrol jet pack, but its got like a 3L engine and is completely carbon fiber and weighs 200kg with out person, and only has a small fly time, very unpractical.

The basic problem with true jet packs – small instances of the class of heavier than air aircraft that don’t have wings, rotors, or other aerodynamic lifting surfaces – is that they’re fuel inefficient. A good ballpark number for small turbofan engines, even high-bypass, efficient ones, is about 0.5 units of weight of fuel per hour of operation per unit of thrust (eg: .05 kg fuel/hr/N). (source http://www.flyingmag.com/very-light-jet/technicalities-speaking-jets)

 

So, starting with an efficient small turbofanjet like the Williams FJ22, with 750 lbs of thrust, you get something like this:

85 lbs engine

50 lbs frame & system

175 lbs pilot

350 lbs fuel

About 1 hr flight duration

 

At nearly 500# fueled weight, this isn’t the sort of “jet pack” you’d be able to walk around carrying. Reduce the fuel to 90#, and you have a 220# pack that you could walk with (though not far or fast), but your time aloft is reduced to 15 min.

 

To get better efficiency, you’ve got to have some sort or airfoil. Assuming you want vertical take off and landing capability, this means some sort of small helicopter. We don’t have to speculate about what such an aircraft would be like, as you’ve been able to buy one since around 2000: the Yanagisawa GEN H-4.

 

This tiny (155 lbs empty) machine is limited for legal reasons to only 5 gallons of gas/oil (2 stroke) fuel (30 lbs), giving it a duration of 1 hr, but at 6 lbs/gallon, you could clearly extend its flight duration as much as you’d likely want. It’s top speed is about 120 MPH – about as fast as your likely like with your body exposed to the breeze!

 

The GEN H-4 is also rather gawky and awkward, with wide-spaced legs/footrests, the better not to tip over and break the expensive thing while learning to fly it, but again, I think this design decision was based on its role as a first-of-its-kind instructional/recreational vehicle. Something like this could be made much more compact and gainly – though even with more, shorter rotors (the GEN H-4’s are 13’ diameter), you’d need a reasonably large takeoff and landing space.

 

it seems to me that the only way to produce a jetpack with a reasonable flight time and no heavy machinery is to use a complex compound which violently and exothermically dissociates into a range of very simple gaseous compounds or elements which can then be fed out through a nozzle.

unfortunately, the only thing that i can think of is some kind of heat-sensitive liquid-form primary explosive …

Most high explosives have very low energy densities compared to ordinary fuels. TNT, for instance, has about 4.6 MJ/kg, C4 about 6.2, some exotic compounds 10 or so, compared to 43 MJ/kg for kerosene. Explosives burn fast (and without external oxygen), but not efficiently. (Source: the oft-cited wikipedia article energy density).

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  • 5 months later...

I saw a cool video on youtube about a water jet pack that can fly for several hours. It's powered by a jet ski like device in the water with a fire hose to carry the water to the jet pack. It can run under water and above water. Limited in speed and altitude due to the length and weight of the hose and the need to drag the power source through the water. Cost is comparable to nearly all other rocket and jet packs, about $100,000. Looks like a blast to fly though.

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