Jet Pack
Jet packs are often things that are struggled with, in a way to try to develop a working mechanism to achieve flight. Jet packs often represent the pinnacle of what could be available to the common man, allowing the average person to fly, or say, fly to work, without the need of an overly large or expensive vehicle. While many solutions have been proposed, and few have succeeded due to practical limitations, I do have a few ideas to how one may work.
One problem with Jet packs seems to be the rocket. Rockets are incredibly inefficient, for many reasons, a large reason being the needed for an internal oxygen source. Since oxygen is relatively heavy compared to other elements often used in combustion, such as hydrogen or carbon, oxygen makes up the bulk of the weight of fuel, and therefore a limit to how long a jet pack can fly (probably, the single largest limitation to jet packs is range and flight time, compared to relative cost). Due to it's large mass in comparison, with the oxidizer of rocket fuel causing the rocket to be roughly 3 times heavier than standard jet fuel, eliminating an internal oxygen supply, or perhaps a molecular one, may make an engine design practical for sustained flight without exceeding the gratuitous weight limit for flight, which could make practical flight impossible if too high, more weight requiring more fuel.For comparison, an F15 is approximately 28,000 pounds, and uses approximately 13,500 pounds of fuel, or around half it's mass in fuel.[1][2] If extra oxygen was required, this might require approximately 40,000 pounds of fuel, significantly more than the mass of the plane.
Additionally, rocket engines by themselves are generally relatively inefficient. General turbine jet engines, that combine oxygen from the air with fuel, tend to be significantly more efficient in their own right, due to their design. A standard rocket efficiency is around 5.9%, whereas a conventional, modern, air-breathing jet engine has an efficiency of around 35%.[1] Thus a conventional jet engine would require six times less fuel to achieve the same distance or time traveled, based on fuel efficiency alone. Generally, rocket fuel is 3 times heavier than standard jet fuel. If the mass of oxygen is factored in, with say, traditional kerosene jet fuel being used, compared to hydrogen peroxide, or a liquid oxygen and hydrogen fuel mix, the kerosene jet fuel absorbing oxygen from the air, then 3 times more fuel is required on this basis alone, given that 3 times more energy is present by using oxygen from the air, rather than within the fuel source. Thus, allowing for approximately 18 times the distance to be traveled, by simply using a more efficient design.
For a base comparison, a Jet pack produced for the U.S. army, for instance, the Bell Rocket Belt, was able to fly for approximately 30 second (in it's improved variant). Therefore, a jet engine design, operating on the simple notion of improved fuel and improved engine efficiency, could theoretically fly for roughly 9 minutes, if it otherwise carried the same properties (aerodynamics, etc.), with a 200 pound person, approximately.
Therefore it is imperative to utilize more efficient engine designs, and more efficient fuels, incorporating external, ambient sources of oxygen, to allow for longer, and more practical flight times. Wings are also an important consideration, considering that wings would allow for higher fuel efficiencies as well, where as traditional rockets (and jet packs, like the Bell Rocket), simply propel themselves without the assistance of wings. By utilizing wings, the design could theoretically be even more efficient, from 2.5 times more with a glider wing suit, or 15 times or more with a proper high efficiency glider.
Fuel
Using something other than rocket fuel, for various reasons, can provide a higher energy to weight ratio for fuel, which takes up the bulk of the weight in most flying devices.
The energy density of Kerosene, or jet fuel, is comparable to gasoline, at around 46 mega joules per kilogram. Jet-A, the standard commercial jet fuel, which must reach ASTM specification D1655 (Jet A), designed with a relatively high flash point, and a number of other safety issues including a stable burn, has around 43 mega joules per kilogram.[1][2][3] I am unsure of the exact energy density of various kind of rocket fuels, although they generally tend to be three times denser than standard jet fuel, producing significantly less energy in terms of over-all mass. This is because they utilize stored oxygen, rather than oxygen from the air, which increases the mass of the fuel. 1 gallon of gasoline for instance produces roughly 20 pounds of carbon dioxide[1]; despite being around 6.8 pounds itself, the oxygen from the air represents the bulk of it's weight in mass, thus producing a 1 to 3 ratio in terms of weight. This means that utilizing the open air jet engines could, based on fuel alone, increase efficiency (in terms of mass) by some 3 times over current designs.
Hydrogen has an even higher energy to mass ratio than kerosene, however. At 43 mega joules per kilogram to hydrogen 123[1][2], hydrogen is approximately 2.85 times more energetic than kerosene in terms of it's energy to mass ratio. This could allow for around 25 minutes of uninterrupted flight using the same basic size and design jet pack, based off of these aspects alone.
However, hydrogen has many drawbacks. Hydrogen is highly volatile, having a potential air to fuel ratio of 4-75%, compared to say gasoline at 6-12%, and a low ignition temperature and energy requirement, capable of being set off by sunlight in the presence of oxygen. Since the oxygen ratio can be extremely variable, hydrogen, even in small amounts, or gratuitous amounts (presumably snuffing out the oxygen) can erupt in giant fireballs. Even at 700 bar, a maximum safe range for a high strength container, hydrogen is still 6 times more voluminous than jet fuel or gasoline, taking up significantly more space and also requiring a high pressure container, which is usually heavy. Since hydrogen is often created from natural gas, and wastes over 40% of the energy, it can be wasteful to produce economically, as well. As a result, hydrogen may not be an ideal fuel source, but it does theoretically provide the highest weight to fuel ratio available, if other factors can be eliminated.
Wings
Wings, perhaps, shed light on how to create the largest range or time of flight increase. Since most rockets lack wings, they generally can be ignored in terms of relative gliding ratios in comparison; particularly, compared to the original jet pack used by the military, or similiar designs.
Right off the bat, a wing suit, that is a commercially available wing suit, can provide a glide ratio of roughly 2.5 to 1, or extend the life of flight by 2.5 times. [1][2] This means that by wearing an awesome glider/wing suit, you can potentially fly 2.5-3 times longer than without one. Ignoring the potential power of hydrogen, this would allow for roughly 27 minutes in flight, or around 75 minutes with hydrogen.
A plane typically has around 12 to 1 glide ratio, while a hang glider has some 15 to 1 ratio. It's easy to see how 30 minutes, multiplied by 12 or 15 could result in a several hour flight, allowing a substantial distance to be crossed. Assuming a substantially efficient glider, even less fuel could be used if a current could be caught, allowing the engine to simply propel the glider up to the maximum height required to catch a current, therefore allowing for a more capable glider operation, or safety issues in case of gliding issues.
Thus if a hang glider, wing suit, or other available high glide ratio is utilized, than even less fuel would be required to travel the same distance. This could be anywhere to 6-8 hours, or more depending on the efficiency of the design. Sail planes for instance often have a glide ratio of 45-70 (although they are substantially larger, and, large planes, of course).
In Conclusion
Jetpacks would be awesome. Getting around the house, on top of the roof, flying to the grocery store, or any other variety of tasks where flight would be useful. Allowing oneself to fly while not being hampered by a large vehicle, allowing hands to be used, could be incredibly useful. Obviously, some kind of suit, protection gear, and helmet and goggles would be required in order to be safe.
The bell rocket belt, used at the Olympics, was later revived in 2000, and was capable of approximately 30 seconds in flight, a maximum speed of 60 mph (96kmph), and a distance of around 350 meters, at 60 kilograms (132 pounds) with 6 gallons of fuel. I am unsure how efficient the design was, the exact energy density of the fuel (water being present in the diluted form, the efficiency of conversion and the catalyst etc.), or other such factors, but it stands to reason that it is not that far off from the basic estimates.
With the initial factors considered, a 6 times more efficient engine, and 3 times more efficient fuel, this would give the jet pack approximately 9 minutes of flight. A jet pack comparative to to the bell rocket, H202-Z
flew for approximately 33 seconds[1][2], while jet pack Jet pack T-73, using a jet engine type design, and kerosene, flew for approximately 9 minutes, indicating that the general concept of an 18 times longer flight time probably holds true in most scenarios.
This means that with a 3 to 1 glide ratio, with a simple, small glider, around the capabilities of a glider suit, a person could theoretically fly for roughly 27 minutes, or around 30 minutes. With a 12 to 1 ratio, similiar to a boeing 747, you could fly for approximately 6 hours. Given the longer time in flight, this could mean a significantly farther range. At 60 mph this would be approximately 360 miles, or perhaps 150 miles to your destination and back, shorter than your average trip to work.
However, the T-73, could fly approximately 18 km; however, it's speed was somewhere around 80 miles per hour, and a slower speed could mean a somewhat higher efficiency. In any case, the maximum range, if 12 times more efficient, would be approximately 216 kilometers, or 135 miles. This would easily be to work and back, or to a store; so 60 miles to a destination, and 60 miles back. While relatively heavy, at around around 130-200 pounds, these jet packs could essentially be significantly lighter and more practical than planes. With a glider suit, the range could be some 45 kilometers, or 28 miles, for a 14 mile trip there and back.
If scaled down to 1/6th their size, if this is potentially possible, it could be a more reasonable 20 to 30 pounds. While it would have a shorter time in flight, this could still be 22 miles. Using a poor glider suit instead, that would be approximately 4.6 miles, and even 4 times smaller, it would be 5-6 pounds, with a range of about a mile. Quite a feat for previous jet packs that would have otherwise been only able to travel a few hundred meters, and last for 30 seconds.
This of course dependent on a variety of factors. But in short, by utilizing wings, a more efficient design, and fuel, it may be possible to travel significantly farther than current jet packs allow, allowing them to reasonable forms of transportation, or at least for moving around the house.
