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In this fictional world a top aerospace engineer has created a carbon neutral jetpack design through two main inventions.

First to improve the efficiency of fuel creation, she has created a more efficient Direct Air Capture (DAC) system using algae coated electrodes that extracts CO2 from collected air at a higher percentage than existing techniques by utilizing the hyper efficiency of algae’s natural CO2 absorption rate.

EDIT: This DAC system will be separate from the jet pack. It is creating the fuel going into the jetpack from a lab, not within the jetpack itself.

And second she has created a more efficient compression chamber system based on the flexible wing propulsion method seen in butterflies.

In summary, the DAC system allows for more extraction of Carbon using less air so more jet fuel can be created from the same air volume. The new compression chamber design will require fewer fans and structural parts which will make it lighter, thereby lessening the amount of jet fuel burned during use.

EDIT: Electrical energy starts up the jet engine fans initially.

So more fuel created, less fuel used.

  • Is it plausible to bio-engineer algae for this purpose and is there any research being pursued to enhance the efficiency of the compression chamber in jet engines?
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    $\begingroup$ I'm confused how these innovations are supposed to apply. How does CO2 capture make your jetpack more efficient? Or wing dynamics? (Does your jetpack fly using wings? Is it a jetpack if it doesn't have jets?) $\endgroup$
    – Cadence
    Jun 5 at 8:48
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    $\begingroup$ Hi @MJ Parker, you've put the hard science tag on this. That demands equations, citations, etc. It's our very most stringent tag and usually doesn't work well with soft or vaguely parameterised sci fi, which this seems to be. Should this instead be 'science based', that is, allowing guesstimates, rules of thumb, etc? If not, could you tell us the weight of the jetpack, thrust, fuel capacity, etc, as well as exactly how fast the algae can convert CO2 to fuel, and % efficiency, as well as any other specs you've figured out? $\endgroup$
    – user86462
    Jun 5 at 9:55
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    $\begingroup$ @MJParker But processing CO2 into carbon and oxygen, and carbon (and hydrogen) into jet fuel both require energy; if you're powering the jetpack by burning that fuel, you're losing energy overall. As for the compressor, I suppose you could design a compressor blade patterned off the butterfly wing (although there would be major challenges), is that what you mean? Or do you mean something about the chamber itself, in which case I don't follow? $\endgroup$
    – Cadence
    Jun 5 at 9:57
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    $\begingroup$ The main problem with extracting carbon from air is that there is almost no carbon dioxide in the air: at a concentration of 0.04%, carbon dioxide is a trace gas. (For comparison, even in the Sahara the air contains about ten times more water vapor than carbon dioxide.) One cubic meter of air contains about 0.1 grams of carbon; to extract sufficient carbon to make 1 liter of jet fuel you need to process something like 10,000 cubic meters of air. $\endgroup$
    – AlexP
    Jun 5 at 12:16
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    $\begingroup$ @MJParker It's not a matter of efficiency losses (although there will be those, too), it's basic thermodynamics: if your engine takes CO2, processes it, burns it, and expels CO2 exhaust, where is the energy coming from? $\endgroup$
    – Cadence
    Jun 5 at 18:13

3 Answers 3

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I think you have a hidden bottleneck here.

In order to capture carbon and make it available for combustion, algae need sunlight. Therefore, even assuming perfect yields in all the steps (which is not the case), your output power will be limited by the sunlight capturing surface you have available: you absorb 100% of the light, 100% of the light energy is converted into chemical energy of the captured carbon and 100% of that energy is converted into kinetic energy of the jet exhaust.

Considering that we get 1 kW per square meter, I suspect your jet engine won't propel that much, unless you equip it with a very large light capturing area.

To throw in some numbers, taken from KLM

For an aircraft like a Boeing 777 with two GE 90-115B engines each engine produces roughly 23 Megawatt of power during cruise flight with a fully loaded aircraft.

At the energy flow given by our sun at sea level, you would need at least 23 thousand square meters of surface to capture all the needed light.

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  • $\begingroup$ Can you back that up with science? You're right, but I do not think the question is answerable in its current state. $\endgroup$
    – Menno van Lavieren
    Jun 5 at 14:54
  • $\begingroup$ I wonder if there's some sort of heat pump type solution that allows greater than 100% efficiency. If the algae is merely moving carbon from the atmosphere into some other form, there is no reason why that theoretically must require at least as much energy as you'd get by burning the carbon. A solar-powered heat pump, for example, can provide more heat energy than is used to power it. $\endgroup$
    – Nuclear Hoagie
    Jun 5 at 15:07
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    $\begingroup$ @NuclearHoagie, As phrased, your suggestion sounds like perpetual motion. Heat pumps aren't more than 100% efficient, they just use solar power to move heat from outside to inside. The heat they release inside is actually less than the combined solar power + heat removed from the outside. $\endgroup$
    – Robert Rapplean
    Jun 5 at 16:26
  • $\begingroup$ @NuclearHoagie Heat Pumps work by moving heat that already exists arouund, they get super high efficiency because they aren't adding energy/destroying energy. They are just moving it. While there is spare heat to borrow throughout the world (as long as T > 0K), there is not random free floating jet engine fuel ready to be borrowed/used. $\endgroup$
    – Questor
    Jun 5 at 16:48
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    $\begingroup$ @NuclearHoagie, don't confuse heat with available energy. Coal is just solar power captured hundreds of millions of years ago. Food is solar power used to collect air, soil, and water, and combine them into useful molecules. When you consider all of the energy inputs and outputs, nothing is even 100% efficient. $\endgroup$
    – Robert Rapplean
    Jun 5 at 17:08
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You need to separate the fuel from the fuel production

Any way you add it up, using algae to make rocket fuel is just another form of solar power, and solar power is limited. While you could put your algae in some sort of wings to increase surface area as L Dutch suggested in his answer, you don't want them so big that they impede your daily activities too much; so, you probably have at most about 1 square meter of "solar panel" you can carry. On Earth, you get an average of 571.2 watts per square meter of sunlight at mid day with an average of 163.2 watts per square meter averaged over 24 hours, +/- a bit depending on your season and latitude. Biological photosynthesis is only about 1-2% efficient. The average human can only sustain about 20-200 minutes of direct sunlight a day depending on their complexion; so, depending on who's waring this thing, they are only going to get about 14-130kJ of solar energy absorption, minus what the algae actually spends on it's own biological functions. If we assume your protagonist is a person of medium complexation, who spends a fair amount of time outdoors, and that the algae needs to consume about 25% of its energy on maintaining itself, you are looking at about 50kJ per day.

Jet fuel has about 43,000 kJ/liter of potential energy meaning that it would take nearly 2 1/2 years of wearing this thing to produce a single liter of jet fuel which is not nearly efficient enough to justify walking around all day with a giant set of solar panels on your back. Even if you replace algae with much more efficient mono-crystal silicon based solar panels, you'd still be looking at only 910 kJ per day which is still too inefficient to justify the wings.

All this said, algae based jet fuel may still be worth producing in special solar farms, and put into a traditional jetpack. First of all, your algae is always going to be dead-weight if you make it part of the flight suit. It takes up extra storage space, water, and hydroponic equipment that you don't need to lug around if you produce your fuel separately; so, unless it is producing fuel in real time, it's not going to be worth the weight. Secondly, an algae pool does not care about how big it is or how much time it spends outside. Just by keeping your algae pool outside instead of strapped to your back, you increase your power generation to about 780 kJ per day. The real cost saver of algae over solar panels is not efficiency per square meter, but that it is self replicating. So, if you have a small 100x100m lake, you could fill it with the algae and produce 180 liters of jet fuel a day at practically no cost... enough to keep a whole platoon of jetpack troopers running regular missions. In contrast, a solar powerplant that could do the same thing would would be smaller, but probably cost a few million dollars.

Unless your algae is not algae...

So, all of this hinges on the idea that you have a photosynthetic tank of something on your back that produces jet fuel... but lets look at the goal instead of the mechanics. What the OP wants is a jetpack that produced jet fuel from the air. If instead of algae, if you have a nuclear powered device that turns the CO2 in the air into jet fuel, this idea might work. The air itself does not contain a significant amount of CO2... but the human breath does. The air that you breath out contains over 400 times as much CO2 as the air you breath in; so, if you assume you are collecting the CO2 from your jetpack trooper, instead of the air, you will have enough CO2 to work with.

The average person exhales about .2 kg of total carbon per day, and jet fuel is about 20% carbon; so, if you combine the CO2 from your breath with liquid water for the extra hydrogen and oxygen you need, then you can produce about a liter of jet fuel a day from your own breath, as long as you have a future tech power source like a miniature nuclear reactor to manage that much conversion.

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  • $\begingroup$ There are some radiotrophic fungi found in the ruins of Chernobyl that are able to use radiation for energy, so a little bit of mixing might let the algae use a radioactive source to generate the necessary energy previously gained from photosynthesis. $\endgroup$
    – Sean Duggan
    Jun 5 at 18:28
  • $\begingroup$ @SeanDuggan Certain radiotrophs certainly can absorb enough radiation for this, but it is unknown how efficiently radiotrophs covert radiation to energy, if they do at all. That said, it would make for a pretty a neat option. $\endgroup$
    – Nosajimiki
    Jun 5 at 19:03
  • $\begingroup$ It doesn't seem explicit from the question as it stands that the fuel has to be generated by algae in the jetpack. It could just be an algal tank at home that the scientist uses to refuel her jetpack. $\endgroup$
    – jdunlop
    Jun 5 at 20:15
  • $\begingroup$ @jdunlop True, but it is very strongly implied. "In this fictional world a top aerospace engineer has created a carbon neutral jetpack design" suggests that this technology is part of the jetpack. Otherwise, this would just be a question about carbon neutral jet fuel. Also the original question said "...Once the jet fuel is created, the jetpack will use this carbon neutral jet fuel..." Suggesting that the jetpack can go straight from making the fuel to using it... $\endgroup$
    – Nosajimiki
    Jun 5 at 20:48
  • $\begingroup$ If you're going to be carrying around a nuclear power plant on your back (!) why bother capturing CO2 and turning it into jet fuel? All jet fuel really does for a jet engine is make air hot. There's almost certainly a better, more efficient way to drive a fan using nuclear energy. $\endgroup$
    – Cadence
    Jun 5 at 21:55
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There seems to be some confusion about carbon neutrality vs. energy efficiency in this answer.

To make something carbon neutral, it has to capture as much carbon as it releases. Normally this would also include the energy required to manufacture the device, but let's just concentrate on the propulsion method.

If you use biofuel kerosine, then you are carbon neutral. If you're thinking of doing the entire process onboard your aircraft (or, worse, inside of a human-sized jet pack), then you're out of luck.

The minimum energy requirement to keep a human aloft is around 250 watts. Bright sunlight can generate up to 1360 w/m^2, so you'd think that you could generate that much power in .2 m^2, but then you have to consider process efficiency. The maximum theoretical efficiency of photosynthesis is roughly 11%, which brings your panel size up to 1.7 m^2.

You can put that 1.7 m^2 onto a wing, but then you need struts to attach the wing to the person, add a few control surfaces, and maybe a little safety equipment to keep bugs out of the pilot's teeth. This all increases the weight, which increases the wattage requirement, which increases the surface area, and all of that spirals upward (learn calculus, it's faster) until you have something like a small jet plane.

Now we have to add in the weight of the algae itself, its container, the liquid it's suspended in, the mechanism for circulating it and the CO2 it needs to interact with, the equipment for extracting glucose, and whatever mechanism you use to actually turn glucose into thrust.

Even by my most generous calculations, this quickly results in a violation of the cube-square law, where the weight required to support the surface area exceeds the power generation required to keep it aloft. You could do this if the entire system were lighter than air, but then you'd have a dirigible, not a jet pack.

And that doesn't even consider the maximum speed of reaction.

In summary, you can get carbon neutrality if you keep your energy-condensing photosynthesis on the ground. You can't get perpetual carbon-neutral flight from just sunlight on Earth.

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  • $\begingroup$ "The minimum energy requirement to keep a human aloft is around 250 watts." <- You seem off by an order of magnitude. The average weight of a human is 80 kg. Gravity is 9.8m/s^2 meaning the theoretical minimum is 784 watts. An decently efficient jet engine is about 30-36% efficient which puts you in the 2.1-2.6kW range. And of course it gets much higher than this once you add in the weight of any gear + jetpack. $\endgroup$
    – Nosajimiki
    Jun 5 at 18:19
  • $\begingroup$ @Nosajimiki, Citation added: shpfn.nl/human-powered-flight-2 $\endgroup$
    – Robert Rapplean
    Jun 5 at 19:02
  • $\begingroup$ Ah, winged flight works differently than a jetpack. Lift is a form of buoyancy, not a reaction force. Your momentum sustains lift which is only reduced by the rate of drag; if drag takes more than 1 second to slow down the energy you just put in last second, then you can accelerate to a faster speed than you were going 1 second ago and achieve a greater lift than you created at your lower speed despite sustaining the same forward acceleration. $\endgroup$
    – Nosajimiki
    Jun 5 at 20:33
  • $\begingroup$ @Nosajimiki, yup. And, as I mentioned in the text, the required surface area immediately puts us into winged flight territory, whether we want that or not. $\endgroup$
    – Robert Rapplean
    Jun 5 at 22:53

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