7 ms·

> What were we expecting from jetpacks though, really? Is it not more fair to suggest that the past was a little bit naïve about what it would take to operate

> What were we expecting from jetpacks though, really? Is it not more fair to suggest that the past was a little bit naïve about what it would take to operate a jetpack? They're still aircraft after all, at the very least you're going to need pilots license. You need a pilots license, but you don't need fine motor coordination skills across your entire body PLUS a lot of core strength, in addition to all the problems with moving in 3d. For this 'jetpack' you are essentially balancing your weight between your back and arms. If you get it wrong for a split second, you'll kiss the ground. It's inherently unstable and tiring since you have jet turbines strapped to your arms. An aircraft is stable. Let go of the controls and it will keep flying. The closest aircraft comparison would possibly be with a hang glider, except even there your weight is supported and you are essentially just shifting it around. Many jetpacks in fiction were envisioned to work more like drones. Tell it where to go and it would do the rest. Iron Man would be similar to this thing, except it is described as a fully mechanized suit (plus artificial intelligence), which magically takes care of some of these problems. Ultimately, 'jetpacks' will probably never exist. There's an inherent physics limitation when trying to strap a whole human body to the side of something small (and generally depicted as being strapped on your back, Bobba Fett style). The center of mass is off.

rictic 5y ago
If battery tech continues to improve at current rates then certainly electric jetpacks will be possible and practical, if there's still demand (and we don't destroy ourselves one way or another). Without doing the math though it might be many decades.
- alisonkisk 5y ago
- mdavidn 5y ago
  Unlikely. The specific energy of jet fuel is more than 40 times that of batteries, and I don't see batteries improving by an order of magnitude. A jetpack would need 40 times more battery mass than fuel to produce the same thrust for the same duration. Of course, the additional weight of the batteries necessitate even more thrust...
  - earleybird 5y ago
    Nailed it. Even in cars where there they do a bunch of gliding (rolling) batteries are just becoming plausible (looking back over time).
  - kragen 5y ago
    Existing aluminum-air batteries have 1300 Wh/kg, or 4.7 MJ/kg in modern units, and it's plausible they could improve to over 20 MJ/kg. The more expensive lithium-air battery already exceeds 6 MJ/kg. Jet fuel is 43 MJ/kg. Pure aluminum is 31 MJ/kg, so to the extent that you can build "new Al-air batteries" during flight with the cathode and electrolyte from "old batteries", you can approach that limit. That sounds ridiculous but we've been doing it with hydrogen-oxygen fuel cells since 01838. Hydrogen has 118 MJ/kg, and there's no fundamental physical reason hydrogen-air fuel cells can't be epsilon above that, though current models are large and heavy. It's true that all rechargeable batteries like Li-ion batteries are so far under 1 MJ/kg, which more than 40 times worse than jet fuel just as you say, but it's extremely plausible that Al-air could beat Li-ion by an order of magnitude, and Li-air already does, except for the best Li-ion cells. It's already most of the way there. And fuel cells can beat jet fuel by almost a factor of 3, and for low-power applications they do. Betavoltaic cells and TEGs have even higher energy density but you can't turn them off, so you need to dissipate the power they produce when you're not using it to fly or whatever. You could build a hybrid system with a large glide ratio that uses chemical batteries for takeoff and a betavoltaic battery or TEG for long-distance cruising, but you might have to only land it on water to keep it from overheating while it was on the ground. https://en.wikipedia.org/wiki/Aluminium%E2%80%93air_battery https://en.wikipedia.org/wiki/Aluminium%E2%80%93air_battery https://en.wikipedia.org/wiki/Energy_density#In_chemical_reactions_(oxidation) https://en.wikipedia.org/wiki/Energy_density#In_chemical_rea... https://en.wikipedia.org/wiki/Energy_density_Extended_Reference_Table https://en.wikipedia.org/wiki/Energy_density_Extended_Refere...
    - FabHK 5y ago
      As you say, practicable batteries in production have specific energy under 300 Wh/kg (1 MJ/kg), 40x worse than fuel. The other technologies you mention don't seem to be practical for now. I don't think it is reasonable to expect any breakthrough making it to market in the next decade. With specific energy doubling every 10+ yrs, electric flight might be sensible for niche applications (training flights, occasional urban transport for example to the airport) in a few years, but for anything else it'll be several decades realistically.
- FabHK 5y ago
  > If battery tech continues to improve at current rates What do you consider current rates? They improve slowly, maybe 3-7% a year. Last I looked, batteries are so much worse in terms of specific energy than fossile fuels that we're looking at several decades before electric flight becomes practicable (except for very short distances and training flights).
  - rictic 5y ago
    7% a year means doubling every decade, 3% means doubling every two. Fossil fuel efficiency isn't moving at nearly that rate. If current rates continue, battery powered jetpacks should be compelling within 60-120 years
dreamcompiler 5y ago
The stability issues could easily be fixed with more closed-loop control. The much more serious issues are heat and the limited fuel supply.
- jonathankoren 5y ago
  Agree. This is the same reason why I'm skeptical on flying cars. There's so many fender benders. I don't want a Jetta crashing through my roof. Sure, general aviation is safer than driving (e.g. less accidents per vehicle, and less accidents per hour/mile), but general aviation is more dangerous than commercial aviation. I suspect there's something going on with maintenance, training, and available safety features. Seems like lowering the barrier of entry to flying is simply going to increase the accident rate because there's just more less skilled people flying. Of course most flights crash on takeoff and landing, and conceivably that would be automated, so maybe I'm just being paranoid. Still seems impractical, if for no other reason space restrictions on the ground.
  - lazide 5y ago
    Commercial aviation gets to pick from the cream of the crop and fully professionalizes it’s pilots - it’s all they do, all the time. That means staying current (keeping habits fresh, remembering key details, etc) happen easier. Considering how low the frequency is for your your typical GA pilot, it’s honestly a wonder it works as well as it does.
    - throw827474737 5y ago
      Flying is not simple and for everyone - it is complex, but many things are.. and when mastered it is actually not that hard - so disagree, no wonder here.
      - Retric 5y ago
        Flying is easy, flying safely is very difficult. GA inherently allows a great many things that significantly increase risks. Going solo inherently makes you a single point of failure so for example many significant but survivable health issue basically guarantee your death. And that’s just the start. Pilots run out of fuel, fall asleep, they land on the wrong runway/field, and so forth across lots of very small risks that end up killing people. I am not saying GA should be more regulated, it’s just important to understand the trade offs in terms of freedom vs safety.
mbrameld 5y ago
> An aircraft is stable. Let go of the controls and it will keep flying. That's true for most fixed-wing aircraft, but it's certainly not true for most rotor-wing aircraft. I'd estimate you've got about a second, if you're lucky, between letting go of the cyclic in a Robinson R22 and unrecoverable loss-of-control.
- evilos 5y ago
  Even with loss of power for rotary wing aircraft don't you have the option to auto rotate to the ground?
  - kevin_thibedeau 5y ago
    You have to have enough forward speed. Auto rotation is a joke in a real emergency most of the time.
  - elpatoisthebest 5y ago
    The problem with Robinson helicopters specifically (older R-22 & R-44 more than most) is their very low mass rotor system, which results in very low inertia. The time you have to recover is significantly shorter than other high inertia systems on other helicopters. Their very low cost makes them pretty common for schools and instruction in general, but the low inertia (and resultant fatalities early in its operation) was significant enough that the FAA requires a Robinson specific endorsement.
XMPPwocky 5y ago
"There's an inherent physics limitation when trying to strap a whole human body to the side of something small (and generally depicted as being strapped on your back, Bobba Fett style). The center of mass is off." While obviously wasteful and goofy, there's always the option of putting an upwards-pointing (i.e. pushing down) thruster at the rear of a jetpack like a lever arm.
- asdfasgasdgasdg 5y ago
  I was thinking that eventually they could design some kind of powered attachment between the arm pods and the backpack. It would accentuate your desired movement while relieving stress. Maybe they could control it with a Neuralink implant so that there won't be accidental movements due to buffeting. Then if you had that you could build an AI to make it mostly user-error-proof. To be clear, this is all pie in the sky nonsense, but it's fun to think about. It would still be terribly impractical, but maybe it could be made reliable enough that using it over land would not be immediately fatal to most ordinary people who would want to try it.
  - rowathay 5y ago
    “ this is all pie in the sky nonsense” Nah, only the Neuralink part.
    - ada1981 5y ago
      I imagine there will be a whole batch of grifters coming post Neurolink… Similar to just add it to blockchain. How about a neurolink to the blockchain? Well I guess we have Doge.
      - asdfasgasdgasdg 5y ago
        I don't work for neuralink nor do I have any economic stake in their success. Please don't make unjustified accusations of grifting. > Please don't post insinuations about astroturfing, shilling, bots, brigading, foreign agents and the like. From the guidelines.
lazide 5y ago
I wouldn’t be so sure. Fundamentally this is a control input/feedback/positioning/power problem - one we don’t currently know how to model or control in an automated fashion. With the right mental model and control/power systems, it wouldn’t have to be much different from riding a unicycle. Helicopters have similar issues (it’s a bit like balancing a spinning bowling ball on the end of a stick in practice - yet many people are licensed and safe helicopter pilots) I’m guessing it’s something that not everyone can do, but something a large percentage of people (20%+?) can do if they spent the time learning and it was systematized usefully.
KineticLensman 5y ago
> since you have jet turbines strapped to your arms So soldiers that use them would need guns strapped to their feet?
CydeWeys 5y ago
> It's inherently unstable and tiring since you have jet turbines strapped to your arms. If you look at the photos in the linked article, you'll see that the turbines are on the backpack. It still is tiring for your arms per the article, but for different reasons.
teh_klev 5y ago
> Ultimately, 'jetpacks' will probably never exist So no Aeroball and Harlem Heroes any time soon :( https://talkingcomicssite.wordpress.com/2018/04/27/the-complete-harlem-heroes/ https://talkingcomicssite.wordpress.com/2018/04/27/the-compl...
orangepurple 5y ago
Minor nitpick. A fixed wing airplane is generally known to be stable. But many other aircraft like multirotor are inherently unstable and require a fast PID loop to stay in the air.

rictic 5y ago

If battery tech continues to improve at current rates then certainly electric jetpacks will be possible and practical, if there's still demand (and we don't destroy ourselves one way or another). Without doing the math though it might be many decades.

alisonkisk 5y ago
mdavidn 5y ago
Unlikely. The specific energy of jet fuel is more than 40 times that of batteries, and I don't see batteries improving by an order of magnitude. A jetpack would need 40 times more battery mass than fuel to produce the same thrust for the same duration. Of course, the additional weight of the batteries necessitate even more thrust...
- earleybird 5y ago
  Nailed it. Even in cars where there they do a bunch of gliding (rolling) batteries are just becoming plausible (looking back over time).
- kragen 5y ago
  Existing aluminum-air batteries have 1300 Wh/kg, or 4.7 MJ/kg in modern units, and it's plausible they could improve to over 20 MJ/kg. The more expensive lithium-air battery already exceeds 6 MJ/kg. Jet fuel is 43 MJ/kg. Pure aluminum is 31 MJ/kg, so to the extent that you can build "new Al-air batteries" during flight with the cathode and electrolyte from "old batteries", you can approach that limit. That sounds ridiculous but we've been doing it with hydrogen-oxygen fuel cells since 01838. Hydrogen has 118 MJ/kg, and there's no fundamental physical reason hydrogen-air fuel cells can't be epsilon above that, though current models are large and heavy. It's true that all rechargeable batteries like Li-ion batteries are so far under 1 MJ/kg, which more than 40 times worse than jet fuel just as you say, but it's extremely plausible that Al-air could beat Li-ion by an order of magnitude, and Li-air already does, except for the best Li-ion cells. It's already most of the way there. And fuel cells can beat jet fuel by almost a factor of 3, and for low-power applications they do. Betavoltaic cells and TEGs have even higher energy density but you can't turn them off, so you need to dissipate the power they produce when you're not using it to fly or whatever. You could build a hybrid system with a large glide ratio that uses chemical batteries for takeoff and a betavoltaic battery or TEG for long-distance cruising, but you might have to only land it on water to keep it from overheating while it was on the ground. https://en.wikipedia.org/wiki/Aluminium%E2%80%93air_battery https://en.wikipedia.org/wiki/Aluminium%E2%80%93air_battery https://en.wikipedia.org/wiki/Energy_density#In_chemical_reactions_(oxidation) https://en.wikipedia.org/wiki/Energy_density#In_chemical_rea... https://en.wikipedia.org/wiki/Energy_density_Extended_Reference_Table https://en.wikipedia.org/wiki/Energy_density_Extended_Refere...
  - FabHK 5y ago
    As you say, practicable batteries in production have specific energy under 300 Wh/kg (1 MJ/kg), 40x worse than fuel. The other technologies you mention don't seem to be practical for now. I don't think it is reasonable to expect any breakthrough making it to market in the next decade. With specific energy doubling every 10+ yrs, electric flight might be sensible for niche applications (training flights, occasional urban transport for example to the airport) in a few years, but for anything else it'll be several decades realistically.
FabHK 5y ago
> If battery tech continues to improve at current rates What do you consider current rates? They improve slowly, maybe 3-7% a year. Last I looked, batteries are so much worse in terms of specific energy than fossile fuels that we're looking at several decades before electric flight becomes practicable (except for very short distances and training flights).
- rictic 5y ago
  7% a year means doubling every decade, 3% means doubling every two. Fossil fuel efficiency isn't moving at nearly that rate. If current rates continue, battery powered jetpacks should be compelling within 60-120 years

dreamcompiler 5y ago

The stability issues could easily be fixed with more closed-loop control. The much more serious issues are heat and the limited fuel supply.

jonathankoren 5y ago
Agree. This is the same reason why I'm skeptical on flying cars. There's so many fender benders. I don't want a Jetta crashing through my roof. Sure, general aviation is safer than driving (e.g. less accidents per vehicle, and less accidents per hour/mile), but general aviation is more dangerous than commercial aviation. I suspect there's something going on with maintenance, training, and available safety features. Seems like lowering the barrier of entry to flying is simply going to increase the accident rate because there's just more less skilled people flying. Of course most flights crash on takeoff and landing, and conceivably that would be automated, so maybe I'm just being paranoid. Still seems impractical, if for no other reason space restrictions on the ground.
- lazide 5y ago
  Commercial aviation gets to pick from the cream of the crop and fully professionalizes it’s pilots - it’s all they do, all the time. That means staying current (keeping habits fresh, remembering key details, etc) happen easier. Considering how low the frequency is for your your typical GA pilot, it’s honestly a wonder it works as well as it does.
  - throw827474737 5y ago
    Flying is not simple and for everyone - it is complex, but many things are.. and when mastered it is actually not that hard - so disagree, no wonder here.
    - Retric 5y ago
      Flying is easy, flying safely is very difficult. GA inherently allows a great many things that significantly increase risks. Going solo inherently makes you a single point of failure so for example many significant but survivable health issue basically guarantee your death. And that’s just the start. Pilots run out of fuel, fall asleep, they land on the wrong runway/field, and so forth across lots of very small risks that end up killing people. I am not saying GA should be more regulated, it’s just important to understand the trade offs in terms of freedom vs safety.

mbrameld 5y ago

> An aircraft is stable. Let go of the controls and it will keep flying. That's true for most fixed-wing aircraft, but it's certainly not true for most rotor-wing aircraft. I'd estimate you've got about a second, if you're lucky, between letting go of the cyclic in a Robinson R22 and unrecoverable loss-of-control.

evilos 5y ago
Even with loss of power for rotary wing aircraft don't you have the option to auto rotate to the ground?
- kevin_thibedeau 5y ago
  You have to have enough forward speed. Auto rotation is a joke in a real emergency most of the time.
- elpatoisthebest 5y ago
  The problem with Robinson helicopters specifically (older R-22 & R-44 more than most) is their very low mass rotor system, which results in very low inertia. The time you have to recover is significantly shorter than other high inertia systems on other helicopters. Their very low cost makes them pretty common for schools and instruction in general, but the low inertia (and resultant fatalities early in its operation) was significant enough that the FAA requires a Robinson specific endorsement.

XMPPwocky 5y ago

"There's an inherent physics limitation when trying to strap a whole human body to the side of something small (and generally depicted as being strapped on your back, Bobba Fett style). The center of mass is off." While obviously wasteful and goofy, there's always the option of putting an upwards-pointing (i.e. pushing down) thruster at the rear of a jetpack like a lever arm.

asdfasgasdgasdg 5y ago
I was thinking that eventually they could design some kind of powered attachment between the arm pods and the backpack. It would accentuate your desired movement while relieving stress. Maybe they could control it with a Neuralink implant so that there won't be accidental movements due to buffeting. Then if you had that you could build an AI to make it mostly user-error-proof. To be clear, this is all pie in the sky nonsense, but it's fun to think about. It would still be terribly impractical, but maybe it could be made reliable enough that using it over land would not be immediately fatal to most ordinary people who would want to try it.
- rowathay 5y ago
  “ this is all pie in the sky nonsense” Nah, only the Neuralink part.
  - ada1981 5y ago
    I imagine there will be a whole batch of grifters coming post Neurolink… Similar to just add it to blockchain. How about a neurolink to the blockchain? Well I guess we have Doge.
    - asdfasgasdgasdg 5y ago
      I don't work for neuralink nor do I have any economic stake in their success. Please don't make unjustified accusations of grifting. > Please don't post insinuations about astroturfing, shilling, bots, brigading, foreign agents and the like. From the guidelines.

lazide 5y ago

I wouldn’t be so sure. Fundamentally this is a control input/feedback/positioning/power problem - one we don’t currently know how to model or control in an automated fashion. With the right mental model and control/power systems, it wouldn’t have to be much different from riding a unicycle. Helicopters have similar issues (it’s a bit like balancing a spinning bowling ball on the end of a stick in practice - yet many people are licensed and safe helicopter pilots) I’m guessing it’s something that not everyone can do, but something a large percentage of people (20%+?) can do if they spent the time learning and it was systematized usefully.

KineticLensman 5y ago

> since you have jet turbines strapped to your arms So soldiers that use them would need guns strapped to their feet?

CydeWeys 5y ago

> It's inherently unstable and tiring since you have jet turbines strapped to your arms. If you look at the photos in the linked article, you'll see that the turbines are on the backpack. It still is tiring for your arms per the article, but for different reasons.

teh_klev 5y ago

> Ultimately, 'jetpacks' will probably never exist So no Aeroball and Harlem Heroes any time soon :( https://talkingcomicssite.wordpress.com/2018/04/27/the-complete-harlem-heroes/ https://talkingcomicssite.wordpress.com/2018/04/27/the-compl...

orangepurple 5y ago

Minor nitpick. A fixed wing airplane is generally known to be stable. But many other aircraft like multirotor are inherently unstable and require a fast PID loop to stay in the air.