Jetpacks have been a holy grail of science fiction for a century but commercial adoption has been elusive. We’ve had them since the 1950s with the Bell Rocket Belt. Popular media fawned over the contraption but there was zero commercial or military interest because it burned through 5 gallons of hydrogen peroxide in 21 seconds of flight time.
Fast forward to 2020 and pop media is fawning over the Gravities Industries’ $440,000 MK2 jetpack despite the fact that it’s more dangerous than the jetpacks we had 60 years ago. The MK2 requires a pilot with the strength and coordination of a calisthenics athlete to operate without a tether because the pilot is balancing his body weight on his hands. Sure, it’s using safer jet fuel or diesel rather than hydrogen peroxide, but that’s no consolation when you smash your head into the ground at 100 MPH. This is why Gravity Founder Richard Browning in a YouTube interview admitted that he avoids flying above 10 feet for safety reasons.
The CNBC video below shows normal people struggling to get off the ground without losing control.
Fuel burn on the Gravity MK2 jetpack is better than the 1950s jetpack, but still absurdly impractical at 5.25 gallons in 4 minutes. Previous videos from Gravity admitted the actual time is more like 2 minutes but even if we go with the 4-minute claim, that’s still a 38% higher fuel burn rate than a 6-passenger 7000-pound TBM 940 turboprop aircraft cruising at 380 MPH!
The 5 turbojets on the MK2 (2 on each arm and one on the back) have to generate 1050 horsepower to levitate a gross flying weight of around 300 lbs. Even if we ignored the lack of safety of the MK2, its fuel consumption and flight duration are absurdly bad. The load on the pilot’s muscles is equally concerning for endurance.
Some of you will be tempted to say that the MK2 is a 10x improvement in jetpack flight time in 60 years so it’s only a matter of time it becomes practical. That’s a faulty conclusion because the Bell Rocket Belt is rocket-powered which is fundamentally less efficient at hovering than turbojets. If you built a rocket-powered jetpack today, it would be almost as bad as the 1950s version.
The propulsive efficiency of a turbojet of similar size has not fundamentally changed in 60 years and it won’t get much better in the next 60 years. Moving a small amount of air at high speeds produces low momentum transfer which means low thrust to horsepower ratios. This is why the 70-year-old B52 bomber will probably keep flying for another 60 years. We’re not talking Moores's Law here folks!
Turbojets are optimized for Mach 2 flight and Rockets are optimized for Mach 5 and above. Any jetpack that uses these engine types will never have a practical useful payload or range.
Bigger Rotors or Fans = Better Efficiency
The human power helicopter flies on a mere 1.3 horsepower generated by human legs. It does this by using impractically large rotors with record-breaking propulsive efficiency. A small 500 lb gross weight helicopter with a smaller and more practical rotor size can fly on 60 horsepower. The MK2 jetpack requires around 1000 horsepower to lift 300 lbs and guzzle fuel.
The Gravity MK2 could double the efficiency by going to turbo fanjets, but that’s still grossly inefficient. Larger turbo fanjet engines would also likely be too bulky for the MK2 anyways. It’s simply a bad design all around not suited for commercial adoption, not unless you count roll-playing Tony Stark for Corporate Events.
CopterPack got it right (but likely faked)
Update 7/4/2021: Sadly, CopterPack looks like a hoax created from video effects because you can see where the wire harness was edited out of the video footage. This is not to say that CopterPack isn’t possible, but this particular video is almost certainly a hoax. Why fake this? Startups frequently employ fake-it-till-you-make-it tactics to get investment funding.
Looking back, there are some obvious problems I should have picked up on. The motor is simply too small to deliver the necessary 15 kilowatts per motor. The battery packs are probably too small as well.
Note that the remainder of this article is based on sound physics but it was written before I learned it was a hoax video.
CopterPack seems to have achieved the first practical wearable twin-rotor electric helicopter. The large rotors have fundamentally better propulsive efficiency because they move a lot more air at lower speeds.
While the specifications are scarce, this looks to be a practical stable design that doesn’t require the strength and balance of a gymnast. It appears to have a flight controller that automatically balances the vehicle by tilting the rotors for pitch control and possibly altering the rotor RPM for roll control. This type of flight control of a tandem rotor helicopter is possible with a $30 hobbyist flight controller.
A reasonable guess on power consumption would be 30 kilowatts (KW) or 40 horsepower with a gross flying weight of 250 to 300 lbs depending on the weight of the pilot and battery capacity. This assumes around 8 lbs of thrust per horsepower delivered to the rotors. The actual number could be a little higher or lower.
If we guessed 6 minutes of flight time at 30,000 watts, that would require a 3000 watt*hour (WH) Lithium Polymer battery weighing around 44 lbs. 12 minutes of flight time would be possible with 6000 WH Lithium-Ion batteries weighing 53 lbs. With more batteries, 30 minute flight time is conceivable.
This much battery capacity can be purchased for less than $1000. The electric motors are around $1000 each. Then there’s the carbon fiber chassis and custom flight controller with a total Bill of Materials (BOM) of less than $5000. The retail price for a finished CopterPack and all the engineering and manufacturing that goes into it will obviously be many multiples higher, but the BOM is far lower than what goes into a Tesla Model 3.
So while jetpacks are totally impractical, electric helicopter packs are very practical because they are safer, longer range, quieter, and more affordable. I could see applications in search & rescue and maybe military applications. Special forces can use the CopterPack and maybe a version that can be remotely piloted to a soldier to extract them from a contested zone.
