The LAX jetpack guy thread made me start to wonder: how would a jetpack work? What would the aerodynamics be? No wings; how would there be any stability?
They aren’t jets like a jet plane (or The Rocketeer), they are jets like a SpaceX first stage, some lumbering thing that is just barely steerable.
Turbofan engines can generate adequate lift.
I wanted to add more and kinda submitted by accident.
This (and the rocket-powered one Darren G. is hinting at) worked by throwing a lot of thrust downwards to overcome gravity.
There’s a different variation where a person used a wingsuit, if memory serves, to generate lift, and a pair of turbofans to generate the thrust. He needed an airlaunch though, as he couldn’t generate enough lift for an unassisted takeoff.
Some use water.
As far as I’m aware, none of them work particularly “well.” I don’t think that there have been many accidents, but that’s probably more a function of their rarity than anything else.
What you need is the military version.
The key to the success of that jet suit was the decision to put the jets on your arms.
And it’s not military per se, they are just trialling it.
Here’s a clip on the development of it:
This. Without airfoils or control surfaces like airplanes or helicopters, how is there any stability, and how are they steered? How does the pilot control where he goes? I haven’t seen any answers to this. IIRC, I asked the same in some earlier thread about jetpacks, and nobody gave any kind of meaningful answer that I recall.
Is this it? Put the jets on the arms so you can flail them about to steer?
Do they have pitch, yaw, and roll jets like spacecraft that operate outside any atmosphere?
Just point your arms in the appropriate direction. They seem to have quite precise control. The guy flying through the forest is obviously well practiced (he is the designer), but he has very good control of it.
How does it maintain stability? What keeps the pilot from tumbling randomly? Airplanes have vertical and horizontal stabilizers for a reason – what equivalent is there in a guy flying a jetpack?
Is weight-and-balance and center of gravity as critical – or even more so – as in an airplane? How does he stay right-side-up?
Did you watch the video about the development of it? There are the jets on his hands and one on his back, they form a tripod which makes it quite stable.
I would think your arms would get quite tired just trying to point the things in the right direction.
I imagine the Royal Marines depicted have somewhat more upper body strength than you or I, and they do not appear to be struggling with control.
I would question what happens when one unit fails (no doubt the reason for dual ducted fans - if they are actually ducted fans) for each hand, but loss of any of the three “legs” of a tripod would be catastrophic. I assume they have some way to trigger a parachute that can be done while spinning chaotically.
Alternatively, if you’re good at balancing…
It has a computerised logic system built in to help with balancing, but it still requires 50-100 hours training to use.
However, Franky Zapata did fly across the English Channel on it at speeds of up 160 km/h (100 mph), and it can go high enough to need an oxygen mask.
Helicopters are inherently unstable. 
Photo of an old-style rocket belt.
In this photo, you can see the traditional design of older models of jetpacks. A heavy fuel tank is on one side. Your body is on the other side. The jet thrusts upward through the vertical centerline of the mass. The jet is above the horizontal centerline of the mass. Your weight keeps the jet pointing downward, and the thrust pointing upward. You steer by shifting your weight, making the jet lean slightly sideways. When you stop actively interfering, your weight will return the jet to a vertical orientation.
Negative flight stability is a defining characteristic of modern high-performance warplanes, going back to the F-16.
That’s true. But in general, if you let go of the stick/yoke in an airplane, it will return to equilibrium. If you let go of the cyclic in a helicopter, you’ll quickly be out of control.
Yep agreed. It is obviously designed for someone with reasonable physical strength. Average lardarses like me would tire very quickly I think.
That is the reason they stay low. At the time the Wired video was made, there was no control or safety device to save a failed engine. It is supposed to be able to fly up 2000’ but nobody wants to fall that far!
This video follows an average Joe who buys a jet suit.