# Jetpacks currently suck. What are the flaws, and how can we overcome them?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Doggzilla, Nov 25, 2017.

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1. Nov 25, 2017

### Doggzilla

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So I was recently reading an article about a jetpack setting the world speed record of about 35mph.

Lets face the truth, jetpacks absolutely suck as we know them today. I think we need to identify the flaws, and try to find solutions.

First off, they either use very small jets with high velocity, or use large ducted fans which make them heavy and bulky.

Secondly, their center of gravity is dominated by the pilot, forcing the jetpack to have its jets offset forward, which makes them awkward and complicates the design.

How can these two problems be overcome?

Firstly, the heavy components need to be moved as far aft as possible to help assist keeping the CG centered and not dominated by the pilot.

Secondly, the powerplant should be transverse, and mounted in a fashion which allows the torque to help lift the pilot.

The torque can be assisted by an exhaust vane as well.

Lastly, the CG automatically centers as the device flies, as leaning forward moves the pilot backward and the pack forwards. This means stabilization is only required when hovering.

As jetpacks become faster, the need for in flight stabilization will be reduced greatly due to the positive stability as the pilot leans further forward.

Anyone notice any other issues that need to be corrected, or have any solutions?

2. Nov 25, 2017

### bmcj

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I think the current jet packs fly close to 60 mph and can fly for 10 minutes or more. Try looking up JB-9 or JB-10 videos.

Also, I suppose Yves Rossi (Jetman) could be considered a “jet pack” and it flies quite fast and has decent maneuvrability.

3. Nov 25, 2017

### pictsidhe

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gas turbines don't scale down very well. A higher pressure ratio would help with their atrocious fuel consumption, but they'd still blow, and more $$. A turbofan would also be an improvement, but now we've got a small and complex engine:$$$. Without moving large amounts of air and being not-small, these things will always blow. Ye cannae break the laws of physics, Jim! The torque won't be useful unless it can react against something that isn't held up by air. Lifting the pilot will push the engine down. No net gain. Possible solutions: 1. $
2. Dream
3. Go find something more sensible to do.
4. If it's just speed you're after, you need horizontal flight. Then you can annoy airliners:

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4. Nov 25, 2017

### Swampyankee

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How, may I ask can torque lift the pilot? It takes force acting antiparallel to they pilot plus jetpack's weight to lift it off the ground.

5. Nov 25, 2017

### TFF

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Personally I hate how my ankles get roasted every time I fly mine. There is always the fuel and weight and that garbage, but the real problem to be solved is fool proof stabilization. Knocking out the hardware and the software and the method is the key. It has to be good enough to get you out of a somersault, or Im not buying one.

6. Nov 25, 2017

### Doggzilla

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That's the problem. We seem to have broken physics.

Jet packs have immense power to weight ratios, yet can barely function!

Any other aircraft with over a 1:1 power ratio is supersonic, or only subsonic due to inlet limitations.

Somehow we have snatched defeat from the jaws of victory.

In this case, Scotty, we do NOT need more power. We need better design.

7. Nov 25, 2017

### Vigilant1

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Helicopters?

No, it's not a design problem. It's a physics fact. It is much more efficient to generate a given amount of thrust by accelerating a large mass of air to a slightly higher velocity (as a helicopter does) than to generate the same thrust by accelerating a small amount of air/gas to a much higher velocity (as a jet pack, or VTOL jet/turbojet, or rocket does). It's all in the equations. More here.

If you want to take off vertically while burning the least amount of fuel/power, you generally need a lot of "fan area." Helicopters (or large "lift platforms") do this well, jetpacks--not so much.

Last edited: Nov 25, 2017
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8. Nov 25, 2017

### bmcj

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That’s why the JB-9 jet pack uses small model jet engines inside of augmentor tubes to draw more air and change the flow from low volume/high speed flow to a higher volume lower speed flow. It also solves the warm leg syndrome by mixing cold air into the flow.

9. Nov 25, 2017

### BBerson

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I didn't know they were using augmentor tubes for that. Would augmentor tubes help for launching a glider?

10. Nov 25, 2017

### Topaz

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There's a very simple answer as to why "Jet Packs", in their current form, are so limited:

The answer is that they come in one of exactly two types. Either they're of the traditional type, where they are operating in powered-lift mode all the time. Unless the powerplant is operating a large rotor (and "backpack helicopters" have their own issues), that means very high fuel consumption the entire flight, with the attendant impacts on range. They're limited in forward speed because the thrust must always be vectored mostly down, because that thrust is the vehicle's sole means support.

The other kind of jet-pack is the Yves-Rossy, kind, where the jet engines just push a wing through the air, strapped to the back of the pilot. Like any fixed-wing aircraft, power-required and therefore fuel consumption is far lower, and forward speed is limited only by drag and the ability/protection for the pilot to withstand the relative wind. It's an airplane strapped to the pilot's back, and behaves accordingly. Of course, this type can't take off or land on its own - you have to dive out of a helicopter or airplane already airborne, and land under a parachute.

Isn't the solution obvious? Combine the two. Take the jet-pack and attach a Yves-Rossy style wing (with aerodynamic controls*) to it. Take off like a jet-pack, convert to horizontal flight, and throttle way back because you don't need all that thrust (and fuel consumption) any more. While this sounds simple, it's not any more simple than any VTOL aircraft - you need the excess thrust and positive six-axis flight control power to get through the transition from vertical to horizontal flight (and back, before landing). The transition is where most VTOL projects fail. It's far harder than simply lifting something vertically under thrust. Now, "traditional" jet-packs have most of that already licked, so they're half-way there to start with, although there's still a tough spot as the vehicle is transitioning out of powered-lift but hasn't quite unstalled the wing yet**. But building in enough excess thrust for the transition is the tougher challenge for them. It's an odd-ball place in the jet engine spectrum, kind of like 40hp is for small piston motors. Very few available options. So maybe quad the model-style jet engines and shut two down in horizontal flight? Dunno. Ducted fans are a complete dead-end for jet-packs. Just not enough thrust-to-weight available with ducted fans to produce a design with enough excess thrust for the transition, without it being a stupidly large system that certainly isn't a "backpack" anymore.

A friend and I were looking at this for a short while for a project of his, and concluded that there absolutely is no "simple" or "inexpensive" solution that isn't a variation on a backpack helicopter, with all the rotor-speed, trip-and-fall, pilot training, and other challenges that involves. If you have the money to blow on developing a "jet-pack with wings", there's a lot of potential there, but it's a bigger design problem than it seems, and certainly a lot harder than simply slapping some wings on an existing jet-pack. Sizing and scaling get to be your enemies. But it's not impossible, and someone will throw enough money at it eventually, and have something really neat.

----------------

*Rossy's setup is controlled by the pilot moving his hands and body around, a reflection of Rossy's skydiving background. Elevons on the swept wing would go a long way towards giving his design really positive control in horizontal flight, and rudders on the verticals even more so. Right now, the design is a circus act, not a practical flying aircraft.

** The spot I mean is when the jet engines are still providing most of the lift for the vehicle, but it's moving forward some short amount below stall speed, and the stalled wing is generating substantial aerodynamic forces because of the forward speed, not necessarily symmetrically because of the stalled condition. You're asking the engines to both maintain all lift, accelerate the vehicle above stall speed for the wing, and provide most of the flight control power about all three rotational axes not only for regular flight control, but also to counter any moments the stalled wing is throwing in. It's the worst part of the flight envelope for the engines, and requires quite a lot more thrust than simply lifting the vehicle up off the ground and moving it forward at typical "jet-pack" speeds. The thrust required spikes sharply, just for that short transition part of the mission, and essentially drives the engine sizing requirements. This is where my friend and I gave up. The "simple and easy jet-pack with wings" was turning into a large and complicated beast, even with jet engines, and any other propulsion technology was far worse. It's almost impossible with ducted fans, and yes, we looked. Controlling precession in a rotor or rotors as the vehicle tilted 90° into forward flight was more than I felt comfortable taking on (helicopters aren't in my skill set), and he was no more comfortable with that than I.

Last edited: Nov 25, 2017
11. Nov 25, 2017

### Giggi

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The problem is that we expect real jetpacks to look and work the same way fictional ones do, and fictional ones don't follow the laws of physics. If you want an actual "pack" that's small enough and light enough to walk around with, the jets (or at least the nozzles) will have to be offset forward because the pilot will count for almost all the mass.

The torque from the powerplant won't help lift anything, cause it won't even generate a torque unless the rotor is accelerating (or decelerating, and then it's in the opposite direction). Also there'd be undesirable torque from gyroscopic effects. You could use secondary air outlets pointed at an angle to counter the torque from rearward-situated lifting jets, but then you'd need tertiary outlets to counter the translational force from the secondary ones and it'd all just be wasted power.

Last edited: Nov 25, 2017
12. Nov 25, 2017

### markaeric

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I think Giggi hits the nail on the head in that the type of jetpack most people envision, and perhaps alluded to in the OP is much closer aligned with sci-fi than practical engineering. Where is the line differentiating a jetpack from other forms of aircraft? To me, if you can't walk around with it on, then its just something strapped to your back and thus isn't any more practical than something that you sit on/in. Sci-fi packs tend to not be very bulky, and if they do have wings, they can be swept back or telescope in for compactness when not in use. The only place where reality and fiction have tended to converge is that they're expected to have VTOL capability.

If you want a jetpack with any kind of endurance and speed, then you really do need something that generates lift from aerodynamic surfaces (other than for TO) like Topaz said.

Propulsion really is the primary challenge. A pack that you can carry means the propulsion system has to have a quite high power to weight ratio. Pretty much the only thing that fits the bill are rockets, but fuel consumption makes them impractical *except* if they're used only for the TO/landing phases, and use something more efficient for flight that doesn't have the same power to weight requirements.

Actually, this has got me thinking about hybrid jet engines; a tubineless unit where an electric motor drives the compressor. Before dismissing the idea outright, consider some of the following: The company Rocketlab has built an orbital-class rocket using electric turbopumps despite the fact such rockets have very demanding mass fractions, and that the penalty for carrying the entire battery mass throughout the burn of each stage wasn't a deal breaker. Second of all, any jet engine with a low PR that doesn't have a regenerator/recouperator has crap efficiency - pretty much every small-scale gas turbine you can buy, so the poorer energy density of batteries vs liquid fuels would be offset quite a bit. Also, thrust for a given compressor mass flow and PR is substantially increased since there no longer has to be a pressure drop across a turbine - thus the maximum pressure and heat can be used to generate thrust. As if that wasn't enough, there's still more potential to increase the thrust in that you can have combustion at a much higher temperatures since you're no longer beholden to turbine inlet temperature limits. Throw on a variable area nozzle and you have a jet engine with wide range of thrust levels.

13. Nov 25, 2017

### pictsidhe

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Probably. Augmentors are about 30% efficient. But since jet velocity is so badly matched to a glider velocity, you may well gain thrust.

14. Nov 26, 2017

### Giggi

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I've put a bit of thought into circumventing turbine inlet temperature limitations, but the method I came up with was using a rankine-cycle engine to drive the compressor, with the boiler running through the jet's exhaust stream then coiling around its combustion chamber in the hope of reducing or removing the need to shield the chamber walls with gas.

Hopefully somebody's already patented that over 20 years ago...

Last edited: Nov 26, 2017
15. Nov 26, 2017

### Vision_2012

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I see jet packs, or preferably rocket packs, working best in non-gravity environments like orbits and deep space. Delete the requirement for lift and there will be successful person sized "flying". See the 1950 movie, Destination Moon.
And if you have just a bit of gravity, smaller "hang gliders" with arm and leg power should work in large caves on earth's moon.

16. Nov 26, 2017

### markaeric

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I'm probably not correctly grasping your idea. Some equilibrium would be reached and the temps would be the same pre and post turbine. So for a given amount of fuel, with your system, turbine inlet temps would be lower than with a standard jet engine, with the opposite being true for the exhaust temps. This has the effect of lowering the power output of the turbine, which would likely be undesirable. And at any rate, a similar effect could be achieved by injecting less fuel into the combustion chamber, and instead injecting the difference into an afterburner. The chamber walls would still need to resist whatever the temps are.

17. Nov 26, 2017

### markaeric

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Efficiency is poor, but they do increase static (and up to low speeds) thrust quite drastically. They make good sense for V/STOL

18. Nov 26, 2017

### Giggi

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Oh no I meant to say compressor, not turbine. That is, the turbine in the rankine engine would drive the compressor of the jet engine. Maybe it makes sense now.

19. Nov 26, 2017

### Doggzilla

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Helicopters and Jump jets work on two entirely different principles.

For jets, its all about having more thrust than weight. Fan area has nothing to do with it.

You are talking about EFFICIENCY. Not thrust.

20. Nov 26, 2017

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