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

### Help Support HomeBuiltAirplanes.com:

#### Aesquire

##### Well-Known Member
There is a reason that super heroes that can fly are also tough. Strong, too, usually, but Tough is a requirement for hard landings.

This IMHO gives jet packs the problem of the Glass Cannon, or Fragile Speedster. See tv tropes.

It's not the machine. It's the man. Skill level, just like helicopters, is required. Maybe even a touch of natural talent.

High kinesthia.... balance sports, surfing, skateboarding, can be learned by idiots. But not all geniuses can learn them. This is a highly arguable point, I admit. Certainly many of you know "natural athletes" and klutzes.

In any event the Real Problem is landing gear. Human legs, hips & spines are just limited by fragility. So look to a WASP platform.Range and aerodynamics are improved, long stroke shock absorbing sloped terrain landing gear are possible. ( though I don't know of a good example. VTOL craft are so weight limited prototypes often have skimpy gear)

#### Aesquire

##### Well-Known Member
That said, the challenge is obvious and seductive

Taking Topaz's skydiving jet wing and adding thrust for VTOL operations has promise. A solution to the high energy requirements for transition flight might be overcome by pilot technique. Zoom to a thousand feet and power dive for airspeed. Swoop into a low altitude vertical climb and back down VTOL. The hard part is the descent, where the wing may object to going backwards. High skill job, tougher than normal VVOL operation. Also it's a bit of a fuel hog. Just because you get away with less power and weight, doesn't change the amount of work to be done. You just stretch out the time a bit. More percentage at full throttle. TANSTAAFL.

#### BBerson

##### Well-Known Member
HBA Supporter
Need a hard surface for both takeoff and landing, or it will suck in rocks and destroy itself.

JB-9 turbine jet pack.

Last edited:

#### Giggi

##### Well-Known Member
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.
Well, the more efficiently you use your available energy, the more thrust you can make.

Jets and helicopters work on exactly the same principles - they both generate force by accelerating mass (F=ma), while also trying to use the minimum amount of energy that their mission requirements allow to do so (E=1/2 mv^2).

#### Doggzilla

##### Well-Known Member
HBA Supporter
Well, the more efficiently you use your available energy, the more thrust you can make.

Jets and helicopters work on exactly the same principles - they both generate force by accelerating mass (F=ma), while also trying to use the minimum amount of energy that their mission requirements allow to do so (E=1/2 mv^2).
That is completely incorrect. Do wings work by accelerating mass? Of course not. They work by producing an area of low pressure. The downdraft is only a small portion of helicopter lift, the main portion being produced by lift.

Jets lift by almost pure thrust and very little lift.

Either way, its clear you are intentionally being combative.

You are intentionally misunderstanding other members, and if you need an outlet, I suggest the youtube comment section. Not HBA.

#### Doggzilla

##### Well-Known Member
HBA Supporter
Need a hard surface for both takeoff and landing, or it will suck in rocks and destroy itself.

JB-9 turbine jet pack.
Well, they dont use any more air than a commercial turbocharger, so perhaps a few of those air filters would help. Theyre only a few pounds, but can almost fit over the entire engine. Give a VERY large surface area with little resistance.

#### Marc Zeitlin

##### Exalted Grand Poobah
Well, I've got a few bones to pick with your claims - let's start with an early one and move through them:

Doggzilla said:
Jet packs have immense power to weight ratios, yet can barely function!
Power comes in units of Watts. Weight comes in units of Newtons. I'm not sure what the ratio of Watts/Newton even means. I BELIEVE that you mean "thrust to weight" ratio, not "power to weight" ratio - at least thrust and weight are measured in Newtons, and if something has a thrust to weight ratio of 1 or more, it can hover or climb based ONLY on the thrust - no lift from aerodynamic surfaces (wings) required.

Doggzilla said:
Any other aircraft with over a 1:1 power ratio is supersonic, or only subsonic due to inlet limitations.
Every helicopter, as has been mentioned by others, has a "thrust to weight" ratio or 1 or larger than 1, else they would not be able to climb in a hover, or even hover. Many aerobatic aircraft as seen at airshows can hang off of their prop and even climb with essentially no lift from the wings - these aircraft also have thrust to weight ratios larger than 1. None of the above can achieve supersonic flight.

The Concorde aircraft had four engines of 169 KN (38K lbf) each, for a total of 676 kN (152K lbf). The MGW of the Concorde was 1.83 MN (187 kg, or 412K lbf). As you can see, the maximum afterburner thrust of the Concorde was about 1/3 of the MGW of the plane - nowhere near a thrust to weight ratio of 1. So while the concorde could not accelerate straight up, nor even hover using it's engines, it could easily achieve supersonic flight. The thrust to weight ratio of an airplane may be correlated with its ability to go supersonic, but there's nothing magic about a T/W ratio of 1 other than for the ability to hover.

That is completely incorrect. Do wings work by accelerating mass? Of course not.
Actually, they do exactly that. The wing has an upwash area ahead of it (in subsonic flow) and an area of downwash behind it. Making air move up and then down is the definition of accelerating mass, since you're changing its velocity. It is precisely this change in momentum of the air by accelerating it that keeps the airplane in the air by making lift.

They work by producing an area of low pressure.
The pressure differential across the wing is part and parcel with the mass acceleration - they are one and the same phenomena.

The downdraft is only a small portion of helicopter lift, the main portion being produced by lift.
I'm not sure what you're trying to say here.

The "downdraft" from the helicopter rotor IS the acceleration of mass through the rotor - I suggest you investigate actuator disc theory to understand what a propeller or helicopter rotor is doing. Here's a link:

http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node86.html

In either case, the thrust is produces by taking low speed air and accelerating it. As others have pointed out, accelerating a lot of mass through a small velocity change obtains far more thrust than accelerating a small amount of mass through a large velocity change. This is why helicopter rotors are large diameter - to capture as much air as possible. With tiny little jet engines or ducted fans, to get the required thrust to levitate the weight of the vehicle requires a lot more power.

A Robinson R-22 can lift 1370 lb (MGW) with 180 HP, while consuming 10 - 12 gph.

Note in the link above in section 11.7.2 the relationship between Thrust, Mass Flow (m dot) and the velocities in front of the disc and aft of the disc (or above and below, for a helicopter rotor), and then the energy required to produce that thrust. Clearly, more M dot and less difference between the two velocities maximizes thrust for minimum power. And just as clearly, the efficiency is highest when the velocity difference is smallest.

Jets lift by almost pure thrust and very little lift.
I'm not sure what you're trying to say here.

Thrust, when pointed opposite to the gravity vector, is lift. Jets produce thrust, and if pointed down, lift, by accelerating a little bit of air a LOT. This is not very efficient, which is why commercial jets now use high bypass ration fanjets, which accelerate a LOT more air by a smaller velocity delta, than the older jets that were pure turbojets with no bypass. It's why NASA spent a lot of time and \$ investigating multi-blade turboprops, because they are more efficient still than the fanjets.

Whether there's a shroud around the blades or not, the blades of the compressor in a jet or the rotors of a helicopter are producing thrust by accelerating mass and inducing a pressure change across them.

Either way, its clear you are intentionally being combative.
Giggi is/was 100% correct in what he wrote and was totally civil about it.

You are intentionally misunderstanding other members, and if you need an outlet, I suggest the youtube comment section. Not HBA.
I'd suggest that before you start expounding on aeronautics and criticizing others, you study actuator disc theory for propellers and helicopter rotors, turbine engine physics and theory for jets and basic aerodynamics to understand wings.

Once you've done that, you'll understand what others have been trying to explain to you as to why itsy bitsy jetpacks will never be (dang physics / aerodynamics) as efficient at lifting weight as a helicopter with a large rotor. It's not because engineers are stupid (although many are, just like the population at large) - it's because the universe has certain rules and regulations that you cannot disobey no matter how hard you may try.

#### bmcj

##### Well-Known Member
HBA Supporter
For the hybrid jetpack/jetwing suggestion by Topaz, perhaps four spaced vectored outlets similar to the Kestral/Harrier would help solve the transition difficulty. Granted, it would be more complex, but it might be possible, especially if you are using small jets with augmentor tubes for increased volume flow.

#### markaeric

##### Well-Known Member
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.
Ah, so you mean exchange heat from the condenser stage to the post compressor stage?

#### markaeric

##### Well-Known Member
Technically you could overcome the inlet temperature issue by driving the turbine off a less intense secondary combustor.

Many rockets drive fuel pumps by simply running a second smaller rocket inside the motor instead of having it driven by the main rocket chamber.
There's no shortage of possible arrangements. TIT isn't the only limiting factor mentioned in my post, but pressure drop too. Yeah, you can drive an oversized compressor (not uncommon with turbochargers), and have parallel combustors. It would work. Efficiency could be increased by adding a regenerator/recouperator/etc to the post-turbine section. Likewise you could use the equivalent of two turbos with one compressor feeding the turbines in parallel or staged arrangement (perhaps utilizing inter-turbine burning if they're staged), and the other compressor being used for thrust generation. Or you can stage the compressors as well to increase the PR.. etc. etc. etc. The end performance and efficiency will be quite similar for all configurations other than one that can recuperate waste heat and produce shaft power to turn something efficient like a fan/prop/rotor to generate thrust. But those things are bulky and thus antithetical to a jetpack, imo.

Using electrics in place of turbines is *A* solution, not *THE* solution to meet performance requirements. I will argue that it has compelling benefits such as the commodity nature of electric motors and related electronics along with the huge knowledge base surrounding the field. You won't find general purpose combustion chambers on eBay. With some cars now being equipped with electric superchargers (at least the Audi SQ7), much of the legwork is already done too.

#### pictsidhe

##### Well-Known Member
Why has no-one mentioned skyhooks yet?

HBA Supporter
Or dog houses.

#### Giggi

##### Well-Known Member
Ah, so you mean exchange heat from the condenser stage to the post compressor stage?
If the compressor is intercooled, yes. I like the concept of intercooled jets, although they're technically challenging to build...

I drew a diagram, please forgive (and/or point out) any violations of diagram-drawing convention I've made.
I don't do this very often :speechles

The reason for the awkward placement of the turbine "ahead" of the compressor is to avoid the need for fancy high-pressure shaft seals, as both shaft penetrations are at the low pressure ends of the turbomachines. Here I'm using water as the whoring fluid for the rankine engine, so it can exhaust close to atmospheric pressure.

The compressor will need some amount of ducting leading to the intake to smooth out the airflow, but the adverse effects on weight and efficiency shouldn't be too significant, and for jetpack application you may want to use some air filtering apparatus in the intake anyway.

Either way, its clear you are intentionally being combative.

You are intentionally misunderstanding other members, and if you need an outlet, I suggest the youtube comment section. Not HBA.
I'm sorry, I didn't intend to come off as aggressive. I love HBA and I agree that it's not a place for hostility. I was just relaying what I know from the physics textbooks I've read.

#### markaeric

##### Well-Known Member
If the compressor is intercooled, yes. I like the concept of intercooled jets, although they're technically challenging to build...

I drew a diagram, please forgive (and/or point out) any violations of diagram-drawing convention I've made.
I don't do this very often :speechles

The reason for the awkward placement of the turbine "ahead" of the compressor is to avoid the need for fancy high-pressure shaft seals, as both shaft penetrations are at the low pressure ends of the turbomachines. Here I'm using water as the whoring fluid for the rankine engine, so it can exhaust close to atmospheric pressure.

The compressor will need some amount of ducting leading to the intake to smooth out the airflow, but the adverse effects on weight and efficiency shouldn't be too significant, and for jetpack application you may want to use some air filtering apparatus in the intake anyway.
Neat! It's kind of a variant of a rocket expander cycle. I actually discussed a similar concept on a rocket forum, but it pumped an on-board oxidizer as expected. It also condensed the steam by running it through a heat exchanger with the propellants, so it was remarkably similar to your design. I like that you went as far as to consider which side of the turbines and compressors you'd run your shaft through. It all certainly seems plausible! I do suspect you would need a pump rather than an injector, though.

#### Topaz

##### Super Moderator
Staff member
Log Member
For the hybrid jetpack/jetwing suggestion by Topaz, perhaps four spaced vectored outlets similar to the Kestral/Harrier would help solve the transition difficulty. Granted, it would be more complex, but it might be possible, especially if you are using small jets with augmentor tubes for increased volume flow.
It certainly would work, the limitation would be pressure losses through the multiple right-angle turns the flow would have to take. Now, obviously that works on a Harrier, so it's not a show-stopper, just would increase the size of the central engine generating the flow.

#### Topaz

##### Super Moderator
Staff member
Log Member
Well, they dont use any more air than a commercial turbocharger, so perhaps a few of those air filters would help. Theyre only a few pounds, but can almost fit over the entire engine. Give a VERY large surface area with little resistance.
Well, no. They put enough mass-flow of air through the engine, combined with the mass of the oxidized (burned) fuel, to produce the thrust that lifts the entire vehicle. That's a considerable mass-flow, and any kind of foam or paper filter as used in automotive intakes would either choke off the thrust considerably, or require a significantly larger engine to maintain enough mass-flow at a high-enough speed to lift the vehicle and payload.

#### markaeric

##### Well-Known Member
Well, no. They put enough mass-flow of air through the engine, combined with the mass of the oxidized (burned) fuel, to produce the thrust that lifts the entire vehicle. That's a considerable mass-flow, and any kind of foam or paper filter as used in automotive intakes would either choke off the thrust considerably, or require a significantly larger engine to maintain enough mass-flow at a high-enough speed to lift the vehicle and payload.
That's probably correct since you often see drag cars that have massive turbos running without filters. Though I could see that with the right packaging, you could create a filter with a large surface area but little weight.

#### Topaz

##### Super Moderator
Staff member
Log Member
A mesh screen with enough surface area would do the majority of the job, at some weight penalty. It would have to have a big-enough surface area to provide the necessary mass-flow, with a small-town pressure drop, even when partially clogged with debris, which makes it all the larger and heavier.

#### BBerson

##### Well-Known Member
HBA Supporter
The heat can recirculate and cause problems. Special louvered decks were used.
Forward flight helps, but the problem of throwing dust and rocks at nearby cars, the noise and fuel flow, all tend to push to lower disc loading.

#### markaeric

##### Well-Known Member
I'm trying to grasp how a winged VTOL jetpack might work. Particularly the transition between vertical and horizontal modes of flight. I'm thinking something along the lines of a flying flea/freewing where the wings are allowed to pivot (AoA set by aero surfaces). That way, a transition from horizontal to vertical flight could be achieved without gaining altitude or transitioning in or near a stall. Pitch, yaw and roll could be accomplished entirely by thrust vectoring and/or verniers, or some/all aerodynamically while in forward flight.

2