Jetpack rides (flies?) again

[Bruno Ogorelec]

I'd just skip lateral movement for simplicity's sake. This needs to be as failsafe a linkage as possible. (snip) A simple bar with hand grip to each nozzle would be safest. If one starts to get sticky, you just pull harder and look for a place to land, preferably near a beautiful farmgirl... Oops, mind is drifting....

You describe it so nicely. However, weight shift (tilting the entire assembly) is so much more intuitive. It gives you freedom of movement in all horizontal directions, effortlessly, almost without thinking.

Mechanically, it is simpler, too. When you gimbal the entire 'wing', there's no control linkage of any kind, unlike the gimbaled nozzles, which have to be steered individually. To me, that spells greater safety, not lesser. I like rigidly set nozzles better.

I don't see two sets of gimbals, each with independent steering linkage, as mechanically safer than a single, relatively massive set without steering linkage.

Those Bell Labs itsy bitsy little things don't look that impressive to me. On the other hand, their gimbals only have to carry the nozzles, not bear the mass of the entire pilot, engines, fuel...

I don't know. I find it very difficult to decide. I have no practical experience that would give me a reliable gut feeling, but weight shift has really worked well gor ages in hang gliding. (Admittedly, it is a slightly different case, but not completely different.)

[Mike Everman]

I don't know. I find it very difficult to decide.

Well, then attack one of the other copious hurdles for now!

You still need to twist them differentially for yaw. No amount of weight shift will do that. Weight shift is really what any approach is going to do, ultimately. But It's like comparing riding a bicycle with and without hands; there's only so much manouverability you'll get with shifting your weight.
On this, I'd rather use my hands and have my CG go where nature requires, rather than haul my legs up to go forward.

[Bruno Ogorelec]

I don't know. I find it very difficult to decide.

Well, then attack one of the other copious hurdles for now!
You still need to twist them differentially for yaw. No amount of weight shift will do that. Weight shift is really what any approach is going to do, ultimately. But It's like comparing riding a bicycle with and without hands; there's only so much manouverability you'll get with shifting your weight.
On this, I'd rather use my hands and have my CG go where nature requires, rather than haul my legs up to go forward.

Ah, but yaw is the easiest of all. That's the only case in which you have a force couple, with a long leverage and almost all the mass in the center (very low polar moment of inertia). So, a simple vane in each nozzle, tilted at just a few degrees, will give you a nice twist.

The way you describe weight shift, it sounds difficult, but everyone I have talked to says it's almost like riding a bicycle, almost totally intuitive. You don't really have the impression you are shifting weight -- you push and pull on the bar as if it were a hamdlebar.

Nevertheless, I try to keep an open mind. The jury is still out.

[Raymond G]

I've been following these threads for awhile, but have been busy elseware so haven't posted. I think it was Mike who wrote in the previous thread that we all want to make a jet-pack. Well, I am certainly no exception to that statement. Jet-packs have been dear to my heart for decades now. My personal preference for design is gas turbine based (i.e. turbofan). In fact, before this thread started, I was unaware of the Williams WR-19 based design that someone has included a picture of. I believe this approach to have one of the best chances of success.

I think for safety reasons, a turbofan design should not have the fan and turbine burst planes intersect the pilot's body, so the Williams WR-19 based jet belt would need to have the engine in a different orientation, which would be bulky. The Monocopter design shown early in the previous thread does a good job of moving the fan plane away from the pilot, though at the cost of considerable complexity. I've personally thought of having some kind of 'pancake' engine which exhausts through a couple of ejectors. Interestingly, such a design might end up looking like the jet packs from 'The Rocketeer' and 'Toy Story' (Buz Lightyear's pack). Another interesting thought about the Buz Lighyear pack is that it resembles a winged WR-19 concept presented in 'Popular Mechanics' over 20 years ago.

Regards,
Raymond

[Raymond G]

While it is certainly fun to talk about this stuff, I've always felt that one of the first steps to making technical dreams a reality is to put a specification on what is required.

So, for a rocket/jet pack what would be the specifications? I can't speak for everyone, but can put out some thoughts of my own:

Configuration: Backpack. Something you can pick up and easily strap on your back!

Purpose: To Have Fun with something that some or several of us could build in our garages!!! Other uses are many, but would make other specs much harder to achieve, esp. flight duration.

Backpack Weight: Human portable, i.e. about 75-100 lbm, depending upon pilot weight. Probably 1/2 pilot wieght is max.

Thrust: Enough to be safe on a hot day at max gross weight i.e. 300-400 lbf. If multi-engined, enough thrust to land safely with one engine out.

Flight Duration: Enough to have some fun! 5-10 minutes min. Greater duration makes for more fun and more uses such as search and rescue, flying ejection seat, transportation, etc. but also makes design much more difficult. A wing may extend this greatly such as old Popular Mechanics design and Buz Lightyear.

Control: Highly stable AND simple design with intuitive interface. High thrust axis is one possibility, like the Williams, Gluhareff, and Bell designs. "Hollywood" designs with low thrust axis are probably unstable like Rocketeer, Buz Lightyear, and Minority Report. Spy Kids design offers an intriguing option to be able to roll in place with 4 vectored nozzles, but complex

Safety: Failsafe for VTOL flight, i.e. if engine fails, or runs out of fuel, can safely recover. Ballistic chute (as used on ultralights) would be an option.

Hope these specs get the thoughts flowing!

Regards,
Raymond

[NanoSoft]

Here is a very interesting and amazing turbine jet pack site. Very cool rear actuating mechanism. Worth a look

http://www.technologie-entwicklung.de/G ... opter.html

[hinote]

So, for a rocket/jet pack what would be the specifications? I can't speak for everyone, but can put out some thoughts of my own:

Configuration: Backpack. Something you can pick up and easily strap on your back!

Purpose: To Have Fun with something that some or several of us could build in our garages!!! Other uses are many, but would make other specs much harder to achieve, esp. flight duration.

Backpack Weight: Human portable, i.e. about 75-100 lbm, depending upon pilot weight. Probably 1/2 pilot wieght is max.

Thrust: Enough to be safe on a hot day at max gross weight i.e. 300-400 lbf. If multi-engined, enough thrust to land safely with one engine out.

Flight Duration: Enough to have some fun! 5-10 minutes min. Greater duration makes for more fun and more uses such as search and rescue, flying ejection seat, transportation, etc. but also makes design much more difficult. A wing may extend this greatly such as old Popular Mechanics design and Buz Lightyear.

Control: Highly stable AND simple design with intuitive interface. High thrust axis is one possibility, like the Williams, Gluhareff, and Bell designs. "Hollywood" designs with low thrust axis are probably unstable like Rocketeer, Buz Lightyear, and Minority Report. Spy Kids design offers an intriguing option to be able to roll in place with 4 vectored nozzles, but complex

Safety: Failsafe for VTOL flight, i.e. if engine fails, or runs out of fuel, can safely recover. Ballistic chute (as used on ultralights) would be an option.

I have some (possibly more realistic) notions about a PJ-powered "backpack":

1. The notion of a true backpack design is probably not feasible; the necessary dimensions to create the desired thrust (at a reasonable [low!] percentage of max thrust--for engine reliability) means that the combo of engine weight, ducting, fuel storage/supply and other ancillaries such as battery, ballistic parachute, etc.--is beyond the dream of a backpack concept.

2. The alternative is a "strap the pilot in" concept, which would have the power unit standing on its own legs, and the pilot would step into the assembly and strap himself in. I don't see a whole lot of difference in the result--except that the poor "pilot" wouldn't have to bear the weight of the whole thing before it gets off the ground.

3. Additionally, several refinements could be incorporated that would improve the quality of the product. These would include impact-absorbing "landing gear", increased fuel capacity, easy on-board start capability, (add your own list to the above).

I see the concept as a tripod, with a mast "backbone", which would be the structural center of the assembly; the engines would be attached to one side of the mast and the pilot would stand on a platform (or possibly sit) on the other side.

It is easy to divvy-up a weight analysis to provide the necessary items listed above. I estimate the total empty weight of the project at something like 200 lb., and the gross takeoff weight about 500 lb. including a 200 lb pilot and 20 gals of fuel--good for about 15-20 minutes of operation including start-up and reserves.

The engines necessary to make this a reality are considerably bigger than those possible for a backpack concept.

I would like to see a realistic concept that could be light enough to be a true backpack concept--but I really don't think it's possible to achieve with our pulsejets. The issue of thrust density prevents the dream of a backpack from being a potential reality.

Bill H.
Acoustic Propulsion Concepts

[hinote]

The engines necessary to make this a reality are considerably bigger than those possible for a backpack concept.

I can only imagine taking the Ecrevisse engine concept, and refining it into a high-density engine to create the necessary thrust. Actually, the thrust created wouldn't need to be a whole lot more than the original bare combustor, because the optimized augmentors would provide the remaining required lift--and static (or low-speed) augmentation is relatively easy to develop.

A multi-folded engine (or possibly an "inside-out" annular concept) to reduce total length, would probably be necessary.

Bill H.
Acoustic Propulsion Concepts

[Mike Everman]

On the whole, I'm with Bill, I think the "rider" is the way to go, and I've been conceptualizing an array of engines around the cabin such that if one cuts out, the others automatically redistribute themselves evenly around. i've thought of simple and light ways to do this.

Otherwise, the backpack is a great thought exersize that forces you to remove all non-essentials, and strive for the highest thrust to weight ratio. Sun Tzu (sp?) said something like "plan for victory, prepare for defeat". I think it's fitting.

[Raymond G]

Certainly a sit in VTOL would make some aspects of the design easier, such as weight and endurance. But I think it would at the same time complicate the control issue. Moller has had the Volanter concept for something like 20 year now and the limiting factor isn't the performance, it's the control. Same with Solotrak. I think the AirBike would also have problems with control. I like the AirBike alot, it reminds me of something I sketched a lot of in gradeschool, and think it is a likely canditate from a performance standpoint, but may not be achievable from a controls point of view. Historically, aircraft have had to solve the duel problems of making enough power AND being controllable berfore any flight success. Happened with the Wright Flyer, the helicopter, the X-1, and the Harrier prototype. If you are Lock-Mart, you just throw money and people at the problem, but I want something that I could conceivably build in my garage, so it needs to be simple. That's why I like the backpack concept. An engine with armstraps, fuel tanks, and exhaust ducting attached. Don't know if you can get any simpler than that.

So I see several classes of personal VTOL:

1. "Portable" Backpack, could be carried by a person. Bell Rocket Belt, Williams jet belt (?)
2. "Non-Portable" Back pack, too heavy to be lifted by a person, but still generally back pack sized. Monocopter and SoloTrek size
3. Sit on Solo or Duel. AirBike
4. Sit in Solo or Multi. Volanter, Air Cars

Raymond

[marksteamnz]

Keep looking and you will find the Williams jet pack morphed into the jet platform for exactly the reasons Bill raised, landing with any sort of velocity tended to sprain or break limbs. The thing was heavy and while a PJ unit might be lighter, with fuel for a 20 minute flight it's going to need the suport cradle / shooting stick affair the jet pack had. As an aside the Williams platform was supoedly very easy to fly. Part of the Army testing was by regular troops so they could confirm you didn't need to be a cross between Tony Hawk and Neil Armstrong to fly it. Worked fine very easy to learn and the 3 nozzles were at the pilots feet

Book re this is WASP by Terry Metzgar.

What happened.
Fantastic solution........just never found the problem to be solved by it.
Plus as we beat to death in the old forum if it goes quiet at 40 ft. The ballistically deployed vechicle recovery parachute will make a nice shroud

[Raymond G]

I believe that it is possible to make a portable jet backpack, at least if it is turbine based. Attached is a spreadsheet detailing a crude weight breakdown. Naturally there are many assumptions:

1. Gas turbine based engine with thrust/weight ratio of 8:1 and 350 lbf thrust for a gross weight of 300 lbm.
2. Most/all exhaust ducting can be made from super thinwalled titanium and CFRP.
3. Very little support structure is required, as the engine is the support structure
4. Batteries, FADEC, starter motor and related hardware is very light
5. I haven't forgotten something important and HEAVY.
6. No margin. This is acceptable, as it can be traded for fuel, ejector ratio (see below) and/or risk.
7. Risk: BRS included. Could be left out for a 16 lbm weight savings if margin is consumed
8. 'Portable' limit is about half of the pilot's weight

Note that in the spreadsheet there is a breakdown of flight duration vs SFC. The significance here is effective bypass ratio of the turbine based engine. I envision a single engine blowing into ductwork and exhausting through ejector nozzles. These nozzles can have the effect of increasing the bypass ratio 2-4 times! This would greatly improve SFC even for a straight turbojet. A SFC of .25-.5 is thus not all that fantastic. This effect would also ease the weight effect of the engine, as it could be significantly smaller than the 350 lbf thrust engine used in the spreadsheet, and could also give better thrust margin.

As an example, consider a 200 lbf thrust turbojet with a 2:1 ejector. The engine weighs about 19 lbm less than the spreadsheet example. Lets say that 10 lbm is needed for the ejectors, so the net savings is 9 lbm. Lets say that the turbojet had a SFC of about 1.2 lbm/lbf*hr (Microjet Engineering HF series) then the Jetpack's SFC is .6! This example would have an endurance of over 10 minutes with about 5 gallons of fuel! That would be a pretty cool toy!

Just some fun thoughts on the subjet

Raymond

[Raymond G]

OK, I just couldn't resist. I've attached a sketch of a concept to illustrate some of the ideas I've been talking about. I've enjoyed Larry's Paint drawings so much, I thought I'd give it a try. Geez its late, I'll type more tomorrow.

Regards,
Raymond

[Mark S.]

"I see the concept as a tripod, with a mast "backbone", which would be the structural center of the assembly; the engines would be attached to one side of the mast and the pilot would stand on a platform (or possibly sit) on the other side."

I have thought about a sit down tripod VTOL for sometime now. A jet powered version of the Hiller XROE1 Rotorcycle is what I have envisioned. Using the basic design you could easily adapt the power configuration you wanted.

[Tom]

Hi all,
I have been watching this, and I have had an idea for quite sometime, and it actually seems like it might go along the lines of this discussion. My idea is of a jet sled of sorts. Those of you who have seen the latest 007 film will know what I am on about. A small, lightweight frame with wings, propulsion that you can lie upon. I drew up a sketch in biology, which I will refine when I get home, after which I will scan it in and upload it to this post. I really am getting more confident that it can be done with all this talk of Jetpacks and the lark.

Tom