A different type of jetpack

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Tetranitrate
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A different type of jetpack

Post by Tetranitrate » Tue Jul 18, 2006 6:13 pm

Hey this is my first post so please overlook any stupidities that I may have included.
I am trying to build a jetpack. However unlike all the other jetpacks discussions Ive read, this is to provide horozintal thrust. I will build some sort of frame/backpack with 2 small pulse jets on my back to push me forward. I am thinking about the best way to acomplish this and would greatly appreciate any advice that anyone could give me, I have some basic questions.
How many lbs of thrust do you think would be the equivalent of a 25 hp motor driving a propellor, I know its hard to compare the 2, but any rough estimate would be great. Can I power each engine by one of those minature coleman propane tanks 16.4 oz. If I used a different tank for each pulsejet how long do you think it would last for a small pulsejet.

My ultimate goal is to get 2 engines powered by some sort of propane tank that can strap onto my leg or something, to provide forward thrust. I am trying to build a jetpack to provide the equivalent of a paramotor propellor which I know run on at least a 25 hp motor. Ive checked the prices of the paramotors and they are really expensive at like 8,000$ which is out of my range so I looked for other methods of thrust. Ive known about pulsejets for a while but never had a reason to build one.

I will take off from a field using the jetpack to provide forward thrust, and once I am airborne I will fly around until the fuel runs out and drift back to earth, at least thats what im hoping for.

Johansson
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re: A different type of jetpack

Post by Johansson » Tue Jul 18, 2006 9:52 pm

Hi!

A 25 hp powered propeller gives quite a bit of static thrust, fitting a pulsejet with that kind of thrust to your backpack will probably make you puke your guts out before your get airborne... (and set you on fire at the same time)

Build an engine first to get used to running it and get a feel of the noice and vibrations generated by it, if you still feel like trying to build the jetpack then do so and keep the forum updated of your progress!

//Anders

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re: A different type of jetpack

Post by Tetranitrate » Tue Jul 18, 2006 10:06 pm

I am going to use the jetpack to provide a forward thrust, which hopefully will be enough to keep me off the ground if I have an inflated parachute above my head. Ill start off on a field or long strip of ground, get the parachute up over my head by running, ignite the engines, and hopefully move fast enough forward that the parachute acts as a wing and will lift me off the ground.
I looked up and saw that one of the weaker paramotors provides 100 lbs of static thrust? whould I be able to acomplish this with 2 50 lb pulse jets or possibly 4 25s.

pezman
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re: A different type of jetpack

Post by pezman » Tue Jul 18, 2006 11:51 pm

Johansson wrote:... will probably make you puke your guts out before your get airborne... (and set you on fire at the same time)
The idea of using puke as part of a Hybrid engine with solid oxidizer seems as if it may have already been proposed, albeit only very recently. We may be seeing the start of a revolution in propulsion.

The whole notion of jettisoning part of a pilot as fuel, (or in this case, the contents of his stomach) strikes me as extremely novel, wth the idea of just plain igniting the pilot being its elegant extreme, fairly screaming, "Why didn't I think of that?".

For the utmost in efficiency, it should be possible to hook a tube to the pilot's "exit orifice" to introduce methane and "miscellaneous solids" (10MJ/kg) into the mix. A one-way valve on this last connection is advisable.

All kidding aside. Sounds like a novel project, but also sounds a little dangerous

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re: A different type of jetpack

Post by vhautaka » Wed Jul 19, 2006 11:17 am

How about...

A small pulsejet or two, balanced on an axle, rotating a propeller of some kind? Maybe put the entire thing inside a duct, with propellers/fans on one or both ends of duct? Guiding vanes to make it propel nice and straight?

Should also help with the sound and vibration problems a bit, and the pulsejet's intakes and exhausts could be set up to enjoy a lot more efficient airspeed than just being strapped to a slow moving parachuter's back.

Also, if the duct were well enough designed it would use the pulsejet to heat and expand a lot more passing air than otherwise possible, effectively making it a sort of pulse-ram-fanjet or something. The fans, lightweight and maybe max. 20" dia, could be ripped of some junk industrial cooling things.

Designing the fuel supply through bearings and then trying to balance the whole thing should be a lot of fun work-hours. But getting fast rotation out of a relatively small pulsejet would also allow for easy liquid fuel pumping or even generating a bit of electricity for the support systems.

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re: A different type of jetpack

Post by larry cottrill » Wed Jul 19, 2006 12:08 pm

It seems to me that what's being proposed (assuming the safety issues could even be worked out!) places the center of drag a long distance above the center of thrust. This is probably unworkable in an aerodynamic design sense. Also, it makes the pilot a sort of "flexible mount" for the engine, which seems to me to create serious control issues. A pilot has enough to do without thinking about how every move he makes alters the direction of the thrust line!

L Cottrill

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Re: re: A different type of jetpack

Post by milisavljevic » Wed Jul 19, 2006 12:27 pm

Larry Cottrill wrote:This is probably unworkable in an aerodynamic design sense.
Some people can never pass up a chance to put their foot in their mouth. Please refer to the attached image.
Consider visiting this website: [ http://www.powerplanes.com/ ]. And yes, there are more examples out there.

The sky is not falling.
M.
no safe haven for merchant scum


for ye merchants who do the prop'r t'ing only if
ye be haul'd-up on charges b'fore ye ship-mates
an' threat'nd wit' forfeiture of all ye precious loot
hear this - so-called stand-up guys YE BE NOT

avast!
Cap'n M.

tufty
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Re: re: A different type of jetpack

Post by tufty » Wed Jul 19, 2006 12:45 pm

milisavljevic wrote:The sky is not falling.
Indeed. That said, I think I'd be significantly unhappy having pulsejet type operating temperatures anywhere near the harness and canopy lines of a paraglider.

That's just me, though. I have this wholly unnatural and somewhat pre-emptive dislike of plummeting thousands of (or hundreds, or even "several") feet to my death...

Simon

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re: A different type of jetpack

Post by Najm » Wed Jul 19, 2006 12:49 pm

You are proposing an idea in which ramjets should work better
A small pulsejet or two, balanced on an axle, rotating a propeller of some kind? Maybe put the entire thing inside a duct, with propellers/fans on one or both ends of duct? Guiding vanes to make it propel nice and straight?

milisavljevic
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Re: A different type of jetpack

Post by milisavljevic » Wed Jul 19, 2006 12:54 pm

tufty wrote:I think I'd be significantly unhappy having pulsejet type operating temperatures anywhere near
the harness and canopy lines of a paraglider.
Nous sommes en l'accord 100%!

M.
no safe haven for merchant scum


for ye merchants who do the prop'r t'ing only if
ye be haul'd-up on charges b'fore ye ship-mates
an' threat'nd wit' forfeiture of all ye precious loot
hear this - so-called stand-up guys YE BE NOT

avast!
Cap'n M.

larry cottrill
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Re: re: A different type of jetpack

Post by larry cottrill » Wed Jul 19, 2006 1:04 pm

milisavljevic wrote:
Larry Cottrill wrote:This is probably unworkable in an aerodynamic design sense.
Some people can never pass up a chance to put their foot in their mouth. Please refer to the attached image.
Consider visiting this website: [ http://www.powerplanes.com/ ]. And yes, there are more examples out there.

The sky is not falling.
M.
OK, I stand corrected. I should not be commenting outside my area of experience; obviously the "problems" of pilot-mounted power have been worked out, at least for operators of reasonable skill.

I did not say, however, that the sky was falling. My only concern was people falling from it to a sudden stop.

I would rather have my foot in my mouth than in someone else's face. My intent is never to discourage someone from trying something, only to encourage them to try things in reasonable ways that have been carefully thought through. When I deliver just plain bad information, it is my own fault alone, and I am readily willing to admit it as such and hope that the corrected view will be accepted.

L Cottrill

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Re: re: A different type of jetpack

Post by larry cottrill » Wed Jul 19, 2006 1:14 pm

Najm wrote:You are proposing an idea in which ramjets should work better
A small pulsejet or two, balanced on an axle, rotating a propeller of some kind? Maybe put the entire thing inside a duct, with propellers/fans on one or both ends of duct? Guiding vanes to make it propel nice and straight?
Najm -

Both pulsejets and ramjets have been proposed for "tipjet" use, and actually tried. I agree that, theoretically, the ramjet seems the better choice. But, several technical problems, especially fuel delivery, make this a difficult idea to implement, no matter what powerplant you use. One problem is that a propeller has to operate over a very wide range of speeds (in order to offer good power control), so the smooth throttling of the ramjet over a wide power range at widely differing air mass flows without flameout is a serious design difficulty.

It's one of those things that can be (and has been) done with some success - but is just a lot more difficult than it looks on the surface.

L Cottrill

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re: A different type of jetpack

Post by Johansson » Wed Jul 19, 2006 1:29 pm

I think that the problems with two hot pulsejets near yourself can be solved by fitting an aerodynamic shell around them that at the same time can act as augmentors, valved engines is probably more suitable due to their smaller overall diameter compared to a bent valveless.

One of the major problems as I see it is the fuel consumption. Two augmented 30 lbs valved engines should provide enough thrust to fly, but they will be quite thirsty. A guesstimated SFC of 2 for the engines gives 120 lbs of fuel per hour or 2 lbs per minute, so a 10 liter fuel tank will last only 10 minutes. An even bigger fuel tank will add too much weight to the backpack to be practical since the two engines with augmentor shells will be quite heavy, then add a strong engine mount and you will have a very heavy backpack...

//Anders

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re: A different type of jetpack

Post by Anders Troberg » Thu Jul 20, 2006 6:03 am

At the speeds we are talking about here, I don't think we have to bother much about aerodynamics.
I have this wholly unnatural and somewhat pre-emptive dislike of plummeting thousands of (or hundreds, or even "several") feet to my death...
Nah, falling is perfectly safe.

Landing, on the other hand...

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re: A different type of jetpack

Post by vhautaka » Fri Jul 21, 2006 8:57 am

A bit more off topic, check:

http://www.rcgroups.com/forums/showthre ... 1653&pp=15
http://www.marksparaglidingpages.com/vi ... ideoid=150

A LiPoly pack + an efficient electric motor would be the way to go if I was to implement a backpack propeller thingy for serious parachute use.

Environmental too, and quiet - current pulsejets are definitely not for low-altitude flight in inhabited areas. Make it regenerating and you can charge the batteries in thermics, extending the range.

Or then just find some kinda chainsaw engine to drive the prop, those things have horsepowers and light weight and would require no brain to convert:

http://www.rcfaq.com/ANSWERS/ENGINES/WEEDIES.HTM

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