Fundamentals of air propulsion?

[rocketman83]

Can anyone break down the fundamentals of air propulsion? thank you. I also have a few other questions.

 

[Langbein]

What do you mean, the the equitation for a rocket engine, or what ?

Rocket engines, jet engines and propeller engines all have different equitations.

Please clarify what you are searching.

 

[rocketman83]

What do you mean, the the equitation for a rocket engine, or what ?

Rocket engines, jet engines and propeller engines all have different equitations.

Please clarify what you are searching.

Jet Engines.

 

[Langbein]

http://en.wikipedia.org/wiki/Jet_engine

 

[Langbein]

For a clean jet engine (Normally not used any more due to noice and high fuel consume.):

The air breathing engine is only useful if the velocity of the gas from the engine, c, is greater than the airplane velocity, v. The net engine thrust is the same as if the gas were emitted with the velocity c-v. So the pushing moment is actually equal to
S = m (c-v)

Most jet engines today is fan engines, but actually the basic formula should be the same ..

 

[Langbein]

OK a little bit more accurate (Depending how you measure the outgiong air speed.)

The low bypass turbofans have the mixed exhaust of the two air flows, running at different speeds (c1 and c2). The pushing moment of such engine is S = m1 (c1 - v) + m2 (c2 - v)

(If you have a exust nozzle or an afterburner like on a military aircraft the two airflow will be mixed so there will be only one airspeed to calculate, and the first formula will be valid.)

 

[rocketman83]

For a clean jet engine (Normally not used any more due to noice and high fuel consume.):

The air breathing engine is only useful if the velocity of the gas from the engine, c, is greater than the airplane velocity, v. The net engine thrust is the same as if the gas were emitted with the velocity c-v. So the pushing moment is actually equal to
S = m (c-v)

Most jet engines today is fan engines, but actually the basic formula should be the same ..

Langbein, its a pleasure to meet you. Thanks for the info. I'm sure you have seen these guys with Jet packs. Now would that be a air breathing engine? Do you know anything about Jet packs?

 

[Langbein]

"Do you know anything about Jet packs?"

No, but I have been working with "real" jet engines F-16 among other.

Do you have a link to the "Jet Pack" ?

 

[rocketman83]

"Do you know anything about Jet packs?"

No, but I have been working with "real" jet engines F-16 among other.

Do you have a link to the "Jet Pack" ?

http://www.jetpackinternational.com/

http://www.engadget.com/2006/07/31/skywalker-jet-packs-in-development/

http://en.wikipedia.org/wiki/Jet_pack

Here are a few websites I have found...

 

[Langbein]

These looks basically like rocket engines and not jet engines.

When I look at the formulas I forst gave you they are not really jet engine formulas but rocket engine formulas (Wikipedia is not precise.)

S = m (c-v)

This should allmost be the formula for a rocket engine, a bit more accurate like this:

S = m*k(c-v)

Where S = trust in Newton, k = some constant, c = gassflow out of the rocket engine (m/s), v= speed of aircraft/rocket, m = massflow out of the rocket engine (kg/s)

Will have to sleap now but will look at the question tomorrow. I might find some of my old books with some more presice info.

 

[rocketman83]

These looks basically like rocket engines and not jet engines.

When I look at the formulas I forst gave you they are not really jet engine formulas but rocket engine formulas (Wikipedia is not precise.)

S = m (c-v)

This should allmost be the formula for a rocket engine, a bit more accurate like this:

S = m*k(c-v)

Where S = trust in Newton, k = some constant, c = gassflow out of the rocket engine (m/s), v= speed of aircraft/rocket, m = massflow out of the rocket engine (kg/s)

Will have to sleap now but will look at the question tomorrow. I might find some of my old books with some more presice info.

Sounds good. Do you think its possible to build a faster or more maneuver jet pack?

 

[Danger]

I didn't check out your links, but I will say that most 'jet-packs' that I've seen were peroxide rockets. They're inherently unstable, but not uncontrollable. The 'mileage', however, is pretty scary.

 

[FredGarvin]

The jetpack is a hydrogen peroxide rocket essentially. There is not a jet engine in it. My company built and flew a version that used an air breathing jet engine.

The basic premise of all of these types is the change in momentum of the flowing fluid creates thrust. This is why they are referred to as "reaction engines."

To be precise, the second equation that was posted by Langbein is pretty close. The net momentum is what counts. Although, to be precise, you need to subtract the inflow momentum of the fuel. It's usually negligible though. You also have to add, what is called, the P/A contribution to thrust due to nozzle geometry. It may be zero, it may not. It all depends on the area of flight that the particular engine is operating.

 

[rocketman83]

I didn't check out your links, but I will say that most 'jet-packs' that I've seen were peroxide rockets. They're inherently unstable, but not uncontrollable. The 'mileage', however, is pretty scary.

I've read that about Jet Packs. Are there any other type of rockets other then Peroxide Rockets?

 

[FredGarvin]

I've read that about Jet Packs. Are there any other type of rockets other then Peroxide Rockets?

There are tons of other types of rocket engines other than HO rockets.

 

[Danger]

Oh man... the variety is almost endless. You could use nitric acid and unsymmetrical dimethylhydrazine if you want to be the next national spokesman for Preparation H. Icing sugar and sulphur and potasium nitrate can be interesting. Then there's the ever popular kerosene/liquid oxygen combination as used in the Saturn V. Good old black powder works reasonably well, but there's an issue with throttleability...

 

[rocketman83]

There are tons of other types of rocket engines other than HO rockets.

Which type would be more suitable for a man operated jet pack?

 

[rocketman83]

Oh man... the variety is almost endless. You could use nitric acid and unsymmetrical dimethylhydrazine if you want to be the next national spokesman for Preparation H. Icing sugar and sulphur and potasium nitrate can be interesting. Then there's the ever popular kerosene/liquid oxygen combination as used in the Saturn V. Good old black powder works reasonably well, but there's an issue with throttleability...

Turbojets
Packs with the turbojet engine work on the traditional kerosene. They have higher efficiency, greater height and a duration of flight of many minutes, but they are complex in construction and very expensive. Only one working model of this pack was made; it underwent flight tests in the 1960s and at present it no longer flies. One report indicates a sighting in 1984 of an anonymous flier in Maryland.

I found this on wikipdeia. As you were saying, they used Kerosene.

 

[Danger]

That would be the peroxide unit, which is why most of the focus has been in that area. As Fred mentioned, his company tried out a turbojet system. Others have as well. There are serious issues with all of them.

edit: Just caught your last post which you sneaked in while I was composing this one. I was actually referring to the LOX/kerosene rocket. Most jet engines use kerosene of some grade or another (Jet A, Jet B, etc.), but they can burn diesel or several other fuels. The same J-34 turbojet that burned premium jet fuel in military fighters is now on the dragstrip sucking diesel. Some others use propane or natural gas. One of the great things about jets is their versatile diet.

 

[rocketman83]

That would be the peroxide unit, which is why most of the focus has been in that area. As Fred mentioned, his company tried out a turbojet system. Others have as well. There are serious issues with all of them.

edit: Just caught your last post which you sneaked in while I was composing this one. I was actually referring to the LOX/kerosene rocket. Most jet engines use kerosene of some grade or another (Jet A, Jet B, etc.), but they can burn diesel or several other fuels. The same J-34 turbojet that burned premium jet fuel in military fighters is now on the dragstrip sucking diesel. Some others use propane or natural gas. One of the great things about jets is their versatile diet.

Thanks for all the info. I'm curious how possible it will be in the future to have a Jet Pack that can self sustain for longer then a few minutes and can go faster and longer distances. When I think of the future of Jet packs, the movie Rocketeer comes to mind.

 

[Danger]

Well, that was fiction after all. You might as well go for Iron Man's microturbo boot jets. I really am going to have to defer to Fred on this. I'm way out of my depth, and this is his area of expertise.

 

[rocketman83]

Well, that was fiction after all. You might as well go for Iron Man's microturbo boot jets. I really am going to have to defer to Fred on this. I'm way out of my depth, and this is his area of expertise.

True, it is fiction but I believe it can be done. I have a gut feeling it not out of our reach as humans to create something so magnificent. Thank you bro, I would really like to talk to Fred. This is what I in-vision for the future below...

 

[FredGarvin]

Well, the realities that have to be dealt with are amount of fuel that needs to be carried and the ability to control the thing. Those are not trivial. The nitrous jet pack has a flight time of about 30 sec IIRC.

 

[Danger]

Did you create that picture, Rocketman? It's really good.
It does, in fact also illustrate another reason for peroxide rockets being favoured over other systems. The exhaust consists of water (well, steam initially, then it condenses). It's hot as hell, but there's no fire hazard.