# How Can I Calculate Pulse Jet Engine Dimensions Using Air as Oxidizer?

• gary350
In summary, Gary does not know how to calculate the dimensions of a pulse jet engine and does not know of any Forumulas or how to calculate the dimensions of a pulse jet engine.
gary350
Does anyone know any Forumulas or how to calculate the dimensions of a pulse jet engine?

I have already built several PULSE JET engines that run by scaling up an existing hobby engine to a larger size. Problem is the scaled up models run but maybe they could run better. I can not find any formulas to design and build a PULSE JET ENGINE. Another problem is pulse jet engines run on air as the oxizider not oxygen like most liquid fuel rocket engines. I can not find any information for using AIR as the oxidizer in a rocket engine. My plan here was to use the formulas to design a liquid fuel rocket engine and use that information to build a Pulse Jet Engine but the physical size of the 100 lb thrust liquid fuel engine is many times smaller than a pulse jet engine so this will not work. I am using Gasoline as the fuel only because it is cheap and easy to get, Alcohol works much better. I am using gaseous oxygen as the oxidizer.

Calculations for a 100 lb thrust liquid fuel rocket engine, gasoline + oxygen.

Isp = Specific Impulse

Wo = lb of oxygen / sec

Wf = Lb of fuel / sec

Wt = wo + wf

Wo = .293 lb / sec

Wf = .117 lb / sec

Wt = .41 lb / sec

Gamma = 1.2

Dc = combustion chamber diameter = .4555"

Lc = combustion chamber length = 2.15"

Dt = nozzle diameter = .238"

Dc = nozzle exit = 1.2"

nozzle angle 15 degrees.

Trial and Error from scaling up a hobby engine to 100 lb Thrust. This works but I think it could work better. I have no idea how efficient my engine is. I have no information on AIR as an oxidizer to determine if my air intake is correct and the combustion chamber is the correct size. Gasoline + Air

Cr = Combustion chamber compression ratio = 2 to 1

Dc = combustion chamber diameter = 5.000""

Lc = combustion chamber length = 6.000"

Ai = Air intake = Dt x .8 = 2.000"

Dt = nozzle diameter = 2.5"

Tl = Tail Pipe length = 27"

Dc = nozzle exit = 3.5"

nozzle angle 15 degrees.

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I have one of about that size. The pipe is missing, so it's difficult to tell. Which means, I have a hunk of aluminum casting, the combustion chamber shell and some reeds.

I have it all figured out. Math is simple and it applies to all size engines. Here are videos. I have run them all for about 2 hours each static thrust no problems reed valves hold up fine. I made some new discoveries how to protect the reed valves from combustion chamber heat. Engines start at 20% thrust and throttle up to 100% static thrust but this is not maximum thrust forward speed produces ram air so the engine can be throttled up to 140% to 150% thrust depending on the speed of the engine and the design of the air intake.

5 lbs thrust. SR-71

Last edited by a moderator:
hi Gary, I've been reading your posts, I'm trying to figure out a pulse jet engine size according to thrust, I built one already according to a ready made plan, it didn't work till I changed the length and diameter of the exhaust tube, but this took a lot of time as a trial and error technique, then I got the right size of the tube without really knowing a basis to relate that size to the thrust, if you got any calcualtions (other than pj calculator 1.4 from the internet) please advice, thank's

I found an interesting link on designing a ramjet engine. Some of the areal formulas are incomplete as shown, but nothing a simple square root won't fix. His calculation was correct he just left off the square root symbol in writing the equation.

http://www.pulse-jets.com/ramjet/ramjet.htm

## What is a pulse jet engine?

A pulse jet engine is a type of jet engine that operates on the principle of using rapid pulses of combustion to generate thrust. It typically consists of a tube with an inlet, a combustion chamber, and an exhaust nozzle.

## What are the main components of a pulse jet engine?

The main components of a pulse jet engine include an inlet for air intake, a combustion chamber where fuel is ignited, and an exhaust nozzle where the hot gases are expelled to create thrust.

## How does a pulse jet engine work?

A pulse jet engine works by taking in air through the inlet, compressing it in the combustion chamber, and then igniting fuel to create a rapid pulse of combustion. This creates a high-pressure wave that travels out of the exhaust nozzle, propelling the engine forward.

## What are the advantages of using a pulse jet engine?

Some advantages of using a pulse jet engine include its simplicity, low cost, and high power-to-weight ratio. It also has fewer moving parts compared to other types of jet engines, making it easier to maintain.

## What are some challenges in designing a pulse jet engine?

Designing a pulse jet engine can be challenging due to its complex aerodynamics and the need to carefully balance air intake, fuel injection, and exhaust flow. It also requires precise timing and fuel control to maintain a stable combustion process.

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