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Help needed to design a pulse jet engine.

  1. Jun 20, 2009 #1
    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.

    Last edited: Jun 21, 2009
  2. jcsd
  3. Jun 20, 2009 #2
    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.
  4. Jul 19, 2010 #3
    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 thurst. http://www.youtube.com/watch?v=QkOR8IZPsFg&feature=related

    5 lbs thrust. SR-71

    10 lbs thrust http://www.youtube.com/watch?v=u2apj002QwY&feature=related

    10 lbs thrust. http://www.youtube.com/watch?v=jqxjgmelP-Q&feature=related

    20 lbs thurst.http://www.youtube.com/watch?v=1VNyTsUT2Xg&feature=related
    Last edited by a moderator: Sep 25, 2014
  5. Sep 20, 2011 #4
    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
  6. Sep 19, 2013 #5
    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.

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