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Liquid rocket gas generator question

  1. Feb 14, 2013 #1
    I am building a liquid fueled rocket engine driven gas generator. I plan to use gasoline for fuel and oxygen gas at a rate of .022lb/sec and O2 at .055lb/sec The chamber is 2.15 in long 1.15 in id with a .238 nozzle. The chamber pressure should be 300 psi with a thrust of 20 pounds. I calculate the thermal output at 2583 BTU/sec. The velocity should be about 1mi/sec. This is a copy of the engine described in ROCKETLABS -How to design,build and test small liquid fueled rocket engines'.T I am trying to calculate the rate of water injection in lb/second required to reduce the temp/pressure of the output of the thrust stream to a high velocity 1200 degree gas stream suitable to drive a turbine. I am open to ideas on the location,angle and configuration of the water injector nozzles and the proper turbine A/R ratio required to optimize this this type of thrust. What thrust nozzle style would be appropriate for expansion of hot gas stream when expanding the thrust to turbine intake. What would be the proper cross section of the entrance to the turbine volute. Any thoughts on the matter would be welcome
    Last edited: Feb 14, 2013
  2. jcsd
  3. Feb 14, 2013 #2
    My thoughts are to:

    1) Get, read and understand one of the Sutton's books" http://www.ebay.com/itm/LIKE-NEW-RO...005?pt=US_Nonfiction_Book&hash=item35b5ff15a5
    2) Get, read and understand http://www.ebay.com/itm/MODERN-ENGI...352?pt=US_Nonfiction_Book&hash=item5aed4f8968
    3) Find all the AIAA papers still in the public domain as possible (many from NASA) about rocketry.
    4) Try designing and testing a monoprop engine first. Biprops are very difficult and very small biprops are even more difficult. Learn and obey the local laws regarding testing (talk to your fire marshall, though they may not be very cooperative). Do NOT do this in your garage or around 'uninvolved' people. Consider a cold gas or hot gas engine design instead of liquids. http://en.wikipedia.org/wiki/Cold_gas_thruster
    5) Find an amatuer rocketry group to join that can help keep you alive.
  4. Feb 15, 2013 #3
    Engine is copy of project motor described in ROCKETLABS book- How to design,build and test small rocket engines. I intend to use it to drive a GARRETT turbo and rework or cast and machine new exhaust side housing to suit application.Turbochargers are designed to operate with high volume/low thermal expansion exhaust gas in an application that is very limited in the back pressure that will allow the turbo to work correctly. Low A/R ratios drive the compressor with much less mass gas flow and higher back pressure. When this is done on a turbo installed on a motor low speed air flow is high but high back pressure prevents higher engine speeds.The turbine maps show that as you reduce the A/R ratio the mas flow required to compress the same lb/min of air to the same pressure ratio drops and back pressure goes up. I realize that I will have to fabricate a housing to suit the radically different flow characteristics involved. In the early 1960s Turbonique of Orlando fl marketed a mono propellant (N-Propyl-Nitrate) powered turbo. I have studied the engineering that went into this set up in detail and have a full set of plans for all their parts. This system burned .127 lb/sec of the n-p=n fuel. This fuel decomposes at 2000-2200 F. This was done at a chamber pressure of 300 psi and gave a gas velocity of 1 mi/sec. At 7420 BTU /lb this generated a thermal output of 942 BTU/sec and a gas flow of.127 lb/sec . Gasoline/O2 has a Is of 260 and at a 2.5/1 mix rate burns at 5750 F and 20,855 BTU/lb This gives a output of 4583 BTU/second @.022 lb/sec. 42.41 BTU=1hp is the conversion factor At 4583BTU/sec the thermal output works out to be 648 horsepower. I do not know how to figure how much kinetic energy is contained in the gas flow of 4.62 lb/min at 1 mile/sec and may need to adjust lb/sec etc to suit the task at hand. On the face of it there would appear to be more than enough energy to do the amount of work in question. This was done 50 years ago with very crude turbine wheels,a lower thermal value fuel and very crude machine tools. I know there is a way to safely harness this power source to do this. I have the skills required to redesign and make the parts and understand the technology well enough to do this. I will find a way to bring this setup into the modern era.
    Last edited: Feb 15, 2013
  5. Feb 15, 2013 #4


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    Thread closed for Moderation...

    [STRIKE]Thread will stay closed because of the dangerous nature of the subject.[/STRIKE]

    Thread is re-opened. Turns out the OP has a lot of experience in this area, so the activity is not as dangerous as if a newbie were asking about it.
    Last edited: Feb 25, 2013
  6. Feb 26, 2013 #5
    NASA made a steam generator for the Stennis center that consisted of the injector head of a flight engine installed in a straight pipe with water injection jets at the bottom to inject water against the gas flow to make steam.
    I want to make a similar setup to make 35 lb/sec@ 1700F steam. I intend to use gasoline/O2gas for fuel
    The NASA unit did not use a nozzle and cooled the chamber with the water in the jacket that surrounded the chamber and fed the water injectors
    I am thinking about 50k BTU/min would be in the ball park. NASA used a spark plug for ignition. Any thoughts on possible sub sonic nozzle to maximize kinetic energy or am I better off with straight steam. The NASA doc detailing this unit is Design and Activation of a LOX/GH Chemical Steam
    G.P. Saunders1
    Jacobs Technology Group, Stennis Space Center, MS, 39529
    C.A. Mulkey2 and S.A. Taylor3
    National Aeronautics and Space Administration, Stennis Space Center, MS, 39529
    Any comments or thoughts would be welcome
  7. Mar 6, 2013 #6
    If you don't have a copy yet, go find an early addition of "Rocket propulsion elements" by George Sutton. The book is a valuable resource.

    Several years back now I built a fairly large (for amateurs) liquid rocket engine and eventually flew a vehicle with it. A video of that flight can be seen here : http://vimeo.com/54742023 Please forgive the video quality.

    Are you just working on a gas generator or do you see it integrated with a larger engine? The reason I ask is you can build a very large rocket engine that is pressure fed with a simple helium tank. For smaller engines, a gas generator is a little over kill unless you plan on using it to run a pump. However, working with turbo machinery adds drastically to the challenge.

    Remember when working with pressurized oxygen or liquid oxygen you have to maintain a strict O2 clean policy for everything. All tools, hoses, fittings, pipes, valves have to be O2 safe. It only takes a little contamination to blow things apart in a bad way. It's good to be paranoid about cleanliness when working with oxidizers.

    Good luck with your project.


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