Hybrid rocket engine uses acrylic as fuel

We are fascinated by the hybrid rocket engine which [Ben Krasnow] built and tested in his shop. It is actually using a hollow cylinder of acrylic as the fuel, with gaseous oxygen as an oxidizer. We’re already quite familiar with solid rocket propellant, but this hybrid approach is much different.

When a rocket motor using solid propellant is lit it continues to burn until all of the fuel is consumed. That is not the case with this design. The acrylic is actually burning, but if the flow of oxygen is cut off it will go out and can be ignited later. This also opens up the possibility of adjusting thrust by regulating the pressure of the oxygen feed.

[Ben] milled the test rig in his shop. It’s a fat acrylic rod through which he bored a hole. There are two aluminum plates which complete either end of the chamber. The intake has a fitting for a valve which connects to the oxygen tank. There is a nozzle on the outflow end. Check out the video after the break to see a full description. You’ll also get a look at the toll the combustion heat takes on the rig.

[Thanks Cesar]

Comments

  1. John says:

    The build is cool, but do you people watch mythbusters? Salami Rocket. Damn near any organic compound is rocket fuel when you add O2 gas.

    • gaioe says:

      Mythbusters is a staged TV show, i would not believe a single word said by the hosts.

      • draeath says:

        You shouldn’t believe everything you see on TV. Likewise, you shouldn’t not believe everything you see on TV.

      • Dra says:

        Mythbusters do not conduct thorough enough experiments, to fully prove or disprove anything, but they do not lie or stage things to present a false positive. You would know this if you’d watch more than a few minutes of any episode.

      • Daniel says:

        Staged? Of course its staged. Thats the whole premise of the show.

        And while many of their results/conclusions are…incomplete, that doesn’t mean the “experiments” are fake.

      • ferdinand says:

        notting is fake by mythbusters but not all things are show on camera things that can kill you like make gun cotton there you not see what to do or use.
        so pepole can not copy it and the 2 thing the legal part its a tv show

      • S says:

        Not like the UK show Brainiac where in the last episode I watched they faked an experiment to confirm their expected result.

        No boom? No worries, just fake it.

      • m says:

        if they don’t get the results they want they usually just say well not what we where expecting but since we rented out the bomb range we are going to blow something up anyways, but point is when they doctor something to get what they want they do it with full disclosure that yeah we packed that with explosives because that’s what every one wants to see.

  2. cbob says:

    A very cool variation of the liquid nitrous oxide/rubber engine used by Scaled Composites.

    Using a rod would eliminate the necessity of having to cast the fuel core. While using the NO2 would simplify it even further.

  3. Ren says:

    I wonder about any noxious gasses produced.

  4. Tom the Brat says:

    Did the Mythbusters get the salami rocket to work? Seemed to me they needed to meter the nitrous. Last I remember if the salami ignited the rocket exploded. (I don’t have a TV any more)

  5. roboman2444 says:

    if i built the same thing, i would have put an aluminum cylinder around the acrylic one. Reduced the risk of the acrylic exploding when it gets further burned, and helps cool it a bit.

    • derp says:

      Usually aluminum is added to the fuel/propellent to increase the temperature inside the chamber.
      The pressure inside is rather low and there isn’t a danger of explosion as long as you don’t let it burn all the way.
      There is no need to cool it off also, you want to get a high temperature(as high as you can) inside to increase thrust and cooling would do the exact opposite(you could simply reduce the oxidizer flow without losing heat).

      We’ve done the exact experiment on a rocket propulsion course, 10 seconds after the engine was turned off you could touch the cylinder with your bare hands and it would feel slightly worm.

      • Philippe says:

        There’s no wiggling your way out of this one: it’s warm, not worm.

      • dude says:

        you could wrap it in aluminum to reduce the amount of warping. The ends are melting because of the amount of pressure put on the acrylic by the tightened bolts. If an aluminum sleeve was around the rod the end might start melting but they would have nowhere to go because the the pressure from the bolts would be placed on the aluminum sleeve. Just a thought.

      • Joe1 says:

        Well if you try to enclose the rod with a metal sleeve… Plastic bursting is a lot less dangerous than metal bursting. Just saying! :)

  6. Joe says:

    Would’ve been interesting to take it outdoors in a safe area, and run it full bore (to the point of the rod consuming itself, and the supporting metal ends have nothing left to support). Guessing the O2 would be propelling molten acrylic a few meters at that point.

  7. Mark Smith says:

    Related?

    Plexi rocket tube, and aerospike nozzle.

  8. Ryan says:

    A very similar setup is used as a lab exercise for an undergraduate aerospace engineering propulsion class at CalPoly (SLO). The fuel grain is a plexiglass rod with a hole drilled through the center; i.e. it’s not cast.
    Check out my former classmates’ senior project video and report: http://www.youtube.com/watch?v=uzvcHr8BTBI, http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1024&context=aerosp

  9. brad says:

    Yo, dawg. I heard you like vises…

  10. Mike bradley says:

    This guy makes great videos! He didn’t show us 10 minutes on threaded Rod, and screwing in each rod.

    I also love the demo, awesome!

  11. Jaybee says:

    It’s certainly the prettiest rocket engine I’ve ever seen. While it’s running, it looks like part of a warp core.
    Certainly not the most important attribute in a rocket engine, but…

  12. Erin says:

    People recommending having a skin around the acrylic are missing the point. It’s transparent! You can see the flame channel inside the rocket! THAT IS COOL.

  13. Montaray Jack says:

    What is happening in the rocket is exactly the same thing that can happen going up the tubes to Oxy tank.
    I’m really not one to be a safty nanny, but Ben, you should really invest in some flashback/flame arrestors for your welding setup.

  14. rachaelcollins01 says:

    Excellent article, very informative.Thanks.

  15. Phil says:

    This is nothing new – I saw the same exact setup (compressed oxygen cylinder, acrylic rod) in 1994 at a technical conference. A piece of steel wool was used as the igniter.

    The problem is performance – you get the words of both worlds in terms of tradeoffs vs. solid and liquid propellants. (in technical terms a low specific impulse and low thrust to weight ratio).

    For some specialized applications it can work (Spaceship One) but overall the few advantages aren’t worth it.

    Makes a great demo though. If you do it with a salami there’s a blue color due to the nitrites.

  16. dz.dice says:

    Well it just came to me that some method to make this a multistaged rocket might be feasable, eg: the core, as soon as it is consumed would detach the front and the back by itself, leaving the rear nozzle(hard to engineer but doable), leaving the sustainer ready to go on it’s second stage… well not as simple as this but it is certainly a thought, also this would or might provide less expensive rockets…!

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