Cast Concrete Nozzles

August 2013

This method of making rocket exhaust nozzles and end-caps is great if you don’t have a lathe to machine nozzles out of steel or graphite. You end up with both convergent and divergent cones, with a steel throat that won’t erode.

I looked at several tutorials online, however none of them allowed me to cast the divergent and convergent cones.

This method works best if you produce motors in batches of 5 to 10 – it takes ~3 hours to produce 10 motors (excluding setting time for the concrete, which is about an hour per motor… but you’re not going to sit around watching concrete dry are you?).

All you need is a printer & some transparency sheet (acetate sheet), hydraulic (expanding) concrete, and bits and pieces that can easily be bought at a hardware store or online.

I cast several of these nozzles and end-caps into thick-walled PVC pipe – the nozzles and retaining method work well and I did not have a single failure (max. chamber pressure ~350psi in a 32mm OD 2.4mm wall case). The nozzle and end-cap are retained inside the motor by gluing a ring of PVC inside the motor casing.

Update May 2014: I have since bought a small lathe and machine nozzles on the lathe, I no longer use this method.

The only problem with cast concrete nozzles is erosion. All of the motors I test-fired showed significant erosion of the concrete just past the steel throat. However, for small motors like these (G motors), I don’t think this will have a big impact on performance.

cast concrete rocket nozzle erosion

Unfired casting on the left, fired nozzle on the right showing erosion just past the steel throat (note that the nozzle on the left has a 4mm throat while the one on the right is 6mm)

Tools & materials:

  • a printer (inkjet, laser, whatever) and transparent/acetate sheets
    • buy the right ones for your printer (there are different sheets for inkjet vs laser)
  • hydraulic concrete – I used Rockite brand but there are others, this type of concrete expands as it sets (unlike normal concrete which shrinks slightly), I found this difficult to get hold of in small quantities
  • for the steel throat, either one of:
    • some ~4mm steel, a drill, and drill bits of the right diameter for your nozzle throat (mine was 6mm)
    • some steel washers with the right internal diameter
  • the Excel workbook (see below) which will draw the cones based on the dimensions you enter – sorry but you will need a Windows system for the VBA code to run (no Macs), alternatively get hold of a free CAD program like SketchUp or OpenOffice Draw and draw them yourself
  • 2 x o-rings – I used 3.5mm thickness rings, get a size that fits snugly (compresses slightly) inside the motor casing, buy a box set from eBay with a range of sizes – one of them will do the job
  • a length of wooden dowel – longer than your motor casing, with a diameter to fit as closely as possible inside the casing (does not have to be exact, but does have to be round)
  • ~100mm length of threaded rod, two normal nuts and one T-nut, one normal washer and a ‘penny’ washer (see pic below) – I used M6 threaded rod
  • medium-sized ziplock bag – to mix the concrete and as a pourer
  • sticky tape – as cheap/thin as possible
  • craft knife, small saw or similar (i.e. Dremel)
  • high temperature (red) RTV silicone
  • (useful to have) a small set of small scales capable of measuring in 0.1g increments

Step 1 – make the cones

Print the cone shapes onto the transparency (acetate sheet), cut them out and tape them up into cone shapes – one each for divergent and convergent cones (do them in batches and build little cities of acetate cones on your desk – people will start to ask questions…).

You can use the Excel file below, or use a free CAD program to draw your own shapes. If drawing your own shapes, include alignment rings to make it easier to tape them up straight.

Make sure that the wide end of the divergent cone is small enough to fit through the retaining ring that we will glue to the inside of the motor casing. Read through the whole post and it will become clear. If you don’t then it’s no big deal to trim it later.

I created some Excel VBA code to draw the cones for given parameters (angle, depth etc).

The Excel file is here, the instructions are in a video here.

Once the shapes are drawn, copy the drawn object to Word and print it out onto the acetate sheet. Don’t print direct from Excel as it will distort the shape. When printing, make sure that you set your printer options to print to 100% scale. Always check the dimensions of the printed shape with a ruler to make sure it hasn’t been distorted.

(If you are running Excel 2007 or later, you will need to click ‘Enable Content’ at the top to enable the code.)

Tape the cones up by wrapping the acetate around your finger until all of the alignment marks are… aligned. Then tape, and trim the ends of the tape using a sharp knife.

[pictures to come]

Step 2 – make and glue retaining rings

I make my retaining rings out of the same pipe as the motor casing. The rings have a segment cut out to allow them to be compressed inside the casing. Other people use rings made out of smaller-diameter pipe – this would also be a good method (if you can find pipe of the right diameter).

PVC rocket motor retaining rings - cut ring

Left to right: ring cut and ready to install, a ring before being cut, and motor casing

Making the retaining rings can be tedious, so I made a little jig to cut rings to the right size easily. This jig is just a piece of PVC pipe that fits over the motor casing pipe, and has slots cut in the sides to slice a piece out of the ring. The grey piece at the top is glued inside the white pipe – this is a ring cut to the correct size.

PVC rocket motor retaining rings - cutting jig

Ring segment cutting jig – slide the ring up from the bottom, insert another piece of motor casing pipe to hold it in place (you can see mine behind and to the right), then cut using the existing slots as a guide…

PVC rocket motor retaining rings - cutting jig cut

…like this. I use a Japanese pull-saw as the blade is very thin. You can see the ‘pusher/holder’ piece of pipe (grey) poking out at the bottom – this holds the ring that is being cut up against the glued piece at the top of the jig.

Getting the rings into the casing can be a challenge, especially when they are smeared with solvent glue (wear gloves and eye protection or you’ll be picking glue off your hands for days). I make my rings so there is a gap of a millimeter or so when they are installed (i.e. the ring does not close up completely) – if you try to cut them too close you will end up with glue and rings pinging across the room.

I found that filing/sanding a little bevel on the edge of the ring and the inside edge of the casing helps immensely. This is especially the case if you are using a plumber’s pipe cutter which produces a raised ridge as it cuts. I leave the ridge on the outside edge of the ring to act as a ‘wiper’, pulling glue along with it as it is inserted into the casing (you can see this ridge in the photo below, next to the word “in” on the left).

To install a ring quickly and reliably, put one end in first and ‘spiral’ it in, pushing the free end into the casing with something hard and flat, like the flat end of a pen. Don’t use anything sharp – your ‘pusher’ device will slip and will poke into your hand or worse.

PVC rocket motor retaining rings - inserting

If you are using a plumber’s pipe cutter, you will need to file or sand off the raised edge that this device creates – both on the rings and the motor casings. Otherwise you will struggle…

We will only glue the nozzle-end retaining ring for now – the ring on the other end gets glued once the fuel has been inserted into the casing.

Practice your ring insertion technique (stop sniggering at the back please…) – when you have it down smooth and reliable then smear glue around the outside of the ring and also inside the casing where the ring will sit. I use a stack of penny washers to set the depth of the ring inside the casing – once the glued ring is inside the casing I quickly push it down onto my stack of washers – this instantly moves the ring to the right depth where it can set in peace. You will need to work fast as the glue sets quickly.

PVC rocket motor retaining rings - nozzle ring

The ring (with glue applied) is inside the motor casing – before the glue sets push the casing down over the stack of washers to set the right height.

Step 3 – steel throat

I drill my steel throats out of 4mm plate. However you can also use a stack of washers.

Drilling your own holes is more work but gives you more options. If you are using washers, note that pop-rivet washers are exactly the size specified (i.e. 8mm = 8.0mm), while normal washers are slightly oversize (i.e. 8mm = 8.5mm).

Whichever option you choose, I would suggest making sure that your throat spans most (but not all) of the internal diameter of the casing. Look at the pics at the top of this article on the topic of erosion – a stack of washers on their own is likely to be spat out with the exhaust gases, as the concrete erodes. I did use washers for a while and just made sure that the one closest to the divergent cone was a penny washer, with a chord cut out of it to allow the concrete to flow through.

 

Step 4 – assemble the casting mold

Before assembling the mold you will need to insert the T-nut into the end of your wooden dowel. To do this drill a hole to suit the diameter of the inside shoulder on the T-nut. Try to drill the hole as central and straight as possible – if you have a drill press then use it. Tap the T-nut in place with a hammer.

cast concrete rocket nozzle - dowel

Make sure that your dowel is a good sliding fit inside the motor casing – I wrapped my dowel with some thick paper (250gsm) until it could slide easily inside the casing without wobbling. With the dowel made now is a good time to check that your o-ring is a good size – put it inside the casing and try to slide it from one end to the other using the dowel as a pusher.

cast concrete rocket nozzle - oring

The Chosen One, The One of Many, The One Ring (etc etc)… it is said that Many are available in a bargain-basement eBay boxed set, but only One actually fits (well, a couple of the others did fit but I liked this nice plump one best).

Now assemble the mold by sliding/screwing the acetate cones, washers, nuts and o-ring onto the threaded rod. I screw the divergent cone on first, then add the other items. From left to right the order is:

  1. nut
  2. normal/small washer
  3. divergent cone
  4. steel throat
  5. convergent cone
  6. penny washer
  7. o-ring
  8. T-nut in wooden dowel

Now would be a good time to check that your divergent cone will fit through the retaining ring we have just glued to the nozzle end. If it doesn’t, just trim it and make a note to yourself to do better next time.

 

Step 5 – pour the concrete for the nozzle

Clamp your wooden dowel in a vice, or find some other way to hold it steady. I find that an extra, thin o-ring slid over the dowel and into the casing at the end-cap end help centre the mold and stop the dowel from falling out (so, the other end to the one we are going to pour). Make sure that the cone is central in the casing.

Mix the concrete with water in the ziplock bag – mix it well as you don’t want any lumps. I use a ratio of 3:1 Rockite to water (by weight & volume) – so 30 grams of Rockite to 10 milliliters of water. I found that ~25g of Rockite is enough for one nozzle.

Snip a corner off the bag and carefully pour it in between the divergent cone and the motor casing. Pour it slowly, and fill it so that the meniscus is above the cone former and casing (it will shrink slightly as it sets).

[add pics]

After half an hour or so, unscrew the nut for the divergent cone end, remove the washer and peel away the acetate former. Gently at first, pull the dowel and threaded rod out from the casing. You will probably need to fish around inside the casing with a piece of wire with a hook at the end to get the o-ring out. I usually leave the acetate convergent cone former inside the casing.

[pics]

Leave the nozzle to dry thoroughly before moving to the next step (overnight is best).

Step 6 – pour the end-cap

Insert your fuel into the casing. On top of the fuel insert a disc of cardboard – this stops the concrete from dripping onto the fuel. I cover my cardboard with a layer of tape to make sure no moisture gets through. Run a bead of the high-temperature RTV silicone around the perimeter of the disc and allow it to dry.

[pics]

Next glue a retaining ring in place. This one should be easier than the nozzle end because it will be flush with the end of the casing.

[pic]

Mix up another bag of concrete, and pour it in. Fill it so the meniscus pokes over the top again, as it will shrink slightly as it sets.

Let it dry (overnight), then fire up your motor. Here’s one I made earlier:

…more to follow sorry… 14 May 2014

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