It is the year of 2020. The
Dutch Government empire has forbidden fireworks. This leaves the rebel forces fresh out of supply of loud noises to ward of evil spirits. Two heroes decide to step in and build something that creates a lot of loud noise. G. Rinder and Weldy. Since the empire has also closed all transportation, shops and the economy in general, they cannot get the help from fancy TIG welders, Ring Rollers or Plasma Cutters. Not only that, they only have a combined budget of 200 Euros. This is the story of their first pulse jet. Will they stay within budget?
FAIR WARNING: Making an running a pulse jet is dangerous. It will try to cut you, burn you, take an eye out and blow your eardrums out. Take PROPER precautions! Leave it to the BuildComics heroes if you are not experienced.
Say Hello to Weldy. A 200A Stick welder. Weldy is a bit uncertain about welding 1.5mm steel. A Youtube video is in order for a bit of at home training, safely from the sofa. Then an exam is in order using some cutoffs from a nearby car exhaust shop. Exam passed, now on to the real thing.
Back from the junk yard, and what a score! Two pieces of 160mm diameter tube with 1.5mm wall thickness, both of 2 meters long. The base design used is a 55 pound Lockwood Hiller Valveless Pulse Jet design by Bruce Simpson. However, the chamber will be straight. Design with drawings, Fusion 360 files and more are available on Thingiverse. First it’s time to make the big Horn. G Rinder cuts of a section that is slightly longer than the 1100 millimeters specified. A piece of angled iron is used to draw a straight center line on it.
Than it is time for some calculations. Take the current diameter (160), and substract the target diameter (133) and multiply the result (27) by pi. This result (84) is the amount to take out of the tube. We draw a line at the exact 1100mm distance marked, and use a thin ruler to mark out half of the result (42) to both sides of the center line. Do the same for the other end with the 76 diameter. A piece of flexible plastic to join the lines, and G. Rinder to do all the hard work of making the cuts.
Time to bring out the band of clamps for some help. With the help of some rookie members, the band clamps. Although this band is a tough bunch, some of the not so sturdy ones got badly hurt. They just couldn’t take the pressure.
Slowly but surely the gap closes. And Weldy can jump in and fix the ends. This little hole burned in didn’t worry Weldy much though, with some patience and enough cooling between tacks, it got closed up again without issue.
Weldy needs to wear proper eye protection though, the tack welds of stainless steel tend to lose their slag quite violently. And Weldy prefers not to lose an eye to a hot piece of slag. More clamping ensues to slowly close the other end of the seam.
Finally, the band of clamps can take a well deserved rest. The entire seam is closed. Time for Weldy to lay in the long bead, and inspect the work inside and out.
With Weldy tired, this pre-made 180 degree bend is a live safer. It is 76mm (or 3 inches in
imperial empire units) diameter and normally used for car exhaust pipes land speeders called a cardan bend. Link for a (Dutch) supplier on the Thingiverse thing. Cutting of one straight end and moving it to the other side exactly matches our intended dimensions! Next up is the chamber. Using a (sleepy) band clamp to create a straight cutting line around the pipe at the correct distance.
And presto, one combustion chamber. Well, part of it at least, next up is the two converging cones. A printed cone template from https://www.templatemaker.nl/en/cone/ is printed, cut and glued on leftover piece of the tube. Time to wake up G Rinder and get to work.
After G Rinder is finished cutting the (slightly longer than needed) cone, Weldy wakes up for a short job welding the cone round. The chamber is used as a guide to mark the correct size. Time for more welding.
One side finished, up to the next. Note that a smaller disk makes as much easier job on G. Rinder for the tight curves. The cutoff of the 180 bend is welded on by Weldy just as smoothly.
Welding the flare has Weldy blowing a fairly big heart-shape hole however. Weldy sure is tired by now, but G. Rinder provides the solution. A triangle small piece of stainless is tack-welded into place.
And after a deep breath, Weldy makes the pass around, and allthough the result doesn’t look pretty on the flare, it works. The rest of the chamber is assembled with the tube between chamber and flare made in the same way as the horn. The 180 bend and horn is attached, and the final shape of the Pulse Jet is suddenly shining in the sunlight!
A hole plus a 3/4″ BSP Stainless Steel Sock is the mount for the fuel injector. Some stands from scrap pieces of stainless tube make up the feed and a strut of the horn. The gas fittings are tricky without using an regulator. This is a DIN R32 LU1 – 1/4 SAE adapter normally used in the cooling/airconditioning business. 1/4 SAE is equal to 7/ UNF, which is the thread used for hydraulic fittings, so with a “PA1014 Pilaar DN10-7/16 UNF” from the planet of “Kramp” we are set to connect a propane tank directly to 6mm propane (20 bar rated) hose.
The injector is made from a stainless steel 1/1″ BSP threaded pipe nipple. Due to a
planning misstake minor oversight, the chosen end-cap doesn’t fit through the 3/4″ sock. G. Rinder is woken up, and the problem fixed. Some holes in the end-cap at different angles are supposedly to work as a good “injector” inside the chamber. A 3/4″ Welding Nipple is welded over the 1/2″ nipple, such that the injector and end-cap sits inside the chamber when screwed in.
The final piece of the plumbing puzzle is a 1/2″ Ball thread with a 6mm to 1/4″ Hose adapter, and a 1/4″ to 1/2″ fitting inbetween. There it is, the entire thing assembled and ready to go. That is, we still need a way to start it. In this case by using a Eurocom 3000 leafblower. It needs a bend at the blowy bit, which in this case is oval. Weldy and G. Rinder are stumped, they have no idea how to make an oval 90 degree (or more bend…)
But don’t worry, 3DP is here to save the day. Behold one halve of an oval bend. Print it twice, and joined with some CA glue it makes for a perfect 120 degree band for starting this Pulse Jet. So enough talke, let’s get jetting!
So that didn’t go quite as planned. And the 120 degree deformed under the heat. Maybe PLA wasn’t the best material, so a new one is made from PolyCarbonate Blend. And for good measure, covered in aluminium tape, better safe than sorry. Next step is to change the injector. Another 1/2″ BSP pipe nipple is cut in half, some triangles sliced out and bend to a point.
Welded, and drilled to 10mm, this point now receives a 10mm stainless steel tube with 1mm walls. A new 3/4″ Welding nipple is welded on to fit in the Pulse Jet.
The pipe is pinched close at the other end, and receives nine 3mm holes through and through. See the drawing for reference. Time to go outside again, and see if this Pulse Jet will finally run…
Now, let’s see if our heres stayed within their 200 Euro budget:
|Scrap Steel Pipe (bought for 30, sold of the rest for 14)||16|
|10 Grinding Discs||12|
|+- 30 Welding Elektrodes||24|
|Injector Socks (3/4″ BSP Inner, 3/4″ BSP Outer)||5|
|Injector Fittings (1/2″ to 1/4″ and 6mm Tailpiece)||8|
|Ball Valve 1/2″ BSP||5|
|Propane Hose 6mm||3|
|Propane Hose Tailpiece||4|
|DIN LU1 Propane Cylinder to 1/4SAE (= 7/16 UNF)||10|
|180 Degree 76mm Stainless Exhaust Bend||37|
|Injector 10mm Tube||12|
Note that these cost don’t include:
Transport (Fuel and Shipping)Teleportation tax
- Fuel (Propane)
- Gas Tank Deposit
- Cost of living for the Heroes
- Apology gifts for the very annoyed neighbors
- Leaf-blower (10 euros second hand)
A special thanks to the guys on the Pulse Jet Forum that helped out with starting the Pulse Jet.
And all the commenters in the first Youtube video helping out!