The Poor Man's Pintle

A bit of a delay since the last post - quite a lot of things going on outside of blog land.

Two posts ago I showed a method for sealing off the end of a stainless tube. But to what end? I was brainstorming on a easy way to make a pintle injector. I took the tube shown in the previous post and drilled 8 holes in the end. The result is less than optimal because my mill's spindle is really too slow for the size hole I need. But the result was promising.

These tests were run with water in the center and air on the annulus.

Data Data Data

Here is some data from the last igniter/small rocket test I did (video in previous post). In this case, I'm sampling chamber pressure at 5kHz. You can see its a little rough which is not particularly surprising given the injection arrangement.

This next plot is a FFT of the above data. You can see a low frequency mode at ~125Hz with multiple harmonics.

Tubes

Lenox Laser makes some really nifty orifice fittings. I recently ordered some of the 1/4" tube fittings because I wanted some orifices for my igniter that are smaller than the 0.014" available as flare jets.

The 1/8" tubes got me thinking about another project and so I was looking at buying some of the smaller orifice fittings and modifying them for my own use. The problem was they really only have up to 3" lengths without added cost. And even at $30 they seemed a bit pricey to buy and carve up. A search of the web found no easy source for the blanks they make so I tried my hand at making one myself.

I first tried to make a mandrel using a 1/8" ball end mill. That didn't work particularly well. I had a ball bearing spinning tool for the lathe left over from another project, so I tried it.

It worked very well. The tube in the picture below is my very first attempt with the technique. I was anticipating having to spot weld the end but it is water tight to 60 psi which should be good enough for what I have in mind.

Additional Testing

I modified the nozzle to a slightly larger throat area and added a diverging section. This test configuration had a 0.014" propane orifice, a 0.027" N2O orifice, and a throat of 0.201". It was running slightly lean at a chamber pressure of 110 psi. I plan to mount it to the thrust stand and get some performance data next.

Nitrous Propane Rocket Igniter

Second video, slightly smaller orifice size, chamber pressure of 110 psi.