Ignition - Part One

There have been several questions about my ignition system. I thought I would start with what I tried that didn't work.

Some time ago, I began researching the idea of initiating nitrous oxide decomposition with spark energy alone. My literature research turned only one relevant reference, documenting work performed by Pratt and Whitney and Rocketdyne.

"Investigation of Decomposition Characteristics of Gaseous and Liquid Nitrous Oxide", Air Force Weapons Laboratory, Kirtland Air Force Base, NM. 1974.

Pratt and Whitney was able to initiate sustained decompositions reactions in pure nitrous oxide with very low spark energies. However, the reaction was only sustainable in large diameter pipes and at pressures of several atmospheres. In addition, diluting non-reacting species such as helium or nitrogen significantly reduced igniteability.

I decided I would try it anyway.

This is the injector head that I designed for this experiment and am still using. The chamber side of the injector (minus the spark plug) can be seen in this post. It has three ports - two 1/4" NPT and one 14mm. The outboard NPT port is for measuring chamber pressure. 14mm is standard spark plug thread.

The spark plug is a standard Autolite platinum plug I had laying around. I'm using a standard ignition module from CH Ignitions. After trying several different approaches, I ended up triggering it with an old HP function generator I had. I found out from Paul Breed - after I ordered my unit - that CH will sell a custom box that sparks continuously when ever it has power.

So, after designing a chamber and a nozzle, casting fuel grains, shaking down a throttle valve, setting up new DAQ software, and getting the spark box to trigger ....

I found out that it doesn't work.

I was unable to measure any indication that the N2O was sustaining a decomposition. No chamber pressure rise, no clearing of the cloudiness in the exhaust. I tried several different throttle settings and spark rates and finally gave up and moved to plan B.

Which I will talk about in Part Two ...

Aluminum Brazing

On the 4.5" hybrid motor I'm currently using NPT fittings in the injector to inject propellant and measure chamber pressure. However, there are many reasons to avoid using NPT fittings in injector heads - packaging, leakage, cost just to name a few. Unfortunately, there are not many good options if you don't have a welding machine, especially working in aluminum.

I am considering starting project which is too small for NPT fittings. I would like to use Aluminum brazing materials for attaching the propellant tubes directly to the injector heads. I bought the brazing material from Durafix some time ago for another fabrication test that didn't work out. McMaster-Carr also sells aluminum brazing alloys. Before I machine the injector, I thought I would test out the joining method with a hydro-test - if it doesn't work I'll have to modify my design.

This is a picture of the finished joint on the test piece. The test article is just a 3/8" aluminum tube brazed to a round rod which was left closed on the end. Both the rod and the tube are made of 6061T6.

The test procedure is simple. I fill the apparatus shown below with water and attach the free end to a standard grease gun. I fill the the grease gun with water as well and then pressurize the entire system. I got the grease gun at an Ace Hardware and its really nothing special. The box says its good to 6000 psi, but I've only ever had it up to 3500. It is very important when doing this kind of test to fill the entire apparatus with water - don't use air!

The end result was no leaks, so it looks as though brazing could work as a low cost method for joining aluminum tubing to manifolds.

More Restarts

Here is a picture of the failed gasket from the hard start.

Other than the failed gasket, there were no problems with the motor. I made a new gasket and reassembled.

Today I tested out some modifications of the start sequencing. The results were good - 4 lights in a row on 5 tries with no hard starts. Unsurprisingly there is some art to getting a sequence which lights reliably without hard-starting. Moving the valve closer to the chamber gave better throttle response as well. I'm working on a post with details of my ignition system (you'll be disappointed). Unfortunately, I'll be out of town this week on business so this movie of today's run will have to do for now.

Random Movie

This a movie of one of the regression rate tests I did to collect the data I showed in this post. Most of my movies are rather unsteady as this one is because the image stabilization feature on my camera was ON. Newer videos are much more stable.

The only interesting thing about this movie (other than being a rocket motor firing) is that it demonstrates "chugging". The popping at the end of the run is a low frequency oscillation in chamber pressure that is most likely caused by elasticity in the propellant feed system. The issue only ever manifested itself at the very end of a run, when the tank held only N2O vapor and the pressure drop across the injector was relatively small.