Wednesday, March 26, 2008

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 ...


Damien said...

So now what about part 2 ? The one that works now :p

James said...

Regarding nitrous oxide decomposition, I'm sure that the Surrey University Small Satellite Technology people had a paper on a small N2O monoprop. They ignited it using a glow wire, and it was self-sustaining in a very small thruster. If you want I can have a look on my computer at home, I may well still mtshave a copy of the paper.