Hybrid Tips

At 8 bars chamber pressure and 3.5 nozzle area ratio, a nozzle is horrifically over expanded. An over expanded nozzle will contribute significantly to poor ISP performance

c* is a better measure of combustion efficiency, being a function of mass flow rate, chamber pressure, and nozzle throat area. At low chamber pressures, use frozen chemistry rather than equilibrium in PROPEP.

It is common practice in hybrid motor design to include a "pre-chamber" upstream of the fuel grain - it will help improve performance and stability.

Back when I was doing N2O/HTPB hybrids, I measured c* efficiencies from 50% to 84% all with the same hardware - changing mainly fuel grains.

Ozone Part II

AC Delco Coil Near Plug - aka "The LS2 Coil"

This is where it gets interesting. The OEM application for this coil is the GM LS2 V8, powering Cameros, Corevettes, and various GM trucks and SUVs over the past decade (PN# D514A). It is termed coil near plug because they are mounted on the valve covers and there is one for each cylinder. The complete setup with coil, weatherproof pigtail, and sparkplug lead can be had off ebay for around $45 (from different vendors, of course). This makes it the second most expensive setup but I think well worth it.

This coil is immensely popular with the DIY electronic engine control community. You can read all about it on this page. The most attractive feature of this coil is that it can be triggered with almost any signal that rises above about 3 volts. In automotive lingo it has a "built-in igniter" meaning the power transistor, protection diodes, and noise suppression components are in the coil package itself and so it can be triggered by a low-current logic level signal. It even has a dwell limiting feature which keeps the coil from burning up if the trigger pin is held high too long. I am triggering it directly from a logic pin on my 3.3V micro-controller - not a single component between the micro's pin and the coil connector. The optimum dwell time is about 5 ms at 12V, which is perfect for a 50% duty cycle at 100Hz - I've run it up to 200 sparks/second with good results as well. Spark energy is visibly higher than the other three inductive systems with a fat, purple spark. It pulls a healthy 2.9 amps at 100 sparks/second and should run on power from 6-16 volts with no issues. When you throw in OEM automotive engineering and reliability (even if it is from GM), this system looks really attractive.

RCEXL CDI System

This is a capacitive discharge ignition module for radio controlled small engine applications. Like the LS2 coil, it may be triggered via a 3.3V low current signal. It is designed to operate over a more narrow range of voltage (4-6V). As a hobby system, I would expect that the engineering, quality, and durability is somewhat less than the LS2 coil. However, it is a CDI system and should deliver higher voltage to the plug. It pulls about 0.5 Amps at 5V and the sparks are pencil thin and bright white. The spark plug cap has a unique metal shell and shield high voltage cable that grounds the ignition system directly at the plug and helps to reduce radiated noise. However, as I discussed in a previous post, this ignition seems sensitive to shorts of the spark plug - it is far more likely to cause processor resets with a short than the other inductive systems. It weighs only 150 grams with cable and boot which is about half of the LS2 coil setup.

Wrap Up

I think the LS2 setup is the obvious winner for most applications given the level of engineering, simplicity of interface and integration, and operating voltage range. It might save a pound over a MSD type system and would probably provide better reliability. The RCEXL system might be preferred if you were on a severe weight or power budget or you were trying to ignite mixtures at high pressures (+200 psi). For my igniter, which lights at just over ambient and runs at 100 psi, all four options ignited with no measurable differences.

The Smell of Ozone

As I mentioned in my previous post, I tried out several different spark ignition systems as potential replacements for the C&H CDI box I have been using.

From left to right are a spare coil for my '76 Ford Pickup, a Tecumseh small engine coil, an AC Delco "coil near plug" module, and the RCEXL CDI System. The first three are all designed to be used in inductive discharge, or Kettering, ignitions systems. Most of the testing I did was at 100 sparks per second. I did not measure spark energy or current as I don't have the equipment necessary.

Ford Can Coil

This is a fairly standard coil used on almost any older car with a distributor and points setup. One can get a coil like this at any automotive parts store by asking for a coil for a random car built before 1975 or so. It is easy to drive with almost any transistor per advice from Thomas McNeill. I actually used a power MOSFET that I have been using to control solenoid valves. I tried this coil while I was waiting for others to ship because I had it already on the shelf. I wouldn't recommend using it otherwise. It generated the weakest spark at the lowest cost of the four options.

Tecumseh Coil

This coil was tested by the folks over at the Rocket Work Bench website (PN# 30560A). It was inexpensive and gave fairly good results so I thought I'd give it a try. As was noted before, it gets fairly warm, especially running at 100 sparks/second. That is not really surprising as it was probably designed for around 3000 RPM or 25 sparks/second. It does seem to produce a fairly energetic spark and is triggered the same as the can coils.

I'm going to make this a multi-parter because its late and I'm being paged.

To be continued ...

Configuration Control

Those of you who work with software know what SVN and CVS are. And you've probably found yourself in the position of having code that used to work several versions ago, but you've messed it up and you pull your hair out trying to figure out what went wrong. So, what you do is go back to the last working version and start over, making smaller changes.

It sure would be nice to have SVN for hardware.

I posted a video of my torch working well. I then welded it onto the chamber for testing and found that every time the spark triggered, the microcontroller would reset. In the standalone configuration, I had never experienced a reset. Unfortunately, I had made several changes at once that contributed to the resets.

First, I had made a new custom PCB to replace the protoboard I was using previously. Because the board was new, I focused on it as the source of the reset issue. I did eventually find an issue with the ground plane design which was contributing to the resets (It is the first PCB i had ever designed). However, the resets continued but now they only occurred when propellants were flowing in the igniter!

At this point I began to suspect that my ignition box was bad. I have been using a CDI box designed for RC engines for some time now and thought perhaps it had been damaged. After perusing this page: http://rocketworkbench.sourceforge.net/igniter/igniter_blog.html and talking with some of my fellow Arocket-eers, I thought I would try a simple inductive coil setup. I bought via mail several different hardware options including a replacement for the CDI I had been using (I'll post a follow on about all the different ignition systems).

The various setups still suffered from similar problems - at this point I was baffled! Eventually, I made an Edisonian discovery on the bench - if the spark plug electrode was shorted to ground when the coil attempted to fire, it would cause a reset every time. Placing a bead of water over the electrode and touching the ground had the same effect.

As it turns out, when I welded the torch to the chamber, I had welded a new spark plug bung onto the igniter. This was done as an afterthought - I wanted to clean up the multiple bungs I had welded onto the igniter when trying different threads for different plugs. My intention was to maintain the exact spacing, but I must have made the new bung slightly shorter than the original.

I added a spacer to the bung to move the tip back just a little --- and everything worked just as it had before. I inject both propellants as liquids and apparently the spark plug tip was seeing too many liquid droplets, causing the spark to not fire and the resets. All of the ignition systems work just fine in this configuration, including the original CDI box.

So, SVN for hardware would have saved me tons of time, headache, and grey hairs.

Stay tuned for a post about ignition options.