February 29, 2004 notes

 

Flameholder engines

 

We did a lot more development with various flameholders and ignition systems.

 

Slitting the tubing lengthwise and making crosses out of it was labor intensive, so we started working with 316 stainless angles.  I had to go someplace other than McMaster-Carr for these -- http://www.supremestainless.com/ had everything I needed in stock, but would only ship mill lengths (20’) by truck.  I got some of each size from ¾” to 2” legs.

 

A single 1.25” angle across the chamber gave a visibly stronger flame than the tubing cross.  The book “Gas Turbine Combustion” by Arthur H. Lefebvre (a good, informative book) has some data on flameholders that follow a few general trends: larger flameholders are more stable than multiple smaller ones, and for angular gutters, increasing the angle (from 60 to 90, for example) increases the stability, but at an increased pressure drop.

 

It was still difficult to get it to ignite consistently.  We tried a couple different spark plug locations, and found that it worked better with the plug about an inch below the angle, which was a bit surprising.  We got a good improvement in ignition when we switched from 100hz sparks to 250 hz sparks.  It would ignite almost all the time, but it sometimes took several seconds.

 

We had been using an MSD small engine controller and MSD DIS coil that came with the XCOR igniter, so we tried moving to a higher energy system: an MSD-7A-AL3 with a ProPower-HVC coil.  This made a strong spark even at 500 hz, but it didn’t seem to improve the ignition characteristics.  We have the top-of-the-line MSD-10Plus system on order, which puts out several times more energy, which we hope improves things.  These big ignition systems will add 5lb per engine, unfortunately.

 

The pipe tap spark plugs we had been using were rusting badly in our chambers, so we got some new Autolite Platinum Pro spark plugs, number APP847.  The bodies are nickel plated, so peroxide won’t hurt them, and platinum electrode surfaces should decompose any peroxide that hits them.  These are gasket seat plugs, so we do need to flatted down the engine side before tapping them, which we didn’t have to do with the NPT plugs.  The change in plugs didn’t seem to have an impact on ignition.

 

We tried using a heavy perf plate (1/2” holes, ¼” thick, 40% open area) with a spark plug under it as a flameholder.  It did hold flame over the surface, but it was much weaker, with more liquid passing by, than the single angle bar, even though it had much more pressure drop.  This follows the guideline of larger flameholders instead of smaller ones.

 

We added two more 1.25” angles below the existing one, at evenly spaced orientations.  When this lit, it had an extremely strong flame, with very little visible liquid.  We were so impressed by it that we tried putting a nozzle below it without any hot pack at all.  It has always been my belief that there isn’t enough energy to thermally decompose the remaining peroxide without any catalyst, but we gave it a try.  It made stable thrust, but the exhaust was filled with undecomposed peroxide, which is basically what I expected.

 

We had switched from solid core spark plug wires to MSD 8.5mm helically wound wires to reduce EMI.  Our data acquisition system no longer immediately spazzed out when the sparks started, and the pressure transducers and throttle feedback channels worked fine, but the load cell was still far to noisy to use.  Once we know the flame is going, we can shut off the spark and get good data.

 

When we added the small hot pack below the triple bar flameholder, the engine could be warmed up and fired cleanly.  It is clearly audible when the flame finally ignites, but it seems to be more difficult in a full engine than in the open tests.  The engine ran smooth at moderate thrust levels, but at high throttle openings it had a severe chug.  We have seen this on several different flameholder experiments now, and don’t understand it yet.

 

We built up a full scale production engine by splicing a 3” deep section of 7” rolled tube with flameholders (two crosses of 1.25” angles) above the hot pack of engine 0, and built a new cold pack with two fresh 600 cpsi monoliths and the 848 hole spreading plate.  Running at full pressure with the big plumbing, it is easy to flood the flameholder early on, so it does still require finding a specific spot to efficiently warm the hot pack.  We made several runs with promising results, but this engine also suffered from chugging above 60% throttle.  If we can work out the issues with this configuration, we have everything on hand to build four of these engines to make a ship-set.