January 3, 2004 Notes

 

Engine Work

 

We built a couple more test engines this week.  We reconfigured the data acquisition to read pressure between the cold pack and the hot pack, and final chamber pressure on all runs.

 

We built a welded engine with 600 grams of ring catalyst in the hot section in a single layer, and a slightly drilled out spreading plate.

 

This started out with great thrust – over 700 lbf, but it tailed off after a couple seconds.  This turned out to be due to many of the rings on the bottom squashing flat under the heat and pressure.

 

Initial: 730 lbf from 293 psi feed, 187 psi mid, 154 psi chamber

Final: 575 lbf from 278 psi feed, 198 psi mid, 120 psi chamber

 

We then ran it at a higher pressure:

808 lbf from 425 psi feed, 300 psi mid, 171 psi chamber

 

The self-preheating behavior of the higher flowing engines has gotten so bad that we started sticking a pole up the nozzle throat during preheat so it could build a little chamber pressure easier.  Our old engines with compressed catalyst bale in the hot section were so restrictive that a slug of propellant could ignite and actually build up enough chamber pressure to make a loud bang when it cooked off, which transferred a lot of heat to the catalyst.  The free-flowing hot packs wouldn’t even build a couple PSI when propellant cooked off, making it very slow and messy to bring them up to operating temperature.  The nozzle plug helped a lot, but isn’t really an operational solution.  We are probably going to look at going back to preheating with a forced propane-air mixture.

 

http://media.armadilloaerospace.com/2004_01_03/nozzlePlug.jpg

 

We received yet another catalyst to try in the hot section, a box full of little ceramic beads impregnated with catalyst.  We didn’t have good luck with the ceramic monoliths that were directly exposed to liquid propellant, but we thought that they might work our well in the hot catalyst section where they are only seeing gas flow during operation.  They should avoid the crushing problem that the rings suffered from.  When heated with a torch some of the little balls crack and pop apart due to trapped moisture, but you can slowly bring them up to red-hot without much damage.  We did several tests preheating these with propane flow in various ways, and the tended to look (and occasionally sound) like nuclear Rice Crispies.

 

http://media.armadilloaerospace.com/2004_01_03/ballCatalyst.jpg

http://media.armadilloaerospace.com/2004_01_03/ringCatalyst.jpg

http://media.armadilloaerospace.com/2004_01_03/900cpsiMonolith.jpg

 

http://media.armadilloaerospace.com/2004_01_03/heating1.jpg

http://media.armadilloaerospace.com/2004_01_03/heating2.jpg

http://media.armadilloaerospace.com/2004_01_03/heating3.jpg

 

 

We cut open the ring engine and replaced the 600 grams of rings with 520 grams of beads, which occupied a much shallower area.  This turned out to be much more restrictive than the rings, but showed no change in behavior over the length of the run.

 

380 lbf from 265 psi feed, 208 psi mid, 83 psi chamber

 

We removed 200 grams of beads, and the thrust went up:

 

515 lbf from 281 psi feed, 210 psi mid, 65 psi chamber (the sensor tube must have been loose, this number can’t be right)

 

We were afraid to pull any more of the beads out, because the layer wasn’t very thick, and any bowing of the supporting plate would let them pull away from the edges and allow some gas to bypass the catalyst.

 

We received a new batch of spreading plates in different sizes laser cut by Global Stencil.  The previous ones have all been 204 x 0.032”.  We built another engine with 204 x 0.040” holes, which is a 60% increase over the old ones, which should show a big improvement if the spreading plate was at all a limiting factor.  This engine used two sections of 200 grams each of ring catalyst, supported by a extra-thick (1/4”) perforated plates with a 10 mesh 316 screen above it to keep the rings from crushing through the larger holes.

 

Performance was steady, without signs of the squashed-ring falloff from the previous motor, but total thrust was not any higher.  This engine also behaved a little odd with the warmup, never seeming to get completely hot during the warmup, but if you just opened the valve it would run cloudy for about two seconds, then clear up for a strong run.  We suspect that added a third layer of 200 grams of ring catalyst would make it start cleaner without hurting the thrust much.

 

610 lbf from 270 psi feed, 180 psi mid, 127 psi chamber

 

I am now officially giving up on trying to increase the chamber pressure for these engine dimensions.  It is probably just barely possible to get 700 lbf out of these engines with supported arrangements of the ring catalyst, but it would be such a near thing that any degradation of performance as they settled could make it non-viable.  A 2.2” throat from a 5.5” diameter catalyst chamber is just too large (6.25 : 1 area ratio).  Our big motors are 9 : 1, which should be a lot more forgiving, but we may want to go even more than that.  The tiny nozzle we tested gave a spectacular chamber pressure, but that was at almost 20 : 1 area ratio.  We can probably get by with much shorter engines with better area ratios, but it will probably still hurt our engine T/W ratio.

 

We are probably going to work out a way to mount one of the big engines in the center, rather than building wider engines for the sides.  We need to build another driver board and rework all the base plumbing to do this.

 

A lot of little work has also happened on the big vehicle – we have made lifting harnesses for transport, our GPS unit is back and installed on the electronics board, new wiring harnesses for the engine pods have been made, the master cutoff watchdog computer box has been built, etc.