February 15, 2004 notes

 

63” Bulkhead

 

The custom honeycomb bulkhead for the next vehicle arrived, and we have started prepping it.  The current cabin has a 40” diameter bulkhead midway up the cone with a fairly sparse 2” thick aluminum honeycomb core.  We have pressurized the cabin to 18 psi without any problems.  The new bulkhead will be going on the bottom of the next vehicle, and is a full 63” diameter, so it needs to be stronger to compensate for the larger span.  We had Teklam build the strongest panel they could conveniently put together, which turned out to be a 3” aluminum core with increased density, and double ply fiberglass facing sheets.  This gives significantly more margin than the current bulkhead, and only weighs 39 pounds.

 

The rolled aluminum cabin cylinder sections is slightly oversize, so we need to build up the bulkhead perimeter some before starting to bond it in.  I got a big roll of 3” wide by 1/8” thick fiberglass, and we have epoxied two layers around the disc after filling in the exposed aluminum core with flox.  We will test-fit on Tuesday to see if we need to add more. 

 

I also got another large sheet of Nomex cored / fiberglass faced honeycomb for our interior work.  The Nomex core avoids any chance of getting bits of aluminum lost in the cabin, and the fiberglass faces come with peel ply, which lets us skip sanding work when bonding to them.

 

We intend to have most of the necessary large scale components for a new vehicle ready for the inevitable time when we crash the current vehicle.

 

http://media.armadilloaerospace.com/2004_02_15/bigParts.jpg

 

Custom Boards

 

Our custom motor drive boards and master cutoff computer boards arrived.  We had these contract built, and had ten of them made, so we have plenty sitting on the shelf for new vehicles and redundant computer systems.  The motor drive boards have 12 very high current bi-directional drive circuits with opto isolation, status LED’s, and fuses.  It is driven by a 50 pin IDC connector from our AccessIO PC104 boards.  Our existing driver board only has four motor drives, which prevent us from using a fifth lifting engine or going to a dual-quad engine arrangement.  The current board also has eight solid state relays, but we will just use motor drives on the new board for uni-polar devices like solenoids.

 

We are potting all the stand-up components right now, so after it cures I will replace the current board.

 

The current master cutoff computer is something that I built up out of our old spare parts, and is ludicrous overkill: it boots Linux on a PC104 stack to basically just listen to a serial port and hit a motor drive when it stops getting sensible data from the main computer.  The new cutoff computer is a simple little microcontroller with four motor drives so it can sequence cutoff valves and various emergency recovery options if the main computer fails.

 

http://media.armadilloaerospace.com/2004_02_15/boards.jpg

 

Engine Work

 

We modified engine 0 some more by rebuilding the cold pack with an additional 600 cpsi monolith and using a smaller area spreading plate, but it still isn’t working flawlessly.  It makes decent thrust on the test stand and doesn’t quench, but it isn’t heating evenly, and it has a substream of clouds in the exhaust.  We might be able to fly like that, but we really want to get completely clear exhausts for visibility and best performance.

 

We fired the 12” engine at some very low pressures, which might be a flight option for us.  All of the catalyst necessary to make nine 12” engines should be arriving in under three weeks, which is faster than ordering any new foil monoliths for 7” diameter.  It warmed quickly and ran at a very even pressure, but at only 40 psi of chamber pressure it didn’t clear itself out completely.  This shows a weakness of all our previous testing: We always opened the valves completely when starting a test run, and many engines were briefly cloudy before clearing.  Trying to creep up the throttle from idle just doesn’t seem to work on the current engines.  Slamming the throttle open isn’t an option for flight vehicles, unless we implement a hold-down system.

 

We have a couple more tweaks to try on the current 5.5”/7” stepped chamber engines, but we might be stuck waiting for new 7” monoliths if we find that the stepped chamber is just problematic.  I have ordered a few thermocouple amplifiers for our data acquisition system, which may let us get a little more insight into the operating characteristics of the engines.  Lots of pressure taps let us finally understand all the thrust questions, so hopefully lots of temperature taps will let us conclusively resolve the warming / start transient issues.

 

The only catalyst that we have ample amounts of fresh material of is the ceramic bead catalyst, so we did several experiments with it.

 

Our previous experience with the beads was using them as a hot pack on January 3:

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 (now 320), 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)

This week, we initially tried making a cold pack with 375 grams of beads over a 600 gram ring hot pack.  We couldn’t get this to preheat.

 

We tried to overkill it by adding a large section above the existing section and filling it with beads, bringing the total to 1197 grams of beads in the cold pack.  This engine was very interesting.  It started making very loud pops and bangs and intermittently stopping completely and occasionally belching flame.  Most interestingly, the chambers were glowing a perfectly even red hot, all the way up through the “cold” pack.  When we took it apart, the engine was a huge mess.  A lot of the beads had been pulverized, many internal supporting screens and plates had been burned away, and bead fragments were cemented all through the rings in the hot section.

 

The prospect of being able to sustain combustion in a single pack section is very enticing, so we built another test engine with a single block of 766 grams of beads.  We left a sizeable gap between the spreading plate and the bead catalyst, and mounted a glow plug there.  The hope was that pulses of propellant would rain into the catalyst, and the vapors would ignite on the glow plug and force combustion down through the pack to warm it throughout.  We used the 1.25” nozzles so that the pack would have a much lower pressure drop than with the big nozzle.  It wouldn’t be a bad trade to get a single catalyst pack even if we had to go to increase our throat to chamber diameter ratio from 3:1 to 4:1, but we couldn’t get this to warm up.  We might try again some time with propane preheating.

 

Even though this wouldn’t burn reliably (it made a few pops), it did seem to make a foamy mess about like the monolith cold packs, so we decided to try putting a 320 gram bead hot pack section below it, with the glow plug in the gap as is our usual practice.  We expected this to just work, because it was the same hot pack layout we had successfully tested before, and the cold pack was probably overkill for the amount of flow we were putting through it.  We were able to get it fairly hot on the preheat, but it quenched when it was opened up.  When we took the engine apart, we had a very interesting sight:  the perf plate above the hot pack was very evenly burned from the outside in.  If we had run it much more, we would have just had fragments inside, but we caught it at an ideal time to see it nearly chewed through at the edges, grading to untouched in the center.  The evidence is now pretty overwhelming that the dust from the ceramic catalysts will catalyze the burning of solid stainless steel as well as various gasses, which makes it rather difficult to make an engine using it…  The bead catalyst is probably a dead end.

 

http://media.armadilloaerospace.com/2004_02_15/burnedPlate.jpg

 

One other random thing we did this weekend was experiment with using a dual stage venturi pump to draw a vacuum in the big tank as an alternate loading method to pressurizing the drums.  The venturi pump is pretty impressive, pumping air out many times faster than our electric vacuum pump, but you do trade having to have a compressed air or nitrogen source for it.  We aren’t sure which we like better yet.  In any case, the new 50’ extra-flex loading hose that arrived is a huge improvement over the very kink-prone smooth bore hose we were previously using.