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Engine development, preparing for flight

December 13, 2003 notes

December 13, 2003 notes


If you haven’t seen our commemorative 100th anniversary of flight video yet, go look on the main page: www.armadilloaerospace.com


Engine development


We went through 70 gallons of peroxide this week.  We have worked out a nice system for extended length testing – we pump four gallons of peroxide at a time into five gallon carboys, and fill one gallon jugs with the right amount of methanol (about 3 liters).  To make a run, all we have to do is dump a methanol jug into a carboy and shake it up.  Because quenching is one of our major concerns, we aren’t bothering with any shorter runs.  Our peroxide supply seems to be all straightened out, so we might as well make the runs long.


For larger runs, we moved the 80 gallon tank onto the trailer and plumbed it up so we could use the same filling equipment for either the small tank or the big tank.  The Structural composite tanks are only rated to take about -4 psi of vacuum, but we didn’t have any problem vacuum loading 20 gallons of propellant into the tank.


The first test we did was an experiment to see if a much lower open area flameholder plate between the cold catalyst and the hot catalyst section would make things work better.  The plate had eight ½” diameter holes, and this was over the same chamber used last week, with 80 grams of catalyst bale on top of two sections of 1” thick x 400 cpsi monolith.  This made a good deal less thrust, basically the same as the restrictive bale chamber, showing that the peroxide decomposition alone was producing enough gas to choke at these hole sizes.




The next runs were retests of the bale engine that we built on November 1, which had started out with three sections of 330 grams catalyst compressed to 1.5” height each, then had the top section removed.  It worked just like it did before.


All of these were right around 350 lbf at 280 psi tank pressure, 400 lbf at 280 psi line clear


We cut the chamber open again, and removed another section of catalyst, leaving only a single 330 gram section.  This still worked fine, but produced little more thrust.  The 330 gram blocks were pretty solidly compressed, and obviously fairly restrictive.


We built a new chamber that had three sections of 110 grams of catalyst each, individually supported by welded perf-plate. 


440 lbf at 264 psi tank pressure, 520 lbf at 264 psi line clear


This is a tiny bit less than we made with the high flowing monolith chamber, but the engine was obviously running a lot hotter, so Isp was back up where we wanted it.  We repeated this, and then ran 10 gallons of it, which it handled with no sign of quenching.


On Sunday, we converted our tank-to-test-stand hose to –16 from –10, which has nearly eliminated the plumbing loss for this size of motor.  We also replacing a leaky valve, changed a tank o-ring, and relocated our data acquisition tank pressure transducer so it would read from both the small tank and the big tank.


The next thing we tried was swapping the 2” throat nozzle for a bored out 2.2” throat nozzle.  It didn’t make any more thrust:


540 lbf at 280 psi (very little line clear difference now)


We saw several interesting things:  The thrust actually rose slightly as the run went on.  The nozzle was glowing red hot, but the catalyst wasn’t red at all after the run.  We conclude that significant combustion was going on underneath the final perforated plate, inside the nozzle converging section, which isn’t optimal.  Even though the exhaust was clear, there was a smell of peroxide in the air, and the run was completed in less time than with the smaller nozzle, implying that Isp was down, even though thrust stayed constant.


This appeared to have crossed the line of how much flow 330 grams of bale catalyst could handle.  If it was actually quenching, then adding more catalyst layers wouldn’t help because it would just progressively quench the additional layers out.  We made a 10 gallon run with the same configuration, and were pleased to see that it showed no sign of quenching, and, in fact, showed signs of combustion getting somewhat better towards the end, with the side of the chamber by the initial flameholder at the top beginning to glow red.  This seems to be a fundamental difference between the bale catalyst and the straight-through monoliths – if the monoliths don’t stay completely hot, they will eventually quench, while the bale seems to be able to partially handle larger flows without becoming useless.


We are going to add a fourth layer of 110 grams of catalyst to the chamber, which will probably bring the combustion quality back up, and should give us increased thrust with the bored out nozzle.  When we cut the chamber open, we could see that the 110 gram patties of catalyst bale had compressed significantly under their own flow resistance, such that they are rattling loose between the perforated plates.  That probably doesn’t hurt anything, giving an open area for turbulent flameholding above each catalyst patty, but it means we can probably precompress it into less space without hurting the flow characteristics.  1” between perf plates is probably what we are going to do on the next fresh engine build.


The next thing that may be limiting our thrust is the spreading plate hole size, which may be a little tight at 204 holes of 0.032” diameter for 0.164 square inches of area.  We might crudely try to drill an extra large hole in the middle of the existing plate, but we will probably have to wait for good experiments on this.  I have a big batch of modular flanged chamber sections being machined, which will allow us to mix-and-match lots of different pieces.  It will be interesting to answer questions like: Is one cold catalyst enough?  Does three improve things?  How much better are the 900 cpsi monoliths than the less dense ones?  Does adding space between the bale patties help?  Would a monolith added underneath three bale sections stay hot? Etc.


If tests on Tuesday go very well with the four patty chamber, we may go ahead and start assembling the four engines for flying the big vehicle, but it would be nice to optimize a little more first.



Preparing for flight


The big vehicle is back together again after insulating.





We installed the electronics, and started doing prep tests.  The valve potentiometer feedback points were calibrated, and everything was leak checked again.  We used the plasma cutter to make big cutouts on the mounting plate so we can get at the flange bolts easier.  This electronics mounting arrangement is just temporary, designed so that we can swap the electronics between the big vehicle and the small vehicle, but now that it looks like we may not fly the small vehicle, we might do something more permanent.


With the electronics board inside the aluminum cabin, communication was almost impossible, even when there was a pretty straight shot through the open hatch, probably due to huge multipath interference inside.  We have an external antenna to mount, we just haven’t installed it yet.  It looks like it will be necessary even for our hover tests.




We built a 50’ –16 braided hose to use to fill the vehicle tank instead of the 2” PE hose we were using.  We may need to upsize when we are filling 850 gallons of propellant, but at this point liquid loading is not the dominant time, and using a high pressure hose allows us to pressurize through the same hose that the liquid loading is done through, saving a manual swapping step.  We are building a custom drum pressurization loading system to push propellant directly from drums into the tank.







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