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Digital IO board, Big vehicle work, 50% H2O2 success

Driver board

June 8, 2003 Update

 

Digital IO board

 

We were doing a dry test run on the small vehicle when we noticed that the drogue ejection actuator was behaving very erratically.  It turned out to be a solder blob that had landed on the PC104 board during the re-soldering of our secondary power input.  Our attempt to repair it wound up killing the entire board, so I replaced it with a newer IO board (a WinSystems UIO-48).  This board didn’t seem to work at all, but after some investigation, we found that it was functioning correctly, but that it just couldn’t drive the 15 mA needed to trigger the opto-isolators on our driver board. Looking at the specs, it only claims to source 2.5 mA, so it isn’t surprising.  What was surprising was that the first board we were using also only listed 2.5 mA source, so we don’t see how that worked at all.  The board (a WinSystems IO-48) must have additional buffering, but we decided to change vendors and get a board that was actually rated for what we were using. 

 

The 104-DIO-48S from Access I/O products sources 32mA, and seems to work fine for us: http://www.accesio.com/go.cgi?p=../104/104-dio-48s.html

 

Big Vehicle Work

 

We built the launch stand for the big vehicle, which is a necessary step before mounting any engines:

 

http://media.armadilloaerospace.com/2003_06_08/launchStand.jpg

 

We did a full fit-up of the cabin and crush cone onto the bonded tank flange, and built the engine wiring harness:

 

http://media.armadilloaerospace.com/2003_06_08/bigVehicle.jpg

 

The sea-catch toggle release (http://ourworld.compuserve.com/homepages/seacatch/ ) arrived, and will be perfect for our helicopter drop tests.  We will have to engineer a little motor drive actuator to use them for the main parachute releases on the big vehicle, but they are going to work great.  You can barely see this dangling under the tank in the launch stand photo.

 

Russ and Neil traveled to White Sands Missile Range to meet with them and AST representatives to discuss the timeline for Armadillo doing space launches from there.  There is a daunting amount of paperwork to be done, but Neil has been diligently pounding away at it for a while.

 

50% H2O2 success

 

We are hopefully very close to getting more 90% peroxide, but, after our many failures to make anything happen with 50% peroxide mixtures, we finally got something going.

 

We are using a platinum impregnated ceramic monolith catalyst from Catalytic Products International http://www.cpilink.com/ :

 

http://media.armadilloaerospace.com/2003_06_08/catalyst.jpg

 

We had been expecting platinum based catalysts to be notably poorer than silver based catalysts at room temperature, probably requiring preheating to do much of anything.  Surprisingly, eye-dropper squirts of 50% peroxide react faster on this than on our silver screens.  There is a good chance we will move to this style of catalyst even for 90% peroxide, because it is much lighter, and has much less pressure drop.  Engine thrust to weight ratio will actually be important to us for the first time on the X-Prize vehicle.

 

The ceramic monolith has 400 fairly large pores per square inch, so it doesn’t offer enough resistance itself to spread out liquid flow from the engine inlet.  We welded in a sheet of stainless that had many small chem.-etched holes at the top of the catalyst chamber, stood off with a few spacers to give the liquid room to spread out.  We tested with water until it was sealed well and distributing evenly.  These holes are still too large to provide an injector pressure drop for stability, but they get the flow reasonably evenly spread out.

 

The catalyst monolith is structurally strong enough that it doesn’t need any kind of supporting plate at all, which is nice.

 

We are testing a 5.5” diameter by 2” thick catalyst pack, with a 1.25” diameter nozzle throat.  There is no post catalyst combustion volume besides the converging section.  Our weld-on flanges are due in soon, so we will be testing various extension lengths to see if it makes a difference.  The idea is that methanol will catalytically burn on platinum, and the mixture is intimate, so it may not need any separate combustion volume at all.

 

We tried flowing some raw 50% peroxide through the engine first.  It was only partially catalyzing, leaving a lot of peroxide foaming on the ground, but this was still a lot more active than when we flowed 50% through our screen based packs.

 

For the first methanol burning test, we used a propane torch stuck in the nozzle to pre-heat the ceramic catalyst pack.  Propane also catalytically burns on platinum, which allows you to do an interesting demonstration:  you can light a propane torch and heat a section of the catalyst until it is hot, then kill the torch and just play the gas onto the catalyst, and you can move a red hot glowing spot over the catalyst without any visible flame.  It is still difficult to heat the catalyst inside the engine, because oxygen is still necessary for combustion, so you can’t just stick the torch way up inside the closed engine.

 

We were using a solenoid for the propellant, so we knew we weren’t going to flow too terribly much through the engine.  The flow started about the same as the raw peroxide, with some foaming peroxide coming out the nozzle, but 1.5 seconds later, the engine roared to life with flame jetting out the bottom.  We had only loaded a small amount, so it ran out in a couple seconds, but we were elated at the first truly positive signs we have seen from a 50% peroxide combination.

 

We repeated that test successfully, then loaded up more propellant for a longer run.  The long run test displayed a truly severe chuffing at around 1 hz intervals, almost as if the engine was being shut down and restarted repeatedly.  This wasn’t too surprising, given that we didn’t have any explicit injection pressure drop anywhere, and the chamber pressure was probably barely 50 psi with the little solenoid feeding that big throat.  We made additional runs with jetted orifices, changed to a small ball valve, and eventually used the little cavitating venturi we had Fox Valve make for our small biprop tests, but the engine hasn’t ran stably yet.  The CV changed the nature of the instability some, but it was still very unsteady.

 

We are assuming that the system has some issues like we have seen in our other catalyst designs, where a very active surface at the top can cause unsteady disruptions of flow before everything settles down to a steady push through the active part of the pack.  Our screen based engines need a number of inert screens at the top to run smoothly, so we probably need something similar for these.  One thing we realized after most of our runs was that the we preheated the pack much more thoroughly on the later runs, such that there wasn’t any wet startup period.  This probably got the heat all the way to the very top of the catalyst, which triggers the flow disruptions.

 

We tried packing a couple discs of inert foam above the catalyst, which increased the frequency of the chug, but didn’t diminish it.  We can’t easily add more at the moment, but when our weld-on flanges arrive, we will have more fabrication options.

 

We may try flowing a significant amount more propellant into the engine and see if it smoothes out. 

 

We can experiment with less aggressive pre-heats.  I have ordered an adjustable heat gun that we will use to blow hot air through the center of the pack, and let it circulate back down around the outsides.  This should give a good, controllable heating.

 

We are going to see if it is possible to use various acids to strip catalyst from the top 1/8” of the monolith, which would give us a flow straightener before it hits active material.

 

Overall, we are very excited about this.  Throttling may not be as good as the screen pack monoprops, but the plumbing is exactly the same, the thrust to weight is much better, the Isp should be much better, and the propellant cost is about 1/10th that of 90% peroxide, with no availability problems.  It is also more gratifying to see flames come out of your rocket engines. J

 

 

 





 






 
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