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Vehicle hot firing

New spreading plates

January 25, 2004 notes


Vehicle hot firing


After a great deal of work this week, we got the entire vehicle together and ran propellant through it.


We got new stainless spreading plates for the engines from http://www.globalstencil.com/ .  They have been turning our orders around from CAD drawing to (local) delivery in under a week, which is really nice for our schedule.  The new plates have 848 holes of 0.022” diameter, which is over four times as many holes, but with a smaller diameter.  We tested the spreading by flowing propane / air through a chamber blocked by the spreading plate and seeing how many 20 mesh screens we needed to add to get a uniformly even flame without hot jets in the spreading plate hole pattern.  I was a little surprised to see that it still took seven screens, down only three from our 204 hole spreading plates.


Russ finished welding all four engines together in this configuration: http://media.armadilloaerospace.com/2004_01_25/finalEngine.jpg


All of the vehicle propellant feed system was reworked to larger diameter plumbing.  The hoses were changed form –10 smooth bore to –16 Extra-Flex (corrugated) Teflon hose.  We have them bent up rather like a pretzel right now, because I had to order hoses before we had the valves mounted.  I may order a set of shorter ones in the future.  The throttling valves are all changed from ½” to 1” KZCO valves, which are faster actuating as well as much larger, so I am going to have to do some simulator work to see if this is going to make the vehicle overcorrect while stabilizing.  The elbows between the valves and engines were changed to smoothly bent tubes instead of machined fittings.


We had to change our drum pressurizing loading pump around a bit because the Air Liquide peroxide drums have a buttress thread instead of an NPT thread, not allowing our 2” tube to fit through it.  We bored out an adapter so it could tightly fit a 1” ID clear flexible PVC hose, and we RTV’d it in.  This is nice, in that the propellant doesn’t flow through any fittings, it only touches the hose, like in a peristaltic pump.  However, even with surface roughing, we got lousy adhesion with the RTV to the PVC.  We lived with it today (holding it in place to minimize the pressure leak), but I am going to try getting PVC pipe adapters that we can use PVC solvent to really bond the hose in with.


Our 50’ trailer-to-vehicle loading hose has gotten kinked pretty badly in several places.  We probably need to replace it with a very long extra-flex hose, which will be a special order.  There is some concern about the amount of fluid that may remain trapped in the corrugations, but the smooth bore hose seems to pick up a worse kink every time we use it.  This hose needs to take the full vehicle tank pressure during loading, so we can’t just use a nice plastic hose.


Lots of wiring work was done on the vehicle to tidy things up.  The glow plug battery was replaced with a smaller sealed lead-acid battery.  I milled a nice mounting plate for the glow plug relays and various distribution blocks coming from the main wiring harness.  Each engine now gathers all ten lines into a single cable – motor drive 1 - 2, pot feedback 1 - 3, pressure transducer 1 - 3, and glow plug 1 – 2.  Par for the course, there were two things miswired by the time I was done.  I added indicator lights for glow plug active and a power light for the master cutoff computer.  We have learned that if you don’t have a bright power light, you will eventually leave a computer on until the battery is completely dead.  Another thing we are doing better is color coding all connectors as well as numbering them.  There have been several cases where the numbers zero through four wind up getting mixed up.  Colors are almost impossible to connect wrong.




Wiring was done so that we can charge all four vehicle batteries (main, actuators, cutoff computer, glow plugs) from a single connector port.


Unrelated to the vehicle work, Phil made improvements to our test stand control board: mounted a voltmeter so we can tell when we have drained the battery with the glow plug, and gave it mounting slots at the shop so we can just pick it up and take it to the remote test site without taking anything apart.


We fabricated nozzle plugs to fit under the engine bells during warmup.  They have a sliding plug that is held up with a pin on a chain, so the chain can just be pulled on to drop the plug and pull it out from under the vehicle.





After working all day Saturday and Sunday, we finally had everything all together:






In theory, this vehicle can now make launch-license-limit flight tests, but we still have bugs to work out.  We went through the loading process and warmed the engines some, but we pulled the plugs out too early, because two of the engines weren’t heated well enough.  With a single engine, we can tell the warmup is complete when the engine stops “spitting”, which is some time after the flow goes completely clear.  It is harder to tell with four engines running at the same time.  It was late, and there are several little glitches we need to address, so we called it a night.  We will be repeating this next week when we can go through the entire process several time.  Once we have everything working perfectly, we will head out for the captive hover test.



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