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Engine roundup, Quad engine setup

ACR = anti-channel ring, currently Smally spiral retaining rings

December 3 and 7, 2002 Meeting Notes

 

Engine Roundup

 

We did several big engine tests today.  It was cold, with 75% humidity, so all the runs were cloudy, but we got good data on a couple new configurations.  Our initial test was a new cleaning of the pure-silver engine pack.  We had noted that after cleaning in the already used nitric acid solution, the screens had not turned the chalky-white color that the last batch had.  We got some new nitric acid, and tried washing some of the screens again.  A 10% nitric acid solution did not seem to have much effect, even after a couple minutes, but a 25% solution almost immediately cleaned the screens.  We cleaned half of a disc for a comparison test, and there was a fairly dramatic difference over the previous cleaning, so the lesson is clear – use fresh acid to wash all screens.  The pure silver screens we get seem to have a fairly tenacious coating on them, and the acid cleans it better than anything else.

 

With the marked difference in activity, we hoped that the silver-only engine would catalyze well, but it still flowed liquid all the way through the pack without fully catalyzing it.  We still don’t really understand the behavior of catalyst packs, where we can have more silver surface area present in this engine, but even with low flow through it due to a mostly closed throttle, it still doesn’t catalyze well.  Removing some silver screens and interleaving stainless screens then allows it to work properly.

 

The second engine we tested was using thickly silver plated stainless screens.  The plating shop needed to do a nickel flash on the stainless before plating the silver, and we also had them sandblast the finished screens for increased surface area.  We were only able to fit 65 of the thicker 20 mesh screens in the engine, even with removing the later anti-channel rings, but somewhat to my surprise, it was still a good performer.  At the lower flow rate due to the long feed line to the test stand, the pack performed about 4% better than the interleaved pack.  The peak thrust as the line cleared was actually lower than the interleaved engine, but we need to repeat the tests more, ideally with a short-feed line, to make any final conclusions.

 

There is a few percent more improvement to be had by using a brazed screen ring instead of the perforated metal plate at the bottom, and welding the intermediate rings in, but it is a bit surprising that plated pack, with less total screens of a larger mesh, didn’t have a more noticeable flow rate improvement.

 

When our coriolis mass flow sensor and new peroxide supply arrives, we will probably go through a couple drums of peroxide to fully characterize a couple different packs over a wide range of pressures, run times, and throttle positions.  We hope to answer most of our questions before going to the 12” engines.

 

We have four different pack combinations in engines at the moment, so it is a good time to recap all the different combinations and results.

 

ACR = anti-channel ring, a Smally spiral retaining ring that fits tightly in the catalyst chamber

Pure silver screens are 32 mesh.  Inert stainless screens and silver plated stainless screens are 20 mesh.

Press force in pounds is approximately twice the gauge psi, and the engine are 5.5” diameter, so the actual psi on the pack is the gauge pressure divided by 12.

 

Lander Engine

-----------------

2 ACR

10 inert stainless screens

ACR

20 silver interleaved with 10 stainless

ACR

20 silver interleaved with 10 stainless

ACR

10 silver interleaved with 10 stainless

ACR

10 silver interleaved with 10 stainless

perforated metal retaining plate

 

After each ACR, the pack was compressed to 5000 gauge psi, and the pack was under compression when the bolts were initially torqued down.  After running a few times, the stress in the pack was relieved, and there was no longer active spring force.  The engine continues to run smoothly, so active compression does not seem to be required after run-in, although we have seen 2” engines start to get rougher after they have run for a while.

 

Used most commonly at 600 psi tank pressure in the sit-down lander.  After nearly two dozen flights, we were beginning to see some apparent channeling from the engine, with a small visible stream under the engine.

 

We had always noticed that the ACR would no longer be a tight spring fit after the engine had been run, and sometimes they would have quite visible gaps around them.  We had previously attributed this to the rings shrinking, but we have since found that it is a combination of ring shrinkage with stretching of the stainless catalyst chamber.  Welding the spiral rings closed did not help the rings stay any tighter.

 

This engine uses one of the old nozzles that was sharp angled, but it has been smoothed out somewhat to about 1.85” throat diameter.  The rest of the engines have 2” diameter throats with a full 1” radius.

 

 

Tube Engine

------------

Because the large tanks do not have the pressure capacity of the tanks on the lander, we began experimenting in hopes of producing an engine with less pressure drop, as well as improving the channeling issue.  Directly welding the anti-channel rings to the catalyst chamber prevents the engine from being disassembled, but provides several benefits:  it is absolutely impossible for any peroxide to get around them, it reinforces the chamber to reduce stretch, it distributes the pressure drop across the pack to multiple supports, instead of allowing the entire pack to compress down on all the layers below it, with the attendant pack oscillation problems.  When the entire pack is a slip-fit, the ¼” thick stainless retaining plate would take on a noticeable bow after running several times.  With multiple welded supports, the only load is divided up among them, such that the we were able to replace the thick and heavy retaining plate with just a stainless screen brazed to the last retaining ring before it was welded in.  After many firings, it is not bowed out at all.

 

2 ACR

10 inert stainless screens

welded ACR

10 stainless / 20 silver alternated

welded ACR

10 stainless / 20 silver alternated

welded ACR

10 stainless / 10 silver alternated

welded ACR

10 stainless / 10 silver alternated

welded ACR with brazed screen

 

Pressed to 5000 gauge psi before welding each ring in, but pressure was not retained during the welding, so there wasn’t any active compression in the pack at all.

 

This engine has been run about ten times, and is completely smooth.  These modifications seem to be completely positive, at the expense of not being able to disassemble the engine.  If you can’t pull the pack out, having flanges doesn’t really make sense, so our future engines are likely going to be top-loaded, with a welded on closure.

 

 

Silver Only Engine

----------------

2 ACR

10 inert stainless screens

welded ACR with brazed screen

10 silver

welded ACR with brazed screen

10 silver

welded ACR with brazed screen

10 silver

welded ACR with brazed screen

10 silver

welded ACR with brazed screen

10 silver

welded ACR with brazed screen

10 silver

welded ACR with brazed screen

10 silver

welded ACR with brazed screen

 

Pressed to 4000 gauge psi before welding each ring in, but pressure was not retained during the welding, so there wasn’t any active compression in the pack at all.

 

This was a big hope for a low-loss engine, but we haven’t been able to make it work yet.  A full pack of pure silver screens gets far too soft, and crushes itself after running a few times, but by supporting every ten silver screens with a welded ring / stainless screen combination, we are able to have most of the pack screens be silver without letting the entire thing crush together.  Unfortunately, even though there are more silver screens than in the interleaved pack, it still won’t fully catalyze.

 

Plated Engine

----------------

2 anti-channel rings as spacers

10 inert stainless screens

ACR

10 plated screens

ACR

10 plated screens

ACR

45 plated screens

perforated metal retaining plate

 

After each 10 screens, the pack was compressed to 6000 gauge psi, and the final assembly with the bolts kept it under fairly near that compression.

 

This pack worked, and produced a small improvement over the interleaved packs, but it wasn’t large, and we are still concerned with the life of plated screens.

 

 

It is tempting to try making a welded engine with silver plated nickel foam, which we know has spectacular catalytic behavior for a given pressure loss, but we never did get foam packs that lasted longer than about 120 seconds, so welding the engines would almost certainly be a bad idea, even though it would solve the other major problem of the foam packs, which is that they got compressed with time, and started to run rough.

 

Quad Engine Setup

 

The vehicle currently in development is going to be our first test of our new propulsion direction – four equal sized, canted, servo throttled engines.  This was the layout for our very first lander that used solenoids, but we moved to big-central-engine/small-attitude-enginess for the subsequent vehicles because of some size issues with our our 500-600 lb vehicles, and because we were trying to mitigate the difficulties of high performance (hybrid / biprop) engines by only having a single one.  Over the last year, we have become more aware that we can’t get the control authority that we will need for larger vehicles with solenoid controlled attitude engines, even if we double them up, and we also found that multiple throttled biprops probably isn’t something to be all that concerned about.

 

A test-fit of four engines with (finally!) a master cutoff valve on a plywood mock-up bulkhead:

http://media.armadilloaerospace.com/2002_12_07/4_engine_mockup.jpg

 

Russ made a new custom board with four bi-directional motor drive circuits on it for the new vehicle, and we let it stress test for several minutes randomly cycling back and forth every 50 msec, without any problems:

http://media.armadilloaerospace.com/2002_12_07/driverboard_test2.jpg

 





 






 
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