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Big vehicle plumbing, more mixed monoprop tests

July 26, 2003 Notes

July 26, 2003 Notes


Big Vehicle Plumbing


We assembled and tested the major fill and drain plumbing for the big vehicle this week.  To be able to load and drain 800+ gallons of propellant in a short amount of time, the plumbing is quite large.  We are using 2” cam and groove hose couplings for connections, and we are welding 2” pipe connections wherever possible, instead of using NPT connections.  We machined custom aluminum flanges to replace the NPT flanges on our KZCO servo valve for the master cutoff, which allows us to do away with rotating connections there.  We still have NPT connections between the stainless steel check valve and aluminum hardware on the loading side.


Fitting a 1/8” thick O-ring to an 18” diameter groove is a pain, because the groove isn’t really deep enough to keep it from wanting to pop out as you move around the circumference.  A thicker O-ring would have made life easier, but then we would have needed to use a thicker flange plate, which would have added weight and required longer flange bolts.  We used Teflon thread sealant to stick down the O-ring as we moved it into position, which worked out well enough.


We got a bit lucky on some of our positioning – it is only just barely possible to actually assemble all the various bolts and fittings of the plumbing on the vehicle.


We loaded 30 gallons of water from a transfer tank into the vehicle tank to check the procedure and look for leaks.  Because we don’t have the engine plumbing assembled we temporarily just put another big hose coupling under the master cutoff valve for draining.  It turns out that we have a leak around the parachute support post, so we are going to have to take everything apart to fix it.  This may have happened when we set the vehicle upright, because our current legs are a few inches too short for everything, and the vehicle hit the parachute shackle, which may have cracked the weld inside the tank.







More Mixed Monoprop Tests


On Tuesday, we made several modifications to our test setup.  We replaced the ¼” ball valve and associated plumbing with a ½” ball valve, we plumbed the pack preheat fuel / air lines in without a manual valve, and we added a chamber pressure tap.  We also moved the engine mount to a new beam for mounting on our permanent test stands, in preparation for high thrust / long duration test burns at our remote site.


The chamber pressure tap has worked perfectly so far.  We welded a swagelock fitting onto the engine, and ran about two feet of fairly small stainless tube from that to a pressure transducer with a porous metal snubber in front of it.  The snubber does act like a low pass filter on the pressure readings, but we didn’t see any signs of temperature affecting the readings, so we are happy with it.


All the runs on Tuesday suffered from extreme chuffing due to the replacement of the ¼” valve with ½” hardware.  The chamber pressure measurements showed that we had very little pressure drop, especially considering that there was still 10’ of –10 hose between the tank and the valve, meaning that much of the measured pressure drop was from the hose, rather than the valve / spreading plate / catalyst pack.  A couple of the runs had smooth enough spots to get reasonable data from, but this was certainly not a goodconfiguration for the engine.


On Saturday, we swapped out the 1.25” diameter nozzles for the big 2” diameter nozzle, which would allow much higher thrust, but would result in a lower chamber pressure, which we hoped would eliminate the chuffing.  We tried twice with this, but we flooded out the catalyst both times, only building a little chamber pressure before dropping off to nothing but a foaming mess.  At a tank pressure of around 200 psi, the initial inrush of liquid with no chamber pressure is two to four times as high as when the engine is operating, depending on how restrictive the plumbing is relative to the catalyst.  This catalyst does not seem to be able to handle that level of flow.  A properly sized cavitating venturi could prevent the excess initial liquid flow, or we could try to just crack open the valve until some chamber pressure has been built.


We added a ¼” restriction before the ½” ball valve to give more pressure drop for smoothness, and made a few more runs.  The low pressure runs were still rougher than the runs from the 19th, because the single ¼” restriction still offered less pressure drop than the ¼” valve and fitting combination that we previously had.  The run at high pressure was smooth, because it had sufficient pressure drop.  The measured Isp from that run was 137 s, consistent with the previous measurement.  I had let it run for a second with the throttle only cracked to allow some chamber pressure to build, which reduced the total Isp somewhat, and we are seeing a little bit of channeling around the outside of the catalyst pack, which also cut a few seconds from it.  We will probably weld in permanent anti-channel rings between the catalyst slices, as we started doing for our screen based engines.


The last run, again at low pressure, did started chuffing again, and the catalyst was quenched.  The next test we need to do is determine if the quenching was a result of the chuffing, or vise versa.


Things are working well enough that I have ordered sufficient catalyst for four 5.5” diameter catalyst packs, which will be able to do low altitude flights of the big vehicle, and one 12” diameter pack for testing the full scale engine.  We are moving from the 200 cells per inch in the current test catalyst to 400 cpi for all the 5.5” catalysts, and we are going to try an experiment with a single 600 cpi catalyst in the big engine, under the theory that the very tiny cells will be too small to support liquid tunneling, and we won’t need to segment the catalyst.


Pulling representative numbers from the three days of testing:


July 19:

62 lbf from 145 psi tank pressure, 0.120 orifice

113 lbf from 132 psi tank, 1/8” NPT restriction

248 lbf from 328 psi tank

163 lbf from 157 psi tank, ¼” NPT valve and fittings

335 lbf from 366 psi tank


July 22:

278 lbf from 227 psi tank, 179 psi chamber, ½” NPT valve

293 lbf from 226 psi tank, 181 psi chamber


July 26:

199 lbf from 168 psi tank, 138 psi chamber, ¼” NPT restriction

553 lbf from 528 psi tank, 345 psi chamber

199 lbf from 167 psi tank, 142 psi chamber


We also did another experiment with hydrogen based preheating.  Flowing hydrogen / air mixtures through the engine does indeed auto-ignite after a short period of catalytic heating, avoiding the need for a manual torch lighting, but once it ignites into a flame, the flame immediately flashes back into the plumbing above the engine, putting a lot more heat there than we like.  We will probably try welding a fitting for the hydrogen directly onto the engine, so that only the forced air flows through the external plumbing. This will still raise the spreading plate to full flame temperature, but the plumbing should stay cooler.




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