March 18 and 22, 2003 Meeting notes

 

Smallest Armadillo Fan

 

We have been selling Armadillo Aerospace T-shirts for the last week or so with PayPal support - previously we hid the fact that we even had t-shirts for sale due to the fact that we’re all busy doing stuff (Anna was willing to take up that task so now we have a paraphernalia page: http://www.armadilloaerospace.com/n.x/Armadillo/Home/Paraphernalia).   The Ward family wanted to outfit their entire crew; including the baby.  Anna is a sucker for such requests (no more, please!), and had a couple tiny shirts made up.  Here are the pics: baby1  baby2

 

Cabin Pressurization Test

 

We had intended to delay the cabin pressurization test until we had mounted the cabin on a full size tank, but I had been thinking about the consequences of having the bulkhead rupture while the cabin was permanently mounted on the tank – we would never have been able to replace it, and it might have forced us to scrap both of them.

 

We went ahead and sealed up the cabin on the boilerplate tank end, and did some tests.

 

We used two-part foam to seal between the cabin cone and the tank end, which worked extremely well, because it poured down, then foamed up.  When we flipped it over, we could see it had foamed through in the couple spots that we knew were a bit of a loose fit.

 

We doubled up the number of bolts we used to secure the cabin to the tank end, because 20 psi over the entire surface area is 60,000 pounds of force.

 

We put RTV around all the bolts securing the seat belt brackets and bulkhead hangers.

 

We finally got the hatch to fit perfectly by a combination of hydraulic press work and banging the hell out of the frame with a big mallet (high tech!).  With a door seal gasket all the way around it, the hatch squashes into place and seals perfectly with only a fraction of a PSI of internal pressure.  We have gone back and forth with all kinds of schemes for hinges, dogs, and latches, but I think we are just going to let it be pressure retained, possibly with a loose fitting chain to hold it when the pressure completely goes away.  We have two removable handles that screw into the outside of the hatch to allow it to be positioned and held while pressurizing.

 

Our early pressurization did show two welds that had some porosity in them, and we had leaking where we had to grind the flox fillet down to install the backup bulkhead hangers.  Russ fixed the welds, and we added some more RTV on the outside of the bulkhead hangers.

 

We put the cabin on the other side of a concrete wall, and started bringing the pressure up.  At 17 PSI, a bubble formed in the RTV by the bulkhead hanger, and popped, giving a pretty strong leak.  It probably would have been fine as is, if we had let it completely cure, but we went ahead and put a good bead of RTV around the entire inner flox fillet, because we believe that it was leaking into the honeycomb from farther away, and only rupturing out at the point on the back that we had to grind the filet away from the wall.

 

The structural elements don’t seem to mind 17 PSI it at all, which is all we expect to see during flight (pressurize to 2 PSIG on the ground, retain full 16 PSIA throughout flight into vacuum).  We are going to do some rapid pressure cycling to 20 PSI next week, but it looks like the cabin sealing is already nailed down, which turned out to be easier than we expected.

 

We still need to test the air tank and back pressure regulator that will maintain a constant, controlled bleed rate for cabin comfort.

 

Another Failed Hover Test

 

We made a bunch of improvements on the small vehicle.

 

Russ finished an O-ringed aluminum piston for the drogue ejection, which works very well.  It seals so well that we have to open the solenoid to allow the piston to be pushed in.

 

Phil picked up our custom nomex parachute bag for the main chute.

 

I removed the potentially-flaky power connector to the Esteem, directly crimping the lines together.

 

The strange behavior with dropping TCP packets while transmitting large UDP packets over the wireless network showed up again, so I changed the wireless channel on both units, and it immediately went away.  There must be another strong transmitter in our area that is sometimes active.

 

Russ finished the new power supply board with real terminal strips, and regulated +12V for all devices.  I also now log the regulated 5v and 12v values, in case that ever tells us anything.

 

The Crossbow IMU was sent off for repair, which looks like it will cost about $1000 (the FOG’s weren’t damaged, just the interface).  Fortunately, our new Crossbow unit was just about done, and it got shipped out overnight for us, so we were able to test with that on Saturday.  The new generation (700 series) units have half the noise in the FOGs, and a few other subtle changes.  The zeroing procedure has changed in a mildly annoying way (no explicit completion signal, just three minutes of active filter time), the angular rate range dropped a bit, and the exact timing of updates at different baud rates changed, but overall it was straightforward to integrate.

 

One thing we have talked about fixing for over a year is to replace the excessively long cable that runs from the Crossbow to the serial port and power source with a proper length one.  We decided to go one step farther and make the cable go directly to the 10 pin ribbon connector on the PC104 module, so the DB9 to DB9 connection went away completely.  I finally got around to buying the proper AMP crimping tools and supplies to make nice DB connectors (DB15 on the crossbow), so I’m not making crappy solder connections on Radio Shack DB connectors.  Professional crimping tools make all the difference in the world.

 

I am trying to get some bonded rubber isolators to mount the entire electronics board with, but I have been playing phone tag with a sales rep.  I had avoided doing this in the past, because I didn’t want to hurt the bandwidth of the IMU, but I realize now that there are a lot of vibrations over a couple hundred hz that would be nice to get rid of, even if we don’t touch the low frequency ones.

 

Several readers pointed me at the Xenarc 7” LCD VGA screen, which I have placed an order for.

 

When we got everything hooked back up for testing, I noticed that the recovery tank (drogue ejection) pressure transducer was reading a ridiculous off scale value (-200 psi).  Russ opened up the bottom access panel and found that the transducer itself was warm to the touch, indicating that it was likely internally shorted.  I went back to the data logs from last week, and found that the sensor did die about a quarter second before the computer did.  We may have had some kind of a short that spiked the power bus, although the battery voltage did not appear to drop in the telemetry.

 

The timeline for last week’s test was:

 

0.00s    begin throttle up

1.00s    crossbow stops updating, code doesn’t properly recognize this, so valves that were opening continue to open

1.65s    recovery pressure transducer dies

2.05s    telemetry ceases

 

We just cut the pressure transducer away for now, but we will replace it with a different brand soon.  This might be an argument for fuses, but I have heard people make the case that they can add more unreliability than the problems they solve.

 

We pulled the vehicle out under the crane and loaded up 2.5 gallons of peroxide for the first test.  150 psi tank pressure is plenty for hovering the vehicle, even with the small nozzles and partial throttle.  The first thing we ran into was that Russ had accidentally closed the master cutoff valve with the manual switchbox (we have the hardware installed, but no microcontroller running it yet), so none of the engines would fire.  I have added a warning light for that condition on the laptop console now, so we know about it before we load any peroxide.

 

It was raining all day, so the engines were very cloudy, but they were making enough thrust to lift off.  We really should have done more thorough test stand testing of each of the four engines before integrating them in the vehicle, but we had hoped that we would just be fortunate and have an “all up test” work out well in the vehicle.  Current questions about relative engine thrust and decomposition efficiency probably won’t be able to be answered until we take it all apart and test each of them individually, but we don’t have enough peroxide on hand at the moment to do that.

 

The first liftoff pitched over about 10 degrees, but it was catching it and correcting as I throttled it back down due to lack of visibility.  It looked like it probably just needed some adjustments to the control gains.  When we tried a second load, we experienced another failure.  During throttle up, just as it registered some positive acceleration, all the voltage signals on the A/D board dropped.  The battery and power supply voltages only dropped a few tenths of a volt, but the valve potentiometer feedbacks dropped by about 0.75 volts, which put them all the way off the bottom of the scale.  This caused the computer to throttle them up, sending the vehicle into the crane again.  The telemetry ceased about the time it hit the forks.

 

http://media.armadilloaerospace.com/2003_03_22/hover1.mpg

http://media.armadilloaerospace.com/2003_03_22/hover2.mpg

 

This time, it came down hard enough that the line stretch and boom flex allowed one of the legs to impact the ground, causing some damage that we will have to repair.  In the future, we will take it up at least another foot.  When we restarted the computer, all the signals on the A/D board were dead, but we could still read them with a voltmeter, so that board seems to be toast.  We will replace the board next week, but we don’t know yet if a bad board was a case or a result of the problems.  I am going to double check the IO and IRQ usages of the boards in our PC104 stack (ampro core module 400, Winsystems A/D16, Winsystems Com4, Winsystems IO48), because this is a very different stack than we used on our previous systems.

 

The fastblock-800 thermal coating on the engines continues to impress.  After five seconds of engine firing, you can go up and grab the engine with your bare hands.  There was also some concern that the coating might be sensitive to water, because the raw coating is water based, but after curing, it doesn’t matter if it gets wet.  The heat on the base plate from exhaust gas blowback is the main thermal issue now.  It would probably be a good idea to coat the entire base plate, because tail section heat may be one of our contributing factors to strange sensor readings.

 

Hanging on a rope does cause the roll angle to drift quite a bit before liftoff, which skews the attitude control a bit.  We had originally intended to pull blocks out from underneath the vehicle at liftoff, but with the big clouds of smoke, we can’t do that accurately.  On the plus side, from the little one second airborne times we are collecting, the angular rates are drastically smoother than we used to see with the solenoid based attitude control systems.  When we finally get it all worked out, it should look a lot more stable than the old binary control system.

 

 

Upcoming small vehicle work:

 

Repair the bottom bulkhead

Replace the A/D board

Figure out what the hell is causing our reliability problems

Figure out why the engines aren’t running very well

Hover test

Cut down cone for longer crush cones

1,000’ flight locally

Assemble TV transmitter equipment bulkhead

3,000’ flight at Oklahoma Spaceport

10,000’ flight somewhere else in Oklahoma

 

 

Upcoming big vehicle work:

 

Pressure cycle test

Test propellant venting times for aborts

Finalize three person cabin arrangements

Install cabin on 800 gallon tank

Fabricate tank end closure for parachute and engine mounts

Helicopter drop test for parachute / crush cone interaction

Test fire the 12” engine

Install four 6” engines and one 12” engine for test flights