January 31, 2004 notes

 

Vehicle work

 

Analyzing the data from last week’s vehicle warm up test showed that while all engines came up to pressure when a big burst was made, with the throttle just cracked open, two of the engines weren’t making any pressure.  I added code to let us individually calibrate the “cracking fraction” of each valve, so we can now expect all of them to open within 1% throttle of each other.  Still, micro throttle cracking isn’t very even, because 1% in the opening fraction of a 1” ball valve is a lot of flow.  If we want to use a tiny idle flow, we may need to add extra valves specifically for that.

 

PVC cementing the drum adapter directly to the 1” PVC feed hose worked better than trying to RTV it, but by the end of the day’s testing it was leaking a bit again.  I need to find a 2” buttress thread fitting in PVC that I can cement both the hose and a support pipe directly to.

 

Our floor scale for weighing propellant inside drums gave us a problem, where it basically stopped reading past about 250 lb, even though it was a 1000 lb scale.  We were able to make an adjustment internally to fix it.

 

We have always had fairly noisy A/D signals on our flight computer, but last week I saw a very strange behavior with several of the signals dropping about 10% after the flight control program had run for a half second or so and was polling them rapidly.  I decided to replace the WinSystems A/D board with a board from AccessIO: http://www.accesio.com/go.cgi?p=../104/104-aio16-16.html  This board also has 16 channels of digital IO, so we were able to replace our dedicated digital IO board (also from AccessIO) as well, reducing our total PC104 card count.  Rewiring everything to a different breakout board was a bit of a pain, but we got a few extra bits of precision out of it, and the odd drop is gone.  I also have an AccessIO serial board that I am going to use to replace the WinSystems serial board that doesn’t seem to work with the AmPro CPU module.

 

Our Ashtech G12 HDMA GPS was still locking up on us, so it went back to Thales for a second time.  They are going to replace the board this time instead of just flashing the firmware and sending it back.

 

I finally figured out two little non-fatal but glitchy things that have been with our flight control system for at least a year and a half.  One of them was pretty silly – I often had to hit enter a couple times to get the flight computer to acknowledge that I wanted to exit the program.  It didn’t seem to have any other effect, so I never chased after it, and just got used to banging enter a few times rapidly when I wanted to quit.  Many old sensor subsystems which are not initialized are still being called to check for updates.  Most of the systems are designed for this, so putting them back in just means uncommenting the initialization line, rather than having to hunt around for multiple places to fix code.  However, in this case (the on-board pilot’s serial joystick), the update wasn’t checking if it had a valid serial file descriptor before polling for input.  Uninitialized descriptor = 0 = stdin.  Makes perfect sense now.  The other glitch was on the laptop side, where I found that occasionally (every few thousand times or so during throttle changes) the USB joystick reading would return a bad value.  No error, just a bad value.  It is a consistent bad value, so I just ignore it now and use the previous value.  This avoids a spurious throttle burp and occasional logging reset.

 

We managed to kill one channel of our lead-acid charger system, so we replaced it with a new one.  Having a single hook up for charging all the batteries is very important.  I need to upsize the main computer battery, because we had to stick the charger on a couple times during testing to keep the voltage from dropping too low.  Having to climb up a ladder and unbolt the hatch to connect the charger each time was a bit of a pain.  Cabin-at-the-bottom on the next vehicle will make several operational issues easier.

 

The propellant loading test we did today had one of the engines start cooking off as soon as propellant was pumped into the vehicle.  It turned out that part of the actuator to valve linkage was held together with just two little spot welds, and they had broken, allowing the valve to slip 30 degrees or so before the plate wedged up against something else.  Russ welded the plates together properly, and we checked out all the other valves.  The 2” valves had a nice weld along the sides already, just like Russ had done, but the 1” valves were all done with two spot welds, which were actually ground smooth, reducing their strength.  We now have three things to do on the 1” KZCO valves: vent the ball, bridge the thermal cutoff, and seam weld the actuator plate.  We use the fastest actuator gearing, which KZCO doesn’t usually sell because it is difficult to manually control, so this may not show up much with their normal customers.  This was a good thing to catch on the ground…

 

If we had good success with the vehicle tests today, we were planning on heading out for hover tests next week, but we didn’t get a successful run.

 

The first test had the valve problem.  The remaining tests functioned properly in all regards except that after removing the preheat plugs, we couldn’t get clear running main engines.  We either got huge clouds or they quenched out on throttle up.  We let one test preheat until the warmup plugs were glowing red hot on removal, so it was certainly quite hot.  We did get the vehicle to hop off the ground into the lift truck forks (we set them up specifically to hold the vehicle down) with only 100 psi in the tank, which means we have plenty of thrust.

 

I have a few theories: The combination of the smooth turn elbows and the much shorter flex hose gives a higher flowing combination than our test stand, so the engines might be getting drowned.  We have never done testing with only 100 psi tank pressure, it is possible that the engines need a higher pressure to “wake up”.  It is possible that the preheat we did, which was a moderately high flow continuously with the plugs in, heated the bottom of the pack quite well, but didn’t heat the top, which would cause quenching behavior.

 

We will do a bunch of test stand work with the flight engines next week, and probably repeat the vehicle warming tests next Saturday.  On the bright side, the electronics were flawless all day, and we are building a lot of experience moving the vehicle around and working with drum scale quantities of propellant.

 

In other work, we have upsized all the plumbing on the 12” engine to 2”, which should drastically improve the performance based on our results with the 5.5” engine.  If we run the tank up to 450 psi, we should see 5000 lbf.  We decided to skip the load cell and hard mount the engine directly to the stand to avoid the shaking problems.  We can calculate thrust from chamber pressure, although we will likely underestimate it somewhat when scaling from the previous run, because the nozzle factor will be better at the higher chamber pressures.  I am getting to be a fan of the cam-lock connectors we are using on the larger hoses, because they have a gasket seal instead of relying on a metal to metal seal like the AN fittings.  However, we have been seeing a lot of corrosion on the aluminum pipes and fittings, even when we haven’t had anything but water in them.  I had hoped to save some money and weight with aluminum in the larger sizes, but it looks like it may be better to just stick with 316.

 

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