August 11, 2001 Meeting Notes

 

In attendance:

 

John Carmack

Phil Eaton

Russ Blink

Bob Norwood

 

General Improvements

 

I made several reliability modifications to the electronics box that were tested today.

 

The small battery that ran the electronics was replaced with another 3.3Ah battery identical to the solenoid driving battery. This lets the computer stay on for almost three hours without a problem. The electronics draws about 1.2 amps constantly, while the solenoids draw up to 35 amps during operation, but that is only during the variable duty cycle of a 10 second or less hop. The electronics battery always needs more charging.

 

I installed a better main power switch. The previous one was a little light for my tastes, and might have caused a voltage drop during operation. Since we moved to dual batteries, we have had the solenoid battery wired directly to the solid state relays without a power switch, but there doesn’t seem to be any measurable current leakage. One effect I do need to watch for is that this means that the SSR board has live current unless I disconnect the battery. I once got some sparks when I accidentally let the board rest against a metal standoff.

 

When we wired in the second battery, one of the wires was crimped to an improper connector gauge. I replaced this with an appropriate one.

 

The bare wire screw terminals powering the PC104 stack were rather loose. On inspection, several screw terminals needed tightening. In the future, I am going to insist on ring terminals for all of our boards, and we will probably replace some of the connectors on the commercial boards we use.

 

We didn’t experience any electronics anomalies, so I hope all the issues are addressed. I also got some copper bar stock that I will probably use for main power and ground distribution in the future, replacing the jumpered terminal strips I am currently using. I have found some cases where moving a power or ground lead from one terminal to another has effected A/D values noticeably, so I am going to give everything a big, fat hunk of copper to move through.

 

Phil outfitted the test stand with a metal spring to keep the slide against the load cell. After we broke two load cells with bounces on the 70 pound thrust engines after the warm up pulse, we had taken to using rubber bands to make sure it stayed taught.

 

Bob fabricated a lever handle to help us pop the fill cart quick connect off under pressure. It is certainly an improvement, but we could still use something better.

 

 

Engine Testing

 

We built all new catalyst packs for all four engines, with 15 discs compressed into the 13 disc sized spacer.

 

It started sprinkling before we got the first motor fired, but we went ahead with it. It performed as expected, still slightly rough, but with good enough characteristics. Oddly, there was what looked like a thin stream of liquid in the center of the otherwise transparent exhaust jet. It wasn’t a typical cloudy run, so we had never seen an effect like that. We meant to take the motor apart and see if it was tunneling all the way through the pack, but we didn’t get around to it.

 

It started raining hard by the second motor test. This one was also odd. It was a very cloudy exhaust, but the thrust was extremely smooth. Performance was down from about 70 pounds to 65 pounds.

 

We don’t know what to make of these results yet. It may be weather related. Our previous tests have been in nearly 100 degree weather and very dry conditions, while it was down to 80 or so and 100% (and then some) humidity for today’s tests.

 

We may just go back to completely filled cat packs, and live with the roughness, or try compressing even more in to keep a lot of pack but smooth it out a bit.

 

http://media.armadilloaerospace.com/2001_08_11/RainyTest.mpg

 

Small Lander Hops

 

We did a couple tests to see if the electronics were holding up ok.

 

On the first hop, something was clearly wrong – it rapidly started to flip over. I was able to set it down before it could, but when I throttled up again, it started to flip the other way. It turned out that two of the engines had been connected backwards…

 

I have just finished changing the electronics box and our venting electronics over to a single eight pin connector, instead of four two pin connectors, so this problem won’t be possible in the future.

 

The second hop worked fine, with five seconds of flight on a liter of peroxide.

 

The third hop was a test with a lower PWM frequency. We have been using 30 hz with the small solenoids, but I am leery about trying to run the larger solenoids that fast. We did this run at 10hz, and it still flew fine for five seconds, but the pulses were now clearly audible, and the control gain probably needed to be adjusted down, because the one push I made on the joystick resulted in a very fast turn.

 

We decided to use 20 hz for the big lander. We may make this the default for future flights of the small lander as well.

 

Big Lander Work

 

Bob brought the big frame back with its fresh powder coat, and, somewhat to my surprise, we managed to get it completely assembled for a test hop.

 

We configured it in its “short form”, with the engines directly connected to the main frame, instead of out on braced extensions. This makes it look almost exactly like an upscaled version of the small lander. We will probably want to use the “long form” that will be much harder to tip over, and has the longer attitude lever arms when we put a pilot on it, but the short form is probably seventy pounds lighter. We will still try flying it in short form with appropriate ballast to simulate a pilot, and see how things go.

 

I ordered four assembled stainless braided Teflon hoses for the vehicle, but they weren’t scheduled to arrive until next week, so we made our own –6 assemblies. In hindsight, we should have remembered to thread all the hose ends onto the hose before we cut it into four parts. Stainless braided hose is a pain to assemble (often literally, due to the sharp little barbs), because you need to fit a hose end around the outside of the braid, then a ferrule on the inside of the braid, then screw everything together. I did the first one, then Phil’s friend Joseph took over and finished the other seven hose ends. We have been a little spotty with our success rates on hose ends, but Joseph seemed to have the nack for it, because none of them leaked a bit.

 

We bolted the electronics box to the pilot’s foot plate, and got all the engine solenoids hooked up to CPC connectors.

 

Bob had steam cleaned the carbon fiber NGV tank we are using as the peroxide tank, and we tested it with some 30% peroxide without any problems.

 

We did a water test with a gallon of distilled water first. It took quite a while to draw the vacuum (five minutes?), and about five minutes to load a gallon of water. We will be moving towards loading the peroxide in the top eventually, but for now we are just using the same manifold as the small lander. On Tuesday, we will probably upsize the main fill cart hose, which will get us some improvement in load times.

 

We need to weigh the big lander in this form, but we estimated that it is at least three times the mass of the small lander. We prepared four liters of peroxide (almost completely running us dry at Long Range), which should be good for five to ten seconds of flight, depending on the total mass. The engines are over four times the thrust of the engines on the small lander, so we were planning on starting at 300 psi instead of the 550 or so we have been using on the small lander.

 

Unfortunately, our nitrogen bottle ran out with less than 240 psi in the lander. A 32 liter tank goes through nitrogen really fast. We are going to move to larger bottles, and have a company start delivering nitrogen to us directly.

 

I throttled it up slowly, because we weren’t sure what the thrust to weight ratio was going to be like. Huge, billowing white clouds came from all the engines. It wound up completely covering the lander, and it hadn’t lifted off. I let the clouds clear, then gave it full throttle, but it still didn’t have enough to lift off.

 

http://media.armadilloaerospace.com/2001_08_11/NoHop.mpg

 

We need to get more nitrogen and more peroxide, then figure out why all of our engines are cloudy. We will probably try to fly it again on Tuesday.