August 28, 2001 Meeting Notes

 

In attendance:

 

John Carmack

Phil Eaton

Russ Blink

Bob Norwood

 

Big Frame Work

 

Bob got brackets mounted for the big engine. It is a tight fit.

 

The triangulated braces are mounted, so the engine arms won’t vibrate up and down like they did in the last hop. That added 18 pounds.

 

Bob got the computer mounting framework added. That will be an aluminum plate, since it doesn’t need to hold nearly as much mass as the pilot plate.

 

I have made a lot of progress with the asynchronous engine control simulation, which we should be ready to fly next meeting.

 

 

Hybrid Motor Testing

 

We tested our hybrid motor again today. The last time we tried, it didn’t light at all. The nozzle was sized far too large, so I don’t think the hot peroxide had any time to combust in the chamber, but we also didn’t see any noticeable grain erosion in five seconds of firing.

 

For this test, Russ modified the nozzle so it could hold an Aerotech phenolic nozzle inside our big copper heat sink nozzle. We also increased the peroxide concentration from 80/20 to 85/15 98% to distilled water ratio. We loaded 500 ml and pressurized to 400 psi.

 

This time, it lit almost immediately, and we saw flame out of our rocket engine for the first time. Amazingly, we managed to screw up both the data collection and the video, so we don’t have much concrete data. Russ did catch a couple pics during firing with his digital camera. The plume got more ragged as the nozzle was being chewed up.

 

media.armadilloaerospace.com/2001_08_28/hybrid1-1.jpg

media.armadilloaerospace.com/2001_08_28/hybrid1-2.jpg

media.armadilloaerospace.com/2001_08_28/hybrid1-3.jpg

 

Some shots of the polyethylene grain after firing:

 

media.armadilloaerospace.com/2001_08_28/poly1.jpg

media.armadilloaerospace.com/2001_08_28/poly2.jpg

media.armadilloaerospace.com/2001_08_28/poly3.jpg

 

The run length was 6.6 seconds, with the hybrid burn being around six seconds long. In this time, the phenolic nozzle was almost completely eroded away. It was a lot different than what the aftermath of a nitrous hybrid looks like. It should be hotter, but we weren’t expecting that radical of a difference. We were considering converting a standard HPR nitrous hybrid into a peroxide hybrid for testing, but this seems to indicate it wouldn’t hold a lot of value.

 

The erosion down the PE grain was very smooth and even.

 

For test work, I think we should stick with pure copper heat sink nozzles, which should hold together for five or six seconds at a time.

 

We weren’t really expecting much on this run, so we didn’t take as many measurements as we should have. Now that we know we are at least in the ballpark, we can get more rigorous. For the next hybrid tests, I propose the following changes:

 

Change the engine design so that the grain doesn’t need the step machined in it. That was going to burn through shortly.

 

Fire in monoprop form to establish a baseline.

 

Weigh and measure the hybrid grain before and after hybrid firing.

 

Weigh and measure the copper nozzle before and after hybrid firing.

 

 

Big Motor Testing

 

media.armadilloaerospace.com/2001_08_28/BigMotorWithPressure.xls

 

We did several runs on the big motor (each run was 2 liters), and learned some good things.

 

Russ made a cavitating venturi that slides inside a 1/2" union fitting. We were just guessing at parameters, and used a 0.25” throat with a 7 degree expansion angle, and a radiused inlet angle.

 

We fired the motor at 400 psi, and it made a little less power than last time due to the restriction, but was just as rough. We also ran it at 200 psi and 600 psi for comparison.

 

I got the extra signal conditioning module yesterday, so all of the runs now have feed pressure as well as load cell force now. You can see the very steep blowdown pressure curve, and the increase in drop when the peroxide is expended. The test stand tank only holds six liters, so pushing two liters out in a second or two is a pretty rapid drop.

 

We were disappointed to not see any benefit from the venturi. After talking about it a bit, we decided to move the pressure transducer down from the tank to right before the engine inlet. We were shocked to see that during the run we had over 150 psi of pressure drop from the tank to the engine, and that the pressure was very rough by the time it got there! That explains why we were seeing lower thrust numbers than we calculated we should from the motor, and would also prevent the venturi from helping anything.

 

We had a fairly involved set of plumbing going from the tank to the engine: a 90 degree fitting off the bottom of the bottle, then five feet of –10 (half inch) Teflon hose, then an AN to pipe fitting going into the ball valve, then some more fittings to stand it off enough that the valve cleared the test stand, then another 90 degree fitting that pressed against the load cell, then some straight fittings to go to the engine.

 

We removed the 90 off the bottle and screwed the bottle manifold directly onto the ball valve and did another run. It wasn’t any smoother, but we gained over 50 psi of pressure at the engine, and thrust went up accordingly.

 

Bob is going to weld up an adapter so we can have the engine secured to the test stand without pressing through the feed fittings. We will then use a short section of hose to gently turn the corner instead of using the remaining 90. The 90 fittings are far from smooth, so we are certain we will pick some more pressure back up, but the big question is if it smoothes out the pressure variation. A vertical test stand would be ideal for this, allowing us to test exactly as the engine will run in the vehicle, basically screwed directly in line with the tank.

 

We had some hint about the effects of plumbing some time ago when we had a long –3 hose cause a large power loss, but this was definitely enlightening to see the data today. This would be one of the reasons the pros but weld everything instead of using fittings (as well as saving mass)… Bob mentioned that sometimes on race car plumbing, they will carefully radius the inlets and outlets of the fittings they use to get better flow. That applies more directly to the rocket plumbing than I was expecting.

 

Phil also mentioned that the early smooth runs we got with the 15 pound thrust motors had check valves in the lines, which we haven’t used since then. We have seen at least a couple smooth runs on the 75 pound thrust motors, so it can’t be a mandatory thing, but we do have a half inch check valve we can try next time.