August 28, 2001 Meeting Notes
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 wont
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
doesnt 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 didnt light at
all. The nozzle was sized far too
large, so I dont think the hot peroxide had any time to combust in the
chamber, but we also didnt see any noticeable grain erosion in five seconds of
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 dont 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.
Some shots of the polyethylene grain after firing:
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 werent 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 wouldnt 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 werent really expecting much on this run, so we didnt
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
Change the engine design so that the grain doesnt 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
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
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 wasnt 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
Phil also mentioned that the early smooth runs we got with
the 15 pound thrust motors had check valves in the lines, which we havent used
since then. We have seen at least a
couple smooth runs on the 75 pound thrust motors, so it cant be a mandatory
thing, but we do have a half inch check valve we can try next time.