This first test of the new 5" hybrid was fairly conservative, I was planning on limiting the N2O to 10 pounds.
In the end, I wound up going a little over that, I guess I just got "caught up in the moment" and filled
the test tank to 12.2 pounds. Setting up for the test was as close to a monumental task as I've ever had for a
static test, with three data channels and associated equipment plus the challenge of managing oxidizer temperature
not to mention setting up the trailer test stand for the first time... It all took a good number of hours.
Here's a shot of the test area as we were setting up.
You can see the building we rammed the front of the trailer into. I'm just getting ready to start the data recording on the PC.
In order to fill the test tank on site, we chilled it in a tub of ice water for about an hour before the test. Once all was set up, cameras were recording, data was recording and spectators safe, I retreated to a temporary bunker we made from sand bags. All clear check, then John did the count down... At ignition the motor belched a small flame out the nozzle and smoked slowly, I wasn't sure if the starter grain was even burning or not, but I decided to give it a try and I opened the nitrous valve. A big whoosh and lots of white gas came out the nozzle, but no flame... We refilled the N2O tank to 11.2 pounds, installed a new igniter and tried again, the igniter lit but again failed to get the APCP starter grain ignited. At least this time I didn't release the nitrous.
Daylight was fading and I was out of igniters. I made a quick dash home, extended the length on two short igniters, then added a few grams of APCP test strands to the end of the igniter with a wrap of masking tape. That should get the bugger going!
Back at the test site the sun had just set, so we had a few minutes of twilight left... Once again John did the count down, shortly after ignition a nice trail of smoke was emanating from the engine nozzle. I gave the N2O valve line a nice tug and...
Ignition! Here's a shot just after ignition at full thrust.
As the N2O pressure drops, mach diamond become visible.
Click Here for a short video of the test from 2 angles.
This chart shows thrust, chamber pressure and N2O pressure.
This chart shows the nozzle Cf in relation to the burn.
N2O Load: 11.2 pounds
N2O Pressure at Start: 687.5 psi
Asphalt Burned:3.6 pounds
Peak Thrust: 683.2 pounds
Peak Chamber Pressure: 374.59 psi
Burn Time: 10.438 seconds
Total Impulse: 2,476.6 pound seconds
N2O only Isp: 221.125 seconds
Isp: 167.337
Nozzle Coefficient Average: 1.389
Oxidizer to Fuel Ratio: 75.7/24.3
This is a view down the fuel grain after removing the forward bulkhead. You can see longitudinal ridges running the length of the grain between the individual injectors.
I was worried I'd have problems getting the fuel grain out. But it really wasn't a problem. The step in the nozzle that allows the cardboard casting tube to slide into was a big help in keeping melted asphalt from coming in contact with the aluminum casing.
The nozzle, bulkhead/injector and nozzle washer were a mess, but undamaged.
I had to use a petroleum distillate to dissolve the asphalt, but it did come off, eventually...
Summary:
In all I'd have to say I was very pleased with the test. While the asphalt isn't fun to work with, it probably wasn't as bad as I had expected. The one problem with asphalt is the fairly high regression rate, which leads to a fuel rich mixture and lower Isp. I know I lost some molten asphalt at the end of the burn when the motor popped several times, I literally saw it fly out of the nozzle. So in reality, the Isp was likely in the low 170's. A greater fill of N2O would lead to better Isp as well, since the nozzle coefficient would be higher for a longer duration, with the same blow down time of gaseous N2O.
When I was designing this motor, I really feared thermal issues. I worried about heat soaking
from the nozzle into the aluminum casing. I went to the engine within 30 seconds of the end of the test. The forward
bulkhead was cool, the fuel grain area at air temperature and the nozzle area of the casing barely warm to the
touch. After 5 minutes or so, the nozzle area had warmed up more, but you could still hold your hand to it for
extended time. So I don't think I'll worry about this design even for longer duration burns.
The chamber pressure was about right on where I wanted it. I was shooting for around 350 psi, and for the duration
of the liquid phase I ran from a high of about 375 psi to a low of about 325 psi. If I were to run this engine
with a full oxidizer load of 30 pounds, it would make for one interesting flight in a sub 100 pound rocket with
a long burn time! Of course, if I use a fuel with a better regression rate and get the oxidizer to fuel ratio better,
not only will performance improve, but I could carry more oxidizer as well...
With that in mind, I see at least two more tests of this engine in it's current configuration.
One test I'd like to try is using a polyester fuel grain with an additive to make it slightly softer. I worry about
brittle grains breaking off a chunk and blocking the nozzle. For the last test I'd like to fire the engine with
a standard R45 HTPB fuel grain, that should give me a standard to go by.