Tank held 26.5 pounds of water full.
Tank has 2.015 pounds of water in ullage, or a net volume of 22.418 pounds in water.
22.418 lbs. / 62.4 = .3593 cubic feet volume, or 10,174 cc.
16.387 cc = 1 cubic inch
65 F. = 48 pounds per cubic foot = 17.2464 pounds N2O
70 F. = 47 lb./c.f. = 16.8871 lb.. N2O
75 F. = 43 lb./c.f. = 15.4499 lb.. N2O
80 F. = 40 lb./c.f. = 14.372 lb. N2O
The vent hole was drilled and tapped for 1/8" NPT, a short nipple and a cap were installed and the cap drilled out to form a .032" vent hole. This is down from the .065" diameter hole used with this tank on the previous test.
After 4 burns the nozzle throat has started to show slight erosion, the throat was measured at 1.265", that's an increase of .015".
The fuel grain is almost exactly the same as in the last test. Only for the protective cap, I used graphite and polyester resin. I realized I may not get chamber pressure data with the cap in place. The space available for the combustion chamber gas to flow into the transducer port is very, very small. The RTV used to seal the cap to the injector bulkhead may preclude any pressure finding the port at all. Well, I guess it's better to have the engine undamaged than record chamber pressure. Although I think I could machine an opening in the cap to allow the gas to flow to the port, but that will have to wait for a future test as the engine is assembled now.
The first try at HR5ST5 had to be aborted due to a low supply tank of N2O. After picking up a new bottle, I was ready to give it a go again. I'm getting a little more comfortable testing hybrids, so I decided to leave the test stand trailer attached to my vehicle. That allows me to test the engine pretty much anywhere I wanted to. While I'm comfortable enough to risk damage to my vehicle, I still keep myself protected by both distance, and in this case an earth berm about 5' high and 12' thick... Always think safety people...
Bill came over to help and in short order we had all the ground support and data acquisition equipment in place. I didn't use the line cutter this time, so we just had to fill using the remote fill valve, ignite the preheater grain and open the N2O valve using the PIRM2 and spring activation.
Click Here for a short video of the test.
Here's the engine at full thrust.
Here's a "com'in at ya" view. Since no one was really impressed with this camera angle last time, I didn't include in the web version.
Good data this time. Here is the graph showing thrust, chamber pressure and N2O pressure. Notice the chamber pressure port opened up .9 seconds into the burn.
N2O Start Pressure: 691.41 psi
N2O Start Temp: 65 psi
N2O Density: 48 pounds per cubic foot
N2O Load: 17.2464 pounds
N2O Only Isp: 213.7 seconds
Fuel Used: 3.2 pounds
Fuel Grain Length: 24.75" with lower lip and upper thermal cap
Isp: 180.27 seconds
Burn Time: ~11.1 seconds
Total Impulse: 3,685.86 pound seconds
Peak Thrust: 715.56 pounds
Full K bottle of N2O weighed 205 pounds, after the test the bottle weighed 182 pounds. 23 pounds were used to net about 17 pounds.
The test went very well, really pretty much exactly as expected. The Isp improved, as expected. The N2O tank with the vent held much more liquid N2O than the unvented tank, even with a smaller internal volume this test had over 1,000 pounds seconds more total impulse than the unvented tank. Let's put the total impulse of the engine into perspective, the 3,685 pound seconds of total impulse is equal to about 1,890 Estes C-6 motors... It's a lot easier to set up a hybrid than to do a cluster of 1,890 C-6's ;)
One thing that was interesting. The bottle of N2O was at ambient temperature of about 80 degrees, yet calculating the temperature of the N2O by the pressure in the flight tank yielded a temperature of 65 degrees. That's not totally unexpected, as the Liquid N2O fills the flight tank and pressurizes, the liquid vaporizes and cools the tank and the liquid. Although I'm a little surprised it cooled it that much. I'm sure there's a little fudge factor there, the vent port, the pressure transducer and so on. What it really boils down to is how much impulse I get from a given tank and engine. The Isp can only be roughly calculated with all those "fudge factors" throw in.
Here's my homemade slide hammer attached the forward bulkhead after removal. Notice not only did the forward fuel grain with the thermal cap come out with the bulkhead, but there is no burn through or even combustion gas flow behind the grain.
I had to whack the grain over the edge of the counter to break it free from the bulkhead. The RTV not only protected it thermally, but kept it glued to thermal cap. Notice how nice and clean the casing is inside.
Here you're looking down the fuel grain from the break between the asphalt fuel grains. The rough texture at the far end is the polyester resin/HDPE post combustion chamber.
The nozzle still looks great after 5 burns as well. I must say I'm very pleased with all aspects of the engine. In fact, I'm so pleased I'm seriously thinking of running it with a full "O" load of N2O, we'll see... For the time being I need the engine available during the construction of the Aestus rocket.