Above is a photo of the completed MB-2 Motor. It's first test used 7 of its 8 grain capacity.
This motor may be pushing the limits for a 2" diameter casing. I have no idea how much erosive burning a motor this long will produce, but, this is experimental rocketry after all. The motor design defies conventional wisdom, I know that. The length to diameter ratio is huge at 15 to 1, and the core to nozzle throat ratio is way too small. But, my theory is; the initial start up phase of the motor enlarges the core diameter before the ends start burning, so the first 10 or 15 hundredths will burn up just bringing the motor to pressure. Well, that's the idea anyway. We'll see about that when the motor is first tested. I may be eating crow in the near future.
My initial calculations have this motor holding about 4.5 lbs of propellant, assuming an Isp of 130 or so, that should should give me a motor with a total impulse of over 580 lb sec. making it a low "L" class motor. The grains are designed for a mostly flat thrust profile, even though experience has indicated it won't be a very flat profile.
This motor is going into a rocket project called Cosmo. I've been a little reluctant to name my rockets, it just seemed a little sappy. When I first started in EX, my rockets were as likely as not to crash or be destroyed in flight from a various number of reasons. But, as I have progressed my reliability has gone up significantly. In fact the only rocket I've lost in the last 10 flights or so is due to a mislabeled deployment charge. This is also a project for the upcoming IEAS convention launch in August, so a name would be needed. There is no real significance to the name Cosmo, other than that's my Border Collie's name. The Cosmo series of rockets will all be personal best altitude attempts. So, if the Cosmo 1 makes it to say 10,000 feet AGL, Cosmo 2 will be an attempt to break my previous best.
The initial design of this motor may change, so if I run into insurmountable problems, I will continue on with the project as MB-3 and so on. MB-1 would be my first 2" motor, the 2"x15".
Here are some of the casing details:
Material: Steel 2" EMT
ID: 2.08"
OD: 2.21"
Wall Thickness: .065"
Overall Length: 33 1/16"
Internal Length: 30 7/8"
Forward Closure: .3" Welded Steel Bulkhead, Tapped for 1/4" NPT port.
Here are some of the nozzle details;
Material: Mild Steel, from 2 1/8" bar stock
Length: 3.48"
Diameter Maximum: 2.06" (at o-ring area)
Convergent Cone: 60 degrees
Divergent Cone: 30 degrees
Throat Diameter: .8"
Exit Cone Diameter: 1.75"
Throat to Exit Cone Ratio (expansion ratio): 4.78
O-ring Grooves: (2) .18" wide x .08" deep
The first test of the MB-2 rocket motor is in the books, and boy do I feel like an idiot (more on that later). The test was performed behind the earth dike of one of our remote farm ponds for safety. My data acquisition hardware/software is still not working with my laptop so I loaded my desktop PC into my van, and powered it with a 400 watt dc to ac power inverter. I suppose the power inverter isn't a very clean power supply, because the signals were a little noisy, although good enough for reasonably accurate figures.
I spent hours loading everything into my van, all the hardware for the data acquisition, the test stand, cords, cables, launch controller, video camera, it goes on and on. But there was one thing I forgot. The "T" fitting coming out of the motor goes to a high pressure hose, and then to the pressure transducer. The other side of the "T" fitting is plugged. Well, it was supposed to be plugged anyway. I test fired the motor with one end of the "T" fitting open. Needless to say a lot of thrust was lost and of course, the pressure transducer didn't get accurate readings. So all of the data is reported with an extra .28" diameter opening out the side of the motor.
Here are the numbers:
Propellant Weight: 4.1069 lb
Grains Geometry: (7) 3.75"x2"
Grain Core: .85" diameter
Burn Time: 1.184 seconds
Total Impulse: 472.46 lb seconds
Peak Thrust: 534.55 lb
Isp: 115
While it's hard to draw any definite conclusions with that open port on the side, the Isp of 115 is better than expected. The burn time is about what would be expected. If you look at the graph of the thrust/pressure/time trace, you will notice a thrust drop and pressure increase at the 51 time mark. Then the thrust goes way up then drops rapidly. I'm going to guess that was nozzle blockage, more than likely one or more of the lower aluminum tape inhibitors going through the nozzle. That's another part of the erosive burning problem. The easiest answer is to limit erosive burning by increasing my grain core diameter, but that means a lower propellant load. Another thought is to make slower burning grains for the aft propellant grains. Or, I could try to find an inhibitor that will not burn, and is strong enough to stay in the chamber through the entire burn.
Here's a capture of the motor at full thrust. I really don't need any e-mails about how stupid I was for leaving the port open, I know that already thank you.
Here's Bill, on left and me checking out the motor post burn.
Click Here for a video of the burn.
Here is another view of the motor before it was tested. Notice the different colors on the ends of grains. I used cherry extract to thin the propellant on some grains, food coloring on other grains and spearmint extract on the last two grains. The spearmint sure made the grain casting more enjoyable as the aroma wafted through my shop. The numbers you can see on the ends of the grains are each grains weight.