- main propellant valve assembly
- bottom bulkhead/thrust structure.
- pressurization system
We use stainless steel ball valves with teflon seals and seats for both of our main propellant valves. We actuate these valves with a common pnuematic cylinder. During our last static test the mounting of the MPVA was a bit shoddy and we may not have had full range of valve motion. There was also a lot of slop in the linkage. We suspected that the fuel valve did not seat closed all the way and we had some leak-by when we pressurized our tanks, and hence a fuel lead instead of the traditional LOX lead. Needless to say we had a hard-start. We want to make sure the MPVA is rigidly mounted this time around and we get full range of motion from both valves. Here's some CAD...
Here they are mounted in the airframe... (the pnuematic cylinder is hidden slightly behind one of the aluminum stringers).
Pressurization System
In the past we have had a hard time with starting our rocket motor. We've had our fair share of hard starts and rapid engine disassembly. Our main propellant valves are not throttleable... just on/off... so our engine just starts full bore. We are building a pressurization system that starts the motor at low chamber pressure then switches to full throttle. Here's how it works...
We're using Mity Mite dome loaded regulators which will initially pressurize our tanks to a low pressure, like 100 psi. When we open the main propellant valves, propellants will flow into the engine with a LOX lead at a lower flowrate than nominal. Once ignition has been detected and Pc attains some minimal value like 70-80 psi we'll then "dump" 500 psi into the dome port of our Mity Mites which will bring our motor up to full throttle (1000 lbf. and 350 psi Pc). This "dump" comes from a 3rd regulator ( a Circle Seal IR10 reg) which is released via a solenoid valve into a spherical accumulator and then into the domes, therby changing the regs output pressure. We're calling this our Soft Start Pressurization System (SSPS).
Here are some CAD models of this scheme...
Here's the real thing....
During testing of the system we found that one of the Mite Mites was installed backwards, but other than that it worked fine at 150 psi when given an artificial external command signal. Next we plan to test it at operational pressures (500 psi) while under load.
Bottom Bulkhead/Thrust Structure
Our airframe is primitive. It is made up of a series of bulkheads attached by 4 stringers. All our propellant tanks, valves, etc... are mounted within this structure.
The bottom bulkhead also serves as our thrust structure since our motor gets bolted to it. This bulkhead is made from 3/4" marine grade plywood which is then skinned with sheet metal. This makes it into a strong, stiff sandwich structure. We usually just use sheet aluminum, however, some titanium sheet was donated to us, so we skinned the bulkhead with it. What we didn't expect was how difficult it would be to cut holes in. None of us are expert machinist, so it took us 2 hours to drill a 2" dia. hole through 1/16" titanium sheet using a hole saw. I looked in the machinist handbook and it said HSS was fine and slow, slow feed speeds which we abided by.
For now I think we'll go back to low-tech materials and leave the exotic stuff to NASA.
My plan is to try to post something at least once a week assuming we make progress. I also want the students to moderate and post updates, questions, etc.
--- Carl
