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During my final year at UCI, I took my senior design class, MAE 151. The way this works at UCI is that professors, external companies, and senior design project advisors (rocket, racecar, etc) can propose projects for students to work on over two quarters - 151 A and B. Since I work with our rocket project advisor quite often, I was able to write a project proposal for him to then propose for the class. So I basically got to define my own project unlike most!
Active Pressure Regulator ⋅ MAE 151 ⋅ Team Under Pressure
IN PROGRESS
Project Overview:
A rocket engine's thrust is proportional to the mass flow rate through the engine. To maintain a constant thrust, you need to maintain a constant propellant flow rate. This can be achieved in a few ways, most commonly with pumps or a regulated high pressure gas. For scale, complexity, and cost reasons, the UCI Rocket Project utilizes gaseous nitrogen to pressurize the rocket’s propellant tanks.
Pressure regulators are required to control the pressure of the rocket propellant tanks. The UCI Rocket Project use a dual regulator configuration, as shown on the right. Each regulator package consists of a Dome Loaded Pressure Regulator (DLPR), responsible for regulating the bulk of the nitrogen flow, in parallel with an in-line spring regulator, used to pilot the DLPRs. The set pressure of the pilot reg acts as a reference pressure to the DLPR, moving an internal diaphragm that controls a poppet. The force balance within the regulator dictates the poppet position and therefore the orifice size and flow rate. These are very capable components, and are used widely in collegiate rocketry; However, these fixed-set-point pressure regulators suffer from a few inherent characteristics; Regulator droop, choked flow effects, the supply pressure effect, and orifice size limitations all contribute to off-nominal regulator output during flight.
UCIRP VTF2 Pressure Curves
Typical Regulator Flow Curve
All of these inadequacies can be resolved with a component that self corrects to maintain a desired outlet pressure, in turn improving engine performance. The goal for the active regulator project is to produce a fluid control component capable of actively regulating and delivering N2 to the rocket propellant tanks.
I elected to take on the mechanical role for my team, taking ownership of the physical valve and control mechanism, and all calculations and analysis to verify our product would meet the extreme operating conditions and requirements set by the UCI Rocket Project.
After evaluating several commercial solutions to our problem, and other universities' solutions, we determined that the best design concept to pursue would be a motor driven ball valve.
A servo was chosen over a standard dc motor or stepper as it has built in encoding and feedback control - 1 less thing we need to worry about ourselves, they typically have high torque at high RPM, and we could create a relatively simple model relating servo/valve angle to mass flow which could simplify our control system.
Since we were working with a tight budget, I wanted to find a suitable valve first as I knew that would be the most expensive component.
PreliminaryDesignReview (1).pdfI came across this valve from mcmaster that met all of our requirements, and after inquiring about the manufacturers datasheet, I obtained more information to help guide the motor decision - the manufacturer provided recommended actuation torque values for their various valve sizes at various differential pressures.
Based on preliminary research, it seemed as though we would need a torque amplifying gear ratio in order to accommodate a standard servo motor. However, a good bit of searching on amazon led to me finding this servo that claims to exceed the 5ftlbs of torque to actuate the ball valve.
This seems to be possible as this servo operates at 16v which is much higher than what is typically on the market. This component only needs to work for <20 seconds while the rocket is flying, so we dont realistically have a power limit! This discovery allowed us to pursue a direct drive design, completely eliminating the added complexity of gearing.
To ensure the ball valve would suit our needs, I performed a bunch of mass flow calculations that are shown below. I calculated the maximum mass flow needed to maintain a constant 475psi in our liquid oxygen tank, the smallest orifice diameter that could achieve the desired mass flow, the mass flow with the limiting area (fittings), the ball valve area as a function of valve angle, and then mass flow through valve as a function of angle.
These calculations show that the selected ball valve can meet our mass flow requirements with ease over a wide range of inlet pressures. In fact, the limiting orifice area would be the fittings connected to the ball valve. With the ball valve and servo selected and theoretically validated, I completed the CAD design for the direct drive servo controlled ball valve. I plan to manufacture the servo mount and valve stem adapter in house! (I know the screws between the valve stem adapter and servo hub are missing in the CAD)
Proof of Concept Videos
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