Hybrid Engine Feed System
Oxidizer Tank Loading:
The first step in the testing of the hybrid rocket engine will be to load the oxidizer tanks with approximately 7 pounds of Nitrous Oxide. To accomplish this, we will use the setup as shown below in Figure 1.
A pressurized supply of Nitrous will be connected to the fill connection of the oxidizer tank assembly (With the manual shut off valve in the OPEN position). The tanks will then be filled to the appropriate level and the manual shut off valve will then be shifted CLOSED. After shutting of the fill line, the connection to the pressurized Nitrous can then be removed. We will use three separate tank assemblies to allow for multiple test firing before having to travel to a Nitrous supplier for refilling.
Nitrogen Tank Loading:
The Nitrogen is a critical part of ground testing as it provides immediate shut down of the hybrid rocket for either a test condition or in the event of an emergency. To fill the Nitrogen tank, first check that the tank to system valve is CLOSED and the fill valve is OPEN. Once these are checked, connect the fill connector to the Nitrogen supply and while monitoring the pressure transducer of the tank, fill the tank with 900 +75, -0 psig. After the desired pressure has been reached, switch the fill valve to CLOSED and then remove the Nitrogen supply connection.
Feed System Description:
Nitrous oxide is the oxidizer of choice when considering different fluids due to its unique property of being self pressurizing at room temperature. To get optimum combustion, the desired combustion chamber pressure is 550 psig. It turns out that Nitrous Oxide will pressurize itself to approximately 800 psig at room temperature. The pressure and density varies considerably with temperature as is shown in Table 1.Temperature (°C) / Vapor Pressure (Bar Abs.) / Liquid Density (kg/m3) / Vapor Density (kg/m3)
0 / 31.27 / 907.4 / 84.86
5 / 35.47 / 881.6 / 98.41
10 / 40.07 / 853.5 / 114.5
15 / 45.10 / 822.2 / 133.9
20 / 50.60 / 786.6 / 158.1
25 / 56.60 / 743.9 / 190.0
Many considerations must be considered for valve selection in order to achieve optimum performance from the hybrid engine. Major valves that are needed in any feed system are solenoid valves, relief valves, check valves and needle valves.
1) Solenoid Valves
A solenoid is a device in that when electrical current is applied, a magnetic field is created that provides an axial force. In the case of a valve, this magnetic force when used with a magnetic metal such as 430 stainless steel will allow for opening and closing of a flow orifice. Sizing of this orifice is critical to the overall system performance. Other design considerations are operating pressure, material compatibility and electrical power consumption.
All liquid flow is a function of pressure drop across the orifice, orifice coefficient and orifice diameter as shown in the below formula.
Cd = Orifice Coefficient
A = Orifice Area
ρ = Fluid Density
ΔP = Pressure Drop Across Orifice
Orifice coefficients can be looked up in any fluids book and it would be shown that for a sharp edged orifice, this coefficient value (Cd) can be approximated as 0.65. Now knowing this value, you can adjust either pressure drop or orifice diameter based on a know flow rate to size the valve orifice. A major design choice that must be closely looked at is the design pressure drop through the valve. To provide for the optimum system performance, the designer must limit this pressure drop to within reason. The entire system must be considered for this value, and in our case we want to use this feed system for many engine sizes / flow rates. If we were use all our allowable pressure drop through the valve, we would not be able to achieve any higher flow rates that the current configuration. As described later, by setting an injector inlet pressure, we want to be able to adjust the pressure drop through the system to thereby change the flow rate for different tests.
Materials used in the solenoid valve and with any other fluid component as seen later must be resistant to corrosion from exposure to various atmospheric conditions. The solenoid valve we have selected is made completely from stainless steel and nickel plated steel, therefore meeting the corrosion resistance requirement.
The amount of electrical power consumed, while not an issue for ground testing has great importance when considering space flight. The size and thereby the weight of a spacecrafts batteries depends mainly of the power requirements of the craft. One then needs to limit the power needed to operate the valve.
2) Relief Valves
The main purpose of any relief valve is to prevent pressure build up in a system that could lead to failure. This therefore makes relief valve a key part of any safety considerations. There are three considerations when sizing a relief valve.
The first consideration that must be determined is what pressure to set the valve to open at. To do this one must consider both the operating pressure and the weakest component in the system. For example, this rocket is designed for a 1000 psig maximum operating pressure and the weakest components can handle up to 1200 psig. Therefore, the relief valves must be set to less than 1200 psig and more or equal to 1000 psig where the 1000 psig pressure will be seen.
The second consideration is what flow rate the valve can handle. If a certain system operates at 50 scfm, the relief valve must be able to vent this flow rate or the system will continue to over pressurize and fail. As shown in Appendix A (Analysis), all relief valves in the rocket system are capable of meeting the flow rate in that part of the system.
A third consideration is material selection. Due to the criticality of the operation of these valves and their exposure to outdoor conditions, no corrosive material should be used in any part of the valve. Corrosion of a part could lead to binding of moving parts and thus no opening.
3) Check Valves
The primary purpose of a check valve is to permit flow in only one direction. Check valves are used in this rocket system to isolate the Nitrogen and Nitrous Oxide and prevent them from mixing. The design considerations for check valves are materials and flow / pressure.
Material selection as metioned above is critical for any fluid system component. The use of any corrosive material such as carbon steel is prohibited from use in any of the check valves.
The only other consideration when sizing a check valve is the maximum operating pressure and flow able to pass through the valve. As shown in the solenoid valve flow explanation, you want to minimize pressure drop through any component exept where necessary.
4) Needle Valves
The purpose of the needle valve in our system is to allow for manual manipulation of the flow rate by a change in pressure drop across the system. As mentioned in the injector section later, we are designing the feed system to provide a fixed pressure at the inlet to the injector. Based on a desired flow rate, we would adjust the needle valve, widening or narrowing the orifice, to achieve this flow. As mentioned above, flow is dependant on pressure drop and orifice size so by changing both of these, one can reduce or increase the system flow rate. In an actual spacecraft, in place of this needle valve is what is called a flow orifice. This orifice is the smallest in the entire system thereby having the greatest influence of the total flow rate. We will not go this route due to our desire to test actual pressure drop lost in the system and test a varity of flow rates.
The injector is the final component in the rocket feed system. The purpose of the injector is to atomize the Nitrous Oxide from a liquid form to a gaseous form. To complete this gasification, there must be a large pressure drop through the injector flow holes. The size and number of these holes is directly dependant on the flow rates desired for testing. For our initial design, a 0.21 kg/sec flow rate was considered. Using this flow rate, we designed for a 150 psig pressure drop across the injector and a 5 hole pattern to help distribute the oxidizer evenly throughout the chamber.