How a Hybrid Works
Combustion
The most fundamental function of producing rocket power is combustion. Combustion is the process where a substance burns in the presence of oxygen, giving off heat and light. The 3 essential ingredients of combustion are fuel, oxygen, and heat. For a hybrid engine, the oxidizer typically used is nitrous-oxide, this compound has the oxidizer molecules bound to nitrogen, making the oxygen only a small portion of the compound. The fuel is a mixture of HTPB, which is a rubber-like compound, and aluminum powder, which burns and creates sparks outside the engine. The combination of these two compounds: oxidizer and fuel, creates the basis for the interaction, the only thing missing is the heat source. Ignition is a common fault of unsuccessful hybrid engines.
Ignition
With the fuel being inert, it is very difficult to pyrolyse the fuel, meaning it is difficult to get the entire surface of the fuel to be on fire. For ignition to occur successfully, the entire surface of the fuel grain needs to be pyrolysed before the oxidizer is injected to ensure a steady burn is achieved. The HADES team chose to use an ammonium-perchlorate composite propellant (APCP) ignition puck as a heat source. This is a small disk of oxidizer-rich solid fuel that is ignited and burnt all the way through immediately before oxidizer injection. This is a common fuel for typical solid rocket motors, it has the oxidizer already imbedded in the fuel and only needs an electrical signal to ignite. With this system, ignition becomes very reliable, the only fault being the danger in handling this fuel type. With the oxidizer and fuel already mixed together, the ignition puck can ignite at any moment if an electrical discharge were to occur.
Main Valve
The main valve mechanism is the most rudimentary design on the system. For ideal performance, we want a slow opening of the valve to mitigate any pressure spikes in the system. This involves a mechanism that may seem rudimentary, but is a balancing act of all the factors at play. The mechanism involves a manual ball valve, rated to the appropriate pressures, with a spring attached to the handle, and a fishing line holding the handle down. The fishing line has a Nichrome burn wire wrapped around the bundle of line, with a 12V relay attached, ready to send the actuation signal. When the main valve command is sent, the relay will flip and send 12V directly to the burn wire, severing the fishing line and springing the main valve open. This sends oxidizer from the oxidizer tank, into the combustion chamber to produce thrust. This rudimentary system has more complexity than is readily apparent. The main valve's actuation characteristics change when it's under different temperature and pressure conditions. Under pressure, the valve becomes stiff and hard to move, but when the valve is cracked, the valve becomes loose and easily releases the pressure from the system. On top of that, when the valve opens, the rapid expansion of the gases rapidly cool the valve to sub-zero temperatures and frosts over the valve. Since the valve is only rated to -40 degrees F, the valve seals are degraded on every use. With all of these factors, getting the correct spring-handle combination is imperative to get the exact slow opening that is desired.
Nozzle
The nozzle is the last bit of the engine need to condition the flow before ejecting it outside the aft end of the rocket to produce thrust. The chosen form of this device was the converging-diverging conical nozzle. This enables the rocket to condition the flow such that supersonic flow is what is ejected outside the engine, producing the maximum thrust possible. The nozzle is constructed out of graphite, due to the massive heat flux going through it, and machined on a lathe. It is given a convergent inlet, and a divergent inlet, the dimensions of which are determined from the isentropic flow equations to have the flow rate at the throat to be supersonic.
