Engine+and+Propellants


 * Important Notes:**

To optimize the thrust of a rocket the pressure of the gas leaving the rocket should equal the ambient pressure. So as you gain altitude the nozzle itself should become longer (ex the stages could have longer nozzles). Once your reach space the nozzle theoretically should be infinite length but due to the mass the nozzle adds there is eventually a point where the thrust generated is lower than the acceleration lost. Hydrogen gas is added to the exhaust gases in rocket engines in order to decrease the molecular mass further


 * Liquid Rocket**

Injector must be a complex series of tubes that alternate between oxidizer and propellant

In order to be able to restart the engine in manned missions nitrogen tetroxide and UDMH are used because they ignite upon contact, leaving no chance that the re-ignition of the fuel fails.

Use hydrogen/oxygen for lower stages then use nitrogen tetroxide in the upper stage for transfer orbit and also for maneuvering near mars.

The pressure created by the turbo pump must be kept high in order to prevent “cavitation where the liquid passing by a vane boils when it passes by the rotating vane causing the liquid to drop in pressure, these bubbles causes shocks which can damage the vane. This does not matter for one use systems but for multiple use systems it can cause the vane to “race” causing it to move faster then necessary and create lower pressure.

“Pogo” is the phenomenon in which the inlet pressure is affected by the thrust pressure of the engine, this increases the time it takes for the gas to reach the combustion chamber by a small amount, but when there is an increase in thrust again then the pressure in the inlet drops once again, causing an oscillation. Due to the very high thrust involved with the rocket these small fluctuations can cause major damage to the space craft. In order to prevent this a chamber containing pressurized fuel Is attached to the inlet, when the pressure in the inlet drops then the chamber adds pressure to the inlet increasing the pressure in the inlet to normal, when the inlet’s pressure increases again then the chamber is refilled with fuel, restoring the chamber and allowing it to constantly preserve the pressure in the inlet. Not necessary in single use rockets because the damage caused to the systems does not matter, however for multi-use systems the effects will build up and irreversibly damage the engine.

Most engines have a combustion temperature of 3,000K, where the melting point of most metals is 2,000K or so. In order to prevent the combustion chamber from melting film cooling is used. This process involves running cold fuel along the sides of the combustion chamber in order to have a layer of cooler gas protecting the walls of the engine. Another possible way of conducting film cooling is by having a hollow cavity in the walls of the combustion chamber that allows for the cold fuel to pass through, this works for hydrogen/oxygen engines, where hydrogen is passed through the hollow space keeping the chamber cool.

Once the fuel is used as a coolant there are 2 possible methods of dealing with the used fuel. One method is “dumping” where the coolant fuel is dumped through nozzles at the end of the combustion chamber. This means that the fuel has not gone through combustion, but it has warmed while cooling the chamber, increasing its velocity. Also by adding hydrogen directly to the exhaust you are decreasing the molar mass of the exhaust, which in turn increases the thrust generated. This method may be inefficient however when compared to “regenerative” cooling. This process cycles the fuel that was used to cool the chamber back into the fuel lines, allowing it to complete combustion. This generates more thrust from the combustion of the gas, however additional mechanical weight is added to the engine in order to allow the gas to cycle, so the thrust generated must exceed the additional force required to lift the rocket due to the excess mass.

Methods of injecting the fuel for cooling include having a solid pipe going around the combustion chamber and ensuring a constant supply of fuel is pumped through it in order to keep fuel in contact with every point of the combustion wall. Another method is having spiraling pipes go around the combustion chamber in order to increase the length of the flow path. It has been shown that if you have a parallel injector for the chamber and a spiral injector for the nozzle a considerably higher thrust velocity can be established, however the system is extremely complicated and any decrease in pressure of the system can prove fatal. Saturn V, parallel only, 4.2k m/s, SSME, parallel and spiral, 4,550 m/s


 * Des****ign**

The fuel should be contained within a spherical chamber in order to maximize mass/volume. A gas should be used in order to keep pressure within the tanks, nitrogen and helium are examples of usable gases.

Source: [] [Rocket Spacecraft and Propulsion]