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The injector is the first stop for our propellants as they begin the journey to the exit of the engine. The purpose of the injector is to inject the 2 propellants into the combustion chamber in such a manner that they mix thoroughly. Thorough mixing of the two propellants is very important, as better mixing leads to more complete combustion. To be clear, the mixing itself occurs in the combustion chamber. The injector serves as a tool to inject the fuel and oxidizer into the combustion chamber in a very specific pattern to facilitate mixing. A very common injector design is one that forces the fuel and oxidizer to flow through a series of small holes (called orifices). The small streams of fuel and oxidizer then collide, or impinge, upon one another, causing the two to mix.
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After the propellants exit the injector and mix, they combust inside the combustion chamber. The combustion chamber must deal with the high pressures of the gas, as well as the high temperatures.
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After the propellants fully mix and combust, they flow into the nozzle. The nozzle features a converging and diverging section, as shown in the above image. This pattern accelerates the flow to supersonic speeds (you will learn why that is in a later psetlset). The primary goal of the nozzle is to accelerate the flow to a high velocity. As we discussed, a higher exit velocity means more thrust (and, as you will find out, higher efficiency).
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Delta-v is an important quantity for launch vehicle and spacecraft design. In order to reach certain ‘destinations’, a certain amount of delta-v is required. For example, reaching low earth orbit from the earth’s surface requires ~10,000 m/s of delta-v. To get Getting from a low earth orbit to a low lunar orbit takes an additional 900 m/s of delta V. It doesn’t matter how large or small the vehicle is. The delta-v requirement is the same.
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This ideal rocket equation shows two very important relations. First, delta-v is directly proportional to the exhaust velocity. This tells us that engine efficiency is highly important. Second, delta-v is proportional to the natural log of the mass ratio, which is the initial mass over the final (dry) mass. This shows that a rocket which that has a very low dry mass will achieve a greater delta-v. The greater your propellant / stuff ratio (“stuff” includes the structure, engines, tanks, etc.), the better performance you will achieve. In launch vehicle design, this is one of the most important principles: every pound of non-propellant weight you add is a drop in performance.
Note: You may be wondering why the rocket equation does not depend on thrust. If a rocket has an engine twice as powerful, shouldn’t it be able to achieve twice the delta-v? Nope. To illustrate why, consider the example of two identical cars that fill up gas at the same station, and drive down a highway until they run out of gas. One car drives at 60 mph, while the other drives at 30mph. Will the car driving at 60 mph go twice the distance before running out of fuel? No, because it is the fuel efficiency (mpg) and the amount of fuel (size of gas tank) that matter. The 60 mph car will simply exhaust its fuel and go the distance quicker. The same is true for rockets. Our exhaust velocity is analogous to fuel efficiency. A more powerful engine with the same exhaust velocity will simply exhaust the spacecraft’s propellants quicker. That being said, thrust output still matters in many scenarios. For example, rockets launching from the ground must be able to overcome the earth’s gravity, so their thrust must be greater than their weight at takeoff. There is a delta-v penalty associated with this need to overcome gravity. It is referred to as “gravity losses”. There is also a penalty associated with drag. It is called “drag losses”.
Questions:
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A rocket is designed for 10 km/s of delta-V. In the next iteration of the design, we want to increase it's capability to 11 km/s. What are some high-level changes we can make to the vehicle to achieve this?
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Given what you know about thrust requirements at launch, and the importance of engine efficiency + mass ratio...why do you think aerospace companies commonly use solid rocket motors for side boosters, and liquid biprop engines for the main and second stages?
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You test a liquid engine and discover that the temperature inside the combustion chamber is not as high as you had predicted/hoped. What component would need to be changed/improved to remedy this?
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