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- Great links (these both go beyond what this learning set covers, but they are terrific resources in general):
- NASA Glenn research guided ‘tours’: https://www.grc.nasa.gov/WWW/K-12/rocket/guided.htm.
- Really good PowerPoint summarizing LRE (liquid rocket engine) basics: https://ntrs.nasa.gov/citations/20140002716 ]
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What is a Rocket?
Put simply, a rocket is a vehicle used to launch people or things into space. Rockets propel themselves by expelling exhaust at a high velocity. This is, at its core, the same way that many propulsive devices work for a variety of vehicles. Aircraft/submarine propellers accelerate the surrounding fluid, leading to the production of thrust. Jet engines intake air, mix it with fuel, burn it, and expel the exhaust at a high velocity. Contrary to popular misconception, you do not need to push “against” anything to produce thrust! Rocket engines work perfectly well in the vacuum of space (in fact, they work even better there than they do on the ground! We’ll talk about why that is in a later LSET).
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There are many different types of rocket engines, but this equation holds true for all of them. Whether we are dealing with solid rocket motors, liquid rocket engines, or cold gas thrusters, the fundamental principle at work is the same.
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Types of Rocket Engines
Cold gas thruster:
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Any combustion reaction requires a fuel and an oxidizer (e.g. for a candle, fuel = wax, oxidizer=air). Aircraft w/ jet engines carry their own fuel and use the surrounding air as the oxidizer. In the vacuum of space, there is no air, so a rocket must carry both the fuel and oxidizer (these, collectively, are the two propellants...hence ‘bi-propellant’). A bipropellant liquid engine works by injecting the fuel and oxidizer into a combustion chamber, where they then mix, combust, and flow out of the nozzle. The diagram below shows a specific variety of bipropellant engines where the propellants are pressurized with a pressurant. Other types of engines use pumps to increase the pressure of the propellants before they are fed to the combustion chamber. We will talk in a later LSET about why having the propellants at a high pressure is desired.
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A (slightly) Deeper Dive into Bipropellant Liquid Engines
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Note: The combustion chamber and nozzle are not always two separate parts, and are sometimes manufactured in one piece.
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The Ideal Rocket Equation
As discussed, rockets must bring their propellants with them. This means that throughout the burn, the rocket will get lighter, as it is expelling the propellants. A lighter rocket means it can be accelerated more with the same force (Newton’s 2nd law: a = F/m). Because the rocket does not have a constant acceleration, the formula for the change in velocity achieved by a rocket is not totally trivial.
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