- Introduction: The Basic Idea
- Chapter 1: Classification
- Chapter 2: Definitions and Fundamentals
- Resources
Introduction : The Basic Idea
Before learning how a rocket works, we need to understand the basic anatomy. This section will give a brief but descriptive idea of just how a rocket works.
The Five Parts of Any Rocket
For any rocket there are at least 5 sections.
- The payload
- The fuel
- The oxygen
- The fins
- The engine(s)
The Payload
This is the thing that will be taken into space. The payload is the reason that the rocket is going up in the first place, usually to send a bit of cargo (or people) to some destination, whether it be the moon, the ISS, or Mars.
The payload is ususally kept at the top of the rocket, furthest away from the damaging flames of the engine and from the other stages of the rocket that will be jettisoned at different stages of flight.
The Fuel
A good portion of any rocket is taken up by the fuel and the fuel tank. The three main sources of fuel from my research consist of RP-1 (rocket propellant 1), which is essentially just a heavily refined kerosene, methane, and hydrogen. The different fuels have their lists of pros and cons, for instance, RP-1 is very weight-efficient but is sooty, while hydrogen is clean but is not very dense. Regardless, that is a more in-depth excerpt.
The fuel is what will ultimately move the rocket in the first place. Up until the recent innovations at SpaceX with Starship, there is no chance of refueling at any point during the flight, so the tank is usually quite large, and in turn, heavy.
The Oxygen
Unlike the duct engines used in aircraft like planes (referring to my recent reading of Rocket Propulsion Elements), there is no present air in the atmosphere for rockets once they are in space. Since combustion requires the presence of oxygen, there needs to be a separate tank that brings O2 with the rocket into space, usually in the form of liquid oxygen (because it’s more dense and takes up less space), commonly referred to as LOX.
From what I know, there aren’t many alternatives to a LOX system. It seems to be the case that the majority of modern rockets only change the fuel source as innovation continues. It makes sense, as there is nothing with a higher density of oxygen than just oxygen.
The Fins
The fins of the rocket are needed to keep the rocket upright and gliding during its time in the Earth’s atmosphere. The fins move in correspondence to the other fins on the rocket to keep the aircraft stable and oriented properly. These fins become less useful as the rocket ascends through the atmosphere of Earth, as the air becomes less dense. Space technically begins at the Karman Line, 100 kilometers in the sky. This is the same distance we are trying to make past for the Texas Rocket Engineering Lab.
The Engine
The engine is where the LOX and the fuel are combusted and directed to create thrust. Rocket engines as a whole are pretty complicated but have a few consistent elements.
First, rocket engines have a converging-diverging shape. This helps create a high-pressure base at the combustion chamber, which makes igniting the propellants easier, before radiating outwards against the back end of the rocket.
Second, in many cases, there are fuel lines that run around the engine. This is used to help pre-heat the fuel before combustion by warming them against the heat of flames. This is used to save fuel and to be as energy-efficient as possible.
Third, they are usually attached to gimbals, which are essentially just heavy-duty joints that are capable of moving in every direction. These are used to help direct the movements of the thrust of the rocket, also helping with orientation similar to the fins, but also working to slow and speed up the rocket in case there is a need to change directions.
Rockets as a whole are not too complicated, though they are becoming more so as we begin to expand from what was previously our only place of inhabitance. Most rockets are not reusable, but in the past decade it is becoming more and more prevalent. Examples such as SpaceX, NASA, and Blue Origin are all developing methods of reusing components, with SpaceX, in particular, leading the field with innovations such as Starship and Falcon.
Chapter 1 : Classification
This chapter is titled Classification, as it goes through the types of propulsion systems and how they differ.
To begin with, let’s define what propulsion means:
“The world propulsion comes from the Latin propulsus, which is the past participle of the verb propellere, meaning “to drive away“. Jet propulsion is a type of motion whereby a reaction force is imparted to a vehicle by the momentum of ejected matter” (RPE, 1)
In other words, a propulsion system is simply a system moved by the movement of something else. If you were floating in space and threw a ball in any given direction, you would be sent flying the other way. By definition, that’s propulsion.
Duct Jet Propulsion
Duct jet engines work by funneling air through the engine to combust the fuel. They do not require a liquid oxygen tank but are limited to only work inside Earth’s atmosphere, so they are commonly found in–you guessed it– jets and planes.
Two jet engines are worth mentioning under this category: the turbojet and the ramjet. Both work under similar conditions with the same general shape of convergence and divergence, just the turbojet also has fan blades that accompany the current of air that flows through the engine while the ramjet does not. This means it is lighter than the turbojet, but it requires extra thrusters to get the initial current of air moving..
Rocket Propulsion
There are many different types of rocket propulsion engines. Of the types, I will be going over liquid propellant rocket engines, solid propellant rocket motors*, and gaseous propellant rocket engines. There are more ways to differentiate engines than by fuel phase, but this is the most relevant.
Liquid Propellant Rocket Engines
Liquid propellants are the most commonly used source of fuel for rocket engines. These engines require a lot of complicated fluid systems with pumps, valves, generators, etc. They use the same converging nozzles mentioned in the previous excerpt to move the liquid at supersonic speed, which ultimately gives the rocket more thrust when the matter is ejected. I plan to better explain this in another post, but here is a video giving an idea of what’s going on.
Solid Propellant Rocket Motors*
*Historically, the word motor is used for solid propllenat rocket propulsion while engine is used for liquid
In a propulsion system using solid fuel, the propellant is held within the combustion chamber, also known as a case. The solid propellant is known as the grain, and it includes chemicals like oxygen so that there is no need for a separate LOX tank.
Solid propellant rockets are generally used once without stopping. Liquid rockets have sensors and valves to detect how much propellant is being used and can be shut off when desirable. Solid motors cannot do this, as once the engine is ignited all of the propellants but be combusted. It means the engine is very simple but less usable. These systems also use a supersonic nozzle to increase thrust.
Gaseous Propellant Rocket Engines
These rocket engines used gasses stored at high pressures for thrust. They require heavy tanks that are capable of not bursting with the high pressure, so that makes them inefficient while still usable. When the rocket is ignited, the already high-pressure gas is ejected out of the nozzle to get air time.
A good way of thinking about gaseous propellant engines is to imagine a balloon blown to maximum capacity. Once the entry hole is opened, the high-pressure air inside of the balloon moves to the lower pressure air of its environment, giving the balloon thrust and flight.
Chapter 2 : Definitions and Fundamentals
This chapter is titled Definitions and Fundamentals. Here we will begin going over the physics of rocket propulsion.
Impulse
First, we have the total impulse, which is essentially the total amount of force given once all of the propellants have been used. One could make a force vs. time graph that would show how the strength of a rocket engine changes over time. Finding the area under the curve of the figure would show the total amount of force applied over the given time. This is measured in Newton seconds (Ns).
Specific impulse is more commonly used, as it measures the efficiency of a rocket engine. Specific impulse tells you how much force is applied by an engine with regards to how much is propellant is used. In the same way that a car would have miles per gallon, rockets use a similar metric to find how much force is given per unit of propellant.
Interestingly enough, the units for specific impulse cancel out to just seconds. Force can also be measured in pounds, and total impulse is pounds-seconds. Divided by pounds (the weight of the rocket), all thats left is seconds.
Effective Exhaust Velocity
The exhaust veloctity of rocket engines are not entirely uniform, so assumptions are made for calculations. The Effective Exhaust Velocity, c, is that assumption integrated into the speed of the exhaust.
EEV is measured in either meters per second or feet per second, just like regular velocity. The math is nothing spectacular, but acts as a mathematical formality.
Mass Ratio
The mass ratio compares the mass of the rocket itself to the mass of the summation of rocket propellants.
The mass ratio can apply under a single or multiple stages of the rocket. Every structure inside the rocket is included in the final mass, including all the flight computers, the tanks, residual propellants, and even the crew. Mass ratios can change based on the use, with “60% for some tactical missiles down to 10% for some unmanned launch vehicle stages” (RPE, 28)
It is important to understand the mass ratios of different stages of the rocket. Different stages of the rocket naturally move upward, as the lower portions fall out during space. In a sense, for the lower stages of the rocket, the later stages are its payload. This gives an idea of how much propellant is left per stage, and how it can be best optimized.
Propellant mass fraction is similar to the mass ratio, but is specific to determining how much mass of the rocket does not make it to its destination, since all propellants will be expelled. The equation is simply the mass of the propellant over the mass of the rocket, whether it be at a single stage or multiple.
Impulse-to-weight ratio is a measurement used to define the proportion of a rocket’s impulse capabilities to its overall weight. This ratio gives another metric of engine efficiency.
The thrust-to-weight ratio is another metric, though it’s more for comparing the power of the rocket to its weight.
Thrust
Thrust is the force given off by the rocket’s propulsion system. Thrust is considered as a reaction force because it is a consequence of the ejection of matter, similar to how a gun may recoil.
Thrust inherently works off of Newton’s Third Law, saying that any reaction has an equal and opposite reaction. By shooting matter out one end of a rocket, the rocket is propelled in the other direction.
The thrust equation shown to the right is daunting but is essentially just the change in the mass (the mass flow rate) multiplied by the exhaust velocity (typically assumed to be constant), which is essentially how quickly the exhaust is in relation to the speed of the rocket.
Imagine a very slow rocket going 50 mph. If the propellant leaves the back end of the rocket at 125 mph, then with relation to the rocket the propellant only left the rocket at 75 mph because 125-50 = 75. This is basic relativity.
Since rocket engines work in environments with changing pressure (the atmosphere, space), there is a continued equation that incorporates thrust with regards to the ambient pressure.
The equation above is the full thrust equation, with ambient pressure included. The first term is called the momentum thrust and the second term is the pressure thrust. The momentum thrust is what we just went over, consisting of how the rocket moves due to ejected mass. The pressure thrust is the thrust given from the difference in pressure from inside the engine and the outside world. If the exit pressure is less than the outside world, the total thrust can decrease, making the pressure thrust negative.
Pressure naturally balances out. If the pressure outside of the engine is stronger than within, the atmosphere will try and fill up the extra space in the engine. This pushes back against the exhaust propellant and lowers the total thrust. In space, the ambient pressure is 0, so there is maximum thrust.
On a side note, the effective exhaust velocity can be calculated here by simply taking the full thrust equation above and dividing it by the mass flow rate. Thrust is a force, but without mass, it is just acceleration (or in this case, since the derivative is mass, all we have left is velocity).
Resources
My learnings have been compiled from three main resources: the book Rocket Propulsion Elements by George P. Sutton and Oscar Biblarz, the Chris Hadfield Masterclass over space exploration, and my time in the Texas Rocket Engineering Lab at the University of Texas at Austin.
David Di-Benedetto 