Rocket Propulsion Systems
Methods of acceleration in and to space
Since the 10th century in Song dynasty China, we have been experimenting with using rockets to propel a system. The idea’s simple; just blow stuff up and launch it out the back of a rocket, but the intricacies behind a rocket propulsion system are amazing when you learn how it works. In this article, we’ll discuss the different ideas behind current methods of getting into space, the science behind a rocket, and up-and-coming developments in rocket propulsion.
The Physics behind A Rocket
When we talk about rocket propulsion, many laws of motion come into play; the law of acceleration, the law of action and reaction, the law of conservation of momentum, and thrust. The whole idea of a rocket is to launch stuff out the back and when we do this, it creates force and has momentum. Since Newton’s second law describes force as mass * acceleration, then when we launch fuel(which has mass) out the back of a rocket at high speeds, we generate force. Newton’s third law states that with every action, there is an equal but opposite reaction, so if we generate force out the back of a rocket, then we would move forward. This is exactly what happens and the way you can calculate this is through thrust, which is calculated by the mass flow rate and the speed at which the mass is expelled.
The next key factor in play is the conservation of momentum. Momentum is a measurement of the quantity of motion, meaning how hard it is to stop something that is moving. Momentum, at a rudimentary level, is described as mass * velocity. Now, in a perfectly closed system(no outside factors), momentum is conserved, meaning that the total momentum in a closed system will always stay the same. Imagine a Newton’s Cradle, when we take one ball at one end and drop it so that it hits the other balls, then on the corresponding side, the other ball will be flown back at the same speed and distance. The momentum of the first ball was transferred to the other ball and the total momentum never changed*.
*You have to note that the conservation of momentum only applies in a perfectly closed system where no energy is lost and our world is definitely not a closed system
The same idea of momentum applies to breaking stuff apart, which is how a rocket works. When we launch fuel out the back of a rocket, it has momentum, and that momentum is conserved to some extent in which the rocket will then move forward at the same momentum. In case this idea still seems complicated, imagine you are in space, you have a toolbox, and your goal is to move forward. Well, if you were to throw your toolbox backwards, then the momentum the toolbox has moving backwards will equate to the momentum you have moving forwards.
Now another three concepts you need to know for specifically rocket propulsion is the thrust to weight ratio, specific impulse, and delta velocity(ΔV). Firstly, the thrust to weight ratio describes how fast you can move a rocket. Since Newton’s second law is that force = mass * acceleration, then acceleration = force / mass. So to move faster, then you need to lower you mass. The thrust to weight ratio essentially tells you how much force your rocket is propelling you compared how much force gravity is pulling you down. You want to maximize the thrust to weigh ratio. Next would be specific impulse which is the measurement of fuel efficiency. This is calculated by dividing thrust by the rate of fuel consumption. Finally would be delta velocity, which breaks apart into change in velocity. This concept is essentially how fast you can get using your rockets and it is calculated using a very boring formula which is:
All of these different principles and values in physics come together to send a rocket into space. These principles define the way we move and are crucial in rocket propulsion.
Chemical Thrusters
The first official type of rocket propulsion is a chemical rocket propulsion system. This method of propulsion falls under the idea of “blowing stuff up” as it works by using combustion. Chemical rockets were first made in 1926 and are still the strongest type of rocket and is the primary go-to for getting into orbit.
The combustion system works just like how a fire would; Combining fuel, oxygen, and heat. For a fire, it would be wood and kindle for fuel, air for oxygen, and a lighter for heat. For a rocket, it would be rocket fuel(this can vary), oxidizers(can also vary), and an ignition spark. Chemical rockets break into 2 separate categories, solid fuel rockets and liquid fuel. The difference is pretty self explanation, one uses liquid fuel and one uses solid fuel, but, they have their other differences that come with different fuels.
A quick fact about rockets is that they have stages. What this means is that there are different levels of a rocket so, once you burn through the fuel for the first engines, you break them off and start the next set of engines. This allows for empty and useless fuel tanks to be discarded, to maximize acceleration.
Solid Fuel Boosters
Solid fuel boosters have been around for a while as it is much simpler to make than liquid fuel rockets. These rockets use aluminum powder(Al) for fuel and mix it with ammonium perchlorate(NH4ClO4) as an oxidizer. If that was a mouthful, essentially, by mixing these 2 ingredients together, you unlock 2 of the 3 things needed for liftoff, oxygen and fuel. Then, by running an electric current through an igniter wire, it provides the heat needed for combustion. Since the combustion takes place where the fuel is, it will continue to fire until it runs out of fuel. On top of this, you can’t control the thrust it creates and it will continue to burn until exhaustion. Solid fuel boosters are generally used for getting out if the atmosphere as you don’t really need to control the thrust when going up. They are typically in the first stage and are attached to the side so, when emptied, can be easily discarded.
Liquid Chemical Thrusters
Although they both are chemical thrusters, liquid rockets are very different from solid rockets. For fuel, the most common fuel source is liquid hydrogen as it has been the most efficient and widespread, but other fuel sources like kerosene, alcohol, hydrazine, and methane is expected to lead the next generations of rockets. For oxidizer, many rockets use liquid oxygen and fun fact, whenever you see rockets during launch expelling steam, that is liquid oxygen boiling and being expelled as liquid oxygen has a low boiling point.
Unlike solid fuel rockets, the fuel and oxidizers are kept separate and are combined in a combustion chamber. Using a lightweight centrifugal turbo-pump*, rocket fuel and oxidizer are pumped into a combustion chamber where it is burned and ignited. Since the combustion is separate from the fuel and you can control the amount of fuel being pumped into the chamber, you can both turn off the engine and control the throttle.
Electric Thrusters
Electric rockets are actually not electric, they still need propellent, but the way they launch this propellent is different. The whole idea is to launch magnetic propellent using electromagnets and this process is much more efficient than chemical rockets. There are 2 types of electric thrusters, ion thrusters(very futuristic), and hall thrusters. Both use xenon for propellant as when you bombard it with electrons, it easily ionizes. An ion is an atom in which there is an imbalance between the proton(positive charge), and electrons(negative charge), creating an electric charge.
For ion thrusters, xenon gas is put into a chamber which walls are coated with electrons, which then bombards the xenon atoms, ionizing it. Then, using electric currents, it propels these ions out at speeds around 20 to 50 km/s. These chambers are put in an array/grid as 1 is way too weak. Even though the exhaust velocity is so fast, the fuel is really light so the thrust generated is not as strong as chemical thrusters. The low thrust is made up for by the fuel consumption rate being very low, so electric thrusters and ion thrusters specifically are the go to for long distance space missions and these thrusters are usually found in the upper stages of a rocket after escaping the earth’s atmosphere.
Hall thrusters are pretty similar to ion thrusters as they both bombard xenon gas atoms and they both use magnetic field, but Hall thrusters create an ionized xenon plasma(a gas where there are electrons flying around) and use the Hall effect. In simple terms, the Hall effect is when you provide a magnetic field at just the right angle and this(in very simple terms) traps electrons which is accelerated with the ionized xenon gas. The main difference is that Hall thrusters provide more thrust but have a lower lifetime and fuel efficiency.
Nuclear Thermal Propulsion
One of the methods of generating heat is through nuclear fission. The idea of nuclear thermal propulsion was first presented in 1961 by NASA and the former Atomic Energy Commission but has not been used in flight yet. The idea is to run a fuel source through a nuclear fission generator which generates heat by breaking apart large and unstable atoms. When the fuel source(in liquid form) runs through this heat, it then boils and expands into gas, being launched out the back. One of the main reasons why this was never used was because we have no way of shielding the extreme heat from the nuclear reactor, risking damage to the whole rocket. On top of this, a NTP rocket is designed to function in orbit so you have to launch a nuclear reactor into space using a big fuel tank and rocket. If something were to go wrong on this initial orbit escape, then what we’ve essentially created is a nuke. NTP is too risky right now and propulsion systems like chemical rockets and ion thrusters are much safer.
Solar Sails
Did you know that light could push things? Discovered by James Clerk Maxwell in 1862, light was known to have momentum before it even was discovered to be composed of proton.you might be wondering “How does light have momentum but has zero mass*?” because we learned that momentum is equal to velocity * mass. Well, light is also a wave and a waves momentum only requires motion, no mass is needed. A solar sail takes advantage of this momentum and uses it to propel it. By launching a big reflective sail the size of a boxing ring(can vary) with a main rocket body the size of a loaf of bread(can vary), you can harness the sun’s light to propel a very light rocket. Another idea being developed is to use lasers stationed on earth to propel this sail. So far only light weight mini satellites can be propelled as a manned rocket would require living space and life support which is really heavy. Solar sails have already begun launching since 2010 and there are many companies currently developing solar sails.
*Einstein’s famous equation e = mc² implies that your energy is proportional to the speed of light, and if you go at the speed of light, then you would have infinite energy
Direct Fusion Drive
We are now entering the futuristic science fiction part where we talk about potential propulsion systems in the future. The main component in this propulsion system would be the fusion. Essentially, when we collide 2 atoms together at insane pressures and heats, they will fuse, giving off tremendous amounts of energy. This is due to the fact that some mass is lost and converted into pure energy(A proton). By using magnets to heat Helium3 and Deuterium(fancy elements), we can create a reliable fusion reaction for maximum output. The energy released can be captured as electricity to power the entire space craft for things like heating, radio communication, etc. By running a propellant around the edge of the reaction and accelerating it using magnets, we can generate trust. The momentum of the propellent is accelerated by ions which expand after leaving the rocket, thus creating an ion thruster of some sort.
Warp Drive
Entering science fiction territory, you may have hear of a “Warp Drive” or “Epstein Drive” being mentioned in popular sci-fi shows like Star Trek. You need a pretty strong knowledge on astrophysics to fully understand it as it is based on a completely theoretical idea. The general idea is to bring your target closer to you. By bending space and time, you can shorten the distance needed to travel and potentially reach superluminal(faster than the speed of light(300000m/s)) speeds. However this is completely speculative as we have no way of bending space time(except using gravity) and traveling faster than the speed of light would mean infinite energy(which is impossible)*.
*Einstein’s famous equation e = mc² implies that your energy is proportional to the speed of light, and if you go at the speed of light, then you would have infinite energy
Where We Are Going
With new developments in science like nuclear fusion, we improve rocket propulsion along side it. Every new advancement in science is an advancement in getting to space. Rocket propulsion allows our civilization to expand as space makes up the majority of out universe. We need to be able to traverse our universe and rocket propulsion is the best way we can. Every day rocket systems are improving and we are inching closer and closer to space, thank you for reading!
Woohoo!
👍👍So proud of you😆