The Ever-Changing Fate of Rocket Propulsion
One of mankind’s biggest achievements was putting a human on the Moon. After decades of research, experiments and technological developments, on July 21, 1969, Neil Armstrong stepped on the surface of the Moon.
One of the most important aspects of the journey was the rocket propulsion system used. In the early 1970s, Chemical Rocket Propulsion systems were groundbreaking and they completely redefined the limits of Space exploration. However, since then, several new rocket propulsion systems have been developed, and with them, new goals for the future.
The main five systems that will be discussed in detail are Chemical Rockets, Ion Thrusters, Nuclear Propulsion, Solar Sails and Antimatter Propulsion.
Chemical Rockets
Traditional chemical rockets are the most basic and most used propulsion system since the beginning of the space race. They have been used on a large majority of space missions due to their simplicity and availability of resources.
A chemical rocket has four major components: the structural system, propulsion system, payload system and guidance system.
The structural system consists of the outer framework of the rocket, the body, nose cone and fins.
The payload system creates the thrust for the rocket to fly. It contains engine, fuel and oxidizer.
The payload system is the people and items that need to be delivered into space. Examples of this are crew, satellites, etc.
Lastly, the guidance system is the computers and machines that control the movement and navigation of the rocket.
The way each of the five types of rockets is different is in the way they produce thrust. Chemical rockets generate thrust by using combustion to create exhaust gases, then releasing the gases at high speeds and temperatures through the back of the rocket, to produce thrust in the opposite direction. There are two main types of chemical rockets, solid propellant rockets and liquid propellant rockets.
In a liquid propellent chemical rocket, the liquid fuel and the liquid oxidizer are stored in separate tanks and pumped into the combustion chamber. The chemicals are ignited and the reaction produces the exhaust gasses mentioned earlier, which are forced out through the nozzle.
In a solid propellant chemical rocket, the solid fuel and solid oxidizers are already mixed and stored as the propellant in a cylindrical combustion chamber, which has a hollow cylinder in the middle that is the same length as the chamber. The hollow cylinder is used to maximize surface area for ignition.
When the propellant is ignited on the surface, it creates a flame front which produces the required gas reactants. They are funnelled through the hollow cylinder and exhausted through the nozzle. Changes in the amount of thrust are achieved by varying the diameter of the nozzle as well as the speed of ignition.
What problems does it solve?
Traditional rockets have been used to accomplish incredible feats of engineering and science including putting a human in orbit, putting a human on the Moon, sending unmanned crafts far into the depths of space and many more.
Not to mention, chemical rockets are currently the most reliable, efficient and safe method of creating enough thrust to break Earth’s orbit. This is because it is currently the propulsion system that creates the highest exhaust velocity.
One of the reasons why chemical rockets won out at the beginning of the space race was because of the availability of the resources required. What this means is that all the materials required for combustion, the fuel itself and the actual rocket structure are readily available and do not require specific conditions. For example, the liquid propellant and the metal to build the combustion chamber are both easily accessible.
Building on the first problem these rockets solve, all the successful missions using chemical rockets prove that they are the safest way to deliver the sensitive payload, like humans into Space. This is due to a variety of factors, but the main one is that chemical rockets do not cause any side effects and minimize the radiation from outer Space.
Once again building on the real-mission experience of chemical rockets, due to their effectiveness in real missions, these rockets have been developed and redeveloped, designed and redesigned, constantly being improved.
Why does it need to change?
Chemical rockets have been able to send people to nearby celestial objects like the International Space Station and the Moon, but we are moving into a new age of Space exploration. With the current technological achievements, humans are keen on exploring the edges of our solar system and even travelling to the nearest star system, Alpha Centauri.
Alpha Centauri is 4.37 light-years away, which is about 36 trillion kilometres. Using our current methods, chemical rockets, it would take 30,000 years to reach Alpha Centauri. This is because chemical rockets are only able to achieve speeds of up to 15 km/s, making us travel far too slow.
All rocket engines are judged by their Specific Impulse. Specific Impulse measures how many seconds of thrust can be produced by a certain amount of fuel. Modern-day rockets have SI’s over 300 seconds and newer rockets being developed have SI’s close to 500 seconds. Not to mention that chemical rockets only have a fuel efficiency of 30%. Remember this for comparison with the other propulsion systems.
Ion Thrusters
Ion Thrusters were first developed in 1964 in NASA’s SERT project. They work by expelling large amounts of positively charged ions out the back to produce thrust.
The engine starts with a neutrally charged noble gas like Xenon. Xenon is often used because it is the heaviest non-radioactive inert gas. This makes it easier to lose electrons. The engine then releases an electron into the chamber with force. The collision between the electron and the large neutral atom causes the Xenon atom to lose an electron, becoming a positively charged ion. Now there are two electrons in the chamber, one that the engine released and one from the Xenon atom. Both the electrons then collide with another Xenon atom, which then releases an electron, repeating the process. In the end, there will always be the one electron that was initially released, allowing the process to be run until the rocket runs out of Xenon.
The positively charged ions are expelled out, at incredibly high voltages, through the use of a negatively charged grid at the back of the rocket. However, due to this, once the electrons pass through the grid, they will get attracted backward.
The solution for this is to re-attach the electrons the Xenon atoms lost. All the electrons in the chamber are captured and the initial electrons that the engine releases are re-inserted into the chamber. Whereas, all the other electrons are captured and then reattached to the Xenon ions outside the rocket.
Ion Thrusters work on the opposite principles of chemical rockets. Instead of creating immense thrust exiting Earth’s atmosphere and then allowing the rocket to glide the rest of the way, Ion Thrusters constantly expel ions, creating much less thrust, but throughout the entire journey. By the third day and more, the craft can reach extremely swift speeds. This is because the longer the thruster is continuously active, the more thrust it produces.
After being active for several days, the Ion Thruster can release atoms at speeds of 90 km/second. The fuel efficiency is 90% of this propulsion system.
Another type of Ion Thruster is the Hall Thruster. It works the same way as normal Ion Thrusters but instead of waiting for the amount of thrust to ramp up over time, it uses strong magnetic fields to accelerate the particles out the back, producing a sizable amount of thrust throughout the entire journey.
One great example of a Hall thruster is NASA’s X3 Hall Thruster, which can produce over 1000 times the thrust of normal Ion propulsion engines.
Why do we need it?
Firstly, as mentioned above, Ion Thruster rockets can reach speeds of up to 6 times as fast as chemical rockets, with 3 times the efficiency.
Since Ion engines can run for absurd amounts of time on low resources, and that the amount of thrust increases the longer you run it, Ion engines are currently the best solution for making long-term, deep space voyages.
Hall thrusters are the more practical and developed Ion propulsion system, since they can accomplish everything normal Ion Thrusters can, with the addition of reaching speeds of about 72,000 km/h. This makes them one of the most likely systems to be used in future space exploration missions.
Are there any drawbacks?
Ion Thrusters use Xenon because it is the heaviest noble gas, meaning it will produce the most thrust per atom, by getting it converted to an ion then forced out the back.
Therefore, one issue is that the engine requires a constant supply of Xenon. However, this can be solved by the 90% efficiency of the thruster. In NASA’s NEXT Ion Engine, they were able to continuously run it for 5 years on only 700 kg of Xenon gas.
Another minor drawback is that the current Ion Thrusters do not produce enough thrust to break Earth’s atmosphere after just starting. However, this is being solved by projects like NASA’s X3 Ion Engine, whose main purpose is to generate more thrust.
Nuclear Propulsion
Nuclear propulsion is broken down into two categories: Nuclear Thermal Propulsion and Nuclear Pulse Propulsion. Both categories use the principles of Nuclear energy generation, but apply it into a rocket in different ways.
Before moving on to nuclear-powered rocket engines, we need to discuss how nuclear power plants work on Earth. In general, there are two main concepts, Nuclear Fission and Nuclear Fusion. We will dive deeper into Nuclear Fission shortly, but to sum it up, it is when parts of an atom are split to release energy. Nuclear Fusion, on the other hand, is when parts of an atom are combined into heavier parts, releasing energy.
Nuclear Fission is almost exclusively used in nuclear power plants and any nuclear-powered engines because it can be controlled very easily and does not require impossible conditions. However, nuclear fusion produces more energy, but the reaction is uncontrollable and it requires conditions of dozens of millions of degrees Celcius.
Moving back to nuclear-powered rocket engines, Nuclear Thermal Propulsion is the more safe, practical and plausible alternative to chemical rockets. The way it works is the rocket starts with an extremely cold tank of propellent, often liquid hydrogen. The engine then uses Nuclear Fission to heat the chamber to insane temperatures, around 2000 degrees Celsius (Nuclear Fission will be explained below). The liquid hydrogen is then sent into the chamber, gets heated up and then expelled out the back of the rocket very high velocities to produce thrust.
Nuclear Fission is what is used in nuclear power plants to generate 10% of all the world’s energy. To understand Nuclear Fission, you need to understand atoms. Each atom has a tremendously dense nucleus, which contains protons and neutrons. A force called the Strong Nuclear Force holds these protons and neutrons in place with one of the strongest forces in nature. The more the number of protons and neutrons, the force decreases. Elements like Uranium are ideal for nuclear energy because it is the heaviest natural element, with 92 protons and can vary around 145 neutrons. Due to this, the Strong Nuclear Force is weaker in the nucleus, making it easier to split apart.
The process (of Nuclear Fission) starts by colliding neutrons with the nucleus of heavy elements like Uranium, breaking the nucleus and releasing more neutrons. Each collision and split of the atom releases large amounts of energy. In the case of the rocket engine, the energy is captured and used to heat the chamber for the propellant.
NASA tested out Nuclear Thermal Propulsion in 1986 with Project NERVA, which stands for Nuclear Engine for Rocket Vehicle Application. Throughout the testing, they were able to develop an engine called the NRX A6, with a Specific Impulse of 869 seconds. Continuing developments, the NERVA XE was a newer model that didn’t burn up because of the extreme heat (from Nuclear Fission) like the NRX A6.
The less effective and less feasible technology is Nuclear Pulse Propulsion and it was first developed in the 1960s when millions of scientists were trying to discover new and unique ways to land people on the Moon.
The way this worked is that it blew up nuclear bombs underneath the rocket to propel the craft upwards. While this idea sounds crazy right now, there were several tests during NASA’s Project Orion in the late ’50s to early ’60s. Project Orion has several drawbacks, which I will proceed to discuss, but it got cancelled in 1963 due to the Partial Nuclear Test Ban Treaty, which prevented all test detonations of nuclear weapons in the atmosphere. It only allowed the tests to be conducted underground.
For both the next sections, I am mainly going to be focusing on Nuclear Thermal Propulsion because it is more safe, reliable and has a higher chance of being used in the future.
Why do we need it?
Using Nuclear Thermal Propulsion, the rockets will be able to reach speeds much higher than traditional chemical rockets, meaning that the travel time for long journeys will be greatly reduced.
Nuclear energy has also been used on Earth for several decades, allowing us to understand all the possible risks of Nuclear Thermal Propulsion and how to avoid them.
As I mentioned in the previous section on chemical rockets, the efficiency and standard of rocket engines are measured by their specific impulse. Nuclear rockets can generate incredible specific impulses into the quadruple digits.
Are there any drawbacks?
One unavoidable risk of Nuclear Thermal Propulsion is that having a nuclear reactor onboard the rocket will be a small source of radiation for the crew. This is in turn solved by the faster travel time, overall resulting in less radiation from Space
The dozens of drawbacks of Nuclear Pulse Propulsion are all the dangers of nuclear bombs. The radiation, blast damage, health effects just to name a few.
Solar Sails
Solar Sails use the same principles as sailing ships, in outer Space.
The way it works is that it uses an extremely large but light material to reflect the Sun’s light. When the Sun’s energy is reflected, it pushes on the reflector slightly because each photon has momentum. In this scenario, the Sun produces such a great amount of energy, that the force produced from flying towards the Sun, then gliding to your destination, is enough for interstellar travel.
According to the Inverse Square Law, if you travel the rocket half the distance towards the Sun from Earth, you get four times the amount of light and energy on the Solar Sail.
NASA is developing this project through its NEA Scout Project. It stands for the Near-Earth Asteroid Scout project and they are working to develop a solar sail-powered rocket.
Why do we need it?
Solar sails are 100% clean and renewable because they harness the always-accessible energy from the Sun.
Solar sails can be used to reach incredibly high speeds outside of Earth’s atmosphere where the energy is not being absorbed and reflected.
This technology is revolutionary because it does not require any engine, fuel or artificial means of creating power.
Are there any drawbacks?
As mentioned before, the two major drawbacks are related to the position of the rocket. These rockets will have extreme difficulty creating enough thrust to break Earth’s atmosphere. This is because the property of our atmosphere that makes Earth sustainable for human life also becomes problematic for solar-powered rockets.
I am talking about the ozone layer. The ozone layer absorbs and reflects solar energy, meaning only a fraction of the Sun’s light and energy enters our planet, which is not enough to create enough the required thrust for liftoff.
One of the, if not the only, solution to this is in the Future of Rockets section when we discuss combining different methods of rocket propulsion.
The second drawback is that the farther away the rocket travels from the Sun, the less thrust is generated per photon. This is solved by travelling closer to Sun, to multiply the amount of thrust, then using that initial momentum to carry the rocket the rest of the voyage.
Antimatter Propulsion
Antimatter is a field of physics that describes how there is an antiparticle for every single subatomic particle in the Universe.
All matter is made up of electrons, quarks, neutrinos, muons and talons. Similarly, there are antielectrons (positrons), antiquarks, antineutrinos and more. These antimatter particles have the same physical properties as regular particles, except the opposite charge.
Think of antimatter in the Universe as the answer to the square root of 4. The square root of four can equal 2, which represents the normal matter or -2, which represents antimatter.
Moving into antimatter propulsion, when antimatter and regular matter particle collides, they explode and destroy both particles. This is because they are the exact opposites of each other. Think of when you add -2 and 2, you get 0.
Unlike common energy solutions, this explosion does not release energy, but rather both the particles transform into pure energy. That is why antimatter annihilation is the Universe’s most efficient way to turn mass into energy.
Just to put the amount of energy in a practical sense, each antimatter particle has 90 megajoules per microgram. In other words, 1 gram of antimatter is equivalent to 80 kilotons of TNT.
Antimatter propulsion was developed in the 1950s by Eugen Sänger. The proposed concept at the time was to trap large amounts of antimatter and regular matter, funnel them through the nozzle, make them collide and then reflect the energy in the opposite direction to produce thrust.
There were three main issues with this concept of antimatter propulsion: producing large amounts of antimatter, trapping the antimatter and reflecting the energy.
The first problem is that there was no reliable way to produce antimatter. The second problem was that, as you know, when antimatter and matter collide, they annihilate each other making it extremely hard to contain large amounts of antimatter. The only way to trap it is by using highly powerful magnetic fields. The last issue is that, when the particle and antimatter collide, they turn into extremely powerful gamma rays. Gamma rays can pass through almost all objects, making them impossible to reflect and direct.
However, a company called Positron Dynamics is working on creating solutions to the challenges and even launching a satellite-powered by antimatter propulsion.
The first step in working with antimatter is to cool it down. The company developed and patented an antimatter array moderator to do so.
The company used a radio-isotope source of positrons, or antielectrons, to constantly generate positrons and feed them through the moderator, solving the first problem.
The second issue of trapping was able to be accomplished by the very efficient moderator because the cooled down antimatter particles were easier to be stored (required more feasible magnetic fields).
The last issue, directing the gamma rays from the collision, was solved by fusion reactions. The physicists transferred the energy from the rays into a charged particle, which can then be directed by a magnetic field.
There are several other designs for antimatter propulsion, but this is the model that won out.
An example of this is highlighted in this Forbes article written by Bruce Dorminey on Antimatter Space Propulsion. The article describes a system that consists of a uranium coated carbon sail, a solid antihydrogen storage unit and an antimatter driven power supply.
When antimatter hydrogen protons hit the uranium on the surface of the sail, it causes nuclear fission and the antimatter annihilation. This reaction creates two identical beams of energy. One beam travels toward the carbon sail and is absorbed. The other beam travels toward the back of the rocket and is accelerated and expelled, generating thrust in the process.
Why do we need it?
Antimatter propulsion is nature’s most efficient way of converting matter into energy. Each antimatter annihilation gets converted into two billion volts of electricity. Therefore, an antimatter rocket will provide exponentially greater thrust than any chemical, nuclear, ion, etc — powered rocket.
General Conclusion
While there are not only 5 types of rocket propulsion, these 5 are the foundation for any other system. For example, laser propulsion has two designs. First, it is when the rocket uses a laser to push a solar sail, which builds off the solar sails. Secondly, it uses lasers to heat the propellant to very high temperatures and then force it out the back, which builds off nuclear thermal propulsion. Another example is plasma propulsion, which uses all the same principles and concepts as ion engines, except for the source.
Future Applications
It is no surprise that the five technologies (four new) mentioned today are not only used as rocket propulsion but rather they apply to several other industries as well.
Chemical
The defining factor of a chemical propulsion system is the Internal Combustion Engine, which ignites a fuel to produce gasses. Therefore, some of the applications are in vehicles that require Internal Combustion Engines. For example:
- Any ground-driven motor vehicle (like cars, motorcycles, busses). They use the combustion engine to cause an explosion, that pushes pistons down, to produce power.
- Planes
- Ships
- Trains
- Smaller machines that require engines (like leaf blowers, snowmobiles, jet skis, lawnmowers): Use a modified version of an internal combustion engine that usually only has two pistons, to produce enough energy to operate the machine.
Predicting the future, Internal Combustion Engines, and then chemical propulsion will become obsolete due to their enormous carbon emissions and the necessity of large amounts of fuel.
Ion
Ionization is the process in which an atom either loses or gains an electron to become either a positively or negatively charged ion. When an atom gains an electron, the increased negative charge causes energy to be released.
Using this, in the future, ionization can become one of the leading energy sources due to the following idea. As we know from how Ion Thrusters work, to create a positively charged ion, you need to propel an electron at a neutral atom, which then causes it to lose an electron and become positively charged. Now there are two electrons in the chamber when you started with one. You can then make the atom gain the electron back, releasing energy. Now you still have the one initial electron, meaning you have an endless cycle
The main drawback is that ion propulsion doesn’t work on Earth because of two reasons:
- There is no friction in space meaning that the small continuous push will increase the thrust over time, which will not happen on Earth
- There is very little to no gravity in space, which would slow the ship on Earth
Projects like the X3 Hall Thruster in the future solve these problems and would allow for ion propulsion to become one of the main propulsion systems.
Nuclear
Nuclear energy is already currently used all over the world, and it produces 10% of all the world’s energy.
Currently, the main downside is nuclear waste, which is an extremely radioactive material that has to be stored in special containment facilities for around 50 years.
One future possibility is a nuclear reactor that runs on used fuel, eliminating the need for nuclear waste facilities.
Apart from that, nuclear energy still requires large amounts of uranium and a powerful reactor, making it implausible to replace other energy sources.
Solar Sails
Solar sails operate best in space and orbit, where there is no atmosphere to absorb and reflect the Sun’s energy.
One future application of solar sails is to figure out a way to launch them into Earth’s orbit without using traditional methods.
A possible solution for that is to reflect and capture the Sun’s energy on Earth for a long period, then funnel it into a beam to provide enough initial thrust to break Earth’s gravity. However, the current solar sails designs are that the sail is folded and then it expands in space so it does not get damaged, meaning this solution wouldn’t work.
Once we can accomplish that, interstellar travel using solar sails could truly become a reality.
Antimatter
Antimatter, perhaps one of the newest and coolest propulsion technologies out there, and as mentioned before, nature’s most efficient way of converting mass into energy.
Antimatter can be produced naturally and artificially. It can be produced naturally by cosmic rays, above thunderstorm clouds, in Van Allen Radiation Belts, near black holes and in neutron stars. This is because positrons (antimatter electrons) are produced naturally in two ways:
- Naturally occurring radioactive isotopes.
- Interactions in gamma quanta, which is a photon of radiation with the most energy and shortest wavelength.
Unfortunately, the locations and specific scenarios in which antimatter is naturally produced are inaccessible from Earth, and even if it is, the scenarios are not predictable and they do not happen consistently.
This leaves us with artificial production of antimatter. The way this works is by forcing high energy protons into a dense material to produce antiprotons. This was tested when scientists used a laser to drive such protons through a gold atoms’ nuclei, producing antiprotons.
The reason this works is that antimatter can be created artificially as resultants of high energy collisions, making use of the fact that the excess energy produces pairs of matter and antimatter subatomic particles. In the illustrated example below, a high energy collision releases electrons and anti-electrons, or positrons.
However, the major issue is that humans have only been able to produce up to 15 nanograms of antimatter throughout history.
In the future, if reliably creating and/or harvesting antimatter is possible, it will revolutionize our entire world. We will have access to the best, most efficient energy creation system which produces no waste and has no drawbacks. Antimatter will be used in everything that needs energy and we would be on the verge of another technological revolution.
Future of Rocket Propulsion
Now you thoroughly understand the five major contenders to replace chemical rockets, but what does that mean for the future?
As you have learned, the major obstacle for several of these rockets is breaking Earth’s gravity and entering orbit. This is because it requires an extreme amount of thrust to escape Earth’s gravity, above 40,000 km/h.
Therefore, the rockets in the future are going to fall in one of three scenarios:
Firstly, a combination of different propulsion systems. For example, a chemical rocket engine to get into orbit, then it switches to an Ion Thruster for the rest of the journey. This will maximize the efficiency of both engines, while drastically decreasing the drawbacks of both.
Secondly, an extremely developed propulsion system. What I mean by this is that as time goes on, more technologies are being discovered and each of these systems is being further developed, to the point where they can overcome their drawbacks. An example of this is a further developed X3 Ion Engine by NASA that can escape from Earth’s gravity. Another example is a nuclear thermal propulsion engine that is able to contain the radiation from affecting the crew.
The last scenario is that there is no standard rocket propulsion system, but rather it varies depending on the journey. For example, in a mission that is required to travel in Earth’s orbit around the Sun, a solar sail could be used to maximize the energy from the Sun. Another example, on an extremely long unmanned mission, antimatter propulsion can be used because it can generate tons of thrust on very little fuel.
Key Takeaways
Chemical
Chemical powered rockets work by using combustion to produce exhaust gasses, then releasing the gasses at high speeds to produce thrust.
Pros:
- Most reliable, well-tested and safe method
- Creates the highest exhaust velocity
- Uses readily available resources
Cons:
- Far too slow — takes to long for deep space exploration voyages
- Low Specific Impulse (Around 300 seconds)
- Fuel efficiency of 30%
Ion Thrusters
The way Ion Thrusters work is by converting atoms into positively charged ions, then using a negatively charged grid to expel the ions, producing thrust.
Pros:
- Can travel six times as fast chemical rockets
- 90% fuel efficiency
- Can run for extremely long times with low fuel
Cons:
- Requires a constant supply of a heavy noble gas like Xenon
- Unable to break Earth’s gravity
Nuclear Propulsion
Divided into Nuclear Thermal Propulsion and Nuclear Pulse Propulsion.
Nuclear Thermal Propulsion works by using Nuclear Fission to heat up propellant and expel it out to produce thrust.
Nuclear Pulse Propulsion works by exploding bombs under the rocket to blast it in the direction it wants to go.
Pros:
- Able to reach faster speeds — faster voyages
- In depth knowledge of Nuclear power — know the risks
- Incredible Specific Impulses over 1000 seconds
Cons:
- Unavoidable radiation on the rocket
- For Nuclear Pulse Propulsion, all the dangers of bombs
Solar Sails
Solar Sails work by reflecting the Sun’s light, which propels the rocket a little per photon of light because it has momentum.
Pros:
- 100% clean and renewable
- Can reach incredibly high speeds outside of Earth’s atmosphere
- Doesn’t require any artificial means of creating power
Cons:
- Cannot generate enough thrust on Earth or inside Earth’s atmosphere
- Needs initial proximity to the sun
Antimatter Propulsion
Works by harnessing the energy produced during antimatter annihilation and reflecting it out the back to produce thrust.
Pros:
- Nature’s most efficient way of converting mass into energy
Cons:
- No reliable way to produce large amounts of antimatter
- Extremely difficult to trap antimatter due to its properties
- Currently no way to reflect/direct the created energy
