We all love rockets - the roar of a man-made contraption leaving our little blue planet is irresistibly compelling, with over 650 million people watching the Apollo 11 moon landing, despite it occurring at midnight. Unfortunately, launching a rocket into space with current hydrocarbon fuels is extremely expensive, time consuming, and unsustainable, not only releasing an enormous 300 tons of carbon dioxide per launch, but also releasing black soot into the atmosphere, and often leaving debris orbiting our planet, making it far more difficult to launch future missions without collisions.
Therefore, in order to still fulfil our dreams of space exploration and progress as a civilization, it is essential that we turn to alternative methods of propulsion. Fortunately, research is already underway in this field, with a wide range of solutions having been suggested. Today, we will be looking at a few of these, checking out their strengths and weaknesses to find out what the technological future of rockets might look like.
Liquid hydrogen combustion
Thermal nuclear engines
Ion thrusters
Electromagnetic railguns
Liquid hydrogen combustion
Using liquid hydrogen as a fuel is very similar to using hydrocarbons like kerosene in traditional chemical engines - indeed it has been used by NASA for decades on many of their missions. Effectively, the fuel (liquid hydrogen), and the oxidiser (usually liquid oxygen), are pumped into a chamber where they combust, releasing the energy stored in their chemical bonds to form extremely hot, high-pressure steam which expands, accelerating out of the nozzle. Isaac Newton's third law of motion states that “every action has an equal and opposite reaction”, meaning that this force exerted on the gas by the rocket engine is balanced by an equally strong force acting on the rocket from the gas, which propels the rocket forward - this force is called thrust.
Hydrogen rockets work in a similar manner to hydrocarbon chemical rockets like the one above
Advantages
Hydrogen has the highest specific impulse (a measure of how efficiently thrust is created) of any known rocket fuel
Like with all liquid rocket fuels, the engine can be throttled, shut down, and restarted
The technology has already been widely used and tested, both in launch from sea level, and in the vacuum of space.
It can be used in conjunction with other sustainable rocket technologies such as reusable rockets - this has already occured with Blue Origin’s BE-3 module.
As a liquid, hydrogen has a high energy density in terms of both its mass and volume.
The main exhaust gas would be harmless water vapour, benefiting the environment.
The hydrogen could be produced sustainably via the electrolysis of plentiful salt water using renewable energy sources.
Disadvantages
Hydrogen can only be liquified at extremely low temperatures, and thus it must be insulated from all heat sources, which poses a challenge.
Metals used as storage containers for liquid hydrogen quickly become brittle due to the low temperatures, and are prone to shattering.
Even when liquid, hydrogen can escape through minute pores in its container.
Liquid fuels and oxidisers (such as our liquid hydrogen and oxygen) require expensive valves, seals, and pumps
Thermal nuclear engines
Thermal nuclear engines expel super-heated exhaust gases just like traditional chemical rockets, but they heat them in a different way. Rather than combusting the fuel with an oxidiser, an onboard nuclear fission reactor, generally using uranium compounds, heats the propellant (usually liquid hydrogen), causing it to expand as a gas and accelerate out of the engine nozzle, thrusting the rocket forward due to Newton’s third law of motion, as described in the liquid hydrogen combustion section.
The heat is created by the nuclear reactor in the following manner: An initial neutron (from a source such as Californium 252), or uranium bullet strikes an atom of the nuclear fuel (usually uranium 235). This briefly turns the atom into an energetic uranium 236 atom, which then splits into lighter elements, also releasing gamma rays, vast amounts of heat energy, and several neutrons which go and repeat the process again, causing a chain reaction, heating the rocket propellant to extremely high temperatures. The reactor itself is cooled by the propellant as it transfers heat to it.
Nuclear rockets would be powered by the same system nuclear fission reactors use.
Advantages
This is true because the exhaust (hydrogen) is far lighter than the combustion engine’s exhaust (water vapour), so it can be ejected at a higher velocity, which is proportional to the specific impulse.
Thermal nuclear systems decrease the travelling times of deep space missions, potentially reducing travel times to Mars by 25%
As described above, the hydrogen could be produced sustainably via the electrolysis of salt water using renewable energy sources.
Disadvantages
There has not actually ever been a nuclear thermal rocket launch, meaning that there is still some uncertainty on how the rocket would perform
Nuclear thermal rocket engines have not been designed to produce the necessary thrust for launch - they would be used in space, and would have to be used in conjunction with another system such as hydrogen combustion to leave earth.
The problems of storing liquid hydrogen (as described in the hydrogen combustion section) still apply.
If the engine is activated whilst still on earth, the release of hydrogen gas into the atmosphere could have negative environmental effects, including the destruction of the ozone.
Ion thrusters
By this point you might be a little fed up with hydrogen - but do not fret, as the next propulsion system on our list is the ion thruster. Instead of ejecting combustion gases to generate thrust, ion thrusters, as their name suggests, use electricity to expel ions. There are many different types of ion thrusters, including hall-effect thrusters, VASIMR, and field-emission electric propulsion, but the most common variant is the gridded electrostatic thruster. Here’s how that works:
The walls of the ion engine (excluding the nozzle wall) are positively charged - they are the anode. The negatively charged electrode (called the hollow cathode) is a rod situated within the engine. Due to the hollow cathode effect, which allows electrical conduction in the thruster at a relatively low voltage, the voltage between the anode and hollow cathode is great enough that electrons flow from the cathode into the chamber (which is filled with an inert gas such as Xenon), towards the anode. Effectively, the hollow cathode acts as an electron gun.
A 3D rendering of a satellite using an ion thruster
While an energetic electron is travelling towards the anode, it may collide with a neutral xenon atom, knocking off one of its electrons, so that there are now two free electrons, and one positively charged xenon ion. This collection of electrons and ions is plasma, and it is the source of the Iron man reminiscent blue glow emanating from ion thrusters. The electrons go and repeat this process several times on their way to the anode, producing numerous positive xenon ions. These ions are very energetic, and as they move around in the plasma, some reach the nozzle wall. The nozzle wall is not merely a positive anode like the other engine walls, instead it consists of 2 parallel grids - one is charged very positively, whilst the other is charged very negatively. The strong electrostatic forces between these grids accelerate the ions to extremely high velocities, ejecting them out of the ions thruster, and just like normal rockets, this creates thrust due to Newton’s third law of motion.
To prevent the spacecraft becoming negatively charged due to the electrons left behind, causing the ejected positive ions to return to it and cancel out the thrust generated, another hollow cathode is placed on the outside of the thruster, moving electrons from the spacecraft to the ions, neutralising them.
Ion thrusters are generally powered sustainably by solar panel arrays, but could also use nuclear electricity generators for some deep space missions.
Advantages
The main benefit of ion thrusters is their efficiency - the velocity of the ions (and thus the engine’s specific impulse) is so high that they can be 10 times more fuel efficient than chemical rockets.
Rockets powered by ion thrusters can also be extremely fast, potentially reaching speeds of over 200,000 mph in contrast to existing space shuttles reaching speeds of 18,000 mph
Their efficiency also means that they can travel great distances on a comparatively small amount of fuel, making them great for long distance deep space missions.
Disadvantages
The biggest disadvantage of ion thrusters is their low thrust (low acceleration). This means that they cannot be used for launch from earth, but only for travel in space, and so would have to be used in conjunction with other engines.
Ion thrusters would have to be used after the rocket has left earth’s atmosphere
Electromagnetic railguns
Now, you might be thinking “did you just say gun?”, but electromagnetic railguns have been seriously considered as a space launch mechanism.
Railguns consist of 3 parts. The power supply, the rails, and the armature (the projectile).
There are two parallel metal rails - one charged very positively, and the other very negatively by the power source. The armature is a conductive rod connecting the two rails, and thus current flows through it between the two rails. If current flows through a substance, a magnetic field is created around it, and so magnetic fields are created around both the positive and negative rails. The field around the positive rail has force lines in a counterclockwise circle, and the field around the negative rail has force lines in a clockwise circle. Therefore, the net direction of the magnetic fields is vertical.
The armature itself has current running through it, and thus it is subject to the Lorentz force - a force perpendicular to both the direction of the aforementioned magnetic field, and the current through the armature. This Lorentz force propels the armature (which contains the spacecraft) along and off the rails, towards space.
A step by step diagram of a projectile leaving a railgun
Advantages
Disadvantages
The spacecraft would be launched at full speed near sea level, meaning that the vast majority of its velocity would be lost to atmospheric drag, so it would be unable to escape earth.
The system would also be unsuitable for manned missions, as the extreme G-forces involved would inflict severe damage on humans
Developing a system which can launch such a projectile at such high speeds would be extremely technologically challenging
Significant amounts of heat would be generated due to drag, which could damage the craft and its components.
After the projectile has been launched, course alterations which may be required would be impossible to achieve without an on-board propulsion system
Although such a system may be unsuitable for earth to space launches, it may be suitable for launches from other planets with no atmosphere and weaker gravities (and therefore lower escape velocities) - indeed it has been recommended for usage on Mercury in the building of a Dyson Sphere.
Clearly, each system has its own pros and cons, and thus the future of space propulsion might not end up using just one of these solutions - perhaps one day we will see a hydrogen combustion engine used for launch, with ion thrusters or nuclear thermal engines used for travel through space. Maybe electromagnetic railguns will be used for launches from other planets, or perhaps another technology, not mentioned here, will take the fore. Regardless of what the future holds for space exploration, it is sure to be exciting.
To learn more about sustainable technologies, whether they are on a domestic scale, industry wide, or even global and beyond, you can check out other articles on this blog.
In fact, why don't you stay up to date by subscribing for FREE!
Free exclusive articles coming to subscribers soon!
Comments
Post a Comment