The Secret Behind Deep Space and Interstellar Travel
The technology that can propel humans far beyond Earth
Many people know of America’s Apollo missions, the climax of the Space Race that saw Neil Armstrong and eleven other men explore the Moon over the course of a few years back in the late 1960s and early 1970s. However, most people don’t know as much about the rocket that took them there.
After the Allied Powers won the Second World War, the government of the United States gave many German physicists the chance to use their skills to help the U.S. design various missiles, and eventually rockets. One of those German scientists was a man named Wernher Von Braun.
Von Braun was the mastermind behind the famous Saturn V rocket, the one used on all of NASA’s lunar missions. It was a behemoth of a machine, with a height of 363 feet and a weight of 6.2 million pounds when fully fueled.
It worked how most rockets still work today, by using high-pressured gas to produce a force that would propel the rocket in the opposite direction. Rocket engines such as those used in the Saturn V are known as chemical engines and are used widely by companies and government agencies looking to send large or heavy payloads into space.
Chemical engines are great for the task of escaping Earth’s atmosphere, where sheer force and quick acceleration are crucial. However, once they enter the vacuum of space they’re not nearly as effective as before. This is where other types of engines come into play, engines designed specifically to be as powerful and efficient as possible in deep space.
Ion Propulsion Engines
Ion engines are by no means a new technology. The first ion engine was built in 1960, nine years before humans would land on the Moon. The first mission to use ion propulsion as its primary source of power was in 1998, one of NASA’s missions to test future spaceflight technologies and to closely observe a comet. However, it’s taken time to get ion propulsion engines to the point where they’re powerful enough to be a viable means of power for deep space missions.
How do Ion Engines Work?
Fundamentally, ion propulsion is very similar to traditional chemical engine propulsion, just at a much smaller scale. It works by ejecting ions, which are atoms with a charge (meaning they’ve either lost or gained an electron and don’t have a neutral charge anymore). Ion engines propel positively charged atoms directly behind a spacecraft, creating a miniscule amount of thrust with each one.
One of the main differences between ion engines and regular chemical combustion engines is that an ion engine can operate continously for extremely long periods of time, while the ones we use for most rockets only last a few minutes at best before all their fuel is used up. Therefore, that small amount of thrust from the ejected ions can add up over time in the vacuum of space where drag is practically nonexistent. Ions engines can operate for so long that modern ones are being designed to continuously fire for about ten years.
Diving into the specifics of the engine, a neutral gas is released into the ionization chamber. Afterwards, the atoms of that neutral gas, usually Xenon, are pelted with negatively charged electrons. When an electron collides with a Xenon atom that causes the atom to release an additional electron, leaving it positively charged.
Towards the back of the ionization chamber, there’s a positive grid which the positively charged Xenon ions are attracted to. This process accelerates them, beaming them out the engine at high speeds to create thrust. Once the positively charged ions are outside the spacecraft, they must be injected with electrons to neutralize their charge. If this didn’t occur they would immediately be reattracted to the positive grid and thus negate any thrust they originally produced.
When are Ion Engines Practical?
With today’s technology ion engines do not deliver enough immediate thrust to escape Earth’s atmosphere, and even if they did they don’t work in environments where other ions are present outside of the engine. As of now, they’re really only practical in the vacuum of space, on missions carrying probes to distant objects.
For example, NASA’s Dawn Mission took place between 2007 and 2018, using an ion propulsion system to capture images of Vesta and Ceres, two of the largest objects in the asteroid belt. It’s thruster ran for about 5.5 years and traveled about 3.5 billion miles since launch on Earth. It did run out of power slightly before engineers believed it would, but overall it was a successful mission that demonstrated ion engines can be quite effective for certain tasks in space.
Nuclear Thermal Propulsion Engines
Similar to ion propulsion, scientists and engineers have known about nuclear thermal power for decades, and NASA has already experimented with it, in partnership with the Atomic Energy Commission. From 1961 to 1973 they worked on a project called the Nuclear Engine for Rocket Vehicle Application (NERVA).
They did lots of testing and experimentation with nuclear thermal propulsion in rockets, but due to budget cuts in the early 1970s were never able to launch an actual mission with the technology. However, NASA has recently shown interest in contracting private companies to develop nuclear reactors for spacecraft.
How do Nuclear Thermal Engines Work?
Harnessing the power from a nuclear reactor is a dangerous challenge, and must be done very precisely. Starting with the basics of how nuclear reactors work here on Earth, they essentially have neutrons bombard uranium atoms. In turn, this creates a chain reaction of more neutrons and mass amounts of heat. For nuclear thermal engines in rockets, the process is very similar.
After the heat builds up from the chain reaction in the reactor, a propellant, usually liquid hydrogen, is driven through the reactor core. This cools the immense amount of heat from the nuclear reactions that were taking place, transferring that heat to the liquid hydrogen. When this occurs, the atoms of liquid hydrogen get excited, causing that hydrogen to enter a gaseous form. Finally, the accelerated hydrogen gas is funneled out of a nozzle for maximum thrust.
What are the Drawbacks to Nuclear Thermal Power?
The main disadvantage of nuclear thermal power has to do with its propellant source, liquid hydrogen. It must be kept at incredibly low temperatures at all times to be safe from a possible explosion, which could be catastrophic on a deep space mission. In addition, because hydrogen is such a small molecule it can actually make its way through surfaces that to humans would be considered completely solid.
If nuclear thermal power is to be used on long-term missions this could pose a fair share of problems, and unfortunately there’s no good alternative for liquid hydrogen as it’s by far the most efficient propellant for the operation.
What is Specific Impulse?
Essentially, specific impulse is a measure of how efficiently rocket fuel is being used.
“The thrust produced per unit rate of consumption of the propellant that is usually expressed in pounds of thrust per pound of propellant used per second and that is a measure of the efficiency of a rocket engine” — Definition of specific impulse, by the Merriam-Webster dictionary
To summarize, the more thrust generated for a certain amount of fuel, the higher the specific impulse and thus the more efficient that engine is in space.
Connecting this to ion and nuclear thermal propulsion, both have a higher specific impulse than regular chemical combustion rockets. Depending on the type of fuel used, most chemical combustion rockets have a specific impulse of around 300–500 second. Nuclear thermal power is projected to give a specific impulse of about 850–900 seconds, and ion propulsion is around 1,200 seconds.
Conclusion
So, if ion and nuclear thermal propulsion are so efficient why haven’t we already used them to send humans to the Moon and Mars? Well, like I mentioned earlier ion propulsion is incredibly slow to accelerate. For example, NASA’s Dawn spacecraft took four days to reach a speed of about 60 mph, and that’s with minimal weight to transport. It would be impractical to send a large crewed spacecraft to Mars using ion propulsion because it would take way too long to accelerate.
Considering nuclear thermal power, as stated earlier the process of storing liquid hydrogen for long periods of time requires tremendous care and can be extremely dangerous for the mission should something go wrong. As of now, traditional chemical rockets are our best bet for manned missions to distant destinations.
In the future it’s quite likely that these technologies will start to be implemented into more missions, specifically those conducted by NASA. It’s very possible that in the next decade there is a sort of hybrid power, for example chemical and ion propulsion for a large transport ship to Mars. The chemical combustion engines could provide the initial burst of acceleration, and from there the ion engines could continuously fire for the duration of the trip, with a deorbit burn by the chemical engines to prepare the spacecraft to land on Mars.
Whatever happens, ion and nuclear thermal propulsion are fascinating technologies that have incredible applications for the future of manned and unmanned spaceflight. Hundreds or thousands of years in the future they may even be a driving force in humanity’s quest to become an interstellar species.
