[Feature: Questioning Japan's Space Strategy] The Future Possibilities of Space Exploration Through the Evolution of Rocket Engines

2019/03/05

To put it simply, the possibilities of space exploration depend on "energy density." As density increases, we can move to the next stage. In this article, I would like to consider the keys to opening up new space exploration in the future, particularly from the perspective of rocket engines.

Generally speaking, when people think of space exploration, they likely imagine "Voyager" or "Hayabusa." Of course, these are representative space probes of the present day. However, if you were to ask a scientist, "What if you could explore freely?" they would likely answer that they want to explore various stars within the Milky Way galaxy, the neighboring Andromeda galaxy, and even as far as 10 billion light-years away. The outer edges of the solar system and asteroids are merely explorations that are "currently feasible."

So, how far can we reach when assuming the near future? My own answer would be, "Whether we can reach Alpha Centauri using near-future technology (about 100 years from now)." Conversely, I believe that is exactly the place we should aim for as "future exploration."

Alpha Centauri, which means the first star of the constellation Centaurus, is the closest star to the Sun in the entire sky, located just 38 trillion kilometers away. While it is a literal neighbor in the vast universe, what does it mean to say we might only "reach it" even with technology 100 years from now? This is simply because the scale is too different.

First, let's look at the scale of the universe. Let's place the Sun at the location of the "Kikyujizo" (commonly known as the Silver Ball) at Hiyoshi Station on the Tokyu Toyoko Line, and place the Earth just before crossing the pedestrian crossing toward the Hiyoshi Campus. At this time, Venus is inside the station building, Mars is near where you cross the pedestrian crossing, and Jupiter is near the fork in the road leading from the tree-lined path into the campus. Saturn is at the campus edge near Fujiyama Memorial Hall (Fujiyama Kinenkan) or Building 6, Uranus is at the entrance of the Yagami Campus, and Neptune is at a point 300 meters before Motosumiyoshi Station (note that on this scale, the Sun is a grain of sand 0.3 millimeters in diameter). Voyager 1, the man-made object that has reached the farthest in human history, is currently just before Den-en-chofu Station. Now, if we ask where Alpha Centauri is located on this scale, it would be Berlin or Los Angeles. It is simply far.

To explore this vast space, "speed" is inevitably required. The key is how to increase speed or accelerate. However, acceleration in space is extremely troublesome. Although we don't usually think about it, on the ground, whether walking or driving a car, we increase "speed" by pushing against the ground. Force is required to accelerate, and to obtain force, it is necessary to push something. However, in outer space, there is nothing around. For this reason, you must bring something to push with you and accelerate by throwing that object outward. This method of acceleration is called rocket propulsion, and a rocket engine is a device for that purpose. The flying objects we usually call "rockets" are abbreviations for "launch vehicles using rocket propulsion." In other words, a rocket does not refer to that characteristic shape, but to its method of propulsion.

And when throwing an object, there is a choice. That is whether to throw a heavy object slowly or a light object quickly. If the value of the two multiplied together is the same, the resulting force—that is, the acceleration—is the same. And since there are no places to replenish objects in space like a gas station, we want to save the objects we throw as much as possible and cover the difference with "throwing speed." In a rocket engine, "throwing speed" is fuel efficiency itself and is the most important indicator.

Another characteristic of rocket propulsion is that you become lighter every time you throw an object. If the same force is applied, the lighter the vehicle, the greater the acceleration obtained. Therefore, even if you are accelerating by throwing objects in the same way, the amount of acceleration increases the more you throw. This is not an advantage. It means that if you load a lot of "objects to throw" to increase the amount of acceleration, the acceleration will decrease by the amount of the increased weight.

The equation that relates this "throwing speed of an object" and "acceleration" is the most famous "rocket equation" in space engineering.

For example, consider a "rocket" with a total weight of 1 ton, consisting of a 100-kilogram vehicle body including yourself, loaded with 900 kilograms of balls. If you continue to throw all the balls at a speed of 50 km/h, the final speed obtained is 115 km/h (ignoring friction). Even if you increase the amount of balls to 9,900 kilograms to accelerate further, the final speed will only be 230 km/h. On the other hand, if you double the throwing speed to 100 km/h, the reached speed becomes 230 km/h even with the original assumption of 1 ton total. You can see the importance of "throwing speed."

However, to increase the "throwing speed," energy is required accordingly. If you think of a pitching machine, you will understand that to eject a ball at double the speed, high power is needed to run a powerful motor. And while a ground-based device can take as much power as it wants from an outlet, that is not the case for a probe. Since it is flying through outer space, the energy to be increased must itself be transported.

So, how should we transport energy? For those who have studied physics, please recall that energy exists in various forms such as electrical, chemical, light, wind, hydraulic, nuclear, kinetic, potential, and thermal, and they can be converted into each other with some loss. However, when it comes to forms suitable for long-term transport, the options are surprisingly few. The realistic options are electrical energy and chemical energy, with batteries and combustion being the specific methods, respectively.

Let's compare these two methods. From the perspective of a probe, the amount of energy per kilogram (called energy density) is important. Regarding batteries, their performance is improving year by year, but the energy of current lithium-ion batteries is about 800 kilojoules per kilogram (kilojoule is a unit of energy).

Next, regarding combustion, let's consider the combustion of hydrogen and oxygen as a representative example in rocket engines. Burning 0.1 kilograms of hydrogen and 0.9 kilograms of oxygen yields about 13,000 kilojoules of energy (note that unlike normal combustion which uses oxygen in the air, oxygen must also be carried). Even considering that tanks are necessary for storing hydrogen and oxygen, as a device, it is about 10,000 kilojoules per kilogram. This is more than 10 times higher than batteries. This overwhelming difference in energy density is the reason why gasoline cars have been used more than electric cars until now.

When viewed as a rocket engine, combustion has another advantage. That is the fact that the "object to throw" and the "energy source" are the same. If you choose a battery as the energy source, for example, you would need both a 1-kilogram battery and a 1-kilogram object. However, in combustion, you can just throw the hydrogen/oxygen itself using the energy extracted from 1 kilogram of hydrogen/oxygen. This is very convenient for a rocket engine that needs both objects and energy. Combined with this characteristic, combustion—that is, chemical energy—has been used for many years for both launch rocket engines and space propulsion rocket engines.

Solar cells are another familiar energy source. This is a device that converts solar energy into electrical energy, and it is slightly different from batteries that perform storage or combustion. However, if we consider "energy density" while also taking time into account, it is possible to treat them on the same level. On Earth, about 1.3 kilojoules of solar energy falls per square meter per second, and converting this to electricity yields about 0.4 kilojoules. Also, the weight of a one-square-meter space solar cell is about 10 kilograms. In other words, it is an energy density of 0.04 kilojoules per kilogram per second. However, this is a story about "one second," and if we assume continuous power generation for one year, the energy per kilogram per year exceeds 1.2 million kilojoules. This is about 100 times the energy density of combustion.

The device that applied this combination of solar cells and long-term power generation to a rocket engine is the ion engine, which became famous with "Hayabusa." If 100 times more energy than combustion can be used, the throwing speed of an object can be increased by 10 times (note that the relationship between energy and speed is not proportional). Overwhelmingly efficient acceleration is possible.

However, this ion engine also has two major drawbacks. One is that the amount of power generation decreases as it moves away from the Sun. It becomes 1/25th at Jupiter, 1/100th at Saturn, and 1/900th by the time it reaches Neptune. Currently, the region where significant acceleration can be achieved using an ion engine is only the area inside Jupiter. The other drawback is that there are cases where long-term power generation is not permitted, such as during a launch from the ground. In a situation where you are decelerated by Earth's gravity, acceleration is needed in a short time, and it is out of the question to take a year to acquire energy. In this sense, there are limits to where solar power generation + ion engines can be used.

Let's return to the speed of the probe. Voyager 1 reached Jupiter two years after launch and Saturn three years after. Its current speed is 17 kilometers per second, boasting the fastest speed for a man-made object leaving the solar system. However, even at this speed, it would take about 80,000 years to reach Alpha Centauri. Some reduction could be achieved by strengthening launch rockets or using ion engines. However, even if you increase the objects to throw by 10 times (meaning using a 10 times larger launch rocket), the speed only increases by a little over 2 times. Since power generation decreases the further you go, the limit would be about 2 times the speed even if solar-powered ion engines were deployed. Even with heavy use of swing-bys, an arrival time of 10,000 years is the best we can hope for. On the other hand, from a human scale, we absolutely need to shorten the time by another factor of 10—in other words, increase the speed by 10 times. For this, the energy density needs to be increased by 100 times. To realize this, nuclear energy will become essential as an energy source that is neither electrical nor chemical.

The use of nuclear energy in space is not a story of the future, but a story of the present or past tense. To begin with, for probes visiting beyond Jupiter, the fact that energy from solar power generation becomes extremely small is a major problem even before the perspective of speed. A probe is meaningless unless it communicates with Earth, and attitude control and temperature adjustment are also essential; electricity is required for all maintenance of the probe's basic functions. However, solar light at Neptune, which is 1/900th of that at Earth, cannot cover these. For this reason, "radioisotope thermoelectric generators" (RTGs) have been used for all explorations beyond Jupiter, with the exception of the latest Jupiter probe, Juno. This is a device that generates power using nuclear energy regardless of solar distance.

The basic configuration of an RTG consists of radioactive material and thermoelectric conversion elements. Certain substances have the property where the nuclei inside the atoms split spontaneously (nuclear fission), and a vast amount of energy is released during this splitting. That substance is called radioactive material, and such a reaction is called radioactive decay. The radiated energy hits the surrounding walls and is converted into thermal energy. The device that converts this heat into electricity is a thermoelectric conversion element. The "thermocouple" frequently used in physical and chemical experiments utilizes the same element (phenomenon) for temperature measurement. Radioactive decay occurs regardless of the distance from the Sun and can be used at Neptune and even beyond. The radioactive decay reaction also gradually decreases, but in the case of a typical RTG, the period until the output is halved is as long as 88 years.

On the other hand, the difficulty with RTGs is their handling difficulty and poor efficiency. A representative radioactive material used in RTGs is plutonium. From one kilogram of this plutonium, 0.5 kilojoules of energy is released per second. This is more than 10 times the value of solar cells, but in the process of converting this to electricity, 95% of it is wasted. Furthermore, when viewed as a device including the container for storing plutonium and the members that receive heat, the electrical energy extracted from one kilogram of an RTG per second is about 0.005 kilojoules. It has dropped to 1/10th compared to solar cells.

Until now, RTGs have been used as the one and only energy source beyond Saturn. However, these are for power to maintain the probe's functions, not for energy used for acceleration. If nuclear energy can be used for the acceleration of the probe, a new world of space exploration will open up. Calculating the total amount of energy obtained from one kilogram of plutonium yields about 2 billion kilojoules, which is three orders of magnitude larger than solar cells (for one year). To date, humanity has experience performing acceleration of 10 kilometers per second through the combination of solar power generation + ion engines, but if the above amount of nuclear energy is applied directly to this example, acceleration of several hundred kilometers per second is possible. If so, it might take less than 1,000 years to reach Alpha Centauri.

However, there are two problems with this calculation. One is the low efficiency of converting heat to electricity at about 5%, and the other is time. If one were to try to extract plutonium's nuclear energy through radioactive decay, it would take nearly 300 years to extract 90% of it. We cannot afford to take 300 years just for the acceleration of the probe. A method is needed to convert nuclear energy into electrical energy more efficiently and in a shorter time.

Methods for this have already been realized on the ground. Instead of radioactive decay, chain reactions are used. Radioactive decay utilized nuclear fission that occurs accidentally, but it is also possible to cause reactions in a chain by using one nuclear fission as a trigger to cause another. By utilizing this, a large amount of nuclear fission can be generated. Furthermore, this chain reaction can be controlled by the number of atoms present in the surroundings and the surrounding conditions (reflection and absorption). A device that generates a nuclear fission reaction at just the right speed in that way is a nuclear reactor, which is the method used in nuclear power plants. In nuclear power plants, the conversion of energy from heat to electricity is performed by spinning turbines, just like in thermal power generation. The efficiency at this time is about 30%, which is larger than thermoelectric conversion elements. In other words, a nuclear reactor is a device with the potential to use nuclear energy more efficiently and in a shorter time than an RTG.

Actually, much research on space nuclear reactors was conducted in the United States and the Soviet Union in the 1960s, and there is even experience using them in space (a different type from ground-based nuclear power). However, no ongoing programs exist. The reasons for this are likely the risk of ground launch of nuclear fuel, the significant improvement in solar power performance, and the lack of necessity for high power in deep space.

However, the realm of human space exploration is expanding, and improving energy density is indispensable for further expansion. As energy, there are also words like nuclear fusion and antimatter, but the essential difference between them, including chemical, electrical, and nuclear, is energy density. And the next high-density energy source that humanity can manipulate will likely be nuclear power (nuclear fission). For this, lightweight and high-efficiency space reactor technology and the establishment of safe launch methods are essential. In acceleration in space, lightness is life, which is very different from ground-based nuclear power generation where weight is not a concern. Also, in space filled with radiation, there is no need to block radiation from the reactor in all directions, and the concepts of decommissioning and accident handling are also different. While based on knowledge on the ground, a design suitable for space will be necessary. The problem of launch is a difficult task, but there are tailwinds such as launch rocket technology and improved reliability. Also, risk diversification by dividing into small portions and methods such as enrichment in space would be effective.

Looking at this background, Japan's proud high-reliability launch rockets and the nuclear utilization and failure experiences cultivated so far can be said to be great weapons. Japan's space budget is extremely small. It is 1/10th of the United States and a fraction of China's, and when considering future exploration, a power struggle is out of the question. In recent years, space-related ventures have been active, and there are several promising companies in Japan. However, this is also two or three orders of magnitude fewer than in the US or China. Since the essence of venture company survival is probability, a power struggle is also questionable here. When considering Japan's winning strategy, rather than moving forward solely with a spirit of challenge like new US rocket companies, it would be a direction of steadily moving pieces like the H-IIA rocket (in this regard, "Hayabusa," which was a mass of challenges, is quite an exception). Perhaps the practical application of space nuclear reactors, which requires caution, is suited for Japan.

However, in reality, in the use of nuclear power, social issues appear more difficult than technical issues. It is a fact that various problems came to light during 3/11, but subsequent discussions surrounding nuclear power have been preceded only by emotionalism and extreme arguments. As long as unscientific discussions like "absolute safety" are rampant, true utilization will be a dream within a dream. Scientifically grasping risks probabilistically and weighing merits and demerits. This is the basis for a decision on any technology. Its adoption is political, but scientific and technical studies conducted in parallel with that should not be hindered. This is not a problem that can be solved overnight, and the only way is for scientists and engineers to continue sincere dialogue.

In this article, I have described "future space exploration" focusing on rocket engines, but this is only a very small part of space exploration. Various functions are required of a probe for space exploration, and while the rocket engine may be the core, it is only one of them. Space engineering is called integrated engineering in academic classification. This refers to a discipline that uses various academic fields to achieve a single goal. Therefore, it can be said that the academic fields and knowledge necessary for space exploration inevitably span all fields. If you are interested in the full picture of space exploration, please refer to my book, "How Far Can We Go in Space? The Capability and Future of Rocket Engines" (Chuko Shinsho).