Because of this speech, thunderous applause broke out in the conference room full of engineers from the National Aeronautics and Space Administration (NASA). Everyone understands this message. In short, develop nuclear-powered rockets and send humans to Mars!
In fact, this is not sudden: as early as a few years ago, NASA reports and research and exhibitions at conferences were full of hints about nuclear energy.
Jim Breeden, director of NASA, often talks about this topic and never hides his excitement. Moreover, the US Congress has also begun to provide funds for the research and development of this technology: in 2020 alone, the funding will be no less than $6,543.8+$25 million. What is certain is that more than a dozen laboratories and enterprises are quietly engaged in related research.
"There is no doubt that the United States hopes to develop a prototype of a nuclear-powered rocket within a few years." Stefan auriol, head of liquid propulsion research at CNES, said.
Atomic launch vehicle? Please note that this is not a small isotope generator with a power of about 1 1,000 watts used by probes such as Voyager, Cassini-Huygens, Curiosity and Mars 2020-those devices can only get weak energy from the heat generated by the decay of plutonium -238.
This time, NASA explicitly considered using a real nuclear reactor, so that the uranium -235 fuel block in it would fission and produce a chain reaction, and output about 500 million watts of heat energy! Then liquid hydrogen vaporizes and expands in such a high temperature environment (the temperature is close to 3000℃), and then it is quickly ejected, and the reaction forms thrust.
Perhaps in people's eyes, the use of a technology that has been opposed on a large scale on a space launch vehicle is more like the scene of a poor science fiction movie, or another provocation by the US government. After all, everyone knows the possible risks of this radioactive device.
However, for the designers of long-distance manned missions in the coming decades, the temptation of this criticized technology is huge, even irresistible. All the calculations are consistent. The thrust generated by a nuclear reactor is as high as 654.38+ million Newton, which is twice as efficient as the current chemical propulsion, that is, the propulsion by hydroxide explosion.
In such an industry that pursues light weight and high speed, the advantages of nuclear power are clear at a glance: the same thrust, half fuel consumption, but the final speed is twice as fast.
In fact, nuclear energy is a highly concentrated energy source: 1 kg uranium -235 is equivalent to burning 2.7 million tons of coal. Using nuclear fission technology, it is not necessary to carry dozens of tons of oxygen for combustion, but only to carry the lightest element hydrogen in the universe and then eject it.
"It is much easier to accelerate the hydrogen at the outlet of the pipeline than to accelerate the combustion of a chemical engine to produce a large number of water molecules, so the former is more efficient." Stefan ohrel pointed out.
Other propulsion methods are also hard to match. It is true that the efficiency of electric propulsion of ion engines is higher than that of nuclear power, but "the thrust generated by ion engines is very weak, only a fraction of Newton's, so it takes a long time to reach the speed required for long-distance missions, which is not suitable for manned missions", emphasized William Emrich, a propulsion engineer at the Marshall Space Flight Center (MSFC) of NASA. Similarly, solar sails can't work.
Light weight, high efficiency and large thrust, using atomic energy can greatly reduce the difficulty of manned trip to Mars. At present, for the flight mission based on chemical propulsion design, material transportation is a nightmare and an impossible challenge;
Engineers can only send up to 500 tons of materials and fuel into earth orbit to maintain the integrity of the spacecraft; They also have to consider the orientation of the planet, that is, a short window period (lasting 3 or 4 weeks) that only appears once every 26 months-astronauts may be trapped on Mars for nearly 1.5 years in order to wait for the best orientation; In addition, it will take a long time to go to Mars-at least six months, which will make astronauts suffering from microgravity, cosmic rays and extreme loneliness face serious physical and psychological problems.
"Chemical propulsion can send humans to earth orbit or the moon or even Mars, but it is unrealistic to go back and forth between the earth and Mars." William Enrich concluded.
The use of nuclear energy may reduce the weight of spacecraft fuel and achieve faster speed. According to the preliminary simulation of NASA, compared with chemical propulsion, the travel time of the Earth to and from Mars can be shortened by 20% to 50%, which makes the launch more flexible. "Even if the relative position between the two planets is not so ideal, engineers can launch, which expands the launch window, and they can also explore routes that cannot be achieved when using chemical propulsion and electric propulsion," Stefan Aurel pointed out.
In view of this, NASA plans to use atomic energy to launch a mission during the period of "Mars rushing to the sun" (Mars and the earth are in a straight line on the same side of the sun) to prevent astronauts from staying on Mars for too long. This move may shorten the total duration of the mission from more than 900 days to 500 days, thus leaving enough time for Mars exploration and reducing the risks faced by astronauts.
More importantly, if people encounter serious problems with the nuclear engine, they have the opportunity to terminate the mission immediately and return to Earth within three months after the flight begins, or even just arrived at Mars. If a chemical propeller is used, once it enters the ground fire transfer orbit, the rocket can only return to Earth within a window period of several hours or at most a few days. The importance of these safety guarantees to engineers is self-evident.
Nuclear propulsion technology still holds the last trump card: it is actually an "old friend" of NASA engineers. In fact, from 1955 to 1973, this institution has compiled a complete research plan on this issue.
Related teams have built and tested more than 20 reactors, mastered the shortcomings of various materials, and cleared many obstacles for research in this field. They even conducted in-depth tests on several prototypes in the Nevada desert.
Later, the project was abandoned, but not for technical reasons, but the United States began to focus on the research of the space shuttle.
Now, "we can use the data collected by this project, which is really a gold mine", said Dale Thomas, a professor of systems engineering at the University of Alabama. However, launch safety, material strength and avoiding radiation damage are all problems that need to be solved. In addition, like other nuclear reactors, possible temperature drop and runaway reactions also need attention. Of course, in theory, nothing is insurmountable.
In the engineer's office, the sketch of the space reactor has been put back on the table. In addition to NASA, the Defense Advanced Research Projects Agency (DARPA) recently launched a nuclear propulsion project to better control the Earth-Moon space and even fight here. The prototype is expected to be released in 2025. "Russia and China are also studying this technology," Stefan ohrel pointed out.
Direct thrust is not the only option: some spacecraft may be able to power ion thrusters by carrying nuclear reactors. This hybrid technology started slowly, but it is extremely efficient, and it can send a lot of goods to Mars or the moon at a very low cost. It can also easily send more than ten tons of scientific and technological materials to satellites of Jupiter or Saturn, or participate in the development of space resources.
"In fact, more than 90% of the space in the solar system cannot be reached by chemical propulsion within a reasonable time range, and as the spacecraft gradually moves away from the sun, the intensity of radiation will become weaker and weaker, so it is impossible to use solar cell technology to reach these places." Stefan ohrel analyzed, "There are endless topics related to the territorialization of planets, planets and asteroids. Solving the transportation of materials outside the Earth and even Mars has become a real strategic issue. Nuclear energy seems to be a necessary means to reach the depths of the solar system." Even now, nuclear energy is regarded as an indispensable source of power for Mars or the moon base.
People will finally understand that today's aerospace engineers have been conquered by the charm of nuclear energy. From the point of view of pure technology, the significance of adopting this scheme is obvious.
In fact, there is no problem at the legal level: the Outer Space Treaty only prohibits sending nuclear weapons into orbit or other celestial bodies. However, how can we convince people that the research can proceed smoothly? How can we ignore the great fear of the people around the launch site and under the flight path of nuclear-powered spacecraft? Many arguments support the use of nuclear energy, but the radioactive launch method is indeed a very sensitive issue, admitted Francis Rocar, head of CNES solar system exploration project.
In the field of aviation engineering, from1970s to1980s, the nuclear reactors supplying power to Soviet satellites left bitter memories for the world. 1978 65438+1On October 24th, the nuclear-powered satellite Kosmos954 crashed in northern Canada. In order to eliminate the pollution caused by it, people have taken many remedial measures. In addition, before the launch of 1997, the Cassini-Huygens probe with a small plutonium generator also met with many protests.
In view of the above concerns, NASA plans not to ignite the reactor during launch, but to ignite it after the spacecraft reaches a stable orbit high enough (above altitude 1000 km) to protect people. 1992, the United nations also voted to pass a resolution prohibiting the spacecraft from carrying out any form of nuclear fission during take-off.
In addition, NASA will not let radioactive rockets return to Earth: the propulsion module will enter the "graveyard" orbit, and astronauts will use the cockpit to return to the ground, just like returning from the International Space Station.
During the ground test, various preventive measures will also be put in place. It's not the age of 1960, when people ruthlessly discharged hydrogen that might be contaminated by radioactive fuel particles into the atmosphere. Engineers of NASA are developing a toxic dust recovery device and considering using it on NASA's A-3 test bench in Mississippi.
In order to quell the controversy, "NASA has also considered conducting research based on the computer simulation results of reactors, just like nuclear tests," Dell Thomas further explained. In addition, the designer of the first unmanned flight test also focused on route planning to avoid the spacecraft colliding with the earth later.
So, are these precautions enough? "Nuclear technology has opened up new areas of flight, but I think no matter what preventive measures are taken, people can't accept it." Stefani Lizy-Doutreix, an associate professor of aerospace systems engineering at the French National Institute of Advanced Aeronautics and Astronautics (ISAE), said frankly, "When I talked with the head of the Guiana Space Center, they said it was still a taboo because they didn't want to trigger a parade around the launch site."
Whether in the laboratory or in the public's cognition, the fate of space atomic technology may usher in significant progress. "The critical moment of sending people and materials to Mars is coming," Francis Rocard stressed. "To achieve the first manned flight over Mars without landing, Americans should choose between nuclear propulsion and chemical propulsion as soon as possible." To this end, the National Academy of Sciences (NAS) has begun to consult experts. "I can't wait to know which technology they will finally recommend in the report." Dale Thomas muttered.
Stick to the traditional and reassuring chemical propulsion technology, or succumb to the temptation of nuclear propulsion technology? It's time to choose.
Author Vincent Nurigart
Compile Wang Juan