 A few weeks ago, a new spending bill passed by Congress in the U.S. earmarking $100 million for development of nuclear propulsion for spaceflight. This isn't the first time that nuclear propulsion for spaceflight has passed by Congress. In fact, Kennedy tried to do it right after he promised the nation a moon landing in the 1960s. So, today, I thought we'd look at how nuclear engines work compared to their chemical counterparts. The one who knows Apollo knows Kennedy's famous speech before Congress in May of 1961, in which he said that this nation should commit itself to achieving the goal before this decade is out of landing a man on the moon and returning him safely to the Earth. What we don't often talk about is what he said directly after that. See, the moon landing was first. Second, he said, an additional $23 million, together with $7 million already available, will accelerate development of the rover nuclear rocket. This gives promise of Sunday providing a means for even more exciting and ambitious integration of space, perhaps beyond the moon, perhaps the very end of the solar system itself. It's not surprising that Kennedy asked Congress to fund nuclear propulsion as well as the moon landing. After all, this whole push to space was an iteration of the Cold War. A Cold War started because America and the Soviet Union each believed the other had better nuclear technology. So, why not combine both? Take on space with the nuclear reaction we're working on for bombs, right? In 1961, the rockets NASA was using were chemical rockets. This is, of course, the redstone and the atlas rockets that were launching the Mercury missions. The Saturn family, then under development, also used a chemical reaction. A chemical rocket, at the core, is really quite simple. Burning highly refined kerosene and liquid oxygen yields a powerful reaction that creates enough energy to lift a rocket off the Earth and into space. In the case of the Saturn V that took astronauts to the moon, the upper stages de-mix of liquid hydrogen and liquid oxygen with the same result. But a chemical rocket's power is limited. They don't actually produce that much energy. That's why, if you want to go to, say, Mars, you have to wait for a better planetary alignment, a so-called ideal launch window, to take advantage of a little bit of a boost from gravity. Otherwise, it takes so long to get there. Another problem with chemical rockets is that they can only carry so much fuel. At some point, the fuel becomes too heavy to get off the ground in the first place, which is why chemical rockets rarely have a sustained burn. Typically, the engine burns to get the spacecraft going, and then it coasts until the engine fires again to make an adjustment. Nuclear propulsion solves these problems. It's a lighter fuel, which means you can take more of it, and it has a more powerful output. When we're talking about rockets, the lighter the exhaust gas, the higher the exhaust velocity, and the higher exhaust velocity means higher thrust. If a nuclear engine's exhaust is hydrogen, the lightest element, its thrust will be very high compared to the exhaust from a chemical rocket at the same temperature. The chemical rocket exhaust contains heavier elements and therefore has less thrust. When we talk specifically about thrust, we talk about specific impulse as the measurement. Specific impulse means the time in seconds that one pound of propellant generates of one pound of thrust. The higher the seconds of specific impulse, the more economic and efficient the rocket. Chemical rockets in the 1960s had a specific impulse of about 300 to 450 seconds. The nuclear rocket engine using hydrogen developed as part of the NERVA program in the 1960s was shown during tests to have a specific impulse in the range of 800 to 900 seconds. That means that nuclear engine with the lighter exhaust would be twice as efficient as a chemical rocket. The nuclear engine for rocket vehicle application or NERVA program was born in 1961 as a joint program under NASA and the Atomic Energy Commission. The prime contractors were the Westinghouse Electric Corporation and Aerojet General Corporation. And this is what the NERVA engine looked like. In the heart of the engine is the reactor, a cylindrical core consisting of graphite elements impregnated with a uranium-235 fuel. Inside the core, fission of uranium atoms produces heat. Keeping that heat in place is a reflector made of beryllium, completely surrounding the core. Inside the reflector are cooling rods, also made of beryllium, but one side is coated with boron. Because fissioning uranium constantly emits neutrons that are reflecting back to the core, the heat is sustained so has to be managed. Turning the rod's boron side in absorbs the neutrons and lowers the temperature, eventually stopping the reaction. Turning the rod's beryllium side in sustains the fission reaction. Hydrogen is stored in a liquid state of minus 420 degrees Fahrenheit above the engine. That hydrogen is then pumped down to the engine nozzle by an external line, then through internal passages back up the nozzle and into the reactor where it passes through small channels in the reflector. This serves the dual purpose of cooling the engine and heating the hydrogen such that by the time it enters the core it's in a gaseous state. The hydrogen then moves through channels coated with niobium carbide to resist corrosion that run through the core from top to bottom. Passing through these channels, the hydrogen picks up heat from the fissioning uranium, heating to at 4000 degrees Fahrenheit. It passes through the exhaust nozzle and expands, producing thrust, moving the rocket. A bleed line carries some of the hydrogen back to the turbo pump to keep the engine running, making it an incredibly compact and powerful nuclear rocket engine for deep space. NASA had plans to harness the power of the NERVA engine in conjunction with the typical chemical rocket by swapping out the S4B upper stage of the Saturn V and replacing it with the NERVA. This so-called Saturn C5N variant could have increased the payload to orbit of the Saturn V from 118,000 kilograms to 155,000 kilograms to low Earth orbit. It could have supported manned missions to Mars by 1980, with a transit time of just four months instead of the eight or nine we get with optimal launch windows. Wernher von Braun even envisioned a mission that would send 12 men to Mars on two nuclear rockets, launching in November of 1981 and getting to the Red Planet in August of 1982. But, as we know, the nuclear propulsion never came to be. The site at Plumbrook that did the most work on developing nuclear reactor cores that could have been put into engines was mockballed in 1973 by the Nixon administration. People developing nuclear propulsion for spaceflight today have the same problems they did in the 1960s. You still have to deal with the insane heat made by the engine, not to mention the crew living near nuclear material. However, the insane efficiency of those engines makes it something that could be very worthwhile solving those difficult problems. If you liked this video, be sure to give it a thumbs up. And if this is your first time watching Vintage Space, welcome and consider subscribing so you never miss an episode. 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