No fast or cheap deep space missions are possible with fuel mass limited chemical rockets. Even Mars takes 8 months with the largest most cost effect SpaceX rockets. Liquid fluoride plus liquid ammonia gas makes the most powerful rocket motor fuel combination, while producing toxic corrosive nasty exhaust emission products so terrible that even the Soviet Union (USSR) abandoned development of this fuel combo, which otherwise enables a runtime or specific impulse of 500+ seconds, unmatched even by cryogenic LH2 plus LO2 or liquid hydrogen and oxygen respectively.
Nuclear fission & fusion propulsion are the only practical way to explore deep space in timeframes relevant to a living persons average lifespan. By human earthly standards, things in Outerspace are very far apart, measured in light years, the distance covered by photons going 3.0x10^8 meters per second, or AU astronomical units, where 1 light year LY equals about 63,241 AU.
The low density of even liquid hydrogen means that cryogenic liquid hydrogen fuel tanks have to be enormously large with huge volume capacity. To give the rocket motor more Isp the fuel tanks have to be larger in size, but as the tank size scales larger so does the mass of the tanks, and as the mass of the rocket increases because of its larger airframe to hold the bigger tanks, more fuel needed to propel the increased mass. This forms what is known as the fuel mass paradox or fuel tank paradox which limits the performance of chemical rockets to just several hundred, perhaps 400 seconds or slightly better in the best cases.
Nuclear fission rockets can increase Isp by 10x or 100x or 1000x depending on the core materials, shielding required, fuel or propellent blown past the core, the core physics, the fissile core neutron flux density, fuel flow performance through the reactor core elements, and other factors like the intense density & thus higher mass of fissile elements used in the fission core of nuclear rockets, the materials to clad these elements like iridium, platinum, gold, tungsten or in future models silicon nitride ceramics developed by Kyocera in Japan, which are half the mass of steel nickel cobalt vanadium superalloys that have excellent high temperature creep resistance or resistance to stretching while spinning red hot inside common commercial airline aircraft turbine engines.
Fusion of hydrogen can produce rocket engines of unmatched Isp that produces only helium 3/4 emissions. The commercial feasibility of this kind of fusion reactor not limited to competing with cheap renewables like wind & solar power on Earth or dirty cheap coal fueled thermal reactors, in aerospace, at the nation state level, fission & fusion rocket propulsion technology technologically contrivable and fission rockets were previously developed by the USA with NERVA & in the Soviet Union, while both programs were abandoned because of the high mass of such 1960's era fission core designs. Thankfully nuclear physics has improved greatly since the 1960s, just as 4th generation fission reactors can obtain core fuel burnup of actinides and other energetic dangerous transuranic isotopes that plague common Gen 2. reactor with long lived waste, that are operating by the hundreds around the world today. Safer, cleaner & more efficient, 4th Gen & 5th & 6th Gen nuclear reactors are our best energy technology as human people on earth, hands down, no debate, just empirical or factually true.
Furthermore, newer IFR & 4th Gen. breeder reactors can get single pass fuel economies of better than 94% by converting that nearly complete percentage of the fuel element fission energy into thermal flux to make steam for the power generating turbines so the reactor can export gigawatts of electricity to power grids so that customers far and wide can make use of the electricity generated by fission energy, 24/7 with great uptime and from reactors with excellent capacity factor. The improved, safer nuclear waste from 4th gen. reactors also easier, less costly and safer to reprocess or recycle into new fuel elements or mixed oxide fuel bundles (MOX). France, South Korea & China are leading 4th Gen. Reactor development today!
Thorium, which is 5x more naturally abundant in Earth surface crust layers where mining is practical, than uranium, means that if thorium mining was as industrialized as uranium, then thorium would cost 5X less than uranium, and Thorium a fertile element can be neutron flux breed in existing & upcoming nuclear reactors to become fissile as Plutonium reactor fuel. The reaction products of this fission can then be PUREX refined into new fuel rods, which along with uranium & new thorium from mining and ocean extraction, can provide reactor fuels for fission reactors to make electricity for 10+ billion people for 4000+ years, until the year 6022 or later. These same reactors can thermal electrical desalinate sea water to make lots of fresh water & similarly produce lots of renewable hydrogen or Green H2. Given that freshwater pollution a growing global problem due to all the acid rain created by fossil fuel combustion emissions mixing with clouds and rainwater, that gets into all waterbodies on Earth.
As we decarbonize energy with more renewables, energy storage & fission energy reactors, we will see less air pollution, less water pollution & have extra energy to make more fresh water for everyone, while also lowering sea levels & refreezing the poles of the Earth. With Fusion energy & ZPE we can have enough surplus energy to completely air condition Earth to our needs, controlling climate & weather effectively, to enable even better agriculture, reducing flooding and droughts, stoping hurricanes & tropical storms. We can master Earth's climate, but we need trillions of trillions of trillions of extra kilowatts of renewable clean electricity to make it happen.
When a space based nuclear fission reactor using helium as the working fluid in a low mass lightly shielded power reactor, produces electrical energy sent to HAL effect engines, gasses like Xenon Xe can be propelled to relativistic velocities to produce amazingly high Isp engines for spacecraft propulsion, for months or even years operating in the vacuum of space continually. This Nuclear Reactor Energized HAL spacecraft propulsion another way to enable deep space missions on shorter time scales.
So yes, chemical rockets are cool, as Everyday Astronaut describes in this excellent YouTube video
https://youtu.be/Owji-ukVt9Ms
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