Can Nuclear Propulsion Take Us to Mars?

In 1965, a young Caltech graduate student named Gary Flaandro was reading his notes in Building 180 of the Jet Propulsion Laboratory, studying the intricacies of using the gravitational fields of planets to launch spacecraft to the far reaches of space without need for additional propellant. gravity assist a method of

propulsion

that uses the gravity of a gravitational body to literally pull the spacecraft toward a new trajectory and speed while examining possible trajectories. Gary noticed something incredible in the late 70s: a window would open where the planet's alignment would open. would allow a single satellite to make its way between Jupiter, Saturn, Uranus and Neptune before eventually being spit out to the outer reaches of our solar system and beyond.

With this knowledge, he began a flurry of activity within NASA and JPL, a once-in-a-lifetime opportunity. It had been discovered that this would not happen again for another 175 years and it was imperative that the opportunity not be wasted. Planning began and in 1977 two Voyager 1 and 2 satellites would launch a plate using a three-stage Titan 3E rocket. Voyager 2 would use a gravitational assist with Jupiter, Saturn, and Uranus before racing past Neptune before it finally exited the solar system, while Voyager 1 completed a more direct path past Jupiter and Saturn before venturing into deep space away. from Earth faster than any other spacecraft.

Voyager 1 is over. 22.8 billion kilometers from Earth, so far away that Voyager 1 is now outside the influence of the sun's constant stream of solar wind, meaning it is now interstellar space that will officially arrive in the region in August 2012. We, as a species, have now made our mark. in interstellar space after a journey of 35 years, an extremely long time on the scale of human life and that was with the help of multiple gravitational assists while we can build and deploy these technical marvels to investigate other worlds, sending us into space is another thing. completely and using the help of gravity to visit our closest neighbors makes little sense to date.

The furthest humans have ventured into space is the dark side of the Moon, just 400,000 kilometers away, if our ambitions to create colonies on Maris are to be realized. We will need a spacecraft that is faster and more efficient than those currently at our disposal. Maris is on average about 64 million kilometers away. The fastest and most efficient method we have to get to Mars using the Homan transfer method with Windows launching every 26 months. It

take

s about nine months to complete. Faster transfer times are possible, but engineers are stuck in a dead end. exit fuel is needed not only to accelerate the spacecraft but also to decelerate it there are no disc brakes in space the more we accelerate, the more we also have to decelerate to carry all that extra propellant.

Engineers will be forced to sacrifice forms of payload, reducing the space available for food, water and other vital supplies for a crude mission to Mars. We can

take

solace in the fact that faster transfer times will reduce supplies needed and reduce the cruiser's exposure to the high radiation of space, but what if we could achieve faster transfer times without sacrificing payload? What if we could achieve faster transfer times with even more payload? Let's first examine current technology to see where things could be improved. The Atlas V rocket that brought Perseverance to Mars used chemical combustion to propel itself.

A method in which a fuel and an oxidizer are combined in a combustion chamber and ignited. The resulting exothermic reaction causes the combustion products to heat rapidly and expand the nozzle design. directs the expanding gas in one direction to achieve thrust by making the most of your fuel and the oxidizer is the first step in maximizing our thrust per unit weight of fuel there is a useful quantity that engineers use to describe this property of fuels and oxidizes the specific impulse that we describe in detail what this value represents in our latest video on the X-15, explaining how it represents the total energy we can extract from our thrusters per unit of waste.

In that video we used this equation where the specific impulse is defined by the thrust force divided by the fuel. flow stroke - This is an extremely important metric for our Maris transfer vehicle, the higher we can push our specific impulse the less fuel we will need to carry which frees up space for the payload or we can carry the same amount of fuel and increase our speed to reach Maris. before or even we will be able to get out of that ideal Omen transfer window so how do we improve the specific impulse? The technology with the best specific impulse currently is ion

propulsion

.

Take the n-star ion thruster aboard the now-retired Dawn spacecraft. This engine used electrical energy to propel ions and achieve specific astronomical impulses. The engine releases xenon atoms into an ionization chamber and then bombards them with high-energy electrons. The collisions produce a positive xenon atom and more electrons. These electrons are then collected by a positively charged chamber wall, while positive xenon atoms. It migrates towards the exit of the chamber which contains two grains, a positive grating called the screen grating and a negative grating called the accelerator grating. The high electrical potential between these grids causes positive ions to accelerate and shoot out of the engine at speeds of up to 40 kilometers. per second, which is much higher than what chemical combustion can provide, which has a typical exhaust velocity of approximately three to four kilometers per second, this exhaust velocity is essential to achieve higher specific impulses, if we play with the specific impulse equation, we can see why.

Specific impulse is equal to thrust force divided by fuel flow. Thrust force is equal to mass flow rate multiplied by velocity, while fuel flow rate is the weight of the fuel on Earth and therefore this value changes to mass flow rate multiplied by acceleration due to gravity. and as we see that the mass flow rates cancel out, leaving us with only the exhaust velocity divided by gravity, it's pretty obvious that to maximize specific impulse we need to maximize the exhaust velocity and ion propulsion is the best technology we have to do that right now. same. times the escape velocity, the ion engine can reach 10 times the specific impulse, which is a phenomenal increase, so why don't we use this technology for interplanetary missions?

Mass flow may not be important for specific impulse, but it is enormously important for thrust, as we saw. At this time the thrust force is equal to the mass flow rate multiplied by the velocity. Chemical combustion occurs extremely quickly; After all, it is a controlled explosion and is therefore capable of accelerating millions and millions of molecules in a very short space of time, generating huge mass. flow stroke High thrust is very important for particular maneuvers such as bird capture, where a rocket will be fired to reduce its speed enough to be captured by a planet's gravity.

The window for this slowdown may only be a few hours, where ion propulsion simply cannot provide enough confidence. in a time short enough to successfully complete the maneuver, ion propulsion simply does not have the mass flow rate necessary to achieve high thrust. It took four days for the Dawn spacecraft to change its speed by just 94 kilometers per hour to increase our ability to change speed quickly. We need to increase the mass flow rate to do this. We need to increase our input power for ion propulsion. That energy comes in the form of electricity which provides the energy to ionize our propellant and accelerate it using an ion-powered current generating electric or magnetic field.

Spacecraft use solar panels to provide that electricity. The Dawn spacecraft has panels capable of producing 10 kilowatts of power when it orbits Earth, which dropped to 1.3 kilowatts when it reached its destination in the asteroid belt three times farther from the Sun, scaling that solar power. becomes impractical very quickly NASA estimates that the Ameris transport vehicle would need at least 400 to 2,000 kilowatts of power to transport astronauts and cargo to and from Mars, so how can we power something like that? Nuclear energy is the only thing that can provide that power density. necessary to make this viable this is not a new concept in 1961 the atomic energy commission and nasa launched the

nuclear

engine for rocket vehicle applications program or nerva for short this program developed and ground tested 20 reactors before It was disbanded in 1973 due to budget constraints, but was recently revitalized when the US Congress approved $125 million in research funding for

nuclear

propulsion.

There are two main types of nuclear space propulsion: nuclear electric, which would power an ion thruster as we saw above, and nuclear thermal, which was the focus of the Nerva program, so let's start there. Nuclear thermal propulsion works by harnessing the heat created during nuclear fission to provide the energy needed to expand and accelerate a propellant through an exhaust nozzle. The nuclear reactors here operate much like a nuclear reactor here on Earthwood. where a chain reaction of neutrons colliding with uranium atoms splits them and creates more neutrons and an enormous amount of heat to capture this heat, a propellant typically liquid hydrogen is pumped through the reactor core, which will cool the reactor core and the heat will pass to the liquid. hydrogen that rapidly expands and accelerates out of the propellant nozzle at high speeds, typically around 8 kilometers per second, twice as fast as chemical combustion and therefore about twice the specific impulse in around 887 seconds .

However, it's not all sunshine and rainbows using hydrogen as a propellant. With some problems, it can attack fuel rods if they are not adequately protected with a material that is resistant to the destructive tendencies of hydrogen. Liquid hydrogen must also be stored at extremely low cryogenic temperatures, and if allowed to rise in temperature, it must be stored. ventilated to prevent an explosion and, furthermore, the tiny molecule is so small that it can slip through seemingly solid materials as it can fit between the spaces of larger molecules, making it unsuitable for long periods of storage and ideally we want a Maris transfer vehicle that can remain in orbit around Earth or Mars for extended periods waiting for the crew to arrive and begin its journey between the planets and then when it arrives the crew can sit in a separate vehicle leaving the transfer vehicle transfer stationed in orbit once again, liquid hydrogen is just difficult to use in this application, so why use it?

Because when it comes to maximizing exhaust velocities and therefore specific impulse low molecular weight exhaust products are important, let's assume for a moment that all the thermal energy we introduce into the system is converted to kinetic energy in the kinetic energy of the exhaust products is equal to half the mass times the velocity squared to find the velocity, we can rearrange this equation so that we now see that the velocity is equal to the square root of two times the energy divided by the mass, here it is clear that increasing the mass of the exhaust particles will decrease the velocity of our exhaust hydrogen is the lightest element and therefore maximizes the specific impulse if we were to use another propellant it would be extremely difficult to make a nuclear thermal propulsion spacecraft with a specific impulse high enough to justify its use.

The next lightest gas is helium, which is twice as heavy as hydrogen and will therefore reduce our specific impulse by the square root of 2, negating almost all of the advantage that nuclear thermal propulsion can provide. lighter that is not solid at temperatures than we What we need is nitrogen, which is 14 times heavier and would therefore decrease our specific impulse by the square root of 14, which is 3.7 times worse, which makes A nuclear nitrogen thermal engine is worse than a traditional combustion engine, so we cannot escapethis hydrogen storage problem. and if we hope to use nuclear thermal propulsion, we're going to need to figure out how to keep hydrogen cryogenically stored for long periods.

If this problem could be solved, the higher specific impulse and thrust could reduce our transfer times to Mars by half or potentially. Open the launch windows outside the ideal home and the transfer window so we can solve this hydrogen storage problem while using nuclear power to achieve higher specific impulses. This is where ion propulsion becomes really attractive again. A major advantage in favor of ion propulsion is its ability to use heavier and easily storable inert noble gases as propellants such as xenon or krypton. This goes against our previous understanding that low escape molecular masses are beneficial for higher escape velocities.

This is possible because we are using electrical energy to launch these atoms at tremendous speeds: the escape velocity of ions. is defined by the charge of the ion, the voltage by which it has been accelerated and the mass of the ion in the charge and the mass of the ion is defined by the choice of propellant, but we can scale that voltage to a very high level before reaching a limit in performance due to material properties or some other physical limit for nuclear combustion or thermal engines we are converting thermal energy into kinetic energy that thermal energy is difficult to scale chemical combustion is limited by the energy we can release from the chemical bonds of the propellants and by the temperature at which our engine can operate before melting, this is a problem for nuclear thermal energy 2, which has to operate with extremely high temperatures in the reactor core, of 2500 degrees Celsius, to achieve exhaust velocities high enough to justify their use.

Specialized nuclear fuel designs are needed to survive these temperatures and any higher temperature would destroy the reactor as a reference. This is an order of magnitude higher than what nuclear reactors here on Earth need to reach, which typically operate at around 300 degrees Celsius, as they are really just boiling water at high pressure. Ion thrusters do not. get closer to the operating temperatures that thermally driven motors have and we can raise that voltage high enough that the added mass of the ion barely matters; We are still achieving 10 times the specific impulse of traditional engines, we could use a lighter propellant to increase the specific impulse, of course, but the advantages of using propellants like Xenon and Krypton are so good that the drop in exhaust velocity and the specific impulse are worth it.

Being inert, they can be easily stored during the long push. The iron propulsion cycles they require make them the ideal propellant for long-duration interplanetary missions. Larger atoms like xenon also hold electrons in their electron cloud much more freely than smaller atoms like hydrogen, so it takes less energy to ionize xenon than to ionize hydrogen. This reduces the electrical energy needed for the first step of our ion propulsion process and, more importantly, greater exhaust mass improves thrust. This equation defines the thrust and the iron propulsion engine can generate where the mass of iron forms the denominator of our specific thrust equation.

It forms the numerator of our thrust equation, meaning that an increase in iron mass will increase our thrust, which is the specification that iron propulsion has the most trouble with. A valuable trade-off for using nuclear energy to generate electricity in space and power our ion engine, we will need to find a way. To cool the reactor core of the nuclear thermal engine, the propellant acts as a coolant for the nuclear electric motor, we will need a closed circuit cooling system in which we do not waste the coolant, but rather keep it in a cycle between the hot engine and a heat exchanger.

The only method we have for discharging heat overboard into space is through radiative cooling, so a nuclear electric powered spacecraft will need massive radiator fins through which this coolant can pass. This is feasible, but we have a long way to go in developing nuclear engines for space and even with this additional power, nuclear-powered ion propulsion would still be on the lower end of thrust, in all likelihood these powered engines by ions will need to be a hybrid engine that can use chemical combustion for high thrust maneuvers or if the long-term problem Hydrogen storage can be addressed with a nuclear hybrid engine which is extremely attractive where our high thrust Burns can be produced by the nuclear heat engine and then through neutron absorption control mechanisms like these rotating drums where one side is coated with a neutron reflector and the other is encoded.

In a neutron absorber, by simply rotating these drums, the engine temperature could be reduced and changed to a closed-loop cooling system that could power our electrical generator and provide an extremely high specific impulse and a gradual increase in speed that could reduce drastically our travel times. Mars or perhaps allow humans to venture further into our solar system and begin our gradual exploration and settlement in the cosmic neighborhood. This is an incredibly complex topic with many complicated and nuanced ideas that I struggled to understand until I found the right equations. understanding how the thrust would require a high mass flow rate yes, the specific impulse is much higher when the molecular weight of the exhaust is low, all the Eureka moments come from dimensional analysis of the equations and their derivations, being able to understand the language of the universe is a vital tool in deciphering the world of physics, which is why shiny is a great partner for these videos. shiny has selected two learning paths that would be perfect for both beginners and experienced learners.

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