Magnets for Space travel - Filling the Energy Density gap
The issue of realistic long term space travel largely comes from the fact there are few good ways to get up to the power levels required to go in to the earth's orbit that are not inefficient. Bridging the energy density gap by various means, such as magnetic-kinetic storage, railgun acceleration, air-breathing rockets and other means could offer insight of how to make space travel practical and cheap.
To understand the problems of space travel, we must first understand the basic problem present with it. Space travel is largely confined by a few criteria, but the primary two are cost and complexity and the energy density of fuels. The simple problem is that the earth's gravity and atmosphere is as such to where space travel is only possible after extreme measures are taken to launch things in or out of orbit, requiring that a rocket or projectile reach over 7.8 km/s so it is capable of getting in to low earth orbit (or LEO), and up to 11 km/s to escape earth orbit; a certain velocity is needed to reach escape velocity in order to get in to space. The primary issue is energy density, as a rocket must be able to propel it's own weight as well as cargo up to certain speeds which increase the energy requirements required exponentially. There is little to bridge the gap in power between chemical and nuclear energy, or for a direct comparison the most powerful chemical available Hydrogen with 140 megajoules per kilogram, and uranium at 80 million megajoules, with the difference being several hundred thousand times the energy per kilogram. Rockets can realistically only send about .3% of their weight in to high earth orbit, and even with far more efficient rocket designs, the inherent limitation of the fuels and the existing efficiency already being over 30% means they likely will not be able to carry much higher payload than this. We are effectively stuck at 1-5% of the rocket's weight in payload, even at the theoretical hypothetical maximum of perfectly efficient rockets. With little to fill the gap between the chemical energy limit theoretically present, and nuclear power, it puts inherent limitations on the capabilities of rockets and thus space travel. There is simply very little in between these two fuel types, other than kinetic energy storage, such as velocity or heat, to bridge the gap. The rockets we are currently capable of sending in to space can only realistically send .3% to 1% of their weight in cargo, meaning that for every pound of cargo sent in to space, such as a person, you need approximately 100-300 pounds of rocket. The bulk of this weight is actually the fuel of the rocket itself, with approximately 90% of the weight of most successful rockets being the weight of the rocket fuel and liquid oxygen, including the tanks and storage systems. For example, to land a 36,000 pound moon lander on the moon, it required an approximately and 6.5 million pound rocket, which was literally hundreds of times heavier than the projectile that could land on the moon. Comparatively, the moon lander was actually capable of flying back to earth, as the much lower gravity of the moon and absence of an atmosphere made this a relatively easy task even for a much smaller and simpler rocket. The earth is in a butter zone where we are just barely able to send things in to space, but on other planets or moons this task becomes far easier.
The extreme nature of these requirements and the absolute precision required to allow rockets to squeeze out just enough power and efficiency to do this are nothing short of miraculous. The United States is the only country on earth to land a person on the moon, and it did it 10 times, over 50 years ago. No other country has even come close. For a time the U.S. also had roughly 890 of the world's 950 or so satellites, essentially being the only country that could reliably launch large volumes of satellites in to space. The problem, primarily rests in the fact that the rocket must carry it's own fuel; if the fuel could be stored externally somehow, this would remove the problem entirely, although this would be impossible in some ways and only practical in a more general sense in others. Because a rocket must accelerate it's own self, and it's own weight is fuel, it requires more fuel to do it; however, this requires more fuel, which requires more fuel, for that fuel, carried on quite a bit, until the rocket is hundreds of times the weight of the intended cargo. This does not continue on infinitely and it is actually possible to launch things in to space, however only with extreme difficulty and clever and generally expensive designs. Rockets typically user booster rockets to get up to speed, however these rockets obviously need rockets several times their size for this to be possible. For example, the third stage of the Apollo mission rockets to go to the moon were just 1/25th of the weight of the rest of the rocket, and the first part of the rocket, the first stage, was approximately 80% of the weight, the majority of which was RP-1 fuel, or highly refined jet fuel. Rockets are only able to go in to space because they get lighter as they fly, losing much of the fuel or dropping previous modules.
Currently, RP-1 (highly refined jet fuel), Liquid Hydrogen, and Liquid natural gas are the best bets for regular space travel, and they work quite consistently, despite their high cost and low payload otherwise. Liquid hydrogen is extremely dangerous, as hydrogen leaks from almost every known container, can be set off with oxygen levels between 4% and 75% with as little as a spark or sunlight, is highly explosive at high velocities unlike most fuels which just burn, and is generally thousands of times more expensive to manufacture and store than fuels like RP-1 and liquid oxygen, which are by comparison far cheaper. Hydrogen is an ideal fuel due to it's high velocity and impulse, but is less ideal in just about every other factor; this is why many rockets utilize RP-1 for a bulk of their weight and mass, with the space missions for example relying upon 80% RP-1 by mass, which is essentially highly refined jet fuel with various compounds such as sulfur removed. Reliability is another issue with rockets, however this is waning to some degree with automation as drones or remote controlled rockets, even with AI, can be used with relative ease by comparison to crewed rockets, obviously not requiring a pilot and thus not endangering a crew, reducing the severity of a rocket explosion that might otherwise kill people. This also reduces necessary cargo as life support systems such as oxygen, food, water, a bathroom, and simply room to store a person are no longer needed and certain safety protocols can be removed. Even with incredible advancements in rocket technology and design and a change in the scope of their mission, such as SpaceX with Elon musk launching micro satellites in to orbit instead of people to locations, with modern electronics being light enough to allow for smaller satellites, and thus much smaller and cheaper rockets, there is still the problem of weight. The fuel can only stretch so far, and without any kind of substantial improvements in efficiency or fuel capability, there is nothing to fill the gap. Had the earth been somewhat smaller, for example the size of mars or the moon, regular space travel may have been possible decades ago. The moon lander took enormous effort to land on the moon, but was able to fly back to earth as the energy required to take off from the moon was substantially lower. Other than the thinner atmosphere, the gravity is 1/6th that of earth, and thus ends up with around 5 times less than what is required to get back to earth, or what ended up being roughly 22 times less energy (as it is actually 4.7 times lower escape velocity of 2.38 km/s vs. 11.2 for earth). This means that if the earth was the size of the moon, we could actually use guns to shoot things in to space fairly safely and realistically without much issue. The same is largely true with Mars, especially if said rocket was mounted on mount Olympus, as it is much taller than any mountain on earth and would have a thinner atmosphere as well. Of course, we must still get off of earth to explore the stars.
Granted atmospheric conditions of such a planet or moon may be wildly different than they are now in this hypothetical world, but it would be conceivably possible to build spaceships and launch them in to space hundreds of times more easily if for example, built and launched from the moon. By itself, this potentially solves an interesting issue of space travel and would allow us to solve our particular problem to a degree, if we could build a moon base that was sufficiently capable of building and launching spacecraft or rockets. This is conceivably possible as the moon is essentially made of roughly the same materials of earth, steel, aluminum, titanium etc. and these same materials could be used to make rockets or space ships that could be launched by railgun or other large guns, or even small rockets made from the aluminum powder of the moon, in to the moon's orbit. Once in orbit, it would only take a relatively small and efficient ion engine in order to get up to speed. For example, the rocket that landed on the moon got up to 7.8 km/s using our rocket engines, but got up to nearly 11, or just under the level needed to get out of the earth's orbit in order to land on the moon, using the ion engine. This equates to roughly the entire energy that it took for the lunar lander to get up in to space in the first place, in just a fraction of the weight, despite having the same energy. However, Ion and plasma thrusters of various types effectively have little to no thrust, despite having extremely good energy to weight efficiency. This means that while they are efficient once they are in space, they can not overcome the earth's gravity with the raw power that is needed to get up in to orbit in the first place. The issue comes from the fact there are few good ways to get up to the power levels required that are not inefficient.
As space has little drag from an atmosphere and no gravity pulling it down, it is far easier once in orbit to use an engine which produces a tiny amount of power, but that slowly builds up, to get up to speed. Even better, said rockets can actually use the gravity of other planets to slingshot themselves, getting up to even higher speeds. However, it is not possible to use such an engine, no matter how theoretically efficient, to get in to space as they simply lack the power, thousands of times less power than is needed. Therefore, the issue is finding a fuel or an engine that is efficient enough to allow for low earth orbit velocities. To get up to 7.8 km/s, there is a need for approximately 60.84 megajoules of energy per kilogram of cargo or rocket. However, a rocket loses it's fuel weight as it gets up to speed and drops booster rockets, thus making it lighter and lighter as it reaches altitude, reducing this requirement once it is actually up to speed. Therefore an exact 1 to 1 is not actually required, although it depends on the rockets flight time and mass at the point of acceleration. RP-1 is only capable of getting up to approximately 40-46 megajoules, which liquid natural gas can get slightly over this at 53.6 megajoules per kilogram. Hydrogen has 140 megajoules per kilogram, but is the most dangerous, least stable and most expensive of all the fuel types. It also has an extremely high volume, which means it requires incredibly large and non-aerodynamic rockets. To get to the moon, the heaviest and first phase of the rocket used RP-1 to get up to 2 km/s, to accelerate the second two phases of hydrogen based rockets up to speed and in to high earth atmosphere, in order to get up to roughly 8 km/s. The atmosphere produces significant drag on rockets, and most objects, and can easily destroy or burn up objects traveling near this speed even relatively high up. Most meteorites burn up at approximately 80 kilometers or 50 miles up, where the atmosphere is up to 10,000 thinner than the surface of the earth. These are often traveling at similar velocities to rockets; if these rockets tried to travel at the same velocities at sea level, they would practically disintegrate instantly upon contact with the atmosphere. Therefore to get up to high enough atmospheric levels to not burn up in the atmosphere, the rockets must slowly accelerate, at first, before getting above this part of the atmosphere and then accelerating. In addition to this, they need high end ceramics and other materials to absorb the heat of the atmosphere, both when going up and coming down. This, by itself, precludes giant guns from being used to launch things in to space easily, unless they could be mounted high enough up, say on a 50 mile tall mountain, to get in to space (by comparison, the tallest mountain in the world is approximately 7 miles tall, where the atmosphere is only 3 times thinner, nowhere near 10,000 times less). However, it also complicates rocket technology, as very little is capable of doing this efficiently and it must travel at different speeds in multiple phases of it's launch, slowly accelerating. The booster rockets not only serve the function of getting the other faster rockets up to speed, but must get them up high enough to avoid the issue of atmospheric drag by going to a thinner level of atmosphere.
What is required is the ability to push off of something physical to get lift off. Driving a car in a straight line is not particularly difficult, and it would only take approximately an hour or two to get in to orbit if for example a road existed that could allow one to drive in to space, or for example a tall enough mountain. An aircraft also similarly can travel the distance easily. What it can not do is reach escape velocity to get off of the earth, and it cannot push off of the lack of air in space. To accelerate oneself or a rocket, one needs to push off of something. With a rocket, this is the rocket exhaust which expands, essentially being hot gases and often water traveling at high speeds which comes from the rocket fuel. Rockets essentially push off of rocket fuel. For an ion engine, this involves tiny microscopic particles such as electrons and protons, which is why it can be so efficient in terms of weight, but it's complete lack of thrust due to these tiny particles means it's incapable of getting up to speed. One way around this is a space elevator, which in theory would provide a cable in to space that an elevator could ride up and down on, thus requiring very little energy to travel up and down and being comparable potentially to a car driving on a road. However, such a cable would not be able to be anchored to anything, and the orbital forces exerted on the cable would be so strong it could easily snap it. As of now, no material is currently strong enough to allow for space elevators to be built on earth, however hypothetically some could be built on the moon with materials such as kevlar, which are fairly cheap and lightweight by comparison to more exotic hypothetical materials such as graphene which would be necessary on earth (being thousands of times cheaper if graphene could be mass produced at all). Thus such an idea is entertaining, but not really plausible as of right now. One way to fill the gap would be with design, which would increase the strength of the cables by designing them differently, however this could only achieve so much extra strength and currently not enough to build space cables.
The main issue, then, comes down to power-to-weight ratio, and the fact few fuels can reach the level needed to get in to orbit. Once in orbit it is easy to stay in orbit or speed up in orbit with an ion engine, but breaching this gap has thus far proven extremely difficult. There is one obvious way around this, however; kinetic energy storage. In theory, kinetic energy levels can reach near the speed of light's worth of energy, and thus it is hypothetically possible to store energy in the form of kinetic energy to be nearly infinite or the highest level possible; it is possible with kinetic energy to store energy near antimatter levels. Already, using hadron colliders, we are capable of using magnetic accelerators to get particles with mass to approach the speed of light, although never actually reach it (which is likely impossible). Railguns with velocities of well over 6 km/s have been used and tested by the military for decades, and while these are usually designed to launch relatively small projectiles with small launchers, it is not actually difficult to get to the required velocities need to get in to low earth orbit using these methods. The key advantage rests in that the energy expended to get the material up to velocity or up to temperature is expended on the ground, by the launching device itself, I.E. a gun or the like, or by the device which powers up the mechanism, rather than the projectile. In this way, this is energy transferred to the rocket or projectile that doesn't require it's own fuel, thus allowing it to get up to these velocities without first requiring an initial rocket. A massive magnetic accelerator accelerates a particle that does not need to accelerate itself. While using some kind of high velocity gun by itself might only be able to impart a relatively small amount of energy, it could pay dividends; rockets are operating on margins of .3% to 1% of their weight in cargo, and increasing this by even a few percentage points, let alone 10-25% would mean a substantially higher amount of weight per rocket could be carried. You could also have rockets starting off at certain base velocities, removing the need for booster rockets when getting up to speed, significantly reducing their size, for example with the first two rockets of the moon missions being 96% of the weight (25 times the weight of the next series of rockets). With something like a scramjet or ramjet engine, which are air-breathing engines which are more efficient, you could also start off at very high velocities and improve the efficiency, velocity or power of rockets, while also putting much of the weight of the air in to the rocket. Liquid oxygen is approximately 2/3rds or more of the weight of the fuel/LOX mixture in terms of weight for most rockets, and therefore transferring as much of the oxygen to the atmosphere as possible, such as with air-breathing rockets, is another way to take weight off of the rocket. A magnetically accelerated air-breathing scramjet rocket could be single stage and get up to high enough velocities to allow for the spacecraft to function without the need for a booster, while also shaving off a large portion of it's weight needed via the high starting velocity, as well.
Most likely, some kind of kinetic hybrid missile would be required in order to get up to velocity. It's not likely easily possible for all forms of cargo to get up to speed with a space gun or some kind at terrestrial levels with thicker atmospheric levels, which could cause the projectiles to burn up at too high of a speed, however getting projectiles up to 2-6 km/s and allowing a rocket to take it the rest of the way, which by itself would remove at least 50% of the necessary energy and a majority of the rocket weight, could work quite well. Bottom line, the cargo payload could be increase dramatically, which by itself is an advantage, and many of the best rockets must start at a certain base velocity such as scramjet or ramjet rockets, solving this part of the issue as well. However, there are a few other ways to store energy via kinetic energy. One is heat and another is velocity. Obviously, it is possible to bring the rocket up to speed itself and use it as the kinetic battery or energy storage device, thus shaving off the weight of fuel needed for the rocket to propel itself. However, it is also possible to use spinning projectiles. While spinning disks are used in flywheels to store energy for satellites and spacecraft already, their weight to strength ratio is quite low given the limited speed flywheels can spin at. By being spun from the center, an enormous amount of torque is imparted on the spinning device, and therefore the device must be extremely strong to resistant breaking; so far, the limits on how strong these types of kinetic storage devices can be are quite low, and are dependent on the strength of the material they're made of, simply to space elevators although using much smaller materials. Another option however is not to spin them this way at all; like how a tipjet helicopter does not exert nearly as much torque on a helicopter by spinning the blades from the outside, if a projectile was spun around in a circle and suspended via magnetic levitation in a donut-like magnetic contraption, the velocities could get much higher without placing undue torque stress on the kinetic battery itself therefore not limiting it's velocity by the strength of the material. While few of said kinetic batteries of these design exist, they could in theory bridge the gap between current chemical and nuclear energy barriers. One could store dozens to hundreds of times the power of a normal rocket in terms of weight in these batteries, if getting them up to spinning velocities of 10-100 km/s.
Regardless of the method used to store it, one method that virtually all rockets use to store energy, if only temporarily, is heat. While purely steam base rockets exist, all rockets essentially use expanding steam and gases to propel them, and with hydrogen based rocket the exhaust of them is often literally water, when the hydrogen mixes with oxygen. The gases are heated to high temperatures and expand, and this expansion pushes off of the rocket nozzle and engine and propels the craft forwards. Despite the gases being lightweight they are propelled at very high velocities, and therefore can give sufficient lift to help the rocket off the ground. Heat energy is one method that allows for kinetic energy to be stored far above chemical energy limits. For example, if a kilogram of water was heated to approximately 40,000 degrees, it would have a higher energy content than hydrogen does chemically at it's theoretical maximum levels of power. However this is more or less impossible as the water would turn in to plasma; what is more practical is to super heat hydrogen to these levels and then confine it via magnets, which require no energy for this task hypothetically or a small amount if using electromagnets. Water has a specific heat capacity of approximately 4200 joules per kilogram per degree of temperature Celsius, while hydrogen has a heat capacity of 14-20 kilojoules, being around 3.5-5 times greater. This reduces the necessary temperature somewhat, but still requires enormously high temperature levels. Both the pressure exerted by the gases and the high heat could destroy it's containers, thus requiring magnetic confinement so that it does not touch the chamber walls. Perhaps ideally, the hydrogen plasma, heated to several hundred thousand or million degrees, could be confined via magnetism, thus only requiring a relatively small amount of propellant to be stored, and could reduce the heat or pressure exerted on the container and the necessary power by the magnets. This is a similar problem presented in fusion reactors, and may be possible given current technological capabilities, in some ways. All materials when heated to a high enough temperature become plasma, which is highly magnetic and also gives off radiation. When the materials turns in to plasma, it would be theoretically possible to confine via magnetism, in the same way a spinning projectile could be stored via magnetic levitation in a vacuum container, although the energy required to do this could be theoretically much higher.
Expanding upon the problem
