Like any good mode of transportation, spaceships require engines. They work… somehow.
How is it that something so ubiquitous to a genre can be so technologically ambiguous? The short answer is that most engines could never accomplish what the stories require of them. Massive warships, limited refueling needs, incredibly fast travel times. These story requirements pose a difficult challenge for even the most ambitious engine designs. The author is left with two choices: 1) put the story first or 2) put the science first at the expense of the story. A story about a small light-driven capsule on a long journey between planets is not without its merits, of course, but it would be hard to cram enough people in one for a compelling soap opera. However, if you are one of those rare few authors that desire to have the best of both worlds, a compelling story propelled both literally and figuratively by accurate rocket science, then this article is for you.
As with all discussions of technology in a Sci-Fi setting, a spacecraft’s technological limitations largely depend on how far into the future the story takes place. Contemporary sci-fi would utilize much of the same technologies we do now. But the majority of sci-fi stories are set far into the future, so far that our modern aerospace technology would seem primitive by comparison. That doesn’t mean physics will change, only that better ways of harnessing it will emerge. For this article, I will do my best to make rocket science sound a little bit less like rocket science.
Spacecraft engine basics.
Designing engines capable of moving a large military frigate or destroyer is harder than it sounds. Physics, specifically the conservation of momentum, imposes some strict limits to this technology. Rocket science is, in effect, the study of moving mass by throwing other mass in the opposite direction. This principle is why solid and liquid chemical rockets continue to be used today. The rocket fuel and oxidizers are massive, and therefore capable of generating useful momentum when burned and propelled out the back of a rocket at thousands of meters per second. There are far more energetic processes, generating particles that move at the speed of light, but most of these particles are effectively massless and the momentum they impart miniscule. Harnessing these more ephemeral particles is certainly not out of the question, indeed some of the most powerful and efficient engines on this list fall into this category, but the technology needed to harness them is, for now, beyond our practical capabilities.
The most notorious equation in rocketry is aptly named the rocket equation, which describes the total change in velocity (delta V) a spacecraft can achieve. It takes into account the thrust of the engine as well as the ratio between the mass of the spacecraft and the mass of the fuel needed to lift the spacecraft (and its own fuel) off the ground. Predictably, fuel with a lot of mass results in a lower delta V, at least until most of that mass is expended. Conversely, the delta V of a spacecraft with less massive fuel and more efficient engines is much higher.
But equations are hard. So, it’s easier to keep in mind two simple concepts: specific impulse and thrust-to-weight ratio. Specific impulse accounts for how many seconds of thrust (e.g., 1lb) can be achieved per an equal weight of fuel (e.g.,1lb). An engine that can produce 1lb of thrust for 200 seconds with 1lb of fuel will have a specific impulse of 200s. This value needs to be very high for the grand star ships we see in the current science fiction. Modern engines, however, struggle to break 500s. Ion engines have a specific impulse over 1,000s. The cap of specific impulse is limited only by physics, but theoretical engines could achieve over 100,000s.
Thrust to weight ratio is defined as a spacecraft’s capacity for acceleration, or its ability to lift its own weight. While this ratio is unitless, because weight is relative to Earth, you can instead express this in gravities of acceleration (Gs). A one-to-one ratio, or value of 1G, would allow the spacecraft to negate the pull of Earth’s gravity but not allow it to gain any altitude. A thrust to weight ratio of 10Gs would provide 9Gs of acceleration directly opposite the pull of a 1G planet, or 10Gs of thrust going horizontally or in orbit. For purposes of spaceflight, a thrust to weight ratio should be capped somewhere between 3-5 to prevent acceleration-related trauma to crew. This can be accomplished by adding more mass to the spacecraft, such as entire decks, a bridge, and turret installations, to name just a few.
The trade-off between specific impulse and thrust-to-weight ratio is that most engine types that produce a high specific impulse have a low thrust to weight ratio, and vice versa. But this isn’t a hard and fast rule. It’s just that it’s difficult to accelerate a lot of mass at high velocities, so most engines will accelerate either a little bit of mass at high velocities, or a lot of mass at low velocities.
Technologies that promise high values for both specific impulse and thrust to weight ratios are required to make most science fiction spacecrafts a reality, especially if interplanetary travel is a requirement. Sadly, the requirement for high specific impulse and thrust to weight ratios removes chemical rockets from consideration, as even the most efficient, a tripropellant combination of lithium, fluorine, and hydrogen, barely achieves a specific impulse above 500. These fuels are also very dense and only convert approximately 1/100,000,000th of their mass into energy that can be used for increasing a rocket’s momentum. Below, I discuss only those engines I believe stand a reasonable chance of being used in a futuristic setting.
Engine types.
The most important consideration when choosing or designing an engine is knowing the purpose of your spacecraft. A probe meant to undertake long journeys to distant planets could very well use solar sails and get rid of engines altogether. Larger spaceships used in space warfare would require massive engines with incredible thrusts and require regular refueling. It is important to consider both the strengths and limitations of each engine to see how they would support or necessitate changes to your story. Indeed, some of these technological features could serve to generate conflict and suspense should something break down or a critical resource be lost.
Light sails- These spacecraft operate by harnessing the momentum of reflected light. This makes the list due to the novelty of not having engines at all, making its specific impulse effectively infinite, though its thrust to weight ratio is abysmal. Photons, while massless, can still impart momentum due to being made up of energy. But this momentum is minuscule. In Earth’s solar orbit, 1 square meter of perfectly reflective sail can only achieve a little over 9 micronewtons of force per second, equivalent to the weight of a small ant on your palm. Of course, sails that are a million times larger, 1kmx1km, could produce the same force as holding a large pineapple. This small force adds up over time, generating velocities that could theoretically reach another start system in a few dozen years.
However, since the light from the sun disappears proportional to the inverse square, by the time the spacecraft reached the asteroid belt or twice the distance between the Earth and the sun, it would only receive a quarter the amount of light. At the orbit of Pluto, the light would be 0.000625x as bright. Not only would such a spacecraft take years to reach Pluto, it would be without maneuverability and thrust on its way to the next star system. To overcome this challenge, a laser could be beamed at the craft from a distance, supplementing its thrust. But considering the power requirements to add about 6 newtons of thrust is similar to the peak power demand of Los Angeles (i.e., 1gigawatt) that kind of infrastructure would far exceed the cost of the spacecraft. Though the expense may be worth it in order to accelerate the craft to as high as 20% the speed of light and reach a nearby star system in someone’s lifetime.
Of course, if you could decrease the mass of the sail, you could afford to increase its area of coverage. That’s the advantage of an electric solar wind sail. By deploying a several kilometer long thin grid of wires and giving the spacecraft a positive charge by ejecting electrons with an electron gun, your spacecraft can be propelled by the much more massive particles in the solar wind (protons).
But even if you could reach fractional light speed over the course of several years, you’d need to slow down once you arrived at your destination, which would add many more years to your journey. And should you miss your target and fail to be captured in orbit, you could be sailing through space forever. Sounds like good story suspense to me.
Advanced ion and Hall-effect thrusters- These engine types function, in simple terms, by ionizing a gas and shooting it out the engine at incredible speeds. This can be done by creating Hall currents, or a cloud of electrons, an electrified metal grid, or high intensity radio frequencies to ionize and accelerate the gas. With some variation in specific impulse (1,500-5,000s) and thrust to weight ratio (0.00001-0.002), they are an excellent example of the tradeoff between efficiency and thrust and are most commonly used in station keeping for very small satellites. Noble gases are the preferred fuel because they have low ionization energies and because their full outer valence shell results in weak interatomic forces, making them very compressible and unreactive for storage. Higher atomic weight noble gases, like xenon, improve the thrust at the expense of cost to procure.
These are unlikely to be used outside of small, slow-moving probes in sci-fi due to their low thrust, but should they be used to move anything a bit more massive, they would likely be unable to escape a gravity well or reach their designation in any meaningful time. They might be capable of reaching another star system, but the journey to the nearest one could take tens of thousands of years.
Nuclear thermal rockets- These rockets function by using the heat of a nuclear reactor to provide kinetic energy to hydrogen gas and accelerate it out the back of a space craft. They can either have a solid, liquid, or gas uranium core, and each has its own advantages and drawbacks. They are thought to be capable of specific impulses of nearly 1000 and thrust to weight ratios between 2-5. There have been several developmental projects and even some test fires of some of these engine types, though development has stalled, largely due to the danger of carrying uranium into the upper atmosphere. This is not without cause, as there have been radiological events in the past, most notable in 1978 when a Soviet satellite with a uranium reactor fell out of orbit over Canada, resulting in costly cleanup operations.
Nuclear pulse propulsion rockets- These are another type of nuclear-powered rocket that relies on supercritical events (i.e., nuclear explosions) to incinerate the propellant (polyethylene, tungsten, or ice) and create a shaped blast of plasma. A pusher plate with shock absorbers would absorb the high heat and momentum of the blast in order to spread out the acceleration over time. Some designs, such as those developed for project Orion, were thought to be capable of generating specific impulses between 10,000 and 100,000, and 10-100 thrust to weight ratios. Unlike nuclear thermal rockets, where the radioactive material was contained, this was instead vaporized in the plasma and allowed to dissipate into the environment. That’s not exactly healthy for anyone living on the planet your characters are trying to leave. The crew and cargo wouldn’t be immune either, as the pusher plate is unlikely to stop all the radiation, especially as the cloud of radioactive plasma expands. Later designs underwent significant modification to instead induce fusion via inertial confinement instead, such as proposed by Project Daedalus and Project Longshot.
Nuclear fusion rockets- These are a mainstay of science fiction, and the first rocket type to be strictly hypothetical. Inertial confinement fusion and magnetic confinement fusion, while possible and even successful at small scales, is still in its early stages and has not yet been adapted into a working prototype engine. Both fusion types function by first initiating the hydrogen or helium isotope fusion and then channeling the generated plasma out the engine in a magnetic nozzle at high speeds, much like ion engines. The theoretical specific impulse is well over 10,000s but with relatively poor thrust to weight ratios averaging around 0.1-1. The low thrust to weight ratio is largely due to the mass requirements of the engine and associated magnetic confinement.
While the hydrogen and/or helium fuels might seem cheap and abundant, this is far from correct. The hydrogen isotopes deuterium and tritium are scarce and difficult and expensive to collect. The helium 3 isotope is even more challenging, as it may have to be mined from the lunar surface where it has accumulated from solar wind for billions of years.
Another challenge is radiation exposure, as fusion energy produces neutrons, which can then go on to induce radioactivity in exposed materials (i.e., neutron activation).
One possible way to both reduce fuel costs and radiation exposure is an engine that uses an aneutronic fusion reaction. Fusion of a proton and boron-11, while requiring much hotter ignition temperatures, creates a short-lived and highly excited carbon nuclei which instantly breaks down into three alpha particles (helium nuclei) with high kinetic energy. As boron and hydrogen-derived protons are very cheap and accessible and the fusion is clean, this could be the future of fusion rockets. Of course, this assumes the challenges of sustaining the reaction can be overcome.
Antimatter engines- While this rocket type could theoretically be lumped under nuclear rockets, matter-antimatter annihilation doesn’t exclusively involve atomic nuclei. These hypothetic engines are also in a league of their own. These are the only semi-realistic engines capable of moving the gargantuan ships seen in sci-fi stories and with comparable fuel efficiencies. Solid core, plasma core, and beam core propulsion are three examples of antimatter engines that promise increasingly higher specific impulses between 1,000-1,000,000s and thrust to weight ratios approaching 100. But with their much higher efficiencies and thrusts, they also have some of the largest drawbacks.
These engines work by containing matter and antimatter ions in an ion trap (e.g., Penning trap), syphoning an equal amount of matter and antimatter into the engine, and harnessing the resulting annihilation energy in some way. The antimatter engine types mostly differ in how that energy is translated into thrust. Due to their higher mass, antiprotons are far more energy dense, capable of releasing over 1800 times more energy than positron annihilation, so this is the preferred reaction mass.
Antiproton annihilation results in the near-instantaneous transition of mass into pure energy as dictated by E=mc^2. That energy first comes in the form of 2-9 pions, of which, about 3/5 are charged and capable of being electromagnetically propelled outside the spacecraft. The neutral pions rapidly go their own way and decay to gamma rays. The charged pions don’t make it very far either, decaying to muons in a bit over a hundred picoseconds. Those muons, however, are still massive, and can be magnetically propelled at near light speed for another couple microseconds before they decay into neutrinos and positrons or electrons, which then annihilate to form more gamma rays. As soon as the “exhaust” becomes nothing but gamma rays and neutrinos, it’s lost most of its efficacy as a fuel, as these particles cannot be directed, penetrate nearly everything, and do not carry any significant mass. Beam core antimatter engines are designed to extract all of this useful momentum as quickly as possible in just a couple microseconds.
Less efficient designs, like the solid core engine, instead attempt to transform the 100% gamma ray mass conversion into heat inside a block of tungsten. They then pass liquid or solid hydrogen through the block to turn it into a heated plasma, which is propelled out of the engine. The exhaust, however, is limited by the melting temperature of the tungsten core. Plasma core engines get around this by instead injecting antiprotons directly into a liquid hydrogen stream to create a plasma and propel the exhaust to even greater velocities.
One of the major challenges to antimatter engines is the radiation. This would require a lot of shielding mass (High Z materials like lead and tungsten) to protect the crew and ship’s contents. The incident heat is also something that would need to be rejected to space through massive radiators.
The second major hurdle is sourcing the antimatter. Fueling a single ship with even a few kilograms of fuel isn’t remotely possible and would require nearly the full industrial resources and harvesting of all antimatter produced in the atmosphere and stored in the magnetic belts of every planet in the solar system continuously for over 10 million years. Any sci-fi using this technology would either have to manufacture particle accelerators on a global scale or harness the power of cosmic radiation to accelerate the process. That’s a lot of work to put in considering a single slip-up could destroy everything in a 100km radius.
Which brings us to the last major hurdle. The storage problem. With something so catastrophically and instantaneously destructive, even a momentary loss of containment would destroy all life aboard the ship, including the ship itself. That’s assuming you could pack enough of it together in the first place. Ion traps are the first logical candidates but require the use of ions. Packing large amounts of ions close together is impossible due to repelling electrostatic forces (Coulomb). Even a frozen plasma (Coulomb crystal) of antiprotons would result in distances of up to 20microns between the particles. Making larger elements, complete with their anti-neutrons, would be an effective way to pack more antiparticles together in larger nuclei, but that would involve smashing together more and more antimatter, an energetically unfavorable task, not to mention finding a way to contain the chargeless anti-neutrons. A better design proposes using frozen antihydrogen in a magnetic minimum trap to keep the density as high as possible yet still confine neutrally charged particles. As these traps are far from strong, containment breaches could occur as the result of minor collisions or weapons fire. As it’s currently possible, albeit difficult, to make antihelium atoms, storing liquid antihelium instead of antihydrogen ice would be easier due to its greater diamagnetism, with the added benefit of it being far denser than frozen hydrogen.
Hybrid engines.
A spaceship can use a combination of these engines to create something that uses the best features of each. For example, instead of using antimatter for propulsion, a significantly smaller amount of it could be used to rapidly meet the energy requirements necessary for fusion. This is called an antimatter-enhanced nuclear pulse propulsion engine.
Similarly, if a material were discovered capable of reflecting gamma rays, a solar sail could be constructed and powered by the gamma rays released from antimatter annihilation. This is called an Antimatter Photon Drive.
Alternative uses for an engine.
Larry Niven coined The Kzinti Lesson, which states that “a reaction drive’s efficiency as a weapon is in direct proportion to its efficiency as a drive.” This makes sense considering thrust is all about the conservation of momentum, meaning the faster you want to go, the faster you have to throw mass in the opposite dimension. This implies that the engines with the fastest engine exhaust could damage a target with the same energy with which they propel the spacecraft in the opposite direction. This assumes, of course, that the energy of the exhaust can be focused at all.
In some cases, the engines themselves can be used to power the spaceship. Heat exchangers responsible for cooling the engine can also provide electricity to the ship via the Seebeck effect or even a system of turbines. However, this also means that the spaceship’s power would rely on the continued use of the engines unless alternative power sources can compensate.
Create your own.
If there is just a single obstacle preventing an engine design from meeting your needs, then it is reasonable to expect some type of technological advancement to eventually meet that need. Fusion is a great example of this, as we have not yet optimized the containment and compression to achieve high efficiency and low engine mass. However, given the growth of the field, it is highly likely this challenge will be overcome. Similarly, the conversion of energy into momentum could be dramatically improved for antimatter engines with the discovery of a single material capable of reflecting or deflecting gamma rays (Antimatter Photon Drive). These possibilities seem a lot more feasible than scientists discovering a way to rip holes into a hyperspace dimension or discovering exotic negative mass.
Why not warp drives?
Wormholes drive (Einstein-Rosen Bridges), classic warp drive (Alcubierre), and reactionless drives (EM drive or Zero point drive) are indeed the quickest and easiest way to suit the needs of your story and minimize the amount of rocket science you need to learn in order to design your spacecraft. However, these drives rely on either impossible and disproven physics, exotic particles or energy states, or phenomena that have never been observed. To say nothing of being overdone and unimaginative. But the real question is, how necessary are they really? In some cases it might be your only option. But if your story doesn’t specifically require faster than light travel, near-immediate teleportation to another star system, etc, then why not stick to real-world physics for your spacecraft?
Personally, I think speculative fiction should aspire to a more noble cause than just entertainment. For centuries, it has been used as an outlet for the scientific imagination, inspiring countless scientists to go on and attempt to make these fictions a reality. And the closer we authors keep to reality, the more likely these innovations will succeed. There is no greater honor in my mind than to be known as a futurist by the next generations, as someone whose work educated and inspired future scientists to make a dream possible. Arthur C. Clarke, was one such futurist, who not only popularized science, but predicted and possibly even inspired the notion of telecommunication satellites, the world wide web and their various uses. So, if your story happens to fit within the bounds of modern physics, I challenge you to imagine engines that could be real one day with the hope that one of your future readers will be inspired enough to create it.
Until next time, write well and science hard.
