A Guide to In-Space Propulsion

The Fundamental Push: How Rockets Work

Every method of moving a spacecraft, from the thunderous launch of a massive rocket to the silent, gentle push of an advanced thruster, operates on a single, elegant principle of physics. This universal rule is Newton’s Third Law of Motion: for every action, there is an equal and opposite reaction. In the context of space, this means that to go somewhere, you have to throw something away. A spacecraft propels itself by expelling mass, known as propellant, in one direction. In reaction, the laws of physics push the spacecraft in the opposite direction.

A simple analogy illustrates this concept perfectly. Imagine inflating a balloon and then letting it go. The air rushes out of the open end—this is the action. The balloon, in turn, zips forward through the air—this is the reaction. A common misconception is that a rocket needs air to “push against.” In reality, the engine works by pushing the hot, high-pressure exhaust gas against the inside of the engine’s bell-shaped nozzle. The vacuum of space is actually a more efficient environment for a rocket engine, as there is no air resistance to slow the exhaust. This fundamental principle, known as the conservation of momentum, governs every propulsion system, whether it’s a chemical rocket burning fuel or an electric thruster expelling individual atoms.

This physical reality leads to a formidable challenge known as the Tsiolkovsky rocket equation. This equation mathematically describes the relationship between a spacecraft’s propellant, its engine’s efficiency, and the total change in velocity it can achieve. The equation reveals a harsh truth: to gain more velocity, a spacecraft must shed a disproportionately large amount of its mass as propellant. The relationship is logarithmic, meaning that for each small, linear increase in desired velocity, the required propellant mass increases exponentially. This creates a situation of diminishing returns, where adding more fuel makes the rocket heavier, which in turn requires even more fuel to lift.

This unforgiving principle is often called the “tyranny of the rocket equation,” and it is the central problem that the entire field of space propulsion seeks to solve. For most ambitious missions, the vast majority of a rocket’s starting weight is nothing but propellant, leaving only a tiny fraction for the actual spacecraft, its scientific instruments, or its crew. The relentless quest for better engines is fundamentally a quest to escape this tyranny by developing more efficient ways to use propellant, allowing more payload to be sent farther and faster. This single constraint explains why there are so many different types of engines and why engineers are constantly pushing the boundaries of technology.

The Language of Propulsion: Key Performance Metrics

To compare different engines and understand their roles, engineers use a specialized vocabulary. Two of the most important terms are thrust and specific impulse. Understanding the interplay between these two metrics is key to understanding why certain engines are chosen for specific tasks.

Thrust: The Measure of Power

Thrust is the raw force produced by a propulsion system, typically measured in Newtons (N) or pounds-force (lbf). It dictates how quickly a spacecraft can accelerate. High thrust is absolutely necessary for launching a vehicle from the surface of a planet, as the engine’s thrust must be greater than the total weight of the rocket to overcome gravity and lift off the pad.

An effective analogy is to think of thrust as the horsepower of a car’s engine. A dragster needs immense horsepower to get off the starting line with blistering acceleration. Similarly, a launch vehicle needs millions of pounds of thrust to begin its journey to orbit. Once in space, high thrust allows for rapid orbital maneuvers, like injecting a spacecraft into orbit around the Moon or making a quick course correction.

Specific Impulse (Isp): The Measure of Efficiency

Specific impulse, often abbreviated as Isp, is the primary measure of a rocket engine’s efficiency. It quantifies how much thrust is generated from a given amount of propellant over time. A high Isp means the engine is very fuel-efficient, getting more “bang for your buck” from every kilogram of propellant it carries. The Isp of an engine is directly proportional to the velocity at which it expels its exhaust. The faster the exhaust, the more momentum is imparted to the spacecraft for the same amount of propellant mass, resulting in a higher Isp.

Continuing the car analogy, if thrust is horsepower, then specific impulse is the car’s fuel economy—its miles per gallon or kilometers per liter. A family sedan might have modest horsepower but excellent gas mileage, allowing it to travel a long distance on a single tank. In the same way, a high-Isp engine can achieve a large total change in velocity with a limited amount of fuel, making it ideal for long journeys across the solar system.

Specific impulse is most often expressed in units of seconds. While its physical meaning is directly related to exhaust velocity (meters per second), the use of “seconds” is a historical artifact. During the early days of the space race, American engineers worked in imperial units (feet, pounds) while European scientists, including many German rocket pioneers, used the metric system (meters, kilograms). To avoid constant, error-prone conversions, they adopted a convention that resulted in a unit—seconds—that was the same for everyone. This convention persists today, a small human quirk embedded in the highly technical language of rocketry.

The Great Trade-Off

The most important concept in understanding propulsion is the trade-off between thrust and specific impulse. For most technologies, high thrust and high specific impulse are mutually exclusive engineering goals. Systems that produce enormous thrust, like chemical rockets, tend to be relatively inefficient. Conversely, systems that are extraordinarily efficient, like electric thrusters, produce only minuscule amounts of thrust.

This inverse relationship is not just a technical detail; it fundamentally defines the architecture of nearly all space missions. It forces a two-phase approach to spaceflight. The first phase is a “brute force” stage, where the primary goal is to escape a planet’s deep gravity well as quickly as possible. Here, high thrust is paramount, and efficiency is a secondary concern. This is the domain of powerful chemical rockets.

The second phase is the “marathon” stage, which takes place in the microgravity environment of space. Here, there is no urgency to accelerate quickly. A tiny, gentle push, applied continuously over weeks, months, or even years, can produce enormous changes in velocity over the course of a mission. In this phase, the main limitation is the amount of propellant a spacecraft can carry. Therefore, efficiency (Isp) becomes the most important factor. This is the domain of high-Isp electric propulsion systems. This fundamental dichotomy explains why we launch spacecraft with powerful, fuel-guzzling chemical rockets but then use hyper-efficient ion thrusters for long interplanetary cruises or for the decades-long task of keeping a satellite in its proper orbit.

The Workhorses: Chemical Propulsion

Chemical propulsion is the most traditional and widely used method of getting to and moving through space. These engines generate thrust by harnessing the energy released from chemical reactions—typically, the rapid combustion of a fuel and an oxidizer. They are the undisputed champions of high thrust and are categorized into two main families: solid and liquid.

Solid-Propellant Rockets

A solid-propellant rocket is the essence of simplicity. The fuel and oxidizer are pre-mixed together into a solid, often rubbery, material called a “grain”. This grain is packed into a sturdy metal or composite casing that also serves as the combustion chamber. At the center of the grain is a hollow channel called the perforation. The shape of this channel is carefully engineered because it determines how much surface area of the propellant is exposed to flame at any given time, which in turn controls the burn rate and the thrust profile of the rocket. To start the engine, an igniter, much like a large firework fuse, is fired, which lights the inner surface of the grain. The propellant then burns from the inside out until it is completely consumed.

The primary advantages of solid rockets stem from this simplicity. With very few moving parts, they are exceptionally reliable and can be stored for years at room temperature, ready to be used at a moment’s notice. They also have a very high thrust density, meaning they can produce a great deal of force from a compact and relatively simple package.

However, this simplicity comes with significant disadvantages. Once a solid rocket motor is ignited, it cannot be throttled, shut down, or restarted. It will burn with its pre-programmed thrust profile until all the fuel is gone. This lack of control makes them unsuitable for maneuvers that require precision. They also have a lower specific impulse than their liquid-fueled counterparts, making them less efficient.

These characteristics define their applications. Their readiness and reliability make them the standard for military applications, such as ballistic missiles. In spaceflight, their high thrust makes them ideal as strap-on boosters. They are often attached to the sides of larger liquid-fueled rockets to provide the extra kick needed to lift very heavy payloads off the launch pad. Famous examples include the boosters for the Space Shuttle, the European Ariane 5, and the American Atlas V.

Liquid-Propellant Rockets

Liquid-propellant rockets are more complex but also more versatile. In these engines, a liquid fuel (like kerosene or liquid hydrogen) and a liquid oxidizer (like liquid oxygen) are stored in separate tanks. During operation, powerful pumps called turbopumps spin at tens of thousands of RPM to force the propellants at extremely high pressure into a combustion chamber. Inside the chamber, the propellants are mixed and ignited, creating a controlled, continuous explosion. The resulting hot, high-pressure gases are then directed out through a nozzle to produce thrust.

The main advantage of liquid rockets is their superior performance and control. They generally have a higher specific impulse than solid rockets, making them more fuel-efficient. Crucially, the flow of propellant can be adjusted by valves, allowing the engine’s thrust to be throttled up or down in real-time. They can also be shut down completely and, in many cases, restarted later in the mission. This level of control is essential for precise orbital insertions, landings, and emergency aborts.

The drawback is their complexity. A liquid rocket engine is a maze of high-speed turbopumps, intricate plumbing, valves, and control systems, all of which must operate under extreme temperatures and pressures. This complexity increases cost and introduces more potential points of failure. Furthermore, the most efficient propellants, like liquid hydrogen and liquid oxygen, are cryogenic, meaning they must be stored at hundreds of degrees below zero. This requires heavily insulated tanks and careful handling to prevent the propellants from boiling away.

Because of their efficiency and controllability, liquid engines are the workhorses for the main stages of most launch vehicles, including the Saturn V that took astronauts to the Moon and the reusable Falcon 9 rockets of today. They are also used for in-space propulsion systems on crewed spacecraft, like the Apollo Lunar Module’s descent and ascent engines, where precise control was a matter of life and death.

The choice between solid and liquid systems is often not a matter of “either/or.” Many of the world’s most successful launch systems use both in a symbiotic relationship that plays to their respective strengths. A common and highly effective architecture combines the cheap, reliable, high thrust of solid rocket boosters for the initial phase of flight with the sustained, efficient, and controllable thrust of a liquid-fueled core stage for the remainder of the ascent to orbit. The Space Shuttle, with its two solid boosters and three liquid-fueled main engines, is a classic example of this hybrid approach.

A significant trend in modern liquid propulsion is the move toward “green” propellants. Traditional propellants like hydrazine are extremely effective but also highly toxic and dangerous to handle, which drives up the cost and complexity of ground operations. This has spurred the development of less-toxic alternatives, such as a hydroxylammonium nitrate (HAN)-based fuel known as ASCENT. This push is driven not just by environmental concerns but by the practical desire to make spaceflight cheaper, safer, and more routine. This research has led to an unexpected and revolutionary breakthrough. ASCENT and similar propellants are ionic liquids, meaning they are salts that are liquid at room temperature. This unique chemical property means the exact same propellant can be used in two radically different ways: it can be burned in a chemical thruster for high-thrust maneuvers, or it can be used as the propellant for a highly efficient electric thruster. This enables the design of “bimodal” propulsion systems that can perform both high-thrust and high-efficiency maneuvers from a single, shared propellant tank, a concept successfully demonstrated by NASA‘s Green Propellant Infusion Mission (GPIM).

The Marathon Runners: Electric Propulsion

While chemical rockets provide the explosive power needed to leave Earth, a different class of engine excels at the long-haul journeys through space. Electric propulsion (EP) systems use electrical power, typically generated by large solar panels, to accelerate a very small amount of propellant to extraordinarily high speeds. They are not limited by the finite chemical energy stored in their propellant; their performance is constrained only by the amount of electrical power available on the spacecraft. The result is a propulsion system with phenomenal specific impulse—often ten times greater than the best chemical rockets—but with very low thrust. A chemical rocket provides a powerful push for a few minutes; an electric thruster provides a gentle, continuous push for months or even years.

Ion Thrusters: Gentle and Steady Acceleration

An ion thruster is the quintessential high-efficiency engine. Its operation begins by feeding a small amount of an inert gas, most commonly xenon, into a chamber. Inside this chamber, a stream of electrons bombards the xenon atoms, knocking away one of their own electrons and leaving behind a positively charged xenon atom, or “ion”. These ions then drift toward a pair of grids at the rear of the thruster. These grids are charged with a strong electrostatic potential, one positive and one negative. As the positive ions pass through the grids, they are powerfully accelerated by the electric field, shooting out of the engine as a high-velocity beam. This beam of ions, though individually tiny, collectively produces a small but constant thrust. A separate device called a neutralizer emits a stream of electrons that recombines with the ion beam as it leaves the spacecraft, ensuring the vehicle does not build up a static charge.

Ion thrusters possess the highest specific impulse of any mature propulsion technology, but they also produce the lowest thrust. The thrust from a typical ion engine is often compared to the force exerted by a single sheet of paper resting on your hand. While this gentle push is useless for launching from a planet, its extreme efficiency makes it perfect for deep-space science missions that require enormous changes in velocity over very long periods. For these missions, the total mass of the propellant is the single greatest limiting factor. A mission like NASA‘s Dawn, which successfully traveled to the asteroid belt and entered orbit around two separate celestial bodies—Vesta and Ceres—would have been physically impossible with chemical rockets. It simply could not have carried enough fuel. The hyper-efficient ion thrusters were the enabling technology for the mission. Ion engines are also used for applications requiring extreme precision, such as the LISA Pathfinder mission, which used them to demonstrate the technology needed to detect gravitational waves.

Hall-Effect Thrusters: A Powerful Middle Ground

A Hall-effect thruster (HET) represents a compromise between the extreme efficiency of an ion thruster and the need for higher thrust. Like an ion engine, a HET works by ionizing a propellant like xenon. However, it accelerates the ions using a different and more compact method. Instead of physical grids, a HET uses a powerful radial magnetic field to trap electrons in a swirling, high-energy ring of plasma inside an annular channel. This circulating cloud of electrons is known as the Hall current. As neutral xenon gas is injected into the channel, it is ionized by collisions with these trapped electrons. The cloud of electrons and newly formed positive ions also creates a powerful axial electric field. This field is what accelerates the positive xenon ions out of the thruster at high speed, generating thrust.

Hall thrusters generally have a lower specific impulse than ion thrusters, but it is still an order of magnitude better than chemical rockets. Their key advantage is a much higher thrust density, meaning they can produce more thrust for a given size and power level, making them more compact and often simpler than ion engines.

This balance of good efficiency and higher thrust has made Hall thrusters the dominant technology for commercial applications. They are the go-to choice for satellite station-keeping—the small, periodic burns needed to counteract orbital perturbations and keep a communications satellite in its precise geostationary slot for its 15-year lifespan. They are also widely used to perform the orbit-raising maneuver itself, slowly and efficiently spiraling a satellite up from the lower orbit where the launch vehicle drops it off to its final operational altitude. They are used extensively in the large satellite mega-constellations being deployed by companies like SpaceX for its Starlink network.

The rise of electric propulsion is a story that is inseparable from the advancement of solar power technology in space. EP systems require kilowatts of power to operate, and their adoption was initially limited by the small, inefficient solar panels available on early spacecraft. As solar arrays became larger, lighter, and more efficient, more power became available, transforming EP from an experimental technology into a mainstream, mission-enabling one. The planned Power and Propulsion Element (PPE) for the lunar Gateway space station represents the pinnacle of this trend. It is a 60-kilowatt class spacecraft, essentially a massive power station and propulsion system in one, designed to maneuver the entire station around the Moon using advanced Hall thrusters.

This widespread adoption, particularly in the commercial sector, is now driving the next wave of innovation. For a one-of-a-kind government science mission like Dawn, the high cost of the ideal propellant, xenon, is acceptable. For a company like SpaceX, which plans to operate thousands of Starlink satellites, the cumulative cost of xenon would be astronomical. This powerful economic pressure has driven the development of Hall thrusters that can operate efficiently on cheaper and more abundant propellants, first with krypton and more recently with argon. This shift is a clear example of how commercial market forces, not just government-funded science, are now a primary driver of innovation in space technology, optimizing for new parameters like cost and supply chain logistics.

The Game Changers: Advanced and Novel Propulsion

Beyond the established chemical and electric systems, engineers are developing a new generation of advanced propulsion technologies. These are not just incremental improvements; they are designed to overcome fundamental barriers and enable entirely new kinds of missions, from sending humans to Mars to dispatching probes to other stars.

Nuclear Thermal Propulsion (NTP): The Key to Mars?

Nuclear Thermal Propulsion (NTP) is a technology that promises to break the long-standing trade-off between thrust and efficiency. Instead of using a chemical reaction to generate heat, an NTP engine uses a compact nuclear fission reactor. During operation, a liquid propellant—typically hydrogen, the lightest and most efficient propellant possible—is pumped directly through the reactor’s core. The core, containing uranium fuel, is heated to thousands of degrees by nuclear fission. This intense heat causes the liquid hydrogen to vaporize and expand explosively, and it is then channeled through a nozzle to produce thrust.

The performance of an NTP system is remarkable. Because it uses pure hydrogen as its propellant, which has a very low molecular weight, it can achieve a specific impulse of 900 seconds or more. This is at least double the efficiency of the best chemical rockets. At the same time, because it generates energy from a powerful nuclear reactor, it can produce a very high level of thrust, comparable to large chemical engines. It offers the best of both worlds: high thrust and high efficiency.

This unique combination makes NTP the leading candidate technology for rapid, crewed missions to other planets, particularly Mars. A journey to Mars using conventional chemical rockets would take six to nine months. Such a long transit time would expose astronauts to dangerous levels of cosmic radiation in deep space, a major health risk and one of the biggest obstacles to human interplanetary exploration. An NTP-powered spacecraft could cut that transit time by 25% or more, significantly reducing the crew’s radiation dose and improving mission safety. For safety reasons, NTP engines would not be used to launch from Earth’s surface; a spacecraft would be lifted to orbit by a conventional rocket, and only then would its nuclear reactor be turned on. NTP is a revived concept from the NERVA program of the 1960s, now made far more practical by advances in high-temperature materials and inherently safe nuclear fuel designs, like TRISO particles, which are engineered to be meltdown-proof.

Solar Sails: Riding on Light

Solar sailing is a truly futuristic concept that does away with on-board propellant entirely. A solar sail is a vast, ultra-lightweight, mirror-like sheet of material that can be deployed in space. It generates thrust by harnessing the pressure of sunlight itself. Light is made of particles called photons. While photons have no mass, they do carry momentum. When a photon from the Sun strikes the reflective surface of the sail, it bounces off, transferring its momentum to the sail and giving it a tiny push. While the push from a single photon is infinitesimal, the Sun emits a ceaseless, enormous stream of them. The cumulative effect of these countless impacts provides a constant, gentle acceleration that can propel a spacecraft through the void.

As a propellant-less system, a solar sail’s operational lifetime is limited only by the durability of its thin-film material. The thrust is extremely low, far lower even than an ion thruster, but it is constant and unending as long as the sail is in sunlight. Over months and years, this continuous acceleration can build up to very high speeds.

The applications for solar sails are as unique as the technology itself. They are ideal for very long-duration missions to the outer solar system or beyond. They also enable entirely new types of orbits that are impossible for spacecraft that rely on gravity alone. For example, a solar sail could be used to “hover” a spacecraft in an unstable position, such as between the Earth and the Sun, using the constant pressure from sunlight to counteract gravity. This could be invaluable for a solar storm warning satellite, giving us an earlier alert of potentially damaging space weather. In the more distant future, it’s envisioned that giant solar sails could be pushed by powerful, ground-based lasers, providing a method to accelerate interstellar probes to a fraction of the speed of light for journeys to other star systems.

These advanced propulsion systems represent a shift in thinking. Chemical and electric propulsion are focused on optimizing the use of a finite resource—the propellant carried on board. NTP addresses the problem by bringing a vastly more powerful and dense energy source—nuclear fission—to the task. Solar sails take the ultimate step by abandoning on-board fuel entirely, instead “living off the land” by harnessing the energy already present in the space environment. These are the technologies being developed to address the next great challenges in exploration, from protecting human life on the way to Mars to sending our first emissaries to the stars.

Choosing the Right Engine for the Job

There is no single “best” propulsion system. The optimal choice is always a complex trade-off between power, efficiency, cost, complexity, and the specific objectives of a given mission. A system perfectly suited for one task may be completely wrong for another. The art of mission design lies in selecting the right tool—or combination of tools—for the job.

A Comparative Look at Propulsion Systems

The diverse landscape of in-space propulsion technologies can be summarized by comparing their key performance characteristics. The following table provides an at-a-glance comparison, highlighting the fundamental trade-offs that mission designers face. It clearly illustrates the inverse relationship between thrust and efficiency that defines much of propulsion engineering, as well as the unique capabilities offered by more advanced systems.

Matching the Engine to the Mission

The practical application of these technologies is best seen through the lens of real-world missions. Each mission’s unique requirements dictate the propulsion architecture.

• Launch to Orbit: Overcoming Earth’s gravity requires immense thrust, making high-power chemical rockets the only viable option. The Saturn V rocket that sent Apollo astronauts to the Moon used powerful liquid-propellant F-1 engines in its first stage, while the Space Shuttle relied on two massive solid rocket boosters combined with its own liquid-fueled main engines to get into orbit.
• Satellite Station-Keeping: Geostationary communications satellites must maintain their precise orbital slot for over 15 years. This requires tiny, periodic, and highly fuel-efficient orbital adjustments. Hall-effect thrusters have become the industry standard for this task, used on countless commercial satellites from providers like SES and on large constellations like Starlink.
• Interplanetary Science: NASA‘s Dawn mission needed to travel to the asteroid belt and enter orbit around two different bodies, Vesta and Ceres. This feat of orbital mechanics required such a massive change in velocity that it would have been impossible with chemical rockets. The mission was only made possible by the extreme fuel efficiency of its three NSTAR ion thrusters, which fired for a cumulative total of over five years. Similarly, the BepiColombo mission to Mercury is using ion thrusters for its long journey through the inner solar system.
• Future Crewed Missions to Mars: The primary challenge for sending humans to Mars is not just getting there, but getting there quickly to minimize the crew’s exposure to harmful deep-space radiation. This requires a propulsion system with both high thrust and high efficiency, a combination that points directly to Nuclear Thermal Propulsion as the leading candidate for the primary transit vehicle.

The following table connects these propulsion technologies to specific, notable missions that have relied on them.

Summary

The ability to move through space is the cornerstone of all exploration and utilization of the cosmos. The methods for achieving this are diverse, each born from a specific need and governed by a fundamental set of physical principles. From the raw, unbridled power of chemical rockets that tear spacecraft from Earth’s grasp, to the patient, hyper-efficient whisper of electric thrusters that guide probes on decades-long journeys, each technology has its place.

The field is defined by a constant balancing act. Mission planners must weigh the brute force of high thrust against the endurance of high efficiency, the simplicity of a solid motor against the fine control of a liquid engine, and the proven reliability of existing systems against the revolutionary potential of new ones. There is no single best engine, only the right engine for the task at hand. The future of spaceflight will rely on a growing and diverse toolkit of these technologies, skillfully combined to push the boundaries of what is possible. The development of advanced systems like nuclear thermal propulsion and solar sails signals a new chapter, one aimed at enabling humanity’s most ambitious goals: setting foot on other worlds and sending our creations to other stars.