Innovation News Network delves into the pros and cons of chemical and electric propulsion, and why slow and steady might actually win the race.
When you think of rockets, you probably imagine a bright flame, a thunderous roar and a vehicle blasting off into the sky. That’s chemical propulsion at its most dramatic — and it’s exactly what we want for launch.
However, once we escape Earth’s gravity well and drift into the quiet of space, a very different kind of engine starts to make sense.
In the realm of deep space exploration, a new generation of spacecraft doesn’t rely on raw power so much as persistent, efficient thrust. Electric propulsion — especially ion and Hall-effect thrusters — can provide that gentle push for thousands of hours. It’s slower, yes, but over the long haul it delivers far more delta-v — the change in velocity needed to get where we want to go — than chemical rockets ever could.
Let’s unpack why that is, how the technologies differ, and why tiny engines with peculiar names are quietly redefining how we explore the solar system.
What makes chemical rockets work
Chemical rockets are straightforward in concept: you mix a fuel and an oxidiser, burn them, and use the expanding gas to push against a nozzle. The energy comes directly from breaking chemical bonds, and because that energy release is so intense, you get a huge thrust — kilogrammes of force over a short period. That’s exactly what you need to leave Earth, punch through the atmosphere and place a payload on a course to orbit or beyond.
The downside is that chemical rockets are inefficient, requiring more propellant. The key metric here is specific impulse (Isp), which measures the thrust per unit of propellant. Chemical engines have an Isp typically around 300–450 seconds — good for short, powerful bursts, but costly if you need a large change in velocity.
Beyond a certain point, loading up on chemical fuel becomes impractical. Carry ten times the propellant, and you need ten times the rocket to lift it. That quickly becomes a losing game.
Introducing electric propulsion
Electric propulsion throws out the old playbook. Instead of burning propellant, it uses electrical power (from solar panels or potentially nuclear reactors) to accelerate atoms — usually xenon — to extremely high speeds. It’s the exhaust velocity that matters here: the faster you can throw mass out the back, the higher your Isp. And electric systems can achieve Isp values an order of magnitude higher than those of chemical rockets.
Two of the most common types are:
- Ion thrusters: These use electric fields to accelerate charged atoms (ions) out of the engine. They can achieve an Isp of several thousand seconds.
- Hall-Effect thrusters: These confine electrons in a magnetic field and use them to ionise and accelerate propellant. They typically have slightly lower Isp than pure ion engines, but can offer higher thrust density in some configurations.
No matter the specific technology, the theme is the same: high exhaust velocity, low thrust.
Thrust vs efficiency: A fundamental trade-off
One of the most confusing things about electric propulsion is that, despite its higher efficiency, it produces very little thrust. The ion engines on NASA’s Dawn mission, for example, created just about 90 millinewtons of force — roughly the push of holding a sheet of paper in your hand. Even the flagship chemical engines on rockets generate millions of newtons over short bursts.
That’s not a flaw in the engineering so much as physics and design priorities. Electric thrusters are usually power-limited: the amount of electrical power a spacecraft can produce and channel into the engines limits how much propellant it can accelerate and how quickly. Space doesn’t require fast bursts or fighting atmospheric drag, so slow and steady wins the race.
Chemical propulsion, by contrast, is thrust-limited by the energy density of chemical reactions. You can get a lot of thrust for a short period, but you quickly run out of reactants.
Why slow works in the vacuum of space
In deep space, there’s no air resistance and no gravity to counteract. That means even tiny forces matter if they act over long durations. If you can keep pushing in the same direction for years, those small accelerations accumulate into very large changes in velocity.
That was precisely the case with the Dawn spacecraft. After launch on a conventional chemical booster, Dawn’s ion engines operated nearly continuously for thousands of days, eventually achieving a total delta-v comparable to the initial launch rocket but using far less propellant.
Electric propulsion also shines when mission designers want to make large orbital adjustments or visit multiple targets. Dawn didn’t just fly to one asteroid — it entered orbit around two separate bodies in the main asteroid belt. That kind of flexibility would have been nearly impossible with chemical systems without exorbitant fuel mass.
Chemical rockets still have their place
Electric propulsion isn’t a replacement for chemical rockets, at least not yet. The high thrust that chemical engines deliver in a short burst is still essential for:
- Launching from Earth’s surface
- Rapid orbital insertion
- Quick manoeuvres in Earth orbit
Nothing electric can do today matches that kind of performance because electric systems simply can’t produce enough instantaneous force. Even if you ran them at full power for days, they wouldn’t accelerate a spacecraft fast enough to serve as a launch vehicle.
That’s why most spacecraft still use chemical propulsion early in their missions, switching to electric systems for long-duration course changes.
Real-world examples
It’s one thing to talk about principles; it’s another to see them in action.
Dawn used gridded electrostatic ion engines to travel to the asteroid belt and visit both Vesta and Ceres. These engines were small in force but incredibly efficient, enabling a cumulative delta-v that rivalled the launch vehicle’s contribution.
ESA’s SMART-1 mission used a Hall-effect thruster powered by solar arrays to reach the Moon. Its thrust was tiny — fractions of a newton — but over months of operation, it achieved orbit around Earth’s nearest neighbour.
More recent missions, like NASA’s Psyche spacecraft, also rely on electric propulsion, using Hall thrusters to push toward an asteroid believed to be rich in metal. This reflects how electric systems have matured to the point of main propulsion on major interplanetary missions.
Limitations and challenges
Electric propulsion looks great on paper, but it isn’t a silver bullet. Some of the practical challenges include:
- Power supply: You need reliable electrical power, usually from large solar arrays or potentially a nuclear source. Far from the Sun, solar power drops off with distance, limiting performance.
- Propellant choices: Xenon is popular because it’s easy to ionise and store, but it’s heavy and expensive. Researchers are exploring alternatives like krypton and even water vapour for small satellite thrusters.
- Thrust limitations: Because thrust is so low, electric propulsion cannot handle time-critical manoeuvres. It’s great for gradual trajectory changes, not for rapid burns or atmospheric flight.
The future: Hybrids and beyond
Engineers aren’t content to choose one or the other forever. Hybrid systems, where spacecraft can switch between chemical and electric modes depending on mission phase, are an area of active research and early development. These could combine the best of both worlds: hard kicks when you need them, and long, efficient rides when you don’t.
Looking further ahead, emerging concepts such as high-powered plasma engines or nuclear-electric propulsion could blur the lines even further, delivering higher thrust without sacrificing efficiency. But those technologies are still in early phases or on the research bench.
It’s all about how far you can keep going
At first glance, it seems counterintuitive that a propulsion system with tiny thrust could outperform a rocket engine.
However, in the vacuum of space, efficiency over time can outweigh explosive power. Electric propulsion delivers that efficiency, making missions possible that would otherwise require untenable amounts of fuel.
Chemical rockets will continue to serve when big, rapid pushes are needed. But for long-distance journeys and missions demanding long-term adjustments, the gentle whisper of electric engines is becoming the dominant theme. It’s not about how fast you can go in a second — it’s about how far you can keep going.
