Space Flight Basics & the Promise of Fusion Propulsion

Fusion propulsion could revolutionize deep space travel

A user on r/scifiwriting asked “What is the maximum efficiency of a fusion engine?”

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Specific Impulse (Isp) and Exhaust Velocity: Measuring Propulsion Efficiency

There is a concept called "specific impulse" (Isp) and it is generally regarded as the index of the "efficiency" of a space propulsion system. Isp is a direct physico-chemical result of the energetic characteristics of the elements and reactions used in the propulsion system. Though it is commonly expressed in “seconds,” it directly reflects the exhaust velocity—which in turn depends on the thermodynamic properties of the propellant (or of the fuel/propellant combination, depending on the system) and the physics of how it's expelled.

Setting aside reactions which involve anti-matter (or some exotic matter as yet undiscovered) fusion, offers the highest theoretical exhaust velocities—because it taps into nuclear binding energy, orders of magnitude more potent than the energy released by breaking molecular bonds in chemical rockets.

Typical chemical rockets achieve Isp in the 300 to 450 seconds ballpark. Ion thrusters ~1,000 + (NASA's NEXT is speculated to reach potentially 10,000). We can only speculate about what sorts of Isp might be possible with fusion powered propulsion, but reasonable models range from 10,000 to 1,000,000 s depending on the design and exhaust configuration.

Isp of a particular space craft drive train, combined with the total mass of the ship when fully fueled and stocked for a journey can be used to determine the spacecrafts Delta-v—the total possible change in velocity the spacecraft can achieve without refueling or resupplying propellant.

Because the fuel for a fusion drive (deuterium, tritium, helium-3) is likely to last for many years of service, and the propellant is likely to be a low-mass substance (whether it's the fusion product itself in a direct "torch" design, or a separate working fluid like lithium heated by the fusion reaction), spacecraft powered by fusion drives are expected to have extremely large delta-v budgets.

As a comparison:

The Apollo Saturn V rockets had ~10.4 km/s of delta-v

A typical early 21st-century unmanned probe had ~14 km/s

A reasonable speculative estimate for a fusion-powered torchship is ~10,000 to 1,000,000 km/s

The Other Half of the Equation: Thrust Matters Too

Now you may be asking: "If ion thrusters already offer Isp values of 1,000 or more—and possibly up to 10,000—why aren’t more spacecraft designed with them?"

The answer is that Isp is not the only meaningful index of a propulsion system’s performance. Thrust—the actual force the engine produces—is just as critical. Ion thrusters achieve high efficiency by expelling ions at extremely high velocity, but they do so in minuscule amounts. Their thrust is measured in milli-Newtons, which means it takes weeks or even months to build up significant speed. They’re ideal for deep-space probes that don’t require rapid maneuvers, but they’re completely inadequate for launch or fast-response applications.

By contrast, chemical rockets have terrible Isp but generate massive thrust (Each F-1 engine of the Saturn V produced about 6.77 MN [Meganewtons] of thrust at sea level), which is why they remain essential for lifting payloads from planetary surfaces.

Fusion propulsion, depending on the design, might offer both high Isp and usable thrust—but usually not at the same time. Many designs require a tradeoff between the two, like shifting gears between “economy mode” and “power mode.” Getting into the details of how fusion drives might achieve various ratios of Isp to thrust is beyond the scope of this already quite lengthy response . . . suffice to say: fusion drives on most manned missions probably never need to achieve more thrust than is necessary to achieve 1 or perhaps 2 g of acceleration—primarily for reasons of human tolerance, which I discuss in more detail below.

Burns, Coasts, and Course Corrections: How Spacecraft Actually “Fly”

Turning now to the concept of burn and coast phases: spacecraft have always, and likely will always, conduct their voyages using scheduled and carefully orchestrated periods of active propulsion—called "burns"—followed by long coasting periods. In missions involving orbital insertion or atmospheric reentry, a deceleration burn may also be required.

In short, once a spacecraft has oriented itself properly to achieve the desired course—whether a Brachistochrone transfer as imagined in The Expanse, or a more traditional Hohmann transfer as used today—it engages its propulsion system. This subjects the ship and its occupants to transverse acceleration, typically measured in G-forces (where 1 g = 9.8 m/s², the acceleration due to gravity at Earth’s surface).

After a sufficient burn, the spacecraft reaches its desired velocity and shuts off its main engines. The craft then enters a coast phase, during which the primary propulsion system is inactive. Attitude thrusters may be used sparingly to reorient the spacecraft (e.g., to point an antenna or position a Whipple shield). While the velocity remains constant, the ship will no longer experience thrust-related acceleration; the crew, unless artificial gravity is generated through rotation, will be in microgravity.

At the appropriate moment, the ship reorients—usually pointing the nose retrograde—and initiates a deceleration burn to slow down for orbital insertion or rendezvous.

Turning back to the topic of thrust: one factor your question may not have accounted for is the inherent biological limits of the human body when it comes to tolerating sustained acceleration.

A healthy human can only endure 2–3 Gs for extended periods (e.g., several minutes) with support. Trained fighter pilots may experience 5–9 Gs, but only briefly, and only with the help of G-suits that prevent blood from pooling in the extremities.

Weeks of high-G acceleration? Unlikely.

Fusion torchships pulling 4–6 Gs for extended periods? Only viable for unmanned payloads, or kinetic projectiles. For crewed ships, such accelerations would be lethal without extreme mitigation measures—like fluid immersion tanks, hibernation, or artificial gravity counter-forces: all of which is quite speculative.

A Back-of-the-Napkin Look at Mars Transit with Fusion Propulsion

To put this all into perspective: if we assume a manned spacecraft being sent to Mars with a total compliment of 20 and sufficient provisions for a long round trip, and assuming some modicum of onboard scientific equipment and means to land on the surface and return, a total mass of 2,000,000 kg seems a reasonable approximation: comparable to a scaled-up version of the ISS with deep-space architecture. If we assume a design like the ships in the Expanse (in which the decks are oriented perpendicular to the axis of acceleration) then accelerating at 1 g (9.81 m/s^2) is entirely reasonable.

Accelerating a mass of 2,000,000 kg at 9.81 m/s² requires only 19.62 MN of thrust—about half the launch thrust of the five F-1 engines of Saturn V, and not enough to lift a massive payload from Earth’s surface, but entirely sufficient for a plausible early-era inner solar system transport ship that doesn’t need to escape Earth’s gravity well. Of course, the question of whether fusion propulsion will ever be viable for launching from planetary surfaces is an entirely separate issue from how it might revolutionize deep-space flight—and it’s in that latter domain where its true potential likely lies.

With that Tsiolkovsky's rocket equation gives us different total dV with different propellant mass:

25% of ship mass as propellant = 565 km/s

50% = 1359 km/s

65% = 2,116 km/s

If we assume a Brachistochrone transfer and the ship accelerates at 9.81 m/s², it will reach 1,000 km/s in about 1,699 minutes (~28.3 hours). (This ignores mass loss from propellant burn, which complicates things quickly.) Given that Mars–Earth opposition ranges from 55 million km to 400 million km, a one-way trip at 1,000 km/s would take between 55,000 and 400,000 seconds—or between 0.64 and 4.63 days.

So, including a 1.2-day acceleration burn, a 1.2-day deceleration burn, and 1 to 5 days of coasting, a fusion engine (thanks to its enormous Isp) could conceivably complete a journey to Mars in under a week—compared to the 6 to 9 months required by Hohmann transfers using conventional propulsion.

That 1,000 km/s acceleration burn at 200,000 Isp would expend roughly 798,078 kg of propellant. (Again, this is a simplified estimate that doesn't account for the fact that the ship’s mass decreases during the burn.) A round trip would double this figure, requiring ~1.6 million kg of propellant—or 80% of the total ship mass. These numbers don’t fully add up—since they're rough, seat-of-the-pants estimates—but what they reveal is that even 200,000 Isp isn’t that much when you're talking about very high velocities like 1,000 km/s.

If you cap maximum velocity at 500 km/s, the total trip time stretches to ~6 to 19 days. At 250 km/s, it's more like 12 to 38 days--which is still much better than the current ideal trip with current propulsion (~180 to 270 days using a Hohmann transfer and chemical rockets). Alternatively, increasing the Isp of the engine allows for comparable trip duration at lower propellant fractions—which is why high-Isp systems are so attractive.

The Promise—and Limits—of Fusion Propulsion

With all that said: although fusion space propulsion remains speculative, the physics behind it are sound. There’s no guarantee that highly efficient, controlled fusion will ever be practically achieved—but the long-term trajectory of experimental progress suggests it may be only a matter of time. The biggest challenge lies in controlling and containing the fusion reaction efficiently and sustainably. So far, fusion reactors have only produced fleeting reactions, and even hypothetical “net energy” events have yet to demonstrate cost-effective, sustained operation.

When—and if—fusion propulsion becomes a reality, it could mark an enormous revolution in human spaceflight: enabling much larger vessels, far greater achievable velocities, and vastly extended mission profiles.