Ad Astra: The future of propulsion technology

For more than sixty-five years, humans have sent payloads and exploration crews to space using vehicles powered by reaction engines - otherwise known as rockets.

The concept is as simple as it is effective, not to mention dangerous. A large, standing vehicle with a fairing initiates a controlled explosion in its interior. The explosion is channeled through nozzles to achieve “escape velocity,” the speed necessary to break free of Earth’s gravity.

On Earth, that means achieving a velocity of 6.96 mi/s (11.2 km/s), which requires a very sudden and significant change in velocity – otherwise known as delta-v (Δv). It takes a lot of propellant to accomplish that, and the heavier the payload, the larger the rocket, and the more propellant is needed.

Is it any wonder why aeronautical engineers, mission planners, and futurists have dreamed of a day when conventional rockets are no longer needed? A lot of research has gone into this very idea, some dating to before the Space Age even began. 

Rocket Equation

No matter how far we've come with the technology, rockets will always be subject to the tyranny of the Rocket Equation.

The foundational bit of aeronautical science is commonly attributed to Russian-Soviet rocket scientist Konstantin Tsiolkovsky (1857-1935), who published it in 1903 (though other scientists independently derived it before and after). 

This equation describes the motion of vehicles that expel some of their mass to generate thrust. This can be represented mathematically as:

Δv = v_e ln (m₀ ÷ m_f) = I_spg_o ln (m₀ ÷ m_f)

Where delta (Δ) is the change in velocity (v), ve is the exhaust velocity, ln is the natural logarithm, I_sp is the efficiency of the thrust generated (specific impulse), g₀ is the force of gravity, m₀ is the initial mass (including propellant), and m_f is the “dry” mass (no propellant). 

This equation has been used for almost seventy years to calculate a rocket's dry mass and the fuel needed to send payloads into space. 

A Vicious Circle

To put it simply, there is a major downside to this equation. Since the beginning of the Space Age, every rocket ever made has been mostly propellant in mass.

To illustrate, consider the most powerful rockets in the world - NASA’s Space Launch System (SLS) and SpaceX’s Starship & Super Heavy.

Whereas the SLS weighs 188,000 lbs (85,275 kg) when it’s not fueled up, its mass increases to  5.75 million lbs (2.6 million kg) once it's fueled and ready for launch. Meanwhile, the Starship launch system has a dry mass of 170,000 lbs (77,110 kg) versus a “wet mass” of 11 million lbs (~5 million kg).

Doing the math, this means that over 96% of the SLS’ full weight consists of propellant. Even worse, the weight of a fully-fueled and stacked Starship & Super Heavy is close to 98.5% propellant.

Now consider the payloads these launch systems can send to Low Earth Orbit (LEO). For the SLS, that’s 209,439 lbs (95,000 kg), and 200,000 to 300,000 lbs (90,718.5 to 136,078 kg) for the Starship. Comparing this to their fully-fueled weight, we see that the SLS and Starship can deploy just 3.65% and 2.72% of their mass to LEO (respectively). 

And keep in mind that this is merely for sending payloads to LEO. To send cargo and crew to the Moon, Mars, or anywhere else in the Solar System, rockets need to generate even more thrust, which means payloads must be smaller. So for missions beyond Earth, an even smaller fraction of a rocket’s overall mass can be payload.

In essence, the bigger the payload, the bigger the rocket needs to be. The bigger the rocket, the more massive it becomes. The more massive it becomes, the more propellant it needs to make it to space. The more propellant it needs, the more massive it becomes. It’s a vicious circle and none too efficient, either. 

And this applies to missions once they reach space as well. To ensure that a spacecraft’s mass isn’t too great, spacecraft designers and mission planners limit the amount of chemical propellants used.

More often than not, spacecraft thrusters will rely on solid chemical propellants. These can provide lots of thrusts, but their limited supply means they must be used sparingly and only for course corrections and maneuvers.

Luckily, there are alternatives, some of which are on the drawing board right now.

Nuclear Propulsion

During the early Space Age, scientists recognized the potential for bringing nuclear power and spaceflight together.

At a time when advanced research was leading to concurrent progress with nuclear bombs, nuclear reactors, and rockets, scientists saw applications for the peaceful use of nuclear energy. 

Between 1963 and 1972, these efforts bore fruit with the creation of the Nuclear Engine for Rocket Vehicle Application (NERVA), a slow-fission solid core nuclear reactor designed for long-range crewed space missions to the Moon or interplanetary destinations.

The Soviet Union also researched the technology, which yielded the RD-0410, a nuclear thermal rocket engine developed from 1965 through the 1980s.

These reactors were intended to become part of a Nuclear-Thermal Propulsion (NTP) system, where heat generated by the slow decay of radioactive isotopes is used to heat liquid hydrogen or deuterium fuel. This causes the fuel to expand, which is directed through nozzles to generate thrust. Between 1972 and today, several concepts for NTP have been proposed, and the technology remains the most commonly-researched application.

In 2017, NASA renewed its attempts to create an NTP system through its Game Changing Development Program. In 2023, NASA and the Defense Advanced Research Projects Agency (DARPA) announced a joint effort to develop an NTP concept called the Demonstration Rocket for Agile Cislunar Operations (DRACO). This will culminate with a demonstration of the DRACO in orbit, which is expected to occur by early 2027.

Since the turn of the century, there have also been proposals for Nuclear-Electric Propulsion (NEP). This method consists of a nuclear reactor generating electrical power for a Hall-Effect Thruster or ion engine, which ionizes an inert gas (like xenon) and directs the charged particles through nozzles to produce thrust.

Efforts to realize a NEP system included Project Prometheus, launched by NASA in 2003. This project yielded the Jupiter Icy Moons Orbiter (JIMO), a proposal for an uncrewed NEP spacecraft that would explore three of Jupiter's largest moons, Europa, Ganymede, and Callisto. The proposal was passed over in 2005 in favor of the Constellation Program.

Throughout this same period, proposals have been made for “bimodal concepts,” which rely on both NTP and NEP systems. Both NTP and NEP offer multiple advantages over traditional chemical rockets. Among them, there’s higher energy density, where a nuclear reactor can extract far more energy per pound than chemical propellants.

What’s more, NTP offers twice the efficiency of chemical rockets, while NEP is 5 to 10 times as efficient. This higher efficiency enables NTP or NEP vehicles to be made at one-third to one-half the size of conventional vehicles.

Famed engineer, NASA technologist, spaceflight expert, and author Les Johnson summarized the potential for nuclear propulsion to Interesting Engineering via email:

"Using a fission rocket to get from near-Earth space (not from the ground to space!) would reduce the required propellant load by 50%, which is significant since propellant for a roundtrip mission to Mars would be the single heaviest element launched and launch costs are driven by mass. It would also reduce the trip time and provide more flexibility in launch windows, making the mission more resilient to potential technical issues and associated delays. For human missions in the inner Solar System out to about Jupiter, nuclear thermal (fission) propulsion is a game-changer."

Fusion Propulsion

Beyond nuclear applications, several propulsion methods are possible using current technology. These include Nuclear Pulse Propulsion (NPP), a method researched through Project Orion between 1958 to 1963. The project was overseen by physicist Ted Taylor at General Atomics and famed physicist Freeman Dyson (who proposed the Dyson Sphere). 

The concept calls for a massive spacecraft loaded with hundreds (or thousands) of nuclear devices. These are released periodically from the spacecraft’s aft section and detonated, creating shock waves that are absorbed by a rear-mounted pressure plate.

This plate transforms the shock waves into forward momentum for the spacecraft, accelerating it to relativistic speed (a speed large enough to make the mass of a body greater than its resting mass; it is expressed as a proportion of the speed of light). 

The project was abandoned in 1963 with the passage of the Partial Ban Nuclear Test Treaty (PTBT), which outlawed testing nuclear devices in space. However, the concept has resurfaced over the years and is still considered a potential means for realizing an interstellar mission.

Shortly thereafter, scientists began researching fusion propulsion through Project Daedalus, conducted between 1973 and 1978 by the British Interplanetary Society (BIS). This project was built on the work of Project Orion and similarly envisioned achieving relativistic speed through nuclear pulses. 

However, this was to be achieved by internal confinement fusion, where electron beams bombed small pellets of deuterium and helium-3 in a combustion chamber. This would trigger reactions akin to tiny thermonuclear explosions. The resulting plasma would be confined and channeled by a powerful magnetic field to produce powerful thrust. 

The idea was picked up in 2009 by Icarus Interstellar, an international organization comprised of members of the British Interplanetary Society (BIS) and the Tau Zero Foundation (TZF), volunteer experts, and citizen scientists. Between 2009 and 2019, they researched a scaled-down version of Daedalus called Project Icarus. 

There’s also the Bussard Ramjet, a fusion concept proposed in 1960 by physicist Robert W. Bussard and popularized in the famous 1970 science fiction novel Tau Zero by Poul Anderson.

In this case, a spacecraft generating powerful magnetic fields would channel hydrogen from the interstellar medium (ISM) into a magnetic confinement chamber, compressing it until nuclear fusion occurs.

While promising, these concepts are prohibitively expensive by our standards today. This includes construction, which must be performed in space to avoid the extreme costs of launching all the prefabricated components into orbit. Second, the cost of manufacturing the fuel would be similarly prohibitive given the rarity of deuterium and helium-3. Dr. Johnson said:

"Fusion propulsion would be truly revolutionary and open the solar system for human exploration and settlement. Before we can seriously consider a fusion rocket, we need to first demonstrate that fusion reactors can operate on the ground and consistently produce significantly more energy than they require to initiate the fusion reaction. While various engineering efforts appear to be close to generating net positive energy, they need to be able to create a lot more energy than they consume, a goal not yet even close to being met. Then there is the issue of scale - the entire fusion reactor would have to be miniaturized to fit in a spacecraft."

Nevertheless, the physics behind these proposals is sound, and the concepts could be realized someday, providing in-space assembly becomes possible, and additional sources of deuterium and helium-3 are procured.

* * *

Taken together, propulsion methods that rely on nuclear fission or fusion are considered the future of spaceflight. However, these concepts are only one part of a much larger constellation of concepts.

With every breakthrough in physics, new ideas are proposed, old ideas are reconsidered, and new attempts are made to realize them.

Stay tuned for Part II of this installment in the Ad Astra series, where we will examine Directed Energy, Antimatter, and even more exotic propulsion methods!