This endeavour demands a considerable acceleration - delta-v (Δv), and an extensive amount of propellant. The payload's weight also influences the size of the rocket and the propellant quantity required. Thus, it's hardly surprising that aerospace engineers, mission strategists, and futurists envision an era where traditional rockets may become obsolete. Numerous studies have focused on this prospect, some even predating the advent of space exploration.
The Rocket Equation
Regardless of technological advancements, rockets are still governed by the infamous rocket equation. This scientific principle, integral to rocket science, is often credited to the Russian-Soviet rocket scientist Konstantin Tsiolkovsky (1857-1935), who published it in 1903, despite other scientists independently deriving it before and after.
This equation, outlining the motion of vehicles that expel part of their mass to generate thrust, can be mathematically expressed as:
Δv = v e ln ( m 0 ÷ m f ) = I sp g o ln ( m 0 ÷ m f )
In this equation, delta (Δ) signifies the velocity change ( v ), ve represents the exhaust velocity, ln stands for the natural logarithm, I sp denotes the efficiency of the thrust generated (specific impulse), g 0 is the gravitational force, m 0 is the initial mass (including propellant), and m f is the "dry" mass (without propellant).
This equation has been a reliable tool for nearly seven decades to determine a rocket's dry mass and the fuel necessary to launch payloads into space.
A vicious cycle
To put it simply, there is a major drawback to this equation. Since the beginning of the space age, the bulk of the mass of every rocket ever made has been propellant.
To understand, consider the most powerful rockets in the world: NASA's Space Launch System (SLS), and SpaceX's Starship+SuperHeavy .
While the SLS weighs 85,275 kg when not fueled, its mass increases to 2.6 million kg once fueled and ready for launch. Meanwhile, the Starship launch system has a dry mass of 77,110 kg against a fully loaded mass of ~5 million kg.
Doing the math, this means that over 96 percent of the total weight of the SLS is propellant. Even worse, the weight of the fully fueled and stacked Starship plus launcher is close to 98.5 percent propellant.
Now consider the payloads that these launch systems can send into low earth orbit (LEO). For the SLS, it is 95,000 kg and 90,718.5 to 136,078 kg for the Starship. Comparing this with their fully loaded weight, we see that SLS and Starship can distribute only 3.65% and 2.72% of their mass to LEO (respectively).
And keep in mind that this is only for sending payloads to low orbit. To send cargo and crew to the Moon, Mars or anywhere else in the Solar System, rockets have to generate even more thrust, which means payloads have to be lower. Thus, for missions beyond Earth, an even smaller fraction of a rocket's total mass can be loaded.
In essence, the larger the payload, the larger the rocket must be. The larger the rocket, the more massive it becomes. The more massive it becomes, the more propellant it needs to reach space. The mo
re propellant it needs, the more massive it becomes. It is a vicious circle and not a very efficient one either.
And this is also true for missions once they reach space. To ensure that the mass of a spacecraft is not too great, spacecraft designers and mission planners limit the amount of chemical propellants used.
Most often, spacecraft thrusters rely on solid chemical propellants. These can provide a lot of thrust, but their limited availability means they must be used sparingly and only for course corrections and maneuvers.
Fortunately, there are alternatives, some of which are being studied at this time.
Nuclear propulsion
During the early space age, scientists recognized the potential for bringing together nuclear power and spaceflight. At a time when advanced research was leading to simultaneous advances 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 manned space missions to the Moon or interplanetary destinations.
The Soviet Union also worked on this technology, and produced the RD-0410, a nuclear thermal rocket engine developed from 1965 until the 1980s.
These reactors were to become part of a nuclear-thermal propulsion system (NTP), in which heat generated by the slow decay of radioactive isotopes is used to heat liquid hydrogen or deuterium. This causes the fuel to expand, which is directed through nozzles to generate thrust. Several concepts for NTP have been proposed between 1972 and today, and the technology remains the most sought-after application.
In 2017, NASA renewed its attempts to create an NTP system through its development program. In 2023, NASA and the Defense Advanced Research Projects Agency (DARPA) announced a joint effort to develop an NTP concept called Demonstration Rocket for Agile Cislunar Operations (DRACO). This will culminate with a demonstration of DRACO in orbit, which is expected to take place by early 2027.
Since the turn of the century, there have also been proposals for nuclear-electric propulsion (NEP). This method uses a nuclear reactor that generates electricity for a Hall-effect thruster or ion engine, which ionizes an inert gas (such as xenon) and directs charged particles through nozzles to produce thrust.
Efforts to make a NEP system included Project Prometheus, launched by NASA in 2003. This project produced the Jupiter Icy Moons Orbiter (JIMO), a proposal for an unmanned NEP spacecraft that was to explore three of Jupiter's largest moons: Europa, Ganymede and Callisto. The proposal was passed over in 2005 in favor of the Constellation program.
During this same period, proposals were made for "bimodal concepts," which are based on both NTP and NEP systems. Both NTP and NEP offer multiple advantages over conventional chemical rockets. These include higher energy density, where a nuclear reactor can extract much more energy per kilogram than chemical propellants.
In addition, NTP offers twice the efficiency of chemical rockets, while NEP is 5 to 10 times more efficient. This increased efficiency allows NTP or NEP vehicles to be produced one-third to one-half the size of conventional vehicles.
Renowned engineer, NASA technologist, spaceflight expert and author Les Johnson summed up the potential of nuclear propulsion: "Using a fission rocket to get from near-Earth space (not from the ground to space! ) would reduce the required propellant load by 50 percent, which is significant since the propellant for a round-trip mission to Mars would be the single heaviest item launched and launch costs are driven by mass, it would also reduce travel time and provide more flexibility in launch windows, making the mission more resilient to potential technical problems and associated delays. (Fission) propulsion is a game changer."
Fusion Propulsion: The Future of Interstellar Travel
Fusion propulsion presents intriguing possibilities for space travel, building on existing technology and research. Nuclear Pulse Propulsion (NPP), a methodology explored in Project Orion from 1958 to 1963, is one such possibility. Supervised by physicists Ted Taylor and Freeman Dyson, the project envisioned a colossal spacecraft equipped with hundreds of nuclear devices.
These devices, deployed and detonated in sequence from the rear of the spacecraft, would generate shock waves. A pressure plate mounted at the back would absorb these waves, converting them into forward propulsion, accelerating the spacecraft to relativistic speed. This is a speed substantial enough to cause an object's mass to exceed its rest mass, expressed as a proportion of light speed.
However, Project Orion was abandoned in 1963 following the Partial Ban Nuclear Test Treaty (PTBT), which prohibited the testing of nuclear devices in space. Despite this, the concept has resurfaced occasionally and is still perceived as a viable method to accomplish an interstellar mission.
Subsequent to this, fusion propulsion research was pursued via Project Daedalus from 1973 to 1978 by the British Interplanetary Society (BIS). The project aimed to achieve relativistic speed through nuclear pulses, similar to Project Orion. The approach envisaged was internal confinement fusion, where electron beams would bombard small deuterium and helium-3 balls in a combustion chamber to trigger reactions akin to miniature thermonuclear explosions. The resultant plasma would be contained and directed by a potent magnetic field to generate a powerful thrust.
This concept was adopted in 2009 by Icarus Interstellar, a global collective of BIS members, Tau Zero Foundation (TZF) members, volunteer professionals and amateur scientists. They studied a downscaled version of Daedalus, named Project Icarus, from 2009 to 2019.
The Bussard Ramjet, another fusion concept proposed in 1960 by physicist Robert W. Bussard and popularized in Poul Anderson's renowned 1970 science fiction novel Tau Zero, envisions a spacecraft that uses potent magnetic fields to guide hydrogen from the interstellar medium (ISM) into a magnetic confinement chamber, compressing it until nuclear fusion occurs.
While these concepts show promise, they are currently cost-prohibitive. This includes construction, which would need to occur in space to bypass the exorbitant costs of launching prefabricated components into orbit. Fuel production costs would also be extremely high given the scarcity of deuterium and helium-3.
Dr. Johnson states, "Fusion propulsion could revolutionize space travel and pave the way for human exploration and colonization of the solar system. However, before we can seriously contemplate a fusion rocket, we need to demonstrate that fusion reactors can operate on the ground, consistently producing significantly more energy than is needed to trigger the fusion reaction. While some engineering attempts appear close to achieving a net positive energy, they need to produce far more energy than they consume, a goal we are still far from accomplishing. Additionally, the entire fusion reactor would need to be miniaturized to fit into a spacecraft."
Nevertheless, the physics behind these proposals are fundamentally sound. Provided we can assemble in space and source additional deuterium and helium-3, these concepts could one day be realized.
Collectively, propulsion methods relying on nuclear fission or fusion are seen as the future of spaceflight. However, these are just one part of a larger constellation of concepts.
With each breakthrough in physics, new ideas are proposed, old ideas are reconsidered, and new attempts are made to realize them.
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