Welcome back to our series on Interstellar Travel, where we examine concepts that have been proposed since the dawn of the Space Age. In our previous installment, we looked at the earliest concepts to emerge from the Space Age. These reflected the nature of the times, where two superpowers were locked in a constant state of competition and advances in rocketry paralleled the development of nuclear weapons.

During this period, NASA and the Soviet Union also built and tested nuclear reactors that would generate heat or electricity for an advanced propulsion system. Their proposed systems fell into two categories: those that harnessed the power of shockwaves produced by nuclear explosions - Nuclear Pulse Propulsion (NPP) - and those that used nuclear reactors to heat propellant or generate electricity for an ion engine - Nuclear Thermal Propulsion (NTP), and Nuclear Electric Propulsion (NEC).

Around the same time, advances in thermonuclear weapons (aka. hydrogen bombs) also led to proposals for fusion propulsion concepts. These ambitious projects offered the prospect of making interstellar journeys within a human's lifetime. Not unlike nuclear concepts such as Project Orion, they also came with monumental challenges, not the least of which was the expense involved in building them. Here's a look at some of the more well-known ideas that came from this era.

Fusion Power (1960 - 2023)

During the 1960s, investigation into fusion propulsion broke off from previous work on NTP systems. Similar to nuclear propulsion, fusion concepts can also be divided into categories: Magnetic Confinement Fusion (MCF) and Inertial Confinement Fusion (ICF). Whereas MCF involves propellant being heated in a plasma to extreme temperatures (over 100 million °C; 212 million °F), ICF relies on powerful lasers to rapidly compress tiny pellets of deuterium, helium-3, and/or tritium (D, ³He, T) to create ultra-hot plasma.

With more advanced reactor configurations, velocities of 3,500 km/s to 7,800 km/s were reportedly possible. However, a spacecraft with such drive systems would still take 364 to 163 years to reach Proxima b, respectively. According to various estimates, a fusion system would be capable of producing a specific impulse (Isp) of between 10,000 and 1,000,000 seconds, translated to an exhaust velocity of 98,067 m/s and 9,806,650 m/s. At this speed, a fusion-powered spacecraft could reach Proxima Centauri in about 130 years.

However, concepts emerged during this period that promised even faster transits, including the extensive work of noted physicist Robert Bussard. In 1955, he joined the Los Alamos Laboratory's Nuclear Propulsion Division, where he worked on Project Rover, the first U.S. attempt at creating an NTP engine. In 1958, he and fellow nuclear physicist researcher R.D. DeLauer published their first treatise on the subject, titled "Nuclear Rocket Propulsion."

In this, and the later treatise, "Fundamentals of Nuclear Flight" (released in 1967), they explained the advantages ship's equipped with nuclear reactors had over chemical rockets.

Famed physicist Freeman Dyson, the chief scientist for Project Orion until its cancellation in 1963, also explored how fusion power could be harnessed for interstellar travel. In 1968, he penned an article in Physics Today titled "Interstellar transport." Similar to the Orion concept, Dyson addressed the possibility of using thermonuclear warheads to generate acceleration, rather than fission devices.

Similarly, he acknowledged that "controlled fusion reactors" could be used instead of bombs, if they could be produced cheaply (which he doubted). According to Dyson's calculations, the "energy density of thermonuclear fuel makes mission velocities in the range 103-104 km/sec reasonable." At speeds of 10,000 to 100,000 km/s, a fusion-powered spacecraft could reach Proxima b in less than 13 years.

Bussard Ramjet

Then, in 1960, Bussard released what would become his most influential paper, "Galactic Matter and Interstellar Flight," in which he proposed the concept for an interstellar ramjet. The proposed spacecraft, named the Bussard Ramjet in his honor, called for a scoop generating a funnel-shaped magnetic field at the ship's front. This field would allow the spacecraft to scoop neutral hydrogen from the surrounding medium as it traveled.

The collected hydrogen would then be funneled into a magnetic confinement chamber, where it would be compressed until thermonuclear fusion occurred. The energy and elementary particles released in the process would then be channeled through nozzles to generate thrust. As Bussard described it:

[B]y abandoning the interstellar rocket entirely, turning to the concept of an interstellar vehicle which does not carry any of the nuclear fuel or propellant mass needed for propulsion, but makes use of the matter spread diffusely throughout our galaxy for these purposes. By rough analogy with its atmospheric counterpart, we call this an interstellar ramjet.

The most obvious advantage, as Bussard noted, was that this spacecraft concept did not require any propellant to be transported along, thus reducing the size and mass requirements of the ship considerably. In addition, as the ship continued to travel, its velocity would increase to the point that it would eventually reach velocities of up to 4% the speed of light. Unfortunately, the concept was limited by the problem of drag, which would accumulate as the ship continued to accelerate.

The ship's potential velocity was also limited to the amount of hydrogen it could scoop up. Bussard's calculations were based on then-estimates of hydrogen in the interstellar medium (ISM), which have since been revised and shown to be much lower than previously thought.

Project Daedalus

Between 1973 and 1978, the British Interplanetary Society (BIS) conducted a feasibility study for an ICF spacecraft known as Project Daedalus. Among the study's criteria, the spacecraft had to rely on existing or near-future technology to reach its destination within a human lifetime. The proposed spacecraft would be powered by an open-cycle fusion engine and consisted of a two-stage uncrewed vehicle with an initial mass of 54,000 tonnes, including 50,000 tonnes of fuel and 500 tonnes of scientific payload (~59,525, 55,115, and 550 U.S. tons).

The first stage, the larger of the two, would operate for 2.05 years and accelerate the spacecraft to 7.1% the speed of light (0.071 c). This stage would then be jettisoned, at which point the second stage would ignite its engine and accelerate the spacecraft up to about 12% of light speed (0.12 c) over the course of 1.8 years. The second-stage engine would then be shut down, and the ship would enter into a 46-year cruise period.

According to the Project's estimates, the spacecraft could achieve an exhaust velocity of 10,000 km/s, allowing it to reach Barnard's Star within 50 years. Adjusted for Proxima Centauri, the spacecraft could make a transit in 36 years. However, the project also identified numerous issues that made it unfeasible using then-current technology. Most of these issues are still unresolved today.

In 2009, members of the British Interplanetary Society (BIS) and the Tau Zero Foundation (TZF) founded Icarus Interstellar in the hopes of revitalizing the project. Like the previous study, Project Icarus envisioned a smaller spacecraft that also relied on ICF and could be realized with existing or near-future technology. In 2013, a design competition was launched that produced multiple concepts (including Icarus Firefly). However, the project was unofficially ended in 2019 when a review determined it had failed to create a workable, updated starship design.

Artist's rendering of the Project Daedalus spacecraft, with a Saturn V provided for scale. Credit: Adrian Mann/British Interplanetary Society

Enzmann Starship

In keeping with the philosophy that interstellar spacecraft should be extremely fast, or designed for the long haul (Generation Ships), another fusion concept to emerge during the Cold War was the Enzmann Starship. The idea was the brainchild of MIT Professor and Raytheon Corporation member Dr. Robert Enzmann, who proposed it in 1964 while working in space-mission designs. This fusion-powered interstellar spacecraft consisted of a mirror-finished sphere measuring 305 m (1,000 ft) in diameter and a cylindrical habitat/propulsion section connected to it.

The sphere would contain frozen pellets of D, which would be fused to create pulses of thrust from 6 to 12 engines, achieving a small fraction of the speed of flight. Long-time space advocate G. Harry Stine popularized the concept in an article, âA Program for Star Flight," that appeared in the Analog Science Fact & Science Fiction magazine in 1973. He described a ï¬eet of Enzmann starships traveling to the nearest stars in a century or so, carrying up to 2,000 passengers each.

Project Longshot

In what was another attempt to realize ICF, the US Naval Academy and NASA pursued the Longshot design-study from 1987 to 1988. It was designed to be built at Space Station Freedom - the planned U.S. space station that gave way to the International Space Station (ISS) - and with existing technology in mind. The spacecraft would be uncrewed and would rely on a fission reactor to power a system of lasers used to trigger fusion in an inertial confinement chamber, similar to Daedalus.

Longshot would have a mass of 396 tonnes (436.5 tons) at the start of the mission, including 264 tonnes of helium-3/deuterium propellant. The main propulsion section would be discarded once the spacecraft's propellant was spent, at which point the remaining spacecraft would have a mass of about 30 tonnes (33 tons). Based on the project's calculations, a trip to Alpha Centauri would take about a century. Adjusted for Proxima Centauri, this would work out to about 97 years.

Once again, research into advanced propulsion produced some very interesting proposals during the Space Age, Cold War, and after. Some of the ideas produced in this period are still with us today, waiting for the day when they will be realizable. The same challenges remain, of course: cost and the need for technology to mature. In the meantime, several other ideas have since emerged that offer possible solutions and alternatives. We'll take a look at some of these in the coming installments.

Further Reading:
- Galactic Matter and Interstellar Flight, Bussard, R. Astronautica Acta (1960), vol. 6, sec. 4
-Interstellar transport, Dyson, F. Physics Today (1968)
- Project Daedalus - The British Interplanetary Society (BIS)
- The Enzmann Starship: History and Engineering Appraisal, Crowl, A., et al. Journal of the British Interplanetary Society, vol. 65, pg. 185-199
- Project Icarus - Icarus Interstellar
- Project Longshot - NASA Technical Reports Server (NTRS)