The Beauty of the Bussard Ramjet
Travel to the distant stars, carry no fuel
The Lacerta constellation. It shares borders with the constellation of Andromeda, home to the Milky Way’s bright and fascinating galactic neighbor. We are often drawn in by the Andromeda Galaxy’s flowerbed of stars situated in its dusty, feathered rings. Yet just to the right of it is a lesser-known treasure trove. Lacerta contains some of the universe’s rarest objects. These O-type and B-type stars are the hottest and brightest of all the spectral classes. A mere 0.00007 percent of our universe’s stars are luminous enough to be classified as an O-type. Here, in the Lacerta constellation, lies one such star.
Lacerta 10 has a surface temperature almost 6 times hotter than the sun. It’s a blistering and exhausting spectacle; the star transcends the usual warm citrus colors of stellar bodies — orange, red, embers of yellow — and is instead an almost glacial blue despite its impressive heat. Though Lacerta 10 is young and in its main sequence, after just a few million years its collapse will erupt in a supernova rich with elements, a fertile ground for the formation of stunning new nebula. But while still active, B and O-type stars such as Lacerta 10 are often surrounded by a Stromgren sphere. This is an area of ionized hydrogen caused by the star’s ultraviolet radiation. The ionized hydrogen is positively charged after losing electrons.
The Stromgren sphere was mentioned in physicist Robert Bussard’s original 1960’s paper where the Bussard ramjet was first proposed. These spheres of ionized hydrogen would be to our interstellar craft what oceans are to our seafaring ships. The hydrogen ions are collected by electromagnetic fields, powering this peculiar spacecraft and demonstrating just what makes it so different from all other interstellar flight concepts.
The Bussard ramjet promises relativistic speeds throughout the cosmos, allowing mankind to reach the nearest star system in under 4 years and the nearest galaxy in under 30. All while carrying no onboard fuel.
The need to carry fuel stunts our ability to explore. With the chemical rockets in use today, we’d need a fuel tank larger than the entire observable universe to achieve interstellar flight. Chemical rockets will never take us beyond the Solar System. This is what makes the Bussard ramjet so revolutionary: it took the idea that we need more onboard fuel for propulsion than there is in the entire universe and reduced it down to nothing. That’s a remarkable feat.
To do this, the ramjet exploits the fact that the large spaces between stars are never really empty. They are a brine of dangerous radiation and cosmic matter like dust and hydrogen atoms. As the craft moves through space it collects this interstellar gas and converts it into energy for the ship.
The ramjet is based on fusion at its heart. Electromagnetic fields would collect hydrogen that would then enter a nuclear fusion reactor and provide the energy for a powerful exhaust jet. Unused hydrogen is also expelled in the exhaust. This fusion reaction propels the ship forward at incredible speeds, simply collecting more hydrogen from the interstellar medium when there is a need to refuel. Bussard aimed for an acceleration of 1g so that the people onboard the ramjet could experience gravity similar to Earth’s. It’s a rate of acceleration safe for human travel yet still fast enough to reach the Andromeda Galaxy’s bewitching edges in just 30 years.
This 1g acceleration requires a lot of hydrogen. The interstellar medium has an average density of about 1 hydrogen atom per cubic centimeter, making it extremely diffuse. Some areas of the universe — like nebula — are more concentrated than others. A 1g acceleration in the average interstellar medium would require a frontal hydrogen scoop of 10,000 square km (3,861 square miles). Even made of ultra-thin material less than a centimeter thick, a scoop of this size would weigh a quarter of a million tons. This is why areas of ionized hydrogen, what we called Stromgren spheres earlier, are a kind of paradise to the Bussard ramjet. Ionized hydrogen could be gathered by much smaller onboard collectors. The collectors would generate a magnetic field that reaches far past the physical extent of the hydrogen scoop. The scoop itself would require less material and could be designed as a mesh instead of a heavier solid. When not traveling through Stromgren spheres, a laser ionizes hydrogen ahead of the craft.
It’s a beautiful idea. Gather the diffuse matter of space, use fusion to create an exhaust jet and propel yourself towards the distant galaxies.
Of course, with its unique beauty and promises so too are there unique challenges to overcome.
Because the hydrogen in a typical area of the interstellar medium is so sparse, the craft will need to be traveling very quickly in order to gather enough of it to use as fuel. This critical speed is about 6% the speed of light. This means that while the Bussard ramjet may not need onboard fuel during travel, it will have needed some at the beginning of its journey to get it coursing fast enough. Once the ship is moving there is the problem of powering the magnetic fields and the lasers needed to ionize and gather the nearby hydrogen.
Designing the scoop itself is complicated; convergence of the magnetic fields might redirect particles and trap them in a way where they couldn’t be injected for use in the actual fusion reactors. And while it’s an essential part of a traditional Bussard ramjet, the hydrogen scoop is also a limitation. As the craft reaches faster speeds a larger drag force is exerted on the scoop, possibly limiting how fast the craft can go before the drag is stronger than the thrust.
The fusion reactors would, in Bussard’s mind, rely on proton-proton fusion — the same fusion chain reaction which occurs inside of stars. Other researchers have found this to be inefficient, proposing instead that the incoming hydrogen be used for fusion with lithium-6 or boron-11. This fusion is not only easier but would result in a higher release of energy. The hydrogen would interact with onboard fuel and give the craft more acceleration in this modification of the Bussard ramjet, known as the ram-augmented interstellar rocket (RAIR).
A different approach to fusion was proposed by physicist Daniel Whitmire in the 1970’s. It would use a catalytic nuclear reaction chain (CNO cycle) over the proton-proton burning suggested by Bussard. The CNO cycle occurs in stars more massive than the sun and is 9 orders of magnitude faster than proton-proton fusion. However, the CNO cycle relies on temperatures and densities we can’t yet achieve with our technology.
Over the years many modifications to the Bussard ramjet have bubbled up. Each attempts to solve a different problem, to make this design more feasible for interstellar travel. It’s easy to see why — using the cosmos itself to fuel our travel feels almost natural, like using the ocean currents to push our vessels along without fearing that we’ll run out of anything along the way. But not only does a fully-functioning design for the ramjet evade us, it’s not a technology we could yet build even if we had the perfect blueprints. Ships such as these are something researchers believe would be more feasible to an advanced alien civilization. They have even suggested looking for alien signatures along dense nebula where ramjets could soak in the higher concentrations of interstellar gas and could use the added energy for acceleration and braking.
Most recently, a version of the Bussard ramjet was used in the design of a stellar engine. This engine would be a marvel. It aims to turn the entire Solar System into a spaceship by way of moving the sun. Because all of our other planetary and lunar bodies are anchored to the sun by gravity, they would move along with the sun anywhere we directed. This “Caplan thruster” uses matter from the solar wind, induces fusion, and creates a jet stream to move the star. Working with a Dyson sphere the powerful fusion reactions could even carry the Solar System beyond the Milky Way, exiting into the nervous black void between galaxies.
We still have, it seems, fascination and hope for the Bussard Ramjet. But there is a great irony to all of this.
Our sun is currently traversing the “Local Bubble”. The hydrogen density here is 10 times lower than the Milky Way’s average, having possibly formed by a supernova explosion that blew most of the interstellar matter away. The sun has been traveling through this cavity for millions of years. Though O-type and B-type stars have been pushing gas towards our sun, these denser clouds of matter won’t reach us for perhaps thousands of years. It’s ironic that we lie in such a paled region of the cosmos. Were we almost anywhere else in the galaxy the Bussard ramjet would have been just that much more realizable for us. That much more in reach. Instead the ramjet remains an unfulfilled design made more fantastical by Earth’s location in the galaxy.
It’s a strange place to be, both in time and location. Today we have the instruments to study the worlds around us only by indirect means. Staring through telescopes at night feels very much like being in a museum. The art is framed by a quiet blackness and veiled by interstellar gas. We can study it, yet we cannot touch it except in our most daring dreams.
