It was in 1960 that the physicist Robert W. Bussard first proposed the interstellar ramjet in his seminal paper [1]. Bussard was born in 1928 and died in 2007. Throughout his spectacular career he had worked on many interesting things, including the thermonuclear rocket program in the 1950s, called Project Rover. He produced two exceptional monographs based on this research. He is also considered the inventor of the Polywell, a type of inertial electrostatic confinement fusion reactor, funded by the US Navy.
The interstellar ramjet method featured in the excellent Poul Anderson book, Tau Zero [2]. The ramjet was a proposed variant of the fusion engine, but rather than carrying along its own fuel, it would use enormous electromagnetic fields to ram scoop hydrogen from the interstellar medium.
The high energy protons enter the ram scoop, confined by magnetic field lines and then meeting under the conditions for fusion reactions to occur, producing a high energy exhaust jet. In theory, if the interstellar ramjet can be made to work, then relativistic star travel will be possible. This means trips to the nearest stars have a travel time of only a matter of years due to the spacecraft continuing to approach the speed of light, but never quite reaching it.
Indeed, trips to the centre of the galaxy, approximately 30,000 light years away, may be possible within a few decades trip-time, although a very long time would have past back on Earth due to the special relativity time dilation effect, as first predicted by Albert Einstein in his 1905 paper.
However, over the years, several physics and engineering problems have been found with the interstellar ramjet making it less credible as a real option for interstellar flight. This includes the fact that in order to ram scoop sufficient hydrogen fuel for the fusion reactions to take place, the spacecraft must already be travelling very fast, which implies carrying some on-board propellant to start with.
Also, maintaining a constant thrust profile for long durations could result in the engine over-heating. The maintenance of a very large electromagnetic field configuration over such large distances presents a real problem for aspiring interstellar engineers. For the on-board crew, there is the problem of finding areas of the interstellar medium where the density of interstellar hydrogen (or other particles) is sufficiently abundant.
The worst problem, which may prevent the interstellar ramjet from ever being realisable, is that the drag force generated as the large scoop passes through the interstellar medium may exceed the thrust generated by the engine.
Over the years variations on the interstellar ramjet design have been proposed. This includes the Ram Augmented Interstellar Ramjet, first proposed in 1974 [3] by Daedalus designer Alan Bond, using the interstellar hydrogen only as a reaction mass. The interstellar hydrogen is not converted to helium in a fusion reaction, but instead it is accelerated by on-board fusion reactions from fuel which the Starship already carries.
Daniel Whitmire made a proposal in 1975 [4] for a catalytic ramjet, which instead uses the carbon-nitrogen-oxygen cycle producing fusion at a higher rate than in the proton-proton chain associated with the direct collection of hydrogen. Aerospace engineers Dana Andrews and Robert Zubrin have also studied the interstellar ramjet in 1985 [5]. Other notable research has also been done which is worth reading [6].
It is often said that the interstellar ramjet is the great hope for interstellar travel. Even the pioneering interstellar physicist Robert Forward, a proponent of propellantless propulsion, spent some time looking at interstellar ramjets. The physicist Greg Matloff vies ramjets as thus:
“…Even if we never build a proton-proton fusion ramjet, the effort spent investigating it will not have been wasted…. As fantastic as these ideas may seem, we should be open-minded about them, not cavalierly rule them out because of their current infeasibility. After all, the march of technology is full of well-known surprises and serendipity. To cite one example: the laser was invented virtually at the same time that the fusion ramjet was proposed. Lasers have since become the most promising way, in theory, to ionize the advancing path of a fusion ramjet and facilitate electromagnetic fuel collection” [7].
Let’s hope that Matloff is correct and the interstellar ramjet will eventually open up the Galaxy and beyond.
At the speeds of today’s spacecraft technology, it would take many thousands of years to reach the nearest stars as measured by clocks on Earth. However, if one could build a spacecraft that could accelerate to relativistic speeds, then it is possible to reduce trip times to the nearest stars to a few years.
This astonishing fact is a consequence of Einstein’s special relativity theory discussed above. To handle the relativity of spaceflight, special equations have been developed. Using these equations, it was demonstrated originally by the astronomer Carl Sagan [8] and discussed later by others, that for a spacecraft travelling a distance in meters of \(S/2\) under an acceleration an of \(1g = 9.81\) ms\(^{-2}\) and then followed by an equivalent deceleration for the same remainder distance, that the time duration of the mission as measured by clocks on board the spacecraft is given by:
$$t = \dfrac{2c}{a_n}\cosh^{-1}\left( a + \dfrac{a_n S}{2c^2} \right)$$
Where the hyperbolic cosine function can be written as the following where \(x = (a_n S)/(2 c^2)\) with \(x>0\):
$$\cosh^{-1}(x) = \ln{\left( x + \sqrt{x^2 - 1} \right)}$$
Accelerating and decelerating at \(1g\) to would bring the spacecraft to the nearest star of Alpha Centauri 4.3 light years away within around 3.5 years. Similarly a trip to Barnard’s star 5.9 ly away would take around 4 years and a trip to Epsilon Eridani 10.7 ly years away would take around 5 years.
These trip times become even more mind boggling when applied to the scale of galaxies. A trip to the centre of the Milky Way at 30,000 light years distance would take around 20 years and to the edge of the Andromeda galaxy a mere 28 years.
How can it be possible that such vastly distant locations in the Universe can be reached within such a short transit time? Well, it comes at a penalty, due to the time dilation effect. What may only seem like years or decades to the crew aboard the ship will be the equivalent of thousands or hundreds of thousands of years back home on Earth.
If the crew were ever able to turn the ship back towards home, they would return to a very different planet, a very different civilization and a species that may not even be aware of their existence. Einstein’s relativity changed our view of the Universe in a radical way. If we can ever build relativistic spacecraft they will come at a price.
References
- Bussard, R. W. (1960). Galactic Matter and Interstellar Spaceflight. Astronautica Acta, 6, 170-194.
- Anderson, P. (1970). Tau Zero. Doubleday.
- Bond, A. (1974). An Analysis of the Potential Performance of the Ram Augmented Interstellar Rocket. JBIS, 27, 674-685.
- Whitmire, D. (1975). Relativistic Spaceflight and the Catalytic Nuclear Ramjet. Acta Astronautica, 2, 497-509.
- Andrews, D. G. and Zubrin, R. M. (1990). Magnetic Sails and Interstellar Travel. JBIS.
- Heppenheimer, T. A. (1978). On the Infeasibility of Interstellar Ramjets. JBIS, 31, 222-224.
- Mallove, E and Matloff, G. (1989). The Starflight Handbook. Wiley.
- Martin, A. R. (1973). Magnetic Intake Limitations on Interstellar Ramjets. Astronautica Acta, 18, 1-10.
- Sagan, C. (1963). Direct Contact Among Galactic Civilizations by Relativistic Interstellar Spaceflight. Planetary and Space Science, 11, 485-498.
- Fishback, J. F. (1969). Relativistic interstellar spaceflight. Astronautica Acta, 15, 25–35.
- Martin, A. R. (1971). Structural limitations on interstellar spaceflight. Astronautica Acta, 16, 353-357.
- Cassenti, B. N. (1991). Design Concepts for the Interstellar Ramjet. AIAA, 91-2537. (27th AIAA/ASME Joint Propulsion Conference, Sacremento, CA, June 24-26, 1991.)
