Tuning Up the Interstellar Ramjet

Catching my eye in the latest Carnival of Space, hosted by Brian Wang at Next Big Future, is Adam Crowl’s write-up of a rethinking of an exotic ramjet technology. Robert Bussard put the interstellar ramjet into the public eye back in 1960 in a paper proposing that a starship moving fast enough would be able to use the hydrogen between the stars as a source of fuel, enabling a constant acceleration at one g. You’ll recognize the Bussard ramjet in Poul Anderson’s classic novel Tau Zero (originally published in Galaxy in 1967 as To Outlive Eternity).

The Problem with Slow Fusion

Anderson’s ‘Leonora Christine’ was a runaway starship, accelerating ever closer to lightspeed until she was punching through entire galaxies in times experienced by the crew as mere minutes. But we don’t have to get quite that extreme with the Bussard idea. It’s built around the premise of gathering fuel along the way so as to avoid the vast mass ratio problems of conventional rocketry. We can imagine an enormous magnetic scoop that might gather up this interstellar hydrogen, but the problem is how to burn it.

Image: The Bussard ramjet concept, as envisioned by the space artist Adrian Mann.

Adam’s discussion nails the problem: Bussard relied on proton to proton fusion, but the problem is that it’s too slow. Assuming we could gather the hydrogen in the first place, we face the fact that proton burning is a kind of reaction that works in stars (like our Sun) because of the sheer size of the stars themselves. Adam describes this well:

The reaction rate of proton-proton fusion at “low” (i.e. an achievable 100 million degrees) temperatures is essentially negligible and only powers the stars because they’re so gigantic. The Sun’s energy production rate is a bit more than 10 Watts per cubic metre of the fusion part of its core, which is far less than the power packed into a battery, for example. Unlike a battery, of course, that energy can trickle out for billions of years – but that’s no good for propelling a starship.

Lighting the Fire: The CNO Alternative

Physicist Daniel Whitmire tackled this problem in a 1975 paper that proposed using hydrogen for fuel but exploiting a catalytic nuclear reaction chain instead of straight proton burning. The so-called CNO Bi-Cycle becomes dominant in sufficiently hot main sequence stars (usually those about 1.5 times the mass of the Sun), while the proton-proton chain is more significant in smaller stars. Here’s Adam’s description:

Basically a hydrogen fuses to a carbon-12, then another is fused to it to make nitrogen-14, then two more to make oxygen-16, which is then highly ‘excited’ and it spits out a helium nucleus (He-4) to return the nitrogen-14 back to carbon-12. Since the carbon-12 isn’t consumed it’s called a “catalytic” cycle, but it’s not chemical catalysis as we know it. Call it “nuclear chemistry.”

Getting the Vessel Up to Speed

Whitmire points out in his paper that using this method, the catalyst fuel, carried along with the spacecraft, is not depleted. Interstellar hydrogen, however, remains the ultimate source of the energy. We still run into the issue of the density of the interstellar medium, which is tricky business. Whitmire comments on the problem of gathering enough fuel:

…independent of other restrictions, nuclear powered ramjets may require nebular regions of space for take off if accelerations greater than 1g are desired. However, once the desired velocity is obtained in a nebula runway, it will be maintained and even increased as the ship moves into less dense regions of space… Nebulae are also required for stopping interstellar ramjets since deceleration requires as much energy as acceleration.

Image: Science fiction has worked often with the ramjet concept, as seen here in this Analog cover from 1978. Artwork by Rick Sternbach.

What do we do about getting up to speed? There being no nearby sufficiently dense nebula, we on planet Earth might find ourselves unable to light the fire. But Whitmire goes on to speculate that technological civilizations living within or near a dense nebula would have no such constraints, and even goes to far as to outline the possibility of searching for the signature of extraterrestrial ramjets that might be tracked while using a nebula ‘runway.’

The Ramjet as Brake

Bussard’s ramjet ideas, followed by Whitmire’s modifications, really did open up the idea of practical interstellar flight some fifty years ago, but recent studies have shown that the ramjet idea has a key obstacle. The ramscoop needed to collect all that hydrogen acts more effectively as a brake. Indeed, we’re now considering similar magnetic scoop ideas as possible braking systems for decelerating a probe into a destination solar system. Is the ramjet through, or are there ways around the braking problem, including other ways of hydrogen collection?

The paper is Whitmire, “Relativistic Spaceflight and the Catalytic Nuclear Ramjet,” Acta Astronautica Vol. 2 (1975), pp. 497-509 (available online, and well worth reading). The original Bussard ramjet paper is “Galactic Matter and Interstellar Flight,” Acta Astronautica Vol. 6 (1960), pp. 179-194. Adam Crowl also points out that Gregory Benford’s starship ‘Lancer’ in Across the Sea of Suns used the CNO cycle, an apparent nod to Whitmire.