Let’s talk about how to get a spacecraft moving without onboard propellant. As noted last week, this is apropos of the design shown in the Netflix streaming video take on Liu Cixin’s novels, which the network titles ‘3 Body Problem.’ There, a kind of ‘runway’ is conceived, one made up of nuclear weapons that go off in sequence to propel a sail and its payload. The plan is to attain 0.012 c and reach an oncoming fleet that is headed to Earth but will not arrive for another four centuries.

This is an intriguing notion, and one with echoes in the interstellar literature. Because Johndale Solem mixed sails and nuclear weapons in a design called ‘Medusa’ that he described in a Los Alamos report back in 1991, although its roots go back decades earlier, as I’ll discuss in an upcoming article. Mixing sails, nuclear weapons and a fusion runway is an unusual take, a hybrid concept that caught my eye immediately, as it did that of Al Jackson, who alluded to runways in a paper in the 1970s. I’ve just become aware of a Greg Matloff paper from 1979 on runways as well.

So let’s start with the runway concept and in subsequent posts, I will be looking at how Medusa evolved and consider whether the hybrid concept of ‘3 Body Problem’ is worth pursuing. Jackson’s paper, written with Daniel Whitmire, is one we’ve considered before in these pages. The concept is to power up a starship by a laser but use reaction mass gathered from the interstellar medium, collecting the latter with a ramscoop. Here the model draws on Robert Bussard’s ramjet notions, originally published in 1960 and more or less immortalized in Poul Anderson’s novel Tau Zero. Jackson and Whitmire’s version was one of several variants on Bussard’s original concept and offered a number of performance benefits.

Image: The interstellar ramjet, as envisioned by British artist Adrian Mann. Variants have appeared in the literature to get around the drag issue induced by the ramscoop design. A fusion runway seeds fuel along a track that the craft follows as it accelerates.

You’ll notice that this is also a hybrid concept, combining ramjet capabilities with laser beaming. Lasers had already been considered for beaming a terrestrial or Solar System-based laser at the departing craft, which could deploy a lightsail to draw momentum from the incoming photons. Jackson and Whitmire found the latter method inefficient. Their solution was to beam the laser at a ramjet that would use reaction mass obtained from a Bussard-style magnetic ram scoop. The ramjet uses the laser beam as a source of energy but, unlike the sail, not as a source of momentum.

Jackson and Whitmire were a potent team, and this is one of their best papers. These methods could be used to reach 0.14 c, allowing the vehicle to switch into full ramjet mode at that point. And because the laser is a source of energy rather than momentum, it can also be used as a means to decelerate on the return trip. For our purposes today, I turn to the last part of the paper, which outlines other starship concepts that grow out of the laser beaming analysis. Here is the relevant passage:

Another possibility would be to artificially make a fusion ramjet runway. Micron-size frozen deuterium pellets could be accelerated electrostatically or electromagnetically beginning several years prior to take-off at which time a fusion ramjet with a relatively modest scoop cross section (perhaps a physical structure) would begin acceleration.

So we have a spacecraft that collects its fuel along the way. As opposed to the ‘pure’ ramjet, which scoops up interstellar material and is dependent on the medium through which it moves, this fusion runway ramjet would know exactly the trajectory to take to collect the needed fuel pellets as it accelerates. Bear in mind the original Bussard ramjet problem of having to reach a certain percentage of lightspeed before being able to ignite its fusion engine. Problem solved.

In recent correspondence, Jackson pointed out that the idea harkens back to the German Vergeltungswaffe 3 (“Vengeance Weapon 3”), which was a gun originally designed to bombard London but only saw use against Allied targets in Luxembourg in 1945 (the bunkers at the Pas-de-Calais were destroyed by bombing raids). Multiple solid-fuel rocket boosters were ignited by the gases pushing the projectile as it moved in staged fashion through the barrel. The French Army had considered plans for such a staged cannon as far back as 1918, and the idea dates to the 19th Century.

Image: The prototype V-3 cannon at Laatzig, Germany (now Zalesie, Poland) in 1942. Credit: Bundesarchiv, Bild 146-1981-147-30A / CC-BY-SA 3.0.

Greg Matloff picked up on the Jackson and Whitmire paper in a 1979 paper in The Journal of the British Interplanetary Society which he was kind enough to pass along to me. The Jackson/Whitmire fusion runway would, he believed, improve ramjet performance and alleviate aerodynamic drag, which is a problem that sharply reduces a Bussard vehicle’s acceleration. He considered in the paper a fusion fuel released as fuel pellets moving in the direction of the destination star, with the ramjet moving up from behind to capture and fuse the pellets. In one scenario, tanker craft would be launched over a 50 year period to produce a runway 0.1 light years long.

Matloff as well as Jackson and Whitmire considered other variations on the interstellar ramjet idea, and I want to just mention these before moving on. From the Matloff paper:

As Whitmire and Jackson have mentioned, the performance of a ramjet might be of interest just above the photosphere of the Sun, n a high-energy, high-particle environment. More prosaically, a ramscoop could be utilized near the Earth to collect fusion fuel from the solar wind over a few decades. Then, if the fuel is utilized to power a ram-augmented interstellar rocket (RAIR), such an approach might be competitive in any discussion of the difficulties and merits of the various ramjet derivatives.

A Sun-skimming ramjet is one I had never seen discussed until I read Jackson and Whitmire. It would make for a lively hard SF tale, that’s for sure. Given the problems with ramjet drag that have been wrestled with in the subsequent literature, it’s worth considering Matloff’s idea of solar wind fuel collection at much lower speeds in the inner system. In any case, the fusion runway notion offers one way to collect a known supply of fuel over the length of a runway that would launch an interstellar craft.

When I wrote my Centauri Dreams book early in the century, I was unaware of both the papers we’ve looked at today, and focused on the only runway concept that was then known to me, the so-called ‘Bussard Buzz Bomb’ of the free-thinking Jordin Kare. Kare is, alas, no longer with us, but I enjoyed a long conversation with him on his runway concept, and I want to cover that in the next post before moving on to Johndale Solem’s Medusa.

The Jackson/Whitmire paper on laser-powered ramjets is “Laser Powered Interstellar Ramjet,” Journal of the British Interplanetary Society Vol. 30 (1977), 223-226. Al Jackson: A Laser Ramjet Reminiscence presents Al’s thoughts on this paper as written for this site. Greg Matloff’s paper on fusion runways is “The Interstellar Ramjet Acceleration Runway,” JBIS Vol. 32 (1979), 219-220. The ToughSF site offers a detailed explanation of runway concepts in Fusion Highways in Space.