A Laser-Powered Interstellar Ramjet

by Paul Gilster on July 24, 2012

Many of the interstellar concepts I write about in these pages take on a life of their own. After the initial brainstorming, the idea gets widely enough disseminated that other scientists take it on, looking to modify and improve on the original concept. That’s been true in the case of solar sails and the more recently devised ‘lightsails,’ which use beamed energy from a laser or microwave source to drive the vehicle. We continue to study magnetic sails — ‘magsails’ — and various nuclear options like the inertial confinement fusion that powered Daedalus and perhaps Icarus. Sometimes insights arise when ideas are grafted onto each other to create a hybrid solution.

The idea I want to examine today, a hybrid design combining a Bussard-style interstellar ramjet with laser beaming — exemplifies this mix and match process. Working with Daniel Whitmire, A. A. Jackson, a frequent commenter and contributor here on Centauri Dreams, pondered the various issues the Bussard ramjet had run into, including the difficulty in lighting the proton/proton fusion reaction Bussard advocated early in the process. Writing at a time not long after he had finished up a PhD in relativistic physics (at the University of Texas), Jackson conceived the idea of beaming energy to the spacecraft and discovered that the method offered advantages over the baseline Bussard design. The laser-powered ramjet is a fascinating concept that has received less attention than it deserves.

Image: Physicist and interstellar theorist Al Jackson, originator of the laser-powered ramjet concept.

Bussard’s ramjet, you’ll recall, lit its fusion fires using reaction mass gathered from the interstellar medium by a huge magnetic ram scoop, which itself has proven problematic given the drag issues such a scoop introduces. The other way to power up a starship using an external source of energy is to beam a terrestrial or Solar System-based laser at the departing craft, which has deployed 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.

Running the numbers and assuming all photons transmitted by the laser will be absorbed by the ship, the authors discovered that the laser-powered ramjet (LPR) is superior to the baseline Bussard ramjet at low velocities, while superior to the laser-pushed sail at all velocities. The Bussard design becomes the most efficient of the three at velocities equal to and above about 0.14 c. The laser-powered ramjet, then, solves at least one of the Bussard vehicle’s problems, the fact that it has to get up to a significant percentage of lightspeed before lighting its fusion reaction. LPR propulsion could be used up to 0.14 c, with the vehicle switching over to full interstellar ramjet mode to achieve high efficiency at relativistic velocities.

The laser-powered ramjet offers other advantages as well. Think back to some of Robert Forward’s laser sail concepts and you’ll recall the problem of deceleration. With the sail powered by a laser beam from the Solar System, it’s possible to reach velocities high enough to take you to the nearest stars in a matter of decades rather than centuries. But how do you slow down once you arrive? Conceiving a manned mission to Epsilon Eridani, Forward came up with a ‘staged’ solution in which the sail separates upon arrival, with the large outer sail ring moving ahead of the vehicle and reflecting beamed laser energy to the now smaller inner sail, thus slowing it down. It would be so much easier if the beam worked in both directions!

But with the laser-powered ramjet, a round trip can be made using a single laser beam because the beam is being used as a source of energy rather than momentum. Jackson and Whitmire showed that the efficiency in the deceleration phase of the outbound journey as a function of velocity is the same as for the acceleration phase. And on the return trip, the energy utilisation efficiency is more favorable in both the acceleration and deceleration phases because the ship is traveling into the beam. In fact, the laser-powered ramjet is superior to both the laser sail and the Bussard ramjet even at high fractions of the speed of light when traveling into the laser beam.

Let’s go over that again: Jackson and Whitmire’s calculations focus on the energy utilisation efficiency parameter, showing that the laser-powered ramjet is superior to the laser sail at all velocities, whether the ship is receding from the beam or approaching (moving into the beam). The LPR is also superior to the Bussard ramjet at velocities less than about 0.14 c when receding from the beam, and superior to the Bussard design at all velocities when approaching. Add to this that the LPR concept requires no onboard proton-burning reactor — the authors assume the use of Whitmire’s ‘catalytic’ ramjet using the CNO (carbon-nitrogen-oxygen) cycle — and that the LPR’s power requirements are less than those of the laser sail.

Image: An interstellar ramjet at work. Credit: Adrian Mann.

You can imagine a future civilization taking advantage of these principles after establishing a presence around another star. A beaming system in the Solar System could eventually be complemented by one around the other star, so that no matter which way the ship was traveling, it would always be moving into the beam on a round-trip flight. I’ve sought in vain for an illustration of this concept in science fiction (and I keep wondering if Larry Niven hasn’t picked up on it somewhere), but I can find no example. It seems prime material for science fictional use.

Jackson and Whitmire looked at still another wrinkle on this notion in a paper the following year, about which more tomorrow. Today’s paper is “Laser Powered Interstellar Ramjet,” Journal of the British Interplanetary Society Vol. 30 (1977), pp. 223-226.

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{ 8 comments }

SM July 24, 2012 at 10:40

What are the power requirements for the laser? and what would be used to focus the beam into the chamber where the CNO Bi-Cycle is taking place?

william July 24, 2012 at 16:45

is Physicist and interstellar theorist Al Jackson the same person as A. A. Jackson ?

James Jason Wentworth July 24, 2012 at 19:03

In “The Starflight Handbook” (my copy of it is, unfortunately, packed away at the moment), mention is made of a laser ramjet that was depicted in a science fiction story. If memory serves, it was “The Mote in God’s Eye” (by Larry Niven?). In any event, the relevant line from “The Starflight Handbook” was, “The cocky, over-confident Kzin are defeated by a human crew riding a laser ramjet.” Perhaps there is an illustrated edition of this story, or maybe the laser ramjet is illustrated on the cover(s) of one or more editions. Also:

To overcome the psychological hurdle of this novel form of propulsion (having its laser energy source so distantly separated from its reaction mass), a “local,” simpler variant could be developed for cislunar and general interplanetary travel (for probes and, eventually, crewed spaceships). This version could use laser-sublimated water ice to produce thrust. In addition:

Later, water ice (and/or methane ice and ammonia ice) to provide reaction mass for the vehicles could be obtained from ice-bearing asteroids and comet nuclei (some asteroids may in fact *be* inactive comet nuclei, whose ices are covered by dark, insulating layers of dust). All during this time, experimental variants powered by laser-initiated fusion (using at least some magnetically-scooped solar wind atoms, perhaps augmented by onboard fusion fuels) could be tested in order to mature the technology for interstellar use.

Christopher Phoenix July 24, 2012 at 21:31

@James Jason Wentworth

I remember the page you mentioned- it was in the chapter entitled “Starflight Between Fact and Fancy”. Niven’s “laser ramjet” is not, in fact, a laser powered ramjet. It is actually a ramjet-equiped photon drive starship. The engine doubles as a means of propulsion and long distance communication. The starship’s drive is essentially a gigantic laser. This ship appears in one of the stories in the Man-Kzin Wars.

The initially peaceful human crew use their photon drive as a weapon when the warlike Kzin attempt to kill them by heating the hull of their starship with an induction heating device. Photon drives require an extreme power output to provide even a gentle thrust, so you can imagine what happened the Kzin ship when they took the full blast of a photon engine capable of at least a 1/10 g.

Wouldn’t any of these laser propulsion schemes require very powerful, efficient laser technology? Modern lasers are rather inefficient and prone to overheating. Even assuming future lasers are better- say 50% efficient- will we have to build elaborate cooling systems for our laser cannons? What about the optics- might they overheat too? And will I get a portable, lethal laser pistol when I board one of these laser ramjet starships? Those Kzin aren’t very friendly, you know… : )

Paul Gilster July 24, 2012 at 22:23

william writes:

is Physicist and interstellar theorist Al Jackson the same person as A. A. Jackson ?

Indeed he is!

Eniac July 24, 2012 at 23:18

Judging from the results, the analysis was done in such a way as to assume that the laser energy could be used efficiently to brake/accelerate oncoming hydrogen atoms without stopping or diverting them first. It is very difficult for me to imagine any actual mechanism that would accomplish this.

If you gather the hydrogen together in one point, somehow, you also will need to gather the laser light energy, somehow. The size of that collector would be enormous, and magnetic fields will not work for light.

Alternatively, you would have a distributed system, a large, thin sail that instead of reflecting light would absorb it and use most of the absorbed energy (somehow?) to accelerate hydrogen atoms that are coming through at extreme velocities.

The more I think about it, the more it seems like pure theoretical fantasy with no hope of an engineering solution in sight.

Eniac July 24, 2012 at 23:34

Consider, also, that the total mass of the ISM between here and a nearby star amounts to a layer of condensed matter only a few microns thick. If you propose to use that as reaction mass, you should see immediately that the mass density of the spacecraft needs to be of that order, preferably less. That is roughly a milligram per square meter that you have to work with to achieve the already steep challenge at hand: Using light energy to efficiently and in a controlled manner transfer momentum to what amounts to ultra-high energy radiation composed of neutral hydrogen.

There is no material in the world that would even withstand such radiation for long, never mind using it productively.

tom November 9, 2012 at 13:27

When I was in my last year of high school back in 1981, I wrote a science fantasy story about a network of ‘luminous energy corridors’ between the star systems in the local group of the Milky Way. The space vehicles in my story were composed of materials that could absorb and radiate the energy to attain instantaneous acceleration curves… the inside of these fictitious craft were in free-fall… traveling decades or centuries within minutes. In the end of the story the return to Earth was more alien than all the traveling put together… 100 millenniums of progress made Earth into a living global city… a lifeform that was a ‘fusion’ of technology, living matter and all species into a ‘group mind’… ‘its as if we are a bunch of Neanderthals who slid down a giant mountain and emerged into the San Diego Zoo with our enclosure ready for us’.
Could this actual happen? I don’t know.. but I hope whatever happens…I hope its interesting.

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