The year after Al Jackson, working with Daniel Whitmire, published his concept of a laser-powered interstellar ramjet, the duo returned in the pages of JBIS with a spinoff design. The issue was obvious: Central to Robert Bussard’s ramjet design was the idea that the spacecraft would carry no fuel, but collect reaction mass from the interstellar medium. There were a number of reasons why this was problematic, including the drag of the ram scoop and the problem of lighting proton/proton fusion. So Jackson and Whitmire decided to look at a space-based laser powering up a starship that carried its own reaction mass onboard.
It turned out that John Bloomer had looked at supplying a spacecraft through an external laser energy source as far back as 1967, though in his case the craft would use the laser energy it received to run an electrical propulsion system (the Bloomer citation is given at the end of this article). Jackson wanted to think in terms of relativistic interstellar flight, and he and Whitmire proceeded to look at how an interstellar rocket carrying inert reaction mass would accelerate. Their paper, “Laser Powered Interstellar Rocket,” was among other things an attempt to explore the areas where beamed energy could interweave with conventional rocket design.
Image: Al Jackson in the heady days of the Apollo program, when he was astronaut trainer on the Lunar Module Simulator.
We can assume hydrogen as the reaction mass onboard the rocket that will be energized by the laser source in the Solar System. Bloomer’s work in the 1960s is interesting — I’ll want to talk about him in other contexts soon. For now, though, it’s the later work of Jackson and Whitmire that first tunes up the laser-powered ramjet and then extends the notion to the modified design the authors call a laser-powered rocket. Like the laser ramjet, the laser-powered rocket has a significant advantage over laser-beamed lightsails in that it can accelerate into the laser beam as well as away from it, since the beam is being used only as a source of energy and not momentum.
You can see how the laser-powered rocket introduces an initial problem into the beamed propulsion concept. As Greg Matloff and Eugene Mallove noted in their book The Starflight Handbook (Wiley, 1989), the beauty of a beamed lightsail is that leaving the propellant behind works wonders on the mass — the same can be said for the ramjet — whereas adding reaction mass works against this trend. But beaming and rocket technologies have been in play since Arthur Kantrowitz (Avco Everett Research Laboratory) first studied using lasers to boost payloads into Earth orbit in 1972. Matloff and Mallove would go on to analyze the performance of a laser-electric ramjet and speculated on using a sail to boost it to an initial velocity of 0.003 c. Laser beaming ideas now extended far beyond Forward’s lightsails. From The Starflight Handbook:
Consider a laser ramjet with a ramscoop, formed with a ‘modest’ 1000-km radius magnetic field…, and coursing through an interstellar medium with 105 ions per cubic meter. The starship’s collector would receive laser energy and use it to increase the kinetic energy of the ion exhaust. We assume that the laser light would be converted to exhaust kinetic energy with an efficiency of 50%. The laser power might be in the range of 104 to 105 megawatts (of the same order as that proposed for Starwisp); the ion exhaust velocity, 5000 to 10,000 km/sec; and the total starship mass, 7.5 X 103 tons.
And what of the transmitting installation? As you would imagine, it’s big:
The required aperture of the Solar System laser transmitter: 100 km for a 2 ly transmission distance and 1000 km for 20 ly. The laser ramjet may be of service for one-way colonization missions requiring a few centuries of travel time.
Looking at Jackson and Whitmire’s numbers and comparing the laser ramjet to the laser rocket, Matloff and Mallove concluded that the laser rocket would be superior to the ramjet for flights to nearby stars like Alpha Centauri and Barnard’s Star, while more distant destinations would demand the ramjet. It’s hard to predict whether a civilization would support a laser installation beaming out huge quantities of power for half a millennium, but Matloff and Mallove note that it would be possible to keep the laser-powered ramjet under the beam for the full duration of the mission, which would allow, by their calculations, an Alpha Centauri crossing in 350 years.
Adam Crowl pointed out to me that Iain Nicholson discussed the laser ramjet in his book The Road to the Stars (William Morrow, 1978), prompting me to dig out the reference (it’s surprising how often this book turns out to be useful, and I suggest finding a copy for your shelves if you don’t already have one). Nicholson runs through the advantages of the laser-powered ramjet, including its ability to fly into the beam when returning (where it becomes far more efficient at energy-gathering) and its ability to use the beam to decelerate at destination using the beamed energy even though the spacecraft is receding from the beam. He adds:
Ignoring the problem of focusing all the energy in the beam onto a starship at a range of several light years (and these problems are formidable), calculations suggest that in principle the system should be more efficient than the laser sail (LPV) which utilizes the momentum of photons. Because the vehicle will be using the energy of the laser beam, high-energy photons will be required, suggesting the need for using X-ray lasers. At relatively low speeds (up to 14% of light speed), it should be more efficient than the ‘conventional’ interstellar ramjet (ISR), but above that speed the ISR becomes more effective.
Jackson and Whitmire anticipated the focusing objection, acknowledging that focusing over multi-light year distances was well beyond forseeable technology and would require, as Nicholson notes, an X-ray laser with a huge effective radiating area. But they suggest that with anticipated technology we might build phase arrays of lasers up to 10 kilometers in diameter with a diffraction limited range of 500 AU for visible light. This would allow what they describe as a ‘small-mass low-relativistic one-way probe,’ our first attempt at an interstellar crossing.
The paper is Jackson and Whitmire, “Laser Powered Interstellar Rocket,” Journal of the British Interplanetary Society, Vol. 31 (1978), pp.335-337. The Bloomer paper is “The Alpha Centauri Probe,” in Proceedings of the 17th International Astronautical Congress (Propulsion and Re-entry), Gordon and Breach. Philadelphia (1967), pp. 225-232.