Hybrid propulsion technologies have emerged naturally as we look at ways to reach the stars. They’re the result of trying to extract maximum performance from each option, and it sometimes turns out that putting two ideas together works better than either by itself. Next week we’ll be looking at one such concept, A. A. Jackson’s idea of combining the Bussard ramjet with laser beaming in ways that turn out to be surprisingly effective. Today I want to start the hybrid discussion – already about a week late because of competing news — by talking about Johndale Solem’s ‘Medusa,’ a combination of sail technologies with nuclear pulse propulsion.
Solem’s work evidently draws on the ideas of Ted Cotter at Los Alamos in the 1970s, which evolved into what George Dyson has described as a ‘rotating-cable pusher.’ Think back to the Orion concept, with its immense pusher-plate and shock absorbers that would withstand the explosion of nuclear devices behind the plate, propelling the vehicle forward while protecting the crew. What Cotter had in mind was doing away with the pusher-plate and instead having the ship, as it spun slowly around its axis, unreel steel cables that would radiate out from the vehicle, in Dyson’s words, ‘like the arms of a giant squid.’ With flattened plates at the end of each, the cables would absorb momentum from the explosions set off behind the vehicle.
I assume the squid reference came from George Dyson’s father, for Freeman Dyson is credited in his son’s book Project Orion: The True Story of the Atomic Spaceship as being the author of a 1958 memo, still classified, called ‘The Bolo and the Squid.’ That places early thinking on this highly modified concept all the way back in the days of active Orion research, which were complemented by a revival at Los Alamos in the early 1970s that resulted in Cotter’s work. It’s natural enough that Johndale Solem, himself working at Los Alamos, should have been the one to take the concept one step further with a design he called Medusa because it would mimic the motion of a jellyfish moving through the ocean as it moved through space. In a 1991 Los Alamos report, Solem wrote:
One can visualize the motion of this spacecraft by comparing it to a jellyfish. The repeated explosions will cause the canopy to pulsate, ripple, and throb. The tethers will be stretching and relaxing. The concept needed a name: its dynamics suggested Medusa.
Thus the scheme: Solem would likewise do away with the pusher plate of Orion, replacing it with a large sail deployed well ahead of the vehicle, with nuclear explosions to be detonated between the two so as to drive the sail and attached vehicle forward. You can see the basic idea in the illustration below. The Medusa idea evolves naturally from some of the problems inherent in the Orion design. No matter how large the pusher-plate, it could only receive a fraction of the momentum from the bomb blast debris, but even so, it had to be massive, and so did the shock absorbers that protected the crew. Solem realized that his sail could create a canopy that could intercept a much larger angle from the detonation point, and that the tethers could be made long and elastic enough to smooth out the acceleration experienced by the canopy.
Solem also considered a combination of tethers working with a servo winch in the space vehicle itself, a method with several advantages, as suggested in the same Los Alamos report:
When the explosive is detonated, a motorgenerator powered winch will pay out line to the spinnaker at a rate programmed to provide a constant acceleration of the space capsule. The motorgenerator will provide electrical power during this phase of the cycle, which will be conveniently stored. After the space capsule has reached the same speed as the spinnaker, the motorgenerator will draw in the line, again at a rate programmed to provide a constant acceleration of the space capsule. The acceleration during the draw-in phase will be less than during the pay-out phase, which will give a net electrical energy gain. The gain will provide power for ancillary equipment in the space capsule…
Image: Medusa in operation. Here we see the design 1) At the moment of bomb explosion; 2) As the explosion pulse reaches the parachute canopy; 3) Effect on the canopy, accelerating it away from the explosion, with the spacecraft playing out the main tether with its winch, braking as it extends, and accelerating the vehicle; 4) The tether being winched back in. Imagine all this in action and the jellyfish reference becomes clear. Credit: George William Herbert/Wikimedia.
Solem was keenly aware of the radiation problem posed by Orion, noting that Medusa would be assembled in space and probably launched from one of the Lagrange points, well out of the magnetosphere so that no charged particles would be trapped into Earth-bound trajectories. Interestingly, he thought of Medusa in terms of interplanetary rather than interstellar flight, noting that a major benefit of the proposal would be to reduce travel times that would lead to crew exposure to solar flare radiation and galactic cosmic rays. Such exposure led to proposals for massive shielding, whereas the swift Medusa would cut travel times by a factor of 5 to 10, with part of the shield being made up of the nuclear bombs that would be used as fuel. He even envisions astronauts using a crawl space inside the fuel as shelter during a solar storm.
Solem believed the best canopy material would be a high-strength aligned polyethylene of the kind that advances in materials technology should make available in the future. In the Los Alamos report he notes that:
We can reduce the mass of the canopy indefinitely by increasing its radius and the number of tethers. The tethers and the canopy material become progressively thinner. Mylar can be fabricated to a thickness of about ¼ mil, but other practical considerations, such as cost, will come into play long before the fabrication limit is reached. I will be conservative and say that we can spin-deploy a canopy 500 m in radius with 104 tethers.
More about Medusa, its possible interstellar applications, and hybrid mission designs on Monday. The Los Alamos report I refer to above is Solem’s “Some New Ideas for Nuclear Explosive Spacecraft Propulsion,” LA-12189-MS, October 1991 (available online). Solem also wrote up the Medusa concept in “Medusa: Nuclear Explosive Propulsion for Interplanetary Travel,” JBIS Vol. 46, No. 1 (1993), pp. 21-26. Two other JBIS papers also come into play for specific mission applications — I’ll give the citations for those next week.