Medusa: Nuclear Pulse Propulsion and the Sail

by Paul Gilster on July 20, 2012

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.

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MrDakka July 20, 2012 at 16:27

Currently UHMWPE (ultra high molecular weight polyethylene) exists; would that be sufficiently strong for the canopy material purposes?

Greg July 20, 2012 at 16:42

I might be missing something but wouldn’t the capsule be dragged through the area where the nuclear device detonates, constantly?

Paul Gilster July 20, 2012 at 16:53

Strikes me as a major problem, and I know Kelvin Long commented on it in his new book. More on this on Monday — right now I’m digging to find out how Solem answered this objection.

A. A. Jackson July 20, 2012 at 19:34

It was Stanislaw Ulam and C. J. Everet who dreamed up Orion tho they only provided a schematic of the process.
I seem to remember that , may be faulty, that Ulam even had the idea for a ‘sort of ‘ Medusa , being the amazing Ulam I would not discount it.
Made me look up some of Ulam’s bio, one question I have had in my mind is who really looked at nuclear propulsion first , well in terms of technical stuff it was during WWII and it was Ulam and Frederick Reines. Don’t know if they documented their investigations.

daedalus2u July 20, 2012 at 20:32

There is nothing left at the point of detonation. The warhead is vaporized and all of the fission products get swept away. Remember this is happening in a vacuum. There is nothing to impede the free expansion of the bomb debris.

This is a pretty expensive way to go. Fissionable material suitable for making bombs has to be either highly enriched uranium or plutonium. As fuel to nuclear power plants it has about a million times the energy density of coal, and the cost of uranium is about a million times higher. ~$40/ton for coal, ~$70 million per ton for highly enriched uranium.

The Isp they get is pretty high for chemical fuels, but you can get ~2x higher than that with ion thrusters.

For a 50 MT capsule, the paper estimates 2x as much fuel to get to a 50 km/s velocity. 100 MT of highly enriched uranium costs about 7 billion. Xenon is the preferred fuel for ion thrusters. Reportedly it is ~$120/100 grams, or $120 million for 100 MT.

Alex Tolley July 21, 2012 at 0:16

@Greg. It looks like the assumption of an expanding gas cloud that is thin enough to be of low radiation hazard to the crew at the end of the tether. The crew is also partially shielded by the bombs.

“…the tethers can be made very long to mitigate radiation hazard to the crew. “

AFAICS, this is not shown in the equations provided, although it might be estimated.

Ken Robbins July 21, 2012 at 0:56

Greg: I hadn’t thought about that. My first “Could this really work?” questions were different.

A Medusa ship would need radiation shielding to protect it from the nuclear pulses. The pulse unit debris has a velocity so high relative to the ship ( approaching or exceeding solar escape velocity ) that it doesn’t ‘stick around’. So accelerating into the vicinity of the detonation point is not a major issue.

If you look at page 10 of the report, Mr. Solem has an example with 25 ton yield bombs at 7.5 km out from the ship. That’s not a big detonation, and the vehicle is well separated from it. Also consider you have a seemingly flimsy polyethylene bag as your engine! My first thought was melting or tearing the drive baggie would be the show stopper.

If anything Medusa’s big objection is a wastefulness of fissionable material if it uses Orion style pulse bombs. If laser fusion is ever perfected ( like in the ever so optimistic SIRIUS project ) the main issue becomes fast neutrons. I regret I don’t have a citation for SIRIUS to give, I saved that report many many years ago and didn’t think to bookmark where I got it from, LANL online library I think ).

I saw Solem’s paper many years ago: The many equations inspired me to quickly make an excel spreadsheet to design all sorts of Medusa ships.

Christopher Phoenix July 21, 2012 at 5:31

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.

This quote seems to suggest a Medusa craft’s swifter flight would allow us to cut down on the amount of shielding the crew capsule requires to protect the crew from solar flares and GCRs, but wouldn’t the residual radiation from the nuclear bomb detonations pose a threat to the crew as well? It seems to me a bit silly to propose lowering the crew’s radiation exposure by setting off a bunch of nuclear bombs in front of their craft and then drag them through the area where the bombs detonated, and even sillier to skimp on shielding the crew capsule on such a craft!!

Radiation from nuclear explosions travels very far in space. I recall seeing papers calculating the possible effects of nuclear detonations on astronauts during a space war. How is the crew of a Medusa craft to be protected from the radiation for their own bombs? Will they shelter in the shielded crawl space during acceleration?

Personally, I want an atomic rocketship like this one. So many proposals for spaceships lack any sense of dignity, class, and beauty. Dangling on kilometer long cables while being dragged through repeated bomb blasts? Spinning around like a bolo while huddled in a cramped bottle-shaped capsule? Cruising around in a cranky amalgamation of orbiting tins? Imagine if the aliens saw us- it would be a complete disgrace!! We’ll look like we have no sense of class at all. A cosmic fashion disaster, that’s what we’ll be. All we’ll see and hear when we land will be the associated twitterings, flashing light patterns, and radio squeaks that signify laughter on other planets.

Michael July 21, 2012 at 9:04

Shielding should not be a great problem -All water/fuel could be stored in cone shape towards the detonation point, both absorb neutrons/radiation well however neutrons hitting nuclear materials such as Plutonium can lead to highly radioactive isotopes which can cause detonation problems. I do see a problem with reflection of radiation at the centre of the canopy which would go back towards the bomb detonation point -a hole there may solve the problem though.

Denver July 21, 2012 at 12:34

“Personally, I want an atomic rocketship like this one.”

Takes off and lands straight up and down, just like God, and Robert Heinlein, intended.

Tony D July 21, 2012 at 16:22

OK, strawman interjection: theoretically how fast could this concept go? The need for speed! Thats all I’m concerned about ;)

Michael the Usually Civilized July 21, 2012 at 18:41

What would it take to build an unmanned version of this?

Greg July 22, 2012 at 10:04

Thinking about it, the solution may be have two sails in front and have the capsule equidistant between the sails. If they were a couple of kilometers apart it could prevent the capsule from coming into contact with most of the radioactivity, possibly, just haven’t crunched the numbers.

A. A. Jackson July 22, 2012 at 12:08

Always sniffing about for firsts , my comments about Stan Ulam sent me to his autobiography …. Stanislaw Ulam, Adventures of a Mathematician, New York, Charles Scribner’s Sons, 1983.
Where I found on page 252 this:
“I think Feynman was the first in Los Alamos during the war to talk about using an atomic reactor which would heat hydrogen and expel the gas at high velocity”
This would have been between 1942 and 1945 , I imagine it was back-of-the-envelope calculations, so I am guessing nothing was published.
There are a multitude of speculations before 1940 on using ‘atomic energy’ for rocket propulsion with out any real technical engineering specified.
Esnault-Pelterie mentions this specifically in his section of relativistic spaceflight in L’Astronautique. Paris: A. Lahure, 1930 , where he mentions his hope that atomic energy would provide the energies needed for interstellar flight.
The greatest concentration of nuclear reactor experts in the world were rubbing shoulders with Feynman every day , so no surprise.
Ulam was also involved with Rover and Kiwi, but what Los Alamos was doing about nuclear thermal rocketry between 1945 and the creation of these programs I have not found out.
Bussard and DeLauer mention only the published works of Shepherd, Cleaver and Tsien* for references before 1950 ( Bussard, R.W.; DeLauer, R. D. (1965). Fundamentals of Nuclear Flight. McGraw-Hill).

*Yes that H.S. Tsien we kicked out of the United States , who became the father of the Chinese space program.

Eniac July 22, 2012 at 18:40

Great concept, in principle. My concerns would be:

The motorgenerator will provide electrical power during this phase of the cycle, which will be conveniently stored.

Makes me wonder if the author considered how in particular a substantial fraction of the energy of a nuclear explosion could be “conveniently” stored. It appears Solem has “reserved” this “for a future paper”. Also, how about the tethers which have to absorb/transmit the blast energy, while at the same time being exposed to its destructive effects? I did not really see the strength and resilience of the tethers addressed as well as that of the canopy.

Geoff July 22, 2012 at 20:47

Wouldn’t the bomb debris stick to the canopy and tethers (by localised melting of the material, perhaps)? A huge, thin sail could soon end up rather less lightweight than it was to start with. Even worse, radiation from stuck-on fission products would degrade the canopy over time. It might be necessary to carry a spare sail, so as to have a fresh one for deceleration upon arrival.

James Jason Wentworth July 23, 2012 at 3:11

Just as the “Putt-putt” test vehicle (which was powered by hand grenade charges, if memory serves) was used to validate Orion’s propulsion concept in an actual flight test by detonating the conventional explosive charges behind its pusher plate, Medusa’s propulsion system could also be flight tested using conventional chemical propellants. It could be done thus:

A sub-scale Medusa demonstrator could be launched into space aboard a multi-stage sounding rocket (a Black Brant XII would provide a long, high sub-orbital flight). After separating from the rocket’s final stage, the Medusa system would deply. The “parachute” could be made of rip-stop Kevlar, metallized on the side facing the Medusa spacecraft and having a thermally-emissive coating on its other side. The “parachute” could be held in shape by a thin steel cable threaded through its rim. The shroud lines could be made of Kevlar or braided titanium strands, as could the main load-carrying cable between the shroud lines and the Medusa spacecraft. Now:

The test vehicle could be powered by hydrazine (fuel) and nitrogen tetroxide (oxidizer), which are hypergolic–that is, they ignite on contact. Two nozzles on the forward-facing end of the Medusa spacecraft (on either side of its cable reel) would be canted inward toward each other, so that their streams of hydrazine and nitrogen tetroxide would impinge upon each other at a pre-determined point (the “focal point”) between the “parachute” and the Medusa spacecraft. The propulsion system would operate in pulsed mode, just like that of the full-scale, nuclear-powered Medusa ship. For the sub-scale Medusa demonstrator, the cycle would be: fuel & oxidizer spray/detonate/pay out cable/reel in cable, then repeat. Such a demonstrator could, in the near term, conceivably lead to a simple LEO/geosynchronous orbit or LEO/lunar orbit transfer vehicle.

Ole Burde July 23, 2012 at 8:05

This is the kind propulsion that could be a necesary solution if we found out the sun would go Nova in 10 years … something that could be quickly glued together from existing parts , more or less .

Martyn Fogg July 23, 2012 at 9:33

Suppose these Solem sails were to have a small hole in their centre, they could be steered accurately, and that nuclear propulsion charges could be lined up perfectly in space, perhaps by laser guidance.
Then one might imagine an ‘Interstellar Solem Sail Runway’ which would impart a jolt of pulse propulsion each time its sail overtook each charge, thereby accelerating the outgoing ship as a whole up to interstellar cruse velocity.
The vessel would only need fuel to decelerate at the target system: a considerable reduction in the mass it would need to carry.

Eniac July 24, 2012 at 23:02

I imagine there would have to be a rather large hole there in any case, since we can’t have tethers going right through the center of the explosion. Such a hole would substantially reduce efficiency. I wonder if this was considered?

Eniac July 27, 2012 at 23:50

The problem with “runway” ideas such as the one expressed by Martyn is that it merely kicks the can down the road: Building the runway is at least as difficult as sending a rocket without one. More difficult, actually, as propulsion is easier for large systems than small.

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