Driving Swiftly to the Stars in our Atomic Space Cars
by John Walker
The following is an analogy I've used several times to explain how I can possibly be interested in so-called propellantless propulsion when it was I who coined the maxim, “Never invest in something that violates a conservation law.”
Robert Zubrin's 1999 book, Entering Space, contains an excellent popular explanation of why any kind of rocket, even fission…even fusion rockets we don't have the slightest idea how to build…are basically hopeless for anything much beyond the solar system.
The key issue isn't the density of your energy source, but rather the mass of the working fluid (or reaction mass) you heave out the back end. The absurdly poor mass ratio of rockets is due to the fact that most of the mass you're accelerating is not payload but rather propellant you'll subsequently eject.
Now consider a submarine with a propeller at the stern, cruising submerged. The submarine needs a source of energy to turn the propeller which can be almost anything—in fact, over the history of submarines, human muscle power, steam, air-breathing diesel engines, electric batteries, rocket-like bipropellant systems (the British experimented with submarines after World War II powered by diesel fuel and high-test hydrogen peroxide), and nuclear fission have all been used. But whatever the source of energy—there's no reason in principle you couldn't build an antimatter-powered submarine, given unlimited funds—the working fluid in which the propeller turns, the material which cancels the momentum it imparts to the sub, is the water in the ocean or, writ large, the Earth as a whole.
Now go back and work out the “rocket equation” for a submarine that's not allowed to use the water which surrounds it but instead is required to carry on-board all the water which passes through the propeller for its entire cruise. You'll find an expression which is similar to the equation for a rocket, and numbers which are just as hopeless regardless of what energy source turns the propeller. More energy doesn't help; what kills you is the need to carry your own working fluid. That's why plausible schemes for interstellar travel usually involve something like a laser-powered light sail in which the working fluid—photons—is supplied from the launch site rather than on board. But such approaches are marginal in the extreme, as Zubrin ably works out in his book.
No, to drive swiftly to the stars in our atomic space cars, we don't so much need a new energy source (though one would be nice for a lot of other reasons as well) as we need a “vacuum propeller” or “space screw” which uses the quantum vacuum in the same manner the propeller of the submarine uses the water in the ocean. There is no violation of any conservation law—you still have to expend the requisite energy for the work you're doing to accelerate the payload and momentum is conserved because the propeller is coupling equal and opposite momentum to the vacuum or, writ large, mass in the universe as a whole, which is fair enough since that's where inertia comes from in the first place according to Einstein.
The actual energy requirements for space travel are rather modest if you don't have to haul your own working fluid on board—look at any of the calculations of the cost of electrical energy needed to raise a payload on a geosynchronous beanstalk. The energy requirements of relativistic interstellar flight remain daunting, but are orders of magnitude better if you don't have to carry all your reaction mass.
I think some people discussing this issue tend to conflate the search for better energy sources and the independent quest for a vacuum propeller to eliminate the on-board working fluid requirement. Finding the latter drastically reduces the need for the former. A vacuum propeller is no more “propellantless propulsion” than the submarine's—it's just exploiting (albeit, in a manner we haven't yet figured out how to do) the ambient medium in precisely the same way.
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