December 5, 2016

EM-Drive, Cannae, and The Implications for Propellantless Rockets

​While much fuss is made over the possibility of EM-Drive, there are actually two propellant-less, low trust concepts being evaluated. Cannae has similar properties, but depends on superconductors to operate. There are plans to do a demonstration satellite in the next few years.

In the near term, these rockets could make extensive surveys of the asteroid belt and outer solar system possible for light spacecraft.  They would provide another strong incentive to build space-rated nuclear reactors. Finally, we could get beautiful images of the planets around the Alpha/Proxima Centari system - by flying a probe in the exact opposite direction.


PROBLEM: The Rocket Equation dictates that the faster or heavier your planned rocket, the more propellant you need to move it. The more propellant you need, the heavier and slower your rocket, due not only to fuel but to bigger tanks and engines. This vicious circle makes reaching orbit difficult and starships using something other than fusion, anti-matter, or laser sails nearly impossible in conventional physics.

Engines used in spaceflight fall into two categories. Purely-chemical engines provide high thrust needed to leave Earth or land on other planets and moons, but only burn for a few minutes. Ion Drive engines are partially chemical, partially electrical. They provide low thrust for long periods of time (basically, like the weight of a sheet of paper on your hand, but nonstop for several years). This is ideal for keeping satellites in place, propelling missions to the asteroid belt, or intercepting comets. Advanced ion engines (with several orders of magnitude capacity) can even capture near-earth boulders and return them to lunar orbit. Very advanced systems (not yet feasible) could slowly transport cargo to Mars. These systems still run out of propellant over time, though. They also have effective speed limits because of the rocket equation.

OPPORTUNITY: These two propellant-less systems, if either one were feasible, would start out as having roughly one-tenth the electric-power-to-thrust efficiency of modern ion drive systems. Note that these new systems are barely viable in the lab, whereas ion drive has been refined for over four decades. It’s quite possible that future systems would be as efficient as ion drive. But the break is that they can basically go until they run out of electricity or break down. In the inner solar system with access to solar power, the only limit is how much you can accelerate until you reach solar escape velocity and no longer have access to solar power. A transport with routine starts and stops could run indefinitely, but would be a relatively slow form of transport. A nuclear-powered system would not depend on proximity to the sun for power and may accelerate for years, thus making a viable near-term interstellar probe.

If either system works, it would be ideal to send a deep space telescope in the direction opposite Alpha Centauri, then look backwards at the sun from 550 AU away, using the sun’s gravity as a lens to view any planets in proximity to the three suns of this system. We would have the equivalent to a distant fly-by mission of all three stars and their planets while going a tiny fraction of the distance (0.009 light years for the telescope versus 4.2 light years for an actual fly-by), and with a much longer window of observation. As the probe flew farther from the sun, the resolution would actually increase for a time until it was no longer viable.  This concept is called The FOCAL mission.

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