Science Forums 2

[CraigD]

Originally Posted by Eclipse Now

anyone read through this page? Extremely optimistic stuff about our ability to get out and explore the universe....

p2pnet news Blog Archive Want to help build a space engine? is certainly optimistic, but – with all due respect to Michael Thomas, who appears to be a distinguished computer engineer – even though many of its links are broken beyond even recovery via archive.org, preventing me from reading all the offered descriptions, I’m pretty sure it’s also unrealistic, and based on an inadequate understanding of basic physics and limited knowledge of spaceflight science fiction and non-fiction.

Thomas’s claim

My Linear Particle Accelerator (LINAC) atom smasher concept for electron particle propulsion is a unique proprietary invention developed over the last ten years and takes space propulsion in a direction never thought of by any scientist or organization in history

notwithstanding, the idea of using a linear accelerator or cyclotron to accelerate the reaction mass of a rocket to relativistic speed is not a new one, appearing in at least occasional science fiction stories no later than the 1960s. It’s a fairly obvious approach to the fundamental rocketry problem of maximizing specific impulse (), a measure of the mass that must be expelled from a rocket (reaction mass) to change it’s momentum: by accelerating the reaction mass to a large fraction of the speed of light (c), then applying force to accelerate it more, the reaction mass is effectively greater when used than when stored as propellant, multiplying the rocket’s , and potentially increasing the amount of useful payload a rocket of a given starting mass can give a given change in velocity (delta V).

A problem with such a system, however, is that accelerating the reaction mass to nearly c requires a lot of energy, and, assuming the source of this energy must be carried by the rocket ship, is also equivalent to mass, so even assuming 100% efficiency (a tremendous overstatement), at some point (which I’ve not yet managed to calculate) increasing the propellant speed results in an increase of the mass of propellant + fuel. The machinery needed for an accelerator also threatens to be prohibitively massive compared to the propellant mass it accelerates – the SLAC, for instance, at about 2 miles (3.2 km) long, and even stripped to its essential components, must mass tens of thousands of tons, yet produces miniscule rocket thrust. So the idea that high propellant speed alone is a panacea, or silver bullet solution, to the problems of rocketry is simply wrong.

This is not to say that using particle-accelerator techniques in rocket motors is a bad idea, or an unexplored one. The VASIMIR motor, which has been under development since 1979 and is expected to be flown experimentally in space in the next few years is a prominent and promising example of such an approach.

This is also not to say that the goal of a spacecraft that can sustain acceleration of 1 g (about 10 m/s/s) is anything but awe-inspiring and worthwhile. However, IMHO, approaching this goal by searching for a radically different rocket motor is not radical enough. I believe it requires completely re-imagining the nature of a spacecraft, abandoning the idea of a self contained vessel reminiscent of an old-fashioned ocean ship, in favor of a system consisting of massive fixed (in solar orbit) facilities in which the payload/crew containing vessel is many orders of magnitude smaller than the total system. The “laser-powered light sail ship Promethius” described in Robert Forward’s 1985 novel Rocheworld is as good a speculative description of such as system as I’ve encountered, though it describes a much more modest performance of 0.01 g acceleration and 0.1 g deceleration for a maximum speed of 0.2 c on a 20 year trip covering 2 light years.