The Emthruster: Thrust without reaction?

[stevebowers]

Photon drives aren't reactionless drives, since photons carry momentum, if not mass. This is because of mass/energy equivalence.

The 'Cannae drive', or the E-M drive, or the Shawyer drive, which all seem to be roughly the same thing, all are supposed to have no net photon emissions in any one direction.
http://en.wikipedia.org/wiki/EmDrive

Science fiction writer Greg Egan distributed a public letter stating that "a sensationalist bent and a lack of basic knowledge by its writers" was making the magazine's coverage sufficiently unreliable "to constitute a real threat to the public understanding of science". In particular, Egan found himself "gobsmacked by the level of scientific illiteracy" in the magazine's coverage of the EmDrive, where New Scientist allowed the publication of "meaningless double-talk" designed to bypass a fatal objection to Shawyer's proposed space drive, namely that it violates the conservation of momentum. Egan urged those reading his letter to write to New Scientist and pressure the magazine to raise its standards, instead of "squandering the opportunity that the magazine's circulation and prestige provides" for genuine science education. The letter was endorsed by mathematical physicist John C. Baez and posted on his blog.[11] Egan has also recommended[11] that New Scientist publish Dr. Costella's refutation[10] of Shawyer's theory paper.[35]

[stevebowers]

Egan's refutation is here, by the way
http://gregegan.customer.netspace.net.au...avity.html

[iancampbell]

Of course photon drives aren't reactionless drives. They are, however, functionally equivalent in that nothing material has to be chucked overboard to get the thrust. And since propellant mass is the major problem for reaction drives...

Incidentally, I don't think antimatter would be quite such a magic fuel as some people think. The reason is that a significant proportion of the energy is generated in forms that are completely useless, such as neutrinos.

[stevebowers]

I agree. Much of the rest is gamma rays, also difficult to manage. That is why I prefer antimatter-catalysed fusion drives, which obtain much of their energy from hydrogen fusion.

[Drashner1]

Of course photon drives aren't reactionless drives. They are, however, functionally equivalent in that nothing material has to be chucked overboard to get the thrust. And since propellant mass is the major problem for reaction drives...

However, you have to consume something material to get the energy to generate the photons. And the efficiency of a photon drive is abysmal - something like 300+ MW of energy needed to generate a single pound of thrust IIRC.

Incidentally, I don't think antimatter would be quite such a magic fuel as some people think. The reason is that a significant proportion of the energy is generated in forms that are completely useless, such as neutrinos.

Suggest you read Forward's work on this. He did a pretty thorough analysis of antimatter propulsion for the Air Force before his death and addresses this issue pretty completely. The result is still a drive that leaves everything else in the dust.

Todd

[stevebowers]

A recent study building on Forward's ideas is here
http://arxiv.org/ftp/arxiv/papers/1205/1205.2281.pdf
Ronan Keane and Wei-Ming Zhang have refined the beamed-core concept, so that it can achieve an effective exhaust speed of 0.69c, more than twice earlier estimates.
I think there would still be a significant problem with radiation (and waste heat) from such a system, but this is significantly better than previously thought.

[kch49er]

A recent study building on Forward's ideas is here
http://arxiv.org/ftp/arxiv/papers/1205/1205.2281.pdf
Ronan Keane and Wei-Ming Zhang have refined the beamed-core concept, so that it can achieve an effective exhaust speed of 0.69c, more than twice earlier estimates.
I think there would still be a significant problem with radiation (and waste heat) from such a system, but this is significantly better than previously thought.

does this affect the setting given that we have 0.25 as the amat clippers (unless these are an earlier verison\) presumbly faster amat ships would let further systems be colonised earlier.

[stevebowers]

Man-rated beamed core ships were not successfully built until 1120 AT, during the First Federation. This allowed the First Fed to expand at a faster rate than the pre-Nanoswarm colony effort.
http://www.orionsarm.com/eg-article/493d299bc818c

In many ways tech development in OA is quite conservative; with technologies like antimatter propulsion and interstellar travel, there are bound to be very long development times and unexpected difficulties.

[Drashner1]

I looked up the beam-core concept on Atomic Rockets and its actually a different design from Forward's (at least I've never heard of him doing work on that design). Beam-core drives use an equal mix of matter and amat and would probably be significantly harder to build and certainly have much higher amat requirements. For this reason, they might have been more expensive to build and therefore less common or taken more time to be built in quantity.

Another factor could be the use of boostbeams and mag-brakes. These might both accelerate and impede the development of beam-core ships. Accelerate in that their use would reduce the amount of amat fuel needed. Impede because they would amplify the performance of existing ships and so raise the bar that a beam-core ship would need to get over to demonstrate a return on investment sufficient to build them in quantity.

Todd

[radtech497]

Another factor could be the use of boostbeams and mag-brakes. These might both accelerate and impede the development of beam-core ships. Accelerate in that their use would reduce the amount of amat fuel needed. Impede because they would amplify the performance of existing ships and so raise the bar that a beam-core ship would need to get over to demonstrate a return on investment sufficient to build them in quantity.

With a maximum velocity of 0.65c, the beamed-core spacecraft would likely have been supplanted by conversion drive ships almost immediately (both were available in the First Federation period), though the amat vessels' dominance might have benefitted from the relatively slow (?) diffusion of conversion technology from its S:2 inventors.

Radtech497