I really enjoyed Stephen Baxter's 'What we could do with it' appendage to the excellent Antimatter article by Frank Close in the October issue and I have no wish to undermine either of them in any way. However I think Stephens description of how rockets work needs a little clarification. I do not mean to detract from his efforts, goodness knows he is evidently doing quite well enough without my interfering input but I do actually know a thing or two about the subject and in the interests of accuracy feel compelled to set the record straight.
At the risk of being accused of splitting hairs over a singularly pedantic technicality it is not true that the speed of a rocket is limited by the velocity of its fuel
, rather it is limited by the velocity of its propellant/exhaust
and the two are not necessarily one and the same thing.
I can happily confirm that the burning of Hydrogen and Oxygen produces an excellent exhaust velocity of some 5 kilometres per second, which is equal to an impulse rating of 2.3, but I would also like to add that Hydrogen and Fluorine produce exhaust velocities of around 6 kilometres per second at an impulse of 2.5 so Hydrogen and Oxygen are not actually 'the best'. However I will concede that Fluorine is a singularly nasty substance to deal with having very unpleasant side effects and although it has been used in various applications is best not considered for future use.
In the case of Hydrogen, which is more correctly the fuel, and the Oxygen which is obviously the oxidiser, the result of burning these two elements creates a lot of heat that excites the resultant molecules, which are basically super heated steam, and these molecules then exit the exhaust nozzle imparting thrust in the opposite direction (every force must have an equal and opposite reaction) pushing the rocket along.
Oberths formulae covers this neatly P = C dm/dt where P = thrust being equal to velocity C multiplied by the mass of the exhaust expelled every second (dm/dt)
Therefore Stephen is correct to point out that the faster the mass of propellant is expelled the less that mass needs to be to impart the equivalent amount of thrust and thus the spaceship does not need to carry so much propellant making the whole process very much more efficient. But this propellant need not be a fuel as such but an entirely inert substance that can be squirted out of the back end. You could for example use a conventional nuclear fission reactor to convert water into steam.
Now to the ideal exhaust velocity being light speed. Two points need clarification here. A photon is a quantum of electromagnetic radiation the energy E which is expressed as E = hv where h = Plancks constant and v = frequency of the electromagnetic radiation.
However although Photons posses energy they have no mass whatsoever and this seemingly minor detail is key because if they posses no mass then they have no actual kinetic energy and can impart no thrust. It is often cited that solar sails are an example of how light can push something along in space and so it is often concluded, incorrectly, that light can produce thrust. The truth is that the solar sail principle is driven by the solar wind which is composed of highly charged particles travelling away from the Sun at hypersonic speeds and as these particles have mass they will impart thrust.
As mentioned in both articles the annihilation of matter and anti matter produces electromagnetic radiation, specifically large amounts of gamma radiation which is a singularly dangerous form of energy that ionises everything it comes into contact with making it radioactive in the process. However, just as with photons gamma radiation has zero mass and thus imparts no thrust and therefore cannot be used as a propulsive medium in the form of a propellant exhaust.
You can test this with a simple experiment involving sunlight and a set of highly sensitive weighing scales, or indeed ask anyone who works with industrial lasers if any form of reaction force has to be compensated for, which I can confirm, it doesn't.
The best way in which antimatter could be used in propelling a space craft (allowing for the huge problems in producing and storing it) is through generating heat energy that could be used to power some form of electric propulsion system such as electro-thermal whereby the propellant is heated causing it to thermodynamically expand and exhaust through a nozzle, or electrostatic when electromagnetic fields act upon charged propellant particles, but then again you could achieve either using far less complex technology than that required by anti matter annihilation.
I was surprised not too see any reference to a Beamed Core system though?http://trs-new.jpl.nasa.gov/dspace/bits ... 0-0212.pdf