A more exciting prospect that nuclear fission is nuclear fusion which is much cleaner (no radioactive by-products) and has a higher energy density, about 108 (10^8 or 100 million) times greater than current chemical systems.
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| From atomicachive.com |
Controlled nuclear fusion is basically the joining of two lightweight nuclei, which produces a single nucleus and a great deal of energy.
There is a difference in mass between the original two
lightweight nuclei and the single product nucleus, and it is this
difference that results in the vast amount of energy released.
Einstein had realised in the early 1900's mass and energy were
interchangeable, and what we basically have here is a certain
initial mass being converted into a smaller mass and some
energy.
So how much energy are we talking about here?
Well it follows Einstein's famous formula
E=mc2,
that is the energy produced is the change in mass multiplied by
the speed of light squared, in other words a serious amount of
energy.
The major problem that we have in nuclear fusion is actually
making it happen, despite the fact that it has been touted as the
power source of the future for more than 50 years now.
The limited success we have achieved, since the late 1930's when
the first experiments were performed, requires such vast amounts
of energy that we are still a long way from the
break-even point.
On the positive side we do know nuclear fusion is a physical possibility because the Sun is a natural nuclear fusion reactor, using this principle to produce the solar energy it fires out through the solar system.
The
specific impulse offered from fusion propulsion systems
could be more than 1,000,000secs (exhaust velocity of
10,000,000m/s), this together with moderately high
thrust levels
allows this propulsion system to open up the entire solar system
to human exploration.
Interestingly hydrogen can be added to the propellant, which
decreases the
specific impulse but increases the
thrust to weight
ratio. This would be ideal for short-range missions. For long
range missions as the
specific impulse is so high it means that
fuel requirements are within possibilities and the high velocity
capability means that time frames are feasible.
The reasons for the difficulty in actually achieving at least
the
break-even point with fusion is that it is extremely
difficult to get the nuclei close enough to fuse.
Why? Nuclei are always positive charged so the electrical
repulsion prevents the two nuclei from getting close to one
another.
If we can get them close together then the strong nuclear force
(the most powerful fundamental force in nature) will overpower
the electromagnetic force and the two nuclei will combine, or
fuse. The strong nuclear force only operates over a very short
range though, so we must get the nuclei close before it will take
over.
So how do we get these nuclei close together?
There are 2 main possibilities and another emerging possibility. These are magnetic confinement fusion (MCF), inertial confinement fusion (ICF) and the latest idea of muon-catalysed fusion.
Magnetic Confinement Fusion (MCF)
MCF, sometimes referred to as continuous fusion, effectively
tries to recreate the Sun's method of achieving fusion, by super
heating the fuel to hundreds of millions of degrees by using a
plasma. The theory is that is as the fuel is heated the atoms
become much more excited and as they rush around at high speeds
there is an increased chance that the nuclei will get close
enough to fuse.
The major problem here is that plasmas are very difficult to
create and even more difficult to control, mainly because they
would simply melt through any structural confinement. The Sun
overcomes this simply by its immense gravitational field
strength, however this is not possible for us to mimic so the
fusion plasma is contained in extremely powerful magnetic fields.
This is possible because the plasma is composed mainly of ions
and electrons, which of course have electromagnetic charges.
Despite this it is still technically very difficult to achieve
for any useful length of time (i.e. for reactions to occur).
Inertial Confinement Fusion (ICF)
ICF, sometimes referred to as pulsed fusion, is a different
idea based on the same principle, in this case a tiny plasma, a
thousand trillion times more dense that that used in MCF, is
created by using blasts from lasers to rapidly superheat fuel
pellets. This plasma then rapidly expands and, due to an equal
and opposite momentum reaction, compresses the fuel pellets
(which increases the fusion reaction rate). This combination of
laser and the compression produces enough heat to induce fusion
to occur.
This system requires no heat containment, as the reaction is so
rapid. This means there is no need for the magnetic fields, the
pellet's own inertia should confine the heat long enough for a
fusion reaction.
There also exists the possibility that particle beams could be
used instead of lasers, as they are more efficient.
Muon-Catalysed Fusion
By contrast to these two approaches muon-catalysed fusion is
much different. A negative muon is an elementary particle similar
to an electron, but about 207 times as massive. Due to this much
larger size the muon orbits much closer to the nucleus than the
electron. So, in terms of fusion, the idea is to introduce these
muons (replacing the electrons) and allow their negative charge
to effectively shield the positively charged nuclei from each
other. This will eliminate the electrical repulsion force and
allow the nuclei to get close enough for the strong nuclear force
to fuse them. Finally the fusion energy ejects the muon and it
goes off to attach itself to another nucleus.
The big advantage here is that no superheating or confinement is
needed, indeed the reaction can occur at virtually any
temperature.
This new theory brings new problems however, firstly it is
extremely difficult to actually introduce these muons and allow
them to move into orbit around the nucleus. Muons also have a
very short lifetime, about 1 millionth of a second, so to be used
to catalyse many reactions the fusion itself will have to be
extremely fast. There is also the problem that no one is sure how
much of the muon will be lost in each reaction it catalyses.
Finally it is a very expensive process producing muons, both in
terms of energy and money, for this system to
break-even the muon
must catalyse many reactions for the system to produce as much
energy as was put in producing and using the muons.
Nuclear Fusion Craft |
| Courtesy MSFC |
The one major point about these systems for propulsion is that no net positive power output will be required, as it will for future power stations. For propulsion the required product is the velocity of the exhaust products of the reaction. Of course advances will have to be made in the inducing of fusion, as there will be a limit to the amount of energy for the reaction on a spacecraft.
It is thought that fusion represents the best possibility of a future propulsion system at the moment, due to the high thrust and exhaust velocity when compared with other possibilities, such as electric propulsion.
MCF Propulsion
MCF as a propulsion system is possibly not the optimum
approach, although it is quite possible that MCF will be the
optimum for power generation, but we will have to wait and see
for a definitive answer.
Before the development of a MCF propulsion system fundamental
scientific understanding of basic components must be achieved
through further research, for example a plasma divertor will be
necessary.
There are two primary problems with this system as a propulsion
system even if we can develop one. Firstly the weight of the
reactors would be prohibitively large due to the huge magnets
that are required for containment. This problem is further
enhanced by the fact that MCF operates a very low density, which
means that much larger reactors would be required than for
ICF.
ICF Propulsion
This propulsion system would operate by detonating pellets in
a chamber at the rear of the vehicle using lasers. Detonation
will have to occur at a rate of anywhere between 30 and 250 per
second.
ICF operates at a much higher density than MCF but it should be
noted that the required banks of lasers are likely to be heavy,
power intensive devices, though probably less so than the magnets
in MFC.
Furthermore there have been various proposals to totally side step this problem in ICF, though no such possibilities exist for MCF.
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Another reason why ICF may hold advantages in space is that the environment (a vacuum) will allow easier operation of the beams and thus the reactor.
Muon-Catalysed Fusion Propulsion
This concept may not be the optimum method for a propulsion
system, the short lifetime of the muon would mean that they would
have to be manufactured on the spacecraft, this would offset the
weight saving of not needing magnets or lasers. Added to this
with current technology the energy required to produce muons is
probably too great for us to generate onboard a spacecraft, which
means to become a realistic possibility much easier and cheaper
methods of production would need to be found. It should be
pointed out that if Zero Point Energy proves to be available the
energy problem could be avoided, but the mass problems would
remain.
(TheSpaceSite.com Zero Point Energy page is here, though we shall get there in a couple of
pages time if you want to wait).
Of course the efficiency of this type of fusion is unknown at the
present time, if a muon proves unable to catalyse many reactions
it is likely the system would be too inefficient for
consideration.
The capability of this type of fusion is difficult to assess, as
there are many unknowns. Even with the required advances whether
it would be superior to the possibilities of ICF is unknown.
Clearly many new advances are needed before this system can be
developed, not least the demonstration of a sustained,
controllable, high-density fusion reaction.
Whichever system is used, the propulsive effect would be produced
by using magnetic fields to channel the fusion reaction to the
exhaust nozzle. The fields then funnel the products out of the
nozzle by compressing the flow, further increasing exhaust
velocity. The optimisation of the process is determined by which
fusion fuel is used.
The other major consideration for fusion propulsion is what type of fuel should we use. In a simple example if we fuse a hydrogen-2 nucleus and a hydrogen-3 nucleus, a helium nucleus is formed, with a spare neutron, and an enormous amount of energy is released.
There are many better fuels though, which are both easier to fuse and produce more energy.
The easiest combination to ignite is deuterium and tritium,
both are isotopes of hydrogen (deuterium has one extra neutron
and tritium has two).
Deuterium is relatively easily refined from seawater, but tritium
has a half-life of only 12 and half years, it is therefore
unsurprising that it is not found anywhere in nature and the
short half life makes it unattractive even when made
artificially.
Deuterium may be fused with itself but this is a much harder
reaction to achieve.
The Problem
The major problem for all these combinations as a propellant
source is the fact that neutrons are produced, together in some
cases with short wave radiation (gamma rays). These are wasteful
and potentially dangerous (as they are radioactive).
Wasteful because the fusion reaction can only propel a spacecraft
if the particles produced can be directed to produce thrust.
These neutrons have no electric charge and thus can not be
manipulated by magnetic fields. This means that crucial
propellant mass is lost, as it can not be directed out of the
exhaust nozzle. Further to this it would require the addition of
both heavy radiators to remove the waste heat that neutrons would
produce by being trapped in the reactor walls and radiation
shielding to protect the crew and passengers. This limits the
effectiveness of fusion for a propellant system (note this is not
a problem for fusion reactors used for energy production).
The Solution
The solution for a propulsion system is to use the so-called
aneutronic fuels (meaning they do not produce neutrons when
fused), though the downside here is they are more difficult to
ignite.
Using these fuels produces high-energy ions (mainly alpha
particles that can be shielded with the thinnest material), which
are of course charged, so all the product can be used for
propellant mass and also the massive amount of waste heat
generated from neutrons is removed.
The resulting propulsion system is both lighter and has a higher
exhaust velocity than the system using deuterium-deuterium as
fuel.
Probably top of the list of these fuels is the mixture of
deuterium and the isotope helium 3 (two protons, one neutron).
The major problem here (besides the achievement of actually
fusing these two nuclei!) is that helium 3 is very rare on the
Earth's surface, though it is abundant in lunar soil and in the
atmospheres of both Jupiter and Uranus, due to billions of years
of solar wind bombardment. To harvest helium 3 would require,
initially at least, mining it from the Moon's surface.
It should be pointed out that there is a side reaction of
deuterium with itself so a small number of neutrons would be
produced.
This propulsion system was originally proposed for the Daedalus Project (see below).
A relatively recent idea is the use of ordinary hydrogen
nuclei (protons) and boron-11. Boron can be refined from
seawater; about 80% of boron is in the form of the boron-11
isotope. This reaction produces no gamma rays or neutrons, the
only product are alpha particles.
This could be the fuel of the future but the energy output is
thought to be quiet a bit lower than the helium 3/deuterium
mixture, but clearly there is no side reactions occurring.
A definite answer to the question of the optimum fuel will not be found until will have tested them both, but there is no question that either will open up new possibilities for exploration.
Project Daedalus| Project Daedalus Spacecraft |
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| Courtesy This is Rocket Science |
During the 1970's the British Interplanetary Society revisited
the Project Orion concept with Project Daedalus, but this time
using the far more powerful and environmentally friendly fusion
power.
Project Daedalus' objective was to send a probe past Barnard's
star (about 6 light years away) with a 50 year trip time.
The design calls for the use of fusion caused by firing electron
beams at fuel pellets in a magnetic "combustion chamber".
Using the magnetic field to confine and shape the plasma would
make Daedalus vastly more efficient than Orion.
The initial mass of the craft would have been 54000t, with
50000t of it being propellant. It was estimated that after 4
years of continuous acceleration the craft would be travelling at
1/8 of the speed of light.
It has been regularly pointed out that there is nothing
inherently absurd in the design of Project Daedalus, it is in
actual fact quite convincing.
It is worth noting that we have still to master the controlled
fusion reaction, while the technology required to build Project
Orion has existed since the late 1960's.
| Bussard Ramjet |
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| Courtesy MSFC |
We should not leave this section on fusion without mentioning the Bussard Ramjet. This was a concept devised in the 1960's that uses a unique way of powering the fusion reactor at the heart of the propellant system. The estimated velocity of the ramjet was up to 25% the speed of light.
The Ramjet attempts to overcome one of the main problems of rockets - that they have to carry prohibitively large payloads of fuel and waste energy carrying it with them.
The basic premise is very simple, why not collect the fuel as
you go?
Space is filled with hydrogen and helium at very low density,
though there are some higher density clouds. This scoped gas
would then be used to power the nuclear fusion reactor.
The original specifications were that for a 1000t spacecraft to generate 1g acceleration the intake would have to be 10,000 sq. km in high density clouds and 100,000 sq. km in low-density areas to scoop enough fuel. Of course the actual structure would not have to be that large as the magnetic fields would extend outwards ionising the gas and channelling the fuel in. The intakes would need to be 100km or 3000km respectively. Today, however, it is generally felt that these specifications are optimistic.
Another major problem is that to scoop enough fuel in the
spacecraft would already have to be moving at high velocity, so
the spacecraft would need to carry initial propellant to get up
to required speeds.
To slow the craft down would, however, be very easy - once the
fusion reactor is turned off the magnetic fields would deflect
gas and cause drag. Only a relatively small amount of fuel would
be required for a final deceleration.
The biggest problem remains the need for a small, efficient
nuclear fusion reactor that can fuse hydrogen with itself, which
is a difficult reaction to ignite. If we manage to achieve this
we still have to get the scooped fuel into the reactor and
address the issue that there will be drag caused by the scoop
even while the reactor is activated.
One final major problem remains, will it be possible to scoop
enough fuel up?
At the present time nobody knows.
Although not a fusion idea it is worth pointing out a variation on this idea known as the ram-augmented interstellar rocket (RAIR) which would be much easier in practice. Here the gas is not used for fuel but as reaction mass, it is in effect a self-fuelled ion rocket. Much work needs to be done on the idea to find out the feasibility of such a design.