Tuesday, 28 July 2015

Random Numbers: How 'Powerful' is a 40 Watt phased plasma rifle?

Science Happens...

   For an Author writing SF can be challenging for many reasons.  Not least is the expectation that there must be some 'science' involved, especially for a 'hard SF' work.  Often this results in lengthy and boring expositions where the technology, setting, or related paraphernalia is explained in detail.  This isn't so bad; it is, after all, the reason many people read that particular kind of SF.  A bigger issue is when the author adds in a random number, not always necessary, in order to imbed more firmly in his reader/audience's mind that this is 'science' fiction.  But, often, the number is not as carefully selected as it should be.  Most SF fans will not care, but for some of us fanatics it is a major annoyance.

   The case in question is from Terminator, one of my favourite SF movies.  The terminator is buying guns and asks the shopkeeper if he has a 'phased plasma rifle in the forty watt range'.  So far so good. A single line that reinforces the fact that the terminator is a robotic killer from the future.

   But I got thinking, is 40W really a good number?  A big problem for hard SF authors wishing to include energy based weapons into their 'Verse is how much power to give them; making sidearms wight eh output of a thermonuclear warhead is an obvious no-no, for example.  So I made the following table, 'translating' the output of several modern kinetic weapons into a 'Power' rating.  It is not a perfect comparison, as directed energy weapons employ a different mechanism to do damage t the target than do KEW, but it provides a rough ballpark.

   It turns out that the terminator's preferred weapon is roughly equivalent to a .22 magnum rifle firing one round every fifteen seconds.  Not a very terrifying prospect.  If it had been forty kilowatts it would be another matter entirely.  While it is not an issue that detracts from the quality of a SF work overall getting details such as this correct is at the heart of hard SF, so hopefully this table will help with those details.  It also shows why multi-barrelled weapons are so deadly, even when they fire a relatively small projectile.  For anyone trying to do their own calculations, they are as follows;

Energy = (Mass * Velocity^2) / 2
mass in kg, velocity in m/s, energy in joules
Power = (Energy * RPM) / 60

   It was pointed out in the comments that I had inadvertently used the wrong equation.  I've fixed that, and the table is updated as well.  If I've made any other errors feel free to point them out

Saturday, 25 July 2015

Myths of SF: Do Nuclear Reactors Explode?

Fission, Fusion, Reactors, and Bombs

   Fear of nuclear devices is deeply engrained in modern culture as a result of the Cold War; years of imminent nuclear annihilation has that effect.  SF often reflects this, with many works from the Cold War period or later revolving around nuclear war as the cause of an apocalypse.  Even though that has been replaced in new works by climate disruption, genetic mishaps, etc. that reflect the newest scientific advances the negative connotation of nuclear devices remains.

   The most common example of this is the use of Fission bombs to show a person or faction as being 'uncivilised' or to show how desperate the situation.  In the Dune 'Verse all the noble families posses Atomics the use of which is seen as unthinkable, while in the movie Oblivion the use of nuclear weapons against the invading aliens was used to indicate the desperation of Earth's defenders.  The second example, more prevalent in movies at least as far as my own experience goes, is that of a reactor exploding.  As well as tapping into people's fear of radiation and their knowledge of the destructive potential of nuclear weapons it is an easy way to add tension to a story.  Alien did this, as is Aliens a classic example with the damaged Atmosphere Processor, and is the B-movie Nuclear Hurricane, although in the latter example the reactor did not explode its use as a literary device is the same.

   For a really fanatical 'hard SF' fan that is a problem.  Nuclear reactors don't explode.  Or more precisely - a nuclear reactor will not undergo a nuclear detonation, producing the feared mushroom cloud.  They can still explode, but this is due to high pressure steam or chemical reactions, and while it may severely damage the reactor facilities and spread radiation, it will not level everything within a kilometres wide blast zone.

   It is also important to understand that in a modern nuclear 'physics package' it is actually quite hard to achieve nuclear detonation.  The explosive compression of the fissile core requires incredible precise timing to achieve the required densities.  While gun-type devices are less precise they are still a mechanism dedicated to achieving a fission explosion.  Thus it is highly improbable that any nuclear reactor is eve able to achieve the conditions for a fission detonation; the required conditions are too precise.

   Yet an immanent catastrophe is the perfect way to spice up an otherwise lagging plot, are to up the stakes just that little bit more, so what can the hard SF writer do?  Firstly, arrange the setting so that a relatively small explosion is catastrophic - "if the reactor goes down the plasma shields fail and the solar flare will kill us all!"  Specify a non-nuclear explosion - fusion reactors could be quite cooperative in this regard, as I will explain later - to avoid the critics, and way you go.  For more specialised situations there is a possibility of nuclear detonation, most revolving around spacecraft due to the inherent danger of a system that can suffer catastrophic failure.

   I'm going to look at the first and second options.  For the non-nuclear explosion a quick look at the Chernobyl and Fukushima disasters will outline the basic failure modes.  Then for more futuristic settings will be a look a Fusion, Antimatter, Black Holes, NSWR, NPP, and more.

Chernobyl after the disaster
Chernobyl & Fukushima

   The key to both Fukushima and Chernobyl hinges, to my (arguable limited) understanding on the fact that a fission reactor cannot be stopped instantly as, for example a car engine can.  A percentage of the power that the fuel outputs is not from the primary reaction but from the decay of short lived isotopes produced in the reactor.  This makes shutting down a fission reactor a tricky matter under the best conditions, as full cooling must be maintained uninterrupted through the process.  In Fukushima the failure of backup diesel genitors compromised the cooling system, and when the backup batteries run out it lead to a meltdown as the containment vessel overheated.  According to the wikipedia page

   "It is estimated that the hot zirconium fuel cladding-water reaction in Reactors 1-3 produced 800 kilograms (1,800 lb) to 1,000 kilograms (2,200 lb) of hydrogen gas each, which was vented out of the reactor pressure vessel and mixed with the ambient air. The gas eventually reached explosive concentration limits in Units 1 and 3. Either through piping connections between Units 3 and 4 or from the zirconium reaction in Unit 4 itself,[27] Unit 4 also filled with hydrogen. Explosions occurred in the upper secondary containment building in all three reactors.[28]"

   A similar situation occurred at Chernobyl.  Although it seems that the problem there was more due to the rapid boiling of the coolant water.  From the appropriate wikipedia page

  "Because of the positive void coefficient of the RBMK reactor at low reactor power levels, it was now primed to embark on a positive feedback loop, in which the formation of steam voids reduced the ability of the liquid water coolant to absorb neutrons, which in turn increased the reactor's power output. This caused yet more water to flash into steam, giving yet a further power increase."

   Basically the coolant flow dropped too low allowing steam to form.  As the steam does not absorb neutrons as well as the water the reaction rate in the core increased rapidly, finally reaching ten time the normal output.  This overpressure blew the containment vessel, venting all coolant and sending lumps of superheated graphite moderator into the air where they fought fire.  The secondary explosion was more powerful than the first and was probably a combination of chemical and steam. 

   The wikipedia pages provide more than enough information for any SF author to write a convincing plot centred around a failed nuclear reactor, and the citation links provide a huge mine of further information, so I won't go any further into the mechanics of a fission reactor failure.

A still of the atmosphere processor from
James Cameron's Aliens          Source
Fusion Reactor Failure

   One of the many advantages a fusion reactor would have over a fusion design would be its relative immunity to catastrophic failure.  The reacting fuel is a thin plasma that can be vented if problems arise, since most fusion fuels are non-radioactive.  Also, if the reaction is allowed to stop the fuel cools very rapidly, unlike solid fission fuel with its decay energy.  So it seems that with a good design catastrophic failure is unluckily in a fusion reactor.  There is, however, one possible medium through which it might occur. 

   Fusion reactors contain plasma through superconducting magnetics.  The superconductivity of such magnets is dependant on their being kept below a certain temperature, called the 'critical temperature'.  Above this point the conductors used in the magnetic become normal conductors, able to carry only a small fraction of the current that they can while superconductive.  If the cooling system was damaged it might be possible for the magnets to reach the critical temperature.  The resistivity of the coils would increase suddenly, heating them.  As the temperature rises so does the resistivity.  If the energy flowing through the coils is high enough it could be released in the form of an explosion as the coils are vaporised.  More energy would be added by the fusion plasma, although I have no idea how much that would be, given the extremely low densities.  If this is possible the effect would be most prominent in reactors with the strongest magnetic coils and high power outputs.

   Of course, any good reactor would be designed to prevent this from happening.  But incompetence, cost cutting, sabotage, and damage all offer an opportunity for any safety features to be circumvented.  The result will not be the nuclear blast of Aliens, but it could be more than enough to destroy a spacecraft or space station, two places where extremely powerful fusion reactors are likely to be found.  And of course boiling lithium or sodium coolant flying all over the place would add to the destruction, especially if there was large amounts of water present, or perhaps a fluorine atmosphere?

Warning sign by Anders Sandberg of
the Lifeboat Foundation

   This hardly needs explaining.  Atomic Rockets has a much better overview of the issues with storing antimatter than I could include here, so follow the link.

   Although not strictly speaking a 'reactor' antimatter might prove to be the only way to achieve certain things.  Interstellar flight, torchships via micro-fission or fusion sparked with minuscule amounts of antimatter.  However it has the fatal flaw of reacting with anything.  Which means no matter how good your containment is, damage through accident or design is a 'bad thing', which is why the containment cylinders on the starship Enterprise could be ejected.

   Superconductors also play into this scenario.  As a superconducting electromagnet does not loose all its power instantly when the power is cut off there might be a short delay between the failure and the magnetically levitated ball of anti hydrogen contacting the containment vessel and vaporising the ship.  It might be only seconds, but those seconds could mean the difference between the crew compartment automatically ejecting or getting atomised.

Robert Zubrin's NSWR from this paper
NSWR: Nuclear Salt Water Rocket

   For the people who think that the Orion Drive is impractical there is a concept known as the Nuclear Salt Water Rocket.  Innocent sounding name, but a rather terrifying mode of operation.  Water containing enriched uranium salts is pumped into the reaction chamber where it undergoes a continuous nuclear detonation.  Premature detonation is prevented by storing the fuel in a matrix of neutron absorbing material.  Once again I refer anyone interested in further details to Atomic Rockets.  The thing is that the NSWR offers such high performance that it might be used despite the obvious risks; military, interstellar probes, and massive commercial spacecraft all have obvious benefits.  They could even form the basis of a power system with the plasma from the exhaust guided through a MHD generator.  But should the neutron absorber be damaged or the 'nuke juice' accumulate to critical mass there will be a low yield fission explosion, perhaps powerful enough to cause detonation of the rest of the fuel.  Slightly safer than antimatter.

Credit Anders Sandberg
Black Holes

   Confusingly these favourites of SF are not, in fact, black.  Through some complicated physics I don't really understand black holes give off Hawking Radiation.  Not only that but they can evaporate.  The rate of evaporation is inversely related to the mass, as is the temperature of the radiation.  If you had a very small, as in atomic radius small, black hole it would give off quite a bit of energy.  If you could stabilise a micro-blask hole by forcing matter into it at the same rate as it lost mass it would be a 100% efficient mass to energy device.  Anyone with the tech to do this has a huge advantage in terms of starship propulsion as well as all the benefits of being a Kardashev level civilisation without having to build a Dyson Shell.  Obviously if the mass input was too low the black hole would 'explode', releasing far more energy than can be contained.  It is also perfectly predictable, if you know the mass of the hole then you know when that moment will come.  This, added to increasing output would be perfect for raising the stakes aboard a post-Singularity starship.  Note that as the amount of Hawking Radiation increases it becomes harder to get the black hole to accept matter due to the sheer energy output, exacerbating the problem.

NPP: Nuclear Pulsed Power

   In pulsed power reactor a tiny nuclear bomb is detonated and the resultant energy turned into electrical power.  A wide rang of techniques are used for both the bomb and the containment/energy capture.

   Most designs would be fairly safe, as under normal conditions the pulses do not put out enough energy to destroy the chamber even under worst case scenarios.  However, a system that had been modified to produce more power, run without spare parts, or one fuel it was not intended to use, could fail catastrophically.   If to the detonation produces to much plasma/debris the containment could be destroyed.  This is most likely in a overpowered magnetic containment design.  If the containment failed the impulsive shock of the detonation on the walls of the chamber could cause massive damage, although an 'explosion' as such is unlikely.

   The two main methods of energy capture are to harvest thermal energy, or to fuse the action of the plasma against magnetic of electric fields to directly generate power.  The former could fail in the method I have already described, but the latter has other modes.  If coolant flow was cut off but fuel detonations continued the coolant could boil, rupturing the system, and causing widespread devastation.  Also, and this applies to a magnetic design as well, the presence of material in the chamber - leaked coolant, gas, or a buildup of reaction products - could magnify the mechanical effects of the explosion, just as the atmosphere does for a nuclear warhead.

Other Systems

   Spacecraft need high performance more than any other application, so it is more than likely that technologies used in space will always be cutting edge, and thus posses more failure modes that tried and true technology.  There is also the slightly cold logic that an explosion in space will probably only harm the crew of one ship even if measured in megatons, while the same detonation on Earth could level a city.  Metastable helium, metallic hydrogen, and similar materials offer vastly increased performance in both spacecraft propulsion and in power generation, but also run the risk of catastrophic failure.  Even further into the haze of a speculative future there will be even more potent dangers.  Anyhow, that should be more than enough information to avoid the common misconceptions surrounding retain failures in SF, and to come up with a more realistic and original scenario.

Sunday, 19 July 2015

Myths of SF: Bioships & Organic Spacecraft

The Fallacy of Organic Technology

   It is integral to the nature of SF(defined in the strictest sense) that the technology it portrays is advanced, or in some way unusual.  It is, after all, the reason that many people read SF over other genres.  Partially because of this biotechnology has become rampant in SF, never achieving widespread attention in the way that hyperdrives or blasters have, but appearing in many and varied works throughout the history of the genre.  Biotechnology of the kind needed to produce a spacecraft, or even part of one, is so far beyond current human understanding that it sets the story firmly in the far future, or ensures that a alien race is seen as more advanced.  And therein lies the problem, although a problem that only hard SF fans such as myself may object to.

   In almost all works biotechnology - especially bioships, which will be my focus - are far more powerful/effective than any comparable tech.  The Yuuzhan Vong(Star Wars), Species 8472(Star Trek), Edenists(Night's Dawn Trilogy), Shadows(Babylon 5), Wraith(Stargate Atlantis), Tyranids(WarHammer 40K), to name a few, all had spacecraft superior or equivalent to those that they faced.  Even when their superiority is not demonstrated through combat the organic spacecraft are often seen as more advanced than their mechanical counterparts, like the TARDIS from Doctor Who, or Moya from Farscape.  And although we have very little knowledge of how a bishop might function it seems certain that it would not be faster, be more resilient, have better weapons, etc than a mechanical ship.

   When confronted with this unfortunate truth the reaction of a SF addict is often to state that "its the future, they know things we don't", or "they're aliens and more advanced", or "its a story".  Of these only the last is a real excuse, and even then is only valid when writing 'soft SF'.  Why is this the case?  Mostly it is due to the difference between the structure of biological and nonbiological materials at a molecular scale, along with several restrictions imposed by the growth of the ship.  Because the non-biological structure is constructed externally it does not have to have provision for growth or del repair - instead of single cells it can be homogenous or structured solely to maximise a particular trait.  The result of this is that any material assembled biologically will be inferior to a nonbiological material.  It is not that simple however, the biological materials will have different properties and so designs will be different to make use of them, somewhat negating the less optimal materials.  The small applies to larger structures or constructs.

   Take rocket engines, or example.  A nuclear thermal rocket, at the low end of practical space travel in term of materials science, uses refractory metals and active cooling to keep from melting, not to mention the effects of radiation.  Any comparable biological system will have to withstand temperatures ranging from the cryogenic to thousands of K, be highly conductive to heat, have good mechanical strength, etc.  It will also need pumps to cycle the cryogenic liquid gas used as reaction mass or suffer the performance penalty associated with water or similar.  For their first requirements they are all characteristics that are increased by the homogeneity of the material, making an organic 'grown' substance unlikely.  For the pump not only does it have to cope with massive torques and insanely high rotational speeds but with the cryogenic temperatures.  Any living tissue will freeze solid and die at those temperatures, and if it is a dead material you loose the biggest advantage of a biological system - self repair.  The same applies to weapons, sensors, etc.  So while it may no be impossible to build a bioship it is unlucky that either the components or the whole will have greater performance than a purely technological system.

   So why bother?  Are there any reasons a bioship could be used?  To answer this it is important to consider this: biological systems are not inferior or superior to technological ones, they are merely optimised for a different scenario.  And this is their advantage.  A standard metal-and-composite hull would take a far amount of technology, resources, and effort to construct, making it an expensive item.  Likewise repairs are probably difficult without the resources used in construction, and may never return full strength or performance.  A bioship side-steps these disadvantages.  For construction it might need only a vat of nutrients, and can self repair to a high standard.  More advanced types might literally grow from eggs or embryos placed in the correct environment, like the Voidhawks of the Night's Dawn Trilogy who grow to maturity in the rings of a gas giant.  If so a fleet could require only time to construct, vasty reducing the const and increasing the huber of vessels available.  In a realistic space war, where it is likely that most hits will disable or destroy a ship, quantity may well be more important than quality.  And of course the whole ship does not have to biological; the Brumallian bioships in Neal Asher's Hilldiggers had implanted fusion drives.  

The Bioship Moya from Farscape 
Biological, symbiote, biomechanoid, cyborg?

   Bioships do not come in a single flavour.  As posited above they will not have the performance of a tech ship they do have the potential advantage of being much cheaper.  The disadvantage can be combated by adding modules of technology - engines, weapons, sensors - but this decreases the advantage.  As it turns out there are four main approaches to this trade-off, each with advantages and disadvantages.  Note that in practice these categories overlaps, some components of a single spacecraft falling under different classifications.  

   In a fully biology-based bioship the spacecraft is one living organism.  It is still alive, perhaps even growing, and requires no external technology to function.  As such it is more an animal than a machine, and may even posses intelligence.  While this is one of the more common variations in SF it is the least likely.  Foremost is the lack of propulsion tech comparable with biological systems, often explained away by giving the bioship the ability to manipulate gravity(Voidhawks and the ships of the Yuuzhan Vong).  If these did occur in 'Real Life' they would likely live in the rings of a gas giant or in its moon system where energy and resources are potentially cheap while deltaV costs are low compared to interplanetary flight.  A fully biological organism could also be used as the basis of an artificial space-based ecosystem, harvested for their concentrated resources by humans or higher level animals.

   While they have the potential to require no human input in growth these bioships suffer from the most flaws.  Not only are they weak in terms of performance they need the most time to grow, need feeding, can get sick, be attacked with biological or chemical agents, and it intelligent suffer mental problems.


   A symbiotic spaceship is similar to a fully organic one except that it is composed of a colony of different organisms rather than single entity, similar to the Portuguese Man 'O War jellyfish.  It has the same disadvantages as the previous version of a bioship with only a few advantages.  The primary advantage is that by dividing the ship into separate 'subsystems' it is more robust against injury or attack, and it one segment fails - a drive unit, sensor cluster, etc. - there is the potential for it to be quickly replaced rather than regrown.  Although, of course, communication and commonality between the segments could be a problem.

   It is also important to realise that any of the other classifications can also be constructed of separately grown systems, although in that case it becomes a mere example of biotechnological engineering rather than a true bioship.


   Biomechanical is a term that is often used to describe the work of H. R. Giger, who designed the alien from the Alien franchise, along with the derelict spacecraft in the first movie.  According to wikipedia it is also a term meaning the same thing as a cyborg.  Its actual meaning - or the most rigorous definition - is a living organism that incorporates elements of mechanical systems, but not as implants in the way a cyborg does.  In other words it is a biological system that rather than finding its own solution to a problem, utilises one that is a at least visually similar to the more technological approach.

   They are the most effective kind of bioship, and probably the hardest to create.  Although grown they are not necessarily still alive, wither in part or whole.  Because of this they can have greater performance.  Structures can be 'layered' in a kind of biological 3D printing.  Coral-like material could be used in rockets, reinforced by fibres on the outside, and cooled by transpiration.  It also makes them more resistant to temperature, radiation, and damage.  They don't need feeding, medical care, or a controlled environment.  And I imagine it is far more comfortable for the crew than  the inside of a living organism.  Of course it loses the ability to heal, but as this is going to be slow in any case, the loss is probably worth the improvement in performance.  It might also be possible to 'reactivate' parts of the ship when they are damaged.  Of greater concern is the fact that many biological materials loose strength when dead.  Many devices such as rotary pumps can be used, which would be hard in a living system, and weapons in particular should be easier.  Sensors and drives should also benefit by the greater degree of optimisation offered by not having living material.


   Self explanatory for any fan of SF the cyborg bioship is probably the most likely ever to be developed or used as it combines the strong points of both biological and mechanical systems.  This approach is exemplified by the Edenist Voidhawks from The Night's Dawn Trilogy, which were sentient bioengineered creatures with the ability to manipulate gravity, and who carried a technological crew compartment, weapons, etc.  While the organism should be alive for it to be a cyborg in the strictest sense a combination of technology and biomechanical systems seems a good approach.  Structure, armour, remass systems, life support, these could all be biological while drives, sensors, communications, and weapons are technological.  The disadvantage is of course the added complexity of getting a biological and mechanical system to interface, and having components that must be manufactured rather than grown.

Aspects of Design

   For bioships in general there are several things to think about, points and suggestions for the way that they could be designed/grown.

Lifespan  Does the bioship age?  Does it have a childhood?  This probably applies only to sentient bioships, but raises interesting questions about how they are 'retired'.  Immaturity might also be a problem with young bioships.

Sickness   Can the bioship get sick?  Even if it cannot there is the possibility of biological attack.  The ship will probably have a immune system of some kind, although it may be closer to a diagnostic system than the immunological setup of a human.  Do they have allergies?  Can they get drunk?  These questions will add interest to any SF 'Verse, and have potential to push the plot in a particular direction without overt handwaving.

Crew   In SF it is common for bioships to 'bond' with a particular individual who then acts as their captain, even to the extent that Voidhawks gestate alongside their future partner.  More realistically the bioship's metabolism could provide life support for the crew or passengers, producing oxygen, food, and warmth, as well as processing waste.

Intelligence   Many bioships in SF are intelligent, making them a character in the story and allowing for many and varied plot twists.  This also brings up somewhat darker questions.  Can the ship feel pain?  Can it have emotions, does it choose its crew?  Do bioships have legal rights, or are they property/enslaved?  This is heavily dependant on the level of sentience - a dog-level ship can be euthanised if injured, but a sapient(human level) ship is another kettle of fish entirely.

   Another fact to consider is the bioship's piloting ability.  If it is sentient, and especially if part of a self-sustaining population, it is likely to be a far greater pilot than any human.  In the way that a bird can fly in winds no aircraft can face the bioship's mind and 'body' are perfectly suited to a 3D environment and the vagaries of orbital mechanics.  Even a AI might have trouble keeping up with them.

Sensorium   While there is no stealth in space a bioship's sensors are likely to be almost pathetically weak if organic in nature.  While 'giant eyeballs' could provide decent optical imaging other frequencies will be difficult to observe.  Communications will also be limited, especially since emitters of any kind of energy, even if possibly, are likely to be weak.  Biological systems do not like high power flows.  However, there is an advantage over tech systems in that sensors should be no more expensive to grow than other modules, allowing high redundancy.  Brightness filters could be in the form of translucent 'nictitating membranes'

Weapons   DEW are going to be impossible to grow, mostly due to the waste heat involved in lasers and the magnetic fields in particle beams.  For the same reason, along with power demand, electromotive weapons - railguns - are unlikely.  Missiles are presumably possible and the ability to grow them in large numbers makes one of their largest current problems, cost, invalid.  Distilling fuel might prove an issue, however.  Chemical guns might be possible, and of course any system can be added as a cybernetic implant.

Landing   While asteroids, low gravity moons, and comets will provide little difficulty to a bioship they are at a disadvantage in a gravity well or atmosphere.  This is to do with the greater performance required, specially in the acceleration area, and brings up another interesting problem.  While most spacecraft can be designed to hold up under far greater acceleration than the crew, a bioship might be limited to the ~5 g that living creatures can stand for short periods.  Reentry into an atmosphere could also pose a challenge.

Drives   Anything using magnetic fields, directed energy, or massive power requirements is a no go.  Thermal rockets will be the oder of the day, the most powerful being variations of a fission thermal rocket.  Being able to 'digest' a asteroid and extract fissionables could allow a ready supply of fuel and remass is only as far away as the next chunk of ice.  Chemical drives are much more likely, and provide adequate perforce for a bioship living in the ring system of a gas giant.  Solar sails are a possibility, although I see no way for the reflective surface to be formed.

Carboneering   Carboneering, the study and use of carbon allotropes and composites is at the forefront of modern material science, and unlike metallurgy and ceramics might be comparable with a biological system.  If carbon nanotubes and graphene sheets can be grown the strength and performance of a bioship will receive a massive boost.

   Doubtless there are many many more aspects to be considered, imagination is really the only limit.  For soft SF anything goes, and for reasonably hard SF all that needs be kept in mind is the poor performance Vs flexibility and cheap production of an organic system.


   Most of these have already been covered, things like the susceptibility to biological attack, possibility of sentience, etc.  Most of the ways they differ from a conventional spacecraft are immediately obvious, as are the consequences.  Also, most of these consequences do not extend beyond the environment in which the bioships are employed.  External effect will be mostly the same as those that a technological ship of similar performance, price, etc would have.  The implications of such advanced biotechnology are wider-reaching, and will be the focus of another post.