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.
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|
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.
Aspects of Design
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.