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Bioships - biological spaceships - appear in a lot of SF, so I thought I'd have a go. However, in most books they are depicted as more powerful than conventional tech in all and any regard. This is unlikely as any given biological system can be outperformed by a mechanical or electrical one. However, if they can be 'grown' without much input then the number in a fleet is massively increased over the more effective but more expensive mechanical ships.

As I'm trying to go for a more scientifically accurate approach several problems have arisen. The most prominent is the nature of the drive system used, both for the ship and for weapons (missiles and torpedoes). So the questions is what propulsion system could a bioship use, based on current understanding, but not necessarily on current ability. In other words, it must not be impossible according to any known science, but very unlikely is fine.

Addendum

It has been pointed out in the comments that the 'terrain' the bioship operates in has a big impact, as does the exact requirement of performance. It will be mainly within the moon/ring systems of gas giants, along with some(rare) interplanetary travel. Definitely no need for interstellar velocities. Also, low DeltaV hohmann transfers are fine. High acceleration is good but not essential.

Addendum 2

All the answers people have given are helpful and complementary, so don't be offended if I didn't pick yours. From the information from this question, along with stuff from around the internet, I have put together some stuff here as a look at bioships for anyone interested.

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  • $\begingroup$ How fast do your ships and missiles need to go? Do your ships need to be capable of interstellar travel? No known propulsion mechanism is able to accelerate a ship anywhere near light speed as they all rely on Newton's Third Law. Stars are generally light-years apart, meaning there are currently no plausible propulsion systems, biological or mechanical, capable of interstellar travel in a reasonable time frame. Many sci-fi universes explain interstellar travel with jump drives or wormholes and use more conventional means for intrastellar propulsion. Will that suffice for your purposes? $\endgroup$ – Mike Nichols Jul 15 '15 at 4:55
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    $\begingroup$ does your ship fart? can it produce electric field to accelerate hydrogen ions for initial burst of speed thereafter switch to magnetic field to pump energy from bioluminescence source to excite the plasma to generate high energy beam which targets normal water instantly converting it into gas producing large amount of energy also for initial burst, remember bioluminiescence and electric field that's double up for microwave drive for cruising. $\endgroup$ – user6760 Jul 15 '15 at 5:45
  • $\begingroup$ I added a comment on your blog post :) $\endgroup$ – Tim B Jul 20 '15 at 18:13
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Science fiction bio-spaceships are always either cybernetic (using technology for propulsion) or have some sort of drive that is currently unknown to us.

In fact bio-spaceships are often given to super-powerful aliens to show just how much more advanced they are than humans. For example the Vorlons in Babylon 5 had living spaceships as just one more way to signal how much ahead of everyone else they are.

The theoretical advantage for living spaceships is in crew, repair and maintenance. They can heal themselves, refuel by scooping up organic matter and processing it, they can produce new versions of themselves without needing massive amounts of infrastructure to support manufacturing. At the same time though they can be mass produced if needed. The weakness though comes from the fact that organics tend to be more vulnerable, weaker, etc than metal. In particular the high temperatures and pressures of conventional engines would be hard to deal with. In fact with currently foreseeable technology you would most likely be able to get the same cargo space for a much lighter frame using traditional technology.

To get back to the question though, in the situations you describe then most likely they would just use compressed gas jets to manoeuvre. They go into the ring around the planet and consume some ice asteroids. They take minerals and water from the asteroid to use, and then split some of the water using electrolysis into hydrogen and oxygen.

That gas is then stored pressurized in bladders all around the hull of the ship. Jets of gas or even water are released from the bladders to accelerate. Possibly burning the hydrogen and oxygen to add energy to the release.

This would not allow large amounts of acceleration but it would allow low energy manoeuvring with high precision and consuming some large ice asteroids would allow sustained thrust over time, which is plenty for moving around in space.

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Biology isn't fundamentally more efficient, better, or different from technology. It just looks that way because it's had billions of years to develop a degree of complexity that our technology hasn't reached yet. But the reality is that to a large degree, the two are interchangeable.

From a certain point of view, a human is a self-directing, self-repairing robot that has subsystems that allow it to automatically repair itself, to take in fuel and resources from the environment for power, and to create more robots.

From another point of view, a robot is a neutered, lobotomized human that's tethered to specific power sources, can't self-repair and lacks flexibility.

Those same concepts apply to spaceships. And when it comes to propulsion, a biological spaceship will use the same propulsion as a technological one.

Physics doesn't change between the two. Mass and acceleration don't care what's being pushed. If a regular ship uses a light sail, that's what the bioship will use. If a regular one uses fusion, then the bioship is only viable if it can also create a fusion chamber and siphon hydrogen from gas giants. And so on.

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Lightsail looks like a good candidate for propulsion, if to solve a problem of growing a living tissue in vacuum.

It may be easier to grow such a structure in place than to deploy it from the package, as with the usual mechanical light sail. Light could provide the energy to grow and transport nutrients to growing regions.

The ship could discard the sail before landing, and then regrow another one after take off.

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Based on current science, the propulsion options are:

  • Rockets: Efficient rockets require a hot, fast exhaust. Temperature and exhaust velocity go hand in hand, so steel and ceramics should be superior to biologicals.
  • Lightsails/Magsails: A biological process might be able to produce a large, thin lightsail. A magsail would require organic superconductor loops, who knows?

When I think of bioships in science fiction, they usually go with a "magic" FTL drive which does not follow science as we know it. Hamilton's Night's Dawn, Farscape, Feintuch's Seafort, the stardrive always levels the playing field.

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    $\begingroup$ What about ion propulsion? $\endgroup$ – Philipp Jul 15 '15 at 7:25
  • $\begingroup$ @Philipp, that would be an example of very hot and very fast exhaust. $\endgroup$ – o.m. Jul 15 '15 at 16:05
  • $\begingroup$ Are ceramics entirely impossible for biological systems to produce? I recognize that there is sort of a chicken-and-egg problem with producing strong metallic structures (where do you forge your metal bones?) but ceramics seem more within reach. For example: (en.wikipedia.org/wiki/Ceramic_engineering#Biomaterials) $\endgroup$ – Deolater Jul 15 '15 at 19:11

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