Dyson Trees are a thing. You could stick an engine onto one of those, and hey presto. Tree ship.

the saga of the seven suns by kevin j anderson has this kind of thing as well... though not as huge transport kind of ships.

Plants on earth evaporate water mainly to transport nutrients from the roots up to the leaves and to suck in CO2 from the atmosphere, i.e. something like 99% of the water is transpired, only a very smal amount is needed in chemical reactions. As far as I understand this is very efficient as the plant just employs the potential gradient (which is in this case mainly gravitational potential) of the water between the soil and the atmosphere to transport the water upwards. It would be much harder to pump the water through the plant. In space ther is no important gravitational potential, therefore pumping via peristalsis might be a reasonable option. This means no transpiration is needed physiologicaly, the stomata of the leaves can just be removed and the plant surface can be sealed to hinder losses to space. Another thing popping up in my head: The ship needs to carry CO2 and H2O with it for the plant to produce biomass, i.e. to grow.

nerdnumber1 wrote:

I don't think comparisons to existing organic mechanisms should be seen as much of a limit. There are biomorphs designed to exist indefinitely in hard vacuum and others that swim in the SUN! A biological solar sail or higher velocity biological jet drive doesn't seem that far fetched when you compare it to whales swimming in the sun. I'm not saying it would necessarily be the cheapest or most efficient option, but some nut job could probably make it work. As for desiccation, that would probably be a problem for any space-worthy biomorph or pod, but apparently they can fix that.

Sure, but it is more fun to figure out how to fix things rather than say "it is Eclipse Phase!" The whales are actually extreme cyborg creatures with magnetic field envelopes, and no doubt the trees will be nearly as artificial. But making a plausible solution is still a balancing act between ease and bio-aesthetics. You cannot do proper rocket propulsion with biomaterials beyond a certain point because of biology's inability to liquefy hydrogen and oxygen (no parts can be cold enough), the strength limitations of biopolymers (set by amino acid or polysaccharide backbone strength) or the limits of heat tolerance (beyond boiling it is not going to work). So a purely biological rocket will be pretty wussy. If you allow nanomaterials created by enhanced cells or cells incorporating nanomachines you can do much more. And if you do a cyborg tree it is fairly easy. Circulation in a Dyson tree might be powered by the emission of water vapour: if it can consistently get enough comet ice to digest in a "stomach" with roots, excess water can then be evaporated from branches and used to draw material - the evaporation also cools the tree. You could solve it by having nano-trains transport nutrients, but then you just have a tree-shaped spacecraft rather than a tree.

nerdnumber1 wrote:

How about a biological solar sail as the main source of propulsion, with either pitiful biological jets or more conventional jets only for maneuvering? Biological systems can be fairly light and strong (though a leafy canopy designed to be a solar sail might have little visual resemblance to a normal tree).

Solar sails have performance inversely proportional to their mass per square meter: they need to be very thin for sunlight reflection to overcome gravity from the sun. Not good for plants, where cells and veins have a lower limit size. Opening a water chamber to space will give you a jet that will propel you, but the thrust (force per expelled kilogram) will be weak. I am less concerned with the wussiness and more with the huge waste.

nerdnumber1 wrote:

Maybe have a symbiotic giant space spider (or other silk-weaving organism) that creates and maintains a vast solar sail made of ultra strong bio-steel silk, possibly using the tree's branches as a framework or as anchor points. The tree can have a mini-ecosystem of organisms that recycle resources and maintain the "ship". The spiders could be smart-animals or the morphs of a somewhat odd group of locals (perhaps slightly odd but social, or perhaps more private and insular).

That sounds reasonable (!) The sail still needs to be very large if the tree is heavy, but it might act as a complement to other drives. Plus, the tree needs to catch volatiles from asteroids - a web is useful. Maybe this is for extreme Brinkers?

nerdnumber1 wrote:

I think the "tree" ship would be much more hollow and thus lighter than a normal tree to accommodate passengers (we are still trying to make a tree-shaped biological SHIP, after all). Also, since the tree is growing and living in microgravity, it may not need to be as dense (then again while it isn't holding up its weight, it is holding in an atmosphere so who knows).

Hollow isn't light enough. The radiation force at Earth orbit is 9.08 μN per square meter, while the gravitational acceleration is 0.006 N/kg. So if you have more than 1.5 grams per square meter the sail will be attracted more strongly to the sun than pushed back. This is not a total counter argument, since you can do clever maneouvering with a heavy solar sail to spiral outwards by gaining momentum, but it forces the sail-tree to do a lot of orbits around the sun when it wants to get somewhere.

Quote:

For the "Sail-makers (the spiders that maintain the sails), I was thinking something along the lines of the Exoglots (Rimward p64) in flavor: insular and withdrawn to their own society, but performing all their duties and showing no sign of outward aggression. They are an integral part of the ship/hab, but they're CREEPY.

Don't mess with them.

Would a solar powered steam rocket work? Mirrored leaves could focus sunlight onto a block of carbonized tubes fed with water to produce a steam jet. The ISP isn't huge but it is immediate, water is easy to store, and the reaction block could grow out like fingernails.