Chemicals for a biorocket

Question

A civilization is very good at bio-engineering, and I was wondering if there was any kind of chemical an organism could produce which would produce viable rocket fuel. It has to do a of couple of things:

1. Work at least half as well as modern rocket fuel. Too much less, and I worry the rocket will not be able to get off the ground. As well as, is there any way to theoretically improve the efficiency?
2. The organism needs to go with the rocket. It's neat if you could create fuel, but if you can't bring it with you, your missions are going to be much shorter.
3. I would prefer that the organism would be able to convert plant matter into rocket fuel, but this is not necessary. The rocket will most likely have a hydroponic farm to feed the crew, and it would be great if the fuel-creators could eat that too. A close second would be some sort of animal matter.
4. The fuel is somewhat easy to replace while in flight. Once you get the fuel-producer up in space, it would be a bummer if you can't use that fuel.

Answer 1

Living organisms can produce hydrogen peroxide, therefore one good choice seems to be high-test peroxide, or HTP

High-test peroxide or HTP is a high (85 to 98 percent)-concentration solution of hydrogen peroxide, with the remainder predominantly made up of water. When used with a suitable catalyst, HTP can be used as a monopropellant, or with a separate fuel as a bipropellant. [...] Some significant United States programs include the reaction control thrusters on the X-15 program, and the Bell Rocket Belt. The NASA Lunar Lander Research Vehicle used it for rocket thrust to simulate a lunar lander.

In the monopropellant usage your organism just need a suitable enzyme to decompose HTP into steam and oxygen.

Answer 2

See Stage Rockets in Larry Niven's Known Space series. They are remnants of the Thrintum/Tnuctipun empire. See "A relic of empire" and a brief mention in "World of Ptavvs"

These were the equivalent of the booster rockets on the shuttle.

---

Half as well isn't very good. A decrease of 50% of specific impulse takes effect in the exponent of the Rocket Equation (It looks innocuous until you solve for Mo)

There are reasons that 60 years after Sputnik we still don't have reusable single stage to orbit rockets.

To create fuel in space, you still need a source of chemicals. E.g. to make a hydrogen/oxygen fuel you need a source of water. Mass is the problem.

In low g environments it starts to make more sense to spend a lot more energy and far less mass. If you can use solar energy concentrators to boil asteroid rock, then use a linear accelerator to accelerate ionized rock vapour to appreciable fractions of the speed of light, you have a ship that can cruise the solar system. You also have a good weapon with that exhaust.

Answer 3

You might want your animal to somehow secrete a combustion chamber of erbium crystals (extremely rare element - so you might have to explain where the food comes from), and generate very high frequency bioluminescence in the range of gamma radiation. Then your animal could generate fusion from heavy water (which also should be on the diet) (http://www.neofuel.com/neutralize/Steinetz%20mse%20catalyzed%20beta%20and%20neutron%20autocatalysis%202017%2006%2026-1551.pdf)

Another alternative is that the animal eats antimatter. It is produced in extremely tiny quantities naturally around planets where incoming cosmic rays first begin to interact with matter. The antimatter could potentially be packaged safely in buckyballs (graphene nanoballs). Assuming your animals were farm-fed from some industrial facility that produces antimatter on a larger scale, they could react the fuel with almost anything.

In terms of weighing alternatives, I think specific impulse is less important than mass energy efficiency. With chemical, it tops out at almost zero; with fusion, the top end is not quite, but close to 1%; with antimatter this could approach 100%

Answer 4

Many common rocket fuels can be created by biological processes. In the case of fossil fuels, they actually were.

Bipropellant fuel

Hydrogen is the most efficient rocket fuel by mass, which is often the most important factor for a spacecraft. It has several major drawbacks, though: it needs to be cooled down to ridiculously low temperatures, has an abysmal density meaning giant tanks, and has a tendency to boil off as well as to escape pretty much any container with time (H2 molecules are so small they slip between the tank's atoms). It can be produced by bacteria, or the civ could use biotech to reproduce today's industrial processes, which for many are relatively straightforward.

Kerosene (specifically highly purified RP1), on the other hand, is compact and doesn't need to be cooled down. It is quite a bit less efficient per mass than hydrogen, but those make up for it enough that it is a rather common rocket fuel. It comes from oil, which has naturally formed from dead organisms, and it is possible to recreate this process artificially from, say, algae. We don't do it today because it would be too expensive, but it may start to appear once we run out of fossil oil - nothing a biotech civ should have any problem with.

Between those, there is methane. Not as compact as kerosene, but not too bad either. Needs to be cooled down, but far from the extremes of hydrogen. Performances are also between those, as its hydrogen ratio in the molecule is higher than kerosene. Methane is a well-known organic byproduct.

Note that gaseous fuel is almost always cryogenically liquefied instead of compressed, as this allows greater density and doesn't require impractically heavy tanks.

For early rockets, ethanol can be used, and such a civ should have ample alcohol-producing capabilities. It is very easy to use as fuel compared to the others, but its poor performances mean that it will be only used for initial experimental rockets (think V2).

Bipropellant oxidiser

As oxidiser, liquid oxygen is the obvious choice. It is cryogenic (though not as extremely cold as hydrogen), but is the most efficient practical oxidiser.

If you hate the world and everything in it, you can use fluorine (aka burning cancer). The stuff is such a nightmare to handle even the Nazis thought it was too dangerous. It will burn anything it touches, produce highly toxic fumes and durably contaminate the zone. There are probably bacteria out there that can produce the stuff, but frankly the world is a scary enough place as it is.

Hydrogen peroxide is not as efficient and will try to explode when given the opportunity, but it is liquid at room temperature, compact and can be useful if you want a simpler, non-cryogenic rocket. Again, there are some organisms that produce it, so it shouldn't be a stretch for a biotech civ to produce it.

Nitrous oxide can also be produced biologically, and fills somewhat the same niche as hydrogen peroxide. You need to cool it somewhat, but not as much as other cryogenic fuels. It is a bit less compact as hydrogen peroxide, but is very stable.

Monopropellant

For things like satellites or small manoeuvring stages, they will want monopropellants, that is rocket fuel that doesn't need oxidisers. You only heat it and/or flow it against a catalyst and it will burn by itself.

Hydrogen peroxide and nitrous oxide can be used as monopropellants. Performances are mediocre compared to bipropellants, but small crafts will need small, simple engines and monopropellant engines are much simpler, and easier to be made tiny.

If you think Nazis were tree-hugging sissies, you can also explore chlorine trifluoride, which is the evil version of fluoride. It will burn water. It will burn sand. It will burn glass. It will burn spontaneously given the slightest provocation (for example, looking at it sideways). And of course its fumes will cancerously burn nearby lungs and contaminate the zone forever. But hey, at least it is efficient at it.