Spacecraft Propulsion

[Kirin Eldridge]

I suppose I should introduce this first.

 

Essentially I am writing a science-fiction story and want it to be quite heavily based on fact. It is set in human timeline in the future. However, one question I need to know, how feasible is methane propulsion for spacecraft?

 

I read a brief article about recycling oxygen for use of breathing, but it did not explain if this was possible on a large scale. The idea is this:

 

Water is electrolysed and the oxygen injected into the air for passengers to breathe, while the hydrogen is combined with the carbon dioxide to form methane and water. The water can be re-used in further electrolysis, while the methane potentially used for fuel. Would it be possible to produce methane on a large enough scale using this method?

[Janus]

Here's the problem I see:

 

In breathing:

O2+C becomes CO2

 

Next step:

 

CO2+3(H2) becomes. CH4+2(H2O)

 

The combustion of the Methane would be:

 

CH4+ 2(O2) becomes CO2+2(H20).

 

Now when you burn the methane as rocket fuel, you use the product of the combustion as the reaction mass that you throw away,

 

It takes 2 oxygen molecules to burn 1 molecule of Methane, But the production of that methane from combining Hydrogen and Carbon Dioxide only produces 2 water molcules, from which you can only extract 1 oxygen molecule from. You end up with a net loss of oxygen, with none left over for re-breathing.

 

Then there is also this:

 

A human uses about 0.786 kg of oxygen a day.

 

This will produce about 0.491 kg of methane by the above process.

 

With the energy of combustion for methane, the best you could possibly hope for is a exhaust velocity of ~3500 m/s.

 

If you take the mass of a human as ~75 kg, then using the rocket equation, you can get an acceleration of only ~ 22 meters/sec/day, if that methane was just used to accelerate the mass of the human (and disregarding the mass of the space ship itself.) Not exactly burning up the spaceways, is it?

Edited December 7, 2013 by Janus

[EdEarl]

Assume your society has perfected fusion of hydrogen, and that you can collect hydrogen from space. Hydrogen is the most common element in space, some of it is free hydrogen atoms, which would be hard to collect and compress, but some is water as ice as part of comets and asteroids. You can electrolyze the water to get hydrogen and oxygen, use some of the hydrogen as fusion fuel, and run an ion engine discarding oxygen or hydrogen ions to propel your craft. With the energy, you can grow food using CO2 in your atmosphere, and the food will convert water and CO2 into food and free oxygen.

[Kirin Eldridge]

Here's the problem I see:

 

In breathing:

O2+C becomes CO2

 

Next step:

 

CO2+3(H2) becomes. CH4+2(H2O)

 

The combustion of the Methane would be:

 

CH4+ 2(O2) becomes CO2+2(H20).

 

Now when you burn the methane as rocket fuel, you use the product of the combustion as the reaction mass that you throw away,

 

It takes 2 oxygen molecules to burn 1 molecule of Methane, But the production of that methane from combining Hydrogen and Carbon Dioxide only produces 2 water molcules, from which you can only extract 1 oxygen molecule from. You end up with a net loss of oxygen, with none left over for re-breathing.

 

Then there is also this:

 

A human uses about 0.786 kg of oxygen a day.

 

This will produce about 0.491 kg of methane by the above process.

 

With the energy of combustion for methane, the best you could possibly hope for is a exhaust velocity of ~3500 m/s.

 

If you take the mass of a human as ~75 kg, then using the rocket equation, you can get an acceleration of only ~ 22 meters/sec/day, if that methane was just used to accelerate the mass of the human (and disregarding the mass of the space ship itself.) Not exactly burning up the spaceways, is it?

Thank you for the info, it was very helpful.

[Enthalpy]

Crews aboard space stations presently live to an amazing extent in a closed materials cycle, to which Sunlight brings the energy.

 

Water is recycled. Extracted from waste and exhaled humidity, purified, and reused as washing, drinking and cooking water. The bottom line is a net production of water resulting from food consumption by humans.

 

Carbon dioxide is extracted from air. I'm not completely sure it's done, but getting the oxygen back is known technology. Again, food consumption by humans results in a net production of dioxide, or after processing, carbon.

 

With the necessary energy input, reprocessing would leave carbon and water, or more easily, some compound of both.

 

----------

 

The amounts are small, in the order of 1kg per day and person, so they won't lift a spaceship from a planet. The available power also limits the rate of reprocessing. So my proposal would not be to transform the waste into a chemical fuel, but instead to use waste as a fuel for a high-impulse propulsion.

 

Such a propulsion takes power, often as electricity, to eject the working fluid at a speed unattainable by a combustion. It's power-hungry but mass-saving, as compared to a combustion rocket. One example is ion propulsion. An other is a resistojet. Have a look at Wiki.

 

The obtained thrust is faint, very few newton at best, but it lasts for long, enabling strong speed variations. As a beginning, it can serve at a space station: reduce the resupply need. It's just that existing engines are picky about their fuel.

 

The benefit of using waste that way would be to save the fuel mass for the high-impulse propulsion. Instead of throwing water out and accelerating xenon, take only food on board, and accelerate the waste. It saves mass.

 

Now I know what happened to Iapetus...

 

Marc Schaefer, aka Enthalpy

[hoola]

if you are saying human timeline being of the next few decades, then a truly advanced propulsion system is maybe not what you want....but if you mean in a few hundred years, then something as a chemical propulsion would seem pretty old fashioned. I have thought about advanced propulsion systems using dark energy as propulsion. That is, by learning the actual fundamentals of the energy source and then building a machine, to cancel out the field's effect on one side of the vehicle, say the front, then using the remaining forces to push the vehicle forward. A partial cancellation of side forces could provide left and right steering and of course, up and down would be controlled in a similar fashion with controlled cancellations above and below the vehicle. This would provide a "green" method of propulsion. No messy exhaust debris to clutter up space....the energy wouldn't be strong enough to get it off a planet, it would have to towed or placed in orbit with conventional chemical thrusters perhaps, but once free of strong gravity fields, it could be a viable option for deep space travel, with small but steady accelerations maybe similar to a solar sail....so not useful for staying in the solar system....edd

[Enthalpy]

The OP wrote: "I want it to be quite heavily based on fact."

[Enthalpy]

On Earth, waste permits us to grow food in a closed material cycle, with Sunlight input. Alas, this has consistently failed in space up to now, and even in the Biosphere 2 experiment, big and on Earth.

http://en.wikipedia.org/wiki/Biosphere_2

 

Until this closed cycle can be reproduced, an imperfect one looks possible. From energy and waste water and dioxide, we can produce chemically glycerol or other rudimentary food. Not healthy complete food, but it's a source of calories as good as sugar, and is permitted and used in industrial cookies. For a fraction of the waste, that would be a more valuable use than propulsion.

 

Ion propulsion prefers heavy ions, and if possible of easily ionized atoms like xenon and rubidium, because ionization takes much energy adding to the useful one that accelerates the ions to propel the craft. Hydrogen, oxygen, nitrogen, carbon available in waste are not the preferred atoms for that use. Though, ion mass spectroscopy includes a generator of molecular ions, which are heavier than atoms. That might benefit to ion propulsion if feasible: waste is first converted to semi-big molecules like toluene, and these are ionized once and accelerated.

 

Marc Schaefer, aka Enthalpy