On Sunday, November 27, 2016 at 4:37:05 PM UTC+13, JF Mezei wrote:
On 2016-11-26 20:22, William Mook wrote:
Methane is pyrolysed using solar energy to reduce methane to carbon and hydrogen again;

CH4 + solar energy --- C + 2 H2

Is there a point in spending energy to do that when methane is the
primary fuel for the vehicle ?

Because the weight of the energy generator is modest when compared to the savings in weight of consumables used.

Please understand what you are saying. You need oxygen to burn methane. I calculated earlier tha an anaerobic digester produces 2.03 kg of methane from the waste produced by 100 people over 280 days of a mission. Each kg of methane releases 55.6 megajoules of heat energy when combined with 8.12 kg of oxygen! Used in this way the CO2 produces 8 Watts per day of heat energy. It also produces 5.58 kg of CO2 and 4.57 litres of water over the course of the mission when used this way.


If you have a byproduce that is useless, it makes sense to spend energy
to convert it to components that are of use for the ship or passengers.

Precisely. The real issue of a designer is to compare the mass of the equipment needed to the mass of consumables saved. Concentrating photovoltaics that are hyper-efficient by use of the full solar spectrum, are very light weight per unit power. Extracting water in a partial vacuum at near room temperature, is likewise very efficient. Oxidising human waste is also efficient weight wise - and very clearn and odour free. Reducing hydrocarbons to water and carbon dioxide - and taking all carbon dioxide and reducing it to methane, not for fuel use, but to reduce that to carbon black - an odour absorber - and hydrogen - a carbon dioxide absorber - and recycling the oxygen back into the cabin - that's the most efficient system people have come up with.


But if you have a by product that can be used be engines, is there a
point converting it to something else ?

Because the weight of material is low compared to the weight of the hardware to make it happen. Methane in conventional systems is produced by bacterial action taking 10 days or more, and involves a very tiny fraction of the total waste.

Methane is processed on board the ISS, but not this way.

Water is electrolysed into hydrogen and oxygen. The oxygen is breathed producing carbon dioxide The carbon dioxide is scrubbed from the air by combining with the hydrogen to make methane and water again. The methane is broken down into carbon black and hydrogen again. The carbon black is used to absorb odours. The hydrogen is used to scrub the air of carbon dioxide. This is a Sabatier reactor.

As far as feces is concerned, there are more direct and less massive and more easily controlled methods One more direct method is to extract water from wastes at low presure near room temperature. Then make hydrogen peroxide from some of the water and some oxygen and solar electricity. Then chemically react the hydrogen peroxide with the dried waste solids forming CO2 and water at near room temperature without a lot of heat. The CO2 and water are processed as described above - and made part of that process.

In the end you carry a few days supply of water and oxygen aboard, and a machine that processes all wastes into oxygen and water. The only thing you consume is food concentrates which are reconstituted using water. You end up with carbon black and phosphate powder and small quantities of nitrogen and oxygen and water - which serve to replace losses from the cabin. The carbon black is used to absorb odours and keep the air smelling fresh. The phosphates are carried to Mars and used as fertilisers there.



Now that for the station, they bag human waste, and any methane that i
produced just accumulates in those bags until Progress burns up. So this
is not part of the equation on the ISS.

That's true, but the ISS does recycle urine for its water content, and does recycle carbon dioxide for its oxygen content using solar energy. This can be extended to all wastes and the highest best use of that waste is to close the consumable loop with a machine that weighs less than the consumables saved.

7 people producing 128 grams of feces per day produce 250 kg over a 280 day period. With a conventional anaerobic digester this produces 128 grams (1/8th kg) of methane gas. Not much at all.


The question is how a 100-pax ship handles human waste on a 3-4 month
journey to Mars.

Well at 128 grams per person per day 100 persons produce 12.8 kg of feces per day. Over 120 days 1,536 kg of feces. In an efficient anaerobic digester this totals about 1 kg of methane gas. When combined with 4 kg of oxygen it produces 2.75 kg of CO2 and 2.25 litres of H2O along with 55.6 megajoules. That's about 5.3 watts per day.

As I've said multiple times already, the highest best use of this stuff is to extract water from it at low partial pressure and room temperature, and then oxidise the dried residue with hydrogen peroxide again at low temperature, and process the CO2 and H2O generated in the waste gas process stage of the air regenerator. This reduces the feces to about 16 kg of carbon black and 4 kg of phosphates over the period - recovering all the water and oxygen for use aboard ship.



The passengers never generate a significant amount of methane relative to the propellant. That's the point.

And when they build airplanes, they try to shave grams off here and
there.

True, but creating and capturing methane purifying it to the standard needed for use as a propellant, recovers far less propellant than the equipment weighs. So the process adds weight. Where waste processing makes sense is recycling all the components where possible. The principal savings is in water usage. The next is recovery of oxygen and production of carbon black.. Third is the production of phosphates.

Small potatoes compared to total weight of aircraft like an A380,
but at the end of the day it does matter. 200 grams removed from a seat,
means 80kg weight saving when you have 400 seats, or the weight or an
extra passenger or 80kg of extra cargo you can carry.

Right, and the same thing applies here. The weight of a propellant line and valve to carry the grams of methane you might produce to the propellant tank is likely to weigh more than the propellant recovered, even if the balance of the system was massless.

100pax over 3-4 months will consume large amounts of food.

True, but most of that ends up in the air as CO2

That is a lot
of mass

True, but they produce primiarly water and CO2. The methane is a secondary process that's far down on the list of things produced. 3.65 kg per day of consumables - 0.128 kg per day of feces - from that MICROGRAMS PER DAY of methane. Got it?

that you have to lift and accelerate out of earth's orbit
towards mars most of which will become waste.

100 people produce 12.8 kg per day. If they spend one day on orbit before boosting into a trans-Mars trajectory they might gather milligrams of methane. This is inconsequential when compared to the METRIC TONS of methane consumed.

Not doing anything with it
means wasting that mass which you spent much fuel accelerating.

You're making a false choice. Just because its not used as propellant doesn't mean its not used. The ISS uses the Sabatier reaction to process water into hydrogen and oxygen. The oxygen is breathed. The hydrogen is used to scrub the CO2 from the air converting it into methane and water.

To this system folks have added other processes. Especially aboard nuclear subs. Methane is easily pyrolysed into hydrogen and carbon black. Water is easily electrolysed into hydrogen and oxygen. Hydrogen is used to continue scrubbing the air, oxygen is breathed, and carbon black is used to absorb odours.

The ISS also processes urine into clean water which recovers the vast majority of water consumed that is not make its way back into the atmosphere.

http://www.iflscience.com/space/amer...ssians-refuse/

https://www.nasa.gov/pdf/146558main_...%204_10_06.pdf