On Saturday, November 26, 2016 at 3:04:23 PM UTC+13, JF Mezei wrote:
On 2016-11-25 19:32, William Mook wrote:

Propelled by Methane and LOX - that's different than the crew systems being powered by methane. The amount of methane consumed during launch, and boost into Mars transfer trajectory is vastly larger than the amount of methane produced from life support processes.

Launch and escape from earth are not a concern as launch is done with
fuel from ground over many launches to bring fuel up.

The amount of methane is very large on the order of centuries worth of waste production, even if you count everything as methane, which it is not. So, as such it is impractical. Further, methane in the life support system is more valued as a source of hydrogen and zeros out the need to bring ANYTHING in the form of water or oxygen.

So, when you drink water, eat food and breathe oxygen you generally get;

2 O2 + food + water --- energy + CO2 + 4 H2O + other waste

Solar powered electrolysis recovers hydrogen;

4 H2O + solar energy -- 4 H2 + 2 O2

Sabatier reaction absorbs CO2 from the air making methane and water again;

CO2 + 4 H2 -- CH4 + 2 H2O

Methane is pyrolysed using solar energy to reduce methane to carbon and hydrogen again;

CH4 + solar energy --- C + 2 H2

Some water is converted to H2O2

2 H2O + O2 + solar energy -- 2 H2O2

H2O2 is used to convert other waste to CO2 and H2O and ash through oxidation

other waste + 2 H2O2 -- 2 H2O + CO2 + ash

That CO2 goes back to the Sabatier process to restore the oxygen and produce carbon black.

The carbon black once produced is used to absorb odors.


And escape from
Earth would happen well before the passenger waste would have generated
significant amoumt of methane.

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

However, the methane generated during the 90 or more days to mars would
help *reduce* amount of fuel needed to get into Mars orbit and then
de-orbit.

While its true that if you throw stuff overboard it makes sense to reuse what you can as propelant, the amounts are minor, and there are better uses in a CLOSED CYCLE SYSTEM. Such a closed cycle system is ALREADY OPERATING ABOARD THE ISS. So, assuming waste is tossed overboard as in Apollo, is wrong.

So we're comparing reusing methane and other wastes in the life support system to close as many loops as possible to reusing methane as propellant. When used as propellant the advantage is not as great as using methane as a closed cycle feedstock. Relative to the cost in life support supply weight when used as propellant you are way ahead using it as closed cycle feedstock.

Remember, methane coming from waste starts out as water, oxygen and sugar or oil. Using methane in the life support process to restore hydrogen supplies at low cost, and converting CO2 to CH4 using hydrogen made in this way (along with water conversion as needed) - radically reduced payload mass, which radically reduces propellant.

Let's look at a worst case situation. Let's look at a METHANE LOX rocket in LEO before departing for Mars. Let's say we look at a 280 day mission for 100 people consuming 3.65 kg per day. That's 28,000 person days and 102.2 metric tons of consumables. The persons themselves mass 85 kg each so that's 8.5 metric tons. We have another 42.5 metric tons of other hardware.. So, this is what you start with. On the 90th day you arrive at Mars and you've used 32.85 tonnes of consumables, and you've held on to it to use as propellant.
LEO--TMI TMI--Mars Mars--TEI
Consumables: 102.2 metric tons 69.35 tons 32.85 tons
Passengers: 8.5 metric tons 8.50 tons 8.50 tons
Structure/tools: 42.5 metric tons 42.50 tons 42.50 tons

TOTAL 153.2 metric tons

So, to land on Mars, requires a delta vee of 1 km/sec with aerobraking. To enter a Mars transfer orbit requires a delta vee of 3.7 km/sec. To take off from Mars and fly to Earth requires 5.9 km/sec from the surface on the 190th day.

Now let's be generous and say ALL the consumables end up as CH4 (which isn't the case) it DOES turn out that the amount of methane used to enter Mars atmosphere and land on Mars, is about equal to the amount of all the supplies thrown away if we were to throw them away;

LEO--TMI TMIMar MarTEI
- 124.83 263.53 O2 needed
- 32.85 69.35 Methane
102.20 69.35 32.85 Supplies
8.50 8.50 8.50 Personnel
42.50 42.50 42.50 Structure
153.20 120.35 83.85 Payload

3.70 1.00 5.90 Delta Vee
0.6484 0.2461 0.8111 u
0.1351 0.0513 0.1690 Methane Fraction
0.5133 0.1948 0.6421 Oxygen Fraction
0.3516 0.7539 0.1889 p

1,905.80 637.27 443.94 Stage Weight
1,235.68 156.83 360.09 Propellant Weight
257.43 32.67 75.02 Methane
978.24 124.16 285.07 Oxygen


Now a 640 tonne payload launched by a Mars Colonial Transport requires three fueling launchers to (the original launch plus three added launches) to accumulate sufficient propellant. The 32.67 tonnes is replaced by the methane in this idealised scenario (which is way more than you'd actually get, and then there's the equipment to recover and store the methane as well).

However, if we use the methane to recover the hydrogen for use in a closed cycle system AND DON'T THROW ANYTHING AWAY as described previously, the mass of consumables is reduced from 102.2 tonnes of consumables to 8.4tonnes of concentrates and other consumables by reusing all the water and oxygen for 280 days! A savings of 93.8 tonnes throughout!

8.40 5.70 2.70 Supplies
8.50 8.50 8.50 Personnel
42.50 42.50 42.50 Structure
59.40 56.70 53.70 Payload

3.70 1.00 5.90 Delta Vee
0.6484 0.2461 0.8111 u
0.1351 0.0513 0.1690 Methane Fraction
0.5133 0.1948 0.6421 Oxygen Fraction
0.3516 0.7539 0.1889 p

1,091.51 381.10 284.31 Stage Weight
707.71 93.79 230.61 Propellant Weight
147.44 19.54 48.04 Methane
560.27 74.25 182.57 Oxygen

Here we have only ONE ADDED fuel flight - cutting launches in half.



If human waste produces methane, does it make sense to use energy to
recycle methane to something else when you can use it almost directly
for engines ?

Yes. Since you cannot use it directly since it has to be cleaned up for use, and since there isn't that much methane anyway, and since it can be used just as easily as a source of hydrogen gas by pyrolysing it.