Using waste for propulsion ?

Le Nov/28/2016 à 9:53 PM, Fred J. McCall a écrit :
Alain Fournier wrote:

On Nov/27/2016 at 10:26 PM, Fred J. McCall wrote :
Alain Fournier wrote:

On Nov/27/2016 at 1:17 AM, Fred J. McCall wrote :
JF Mezei wrote:


100pax over 3-4 months will consume large amounts of food. That is a lot
of mass that you have to lift and accelerate out of earth's orbit
towards mars most of which will become waste. Not doing anything with it
means wasting that mass which you spent much fuel accelerating.


I know it's hard for you, but think about it. Most of the mass of
food (and feces) is water. You're going to get the water back for
recycling on the back side of the process. That means each person
will generate 1-2 ounces of solid waste per day once the water has
been recovered (and you'll get 3-6 ounces of water out of the same
waste stream). Let's use the larger number as more 'favorable' to
your case; 100 people (not sure what 'pax' are when they're up and
dressed) will generate around 12.5 pounds of solid waste per day. That
waste is a mix of dead bacteria, indigestible food elements like
cellulose, minerals, and indigestible fats. You're not going to turn
it into methane without giving up a lot of the recovered water and
even then most of it isn't going to 'convert'. Recovering the water
is more valuable, since you can make things like breathing air out of
that stuff. So you're going to accumulate a little over half a ton of
such cruft during the course of the trip.

It's not one or the other. You can very well recover the methane and the
water and grow food. Plants don't need the methane from human waste to
grow. So after extracting methane, the waste isn't any less fertile than
it was before extraction.


What 'methane' is there to recover? To get methane from ****e, you
have to process the ****e, removing carbohydrates. That makes it less
fertile because you've removed all the carbon and hydrogen.


Plants don't need carbon in soil, removing carbohydrates is not a
problem. Plants get their carbon from CO2 in the air.


Try growing plants in soil with no carbon in it and see how that works
for you (it mostly will work very poorly, if at all).

Do you have a site to support that claim.
https://en.wikipedia.org/wiki/Plant_nutrition#Carbon
Doesn't seem to agree with you.

You can't
'recover' the water because you need it as part of processing the
****e.

To recover the water see what I wrote just after:


So why make the methane in the first place if you can only burn it in
a condensing chamber? I did read what you wrote, but it made no
sense.

As I said, I don't favour doing so. I was just stating that it can be
done. Also, I wasn't suggesting to only burn it in a condensing chamber.
I said you could make electricity with it.

If you want to recuperate the water that was lost in the fermentation,
you can burn the methane and make electricity, water and CO2. The CO2
will be taken by the plants you want to grow. Of course, if you do so,
that means you can't use Mr Mezei's idea of burning the methane as
rocket propellant. Which probably isn't worth the trouble anyway.


Indeed. How much power, equipment, space, and effort are you going to
expend to try to make half a tonne of ****e 'useful'.


Even growing food on the spaceship probably isn't worth the trouble. The
trip is not long enough to do serious farming. I think that the best use
of human waste on a spaceship bringing colonists to Mars is to store it.
Land it on Mars. And then, once on Mars compost it and use it to grow
food. You will want to have lots of fertilizer handy for your colony on
Mars.


I don't think so, either. Probably makes more sense to just start
dumping it in a crater someplace.


That is not what farmers do with their manure here on Earth. Earth soil
is typically more fertile than Martian soil and will not need less
fertilizer.


I thought it was obvious what happened next, but apparently not. Once
you collect enough **** to be useful (it will take a lot more than one
trip) you can process it with in situ resources. Doing it half a
tonne at a time would just be stupid.

I'm glad to see we mostly agree on that point. I think that the first
half tonne can be useful. You want to start the process of growing
plants as fast as you can. Of course you would later move up to a larger
scale.


Alain Fournier