Running multiple HET in parallel?

"Allen Thomson" writes:

John Schilling wrote:
"Allen Thomson" writes:

I checked on MREs and the Army's nutrition lab says that they're
nutritionally good beyond 10 years if held unfrozen at 15 C
(60 degrees 'murkin). So I agree, the claim that a few-year
mission couldn't get along on preserved food plus some
supplements looks pretty odd.

The Army's nutrition lab also says that MREs, new or old, are
nutritionally *bad* if they are the entirety of one's diet for
more than a few weeks. If you believe the Army, you can't just
stock your Mars ship with a three-year supply of MREs and imagine
the problem has been solved.

Er, I used the MREs as an example of the preservability of food
while retaining whatever nutritional virtue it started out
with. In the case of MREs, that virtue seems to be considerable,
if not long-term adequate.

Right. The issue isn't "retaining whatever nutritional virtue
it started out with"; a hermetically sealed container of pure
cane sugar will do that. Lots of foods, we know how to store
more or less indefinitely.

The issue is whether a long-term adequate diet can be constructed
exclusively from storable foods. The answer is probably yes, but
not yet AFIK *proven* to be yes.


However, the viewgraph that started this seemed to say that
the state of the art isn't here for preserving nutritionally
adequate food for multi-year Mars missions. So is there some
ingredient of a multi-year nutritionally complete diet that
can't be preserved by chilling or freezing or dehydration or
whatever? If so, what might it be?

Don't know; I can't find the original references at my local
library or online, just abstracts and summaries. What I can
find suggests it isn't anything as simple as a missing vitamin
or amino acid or whatnot. But in the course of developing the
MRE, the Army did *something* to the mix that resulted in a
food that would last indefinitely if kept cold, but will lead
to unexplained weight loss and mental deterioration if used
exclusively for more than a few weeks.

If it were trivial to produce a ration that could be stored
indefinitely and consumed exclusively, I would assume that
the Army would have done so in developing the MRE. It would
certainly have been a useful enough combination to be worth
a little bit of extra effort.

So I take the MRE, and similar rations, as evidence that it is
not trivial to produce rations suitable for long-term space
flight. I don't expect it to be hugely difficult either; like
the zero-gravity toilet it's just one of those things that has
to be done and will be done but isn't quite as easy as it sounds
like it should be.


At very least, three or four years before the first manned Mars
mission we ought to take our best guess and put it to the test.
Round up a batch of graduate students or other suitable guinea
pigs, give them the locker or freezer full of astro-MREs, and
say "This is all you get to eat for the next three years. Let
us know how it works out..."


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-951-9107 or 661-275-6795 * -58th Rule of Acquisition *

"Allen Thomson" writes:

Michael Smith wrote:

It would be interesting to work out how much of a
spacecraft you would have with a couple of submarine
style fission reactors and as many ion or hall thrusters
as you had power for.

Given the lack of enthusiasm for this approach I can
only assume that it doesn't deliver transit times short
enough to be safe for humans.

It would be interesting to know if there is currently
any propulsion approach available that would allow
significantly faster than Hohmann trips for humans
to other planets/moons/major asteroids. (Our moon
excepted, of course.) "Currently available" can be
interpreted to mean "available by 2025 at a development
+ procurement cost of no more than $10G in 2004 dollars
per year between now and then."

If by "significantly faster" you mean a factor of two or
so, that can be done.

A minimum-energy transfer to Mars takes somewhere between
240 and 260 days, depending on what launch window you use.
Looking at payload delivery from low Earth orbit to low
Mars orbit using various propulsions systems, we get:

LOX-LH2 Chemical Rocket: 240 days 25% payload fraction
Nuclear Thermal Rocket: 240 days 45% payload fraction
Nuclear Electric Drive: 300 days 45% payload fraction
Solar Electric Drive: 300 days 40% payload fraction

"Nuclear Thermal", means running liquid hydrogen through a
hot reactor core and exhausting the gas through a nozzle -
same principle as a chemical rocket, but different energy
source and lighter working fluid. This has been tested
on the ground, back when open-air nuclear reactors were
an acceptable thing, but never flown.

"Electric Drive" refers to an ion or plasma thruster system
similar to what I described in an earlier post, using either
a nuclear reactor or advanced solar arrays as a power source.
The longer trip time comes from the necessary acceleration
period using low-thrust propulsion, and these are systems
that have flown at a smaller scale and on solar power.

If we're looking for a factor of two improvement in speed,
we can use moderately high energy orbits using any of these
propulsion systems.

LOX-LH2 Chemical Rocket: 120 days 10% payload fraction
Nuclear Thermal Rocket: 120 days 30% payload fraction
Nuclear Electric Drive: 180 days 30% payload fraction
Solar Electric Drive: 180 days 20% payload fraction

So, even if we are stuck using chemical rockets, we can get
four-month trips if we are willing to accept 10:1 mass
ratios. And we can do better than that if we are willing
to go nuclear, using reasonably well established but of
course politically controversial technology. Even if we
have to use fluffy green solar power, we can still beat
chemical rocketry and the Hohmann orbit by a fair margin

If you want weeks instead of months, no go using any
technology we can forsee for the next two decades. And
note that for all of these, launch windows open every
2.15 years. Interplanetary travel without regard for
launch windows is another thing we aren't going to be
doing in this generation, though in the case of Mars
missions you can frequently get an off-year window at
tolerable cost by using a Venus flyby.


Equally intresting would be to know about the technology
for life support systems that would reasonably reliably
sustain a half-dozen people for two or more years in
space without help from Earth.

For a mission of that scale, you'd keep it simple and use
industrial chemistry to close the air and water loops.
Roughly speaking, the human body turns clean water into
dirty water, water content of food into dirty water, and
dry food plus air into carbon dioxideand clean water with
a little bit of solid waste on the side.

That last step is critical, because it means there is
a surplus of water in the output stage which can be
used directly to make up for inefficiencies in your
water recycling system or electrolyzed to make up for
inefficiencies in your oxygen recycling system.

A physiochemical life support system for a two- to
three-year mission would consist of six major elements:

A vapor distillation unit to turn dirty water into
clean water, with the impurities concentrated in a
brine that is vented overboard (along with a little
bit of water, but as noted we can make that up).

A molecular sieve or other regenerable physiochemical
system for extracting carbon dioxide and other trace
impurities from the cabin atmosphere

A Sabatier reactor for turning carbon dioxide plus
hydrogen into methane plus water. The methane we
might be able to make use of or might just vent,
the water we for sure can make use of.

A water electrolysis unit to turn the surplus water
(both from human respiration and the Sabatier reactor)
into oxygen and hydrogen. The hydrogen feeds back
into the Sabatier reactor, and the oxygen goes into
the cabin air.

An incineration or other oxidation unit to reduce
the solid waste to ash and recover what water and
carbon dioxide we can.

And a stockpile of canned, dehydrated, frozen, or
otherwise preserved food, details yet to be determined.
If it isn't fully dehydrated, that's still more surplus
water that can be used to compensate for inefficiencies
elsewhere.

The hardware would mass about 200-500 kilograms per man
with current technology, and consume about one kilowatt
per man of electric power.

Stored consumables would ammount to about two kilograms
per man per day, mostly food. Minor consumables would
be nitrogen to make up for atmospheric leakage, hydrogen
for the Sabatier reactor (that loop can't be fully closed
without a very large excess of water to electrolyze),
maybe ammonia or hydrazine in place of seperate nitrogen
and hydrogen if the mass balance is right, various personal
and environmental hygene supplies, filters and other parts
for the machinery, and packaging for all of the above.

With a bit of effort and austerity, it might be possible
to get the consumables requirement down to 1 kg/man-day,
but almost certainly no further unless you start growing
your own food. And that's not worth the bother for only
a few people and a few years.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-951-9107 or 661-275-6795 * -58th Rule of Acquisition *

In article ,
Herman Rubin wrote:
There is another problem, which is much more serious than
it might appear. If we reduce the food supply to those
nutrients about which we know, we may end up missing
something important over moderately long periods.

Except, of course, that's not what anyone is proposing. The only way you
*could* do that would be chemical synthesis of food, which is out of the
question. Realistic proposals all involve preserved normal foods, and the
only significant question is whether absolutely everything of importance
survives preservation.

And as has already been noted, anything that will survive freezing *will*
survive essentially indefinitely in cryogenic refrigeration. So there
should be no difficulty with anything that's needed only in trace amounts,
given a modest amount of frozen whole food as a supplement to dehydrated
stuff.

People have lived on such diets -- in fact, on diets considerably inferior
to that -- for periods of a year or more in places like the polar regions.
Extending this to Mars-mission durations would require some confirming
experiments to be absolutely certain, but major trouble is most unlikely.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

John Schilling wrote:

If we're looking for a factor of two improvement in speed,
we can use moderately high energy orbits using any of these
propulsion systems.

LOX-LH2 Chemical Rocket: 120 days 10% payload fraction
Nuclear Thermal Rocket: 120 days 30% payload fraction
Nuclear Electric Drive: 180 days 30% payload fraction
Solar Electric Drive: 180 days 20% payload fraction


I'll point out that you can start the mission from highly eccentric
earth orbit and reduce the delta-V requirements. You could
send cargo, propellant and supplies up into that orbit, perhaps using
low thrust electric rockets, adding the crew only when you
were ready to launch to Mars.

(If radiation in HEEO is too high you can start in a much higher
orbit and do a small burn to pass again near the Earth, where
you make your main burn to inject to Mars.)

Paul

"John Schilling" wrote in message
...
"Allen Thomson" writes:
...
However, the viewgraph that started this seemed to say that
the state of the art isn't here for preserving nutritionally
adequate food for multi-year Mars missions. So is there some
ingredient of a multi-year nutritionally complete diet that
can't be preserved by chilling or freezing or dehydration or
whatever? If so, what might it be?

Don't know; I can't find the original references at my local
library or online, just abstracts and summaries. What I can
find suggests it isn't anything as simple as a missing vitamin
or amino acid or whatnot. But in the course of developing the
MRE, the Army did *something* to the mix that resulted in a
food that would last indefinitely if kept cold, but will lead
to unexplained weight loss and mental deterioration if used
exclusively for more than a few weeks.

I think the "cherry box" on the viewgraph:
" Improvements in food storage technology or production technology are also
needed to reduce overall mass and ensure crew health."
states the issue accurate, but the other sentence on the slide:
"Current food preservation technology is not capable of providing
nutritionally viable food for the longer mission durations under study"
is a bit of a misstatement.

Ensuring crew health requires a diet that is varied and palatable so that
the crew eats properly, and the food itself is not a source of stress on the
mission (psychological health).

And the trick is to do it with low mass foods (i.e. dehydrated).

Also, nutrition science is far beyond the RDA stage - finding the essential
individual components in a diet required for health. We all know about the
debates about what makes an *optimum* healthy diets:
What kind of fats and in what proportion?
How much and what kind of fiber?
How much flavonoids and carotenoids, and what kinds, with what ratios?
Etc.

I think it is the combined problem of satisfying all of these together, and
quite clearly no one has ever developed a food system like this before.

The whole viewgraph presentation is about design trade-offs, and the dietary
aspect of a mission is going to involve trade-offs of its own. For a
palatable, optimally healthy, indefinitely storable diet a solution is at
hand right now - just prepare thousands of excellent meals and freeze them
in ready-to-eat form. But this is quite heavy with all that water.
Maintaining the good qualities of those meals but getting rid of the water
mass, not so easy.

Carey Sublette

In article ,
Henry Spencer wrote:
In article ,
Herman Rubin wrote:
There is another problem, which is much more serious than
it might appear. If we reduce the food supply to those
nutrients about which we know, we may end up missing
something important over moderately long periods.

Except, of course, that's not what anyone is proposing. The only way you
*could* do that would be chemical synthesis of food, which is out of the
question. Realistic proposals all involve preserved normal foods, and the
only significant question is whether absolutely everything of importance
survives preservation.

And as has already been noted, anything that will survive freezing *will*
survive essentially indefinitely in cryogenic refrigeration. So there
should be no difficulty with anything that's needed only in trace amounts,
given a modest amount of frozen whole food as a supplement to dehydrated
stuff.

You are underestimating the nature of the problem. If a
large enough variety of foods, from enough regions, are
used, this might not be a problem. One nutrient which I
mentioned, selenium, is a problem in both directions; one
needs enough, and not too much, and where food is grown can
make a large difference.

People have lived on such diets -- in fact, on diets considerably inferior
to that -- for periods of a year or more in places like the polar regions.
Extending this to Mars-mission durations would require some confirming
experiments to be absolutely certain, but major trouble is most unlikely.

The polar regions might not be the worst. The American
Midwest was known as the "goiter belt" because of a
shortage of iodine. In this case, the danger of too much
is not that great, but I do not know what the effect would
be if too much accumulates in recycled water.

There can be some problems which cannot occur on earth;
in these cases, our ignorance is immense. We have no
idea about the need, if any, for aluminum and silicon,
and as soil is mainly composed of aluminum silicates,
a shortage is not a problem. I know an excess of
aluminum can be toxic, and is difficult to test for.

--
This address is for information only. I do not claim that these views
are those of the Statistics Department or of Purdue University.
Herman Rubin, Department of Statistics, Purdue University
Phone: (765)494-6054 FAX: (765)494-0558

That said, there are people that have survived long periods of time
eating only one type of meal.

Um, personally I spent a year eating only curry rice and oatmeal - and
a different year eating mainly hamburgers... not that I am
particularly healthy, though.

The solution is obviously to send only teenagers - they can survive
eating anything!

-David

"Paul F. Dietz" writes:

John Schilling wrote:

If we're looking for a factor of two improvement in speed,
we can use moderately high energy orbits using any of these
propulsion systems.

LOX-LH2 Chemical Rocket: 120 days 10% payload fraction
Nuclear Thermal Rocket: 120 days 30% payload fraction
Nuclear Electric Drive: 180 days 30% payload fraction
Solar Electric Drive: 180 days 20% payload fraction


I'll point out that you can start the mission from highly eccentric
earth orbit and reduce the delta-V requirements. You could
send cargo, propellant and supplies up into that orbit, perhaps using
low thrust electric rockets, adding the crew only when you
were ready to launch to Mars.

If the mission is payload delivery, you almost certainly can't start
that from a highly eccentric earth orbit, because very few payloads
are going to originate in highly eccentric earth orbit. For that
mater, very few spaceships are going to be built in highly eccentric
earth orbit. You have to count boosting the ship and payload to
the highly eccentric earth orbit as part of the mission.

Which is still quite efficient if you're using a low-thrust propulsion
system, and is part of the assumed mission plan for the SEP and NEP
cases above. Passengers, yes, can join the ship a bit later, but
passengers are only a minor portion of the payload even on a passenger
ship.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-718-0955 or 661-275-6795 * -58th Rule of Acquisition *

"Carey Sublette" writes:

"John Schilling" wrote in message
...
"Allen Thomson" writes:

However, the viewgraph that started this seemed to say that
the state of the art isn't here for preserving nutritionally
adequate food for multi-year Mars missions. So is there some
ingredient of a multi-year nutritionally complete diet that
can't be preserved by chilling or freezing or dehydration or
whatever? If so, what might it be?

Don't know; I can't find the original references at my local
library or online, just abstracts and summaries. What I can
find suggests it isn't anything as simple as a missing vitamin
or amino acid or whatnot. But in the course of developing the
MRE, the Army did *something* to the mix that resulted in a
food that would last indefinitely if kept cold, but will lead
to unexplained weight loss and mental deterioration if used
exclusively for more than a few weeks.

I think the "cherry box" on the viewgraph:
" Improvements in food storage technology or production technology are also
needed to reduce overall mass and ensure crew health."
states the issue accurate, but the other sentence on the slide:
"Current food preservation technology is not capable of providing
nutritionally viable food for the longer mission durations under study"
is a bit of a misstatement.

Ensuring crew health requires a diet that is varied and palatable so that
the crew eats properly, and the food itself is not a source of stress on the
mission (psychological health).

And the trick is to do it with low mass foods (i.e. dehydrated).

Also, nutrition science is far beyond the RDA stage - finding the essential
individual components in a diet required for health.

Right, and the failures of known long-term stored-food diets are not at
the RDA, individual-componet level either.


I think it is the combined problem of satisfying all of these together, and
quite clearly no one has ever developed a food system like this before.

The whole viewgraph presentation is about design trade-offs, and the dietary
aspect of a mission is going to involve trade-offs of its own. For a
palatable, optimally healthy, indefinitely storable diet a solution is at
hand right now - just prepare thousands of excellent meals and freeze them
in ready-to-eat form. But this is quite heavy with all that water.

It's not that easy, alas. Not all foods retain everything that used to
make them nutritious and palatable through a freeze/thaw cycle, at least
using known freezing and thawing processes, and I'm not aware of any
successful attempt at providing a healthy long-term diet using only those
foods known to freeze well.


Maintaining the good qualities of those meals but getting rid of the water
mass, not so easy.

Getting rid of most of the water would be a plus, but *all* of the known
preservation methods - freezing, canning, dehydrating, and the rest - have
limits. The early arctic and antarctic expeditions were not seriously
mass-limited, at least for their base camps, and certainly did not lack
for refrigeration. They could and did carry their choice of canned and
frozen food, all fully hydrated, and yet found that a full overwinter
stay was testing the limits of endurance on such a diet.

Again, I don't think this is likely to be a major problem. We know more
about food preservation than we did in the early 20th century, and we
know from their experience what doesn't work and what the problems are.
But we also know more about transport logistics than we did in the early
20th century, and have used that to ensure that we never had to test our
presumed ability to have people live for years without any resupply.

That's something we are going to have to test before we sent people to
Mars with an assortment of canned, frozen, and dehydrated food. Tweaking
the diet to increase the fraction of dehydrated food is a secondary goal.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-718-0955 or 661-275-6795 * -58th Rule of Acquisition *

John Schilling wrote:

If the mission is payload delivery, you almost certainly can't start
that from a highly eccentric earth orbit, because very few payloads
are going to originate in highly eccentric earth orbit. For that
mater, very few spaceships are going to be built in highly eccentric
earth orbit. You have to count boosting the ship and payload to
the highly eccentric earth orbit as part of the mission.

However, the ship without payload can be placed into that orbit
slowly, then the payload added near the time of injection to Mars.
This is not possible if that delta-V were instead used to send
the vehicle directly to Mars.

Whether this is significant may depend on just where the concern
about transit time is coming from. For crews, it may come from
concerns about reliability of the systems and/or radiation exposure,
and in that case it would be significant.

The system that boosts payloads into HEEO can also be staged
and left in HEEO, where it is more easily reused. One could
also imagine using schemes such as power beaming from Earth or
other sources in cis-lunar space, which again are also not possible
if the vehicle had been injected directly to Mars from LEO.
Breakdowns during the boost to HEEO are more easily repaired
than if the vehicle is en route to Mars.

Paul