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#176 2017-10-11 08:25:56

Terraformer
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Re: Light weight nuclear reactor, updating Mars Direct

It seems like the only way solar will work is  if we manage to develop photolytic cells that split the water directly using sunlight.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#177 2017-10-11 08:35:44

Oldfart1939
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Re: Light weight nuclear reactor, updating Mars Direct

Terraformer--That's highly improbable due to the energetics of the reaction. Like it or not--both Elon and Louis are stuck with a nuclear requirement.

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#178 2017-10-11 09:33:43

GW Johnson
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Re: Light weight nuclear reactor, updating Mars Direct

Actually it depends on your thinking-box for what you will be doing on Mars. 

If you assume from the outset that return and ascent propellants will be sent from Earth,  then almost regardless of the detail architecture,  all you have to supply with electricity while there is a handful of habitats to heat and light,  plus a couple of small,  slow rover cars to keep charged,  and some small scale experiments to run.  That gets you down into the 10's to 100's of KW range,  which is what SAFE-400 and "kilopower" are all about. 

If on the other hand you make the assumption that you produce at least your ascent propellant (and maybe your return propellant,  depending on the detail architecture) in situ on Mars,  then you need something like 2 or 3 orders of magnitude more electricity to support that activity on a 2-year timescale.  That is just WAY OUT OF THE BALLPARK for SAFE-400 size sources,  and for nothing-but-solar,  as we currently know it.  You are literally looking at the need for 10's to 100's of MW of electricity. 

As pointed out,  the machinery to support propellant manufacture on that scale is going to be both heavy and voluminous.  That is not the sort of thing you land in 10-ton items.  It comes in 100-ton lots inside vehicles larger still by far.  That is what drove Musk's people to the very large BFR/ITS combination,  even downscaled as it is.  The larger Guadalajara version required some 1900 tons of propellant per ship.  The smaller version uses less,  but it's still in the 1000-ton class.  I just don't know the exact current value.  Doesn't matter.

Those two scenarios are simply 3 orders of magnitude different in their electricity needs.  They always will be.  It's inherent,  given what we know about electrolysis. 

Your source of Martian water can actually make the problem very much worse than that.  It's fairly easy to use heat to mine a massive buried glacier up a well,  and that heat can come from the waste heat from electricity production. On the other hand,  mining moisture from damp regolith is a gigantic dirt-moving operation similar to pit mining,  with giant,  power-hungry machines,  and no practical way to power them except with battery electricity.  Plus,  somebody has to drive and operate those machines,  and while they are doing that,  they are not doing anything else.  Which "else" is why we sent them to Mars in the first place!  I find that entirely self-defeating. 

That adds at least another order of magnitude or two to the electricity requirements while there.  That unfortunate result comes about precisely because the moisture content of Martian regolith is a single-digit percentage,  just like the energy efficiency of electrolysis.  It's inherent.  There is no way around it.  You just deal with it by brute force,  just like powering electrolysis.

To avoid that,  you must pick the site with the massive buried glacier "for sure".  Which means you need a way to verify its presence before you mount the mission.  I don't trust remote sensing for that:  ground truth has always been different from conclusions drawn from remote sensing,  and quite often vastly different.  The hydrogen you sensed is there,  yes,  but there is NOTHING to guarantee it is water,  and not some other compound. 

Screw that up,  and your crew dies for lack of a way home.  Period.  Note the trite saying below my by-line.  Maybe it's not so trite.  Apollo-1,  Challenger,  and Columbia say it is quite the serious consideration. 

GW

Last edited by GW Johnson (2017-10-11 09:59:18)


GW Johnson
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#179 2017-10-11 10:28:46

Terraformer
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Re: Light weight nuclear reactor, updating Mars Direct

If you leave the bulk of your return mass in orbit, taking the return propellent with you isn't going to be as much of an issue as it would be if you insisted on direct launch from the surface.

Given that the crew will be there for around 1.5 years, basing out of orbit for the first half, and then landing after we've explored a few sites and found somewhere suitable for a base, seems to be best?


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#180 2017-10-11 11:24:42

RobertDyck
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Re: Light weight nuclear reactor, updating Mars Direct

My mission plan is a modification of Mars Direct. I consider it a "tweek" of Mars Direct. Furthermore, when I first presented it at the Mars Society convention in 2002, that vast majority of attendees strongly criticized my plan for being at all different than Mars Direct. They wanted Mars Direct, period.

Mars Direct (and my plan) uses ISPP for return propellant. All return propellant. The SP-100 nuclear reactor was developed in the late 1980s, completed 1992. It was included in Mars Direct, developed in the last quarter of 1989 and first half of 1990, presented to NASA in June 1990. So Dr. Zubrin and his partner David Baker used the latest bleading-edge technology of the time. I have argued the exact same team who developed SP-100, were the ones who developed SAFE-400. Designed to operate in exactly the same environment, produce exactly the same amount of power. But SAFE-400 was completed in 2007, and has lower launch mass. So it's just the newest, latest, lighter version of the same thing. Mars Direct used SP-100 for multiple months to produce enough propellant for return. My discussion thread "updating Mars Direct" proposed SAFE-400 instead. We could definitely produce return propellant by operating for multiple months, possibly more than a year, to fill propellant tanks for return.

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#181 2017-10-11 13:41:38

GW Johnson
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Re: Light weight nuclear reactor, updating Mars Direct

You could possibly launch something small that way,  with ISPP propellant in the 10's of tons not 1000-ton class.  That reduces your electrolysis power level and integral-over-time by perhaps an order of magnitude,  or maybe two.  So it's still around a MW or so,  maybe 10 MW.  That's more-than-ten to maybe 100 SAFE-400's to power it.   

And you still have to power the methane maker,  heat and light the camp,  and you have to find the massive ice,  because you cannot afford the electric power (or the operator time) to run the regolith-processing diggers to find enough water at 6%-class moisture content. 

If you intend on massive ISPP (even for small vehicles and only-10-ton propellant quantities),  you open that MW+ scale of electricity can-of-worms,  it is inherent.  Precisely because both the regolith moisture content and the efficiency of electrolysis,  are single-digit.  QED.

It gets out of hand very quickly,  pretty much similar to the exponential troubles we get into with the rocket equation.  If your mission is small scope with smaller vehicles,  it is really better just to land the ascent propellant from Earth to reduce your on-site electricity needs to a SAFE-400 or two.  Experiment with ISPP,  yes,  but do NOT count on it to come home.  That way you can land smaller experimental items. 

If you don't believe my numbers,  run them for yourself.  Most of y'all know more about this ISRU/ISPP stuff than I do.  I just pointed out the 6% efficiency bottleneck with electrolysis.  Comes right from thermochemistry basics and the knowledge that there is electrolysis available in large-enough scales to do this job,  but not much else.

GW

Last edited by GW Johnson (2017-10-11 13:46:11)


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#182 2017-10-12 08:33:09

JoshNH4H
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Re: Light weight nuclear reactor, updating Mars Direct

GW:

Where do you get 6% efficiency from?  Every source I've seen shows that modern electrolysis units have efficiencies much closer to 60% (sometimes higher!) than 6%.

http://hydrogenoman.com/docs/click%20on … tached.pdf

In this paper the author discusses electrolytic efficiencies as high as 92% in lab settings for high-temp electrolysis.  I generally use 60% as a reasonable but conservative value for electrolytic efficiency, and 75% as a target for industrial-scale electrolysis, both on the conservative end of what we get on Earth.


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#183 2017-10-12 10:11:21

GW Johnson
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Re: Light weight nuclear reactor, updating Mars Direct

Hi Josh:

I think it depends upon what you do,  and how you book-keep all the data.  Open-beaker 1-bar rigs like what I played with in high school chemistry about half a century ago are indeed very low efficiency.  But they are simple,  and very lightweight relative to the mass of the water being processed. High pressure / high temperature equipment is very big,  and very heavy,  for the mass of water processed.  That's big heavy stuff like what's in the typical chemical refinery.

The report you linked to,  indicates the electrolysis reaction itself at high pressures and temperatures is far more efficient,  but that report also explicitly states that there is a whale of a lot more energy needed to create the high temperature / high-pressure conditions.  My guess is that their definition of "efficiency" for the electrolysis reaction does not include that enormous energy to create the conditions. 

That's fairly typical of lab reports in the literature,  by the way.  The comparisons between experimenters are "fair" and "honest",  as long as everyone "cheats" the same way.  And they do.  It is precisely why actual commercial products rarely live up to early scientific experiment claims. 

Even so,  60% vs 6% is only one order of magnitude out a 3-order-of-magnitude disparity between the electricity requirements needed to support propellant manufacture or not on Mars,  which calculations are what I posted above for only electrolyzing water into hydrogen and oxygen.  I gave no figures for their capture,  their liquifaction,  or for the use of some of them with Martian "air" to create the methane. 

All of those processes cost energy.  Some are efficient,  maybe,  but they pretty much all increase the electricity requirements beyond what I calculated.  My basic point is that if you count on ISPP to come home,  you must have electricity available in the multi-MW range,  not the multi-KW range. 

My question is this:  why are we proposing to count on this for the very first landing,  when we don't know "for sure" the water is actually there,  with which to create the oxygen that is 75% of the needed propellant mass,  and when no one at all is working on MW-range power sources for use on Mars?  And only Musk is proposing a vehicle big enough to carry the necessary machinery?

None of the answers I have seen so far,  make any good,  basic common sense. 

Getting this wrong for that first landing pretty much guarantees a dead crew,  and that makes no sense at all.

GW

PS --  The bench-top demonstrators for ISPP make quantities on the order of a kg or so per day,  as best I understand it.  Maybe.  If they could run continuously at full throughput without breaking down (something I doubt) 24/7/365,  then if you need 10 tons (10,000 kg),  that's 10,000 days (27 years) to make your propellant with one device at low power draw,  in a package light and small enough to land in a dinky vehicle.  If it's 100 tons,  270 years.  if it's 1000 tons,  2700 years. 

You have slightly less than 2 years to get the job done,  which means you need 14 such units for the 10-ton job,  or a unit 14 times larger in terms of throughput,  and you had better have spares,  because machinery breaks down!  It's even worse if you need more tons.  By the time you do all that,  you're back into the multi-MW electricity requirement again,  and a requirement for multiple tons of machinery to be landed,  not even counting the power supplies. 

Whether it's 10,  100,  or 1000 tons of propellant to manufacture doesn't make a lot of difference to that outcome.  It gets ridiculous very fast,  regardless.  As do the vehicle transport requirements,  none of which seem very compatible with min thrown-mass concepts like Mars Direct or any of the variations on it. 

I think this conundrum is what drove Musk to the giant ship idea.  But even so,  he's still running the risk of whether the required water is really there in a form he can actually use.  And single-digit moisture content in regolith is not the answer,  for the reasons I already gave.

Last edited by GW Johnson (2017-10-12 10:49:46)


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#184 2017-10-12 12:05:10

RobertDyck
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Re: Light weight nuclear reactor, updating Mars Direct

One energy conservation trick that Robert Zubrin used was to only harvest CO2 from the atmosphere at night. The temperature at night is only a few °C above the freezing temperature for dry ice, so use a freezer to drop those last few degrees to accumulate dry ice. At dawn, close the collection container, reverse the refrigeration coils to warm the dry ice and sublimate it. This concentrates CO2, producing almost pure CO2 gas, and self-pressurizes in the phase transition from solid to gas. Resulting CO2 gas is pure enough for the Sabatier reactor.

A 100 kWe reactor produces more than enough to produce 108 metric tonnes of LOX/LCH4 propellant, plus 12 tonnes for an internal combustion vehicle to drive around the surface. I got these numbers from "The Case for Mars" first edition, paper back, copyright 1996, published 1997, page 5. Table 4.5 on page 93 lists the ERV as 28.6 tonne launch mass, but it also lists the SP-100 producing 80 kWe. When Dr. Zubrin and his partner wrote Mars Direct in 1990, SP-100 was still under development. Dr. Zubrin hoped a lower power reactor would mass less, but when SP-100 was complete in 1992 the nuclear engineers said an 80 kWe reactor would have exactly the same mass. So you may as well stick to the standard 100 kWe design. Filling propellant tanks for a vehicle in 10s of tonnes will take months, but less than a year. Remember, the ERV is launched first, astronauts ride a hab on the next launch opportunity 26 months later.

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#185 2017-10-12 12:46:27

JoshNH4H
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Re: Light weight nuclear reactor, updating Mars Direct

I have to admit I did not read most of the posts in the thread before responding.  As far as energy needs go, though, I'll do some BOTE calculations now to see if my numbers agree with yours.

Launch windows for Mars come every 26 months, but since astronauts will take a quicker trajectory than cargo let's say you have 24 months to generate your return propellant (Zubrin says you want it all to be generated by the time crew arrives and I agree with that). 

Zubrin proposes the following reactions for ISPP:

1A) 4 H2 + CO2 -> CH4 + 2 H2O

1B) 2 H2O -> 2 H2 + O2

And, to generate the additional LOX (you need twice as much as you get from this process):

1C) 2 H2 + 2 CO2 -> 2 H2O + 2 CO

1D) 2 H2O - > H2 + O2

In this thread it seems like there has also been discussion of mining the water on Mars.  This is substantially more risky, but in that case the reaction would be:

2A) 4 H2O -> 4 H2 + 2 O2

2B) 4 H2 + CO2 -> CH4 + 2 H2O

In either case, you need to electrolyze 4 moles of H2O per mole of Methane.  Reactions 1A, 1C, and 2A are roughly energy neutral so the power input for electrolysis dominates.  H2O has an enthalpy of formation of -286 kJ/mol, and one "mole" of propellant (CH4 + 2 O2) has a mass of 80 grams.  Assuming 60% electrolytic efficiency (all-in), the electrolytic energy input is 14.3 MJ/kg of propellant.  This works out to 166 W/kg-day* for electrolysis.  Over 24 months, it's 226 W per tonne.  Given all the other energy requirements, I will more than double this to 500 W per tonne.  I think this is generous.  14.3 MJ/kg is a lot to play with for physical transformations.

If you have a 100 kWe nuclear reactor working 24+ hours per Martian day, you will be able to generate 200 tonnes of propellant over the course of your 24 month generation window.  Assuming a Vex of 350 s and a total delta V of 7 km/s (roughly speaking, what it takes to get from Mars's surface to home), you need a mass ratio of 7.5, which gives you a theoretical dry mass of up to 26.7 tonnes.  If you can get a higher (but matched by Russian CH4/LOX engines) Isp of 375 s, that mass ratio falls to 6.5 and the theoretical dry mass rises to 31 tonnes.

It seems to me that we're playing with numbers in the right ballpark, unless you can point out somewhere where I've gone wrong?

*Earth day, e.g. 86,400 s


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#186 2017-10-13 09:04:36

Oldfart1939
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Re: Light weight nuclear reactor, updating Mars Direct

We seem to have forgotten the Moxie unit in this discussion. That system is not reliant on having ANY water present. The energy required to "mine" ice or melt glaciers is enormous, in addition to the conversion of H2O to elemental hydrogen and oxygen via electrolysis. The original Zubrin proposal included bringing H2 along for CH4 production from the Mars atmosphere.

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#187 2017-10-13 09:21:22

GW Johnson
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Re: Light weight nuclear reactor, updating Mars Direct

Josh:

The difference isn't really in the theoretical electrolysis energy.  You and I are getting the about the same energy per ton to split water,  even though I am just looking at the classic textbook reaction.  The real difference is in the overall process efficiency and in the quantities to be made. 

High efficiency equipment is going to operate at high temperatures and pressures inside,  and I rather doubt the laboratory efficiency results for prototypes includes the energy necessary to maintain those conditions.  That is because there are so many different ways of doing that,  all of different energy requirements. 

Low efficiency equipment will be smaller and lighter,  and will not have energy requirements to maintain such demanding conditions.  But the efficiency will be far lower,  closer to the chemistry lab equipment I played with half a century ago. 

The net effect of that issue is that overall efficiency of a working system is just not going to be what gets reported in the literature.  I've never seen any real systems that did,  in any area of endeavor. 

My efficiency was an order of magnitude lower than yours,  and my quantity was an order of magnitude higher.  That's why your 100's of KW electric was my MW's of electric.  Simple as that.

Whether one calls that pessimism or skepticism makes no difference.  Having a "jaundiced eye" regarding claims in academic papers has served me very well for many decades.  I had to actually go and build stuff that had to work. 

Which is also why I am skeptical of taking the risk of relying on ISPP to get home from that very first landing.  If it doesn't work as advertised,  the crew dies,  and from a very bad management decision.  We all know where that leads. 

The RMS Titanic was not as unsinkable as advertised.  The B-1B avionics system wasn't integrated properly by USAF the way they advertised.  The highly-touted super-stealth qualities of the F-117A didn't save one from being shot down in the Balkans in the 1990's by an obsolete SA-6 with a 1955-vintage radar seeker.  Nuclear-generated electricity did not turn out to be "too cheap to meter" the way it was advertised. 

So why should we believe we know enough about that first landing site on Mars to claim that ISPP will work "for sure" and "exactly as advertised" and so we can bet lives on it?  Our history as a technological species says that claim is bullshit.

The notion that we cannot afford to send and land ascent propellant from Earth is based on the dual thinking boxes of "we can only afford min thrown mass" and "only a direct shot with an expensive giant rocket is feasible".  With commercial launchers an order of magnitude cheaper than SLS is advertised (and it will be more expensive than that),  and a 2-decade history of orbital assembly by docking,  both those thinking-constraint boxes are now bullshit.  Times really have changed.

The real objective should now be (1) make damned sure that first crew has every possible chance to get back home safe so that a return visit is acceptable to everybody,  and (2) try out your ISPP/ISRU stuff on that first mission so that you really can rely upon it for the return visits. 

Maybe even leave a small facility running on automatic for the next crew.  It really does reduce the expense by reducing thrown mass,  no doubt about that (I never said it didn't).  Even with orbital assembly and far cheaper smaller launch rockets,  that is still true. 

But that savings is not worth the risks on that first trip.  There's a reason a new airplane's first flight test does not explore its highest speeds.  Or the first sea trials of a new ship are not high-speed runs. It's called "not taking chances that you can reasonably avoid".

GW

Last edited by GW Johnson (2017-10-13 09:47:09)


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#188 2017-10-13 10:15:48

JoshNH4H
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Re: Light weight nuclear reactor, updating Mars Direct

Depends on the savings, I guess. It definitely made more sense when launch costs were $15,000/kg instead of >$2000/kg like we're looking at for the next few years, and maybe less beyond that.

If you assume your mass to Mars will cost 5 times more than mass to LEO, AMD that mass to LEO is $2000/kg, you're looking at $10,000/kg to Mars. For 150 tonnes of fuel, that's 1.5 billion. A lot, but maybe not a mission-killer.

As far as electrolytic efficiency goes, it's my understanding that 60% is the efficiency of real-world units. I need to look into it again because I picked out a few years ago, but it's based on the efficiency of industrial-size electrolytic units. And of course my 500 W per tonne gives a multiplier of 2.2 over my calculated electrolytic requirements.

I'll do some research into real-world models and report back.


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#189 2017-10-13 11:03:12

GW Johnson
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Re: Light weight nuclear reactor, updating Mars Direct

It depends upon what gets book-kept into the efficiency.  In the academic publishing arena,  just about everybody "cheats" pretty much the same way,  which makes the comparisons fair,  but not necessarily realistic.  You really have to "read the fine print" to see it,  though. 

I dunno,  I've been obsolete and retired out of that game for a long time now. You know a lot more about such stuff than I do.  But rockets and launch costs I understand. 

If we build,  say,  2000 tons of exploration fleet (manned and unmanned vehicles) in LEO,  and send that Mars with conventional rocketry using your $10M/ton to Mars,  that's $20B.  There's landers and other vehicles to develop and prove out,  so assume launch costs are 20% of your program (for a well-run program).  That's $100B to send a big crew to Mars,  not the half-trillion-$ BS put forth repeatedly by NASA and "old space". 

If the unmanned stuff goes by electric propulsion,  then launched mass to LEO for assembly greatly reduces.  Which is where my $50B price tag for sending a crew of 6 to Mars,  and visiting 8 different sites in the one trip,  comes from.  I guess nobody believes me,  but that really does look realistic to me.  That's what I posted over at "exrocketman" last year.

GW


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#190 2017-10-13 12:33:32

JoshNH4H
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Re: Light weight nuclear reactor, updating Mars Direct

That seems like a reasonable range to me.  I like the rule-of-thumb that 20% of total costs are launch costs, but I don't know that it necessarily applies when you've got electric propulsion involved, which reduces the launch costs without reducing other procurement or development costs.

But maybe such nitpicking is missing the point of rules of thumb.

Anyway, as far as efficiency goes, I offer the following pdf from a company that appears to be one of the world's foremost suppliers of electrolysis units.

http://nelhydrogen.com/assets/uploads/2 … ochure.pdf

These being marketing materials, they are less helpful than they could be.  Your point about cheating on energy consumption is well-taken too, and I will account for that.

In this brochure they use the unfortunate unit "Nm3".  This is "Normal Cubic Meter of Hydrogen Gas", or the amount of H2 that would occupy one cubic meter at 0 C and 101.325 kPa.  This corresponds to 0.090 kg.

They claim that their systems consume 3.8-4.4 kWh per Nm3.  This corresponds to 13.7-15.8 MJ per Nm3 or Hydrogen, or, alternatively, 152-176 MJ/kg of H2.  Including Oxygen mass, that's 15.9-19.5 MJ/kg of water.  Compared to an energy of formation of 15.9 MJ/kg, that's quite a high efficiency indeed.  (82%-100%).

To be clear, I don't think the actual efficiency is ever 100%, for most of the reasons GW mentioned.  High temperature, pressurization, etc. are energy inputs to the system that are probably not measured as part of this efficiency measure. 

While these energy inputs are substantial, the electrical energy of electrolysis strictly dominates, which is why I am happy with the number of 60% all-in efficiency for a commercial electrolysis unit.

For comparison, these units have hydrogen outputs of 50-500 Nm3 per hour.  Using my 60% practical efficiency number, this indicates a total energy consumption of 33-330 kW, likewise right in our ideal range.


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#191 2017-10-13 14:05:09

GW Johnson
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Re: Light weight nuclear reactor, updating Mars Direct

Looks like we could have them custom-add oxygen capture to the small one,  and it might easily fit aboard one of Musk's ships,  even being commercial chemical plant equipment like it is.  Maybe right along with a SAFE-400 unit.  It is better than I thought.  Nice brochure.

GW


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#192 2017-10-13 20:06:48

JoshNH4H
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Re: Light weight nuclear reactor, updating Mars Direct

Yeah, definitely.  We would probably have a look at investing just a bit in mass reductions but I think a commercial product could do the job with relatively little modification.


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#193 2017-10-21 15:20:59

Dudevycy
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Re: Light weight nuclear reactor, updating Mars Direct

The vehicle obviously has an aerodynamic shape. Aerodynamic shapes are designed to go through air. Air resistance is a major problem on Earth but Mars is a near vacuum, so it is the wrong shape.

The big problem on the Moon and Mars is life support. Has space been left aside for an ECLSS?
I do not see an oxygen tank.

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#194 2017-10-23 20:17:59

kbd512
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Re: Light weight nuclear reactor, updating Mars Direct

When you slam into the atmosphere of Mars at multiple kilometers per second, aerodynamics becomes a factor, even when the atmosphere is as thin as Mars happens to be.  SpaceX has learned a thing or two about supersonic and hypersonic flight in Earth's thin upper atmosphere.

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#195 2017-10-25 11:27:28

GW Johnson
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Re: Light weight nuclear reactor, updating Mars Direct

They re-learned it the hard way with staging difficulties flying Falcon-1.  That comes from hiring no one over the age of 45.

For the same speeds otherwise,  100,000+ feet on Earth is the same density atmosphere as low in the atmosphere at Mars.  That's why the Viking probe chutes were tested at full deployment speed a bit above 100,000 feet.  About the same as coming out of hypersonics around 5-10 km on Mars. 

GW


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#196 2017-10-25 19:35:40

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Re: Light weight nuclear reactor, updating Mars Direct

They have also been learning as they go on metals which did not do so well on the stage....

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