Author Topic: MCT Speculation and Discussion Thread 4  (Read 618055 times)

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2040 on: 05/13/2016 04:00 PM »
Is there a guess on the minimum power required to maintain basic functionality of the craft? (I.e. basic life support, basic thermal control, minimal comms, emergency lighting only, basic GNC, etc.)
Passive thermal control, manually operated life support or oxygen candles and chemical scrubbers. Windows. gravity-stabilized orientation or manual operation of thruster valves (not a terribly good idea, but possible). Except for comms, basically everything could be done with zero electrical power (though relying on manual operations).

But that's unnecessary.
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Offline Jet Black

Re: MCT Speculation and Discussion Thread 4
« Reply #2041 on: 05/13/2016 04:01 PM »
Do you have an IC in your rebreather to act as a backup to the battery? No? Of course not, because that'd be ridiculous. It'd be dangerous, complicated, and would probably end up adding more failure modes than it'd save. You use backup batteries.

NO ONE is saying not to have redundancies and margin. What we're saying is ICE is the wrong solution.

Yeah I'm struggling to think what other options there are beyond solar/battery power combo and an ICE, unless there is some useful power that can be dragged out of the cooling system (I'm assuming that there won't be nuclear power or RTG on the crew transports)
Methane/oxygen fuel cells
Hydrogen/oxygen fuel cells
Stirling engine based on catalytic Methane/Oxygen reaction
Turbine based on Methane/Oxygen combustion

I was subconsciously including all combustion in that... but yep. Once you can split your water back into Hydrogen and oxygen again (using solar) then the fuel cells are an interesting backup, since it's in principle replenishable and probably easier to do than making more methane and the components either side of the reaction are useful.

There's ammonia to burn too.
For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled. -- Richard Feynman

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2042 on: 05/13/2016 04:01 PM »
Do you have an IC in your rebreather to act as a backup to the battery? No? Of course not, because that'd be ridiculous. It'd be dangerous, complicated, and would probably end up adding more failure modes than it'd save. You use backup batteries.

NO ONE is saying not to have redundancies and margin. What we're saying is ICE is the wrong solution.

Yeah I'm struggling to think what other options there are beyond solar/battery power combo and an ICE, unless there is some useful power that can be dragged out of the cooling system (I'm assuming that there won't be nuclear power or RTG on the crew transports)
Methane/oxygen fuel cells
Hydrogen/oxygen fuel cells
Stirling engine based on catalytic Methane/Oxygen reaction
Turbine based on Methane/Oxygen combustion
Or use independent batteries. The simplest, most reliable solution.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2043 on: 05/13/2016 04:02 PM »
Something like a Redox low temp solid oxide fuel cell. A 1 m3 450kg Cube runs on NG and delivers 25kw. Much more efficient than an ICE generator.
DM

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2044 on: 05/13/2016 04:02 PM »
Do you have an IC in your rebreather to act as a backup to the battery? No? Of course not, because that'd be ridiculous. It'd be dangerous, complicated, and would probably end up adding more failure modes than it'd save. You use backup batteries.

NO ONE is saying not to have redundancies and margin. What we're saying is ICE is the wrong solution.

Yeah I'm struggling to think what other options there are beyond solar/battery power combo and an ICE, unless there is some useful power that can be dragged out of the cooling system (I'm assuming that there won't be nuclear power or RTG on the crew transports)
Methane/oxygen fuel cells
Hydrogen/oxygen fuel cells
Stirling engine based on catalytic Methane/Oxygen reaction
Turbine based on Methane/Oxygen combustion

I was subconsciously including all combustion in that... but yep. Once you can split your water back into Hydrogen and oxygen again (using solar) then the fuel cells are an interesting backup, since it's in principle replenishable and probably easier to do than making more methane and the components either side of the reaction are useful.

There's ammonia to burn too.
Regenerative fuel cells have very low round-trip efficiency. Use frakking batteries, you'll get a lot more power for your trouble and no moving parts (regenerative fuel cells need pumps, etc).

There seems to be some sort of allergy to batteries here.
« Last Edit: 05/13/2016 04:03 PM by Robotbeat »
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #2045 on: 05/13/2016 04:07 PM »

I was subconsciously including all combustion in that... but yep. Once you can split your water back into Hydrogen and oxygen again (using solar) then the fuel cells are an interesting backup, since it's in principle replenishable and probably easier to do than making more methane and the components either side of the reaction are useful.

There's ammonia to burn too.

There is methane, and oxygen, for propellant. Methane is currently a byproduct of the oxygen generation of one LSS system in the ISS IIRC.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Online Lar

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Re: MCT Speculation and Discussion Thread 4
« Reply #2046 on: 05/13/2016 04:25 PM »
Guys, be excellent to each other. I won't single any one person out, although I could.

Terms like "zombie idea" are not helpful.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
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Offline baldusi

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Re: MCT Speculation and Discussion Thread 4
« Reply #2047 on: 05/13/2016 05:43 PM »
Do you have an IC in your rebreather to act as a backup to the battery? No? Of course not, because that'd be ridiculous. It'd be dangerous, complicated, and would probably end up adding more failure modes than it'd save. You use backup batteries.

NO ONE is saying not to have redundancies and margin. What we're saying is ICE is the wrong solution.

Yeah I'm struggling to think what other options there are beyond solar/battery power combo and an ICE, unless there is some useful power that can be dragged out of the cooling system (I'm assuming that there won't be nuclear power or RTG on the crew transports)
Methane/oxygen fuel cells
Hydrogen/oxygen fuel cells
Stirling engine based on catalytic Methane/Oxygen reaction
Turbine based on Methane/Oxygen combustion
Or use independent batteries. The simplest, most reliable solution.
I might have missed the lines in so many posts. But what are the requirements exactly on this discussion? Backup for what phase of the trip, for how long, considering which contingencies? I understand that nuclear power will be prepossitioned. And I'm assuming a certain amount of CH4/LOX is supposed to be on the Mars pad, but the MCT would have some margin on its own storage tanks.
So again, what are the requirements. I don't feel like stating an educated guess (some call it an opinion) if I don't fully understand the requirements.

Offline Jet Black

Re: MCT Speculation and Discussion Thread 4
« Reply #2048 on: 05/13/2016 07:56 PM »

I was subconsciously including all combustion in that... but yep. Once you can split your water back into Hydrogen and oxygen again (using solar) then the fuel cells are an interesting backup, since it's in principle replenishable and probably easier to do than making more methane and the components either side of the reaction are useful.

There's ammonia to burn too.

There is methane, and oxygen, for propellant. Methane is currently a byproduct of the oxygen generation of one LSS system in the ISS IIRC.

sure, but is the methane as easy to replenish as hydrogen/oxygen would be? If something is going to be used in a burning system, then you want to be able to replenish it when you have spare energy again.
For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled. -- Richard Feynman

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2049 on: 05/13/2016 08:00 PM »

I was subconsciously including all combustion in that... but yep. Once you can split your water back into Hydrogen and oxygen again (using solar) then the fuel cells are an interesting backup, since it's in principle replenishable and probably easier to do than making more methane and the components either side of the reaction are useful.

There's ammonia to burn too.

There is methane, and oxygen, for propellant. Methane is currently a byproduct of the oxygen generation of one LSS system in the ISS IIRC.

sure, but is the methane as easy to replenish as hydrogen/oxygen would be? If something is going to be used in a burning system, then you want to be able to replenish it when you have spare energy again.
But then you have to reclaim the exhaust, include an electrolysis kit, a compressor... which adds a bunch of mass. So much that you get better storage capacity per mass with... *dundundun* ...a lithium rechargable battery.

(not to mention much better round-trip efficiency)

If you're using a fuel cell (which is much better than a small IC, though still bad IMO), just vent the exhaust or possibly use the water for crew use (like on Shuttle). Anything else will just result in a lithium battery being a much better option.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #2050 on: 05/13/2016 08:06 PM »

I was subconsciously including all combustion in that... but yep. Once you can split your water back into Hydrogen and oxygen again (using solar) then the fuel cells are an interesting backup, since it's in principle replenishable and probably easier to do than making more methane and the components either side of the reaction are useful.

There's ammonia to burn too.

There is methane, and oxygen, for propellant. Methane is currently a byproduct of the oxygen generation of one LSS system in the ISS IIRC.

sure, but is the methane as easy to replenish as hydrogen/oxygen would be? If something is going to be used in a burning system, then you want to be able to replenish it when you have spare energy again.
But then you have to reclaim the exhaust, include an electrolysis kit, a compressor... which adds a bunch of mass. So much that you get better storage capacity per mass with... *dundundun* ...a lithium rechargable battery.

(not to mention much better round-trip efficiency)

If you're using a fuel cell (which is much better than a small IC, though still bad IMO), just vent the exhaust or possibly use the water for crew use (like on Shuttle). Anything else will just result in a lithium battery being a much better option.

Unless of course your LSS is already processing the CO2 and water vapor in the air into CH4 and O2 then there is no added mass. I would favor a methane fuel cell if it had a decent electrolyte lifespan.

It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline kaoru

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Re: MCT Speculation and Discussion Thread 4
« Reply #2051 on: 05/13/2016 08:37 PM »
As I said, the backup would be batteries. But you'd also build-in redundancy in the solar array. For life support, no doubt you'd actually have a few oxygen candles and chemical scrubbers, or rely on the large volume of the MCT as a buffer.
As mentioned previously, the concern is power "generation" for all modes/situations; in space or on Mars.  In a life critical context, having two or more independent power generation capabilities is obvious.  However, batteries are for power "storage", not generation.  Sure you can use batteries when your power generation fails but only to the extend of the amount of power stored in the batteries and the number of physical batteries you have.

In other words, we're talking energy density, aka how much power you can store or is stored.  Granted that batteries are extremely efficient, for their mass, in providing the stored energy but fails in another metric, energy density.  Batteries just don't have the energy density to even come close to Methane LOX, either by combustion or reaction, mass for mass.  That's why a ICE sedan (aka Audi or BMW) similar to Tesla Model S weighs half as much and has twice (give or take) the range.  It's also why Methane LOX is rocket fuel and a battery powered rocket engine will never exist.

In short, batteries are excellent "buffers" in power generation to maintain supply during deterministic lapses in power generation.  They are useless for "non-deterministic" lapses (in power generation) as there is always the possibility of running out of energy.  A dust storm on Mars (which have covered the entire planet) is possibly one of those "non-deterministic" lapses that could last months.  Though I'm not for or against any technology, I do hold the opinion that an ICE (or derivative technology) genset is the best, simplest, non-nuclear solution.

Kaoru   

Offline CuddlyRocket

Re: MCT Speculation and Discussion Thread 4
« Reply #2052 on: 05/14/2016 01:16 AM »
What about in a dust storm on mars? Your solar panels probably aren't going to be much use then.

Dust storms don't completely block the Sun. The maximum reduction in power from the Mars rovers' solar cells was ~80%.

In a life critical context, having two or more independent power generation capabilities is obvious.

I agree; for safety reasons there needs to be a source of electric power that is not derived either directly or indirectly from your main source. It may be less efficient, but it is independent and that is a worthwhile feature in its own right to be factored into any engineering trades.

Unfortunately, it is difficult to think of any independent source of power to solar other than nuclear, or vice-versa. (For very small amounts of power there are the crew members.)

Quote
Though I'm not for or against any technology, I do hold the opinion that an ICE (or derivative technology) genset is the best, simplest, non-nuclear solution.

ICE and associated fuel/oxygen stocks is a power storage method and needs to be traded against other storage methods. That said, if on the surface of Mars you're going to be producing large amounts of methalox propellant, it would seem a shame if you had no means to use it to generate electricity in an emergency! Zubrin noted the back-up possibilities in his Case for Mars, although he assumed ICE-powered vehicles whose engines could do dual duty as generators. How much this applies to the MCT is another matter.

Offline biosehnsucht

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Re: MCT Speculation and Discussion Thread 4
« Reply #2053 on: 05/14/2016 03:48 AM »
TL;DR: If there's an ICE to be found, I think it's most reasonable to put it in a long/medium range rover (for distances greater than half a day's walk, or less if your backpack life support doesn't last that long) to be used for recharging the batteries (sized for normal close range stuff but not for long range, due to mass) in a series hybrid arrangement (it would be a plug-in as well, and can use ICE to feed power back to whatever and can be refilled from either ground side tanks of liquid or gaseous CH4/O2 or from MCT's tanks)

--

In space, ICE just as backup (or really, any) source doesn't make sense unless it's part of your overall IVF system and a necessary part of it (I recall reading about an ICE in the IVF of ... was it the ACES upper stage?).

On the surface of Mars, ICE isn't a terrible backup system if you want to eliminate "eggs in one basket" scenario, but obviously you can't take enough of it to power everything at once (would mass too much plus where would you get the fuel?). You'll have to generate it's fuel from ISRU which means it's really another kind of solar power.

So, it would seem obvious that batteries make a better choice, but batteries have mass, and an empty tank doesn't have near as much.

If for example you can pull the fuel back out of your partially-refueled MCT to run the ICE, then the overall additional mass of the ICE and it's available runtime is far more mass efficient than batteries. You might also be able to have some kind of expandable or otherwise lightweight gaseous (rather than liquid) storage for CH4 and O2, which might or might not be better than batteries for mass (since it's now something you weren't going to bring). Whether an ICE and storage (either additional or siphoning off the MCT) is more mass efficient than enough batteries to outlast whatever issue (may not just be storms, maybe damage to solar panels etc) is dependent on how "bad" you think the bad can possibly be.

The "obvious" choice for where to make the ICE useful for more than just backup would be to have your surface vehicles use ICE as some kind of series hybrid setup and be able to feed power into whatever else you need it for, but realistically you're likely to find batteries a better use for such a use.

However, a longer range rover (in which you might possibly go more than half a day's walk) might be a good place to put an ICE. The energy density of ICE without the mass (at launch from earth) of batteries (you'd still need some, but not as much) might make that worthwhile. A large enough solar array to recharge it for such a range may not be feasible to make portable, so an ICE would win if you want to risk venturing too far to walk back safely, then when you need emergency power (more than say overnight or a day) you can by then get the rover back and use it's ICE to convert fuel (either stored onboard or via ground side tanks or the MCT's tanks) back into energy.


Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2054 on: 05/14/2016 04:17 AM »
Regenerative fuel cells have very low round-trip efficiency. Use frakking batteries, you'll get a lot more power for your trouble and no moving parts (regenerative fuel cells need pumps, etc).

There seems to be some sort of allergy to batteries here.

Their is one motivation I can see for chemical energy storage.  When a solar powered ISRU process is being run continually and we want to avoid the need to store the large amounts of energy needed to run the process at night the best solution is to immediately use all solar power for water electrolysis (which is most of the energy input to the whole process) and store the excess hydrogen during the day as it is being produced faster then it is being consumed.  Because the remaining chemo-synthesis steps to create methane are exothermic after Hydrogen creation it can run with little power input (compressing the methane will require some power).

Now admittedly you could just size the system such that all hydrogen produced during the day is immediately consumed and then their is a full shutdown at night.  But this would introduce thermal cycling to the synthesis equipment as well as a daily startup process both of which would certainly cause wear on it, so keeping the process continuous would be advantages to both mass and lifespan of the equipment.  Depending on latitude a seasonal shutdown due to lower solar power might still be necessary and their are dust-storms too, but that's still about a 700 fold reduction in cycling.

Now all of that being justified on it's own ground I can running the stored Hydrogen/Oxygen through a fuel-cell as a back up power-source after batteries are depleted, at any one time their will be only 1 day's worth of Hydrogen stored but this may constituted several days worth of emergency power needs if their is a sudden failure of solar and or batteries.  Hydrogen/Oxygen fuel cells would be preferable to a Methane/Oxygen ones because the source fuel Hydrogen has gone through less conversions steps and thus has a higher storage efficiency, Hydrogen/Oxygen fuel-cells are also a more refined technology.

Internal combustion is an absolute non-starter as it would be just a fraction of the efficiency of a fuel-cell and as the only conceivable use is as an emergency backup you want electricity to run a base while repairs are made a stationary system is ideal, not one mounted to a vehicle which might be absent right when it is needed most.
« Last Edit: 05/14/2016 04:29 AM by Impaler »

Offline randomly

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Re: MCT Speculation and Discussion Thread 4
« Reply #2055 on: 05/14/2016 05:47 AM »
Just remember it takes about 75-80kw/hr of electrical energy to produce 1kg of hydrogen, and a fuel cell only extracts about 16 kwh from that 1kg of hydrogen. so you are throwing away 80% of your energy.
as Robotbeat mentioned, the round trip energy efficiency of fuel cells sucks. You'd have equivalent results if you only stored 20% of the power in lithium batteries.

I wouldn't dismiss methane fuel cells. The hydrogen storage problem is substantial. It may be a better over all system trade to use methane/oxygen.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2056 on: 05/14/2016 08:29 AM »
Batteries just don't have the energy density to even come close to Methane LOX, either by combustion or reaction, mass for mass.  That's why a ICE sedan (aka Audi or BMW) similar to Tesla Model S weighs half as much and has twice (give or take) the range.

Cars on Earth don't have to carry their own oxygen. As RB said, you are letting your experience on Earth mislead you.

It's also why Methane LOX is rocket fuel and a battery powered rocket engine will never exist.

Solar electric propulsion has vastly better "range" than any chemical rocket.

A dust storm on Mars (which have covered the entire planet) is possibly one of those "non-deterministic" lapses that could last months.

Solar panels still work during dust storms. (Dust is actually easier for modern solar panels than water-droplet clouds on Earth.)

The power generated will drop, of course. But most of the power required on Mars will be for ISRU. You simply suspend power hungry ops during the worst of the storm. (Which, judging by MER-Opportunity, is only a few days even in a month long dust storm.)

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Re: MCT Speculation and Discussion Thread 4
« Reply #2057 on: 05/14/2016 02:27 PM »

A dust storm on Mars (which have covered the entire planet) is possibly one of those "non-deterministic" lapses that could last months.

Solar panels still work during dust storms. (Dust is actually easier for modern solar panels than water-droplet clouds on Earth.)


The biggest concern is that the "Mars Dust" is more of salty (sticky), abrasive (sharp edges) and much smaller particles (talcum powder sized) which is electrically charged attracted to any man-made elements, not the "Earth dust" we think of ones from the dry deserts.

Offline aameise9

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Re: MCT Speculation and Discussion Thread 4
« Reply #2058 on: 05/14/2016 05:54 PM »
1) Mars surface is reached by a larger version of the Dragon, with engines suitable for precision landing.

2) Mars surface is reached by a larger version of the F9R, with engines suitable for both orbital launch and precision landing.
Neither. Or rather, something in between.

You need maybe 6-7km/s to return to Earth from the surface of Mars. You can do that in a single stage with TPS, though it is hard.

Thank you!  So something in between a squat capsule and a pencil-shaped booster.

Presumably with short, stubby legs to support the weight of a fully tanked craft?

Presumably with engines angling outward (Dragon-style) to direct blast debris away from craft?

I am not sure that I understood the relevance of TPS (temperature protection system)?


Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 4
« Reply #2059 on: 05/14/2016 06:21 PM »


The biggest concern is that the "Mars Dust" is more of salty (sticky), abrasive (sharp edges) and much smaller particles (talcum powder sized) which is electrically charged attracted to any man-made elements, not the "Earth dust" we think of ones from the dry deserts.

So effectively nothing you want in joins, bearings, panels engines or lungs.
« Last Edit: 05/14/2016 06:21 PM by The Amazing Catstronaut »
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