Author Topic: How does shielding mass trade with trip times to Mars - MCT related.  (Read 13171 times)

Offline aero

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The MSL reported measuring 1.8 milliSieverts/day radiation dose during cruise to Mars. One Sievert total dose increases risk of fatal cancer by 5%. What is the trade between shielding mass and shortened trip times to constrain radiation dose to acceptable levels?

It must be correct that halving the trip time would halve the total radiation dose on any crew. How much shielding did the MSL provide to the instrument and is it necessary to double the shielding to halve the dose with shielding?

I posted this in SpaceX because of the MCT fast trip to Mars discussion but not in that thread because it would be quickly buried by the main topic posts. Move it if needed.
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Offline Robotbeat

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The MSL reported measuring 1.8 milliSieverts/day radiation dose during cruise to Mars. One Sievert total dose increases risk of fatal cancer by 5%. ...
...in the linear, no-threshold model based on acute, very high doses and extrapolating back to very low doses...
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Offline Robotbeat

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...
It must be correct that halving the trip time would halve the total radiation dose on any crew. How much shielding did the MSL provide to the instrument and is it necessary to double the shielding to halve the dose with shielding? ...
The relationship between shielding level and radiation level is complicated. Also, there's the possibility of simply shielding the crew sleeping quarters and making them spend most of their time in there if they want lower doses of radiation.

...Elon Musk should know that his idea of shielding just with a column of water pointed at the Sun won't work for many reasons. I should shoot him an email.
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Offline smoliarm

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...
How much shielding did the MSL provide to the instrument and is it necessary to double the shielding to halve the dose with shielding?

...

Basically, there was no specific radiation shielding in MSL design, just whatever was provided by construction withstanding launch loads, nothing more.
Therefore, it's incorrect to transfer directly the dose measured by RAD in MSL to human-rated spacecraft.
For instance, if we put Apollo CM in trans-Mars injection with RAD inside - I would expect the cumulative trip dose to get lower by factor of 2.

Offline guckyfan

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...
How much shielding did the MSL provide to the instrument and is it necessary to double the shielding to halve the dose with shielding?

...

Basically, there was no specific radiation shielding in MSL design, just whatever was provided by construction withstanding launch loads, nothing more.
Therefore, it's incorrect to transfer directly the dose measured by RAD in MSL to human-rated spacecraft.
For instance, if we put Apollo CM in trans-Mars injection with RAD inside - I would expect the cumulative trip dose to get lower by factor of 2.

Thanks, interesting. That would put the total dose well into the range considered safe by NASA. It is also safe to assume that the shielding even with very low added weight will be better than it was in Apollo. So it would be even lower than that.

Offline smoliarm

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...

...Elon Musk should know that his idea of shielding just with a column of water pointed at the Sun won't work for many reasons. I should shoot him an email.

It's not that simple  :) although, I'm going to present simplified picture too (softly speaking :)

There are TWO major radiation threats for a trip to Mars: GCR and Solar flares

******* GCR *******
= constant level, low fluctuations (or "no events");
= isotropic (comes from all directions);
= VERY high energy (makes passive shielding ineffective);
= while present serious health hazard at long exposures, it is not life-threatening in short-term.

*** SOLAR FLARES ***
= catastrophic events (up to 4 orders of magnitude in range);
= uni-directional (from the Sun, roughly - plus/minus magnetic field distortion for p+ and e- fluxes);
= low-mass particles, mostly - n, p, e, and gamma (makes common light liquids like LOX, RP1, CH4 highly effective shields);
= deadly dangerous - can kill in a matter of minutes.

Considering the above two lists (simplified, yes, but correct in the first approach) - the idea of Sun-pointed liquid shield is reasonable and working ( and btw - it is not Elon's ;) ).
Yes, it's working, just like body armor - it won't make you 100 % safe in battlefield, but it protects you from deadly hits of small arms fire and shrapnel.

Online docmordrid

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Using the TransHab/Bigelow expandable habitat tech is a good place  to start. Their 16+ inches of high hydrogen polymers  plus water containers in the walls, perhaps some layers with soluble beryllium in the mix for neutrons, should  make for a quite good shield. 
« Last Edit: 12/31/2013 08:45 am by docmordrid »
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Offline Elmar Moelzer

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I have been a long time proponent of short trip times as opposed to a more complex spacecraft. The collumn of water makes sense for solar radiation and events. The rest is IMHO better solved with reduced trip times.
A technological breakthrough in the area of propulsion should IMHO be a focus of research, much more than shielding and in situ resource use.

Offline aero

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I have been a long time proponent of short trip times as opposed to a more complex spacecraft. The collumn of water makes sense for solar radiation and events. The rest is IMHO better solved with reduced trip times.
A technological breakthrough in the area of propulsion should IMHO be a focus of research, much more than shielding and in situ resource use.

I have to concur. There are innumerable benefits to shorter trip times, and that goes without even mentioning the savings of life time for the crew.

Given an average 80 year life span, 200 days avoided travel time gives .7% of the life span for productive work on Mars. Then consider that productive life span for crew is more like 40 to 50 years that means 200 days saved on trip time gives 1.1 to 1.4 % additional productive life time. Now, consider that time on Mars for a crew member may well be something like 10 years and then 200 days becomes a significant fraction of his/her production time on Mars, over 5%. That means that for every 20 crew sent slowly, the productivity of one additional crew member could be available on Mars just from reduced trip time.

edit add: From Wikipedia -
Quote
typical Mars mission plans have round-trip flight times of 400 to 450 days.[21] A fast Mars mission of 245 days round trip could be possible with on-orbit staging.

With that, you can see that a 6 week trip time, or even a 90 day trip time might double the Mars crew productivity numbers given above.
« Last Edit: 12/31/2013 11:52 pm by aero »
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Offline MickQ

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So, as Hyperion5 said in the vehicle config/Raptor thread, we need to use a BIG cluster of ion engines, preferably powered by a reactor, for transit propulsion and use Raptors for TMI, MOI, TEI and EOI.  This would also have the bonus of providing some small amount of gravity for the majority of the trip each way.

Mick.

Offline CuddlyRocket

^ Forget about a reactor. SpaceX would probably go bust fending off legal actions and other politically-instigated regulatory actions!

Not much you can do about GCRs. As for solar flares; point the engines at the Sun - those, together with the propellant tanks should provide sufficient shielding.

Offline Hyperion5

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^ Forget about a reactor. SpaceX would probably go bust fending off legal actions and other politically-instigated regulatory actions!

Not much you can do about GCRs. As for solar flares; point the engines at the Sun - those, together with the propellant tanks should provide sufficient shielding.

Considering I haven't seen NASA get sued for flinging plutonium-fueled RTGs up into orbit (mounted on various probes), your point remains just a hypothesis.  We would need to see Spacex actually go forward with that approach to test it.  Also, given that Big Tobacco is still in business today despite losing a huge lawsuit, I remain dubious about Spacex getting sued out of existence.  If they're dominating the world launch market with their Falcon 9 family in 5 years, the worst that could possibly happen is they'd have to file for bankruptcy and reorganize.  Much like Sea Launch they'd probably be right back in business, only far more successfully. 

In an interesting possibility, you might be able to use the reactor shielding to also protect the crew from solar flares.  Think of it as a 2 for the price of one deal. 

Offline CuddlyRocket

^ Forget about a reactor. SpaceX would probably go bust fending off legal actions and other politically-instigated regulatory actions!

Not much you can do about GCRs. As for solar flares; point the engines at the Sun - those, together with the propellant tanks should provide sufficient shielding.

Considering I haven't seen NASA get sued for flinging plutonium-fueled RTGs up into orbit (mounted on various probes), your point remains just a hypothesis.

NASA had to fight off court cases for both the Galileo and Cassini missions, and that was for just a few kg of plutonium. Imagine the fuss over launching a nuclear reactor! Of course, you could launch it without the fuel and then send the fuel up separately in lots of individual launches. But imagine the insurance costs for a private company.

Quote
We would need to see Spacex actually go forward with that approach to test it.

I don't think they fancy being the guinea pigs. Leave it to governments (in fact, probably non-democratic governments) with their much deeper pockets (and lack of concern for public opinion).

Quote
Also, given that Big Tobacco is still in business today despite losing a huge lawsuit, I remain dubious about Spacex getting sued out of existence.

They haven't lost yet, it's still on appeal. And slight difference in size; a big tobacco company's annual profit is more than SpaceX's entire sales to date. Also, there are lots and lots of tobacco companies so court cases are spread around; different to being the only company trying to launch a nuclear reactor.

Quote
If they're dominating the world launch market with their Falcon 9 family in 5 years, the worst that could possibly happen is they'd have to file for bankruptcy and reorganize.  Much like Sea Launch they'd probably be right back in business, only far more successfully.

The company may survive, but the present shareholders wouldn't - I'm not sure they're that keen on risking their present investment and control; and the money men who'd then control the company might want a different kind of leadership than visionary. 

Quote
In an interesting possibility, you might be able to use the reactor shielding to also protect the crew from solar flares.  Think of it as a 2 for the price of one deal.

If you did have a reactor that would be true. But I suspect SpaceX is going to try and think of other, less fraught methods first.

In any event, there is absolutely no sign of them developing a reactor, which would be quite a long lead-time project. As such, it's more of an advanced concept than speculation as to what SpaceX might be doing at the present time.

Offline Robotbeat

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...

...Elon Musk should know that his idea of shielding just with a column of water pointed at the Sun won't work for many reasons. I should shoot him an email.

It's not that simple  :) although, I'm going to present simplified picture too (softly speaking :)

There are TWO major radiation threats for a trip to Mars: GCR and Solar flares

******* GCR *******
= constant level, low fluctuations (or "no events");
= isotropic (comes from all directions);
= VERY high energy (makes passive shielding ineffective);
= while present serious health hazard at long exposures, it is not life-threatening in short-term.

*** SOLAR FLARES ***
= catastrophic events (up to 4 orders of magnitude in range);
= uni-directional (from the Sun, roughly - plus/minus magnetic field distortion for p+ and e- fluxes);
= low-mass particles, mostly - n, p, e, and gamma (makes common light liquids like LOX, RP1, CH4 highly effective shields);
= deadly dangerous - can kill in a matter of minutes.

Considering the above two lists (simplified, yes, but correct in the first approach) - the idea of Sun-pointed liquid shield is reasonable and working ( and btw - it is not Elon's ;) ).
Yes, it's working, just like body armor - it won't make you 100 % safe in battlefield, but it protects you from deadly hits of small arms fire and shrapnel.
Solar flare particles are most definitely NOT unidirectional. They spiral around the magnetic field lines at various angles, thus end up fairly isotropic (though certainly not perfectly so).

It's like shielding from light on a clear day versus a cloudy day.
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Offline jg

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Uranium reactors aren't particularly radioactive until you take them critical; it's the fission byproducts that are nasty (including plutonium).  I grew up eating off of beautiful yellow glass plates (uranium used to be used to make a very pretty glass before it got a bad name).  My father just took them into a lab and checked with a Geiger counter when we got them.

Plutonium is just plain nasty; much more radioactive than uranium (much much, shorter half-life), and very poisonous, and it doesn't like to stay put.

So any reactor had better be powered up *off* the earth (and keep it on a trajectory where it won't re-enter).  And I'd expect you'd use a uranium reactor....

Now, whether you can convince the public of the safety of such a thing is a different question....

Offline Ludus

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I suspect any realistic Mars settlement ambitions require reactors shipped to Mars so the politics aren't going to be avoided indefinitely.

If it succeeds in it's ambitions, SpaceX will be a very global launch services provider by then and it won't be limited by the reach of lawsuits in US courts. If the US is still a problem they just launch via a foreign subsidiary with a friendly nuclear regulatory environment like SpaceX India. Another alternative in the US as with Flibe is to operate under the independent nuclear regulatory authority of the military that's much more insulated from civil court actions.

Offline smoliarm

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Solar flare particles are most definitely NOT unidirectional. They spiral around the magnetic field lines at various angles, thus end up fairly isotropic (though certainly not perfectly so).

It's like shielding from light on a clear day versus a cloudy day.

Your picture is wrong:
Speaking of strong solar flares, at these energies charged particles do dot "spiral" - the magnetic field just curves their track. Those guys who go spiral - they do nor reach Earth's orbit or at least do not make catastrophic events there.

Yes, Sun's magnetic field will effectively 'un-focus' initially narrow beam, but when this cloud reaches craft on trans-Mars orbit - it will be UNI-directional from the craft's point of view. The apparent origin of these beams will not coincide with Sun, moreover - it will be drifting with time (the early hits will be closer to the Sun, late hits will be more away).

The parallel with "cloudy day" is wrong just as well. There are no "clouds" in the Solar System. As particles leave Sun's magnetic influence, they move straight.

This is not my speculation, this is a well established fact.

Offline Robotbeat

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I sincerely doubt Musk is planning nukes.

I agree it'd be helpful vs solar on the surface, but NOT in transit. For high-thrust rockets, NTRs aren't worth it compared to hydrolox. For electric propulsion, solar beats nuclear until you get to Jupiter. This is BEFORE politics has anything to do with it.

And even with surface power, if you're using a nice lithium-sulfur rechargeable battery (they're starting to enter the market) with unrollable solar arrays, you still get very close to nuclear power. Personally, I think nuclear power is still quite useful for a colony (it's a very stable, low-maintenance power supply that could be designed to last a century before needing a fill up or maintenance http://terrapower.com/ ), but you could certainly still do it with solar power.
« Last Edit: 01/03/2014 03:14 am by Robotbeat »
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Offline Lobo

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I sincerely doubt Musk is planning nukes.

I agree it'd be helpful vs solar on the surface, but NOT in transit. For high-thrust rockets, NTRs aren't worth it compared to hydrolox. For electric propulsion, solar beats nuclear until you get to Jupiter. This is BEFORE politics has anything to do with it.

And even with surface power, if you're using a nice lithium-sulfur rechargeable battery (they're starting to enter the market) with unrollable solar arrays, you still get very close to nuclear power. Personally, I think nuclear power is still quite useful for a colony (it's a very stable, low-maintenance power supply that could be designed to last a century before needing a fill up or maintenance http://terrapower.com/ ), but you could certainly still do it with solar power.

Agreed.

I think if there's a budding colony on Mars, and things seem to be going good for SpaceX and just Mars PR in general, and the colony is really hitting the limit of what solar arrays can do, then I could see the possibility of a small reactor that could be buried and shielded a ways off to the side of the colony developed for Mars.  Probably in conjunction with NASA, as they've already had studies on it, and they could do development and "ownership" of it.  That could be they way NASA astronauts get a place in the colony to stage their own research and operations some.  Although I'm sure Elon would be eager to have NASA on board anyway, supplying a small reactor for base power, along with some other high-tech elements could be for the colony what JAXA and the ESA supplying some ISS elements was for the ISS. 

But I just don't see an NTR powered MCT.  Perhaps a NEP module that would be used to transport MCT to and from Mars, but even then SEP is still a viable option within the inner planets.  And SEP would keep the possibility of a NEP module from malfunctioning and plunging to the Earth on a mission return.

Obviously either SEP or NEP have much better ISP than NTR. 

But even with a base reactor, keep in mind you have a LOT of realestate on MArs.  You have lots and lots and lots of room to just lay out big, dumb thin solar arrays on the ground and wire them together.  you can walk by them occasionally with a broom and dust them off.  you don't necessarily need heavy and complex tracking mechanisms or anything.  Just unroll them on the ground like carpets and stake them down.

Each mission could bring a few more and adequate cable to hook them into the grid.

Another 20 years by the time there might be need for such arrays should see a lot more advancement in the technology.  You want something lightweight and flexible so it can be rolled up and deployed.  (or stacked and deployed, however). 
You might be able to support quite a large base with just solar.  Making large quantities of methalox to refuel MCT's rapidly could require power from a reactor though.  I have not idea what sort of power that would require.


Offline Robotbeat

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No reason to bury the reactor on Mars, just build up a derm around it. That way you can still use the atmosphere for heat transfer without requiring a big pipeline to circulate water to the surface and back.
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Offline gbrocki

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I think sending a reactor to mars is making the same mistakes again. Elon is sure that the energy of the sun is the energy of the future and that is what i think. By the way there are other easier ways to use the sun than "only" producing electrical energy: what about using the thermal energy (efficiency over 90%).

Offline CuddlyRocket

Let's keep this thread on-topic folks - otherwise this thread will be reviewed (like the MCT speculation thread) or closed!

The design of a Mars colony itself is surely off-topic for the SpaceX forums (SpaceX is about creating the transportation infrastructure to Mars, not about precisely what is transported), let alone this particular thread. As for nuclear propulsion, shouldn't that be in advanced concepts? IMO we should limit speculation here to what we think SpaceX might be doing based on what we know it's doing; not about what we think it should be doing! :) And there is absolutely no evidence that SpaceX is planning on anything nuclear.

Offline guckyfan

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I think sending a reactor to mars is making the same mistakes again. Elon is sure that the energy of the sun is the energy of the future and that is what i think. By the way there are other easier ways to use the sun than "only" producing electrical energy: what about using the thermal energy (efficiency over 90%).

Welcome to the forum. But your question is in the wrong part. There is a whole dedicated section for Mars including discussions on colonies and how they are built and supplied.

http://forum.nasaspaceflight.com/index.php?board=21.0

Offline go4mars

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This thread will get a lot more interesting and relevant when SpaceX 3D print's their first pusher plate.  8)  :o   ;D
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Offline Robotbeat

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SpaceX isn't pursuing nuclear technology. The only SpaceX guy who talked about it isn't even working there anymore.
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Offline gbrocki

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Can anyone tell me which constant power (in kW) it needs to reduce the travel-time to mars by 10%, 25% or 50% let's say for a 100t spaceship (without loss of weight by the propulsion).

Offline Robotbeat

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Can anyone tell me which constant power (in kW) it needs to reduce the travel-time to mars by 10%, 25% or 50% let's say for a 100t spaceship (without loss of weight by the propulsion).
No, because you'll always be losing weight via expelling it as propellant. ;)
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Offline gbrocki

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ok - that's right - i just want to get a feeling for the power needed. If you want let's say 20t at mars.

Offline Robotbeat

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ok - that's right - i just want to get a feeling for the power needed. If you want let's say 20t at mars.
...power required is a function of time, payload size (which you gave), and propellant Isp. The higher the Isp, the more power is required (...generally... but if you get too high on the exponential side, I suppose it may be possible to INCREASE the power required by reducing the Isp).
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Offline gbrocki

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thanks for this little lesson in physics - my lessons at the university (at the time of the first lunar landing) are partly forgotten. So my question changes to the continuous thrust needed to achieve the reduction of travel-time.

Offline Lourens

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gbrocki, this post in the MCT Speculation Thread sums up a brief discussion on this, and should give you a feel for the numbers.
« Last Edit: 01/04/2014 12:27 pm by Lourens »

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