NASASpaceFlight.com Forum
SLS / Orion / Beyond-LEO HSF - Constellation => Missions To The Near Earth Asteroids (HSF) => Topic started by: rdale on 10/19/2011 04:12 pm
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Neat graphics showing concepts for long timeframe missions...
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110016277_2011017267.pdf
National Aeronautics and Space Administration (NASA) has begun
evaluating various mission and system components of possible implementations of what the U.S. Human Spaceflight Plans Committee (also known as the Augustine Committee) has named the flexible path (Anon., 2009). As human spaceflight missions expand further into deep space, the duration of these missions increases to the point where a dedicated crew habitat element appears necessary. There are several destinations included in this flexible path a near Earth asteroid (NEA) mission, a Phobos/Deimos (Ph/D) mission, and a Mars surface exploration mission that all include at least a portion of the total mission in which the crew spends significant periods of time (measured in months) in the deep space environment and are thus candidates for a dedicated habitat element. As one facet of a number of studies being conducted by the Human Spaceflight Architecture Team (HAT) a workshop was conducted to consider how best to define and quantify habitable volume for these future deep space missions. One conclusion reached during this workshop was the need for a description of the scope and scale of these missions and the intended uses of a habitat element. A group was set up to prepare a concept of operations document to address this need. This document describes a concept of operations for a habitat element used for these deep space missions. Although it may eventually be determined that there is significant overlap with this concept of operations and that of a habitat destined for use on planetary surfaces, such as the Moon and Mars, no such presumption is made in this document.
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As a boundary layer physicist, I appreciate the contributions from the Chef Meteorologist.
Wonderful planning document. I noticed that the hybrid approach (pg 18) could also work for a lunar mission, at least an orbital station, if not a lander and hope that future detailed planning would incorporate a moon orbit/lander configuration. I also noted with pride that NTR is well positioned in NASA thinking for deep space missions.
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Great document but it looks like NASA is once again moving away from a zero pre-breathe suit. :-\
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Interesting little note from page 17 (see attached screenshot)
So, Orion will have to beefed-up TPS-wise to survive re-entry from a NEA mission.
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I'm curious as to why the Phobos/Deimos mission can go as close as Venus but the Mars surface mission cannot go inside Earth's orbit... Kind of constrains the mission classes (i.e. no short-stay) pretty severely. I mean, it's fine if you're ONLY looking at long stay missions, but overly constraining otherwise.
Also, if 4 crew are adequate for a Phobos/Deimos mission, why are 6 needed for a Mars surface mission? I understand it's helpful for comparison purposes (since many architectures already assumed that), but it's still kind of weird. I think that if 4 crew are sufficient, we should probably stick to only 4 crew for the first time... Mars surface missions are far off enough as it is...
I like the real similarities between NEA missions and Phobos/Deimos missions... Should be able to use the same exploration stack for the most part (though short-stay would need extra delta-v... at least 8km/s total versus probably 7km/s or less for NEA... and long-stay would need considerably more consumables and probably extra space). I sort of wish more thought was given to make the Mars surface mission more integrated with the Phobos/Deimos mission... With pre-placed assets like an orbiting lander and an already-ISRU-fueled ascent vehicle on the surface, a Phobos/Deimos short-stay mission (which would be just 50% longer than the standard NEA mission) could become a Mars surface mission. All it'd take is pre-placing assets (perhaps by SEP slow-boat) with an extra couple launches (doesn't even necessarily need to be with SLS, though if it's around, that'd likely be used), a couple of modest SEP tugs, a couple landers with an ISRU package. You could even skip the advanced EDL if you had to and rely mostly on propulsive landing (though I wouldn't recommend).
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Interesting little note from page 17 (see attached screenshot)
So, Orion will have to beefed-up TPS-wise to survive re-entry from a NEA mission.
Or switch to PICA.
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The only ones I can see them actually doing with this setup is the NEA and short stay Phobos and Demos mission as they keep the microgravity time close to something we know we can deal with.
I'm going by the condition Valeri Polyakov was in after a 437 day stay on Mir.
They probably could hopped up the delta V end to get it back under the 437 to 450 day limit.
The long stay missions you'll almost certainly need something more along the lines of Nautilus-X or a large pre landed hab on Mars.
Mars at least has some gravity and some protection from radiation which makes a long stay there less dicey then in Mars orbit.
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The only ones I can see them actually doing with this setup is the NEA and short stay Phobos and Demos mission as they keep the microgravity time close to something we know we can deal with.
I'm going by the condition Valeri Polyakov was in after a 437 day stay on Mir.
They probably could hopped up the delta V end to get it back under the 437 to 450 day limit.
The long stay missions you'll almost certainly need something more along the lines of Nautilus-X or a large pre landed hab on Mars.
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Not necessarily... We're finally starting to test some (anti-osteoporosis) drugs on ISS which could allow extension beyond the 437 days of Valeri's trip. And providing enough delta-v is VERY difficult for anything shorter than the trip times they give for a short-stay Mars orbit mission. There's also the possibility of getting some benefit out of very small, "human-powered" centrifuges which could easily fit in a deep space hab. Also, we've learned more about exercise techniques since then. I agree the long-stay missions will need probably 50%+ more volume (partly for habitable space partly for more consumables).
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Are there any studies about converting ESA ATV into a Hab module? Seems like a perfect candidate for such a mission.
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Are there any studies about converting ESA ATV into a Hab module? Seems like a perfect candidate for such a mission.
Too small. 4 to 6 people in a can for a year need space - lots of it.
Maybe as one module among many, but alone - no.
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{snip}
I like the real similarities between NEA missions and Phobos/Deimos missions... Should be able to use the same exploration stack for the most part (though short-stay would need extra delta-v... at least 8km/s total versus probably 7km/s or less for NEA... and long-stay would need considerably more consumables and probably extra space). I sort of wish more thought was given to make the Mars surface mission more integrated with the Phobos/Deimos mission... With pre-placed assets like an orbiting lander and an already-ISRU-fueled ascent vehicle on the surface, a Phobos/Deimos short-stay mission (which would be just 50% longer than the standard NEA mission) could become a Mars surface mission. All it'd take is pre-placing assets (perhaps by SEP slow-boat) with an extra couple launches (doesn't even necessarily need to be with SLS, though if it's around, that'd likely be used), a couple of modest SEP tugs, a couple landers with an ISRU package. You could even skip the advanced EDL if you had to and rely mostly on propulsive landing (though I wouldn't recommend).
Or the Phobos/Deimos mission creates a base for the Mars mission that follows. Possible including a hanger for the reusable lander.
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The only ones I can see them actually doing with this setup is the NEA and short stay Phobos and Demos mission as they keep the microgravity time close to something we know we can deal with.
I'm going by the condition Valeri Polyakov was in after a 437 day stay on Mir.
They probably could hopped up the delta V end to get it back under the 437 to 450 day limit.
The long stay missions you'll almost certainly need something more along the lines of Nautilus-X or a large pre landed hab on Mars.
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Not necessarily... We're finally starting to test some (anti-osteoporosis) drugs on ISS which could allow extension beyond the 437 days of Valeri's trip. And providing enough delta-v is VERY difficult for anything shorter than the trip times they give for a short-stay Mars orbit mission. There's also the possibility of getting some benefit out of very small, "human-powered" centrifuges which could easily fit in a deep space hab. Also, we've learned more about exercise techniques since then. I agree the long-stay missions will need probably 50%+ more volume (partly for habitable space partly for more consumables).
I have to say even with those findings 600 days will likely be the max as most of those have been found to be at best only partially effective.
If anything the experience since then and testing of those solutions has only reaffirmed there is a limit and you'll need AG for anything more then 500 days.
As for getting under 437 days it's easier then you think as seen in the Ride report.
In fact it may be the easiest solution at the moment.
http://www.astronautix.com/craft/rideport.htm
After that tethers would be second.
I think they're either not really serious about this plan ie it's mostly an attempt at PR to sell SLS or it's simply unfinished at this stage and will evolve over time as they refine it and fix the glaring issues.
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Are there any studies about converting ESA ATV into a Hab module? Seems like a perfect candidate for such a mission.
Too small. 4 to 6 people in a can for a year need space - lots of it.
Maybe as one module among many, but alone - no.
Agreed, but could you not stack 3-4 or however many you need of them together and attach an Orion on one of the ends?
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Are there any studies about converting ESA ATV into a Hab module? Seems like a perfect candidate for such a mission.
Too small. 4 to 6 people in a can for a year need space - lots of it.
Maybe as one module among many, but alone - no.
Agreed, but could you not stack 3-4 or however many you need of them together and attach an Orion on one of the ends?
DSH would do a better job for much less mass.
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Are there any studies about converting ESA ATV into a Hab module? Seems like a perfect candidate for such a mission.
Too small. 4 to 6 people in a can for a year need space - lots of it.
Maybe as one module among many, but alone - no.
Agreed, but could you not stack 3-4 or however many you need of them together and attach an Orion on one of the ends?
DSH would do a better job for much less mass.
A transhab type module would have still better mass numbers.
Maybe something like a BA-330 but with two redundant pressurized sections or two Sun dancer sized modules.
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Are there any studies about converting ESA ATV into a Hab module? Seems like a perfect candidate for such a mission.
Too small. 4 to 6 people in a can for a year need space - lots of it.
Maybe as one module among many, but alone - no.
Agreed, but could you not stack 3-4 or however many you need of them together and attach an Orion on one of the ends?
DSH would do a better job for much less mass.
Hmm are you referring to this little beauty?
http://www.nasa.gov/multimedia/videogallery/index.html?media_id=85841491
It looks more for planetary missions than long space travel, but I could be wrong. The video was pretty cool though :P
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Are there any studies about converting ESA ATV into a Hab module? Seems like a perfect candidate for such a mission.
Too small. 4 to 6 people in a can for a year need space - lots of it.
Maybe as one module among many, but alone - no.
Agreed, but could you not stack 3-4 or however many you need of them together and attach an Orion on one of the ends?
DSH would do a better job for much less mass.
Hmm are you referring to this little beauty?
http://www.nasa.gov/multimedia/videogallery/index.html?media_id=85841491
It looks more for planetary missions than long space travel, but I could be wrong. The video was pretty cool though :P
Looks a lot like Zurbin's Mars Hab that's supposed to be modular in that you could dock multiple units together building a Mars base.
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For long duration missions just build the Nautilus
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Are there any studies about converting ESA ATV into a Hab module? Seems like a perfect candidate for such a mission.
Too small. 4 to 6 people in a can for a year need space - lots of it.
Maybe as one module among many, but alone - no.
Agreed, but could you not stack 3-4 or however many you need of them together and attach an Orion on one of the ends?
DSH would do a better job for much less mass.
Hmm are you referring to this little beauty?
http://www.nasa.gov/multimedia/videogallery/index.html?media_id=85841491
It looks more for planetary missions than long space travel, but I could be wrong. The video was pretty cool though :P
That does look cool. Nice to see ATHLETE at work. JPL is awesome.
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For long duration missions just build the Nautilus
I agree for a long mission Nautilus-X is probably the best option.
Now this would be a good starting point to work towards Nautilus sorta like how Gemini lead to Apollo.
Start with easier missions such as close NEOs six month mission time and sprint class Mars missions and Venus flybys.
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For long duration missions just build the Nautilus
I agree for a long mission Nautilus-X is probably the best option.
Now this would be a good starting point to work towards Nautilus sorta like Gemini lead to Apollo.
Start with easier missions such as close NEOs six month mission time and sprint class Mars missions and Venus flybys.
Nautilus X is probably too massive. It'd be quite limited in what sort of missions it could do with a reasonable sized chemical EDS. And the centrifuge they showed probably has a whole host of technical issues just waiting to rear their heads (not that they couldn't be solved, but it'd be difficult). Something built out of more ISS or Space Station Freedom-sized chunks (at least for the core) would probably be better, since it'd allow earlier and cheaper access to more destinations and more delta-v for the same EDS.
The Deep Space Habitat needs to be right-sized in order to ever get off the ground, just like the Apollo mission stack was. It'll have to go through a very large delta-v, so you have to be very frugal with mass, more than you normally would for something in LEO.
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For long duration missions just build the Nautilus
I agree for a long mission Nautilus-X is probably the best option.
Now this would be a good starting point to work towards Nautilus sorta like Gemini lead to Apollo.
Start with easier missions such as close NEOs six month mission time and sprint class Mars missions and Venus flybys.
Nautilus X is probably too massive. It'd be quite limited in what sort of missions it could do with a reasonable sized chemical EDS. And the centrifuge they showed probably has a whole host of technical issues just waiting to rear their heads (not that they couldn't be solved, but it'd be difficult). Something built out of more ISS or Space Station Freedom-sized chunks (at least for the core) would probably be better, since it'd allow earlier and cheaper access to more destinations and more delta-v for the same EDS.
The Deep Space Habitat needs to be right-sized in order to ever get off the ground, just like the Apollo mission stack was. It'll have to go through a very large delta-v, so you have to be very frugal with mass, more than you normally would for something in LEO.
Exactly, thats why I brought up stacking bunch of ATVs together. Seems (in my limited knowledge) like the most direct path forward for NEO and Mars sprint missions while adding another partner into the fold.
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The BA-330 already is close that what you'd need for a sprint class mission I think it would be a good starting point.
Take that and figure out what changes would be needed for deep space.
Unlike an ISS module it is a fully self contained space station.
It's only 25,000kg you wouldn't even need depots though I'd get depot technology down before even attempting a Mars mission.
A lot of the same technology needed for depots is needed to keep the propellant for the return trip.
Plus it frees the EDS max fueled mass from the LEO payload limits of the LV.
For long stay missions you're pretty much driven to a Zurbin Mars direct or Nautilus-X architecture depending where you're planning on waiting for the next alignment.
Mars surface is easier in some ways as you don't need fully closed loop life support as there is ice,soil, and atmosphere to be used.
Demos and Phobos can provide some volatiles as well but you'll be processing it in near zero g.
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ATVs have about half as much pressurized volume for about the same mass as other (ISS) modules have. Not a very efficient way to provide livable space.
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The BA-330 already is close that what you'd need for a sprint class mission I think it would be a good starting point.
Take that and figure out what changes would be needed for deep space.
Unlike an ISS module it is a fully self contained space station.
It's only 25,000kg you wouldn't even need depots though I'd get depot technology down before even attempting a Mars mission.
{snip}
A lot of the facilities of a spacestation can be tested for several years unmanned. These include temperature control, electrical power generation and ensuring that it stays air tight. The rest of the life support and hygiene can wait for the people to arrive.
An Atlas V or Delta IV Heavy can lift a BA-330 to LEO. Some sort of upper stage would be needed to lift it to high Earth orbit. This stage could use a chemical thruster or SEP.
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For long duration missions just build the Nautilus
I agree for a long mission Nautilus-X is probably the best option.
Now this would be a good starting point to work towards Nautilus sorta like Gemini lead to Apollo.
Start with easier missions such as close NEOs six month mission time and sprint class Mars missions and Venus flybys.
Nautilus X is probably too massive. It'd be quite limited in what sort of missions it could do with a reasonable sized chemical EDS. And the centrifuge they showed probably has a whole host of technical issues just waiting to rear their heads (not that they couldn't be solved, but it'd be difficult). Something built out of more ISS or Space Station Freedom-sized chunks (at least for the core) would probably be better, since it'd allow earlier and cheaper access to more destinations and more delta-v for the same EDS.
The Deep Space Habitat needs to be right-sized in order to ever get off the ground, just like the Apollo mission stack was. It'll have to go through a very large delta-v, so you have to be very frugal with mass, more than you normally would for something in LEO.
Exactly, thats why I brought up stacking bunch of ATVs together. Seems (in my limited knowledge) like the most direct path forward for NEO and Mars sprint missions while adding another partner into the fold.
A bunch of stacked ATVs are way heavier than one inflatable with the same volume.
BA-330 for DSH!!
Swallow: Falcon Heavy could launch BA-330 to HEO.
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For long duration missions just build the Nautilus
I agree for a long mission Nautilus-X is probably the best option.
Now this would be a good starting point to work towards Nautilus sorta like Gemini lead to Apollo.
Start with easier missions such as close NEOs six month mission time and sprint class Mars missions and Venus flybys.
Nautilus X is probably too massive. It'd be quite limited in what sort of missions it could do with a reasonable sized chemical EDS. And the centrifuge they showed probably has a whole host of technical issues just waiting to rear their heads (not that they couldn't be solved, but it'd be difficult). Something built out of more ISS or Space Station Freedom-sized chunks (at least for the core) would probably be better, since it'd allow earlier and cheaper access to more destinations and more delta-v for the same EDS.
The Deep Space Habitat needs to be right-sized in order to ever get off the ground, just like the Apollo mission stack was. It'll have to go through a very large delta-v, so you have to be very frugal with mass, more than you normally would for something in LEO.
Exactly, thats why I brought up stacking bunch of ATVs together. Seems (in my limited knowledge) like the most direct path forward for NEO and Mars sprint missions while adding another partner into the fold.
A bunch of stacked ATVs are way heavier than one inflatable with the same volume.
BA-330 for DSH!!
Swallow: Falcon Heavy could launch BA-330 to HEO.
Ya I understand it's not the most efficient hab, but creating a DSH or BA-330 is far from reality at this point. Just looking at things that will work. But I guess we wouldn't really need one until we're going to Mars any way.
What does a mission to a NEO look like any way, similar to lunar?
Edit: ah from one of Chris's articles it looks like two Orions, MMSEV and DSH. Seems like an awful lot still needs to be developed before we can even attempt NEO. Perhaps thats the reason for more lunar emphasis as of late?
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For long duration missions just build the Nautilus-X.
I agree for a long mission Nautilus-X is probably the best option.
Now this would be a good starting point to work towards Nautilus sorta like Gemini lead to Apollo.
Start with easier missions such as close NEOs six month mission time and sprint class Mars missions and Venus flybys.
Nautilus X is probably too massive.
I disagree. It's not big enough. It needs to be at least double that size.
This needs to be a general purpose deep space vehicle, not a mission specific design. Designing a spacecraft for a specific mission is something we have to get over. That was fine when our capabilities were extremely limited. You *had* to do it that way to make sure you could do the mission. But our capabilities have grown well beyond that now. We are now setting out on a path of general exploration, going to many different destinations in several different places in the solar system. We need a general purpose spacecraft design that can handle a large crew, perhaps as many as 12. And we need a small fleet of them.
While the trucking industry still design specific-purpose trucks, their largest truck production facilities are in the long-haul general purpose heavy vehicles, because they don't have any idea what the people will use them for except for long haul heavy trucking. NASA needs to *begin* to adopt the same mindset. We will always have a need for small one-of spacecraft, but NASA's main deep space spacecraft future lies in fleets of general purpose heavy haulers. Nautilus-X is a good place to start but will prove to be too small for general purpose exploration. It's time to put the harbor sloop away and move to the ocean-going sailing ship.
And just to stir this pot up some more it's also time to get away from chemical propulsion for these deep space designs. Go VASIMR or NTR for the MPS.
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A modular approach using ISS-like modules means you don't have to design everything all over again for each mission.
I agree going with high-Isp propulsion (especially electric propulsion) would remove a lot of mass constraints, but I'm pretty sure that designing a spacecraft for 12 occupants to deep space is getting a little ahead of ourselves. It's not needed even when we have a Martian base, only once we want to start a settlement.
Designing it for 12 people would mean it's far less likely to ever happen. It'd push off the first NEA mission (and the first Mars mission) considerably to the right. The closer to the left in schedule a mission is, the greater chance it has of avoiding cancelation (and cost over-runs, etc). We shouldn't need a spacecraft bigger than ISS just to go to a NEA. With a smaller, 60mT or less exploration stack (Mars version of Orion, SEV, DSH), a 260mT CPS could launch it to multiple different NEAs, and if staged from HEO or EML1 or EML2, it would have enough delta-v to push that whole stack through the 8+km/s needed for a short-stay Mars mission. With a 150-200mT exploration stack, you'd need a far bigger CPS (quite possibly requiring orbital assembly) or high-isp propulsion would be on the critical path (and you'd need a much larger high-Isp propulsion stage than for the much smaller 50-60mT 4 person exploration stack).
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There are two points I want to make.
Rotating sections
Although I agree that rotating sections to provide simulated gravity are technically challenging, I think they're more-or-less essential for long-duration spacecraft or space stations. There are two main reasons for this.
Firstly, contribution to crew health. If the crew can sleep and spend much of their working time in a macro gravity environment (say, Mars-equivalent or higher), then muscle and bone mass loss will be reduced. Now, admittedly we don't yet have models for how much that loss will be reduced by, but that's partly because no-one's done the experiments...
Secondly, reduction in consumables and trash generation. If you have a macro gravity environment, it's much easier to e.g. wash and dry clothes and eating utensils, cook with bulk ingredients rather than needing everything in individual packages, etc.
Propulsion and power
Propulsion is a major challenge. It's desirable to have very high Isp, to maximise the mass fraction of your spacecraft, but the power consumption of the spacecraft is important to consider as well. The ISS requires enormous solar arrays to function, and it doesn't have an huge, constant propulsion power requirement.
There are a lot of trade-offs to be made between specific impulse and specific mass, and I don't think anyone's found the "right answer" yet. VASIMR seems to be an unhappy compromise to me; it scales well, but it has significantly lower Isp and lower power efficiency (~60%) than other electric propulsion systems (e.g. HiPEP, which manages ~80%).
Studies done by Ad Astra on VASIMR-based Mars mission designs seem to assume power supplies that achieve 4 kW/kg, and I'm pretty certain that can't be achieved with solar power in Mars orbit; it seems like either we need to have a breakthrough on extremely lightweight solar cells and structures for microgravity/low gravity applications, or we need to actually get on and build fission reactors for space applications...
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Studies done by Ad Astra on VASIMR-based Mars mission designs seem to assume power supplies that achieve 4 kW/kg, and I'm pretty certain that can't be achieved with solar power in Mars orbit; it seems like either we need to have a breakthrough on extremely lightweight solar cells and structures for microgravity/low gravity applications, or we need to actually get on and build fission reactors for space applications...
Which is why I mentioned VASMIR and NTR in the same sentence. I believe we need to have all our electrical power generation done by a nuclear reactor of some type, and use that to power the VASMIR during cruise phase, taking advantage of the high isp and the rapid transit times that makes possible. But do it in such a way that we also have on-demand high thrust NTR for rapid acceleration or deceleration; i.e., entering or leaving orbit and emergency rapid collision avoidance maneuvering.
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For long duration missions just build the Nautilus-X.
I agree for a long mission Nautilus-X is probably the best option.
Now this would be a good starting point to work towards Nautilus sorta like Gemini lead to Apollo.
Start with easier missions such as close NEOs six month mission time and sprint class Mars missions and Venus flybys.
Nautilus X is probably too massive.
I disagree. It's not big enough. It needs to be at least double that size.
This needs to be a general purpose deep space vehicle, not a mission specific design. Designing a spacecraft for a specific mission is something we have to get over. That was fine when our capabilities were extremely limited. You *had* to do it that way to make sure you could do the mission. But our capabilities have grown well beyond that now. We are now setting out on a path of general exploration, going to many different destinations in several different places in the solar system. We need a general purpose spacecraft design that can handle a large crew, perhaps as many as 12. And we need a small fleet of them.
Seems like you're looking at a different budget than everyone else.
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A bunch of stacked ATVs are way heavier than one inflatable with the same volume.
BA-330 for DSH!!
Swallow: Falcon Heavy could launch BA-330 to HEO.
KISS.
When both the BA-330 and Falcon Heavy have been flow separately marrying them will probably be a good idea. 8)
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Studies done by Ad Astra on VASIMR-based Mars mission designs seem to assume power supplies that achieve 4 kW/kg, and I'm pretty certain that can't be achieved with solar power in Mars orbit; it seems like either we need to have a breakthrough on extremely lightweight solar cells and structures for microgravity/low gravity applications, or we need to actually get on and build fission reactors for space applications...
Which is why I mentioned VASMIR and NTR in the same sentence. I believe we need to have all our electrical power generation done by a nuclear reactor of some type, and use that to power the VASMIR during cruise phase, taking advantage of the high isp and the rapid transit times that makes possible. But do it in such a way that we also have on-demand high thrust NTR for rapid acceleration or deceleration; i.e., entering or leaving orbit and emergency rapid collision avoidance maneuvering.
Small delta-V, high thrust with space storable propellant. Is the parking thrusting on the SEP best done by NTR or a methane thruster?
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Seems like you're looking at a different budget than everyone else.
To quote a certain well-known Senate Staffer: "It's only money".
Don't let the doom-sayers running amuck inside the beltway fool you. There is more money in this country than you can shake a stick at. If you go into Congress and ask for $20 billion, you'll get $12-$15. If you ask for $30 billion you'll get $20 to $25 and so forth. If this kind of effort required $100 billion then you go in and ask for it. You won't get it all but you will get most of it - guarenteed.
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Studies done by Ad Astra on VASIMR-based Mars mission designs seem to assume power supplies that achieve 4 kW/kg, and I'm pretty certain that can't be achieved with solar power in Mars orbit; it seems like either we need to have a breakthrough on extremely lightweight solar cells and structures for microgravity/low gravity applications, or we need to actually get on and build fission reactors for space applications...
Which is why I mentioned VASMIR and NTR in the same sentence. I believe we need to have all our electrical power generation done by a nuclear reactor of some type, and use that to power the VASMIR during cruise phase, taking advantage of the high isp and the rapid transit times that makes possible. But do it in such a way that we also have on-demand high thrust NTR for rapid acceleration or deceleration; i.e., entering or leaving orbit and emergency rapid collision avoidance maneuvering.
Small delta-V, high thrust with space storable propellant. Is the parking thrusting on the SEP best done by NTR or a methane thruster?
I'd use the reactor in NTR mode. It is *deeply* throttlable.
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Studies done by Ad Astra on VASIMR-based Mars mission designs seem to assume power supplies that achieve 4 kW/kg, and I'm pretty certain that can't be achieved with solar power in Mars orbit; it seems like either we need to have a breakthrough on extremely lightweight solar cells and structures for microgravity/low gravity applications, or we need to actually get on and build fission reactors for space applications...
Which is why I mentioned VASMIR and NTR in the same sentence. I believe we need to have all our electrical power generation done by a nuclear reactor of some type, and use that to power the VASMIR during cruise phase, taking advantage of the high isp and the rapid transit times that makes possible. But do it in such a way that we also have on-demand high thrust NTR for rapid acceleration or deceleration; i.e., entering or leaving orbit and emergency rapid collision avoidance maneuvering.
Another option is to use a large solar concentrator mirror and concentrator type PV. The focused sunlight may be used directly to power a solar thermal rocket for orbit raising to a highly elliptical orbit in a reasonable time v. electric propulsion; and then switch to solar electric after a departure thrust to just over escape velocity. Oxygen injection is also a possibility for thrust increase for the departure thrust.
The concentrator can potentially have specific power much higher than a solar cell array although the power is in the form of light not electricity. This means that at the distance of Mars a larger concentrator is required for a concentrator type PV system than at Earth's distance from the Sun, but this concentrator potentially adds much less mass than having a roughly 2.5 times larger solar cell array. A combination of solar thermal and electric can be used to provide a large range of Isp's like VASIMR and both can use lithium effectively as propellant although solar thermal can use almost anything and hydrogen is of course best.
If the Prometheus project is any indication, whatever advantage reactors confer comes at quite a cost! Also isn't there a problem with using a NTR reactor for electric power generation? It certainly is different in design than reactors designed for electric power production.
Steve Mickler
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....
Studies done by Ad Astra on VASIMR-based Mars mission designs seem to assume power supplies that achieve 4 kW/kg, and I'm pretty certain that can't be achieved with solar power in Mars orbit...
It actually can:
EDIT:forgot the link! http://www.spacefuture.com/archive/early_commercial_demonstration_of_space_solar_power_using_ultra_lightweight_arrays.shtml
"Manufacturing capability is critical. Ultra-lightweight carbon fiber-reinforced polymer, thin-film solar cell (CFRP TFSC) arrays provide, low payload volume and low payload mass space solar arrays that are the key enabling technology for SPS. The ability to make such solar arrays has been demonstrated at laboratory scale in the form of record power density 4300 W/kg, 9.5% stabilized efficiency AM0 (1357 Standard) 122 W/m2 thin-film solar cells on 6 micron thickness CP1 polyimide superstrates (polymer film towards the light).
In-space conditions offer temperatures which have been shown to self-repair a-Si cells by heat annealing. These same 6 micron thickness a-Si:H TFSC, which are 9.5% efficient under room temperature conditions, are 12.4% efficient and will provide 5950 W/kg and 170 W/m2 under 100oC space heat annealing self-repair conditions. Depositing these TFSC on 2 micron thickness CP1 or CORIN polyimide will reduce the superstrate weight by 2/3 and increase the area per kilogram of the array to approximately 100 m² per kilogram. 100 m² of TFSC that produces 170 Watts/m² predicts bare solar arrays with power density 17,000 Watts/kilogram."
and
"2007-2008 R&D plans at Welsom includes the deposition TFSC on 2 micron thickness CP1/CORIN. TFSC on 2 microns thickness polyimide superstrates and 15.2% efficient TFSC expected under space conditions of heat annealing and self-repair, predict the manufacture of record power density solar arrays in excess of 10,000 Watts/kilogram."
(you need ~8000W/kg at Earth to equal ~4000W/kg at Mars, of course)
Now, there are improvements in efficiency all the time. These figures are for ~9-15% efficient cells. Once efficiency improves (and there should be room for considerable improvement in efficiency since it's much lower than state of the art) in a few decades, I wouldn't be surprised if you didn't get 30-45% efficiency, so you could get even higher than 10kW/kg at Earth.
There is a ways to get there, but it seems well within the bounds of possibility (and actually, IKAROS demonstrated recently an equivalent specific power of about ~1kW/kg at 1AU without a lot of attention given to either efficiency or using the very lightest sort of solar array/sail material).
In 20-30 years, 8kW/kg at 1AU is feasible, IMO.
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The VASIMR paper assumed a 12MW power source. At 675W/m² of solar energy density at Mars distance, and assuming an efficiency of 9%, that's an effective energy input of 60.75W/m². So, you'd need around 200,000m² of effective light gathering surface, i.e. assuming no losses due to light incidence angle. That's roughly a 500m x 500m solar panel or a circle of 520m in diameter. Kind of too big, isn't it?
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The VASIMR paper assumed a 12MW power source. At 675W/m² of solar energy density at Mars distance, and assuming an efficiency of 9%, that's an effective energy input of 60.75W/m². So, you'd need around 200,000m² of effective light gathering surface, i.e. assuming no losses due to light incidence angle. That's roughly a 500m x 500m solar panel or a circle of 520m in diameter. Kind of too big, isn't it?
Why is it too big? Surely you should have plenty of... ahem... space! ;)
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The VASIMR paper assumed a 12MW power source. At 675W/m² of solar energy density at Mars distance, and assuming an efficiency of 9%, that's an effective energy input of 60.75W/m². So, you'd need around 200,000m² of effective light gathering surface, i.e. assuming no losses due to light incidence angle. That's roughly a 500m x 500m solar panel or a circle of 520m in diameter. Kind of too big, isn't it?
Yes it is big, but not impossibly big. When the ship is in LEO you may even be able to see the array from the ground.
Solar panels fold up so only a few launches will be needed to lift the array to orbit.
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The VASIMR paper assumed a 12MW power source. At 675W/m² of solar energy density at Mars distance, and assuming an efficiency of 9%, that's an effective energy input of 60.75W/m². So, you'd need around 200,000m² of effective light gathering surface, i.e. assuming no losses due to light incidence angle. That's roughly a 500m x 500m solar panel or a circle of 520m in diameter. Kind of too big, isn't it?
Yes it is big, but not impossibly big. When the ship is in LEO you may even be able to see the array from the ground.
Solar panels fold up so only a few launches will be needed to lift the array to orbit.
Have you made structural considerations? Pointing considerations? The effects of solar flux push? It's not that it can't be done. The question is if it can be done while keeping the density above 4kW/kg. You might be able to get it with a tension design, but you'd probably need to discard some structured needed during assembly. And you'd need a ship about that long. And the whole structure must be kept pointing to the Sun, else you'd get incidence angle losses.
Then, there's the issue of power distribution. You might have cells, and even panels, with that density. But you not only need a frame, but to transport the electricity. Please remember that you want to transport some 12MW. Let's say that we distribute in 96 segments, that's 125kW at 120V (the natural voltage of a cell is 0.5V, but let's say we group them in 240 cells), that's 1,041 Ampere :o. Are you aware of the section that you'd need? What super material will you use that won't weight a lot? If you use a transformer, you'd have to add a lot of weight. And you can't keep it like a cloth, but will have to add structural elements. Individual lightweight cells, even panels, are easy. Whole power systems, are not.
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The VASIMR paper assumed a 12MW power source. At 675W/m² of solar energy density at Mars distance, and assuming an efficiency of 9%, that's an effective energy input of 60.75W/m². So, you'd need around 200,000m² of effective light gathering surface, i.e. assuming no losses due to light incidence angle. That's roughly a 500m x 500m solar panel or a circle of 520m in diameter. Kind of too big, isn't it?
Yes it is big, but not impossibly big. When the ship is in LEO you may even be able to see the array from the ground.
Solar panels fold up so only a few launches will be needed to lift the array to orbit.
Have you made structural considerations? Pointing considerations? The effects of solar flux push? It's not that it can't be done. The question is if it can be done while keeping the density above 4kW/kg. You might be able to get it with a tension design, but you'd probably need to discard some structured needed during assembly. And you'd need a ship about that long. And the whole structure must be kept pointing to the Sun, else you'd get incidence angle losses.
Then, there's the issue of power distribution. You might have cells, and even panels, with that density. But you not only need a frame, but to transport the electricity. Please remember that you want to transport some 12MW. Let's say that we distribute in 96 segments, that's 125kW at 120V (the natural voltage of a cell is 0.5V, but let's say we group them in 240 cells), that's 1,041 Ampere :o. Are you aware of the section that you'd need? What super material will you use that won't weight a lot? If you use a transformer, you'd have to add a lot of weight. And you can't keep it like a cloth, but will have to add structural elements. Individual lightweight cells, even panels, are easy. Whole power systems, are not.
Yeah, you very well may need a higher voltage than just 120V. Also, there are conductors which could be considerably lighter than copper (for the same resistivity and current), such as those made from Lithium (or is it Sodium or Calcium?). Even Aluminum is lighter than equivalent copper wires.
There's also the possibility (in the further future) of superconducting wires able to have a far higher current carrying capacity (and this keeps increasing as we make solid state physics advances) for the same mass (all inclusive) as copper. Nanotubes can also reportedly carry a lot of current for their mass (though this is still a research area, not yet practical).
Anyway, this is wandering off-topic, but the point is that solar arrays can have pretty darned good specific power inside the asteroid belt (and, as Juno is showing, usable power to Jupiter at least), and that they have lots of room to grow in that area ten or even a hundred times better than current state of the art (which is ~100-150 W/kg at 1AU). Even 4kW/kg at 1.5 AU (i.e. Mars) can't be dismissed out of hand.
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Secondly, reduction in consumables and trash generation. If you have a macro gravity environment, it's much easier to e.g. wash and dry clothes and eating utensils, cook with bulk ingredients rather than needing everything in individual packages, etc.
Not really. wash and dry clothes can be done just as easy in zero g.
Eating utensils are not different in zero g.
Bulk cooking is not easier. Requires more ingredients and more logistics. Prepackage is the way to go.
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Not really. wash and dry clothes can be done just as easy in zero g.
Eating utensils are not different in zero g.
Bulk cooking is not easier. Requires more ingredients and more logistics. Prepackage is the way to go.
Can you direct me to any additional information about research that has been done on these topics? I would be fascinated to find out more about it.
I wonder how much mass can be saved on toilet consumables because gravity permits the use of a "normal" toilet... :)
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As for bulk cooking, look at the size of army formation that requires a cook
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Well, my 100 man tank company had 5 cooks. My 72 man tank company had 3 cooks. Of course, I didn't control it directly, it was the S-4's (supply officer's job) and in the field we usually got one hot meal a day (and it was usually a "B" ration which meant generlly canned food).
The other two meals were either C-rations (early) or MRE's (Meal Ready to Eat, which was sometimes thought to be a triple like).
In an assembly area or rest area, you might get two hot meals a day and sometimes they would be "A"s - real food. Usally breakfast and dinner/supper.
Today there are probably even fewer cooks and the hot meal is likely to be a unit ration, sort of like airplane food used to be.
So, I don't see a need for a dedicated cook, but make sure you bring hot sauce and spiced to vary the taste of the food!
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British infantry squad firebases in Helmand (squads of 8-10 men IIRC) have cooked food (they don't eat from ratpacks). They don't have a dedicated cook (squaddies take turns). But I don't recall ever suggesting that a Mars mission should have a ships cook, just that there should be a galley. :)
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British infantry squad firebases in Helmand (squads of 8-10 men IIRC) have cooked food (they don't eat from ratpacks). They don't have a dedicated cook (squaddies take turns). But I don't recall ever suggesting that a Mars mission should have a ships cook, just that there should be a galley. :)
A galley? A tiny refrigerator/freezer, a food warmer, a tiny microwave, and a tiny toaster oven would be fine, perhaps also an area to wash stuff. It could all fit in a cube 2 feet on a side. Lots of those are also useful for repairs or sample storage or drug storage or tool/utensil sterilization, so they may either serve dual purpose or a similar non-food set may also be needed.
But not a fully galley. Food preparation from bulk food also is quite time-consuming, and the crew will need all the time they can get, at least once at the site.
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A galley? A tiny refrigerator/freezer, a food warmer, a tiny microwave, and a tiny toaster oven would be fine, perhaps also an area to wash stuff. It could all fit in a cube 2 feet on a side. Lots of those are also useful for repairs or sample storage or drug storage or tool/utensil sterilization, so they may either serve dual purpose or a similar non-food set may also be needed.
Sound like exactly what I had in mind. :)
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I think there's room, and arguably a place for pre-packaged, semi-bulk, and possibly bulk food store and preparation. While pre-packaged has a number of advantages, you're stuck with whatever was put in the pantry at launch.
THe addition of fresh ingredients and meal variety has been the subject of some NASA research, and part of the rationale is crew morale. Semi-bulk or bulk allows a potentially far greater variety. While not necessarily for every day, maybe a couple days a week or a few days a month is appropriate.
A zero-g bread, rice, bean, etc. cooker (semi-bulk or bulk) sounds feasible (altho I admit I've never considered the problem), and should require very little crew time. Stock the pantry with some pre-packaged ingredients and spices to add to the pot and vary the results..
How about some fresh-baked bread to make that umpteenth pre-packaged eggs-and-sausage, oatmeal or whatever breakfast more palatable? Maybe some fresh-from-the-pot beans or rice (with a dash of cumin and red pepper at your option) to make that 50th Salisbury steak a bit more attractive?
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A galley? A tiny refrigerator/freezer, a food warmer, a tiny microwave, and a tiny toaster oven would be fine, perhaps also an area to wash stuff. It could all fit in a cube 2 feet on a side. Lots of those are also useful for repairs or sample storage or drug storage or tool/utensil sterilization, so they may either serve dual purpose or a similar non-food set may also be needed.
Sound like exactly what I had in mind. :)
That isnt for bulk cooking
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British infantry squad firebases in Helmand (squads of 8-10 men IIRC) have cooked food (they don't eat from ratpacks). They don't have a dedicated cook (squaddies take turns). But I don't recall ever suggesting that a Mars mission should have a ships cook, just that there should be a galley. :)
They don't have the manpower to spare to prepare bulk food like your example
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What exactly is the manpower being used for in the 180 days it will take a crew to get to Mars?
They can't cook one meal?
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What exactly is the manpower being used for in the 180 days it will take a crew to get to Mars?
They can't cook one meal?
With the shuttle and ISS they don't cook meals, they warm them up.
One crew member is given the task of making the meal for the crew(and this job rotates among the crew). Space food is designed to need as little prep. as possible. The bigger issuse is clean up after the meal (stuff floats everywhere) and there are issuses of waste disposal (one upgrade to the shuttle gave the shuttle a garbage compactor).
In space cooking could pose problems (i.e. gravity is quite nice at keeping things contained....). I think for moral reason they might but don't expect complex dishes.
For instance white bread is not allowed due to the crumbs it generates tortillas are prefered.
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In space cooking could pose problems (i.e. gravity is quite nice at keeping things contained....). I think for moral reason they might but don't expect complex dishes.
Well, getting back to the point, I was arguing that one advantage of providing a centrifugal section for a Mars transit habitat would be the ability to cook real food, with the concomitant crew morale benefits. I still think that the health/acclimatisation benefits would turn out to be the more important factor, however.
I'm still waiting for Jim to back up his supremely confident assertions w.r.t. zero-g space laundry and the packaging efficiency of prepackaged food vs. bulk cooked food. It sounds like there's been some research/experiments done that I'm not aware of, and as I said, I am genuinely interested to learn more...
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Not really. wash and dry clothes can be done just as easy in zero g.
Eating utensils are not different in zero g.
Bulk cooking is not easier. Requires more ingredients and more logistics. Prepackage is the way to go.
Can you direct me to any additional information about research that has been done on these topics? I would be fascinated to find out more about it.
I wonder how much mass can be saved on toilet consumables because gravity permits the use of a "normal" toilet... :)
I once saw a concept for a zero G clothes washer proposed for ISS.
As for a toilet they have much better units then the one that went to ISS.
Still for any long 500+ day mission sans landing a centrifuge is probably a necessity.
Really though if you going to hang around Mars for any length of time you might as well land on it and make use of the resources there.
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Gravity just makes a lot of things easier. No need to have a fan blowing on your face to prevent you waking up choking in a bubble of your own CO2. Surgery is easier: an astronaut-doctor used to practicing on Earth will have a much easier time of it. No head restraint because your carotid artery bounces your head around when you're trying to sleep. You can actually taste your reconstituted salisbury steak because your head isn't bunged up with fluid.
No explosive flatulence.
Yes, they are astronauts who can suck it up and take risks (most forumites I bet wouldn't hesitate to spend 500 days in zero G if it meant just being in space) but we also need data on AG anyway if we're going out any further than Mars. Also, prolonged exposure to low gravity. That data is in itself valuable.
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In space cooking could pose problems (i.e. gravity is quite nice at keeping things contained....). I think for moral reason they might but don't expect complex dishes.
Well, getting back to the point, I was arguing that one advantage of providing a centrifugal section for a Mars transit habitat would be the ability to cook real food, with the concomitant crew morale benefits. I still think that the health/acclimatisation benefits would turn out to be the more important factor, however.
I'm still waiting for Jim to back up his supremely confident assertions w.r.t. zero-g space laundry and the packaging efficiency of prepackaged food vs. bulk cooked food. It sounds like there's been some research/experiments done that I'm not aware of, and as I said, I am genuinely interested to learn more...
Space laundry looks like a front loading washer. there was one designed for the ISS.
Every Mars mission is planned with no centrifuge. Too short of duration to need one.
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In space cooking could pose problems (i.e. gravity is quite nice at keeping things contained....). I think for moral reason they might but don't expect complex dishes.
Well, getting back to the point, I was arguing that one advantage of providing a centrifugal section for a Mars transit habitat would be the ability to cook real food, with the concomitant crew morale benefits. I still think that the health/acclimatisation benefits would turn out to be the more important factor, however.
I'm still waiting for Jim to back up his supremely confident assertions w.r.t. zero-g space laundry and the packaging efficiency of prepackaged food vs. bulk cooked food. It sounds like there's been some research/experiments done that I'm not aware of, and as I said, I am genuinely interested to learn more...
Space laundry looks like a front loading washer. there was one designed for the ISS.
Every Mars mission is planned with no centrifuge. Too short of duration to need one.
Well that is not entirely correct most Mars mission concepts do have some sort AG usually a tether or in the case of the last serious study the BNTR concept the entire ship acted as a centrifuge during cruise.
Also Mars direct used the old discarded departure stage as a counter weight for ag.
DSH is extremely early and what is shown probably will never fly let alone go to Mars.
The data points that do exist for very long space flights suggest prolonged zero gravity should be avoided.
It's also why I don't see DSH as a serious concept and instead as mostly a political ploy.
If the money for a Mars mission shows up then we may see something that can really go there such as BNTR or Nautilus-X.
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The data points that do exist for very long space flights suggest prolonged zero gravity should be avoided.
It's also why I don't see DSH as a serious concept and instead as mostly a political ploy.
Ploy - "a maneuver or strategem" with the purpose of . . . ?
You're discounting non-AG technologies that could help, such as stretchable suits.
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What responsibilities would the crew have during the cruise phase of a Mars mission? It seems to me that boredom would be a dominant feature of that transit.
Cooking could help relieve the tedium of a transit lasting several months with essentially nothing to do (unless something goes wrong).
On the other hand, once on the surface of Phobos or Mars time spent cooking would be time not available for exploration (or sleeping).
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What responsibilities would the crew have during the cruise phase of a Mars mission? It seems to me that boredom would be a dominant feature of that transit.
An interesting question that has come to mind in connection with lunar outposts also. After you've got done with primary duties (collecting rocks and the like), secondary but vital ones like maintaining the station, what do you? Read, watch movies, play poker, take correspondence courses, surf the Web and so on, I suppose.
I trust that any Mars expedition will have a bunch of terabytes of storage on board and a wide-band link to earth, so their information needs will be pretty well taken care of. Speed of light lag would rule out Skype, but email should be just fine and Google work on local databases and, with lag, those back on Earth.
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Every Mars mission is planned with no centrifuge. Too short of duration to need one.
Well that is not entirely correct most Mars mission concepts do have some sort AG usually a tether or in the case of the last serious study the BNTR concept the entire ship acted as a centrifuge during cruise.
Also Mars direct used the old discarded departure stage as a counter weight for ag.
DSH is extremely early and what is shown probably will never fly let alone go to Mars.
So Jim is wrong again? ;)
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Every Mars mission is planned with no centrifuge. Too short of duration to need one.
Well that is not entirely correct most Mars mission concepts do have some sort AG usually a tether or in the case of the last serious study the BNTR concept the entire ship acted as a centrifuge during cruise.
Also Mars direct used the old discarded departure stage as a counter weight for ag.
DSH is extremely early and what is shown probably will never fly let alone go to Mars.
So Jim is wrong again? ;)
Again?
Actually, Mars Direct was without AG, but it was an option
And I was talking about plans in the last few years, like ESAS
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I'm still waiting for Jim to back up his supremely confident assertions w.r.t. zero-g space laundry and the packaging efficiency of prepackaged food vs. bulk cooked food. It sounds like there's been some research/experiments done that I'm not aware of, and as I said, I am genuinely interested to learn more...
Depends in part on mission duration; e.g., current thermostabilized pre-packaged shelf life can be a problem beyond ~18mo. There have been numerous studies over the last decades on various food packaging options and tradeoffs, from various forms or pre-packaged to bulk. A good (and recent) article with lots-o-references:
Developing the NASA Food System for Long-Duration Missions (http://onlinelibrary.wiley.com/doi/10.1111/j.1750-3841.2010.01982.x/full), Journal of Food Sience, Mar 2011
There is also a project that started in 2010 and is suppose to run for two years to look at various options, including "Trade Study – Food Processing vs. Packaged Food System". Not sure if it's still going but they haven't published anything that I can find; see:
NASA Advanced Food Technology Project (http://www.nasa.gov/centers/johnson/slsd/about/divisions/hefd/project/advanced-foods.html)
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It's drifting off topic a bit to point this out, but just about every terrestrial clothes washing machine uses a centrifuge and artificial gravity during its spin cycle....
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Again?
Actually, Mars Direct was without AG, but it was an option
And I was talking about plans in the last few years, like ESAS
I read some of Zurbin's published reports including several of his books on the subject and a lot depended on findings on whether or not would Mars gravity be enough to get the crew back into shape after the five to six month trip out.
One reason for the later Mars gravity biosatellite concept was to answer those question which we still don't know.
http://en.wikipedia.org/wiki/Mars_Gravity_Biosatellite
I think a Dragon lab should be purchased for Mars gravity research as it might be the cheapest possible way to do that research.
We really don't have any data on the effects of partial g on mammals.
Without that data you pretty much have to error on the side of crew health.
As for the ESAS plans they seemed to be ignoring an entire herd of elephants in the room.
Plus pretty much ignoring the fact orbital rendezvous and assembly is routine now.
In short it looked like a Mars mission concept from the 1970s.
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There have been numerous studies over the last decades on various food packaging options and tradeoffs, from various forms or pre-packaged to bulk. A good (and recent) article with lots-o-references:
Developing the NASA Food System for Long-Duration Missions (http://onlinelibrary.wiley.com/doi/10.1111/j.1750-3841.2010.01982.x/full), Journal of Food Sience, Mar 2011
Quite an interesting paper, with lots of useful numbers. Thanks for pointing it out!
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A couple additional references from the archives that may be of interest, albeit somewhat dated (I need a better memory or a better indexing system).
These include discussion and evaluation of advanced vs. baseline life support technologies (e.g., food, clothing, waste management, etc.) for various mission classes (e.g., ISS, lunar, mars transit, mars surface, etc). The second provides additional info on some of the evaluation metrics and their assumptions and derivations.
Advanced Life Support Research and Technology Development Metric (http://ston.jsc.nasa.gov/collections/TRS/_techrep/CR-2006-213694.pdf), NASA CR-2006-213694, 2006
Advanced Life Support Baseline Values and Assumptions Document (http://ston.jsc.nasa.gov/collections/TRS/_techrep/CR-2004-208941.pdf), NASA CR-2004–208941, 2004
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... If the Prometheus project is any indication, whatever advantage reactors confer comes at quite a cost! Also isn't there a problem with using a NTR reactor for electric power generation? It certainly is different in design than reactors designed for electric power production.
Have a look at the PWR Triton tri-mode reactor concept. Operates at about 1% of the full NTR power mode to produce about 25 kWe. That's a little more than the max continuous output of the Space Shuttle's 3 fuel cells combined.
At 15klbf (30klbf with LOX augmentation) a Deep Space vehicle leaving L1/2 might use several to reduce gravity losses, and increase propulsion & power redundancy later in the mission.
As well as providing DSH power, they could also supply conventional ion drives for course correction and to speed up the trip. IMO a VASIMR is not necessary if you already have a high thrust engine.
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Looking at the pdf in the OP, I'm wondering about the hardware that gets discarded. Sticking to just the NEO missions, at the end, the SEP and the DSH get discarded. It might just be beyond the scope of this paper, but could they be saved? Could they be parked in a high orbit and be available for some future purpose? could the SEP slow down the craft on the return trip to lower that 11.8km/s velocity (while also extending the trip to greater than 210 days however)?