Author Topic: MCT Speculation and Discussion Thread 4  (Read 610757 times)

Online spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #280 on: 06/25/2015 04:18 PM »
Sorry about the negative signs.  I'm not a rocket scientist and have never used kelvin.  When I looked up the temps, they were not listed in kelvin but C and F.  I know kelvin is from absolute zero, but a lot of people here are not rocket scientists but all should know degrees C or F. 

Offline RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #281 on: 06/25/2015 04:19 PM »
I worked for a natural gas company and we liquefied natural gas in the summer for winter peeks.  Boil off was not that big of a problem on the ground, and space is colder.  Tanks on the ground were doubled like a thermos bottle, with a vacuum pulled between the inner storage tank and outer shell.  There was about 3' of space between them (1m), so keeping cold wasn't hard, and that is in the deep south.

Taking advantage of the vacuum of space, a sunshield keeping the tanks in the shade would have the same effect. It shouldn't be difficult to minimize boil off for LOX and liquid methane.

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Re: MCT Speculation and Discussion Thread 4
« Reply #282 on: 06/25/2015 04:28 PM »
On another question, could one of these large fuel depots be towed to L1 or L2 for say fueling some MCT's going to and from Mars without them landing every time?  Seems like a lot of cargo, in cargo containers that could fit in an MCT could be brought up in Falcon Heavies, say two 50 ton containers.  Then towed with SEP tugs to L1 or L2 to be loaded into an MCT to be sent back to Mars.  Fuel and LOX in 50 ton units could also be towed to the fuel depot for refilling.  If Vulcan comes on line, it too, could send up shipments of cargo/fuel to be loaded and sent to Mars.  SpaceX wouldn't have to provide everything.  Everyone might eventually get involved in Mars colonization, ESA, Russia, China, India, Japan, NASA, and other American companies.  SpaceX just seems to be leading the way. 

Offline nadreck

Re: MCT Speculation and Discussion Thread 4
« Reply #283 on: 06/25/2015 04:52 PM »
Also because of the intensity of the black body radiation of the earth and its daytime reflection of heat, I see the depot needing far more active cooling than the MCT which will only need to keep its propellant from boiling off near Mars and between Mars and Earth but will not need to keep it cool for long in the 10 radii range of the Earth.
Not sure this is a problem. Other depot studies have noted this as an issue for hydrogen but methane is quite a mild cryogen in comparison. With solar power a methane prop depot should be able to be zero boiloff anywhere.


There was a paper I read in the last month (and I know it is linked to here on NSF and I will look for it later) that suggested LOX and Methane would be fine more than 10 radii from Earth at Earth's distance from the sun with simply passive cooling, but that near Earth and potentially Mars more cooling would be required. And remember a LEO depot will spend roughly half its time above a sunlit Earth that is radiating significantly more than its black body night time amount and that it will cover a significant fraction of the visible area around the depot.

Only had to go back 119 posts to find where I replied to a message with the excerpt from that paper quoted:

http://www.permanent.com/space-transportation-propellants.html
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 R7

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Re: MCT Speculation and Discussion Thread 4
« Reply #284 on: 06/25/2015 05:01 PM »
True, it is much more difficult in LEO, but an actuated passive system is very flexible, and even a static passive system can be done.  Mount a cone-shaped reflective thermal shroud around the tanks, and point it normal to the orbital plane, and so long as your choice of orbital plane isn't very far from the ecliptic, you can be mostly in radiative thermal contact with deep space rather than the Earth or the Sun.

So, basically use Webb telescope shielding technology to build a "crater" in orbit, well insulated from Earth. Place propellant tanks in the bottom of that crater.  Add a sun shield to make it permanently shadowed and you're set. Illustrated simple setting with articulating boom. Some other geometry might not even need that. Having openings to "vent" the thermal radiation into 2.7K space improves shield efficiency a lot. In regular MLI it just bounces between layers without escape.
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #285 on: 06/25/2015 05:18 PM »
True, it is much more difficult in LEO, but an actuated passive system is very flexible, and even a static passive system can be done.  Mount a cone-shaped reflective thermal shroud around the tanks, and point it normal to the orbital plane, and so long as your choice of orbital plane isn't very far from the ecliptic, you can be mostly in radiative thermal contact with deep space rather than the Earth or the Sun.

So, basically use Webb telescope shielding technology to build a "crater" in orbit, well insulated from Earth. Place propellant tanks in the bottom of that crater.  Add a sun shield to make it permanently shadowed and you're set. Illustrated simple setting with articulating boom. Some other geometry might not even need that. Having openings to "vent" the thermal radiation into 2.7K space improves shield efficiency a lot. In regular MLI it just bounces between layers without escape.

That's one way, but you would have to actuate that every orbit.  A non-actuated passive thermal shield is also possible so long as you control orientation, tightly in LEO and less tightly as you gain altitude.  It would get a *little* incident radiation, but not enough to matter.  This is a matter of proportions: You can tolerate small amounts of boiloff, a 99% reduction is fine.

Attachment has reflective thermal shield as a gold conical foil around a blue-grey tank.  Emission direction is towards the normal of the orbital plane, the only area that remains non-occluded by the Earth for the whole orbit.  The orbital plane is chosen so the Sun never drifts into this narrow beam of heat emission/absorption.

An elliptical reflector would work better (for the same reasons spotlight reflectors and other reflective geometrical optics work), but may have issues with structural packing.  The cone illustrated is probably not long enough to work well, but suffices to show the concept.

« Last Edit: 06/25/2015 05:30 PM by Burninate »

Offline R7

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Re: MCT Speculation and Discussion Thread 4
« Reply #286 on: 06/25/2015 06:11 PM »
That shields the sun completely but the "mouth" of the cone still sees half of earth. Wondering if some finetuning would help that. Might be quite sufficient for methalox but passive LEO system capable to cope with hydrogen would be nice too.

Extend a flat reflector (normal pointing towards earth) from the earth-side lip of the cone? If the orbit is at ecliptic it won't reflect sunshine into the cone.

Fancier would be to do elliptical conical cut, the longer "lip" facing earth. Sun would shine into the cone but it should reflect it away before it reaches the bottom of it, no? (Not sure, seems that way)

With elliptical reflector do you mean those which attempt to focus the beam  (parabola being optimal) ? Wondering if inverted shape would be even better, like a trumpet? After all the goal is a shape which rejects radiation fed to it from all angles as much as possible before it reaches the "bottom" where the tank(s) are, which see only black space.
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Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #287 on: 06/25/2015 06:43 PM »
Sorry about the negative signs.  I'm not a rocket scientist and have never used kelvin.  When I looked up the temps, they were not listed in kelvin but C and F.  I know kelvin is from absolute zero, but a lot of people here are not rocket scientists but all should know degrees C or F.

Too funny Just asked 6th grade daughter and she knows the Kelvin scale 😀
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Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #288 on: 06/25/2015 09:04 PM »
Sorry about the negative signs.  I'm not a rocket scientist and have never used kelvin.  When I looked up the temps, they were not listed in kelvin but C and F.  I know kelvin is from absolute zero, but a lot of people here are not rocket scientists but all should know degrees C or F.

Too funny Just asked 6th grade daughter and she knows the Kelvin scale 😀

Yes, you don't have to be a rocket scientist to use Kelvin, but most here do have some kind of STEM background and fully understand it. Kelvin is taught in the younger grades and it is the metric used in high school sciences. In high school chemistry, basic calorimetry is measured in Kelvin. Thermal calculations in high school physics are done in Kelvin. Celsius and Kelvin both have a 100 degree difference between the state change temperatures of pure H2O @ STP (Standard Temperature and Pressure), i.e. solid/liquid and liquid/gas. Thus, Celsius and Kelvin scale on a 1:1 ratio. Since absolute zero is 273.15C below the first state change temperature of pure H2O @ STP, given y = temp K and x = temp C, y = x +273.15. Kids do learn how to do this in middle school.

Online AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #289 on: 06/25/2015 09:25 PM »
On another question, could one of these large fuel depots be towed to L1 or L2 for say fueling some MCT's going to and from Mars without them landing every time?  Seems like a lot of cargo, in cargo containers that could fit in an MCT could be brought up in Falcon Heavies, say two 50 ton containers.  Then towed with SEP tugs to L1 or L2 to be loaded into an MCT to be sent back to Mars.  Fuel and LOX in 50 ton units could also be towed to the fuel depot for refilling.  If Vulcan comes on line, it too, could send up shipments of cargo/fuel to be loaded and sent to Mars.  SpaceX wouldn't have to provide everything.  Everyone might eventually get involved in Mars colonization, ESA, Russia, China, India, Japan, NASA, and other American companies.  SpaceX just seems to be leading the way.

Fuel depots would optimally have their own vacuum engines (and they already have plenty fuel).  This would allow one to be filled in LEO and then cruise on to EML-1/2 to either await customers or transfer its load to another depot.  The ULA-proposed ACES model, adjusted to store liquid methane instead of liquid hydrogen, is the best example I've seen of a functional system.  Additionally, a full depot at EML-2 would be only 0.14km/s from Earth departure, so could also reposition itself to Mars orbit for fueling ops there.

Cargo would be tougher to move than fuel, so modular containers/tugs could work well.  This presupposes that the MCTs don't land on Earth to load cargo...
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Online spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #290 on: 06/25/2015 09:51 PM »
I too know the kelvin scale and I too have used formulas for gas flows using the kelvin scale.  I was just trying to keep it simple and I saw the degrees C and just transferred it in my text.  Most people here at lest can relate to 0 deg C to freezing water.  However there are a lot of newbees, and a lot of foreign people reading this and know centigrade and do not use the F scale.  Some of you guys obviously are just plain rude and don't care if a lot of people read this forum.  I for one, would like for more to read it and get interested because they VOTE and can have a say in how much is spent on the space program.  It has to be focused and exciting for most to understand.  Sorry if I have offended you Kelvin only users. 

Offline R7

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Re: MCT Speculation and Discussion Thread 4
« Reply #291 on: 06/25/2015 10:18 PM »
Lets just correct the signs if they are wrong and not mock people for their chosen temperature scales. An american using celsius has already shown generosity towards rest of the world. Not long ago I read scientific paper about gas turbines which had temperatures in rankines.
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Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #292 on: 06/25/2015 10:44 PM »
Sorry about the negative signs.  I'm not a rocket scientist and have never used kelvin.  When I looked up the temps, they were not listed in kelvin but C and F.  I know kelvin is from absolute zero, but a lot of people here are not rocket scientists but all should know degrees C or F.

Too funny Just asked 6th grade daughter and she knows the Kelvin scale 😀

Yes, you don't have to be a rocket scientist to use Kelvin, but most here do have some kind of STEM background and fully understand it. Kelvin is taught in the younger grades and it is the metric used in high school sciences. In high school chemistry, basic calorimetry is measured in Kelvin. Thermal calculations in high school physics are done in Kelvin. Celsius and Kelvin both have a 100 degree difference between the state change temperatures of pure H2O @ STP (Standard Temperature and Pressure), i.e. solid/liquid and liquid/gas. Thus, Celsius and Kelvin scale on a 1:1 ratio. Since absolute zero is 273.15C below the first state change temperature of pure H2O @ STP, given y = temp K and x = temp C, y = x +273.15. Kids do learn how to do this in middle school.

Real rocket scientists use Rankine. 

;-)

Lets just correct the signs if they are wrong and not mock people for their chosen temperature scales. An american using celsius has already shown generosity towards rest of the world. Not long ago I read scientific paper about gas turbines which had temperatures in rankines.

Well said.  You don't need to be a rocket scientist to use kelvin, and rocket scientist I'm sure use celsius and Fahrenheit an Rankine depending on how old they are, where they are located in the world, and person preference. 

I had a Mechanics of Materials professor who hated all metric scales and so often had Imperial/US Standard on his tests, under the premise that we here (in the Northwest US) don't have a feel for metric units. They're just numbers with no "gut check" ability.  If we got an answer in pounds, we had a feel of our result was withing the ballpark, or completely out of whack meaning we probably screwed up a calculation somewhere, and needed to go back and check our work.  But what does a Newton feel like? What does 21,409 Newtons feel like?  Do you have an idea right off the top of your head?  Does your answer feel right?  Probably not, if you grew up in the US.
But you do if it's 1 pound or 21,409 pounds. 

Or a Pascal vs. a PSI.  or an acre vs. a hectare, etc.

I know that will make some people's head's explode here, but he was a PhD in Mechanical Engineering and a very big brain.  It was his personal preference because he noticed student working in metric who made a calculation error usually had no feel for if their answer made any sense or not, as they usually did with US Standard/Imperial.
And I had other professors who were dead set on making sure we all -only- used SI units, because they thought America should already be using them.

So...use of units isn't any indication of anything other than preference usually.  Nothing to get hung up on.  :-)

« Last Edit: 06/25/2015 11:01 PM by Lobo »

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Re: MCT Speculation and Discussion Thread 4
« Reply #293 on: 06/26/2015 01:37 AM »
I'm retired and grew up on the English system.  I even had a professor who would not let me use my new $100 calculator in class, had to use a slide rule.  Said the slide rule didn't use batteries and always worked.  I also worked most of my career using paper drawings instead of computer.  I was only trying to use what I thought most who might read these forums would use, Centigrade.  I try to be simple so MOST people can easily understand.  There are thousands of people who access these forums who are not scientists or engineers, but who just are interested in the space program.  Also, this is s Speculation thread, not one working out problems.  I saw that someone posted about not using methane for rocket fuel because of the DEEP cyro cooling of methane which is warmer than lox.  Hydrogen is the problem, not methane.  I worked as an engineer with a natural gas company for 39 years, and we liquefied natural gas on the ground in large tanks, no problem.  Just trying to let him know space could be easier since it is colder. 

I have two college professor relatives, and they don't use kelvin as their fields are not engineering or physics, but they are interested in what NASA is doing. 

Offline Lampyridae

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Re: MCT Speculation and Discussion Thread 4
« Reply #294 on: 06/29/2015 09:22 AM »
When I was in primary school, sometime around Grade 3, as part of our science class we measured out 1 kg of sand in a plastic bag and passed it around, getting the heft of it. That stuck with me throughout my life, although it weighs "less" now that I'm an adult. Amusingly, I was also about 1 metre tall at the time. I don't remember learning about Kelvin, it must have been in Jerry Pournelle's A Step Farther Out which taught me a lot of my physics before I learned it in the classroom.

How much does a kilo weigh? Same as a litre of coke. How much is a Newton? A 100gm bag of chips. Metric's great if you grow up with it.

Back on topic, metholox is also a lot less volume to shade than LH2... but you will have to reboost against atmospheric drag in LEO, which counts against your boiloff issues. Staging from L2 you could use the recovered spent stages

The size of depot you're going to be looking at to support (eventually) 800 transports is huge. O'Neill colony huge. So after the first few depots you want something that is pretty daunting, and the economics will only work if you scale up quickly. Maybe a long chain of these things (although tidal effects come into play here too - maybe vertically oriented). Debris is also going to be a consideration. It would probably have to be manned anyway, and with so many stages coming and going shouldn't be a problem. There's going to be all sorts of work needed to be done all the time and robotics won't be able to handle all of it, not beyond the first dozen or so tanks.

So perhaps a popcorn box-shaped sunshield, with a reinforced "fore" panel and removable "aft" panel that simply gets transferred rearwards as the depot grows.
« Last Edit: 06/29/2015 09:52 AM by Lampyridae »
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Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #295 on: 06/29/2015 10:06 AM »
With the size of a large depot the boiloff problem in LEO may go away. The square cube law helps. Plus constantly arriving sub cooled propellant. What's left of boiloff may justbe accepted for the sake of simplicity of operations.

Offline lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #296 on: 06/29/2015 11:44 AM »
If you have a sunshade you also automatically have a place for solar cells.  You can use these to power compressors than can cool the fuel and eliminate boil off altogether.  Boil off is using phase change to cool the fuel to offset solar gains.  It seems better to me to do this in a controlled way, that will eventually offset the cost of lifting the compressors to orbit.

Offline Lampyridae

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Re: MCT Speculation and Discussion Thread 4
« Reply #297 on: 06/29/2015 01:34 PM »
With the size of a large depot the boiloff problem in LEO may go away. The square cube law helps. Plus constantly arriving sub cooled propellant. What's left of boiloff may justbe accepted for the sake of simplicity of operations.

True but your tank size is going to be constrained by the size of the launch vehicle payload, unless you weld it together in orbit (also a possibility). A 12m diameter tank should be fine though - and hold plenty of propellant.

Plus you have to make ullage burns to get the stuff flowing, or else use something clever like low-temperature bladders. Rotate for (very weak) artificial gravity? That'll probably cause more problems than it solves.
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Re: MCT Speculation and Discussion Thread 4
« Reply #298 on: 06/29/2015 02:21 PM »
True but your tank size is going to be constrained by the size of the launch vehicle payload, unless you weld it together in orbit (also a possibility). A 12m diameter tank should be fine though - and hold plenty of propellant.

There are ways around that. One, it was mentioned somewhere on the forum that inflatable tanks for LOX and methane are possible. The second, the square cube law still applies if you bundle tanks. They see the temperature of the fuel everywhere, where a tank is.

Plus you have to make ullage burns to get the stuff flowing, or else use something clever like low-temperature bladders. Rotate for (very weak) artificial gravity? That'll probably cause more problems than it solves.

You are right, I forgot ullage. If the tank becomes really large that becomes an issue.

Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #299 on: 06/29/2015 03:15 PM »
Plus you have to make ullage burns to get the stuff flowing, or else use something clever like low-temperature bladders. Rotate for (very weak) artificial gravity? That'll probably cause more problems than it solves.

You are right, I forgot ullage. If the tank becomes really large that becomes an issue.

A very large tank will have a significant gravity gradient (and by design could have a larger one), I don't known whether this is significant enough to avoid needing other forms of settling.

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