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.
Quote from: ArbitraryConstant on 06/25/2015 02:11 pmQuote from: nadreck on 06/21/2015 06:05 pmAlso 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.
Quote from: nadreck on 06/21/2015 06:05 pmAlso 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.
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.
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.
Quote from: Burninate on 06/25/2015 04:09 pmTrue, 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.
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.
Quote from: spacenut on 06/25/2015 04:18 pmSorry 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 😀
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.
Quote from: philw1776 on 06/25/2015 06:43 pmQuote from: spacenut on 06/25/2015 04:18 pmSorry 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.
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.
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.
Quote from: Lampyridae on 06/29/2015 01:34 pmPlus 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.