Quote from: guckyfan on 09/07/2015 08:59 pmThe depot will need that capability. It must be able to do debris avoidance, when no MCT is docked. But that's not hard. It has the fuel and can easily have a few methane RCS engines.Well, the engines on the depot cant use the large tanks because they cant get the fuel to the engines. Its the same problem as with tanking. They need pressure fed engines, which must draw fuel from different tanks. Why would you need Methane if you need extra tanks anyway? Cant it be any fuel, Hydrazine for instance?
The depot will need that capability. It must be able to do debris avoidance, when no MCT is docked. But that's not hard. It has the fuel and can easily have a few methane RCS engines.
Quote from: Burninate on 09/07/2015 06:17 pmA small linear acceleration requires constant fuel expenditure per time for pumping, and modifies the orbit. Pumping needs to be very fast.A rotation only requires fuel expenditure while spinning up or spinning down, and can be done without modifying the orbit. Pumping can be done at leisure.If we employ rotation at the 6-berth Depot, what should be the center of rotation? Should the propellant flow toward the forward end, the aft end, or the sides of the MCT tanks lying in berths?Would it be necessary to spin down or terminate rotation to accommodate a vehicle arriving or leaving the Depot? Thank you for your insights.Edited: grammar.
A small linear acceleration requires constant fuel expenditure per time for pumping, and modifies the orbit. Pumping needs to be very fast.A rotation only requires fuel expenditure while spinning up or spinning down, and can be done without modifying the orbit. Pumping can be done at leisure.
Forward frameGiven the geometrical layout of a berth, the following is the layout of the struts of the most forward of the frames:
.........So let's say we use a deviation from the 6-shooter depot: We cant the tanks outwards by just 2 degrees or so, so one end of each tank is sticking out at a slightly larger diameter. Then place the pumps on the greatest-diameter section on the outer sidewall.Now, when the vehicle rotates, the propellant flows towards the side of the tank, and then slowly rolls "downhill" to a spot near where the hemispherical endcap curvature begins. The slight cant guarantees you can get the thing 99.9% empty instead of only 99% empty; Just placing it on the outermost sidewall of parallel tanks should work most of the way.
A whole series of questions ;-)What might be the acceleration required to settle down the fluid? I expect the fuel transfer operation might take many hours and I wonder what might be the final deltaV change. As someone pointed out, though, we can accelerate and then decelerate. That would be the simplest, if the acceleration is low..........Regards,Michel Lamontagne
Quote from: Burninate on 09/08/2015 07:05 am.........So let's say we use a deviation from the 6-shooter depot: We cant the tanks outwards by just 2 degrees or so, so one end of each tank is sticking out at a slightly larger diameter. Then place the pumps on the greatest-diameter section on the outer sidewall.Now, when the vehicle rotates, the propellant flows towards the side of the tank, and then slowly rolls "downhill" to a spot near where the hemispherical endcap curvature begins. The slight cant guarantees you can get the thing 99.9% empty instead of only 99% empty; Just placing it on the outermost sidewall of parallel tanks should work most of the way.What a great idea!With your permission I would like to incorporate this approach into the next iteration of the Depot. I already have a sketch in mind.
A whole series of questions ;-).........A full MCT requires something like 5-700 tons of fuel depending on how much of a hurry you are. However, most BFR designs can only deliver 250 tons to orbit. So the depot would hold the fuel for six Mars transfers, lifted up by 17 or 18 tanker flights? .........
.........Sure!Tossing out an abortive idea I had for something fairly similar:Instead of canting the tanks outwards at 2 degrees, keep the tanks parallel, and put some sort of lightweight closed-cell foam in place on the interior of each cylinder as a ramp at a 2 degree cant. The pressure vessel still needs those cylindrical walls to hold together efficiently; This foam ramp would not be resillient under bending loads, it would simply transfer pressure to the outer cylindrical pressure vessel.The problem is in the materials.Things that I would prefer to use:*A solid form, shaped like a ramp insert; But too heavy.*An inflated / sealed composite ballast tank, shaped like a ramp insert and with plenty of internal pillar supports, like an inflatable mattress; But then you need to mess with ideal gas expansion of the internal reservoir over varying pressures, and worse you would need to plausibly deal with phase change behavior over a thermal gradient.*A foam of some sort. Can you make a closed-cell aerogel strong enough to hold pressure of a gas with lower boiling point than liquid oxygen, while still being lightweight? Probably not.
Oh, another note:How are you going to handle LOX vs CH4?You could go with common sidewall tanks, but they're fairly close to needing a 2:1 volume ratio at Musk's specified 3.8:1 mass ratio, which needs one less sidewall if you're starting out with a number of tanks divisible by 3. Six tanks gives you this, and gives you a balanced center of mass.
A whole series of questions ;-).........Could there be some tanks dedicated to ion propulsion for different types of missions, or SEP assist for Mars transfer? and what might the Ion propellant be? Argon would be nice, since it can be replaced on Mars..........
A whole series of questions ;-).........Regarding rotation, what might be the best solution?1- We can rotate the MCT and depot assembly around a common center of gravity on the short axis (this is particularly useful for fuel transfer from tankers) 2- Rotate the tanks as a group along the long axis, and rotate the MCT, with a very precise rotation system the MCT might even dock at the center point of a rotating tank array, like the 2001 shuttle. that way we wouldn't need to accelerate and decelerate the depot, 3- We can have a non rotating dock, or six contra rotating tanks in a fixed frame, although there may be some unsolvable seal problems with that.The angle for the tanks might be quite small? Liquids flow very well with just 1 or 2% slope, so this might not affect construction much..........
.........Seems I was using the wrong estimate.Correction:Musk said O:F mass ratio was 3.8:1Density of liquid methane is 422.4kg/m3Density of liquid oxygen is 1141kg/m3Therefore -A measurement unit of 1 ton methane will require 1000/422.4 = 2.367m3A measurement unit of 3.8 tons oxygen will require 3800/1141 = 3.333m32.367+3.333 = 5.700m3 total for the measurement unit of 4.8 tons total2.367/5.700 = 41.53% methane by volume3.333/5.700 = 58.47% oxygen by volumeDensity of methalox combination: 4800/5.700 = 842.1kg/m3
Quote from: Burninate on 09/09/2015 06:31 am.........Seems I was using the wrong estimate.Correction:Musk said O:F mass ratio was 3.8:1Density of liquid methane is 422.4kg/m3Density of liquid oxygen is 1141kg/m3Therefore -A measurement unit of 1 ton methane will require 1000/422.4 = 2.367m3A measurement unit of 3.8 tons oxygen will require 3800/1141 = 3.333m32.367+3.333 = 5.700m3 total for the measurement unit of 4.8 tons total2.367/5.700 = 41.53% methane by volume3.333/5.700 = 58.47% oxygen by volumeDensity of methalox combination: 4800/5.700 = 842.1kg/m3Burninate, I see you will be analyzing the mass imbalances of the Depot using the appropriate data.We have introduced a method for spinning the Depot to place a force on the fuel while pumping it out of a tank. However, the various MCTs docked at the Depot will have propellant tanks in various conditions of fullness, from empty to full and states in between. Furthermore, vehicles will be coming and going frequently, thus changing the mass balance of the Depot profoundly while doing so. Spinning the station will inevitably introduce wobbles in the deportment of the Depot.What can be done about Depot wobble, if anything? As usual I will refer to an Earth analog.Dynamic On-the-Car Spin Balancing (from Autos.com)“This is the most accurate, yet most dangerous method of tire balancing. The tire and wheel are left on the car, and a ‘hat’ is placed on the wheel and secured. A motor with a wheel is placed against the tire, causing it to spin. Balance mechanisms contained within the hat determine where the unbalanced condition is occurring. There are six friction rings on a snout protruding from the hat. These friction rings are held lightly by the technician, while he rests a hand against the fender, moving weights inside the hat. When the most ideal combination of inner and outer weights is achieved, perceived vibrations will be minimized. The technician then reads the display, which reads graphically, and applies the required weights in the specified positions. He will then usually spin the tire up once again in order to verify his work. If the proper weights and positioning were used, the least amount of vibrations will occur with both inner and outer indicators reading zero or close to it. This type of balancing is highly accurate because the tire and wheel are spun on the car, allowing any vehicle specific issues to be accounted for, such as an off center lug pattern or hub.”The point is that a force is applied at key location(s) on the rim of the wheel in the form of small weights to counterbalance the wobble. In the case of the MCT Propellant Depot, could the RCS thrusters, using variable thrusts in variable directions instead of weights, counterbalance a wobble in the spinning Depot?