Quote from: guckyfan on 06/29/2015 10:06 amWith 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.
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.
Quote from: Lampyridae on 06/29/2015 01:34 pmQuote from: guckyfan on 06/29/2015 10:06 amWith 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.With two sets of tanks, it should be possible to arrange for flow of the liquid in the tanks using low velocity mixers. If the mixers were contra rotating, the angular acceleration would tend to cancel out. It would be important to keep velocities low, and to be able to vary the velocity of the mixers for operation with mixed phase fluids. Rotating the whole tank seems complex, and might require rotating joints for fluid transfer, which are weak points. As a negative point the mixers would add energy and possible increase boil off because there would be constant friction, but this could be offset by using active mechanical cooling of the fuel. Careful operation should create laminar flow; that would create the lowest friction.
Rather than guessing this or that, how about some math?The solar constant is about 1300 W/m2. Since the depot is in the Earth's shadow 50% of the time this can be reduced to 650 W/m2 average. A good reflector foil wrap with a low emissivity of e=0.1 will reduce this to 65 W/m2. Depending on insulation effectiveness, this energy will either be absorbed into the fuel, or radiated back out into space. If the insulation was 100% effective, the exterior hull temperature would be determined by Q=Be(Ts^4-ta^4), where B is 5.7e-8, e is the emissivity of 0.1 and ta is the average ambient temperature in earth orbit, that I believe NASA usually sets at about 200°K. Solving this gives a surface temperature of 337°K, or 65°C.
Quote from: lamontagne on 06/29/2015 04:30 pmRather than guessing this or that, how about some math?The solar constant is about 1300 W/m2. Since the depot is in the Earth's shadow 50% of the time this can be reduced to 650 W/m2 average. A good reflector foil wrap with a low emissivity of e=0.1 will reduce this to 65 W/m2. Depending on insulation effectiveness, this energy will either be absorbed into the fuel, or radiated back out into space. If the insulation was 100% effective, the exterior hull temperature would be determined by Q=Be(Ts^4-ta^4), where B is 5.7e-8, e is the emissivity of 0.1 and ta is the average ambient temperature in earth orbit, that I believe NASA usually sets at about 200°K. Solving this gives a surface temperature of 337°K, or 65°C.I don't quite understand this. The effective temperature of a blackbody at 1AU should be 254K. You can set the emissivity fairly high (for rocky Earth, emissivity approaches 1.0), but albedo is the variable you can tweak heavily, and with higher albedo should come lower hull temperatures.In Low Earth Orbit, you have the added factor of a hemisphere radiating at ~288K (may be subject to some corrections). But nothing, as far as I understand it, should raise the hull temperature up to 337K.
The fuel depot has low emissivity and likely fairly high albedo, these are usually more or less inversely proportional. So it reflects a lot of radiation (high albedo), and therefore absorbs very little , but for the radiation it doesn't reflect, it had a lot of difficulty getting rid of, so it has to heat up quite a bit.
Quote from: lamontagne on 07/01/2015 12:36 amThe fuel depot has low emissivity and likely fairly high albedo, these are usually more or less inversely proportional. So it reflects a lot of radiation (high albedo), and therefore absorbs very little , but for the radiation it doesn't reflect, it had a lot of difficulty getting rid of, so it has to heat up quite a bit. In another discussion it was mentioned that there is something as simple as a paint that combines both properties. It is high albedo - white - in the visible spectrum where the sun emits most of its energy and at the same time low albedo in infrared where a depot needs to get rid of excess energy.
The clearest proof that landing is the plan was the statement that for the first crews MCT would be the habitat on Mars.
Quote from: guckyfan on 07/04/2015 07:36 pmThe clearest proof that landing is the plan was the statement that for the first crews MCT would be the habitat on Mars.And their is nothing inconsistent with that statement and having a SEP transit stage. Musk calls it the "Mars Colonial transport SYSTEM" which clearly implies multiple parts such as the BFR first stages and what ever LEO propellent depots are need, neither of which will go to Mars.
......I recall Elon Musk saying something like it can be done with chemical propulsion, no advanced propulsion systems are necessary. As far as I know he never repeated that statement and it does not preclude SEP. I would not be too surprised if it is added at some point in time to improve efficiency. But I am very sure it will not be part of the initial system at the time when a first base is set up because it adds complexity.
Four fully-fueled Tanker-MCT (TMCT) are clustered around and attached to a Mars-Bound-MCT (MBMCT). The engines of the four TMCTs are lit and push the cluster to HEO using about 1/2 of their fuel. The MBMCT is released and begins its TMI burn while the remaining four return to LEO and then individually RTLS. Alternatively, just to a fuel depot in LEO.Feasible?Edit: Starting point is LEO.
......I don't think it is a very efficient architecture. It means several MCT with all their mass would need to be accelerated a significant part of TMI. Also you mention using half of their fuel. It would not be necessary to reserve half of the fuel for return. Injecting into a highly ellicptic orbit would give the Mars bound MCT much of the needed delta-v and brings the booster MCT back to earth basically free. Why do you propose to get them back to LEO? More efficient to land them for a new launch with payload.I think the most efficient way is giving MCT tanks large enough to do TMI burn and Mars EDL by themselves. Use tanker MCT to refuel in LEO either directly fuelling up an MCT or filling depots. They need that tankage and the delta-v to get back to earth from the Mars surface.
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.Maybe a totally crazy idea. But could a fan be used to herd the propellant to the pumps? That would save the need to accelerate a depot with thousands of tons of propellant so it can settle.