natsirt721

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  1. For equatorial launches, timing isn't important, except for reducing travel times. Phasing is pretty cheap if you're willing to wait for it, so I usually try to get close but err on the side of behind. For LKO intercepts like saving kerbals in <80km orbits, you probably have to go high to phase anyway, so timing is less important. For inclined intercepts, timing is incredibly important. Plane changes are hands down the most expensive maneuver, so if you can inject close to (preferably directly into) the target's orbital plane, you can save a ton of propellant.
  2. The reason you need tensile strength is to overcome tensile forces. Whether those forces are because of gravity or due to pressure across a membrane doesn't matter. This is why, in the thought experiment, the plastic can survive as long as the area is small - the total amount of force is small. When the area increases, even though the pressure is the same, the forces increase. At some point, the plastic will fail - not because of the pressure, but because the total force on the plastic is too great. The experiment may have been a little misleading, because I emphasized the weight of the water - the critical piece isn't the weight necessarily, it's the pressure * area on the plastic that causes the failure. It doesn't matter that the pressure is caused by the weight of the fluid. In space, there's no weight, but there is fluid pressure. Fundamentally, it's the same problem, just with different geometry. The 2d plastic across the bottom of plexi column becomes the surface of a sphere, and the pressure gradient is from the vacuum of space (0 atm) to the internal pressure of the tank (1 atm, or whatever) instead of from the weight of the water. Yes, I would too. Plastic is good for containing things when the pressure gradient is low. But in space, the pressure gradient is not low, so you need a stronger material. Here's where the confusion lies. Materials don't withstand fluid pressure, they withstand stresses. Structures withstand fluid pressure, by determining how the pressure is translated to stress, and where the concentrations of stress are. The '6mil plastic at 20-60psi' figure is confusing - fundamentally it doesn't make sense because of the above. It could be that you are quoting the tensile strength of the plastic, but 140 kPa is a pathetic strength. Frankly, the cryostorage solution is a pretty good tank-fluid mass ratio - as long as your polymer can withstand cryotemperatures without being structurally compromised and you have a practical way to deal with debris strikes, it's not a bad solution for moderate amounts of propellant. A 2 cm thick bag 4 m in radius is certainly plausible.
  3. Wavelength and pupil shape are not related. Both are determined by need, but affected by different factors. For xenobio coloration, there are two important factors. Primarily, the star of the homeworld. The human vision spectrum is attuned to center on the 380-740 nm band, from violet to red, with yellow-green (555 nm) in the middle and black on the edges. Why this wavelength? The sun's peak energy output, per wavelength, is somewhere between 500 and 600 nm. The human eye is attuned to detect green(ish - we'll say its green although its a little closer to yellow-green) light because there are more energetic photons hitting the retina at those wavelengths. This is also why plants (chlorophyll) is green - peak power absorption. Statistically, the most common stars in the galaxy are red dwarfs (M-type) , with surface temperatures near 4000 K - this is cooler than our sun (G-type, surface around 5800 K), so (assuming an equal chance of existence of life around both G and M type stars) in all likelihood aliens will be able to see well into the infrared, and may not even be able to perceive any wavelengths in the visible spectrum. Additionally, their pupils will likely be larger in order to pick up a larger amount of the weaker sunlight, and may easily be blinded by Earth-normal brightnesses. Frankly, I'm too lazy to do too much research into pupil shape, but a quick google search suggest the following: Pupils with horizontal aspect ratios are prey, while pupils with vertical aspect ratios are predators. This also correlates with the placement of eyes (either for stereoscopic or wide-field vision) for on the sides vs the front of the head, respectively. In general, it is probably safe to assume that your xenos are predators, because prey seldom have time for the advanced habits that advanced beings need to cross the thin vermeer from animal to sentient (of course, this could be circumvented with clever storytelling, e.g. Pournelle/Niven's Footfall)
  4. I believe wumpus is correct here. I can't say for sure, but I remember solving similar exercises as this one in my undergrad EE class, and nothing extreme ever resulted. I'd have to check my notebooks (I don't have them on hand) but I'd guess the max voltage you're going to get is a function of the capacitance and inductance, and the resistance of the wires if you're counting that (which at high voltage you probably should be). The classical equations break down because 'derivative' and 'instantaneous' don't play nicely together, ever. Usually you end up with infinite voltage or current - practically, that means that some component burns out and opens the circuit/explodes/starts fires/other bad things. I've seen and LC loop (the RHS of your circuit there) used explicitly to prevent that component burnout in circuits with capacitors when the circuit is opened - the LC absorb the 'infinite' jump in voltage and dissipate the current because of their innate resistance. IDK how that would play out with superconductors though. I think that was with AC circuits though - its been a few years since I've done EE work.
  5. Well, strictly speaking no. Technically there is nothing limiting you from building such a ship in Earth orbit today - it would be incredibly expensive and probably take a decade with current launcher technology and availability, but its not technically restricted (on-orbit construction woes aside). The problem is that while your engine choice dictates how much propellant you need, it also dictates how much propellant per kilogram of non-fuel you need for a given mission. That's where the problems set in. If you only need 2 kps of dV, you can achieve a mass ratio around 1.5 with hydrolox propellants. Strictly speaking this is not too bad, but for you that means 40 kt of propellant (assuming 97 kt is wetmass). That's the beauty of the rocket equation - it doesn't really care about mass, it cares about mass ratio. You could make a ship that weighted 9.7 grams and it would need 4 grams of propellant. You could make a ship that weighed 9.7e13 tons, and you would need 4e13 tons of fuel. I read somewhere that for a non-bulk ship (i.e. a passenger liner or quick cargo vessel) you should try to keep the mass ratio below about 4. For you, this means about 80 kt of fuel maximum. From there, you can work backwards and try to figure out how much dV you think you need (in this example, basically none) and then how high your exhaust velocity can be to achieve that. The handwave drive and the gravity inverter kinda throw a wrench in this process - most of your dV expenditures are still going to come during liftoff, but if you don't have to fight gravity then your engines can be significantly weaker. I would SWAG about 10 kt of chemical propellant for the main engines, and a few kt for attitude control. Edit: I fully concur with KSK's analysis above, but it is important for the author to have a good grasp on the technical capability of the vehicle so they don't do anything too absurd. I would say that having translational drives and gravity inverters makes this sort of technical analysis meaningless, but if they are salvaged parts I can get behind it.
  6. This is called 'translating' I believe. cantab is right - assuming that this vehicle is visiting infrastructure designed for other vehicles of its size, and using 500m as an estimate for length (COADE ships are usually spindly, a liner would be thicc) you're already very close to the station. The real question is, why bother with docking at all? Most of the ship is probably dead mass - that is, for all intents and purposes it is part of the ship. It ain't going anywhere. Usually an appreciable fraction of the mass is fuel, but if you can translate outside of the atmosphere you don't really need a lot of fuel - we can put the upper boundary on a few tens of kilotons. Passengers and baggage mass is basically negligible. Cargo is probably similar to fuel, a 10-20 kt. Rather than bringing the behemoth to the station to transfer passengers/fuel/cargo, bring the passengers/cargo to the station and the fuel to the ship using tankers (or a really long hose :p). Cargo and fuel might be a little tricky - cargo is probably carried in standard containers, or at least not in bulk so you only need a ship (probably a fleet owned by the station) large enough to carry the largest denomination of cargo container. Tankers can carry a few kt of fuel at a time and make several trips - sure its not as convenient but provided there are enough tankers to be continuously filling your ship, it isn't going to be significantly slower. Passengers will wait however goddamn long you tell them to wait - I've spend two hours in an airliner on the tarmac waiting for a gate. But it's easier just to get them a shuttle. I'll tackle the 'what is the RCS composed of' and hope that provides a good answer. Exotics (antimatter, fusion, etc.) are out of the question. Too much dry mass per thruster. Electric is too weak - even if you can a kilonewton per thruster, its going to take a while to build up any appreciable angular velocity, and equally long to negate it. You can basically forget about linear acceleration - even 10 of those 1 kN thrusters firing at once gets you about one-tenth of one millimeter per second squared. Nuclear thermal is probably impractical - you would need one core for every thruster block - between 6 and 8 reactors minimum. If you can afford the technological burden, this is probably your most fuel-optimal choice. Also, as cantab mentioned, throttle response would be poor, either sacrificing efficiency as you expel underheated propellant or mass for an active cooling system. Chemical is easy. LOX fuels are a decent choice, but cryocooling can be a pain. Restarting them can also be problematic (but you can probably hand-wave that away). Personal bias steers me towards hypergolics, like N204/MMH. Isp near 330 is pretty poor, but you don't need any heavy turbopumps or cryocoolers for the fuel, and the engines are basically nozzles with a box on one end where the fire happens. Thrust is good enough - it won't be great, and you will have to fill up more than you would for LOX or NTR, but responsiveness and dry mass are stellar. If you had a big ole nuclear reactor (or other convenient source of electricity), you could probably do an electrically-pumped thermal solution - like a steam rocket pumped by a megawatt laser. Those can be as efficient as NTR and you don't have to deal with nuclear problems.
  7. That's exactly what I mean when I say 'they want the biosphere'.
  8. RIght, I forgot that a mil is a unit of measure. Well, that's even worse that I thought. I completely agree that hydrogen is not a good candidate, neither as a gas nor a liquid due to its abysmal density across the range of feasible pressures and temperatures. Your cryofuels are going to be well below -100 C. Unless we're talking about storing them as a gas (presumably near STP), but I bet the decrease in density and larger volume requirements outweigh the mass savings from a cheaper material. Let's examine that real quick. Say we want to store 100t of methane. At STP, methane has a density of 0.717 kg/m3. 100t would require a volume of ~140,000 m3, or a sphere with a radius of 32.2m and a surface area just greater than 13,000 m2. Surface stress on a sphere under pressure is (p*r) / (2*t). Some quick googling puts the tensile strength of LDPE around 14 MPa, and HDPE around 30MPa, which (including a safety margin of 1.2, which is pretty conservative for space applications) gives a thickness of 0.139 m of LDPE, and 0.065 m for HDPE. Density for plastics vary a lot by manufacturer, this source says 0.92 t/m3 for LDPE which seems high so let's call it 0.90 t/m3, and 0.95 t/m3 for HDPE. Given our surface area, the total container has a mass of 1625 metric tons for LDPE, and a better-but-still-awful 800 t for HDPE! Holy crap, I expected a high mass, but that was truly horrible! Now, we can bring that down a bit if we expect the bag to stretch slightly, but then the bag gets thinner and larger every time it gets deflated (which might not be ever, if you can keep enough propellant on-site). We can also lower the pressure of the gas, but gasses really like to expand, so I think the volume might increase faster than the lower pressure saves you from your thickness. Either way, its going to be a fuel-mass ratio of 1/7 for your tank, which is not really practical. At -162 C, 1 atm, liquid methane has a density of 422 kg/m3, which is about 600 times more dense. Repeating the same same exercise as above gives us a total volume of 237 m3, sphere radius 3.9 m, surface area 191 m2, wall thickness 0.016 m for LDPE and 0.0078 m for HDPE, total mass 3.41t for LDPE and 1.77t HDPE. So yeah, storing gasses on-orbit is not worth it - the material definitely needs to hold up at cryogenic temperatures. You not only need a material that is flexible at low temperatures, but that doesn't outgass, doesn't degrade under full spectrum light, and can radiate away more heat than it absorbs at its operating temperature. That's a pretty tall order for plastics alone, and you said that PE already fails at sub -100 C temperatures. Composite materials are probably going to be what you want, but they ain't gonna be cheap like PE. The reason weather balloons and greenhouses (and sandwich bags) can get away with sub-millimeter amounts of plastic is because the pressure gradient (or gauge pressure of the inside fluid) is very small. In space, the pressure gradient is just whatever your contents are pressurized to, and 101 kPa is a lot of Pa. Consider this thought experiment: static water pressure increases at about 1 atm per 10 m of depth. If you take a plexiglass tank open on the top 1m by 1m by 10 m tall and fill it with water, the pressure on the bottom is about 2 atm (1 atm of atmosphere plus 1 atm of water). Now replace the bottom panel with greenhouse plastic and lift the column off the ground, so the pressure gradient is 1 atm (the pressure from the atmosphere cancles, and you get just the water). Does it survive (assume that the plastic is perfectly adhered to the tank)? It might, after all the total area is only 1m2. How wide can you make it before the plastic fails? Does 10x10 m survive? That's only 100 m2, not even enough for our liquid methane experiment. Our original column held 10 tons of water, this one holds 1000 tons - probably too much for the plastic to withstand. Plastics are really only good for volume containment when the pressure requirements are low. For anything else, you need a higher strength than most commercial plastics can provide - that means metal alloys or composites.
  9. At 1 atm LOX has a density around 1.14 t/m3, so each bag (10m edge cube) would actually hold 1140 tons. LH2 has a density around 0.07 t/m3, so each LH2 bag holds 70 tons. Cubes are poor structures for storing things under pressure, because all of the stresses on the panels converge at the edges and vertices. Spheres (and to a lesser extent cylinders) redistribute stress better across the surface of the structure, so you need less mass reinforcing the stress concentrations. A cube also has more surface area per volume than a sphere, increasing overall mass for the same containment value. After doing a little bit of research, I don't think that pure PE is the right material for the job. Consider that one side of the bag is exposed to a vacuum, which can lead to outgassing for many plastics. The other side is a cryofluid, and plastics typically become significantly more rigid as they cool. Either of these effects will severely compromise the structural integrity of the bag, even if it isn't hit by debris. I don't think any cog-e is going to sign for 6 mm of plastic between 1100 tons of valuable propellant and the unforgiving vacuum of space.
  10. If you're interested in this topic, I suggest reading the books Footfall by Niven/Pournelle and The First Formic War trilogy by Johnston and O.S.Card, plus Ender's Game and Speaker for the Dead by O.S.Card (for more perspective). Both give interesting and unique approaches to alien psychology and tactics when it comes to conquering the Earth. It also gives both aliens the upper hand and forces the humans to react, because lets face it - if they are coming to us, they're at least 50 years (optimistically) but probably a century or two more technologically advanced than we are (Footfall bends the rules a bit here, but that a plot point so I won't go into it). I'll drop another projectrho link, because goddamn it's so useful. The page on Aliens page is a good place to start for physiology, and from there you can check out tech levels and contact scenarios. For any aspiring sci-fi author or rocket enthusiast that website is a goldmine, I highly recommend reading into any topics you have questions about. <br> To the point at hand, there is really only two reasons for aliens to 'conquer' earth. They either want the biosphere (colonization), or they want slaves. The former has some implications about the biosphere the aliens evolved in, which is a goldmine for plot elements. That latter implies something about the needs of the aliens, which can also be useful. The question you have to ask yourselves before asking questions like this is 'why'. If your 'why' isn't compelling enough, the conquest is going to seem totally contrived and the entire premise of the story will collapse. That being said, if you can build a strong enough case, the 'why' can be pretty much anything you want - but the further from those basic reasons you diverge, the stronger your case will have to be. Here's some bad reasons for conquest, and why. They want our water. Water is one of the most abundant resources in the universe, being composed of simple elements hydrogen and oxygen. Sure, we have a ton of it, but a) at the bottom of a hefty gravity well (if they aren't planning to bring it somewhere, then they want our biosphere) and b) no long-lived decision-making culture is going to choose to fight a war, tens of lightyears from home, against us on our own turf rather than just go somewhere else unless they're certain of victory. They want our mineral resources. See above. Cheaper, easier to access minerals are all over the asteroid belt (and presumably equally available in other, uninhabited systems), there is no reason to come fight us for our mostly-depleted stock here on Earth. They see us as an existential threat. There are two solutions to this, and neither ends well for us, nor in conquest. If they're interstellar aliens (and they probably are) they can afford to build a planet-cracker kinetic warhead. Even at a few percent c, it doesn't take much mass to destroy the biosphere and obliterate modern society. Optimistically for us, that sets us back a few centuries, if we survive at all. Alternatively, if they're uncertain about their ability to hit us with a planet-cracker, then a crewed vessel with a few hundred gigaton nuclear bombs will almost certainly get the job done. Neither makes for a particularly compelling novel. The thing about inter-species war is that, if there are at least two species, there are probably a lot more. Species are not going to make risky decisions about interacting with others when one of the outcomes is the annihilation of your entire genetic pool. Game theory kicks in here, and again, I'll point to one of the links above for working through that and the implications that you have to accept.
  11. So, that depends on the station I guess. For a large station with access to resources, smaller inflatable bubbles might work. String a few dozen on a truss, and if one gets punctured, pump the fuel to a different one and bring the damaged one in for repairs or replacement or patch it in-situ. For anything smaller or less well equipped, yeah, shielding is a necessity. Fractals help a little bit if you don't have shielding, but repairing the damaged internal cells is nigh impossible, so eventually you need to replace the whole thing. I'm no expert on hypervelocity impacts but low density plastic shielding is not going to stop an impactor. 10 kg/m2 is probably high - again, no expert. You could probably use PE panels, if the station is so equipped to produce them, but a naked bag is not going to last. The reason we don't see debris strikes as more of a hazard is that the ISS is pretty well armored. Most of the surface is either structural (trusses or pressurized areas) or redundant (radiator panels or solar arrays). Microdebris impacts against these have relatively little effect on the operational capacity of the vehicle. Against a pressurized plastic lung, the same debris could destroy it. You said yourself that PE tears rather than shattering - an impactor could create a tear which totally destroys the gas bag, like popping a balloon. This storage solution is feasible under different conditions, but in the near term, I don't think it is practical.
  12. Skids also let you touch down with a bit of lateral velocity (just a bit, mind you) which might be helpful when landing in windy conditions. That being said, I suspect that there simply isn't enough energy flux on Titan to create meaningful winds. Thermal management will be heavily controlled, with only the parts that actually require it being heated. The RTG provides a convenient source of heat, so I suspect something like a 'thermos' configuration where all of the electronics and batteries are stored in a heavily insulated box with heat pipes from the RTG to keep warm. The electric motors will probably use electric heaters while the batteries are charging, and to supplement them while in flight. The rotors themselves will probably be as cold as the air, plus conductive heating from the motors (this will probably be the largest source of convective cooling because of the mass flow across the blades). The lower structure will probably be nearly as cold as the surface, because if they aren't that means you're wasting heat.
  13. The reason the whipple shield works is because each layer has enough mass per unit area to vaporise the impactor before it passes through. 1 gram/cm2 in spaced layers is sufficient, but a few milligrams of plastic is not going to cut it. For the same reason, the gas will likely be as ineffective as the plastic for stopping impactors. For smaller objects like dust or paint chips the plastic might be effective (if deployed in spaced layers), but anything else is going to go clean through. You can make whipple shields out of pretty much anything because again, it's about mass per unit area and spacing that makes it effective. For something sacrificial like a whipple shield, I would probably use something like aluminum thats cheap to replace.
  14. This sentence makes me cringe. The word you're looking for is 'propellant'. I agree with 1, but rather than an arbitrarily configurable LF/OX ratio, just a toggle between LF, OX, or LF/OX at ratio. Finer ratios would only be useful for spaceplanes, and it doesn't really jive with SQUAD's lego-esque approach to the editor. 1.1 is impractical - you can't just use a 'jet engine' with any old hydrocarbon mix as working fluid. You would need specialized parts, probably one per atmosphere. Especially because conventional jets use the atmosphere as an source of oxidizer, whereas these would use it as a source of fuel. As for procedural parts, it ain't gonna happen. SQUAD already said they don't want to do that, and fairings were as far as they wanted to go. Not that I don't agree that they could make the game 'better' for those of us who would like more precision, but thats why there are mods.
  15. What happens when a 1 gram bit of rock or old space-grade aluminum hits your polyethylene bag at LEO velocities? Even if the impact is non-catastrophic, its still going to put 2 holes in the lung. This would require a full-time EVA team on standby to prevent significant loss in event of a strike. You're at least going to want a layer around it to act as a whipple shield to mitigate damage. But I don't really see the need for an at-scale LEO fuel dump until we get fuel from the moon or NEAs. At first, your mission schedule is going to be pretty sparse, so there's plenty of time between missions to take a few months/launches and just directly refuel whatever your vehicle is. At most, you keep enough propellant on-site to fuel up the ship once, and only start filling that from the ground when absolutely necessary - it spends most of its time empty rather than full. What you really want to do is ship up and store water, and then electrolize it into LH2 and LOX using free solar power or an onboard nukie when you need it. Water is way more dense than either propellant, so you can fit way more into a payload fairing. When you get it to the depot, just let it freeze into a block and be done with it. Sure, the electrical overhead is significant to melt and then convert it (water has a stupid high specific heat and enthalpy of fusion, but waste heat from the nukie and solar heating can help), but you're basically immune to propellant loss due to impact, and you don't have a constant need for cryocooling. This makes sense for a lung - when your new propellant arrives in a fluid state, pump it around the outside of the current icecube and let the bag expand. Once it freezes, forget about it. Before the next shipment arrives, send an EVA team to patch all the holes. Best part is, the crew (if the station is crewed, which is probably should be for maintenance purposes) gets free ox and drinking water from the propellant store. Centrifuge gravity would also help with managing the fluids, but that would be hard to manage when your mass is constantly changing. Edit: methane is only in vogue right now because Mars has CO2 in spades and a Sabiter reactor is practical. Any other destinations and you're going to want water derivatives (i.e. hydrolox).