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Posts posted by sevenperforce

  1. 4 minutes ago, SomeGuy123 said:

    Why bother?  As your vehicle picks up speed, even if it worked, you'd very rapidly leave the range of your laser.  It would be very short range because once the light intensity drops below the threshold for fusion you stop getting any thrust.

    I think you missed the point. I suggested a laser ground array to initially ignite the first pellet, but after that, the light from the fusion of the first pellet is supposed to ignite the next one using a parabolic mirror which doubles as the exhaust bell.

  2. 14 minutes ago, Chewy62 said:

    Are we allowed to take anything back with us?


    Nothing that will be used in the actual rocket, or in its physical manufacture. For example, blueprints are fine; programmable control modules are not. Though if you can somehow construct a control board compatible with modern memory storage then you're free to take a USB stick back with you.

    10 minutes ago, legoclone09 said:

    1850-1900, that was when RP-1 and LOX could be made.

    Could a liquid-fueled rocket be built at that time?

    Honestly, I'm thinking your best bet is to build a multistage rocket with a mix of solid-fueled and hybrid-fueled boosters. Consider a balloon launch for altitude assist. Liquid engines are going to require materials and machining techniques far too advanced to get before the modern era.

  3. 7 minutes ago, Emperor of the Titan Squid said:

    Apart from problems caused by not being able to launch a rocket from a blimp, unreliability of blimps, corrosive atmosphere and high pressure, Venus is better, but before we stop all of that,  Mars is better, and poses an advantage over asteroid colonies by having an ataatmosphere.

    Are blimps necessary, per se? If you brought a bunch of simple cylindrical fuel tanks (accumulated from earlier launches) and filled them with a nominal amount of lightweight gas, you could build a stable floating platform in the Venusian sky.

  4. Suppose you travel back in time and decide you want to build a rocket to reach orbit. The rocket merely has to reach a moderately stable orbit on its own; there are no payload requirements in particular. 

    Suppose further that you have access to all the world's knowledge at the present day.

    Finally, suppose that you are able to convince the dominant world power of that era to assist you in collecting and refining raw materials.

    What is the earliest point in history at which you could reasonably be able to construct an orbital rocket? 

  5. On the subject of pure fusion propulsion....

    Assuming, for the sake of argument, that you can use a ground-based laser array to initally ignite a tritium-deuterium pellet into fusion, would it be possible to build an exhaust/recoil bell which was also a parabolic reflector to focus the visible-light portion of the fusing pellet's radiation to ignite the next pellet? 

  6. An impulse of 617 seconds affords a propellant mass fraction of 72.5%. Of course, actual performance would be lower, but atmospheric augmentation/combustion could probably compensate enough to bring it back up to that; hydrazine exhaust is 13% pure hydrogen by mass (67% by volume). 

    A delta-body with a parabolic-concave surface on one side (painted with a super-low-density reflective paint) and a blunt-body re-entry surface on the other side could probably make it work with structural integrity to spare.

  7. 6 hours ago, Shpaget said:

    You would run hydrazine engine in the atmosphere?

    ...yes. Why not?

    Hydrazine is deadly toxic in its liquid form, but its exhaust is completely harmless. 

    A solar-thermal hydrazine rocket would have a theoretical maximum specific impulse of 617 s, even without any air augmentation or secondary atmospheric combustion. 


    The intermediate step would be chemical propellant, non chemical fuel. In other words, chemicals must still be used as propellant, but the energy comes from somewhere else. For example, hydrazine has a crappy specific impulse of 220 seconds but if you heat it up with an electric arc, it gives you a specific impulse of over 500 seconds.

    The difficulty is finding a source of heat energy that is lightweight other than an unshielded nuclear reactor.

  9. On discussion of Venus...

    I don't know if this would be viable or not, but could you use a low-temperature reservoir as an energy source on a robotic Venusian lander? A tank of slush hydrogen could be used as a heat sink, expanding through a manifold heated by ambient temperature in order to perform work. Proper design would also allow it to cool sensitive components, and once the mission was completed, the remaining liquid hydrogen could inflate a balloon to return to the clouds. 

  10. Of course the US was lightyears ahead of the Soviets when it came to thermonuclear weapons design. The Soviets derided Ivy Mike, the cryogenic hydrogen version of the H-bomb, as a "thermonuclear installation". But they didn't even come up with third idea until after Castle Bravo. In fact, if they had been a little more careful about taking atmospheric samples after Castle Bravo, they would have figured it out sooner. 

    I wonder just how much of the thermal nuclear weapon design remains classified. The United States has a weapon design with some intermediate material that requires extremely careful and precise properties. Obviously there are details like timing and ratios. But I can't help thinking there are some other design elements which have remained secret all this time.

    Building a gun fission weapon is easy enough. Building a spherical implosion  weapon is a little tougher, but still doable. Two-point implosion requires some insanely smart scientists. Fusion boosting isn't much of a challenge at all. Thermonuclear weapons are the really hard part. 

    And, of course, miniaturizing thermonuclear weapons is flat-out impossible unless you happen to be the United States.


  11. 24 minutes ago, Nothalogh said:

    Also, a question for those who might know.

    Is the fission-fusion-fission design called a Sakharov device in Russia, in line with how we named it after Teller and Ullam? 

    They didn't name it after Sakharaov. Sakharaov proposed First Idea, Second Idea, and Third IdeaFirst Idea, nicknamed sloika, was a layer-cake design which was fusion-boosted fission. Second Idea involved the use of lithium deuteride as the fusion booster. Third Idea, developed by Sakharaov, Zeldovich, and Davidenko, separated the primary and secondary and use x-rays from the primary to compress the secondary; this was equivalent of the Teller-Ullam design. Third Idea was first tested in the RDS-37 shot.

    Teller-Ullam is technically still classified, and although the basic principle is fairly clear, there seem to be critical details enabling the actual process which remain secret. The US didn't share any of the details with the UK until after UK's Grapple X test demonstrated that they had unlocked the secret.

  12. 2 minutes ago, Shpaget said:

    There is no way I would choose set B in any sane everyday scenario.

    If I want efficiency, I would certainly opt to quickly take ten scoops of 1l, rather than to fiddle with various combinations of different scoops.

    You are inventing an artificial and unrealistic scenario to give unfair advantage to your arbitrarily chosen scoops.

    Then you would lose. By a huge margin.

    There's nothing unrealistic about wanting to reduce the number of steps in a process, and there's nothing arbitrary about using powers of 2.

    3 minutes ago, kurja said:

    You know, if it's more of different sized measuring cups that make imperial better(?), there are cups and spoons in kazillion different sizes regardless of what units are labelled on them. I have a set with 5ml, 15ml and lots of different sizes in my kitchen. 

    Right. Those exist because measuring by factors smaller than 10 is more efficient for a given range of amounts. The advantage of Imperial is that the units themselves exist in a range of prime factors of each other.

    5 minutes ago, kurja said:
    30 minutes ago, sevenperforce said:

    Maybe you can visualize it like this. Suppose you are on a game show, and you need to fill up a series of 10L buckets with varying volumes of sand (to 1 mL precision) in order to trip pressure-sensitive plates and unlock a prize. The sand must be measured using a set of labeled scoops provided by the game; you can take as long as you want, but you need to complete the challenge in as few steps  (each time you dump a scoop of sand is a single step) as possible.

    Set A contains a 1 mL scoop, a 10 mL scoop, a 100 mL scoop, and a 1 L scoop.

    Set B contains a 1 mL scoop, a 2 mL scoop, a 4 mL scoop, an 8 mL scoop, a 16 mL scoop, a 32 mL scoop, a 64 mL scoop, a 128 mL scoop, a 256 mL scoop, a 512 mL scoop, a 1.024 L scoop, and a 2.048 L scoop.

    Which set will you take?

    Notice how in both cases the units are metric.

    I gave you the units in metric so they'd be more familiar to you, but any arbitrary base unit v works. The point is that measuring by v*2n or even v*3n will always offer more flexibility at a given level than measuring by v*10n.

    9 minutes ago, kurja said:
    35 minutes ago, sevenperforce said:

    Of course metric is more useful in a great many situations. I never denied that.

    Along the lines of it's more useful as long as everything's in multiples of ten and made of honey, was it? :P

    No, it's much more useful for converting between different types of stuff, like energy and velocity or mass and density. It's also much more useful when you're jumping wildly between different orders of magnitudes. Doing science is generally easier in metric.

  13. 37 minutes ago, kurja said:

    Do you have a separate measuring cup for every 3/8 of a spoon and 9/16 of a cup? Hardly. I have 1dl and 1l measuring cups with scales on the side. If you wanted you could think of l, dl, cl, ml and so on as different units if that makes it easier for you to perceive what kind of quantities they represent.

    Oh, perception isn't the problem; I'm quite familiar with the use of metric. I just also happen to be familiar with the use of Imperial. And so I know the comparative advantages and disadvantages.

    Graduated cylinders/measures are fine for most liquids, but they're a pain to use with dry ingredients or viscous liquids. And that's where Imperial has a distinct advantage, because it has a great many units which are all prime factors of each other, on a scale which matches the amounts of those ingredients which humans regularly deal with.

    Maybe you can visualize it like this. Suppose you are on a game show, and you need to fill up a series of 10L buckets with varying volumes of sand (to 1 mL precision) in order to trip pressure-sensitive plates and unlock a prize. The sand must be measured using a set of labeled scoops provided by the game; you can take as long as you want, but you need to complete the challenge in as few steps  (each time you dump a scoop of sand is a single step) as possible.

    Set A contains a 1 mL scoop, a 10 mL scoop, a 100 mL scoop, and a 1 L scoop.

    Set B contains a 1 mL scoop, a 2 mL scoop, a 4 mL scoop, an 8 mL scoop, a 16 mL scoop, a 32 mL scoop, a 64 mL scoop, a 128 mL scoop, a 256 mL scoop, a 512 mL scoop, a 1.024 L scoop, and a 2.048 L scoop.

    Which set will you take?

    If you're smart, you'll take set B. Sure, the numbers aren't all rounded nicely to multiples of 10...but once you realize that they're all just powers of 2, you can combine them to reach any value between 1 mL and 10 L in a far lower number of steps.

    For example, if you need to fill the first bucket to 4.894 L, and chose Set A, you'd have to use 4x1mL + 9x10 mL + 8x100 mL + 4x1 L = 25 scoops. Compare that to 2x2.048 L + 1x512 mL + 1x256 mL + 1x32 mL - 1x2 mL = 6 scoops using Set B. Insisting that metric is always better than Imperial is like choosing Set A because Set B is heavier to carry around.

    37 minutes ago, kurja said:

    No, what the exact dimensions are is irrelevant. Dimensions of containers/vessels are what they are. Calculating 10*10 or 665*174 or whatever with or without a calculator is just as easy or difficult regardless of what units they are. I need to know the volume and height of the vessel and density of the contained liquid, then I can measure the hydrostatic pressure at bottom level and from there it's trivial to give level height and volume and mass of contained liquid. You'd see how and why using metric is an advantage in cases like this if you cared to think of it a little but I see you're more interested in trying to argue.

    Of course metric is more useful in a great many situations. I never denied that.

  14. 14 hours ago, AngelLestat said:

    Heh, so the imperial system is  powerful if you are using imperial units?    Is like said the Bible is true because the Bible said that is true.

    For start, you need to give your result using many units instead one.  

    Poor comparison. It's more like you're saying "You only think that it's possible to use the tax code because you're a tax attorney!"

    Using many units instead of one is a feature, not a bug, if you're doing mental math. A computer program will be smaller and run faster if it can be written entirely with unsigned int variables rather than float variables, even if it takes a few extra lines of code in certain places. In the same way, humans remember and manipulate single-digit integers more easily than long numbers with lots of decimal points. You probably don't understand this because you don't know how to use Imperial, but having more units makes quantities within the single-digit range of those units much easier to manipulate.

    14 hours ago, AngelLestat said:

    Is based on SI units (metric).
    What you think that E means in metric?  Or C?   C= coulomb? 
    All those variables are based on SI.
    C=speed of light in meters/seconds 
    Take a look to formulas in Imperial, they are full of weirds constants that are impossible to remember.


    E = mc2 is not remotely metric. It is simply an equation. Energy can be represented as joules or as calories or as TNT-tonne equivalents; mass can be represented in kilograms or in grams or in slugs or in Jupiter masses; the speed of light can be provided in meters/second or feet/minute or lightyears/year. It all works.

    Sure, metric units are built in such a way as to simplify conversions between different types of things. But that doesn't eliminate weird constants in certain formulas. For example: who wants to deal with 8.3144598 J/mol*K, or 6.67408e-11 m3/kg*s2, or 96,485.33289 C/mol?

    13 hours ago, kurja said:

    I wouldn't divide fractions of cups and spoons at all because I'd be dividing 9dl in half which I can tell without blinking to be 4 and a half dl, it just isn't any more difficult than that. I just find the notion that dividing a number would be any easier if it's followed by a different unit suffix to be silly :/

    Well, do you have a 3dl measuring cup and a half-dl measuring cup and a 1dl measuring cup?

    The advantage of dividing amounts and ending up with multiple units is it makes actually measuring out those amounts much easier, because you have those units represented physically right in front of you.

    13 hours ago, kurja said:

    Not really; for different materials you always have to know their density (regardless of unit system), for water it happens to be "1". So you add that one multiplication depending on material. I actually do this on a regular basis at work, it is highly convenient for level measurement by pressure transmitters and such applications.

    Which is easy when you're calculating the mass of a 10m*20m*100m swimming pool full of honey, but less so when your dimensions aren't multiples of ten.

    10 hours ago, YNM said:

    For mass, of course you would divide the number first before massing (weighing) it.

    Requiring you to pull out a calculator in all but a handful of cases.

    9 hours ago, Kertech said:


    Been watching this unfold, getting the same arguments, basically Imperial: easier to factorise but a nuissance when doing conversion and science. Metric: good for conversion and scaling but factorising is more difficult. 

    Metric allows easier conversion between different types of measurements (volume vs length, energy vs velocity, etc), but since all the units for a given type of measurement are limited to powers of 10, doing mental arithmetic is more challenging unless all your values just arbitrarily happen to be given in multiples of 10.

    4 hours ago, Tex_NL said:

    A certain distance, volume or weight will always stay the same no matter what yardstick you use and can be calculated to the same accuracy. What matters... the ONLY thing that matters is the ease of conversion from one unit to another.

    If you're doing everything with a calculator, sure.

  15. Yikes. So much misinformation. Where to begin...

    Yes, there are a bunch of pyramidal chemical charges with detonators inside nuclear weapons. At least, there were in the earliest spherical implosion warheads. Later designs used more advanced two-point implosion. But in all these cases, the role of the chemical explosive was to precisely implode the fissile mass with enough inertial confinement to allow supercriticality to be reached. Once the actual nuclear chain reaction was triggered, the chemical explosives were utterly useless.

    Chemical shaped charges and nuclear shaped charges serve the same function, to a degree, but they use completely different and unrelated mechanisms. A chemical shaped charge uses an explosive material molded into the form of a concave lens. When the explosive is detonated the combustion takes place on the surface of lens so that the supersonic, superheated gases from the explosive are inertially focused into a thin, penetrating stream that cuts through pretty much anything. 

    In contrast, you cannot really shape or mold a nuclear explosive to produce a comparable effect, because the nuclear explosive depends on a particular geometry in order to achieve criticality. A nuclear device can only be spherical, as in the case of a fission warhead, or cylindrical, as in the case of a Teller-Ullam thermonuclear fusion bomb. In either case, the explosion is so viciously energetic that it will smooth out any inhomogeneities into a gigantic spherical plasma death ball every time. No way to shape that.

    However, it is possible to use a portion of the energy from the exploding nuke - usually the xrays - to redirect momentum in a particular way. That is how a Teller-Ullam device works, after all. Something very dense, like tungsten, will be able to absorb a large portion of the x-ray spectrum energy even though it isn't going to be able to physically confine the explosion. It's like being on a trampoline and getting a bounce from a really really fat kid: you are going to get launched, but you can control what direction you'll be launched in if you know what you're doing. By giving the tungsten plate a particular shape, you can cause its momentum to have a particular vector.

  16. 4 hours ago, Camacha said:

    This is an argument that has been brought to bear a couple of times now, but has never been substantiated. Please indicate for what applications the imperial system would be more powerful. There does not seem to be a scenario that is not highly trivial. In reverse, there are many examples of how the metric system makes calculations that are complex in imperial a breeze.

    The whole concept of the metric system is that it is both powerful and easy. You can traverse the numbers horizontally (quickly changing scale, so going from kilometre to centimetre without making a single calculation). The real beauty, however, is that it also works vertically. If you tell me a pool is 800 centimetre long, 0,006 kilometres wide and 2 metres deep, I can immediately, without a calculator, tell you the respective dimensions in any of those units (8 metres by 6 metres by 2 metres). This also means that calculating the volume is trivial and easily done without a calculator: 96 cubic metres.  Without any additional calculations, I can tell you the pool contains 96.000 litres and filled with water weighs about 96.000 kilos or 96 tonnes.

    Try telling me quickly and without pen, paper or calculator what the volume and the weight of the water in a pool 0.003 miles by 20 foot by 120 inches are. As soon as you calculated it in gallons and pounds and someone tells you he needed it in stone and pints, you can whip out the calculator again. We are not talking yet about the fact that there are several standards for ounces, miles and other units.

    Imperial seems to work in the select countries it is used, so it certainly is not broken. However, powerful is not a term that springs to mind. It is a system that happened to become whatever it became.

    I actually gave a few examples already. In particular, the question was posed of how you'd divide 3 3/4 cups in half; as it turns out, I knew without blinking that that's 2 tablespoons less than 2 cups. 

    The Imperial system is more powerful when you're converting between units on the level of measurements around which the Imperial system developed, IF you know how to use it. If you grow up with the metric system, it is hard to understand the advantages of the Imperial system. 

    If you're trying to jump between orders of magnitude, then yes, the metric system's dependence on base 10 is convenient. But if you need to multiply and divide by single-digit integers, having a system with a lot of units that factor into each other by primes is really useful. For example, let's say you're trying to make matching suits for a father and son, and you need to scale down the father's measurements to fit the child. If you are equally well-acquainted with metric and imperial, I guarantee you will find it simpler to scale it down in Imperial.

    The density of water is a bit arbitrary. If it was filled with any other fluid, you're screwed. 

    3 hours ago, AngelLestat said:

    Celcius is perfect to set temperature for humans, I guess nobody can feel temperature with the skin and predict the real temperature with more accuracy than celcius without decimals. In fact there is other factor that changes everything.. humidity..  Which give you a different thermal sensation, sometimes can be 10 degrees of difference or more. 
    Then wind or a heat surface by radiation also change your thermal sensation, in addiction as someone else said: a room has no constant temperature either.
    So is completely pointless.

    And in case someone needs decimals for some unit, what is the problem?  Is so hard to understand than 3,5 is the half step between 3 and 4?

    Now lets go back to the basic..  why someone would not like to change to the metric system?  Or one step further.. to adopt the whole SI of units.

    Just take a look how complicated it is the imperial system just to manage distances (no even mention all the other units)

    You can see the Meter tens on the right (I know that you use only 6 or 7 of those, but still..)

    With imperial system you also need to learn all the other units like PSI, pound force, BTU, horsepower, slug and try to find some coherence between them...
    E=mc2 does not need extra constants because all the units has coherence between them. 
    If someone adopt the metric system (SI), it does not need to renounce to call some products or stuff by its mundane unit..
    We use those units all the time even if we dont know it, a cement bag, a bottle of soda (they always add 0.25 liters in promotions until they reach 2,25 liters which become the standard on coke or other soda), 4 limons, etc.

    But then, the SI makes you life super easy in everything else.


    E=mc2 is not a "metric" equation. Unless you think the m stands for meters.

    Yes, imperial has a lot of units for length. With a lot of different prime factors relating them. That's the point. Feature, not bug. 

  17. 10 minutes ago, DerekL1963 said:

    o.0   It doesn't actually work that way.   On top of which, you'll need to make your nozzle and chamber cooling passages much more structurally robust to withstand the increased temperatures and pressures post disassociation.  (As well as somehow rigging things so the disassociation doesn't propagate upstream.)  And you'll need cat packs anyhow to start the engine and bring it up to the temperature where it auto-dissociates in the cooling passages.  (Which auto-dissociation also severely limits your ability to throttle and introduced considerable startup and shutdown lag.)

    Easier to just not let the auto-dissociation get a foot in the door in the first place.

    Yeah, probably right.

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