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Posts posted by RCgothic

  1. Nuclear engineer here. 

    No, no fission reactor design can detonate like a nuclear explosion. When the reaction runs away out of control the fissionable material heats up, and there are several mechanisms by which that makes the fuel less reactive, bringing the reaction to a new equilibrium state. The fuel may melt, but if there's an explosion it's going to be as a result of other materials present in the core (hydrogen, steam, molten salt explosions).

    Designing a bomb to detonate (even to get it to fizzle) is very difficult.  You need to convince most of the fuel to react before heat increase brings the reactivity down. This requires both a very dense concentration of fissile material (else neutrons won't propagate through the entire core fast enough) and a very low concentration of non-fissile nuclei (which absorb valuable neutrons). Additionally, the need to exclude non-fissile nuclei also generally excludes the use of a moderator. A moderator is a material that slows down the 'fast' neutrons emitted by fission events to a 'thermal' level which more readily react with fissile nuclei. Without a moderator the fissile material is less reactive, so yet greater density of fissile nuclei is required.

    This all add up to very high enrichment, typically 95-98%. Even fast reactors don't normally get this high. Most reactor fuel is uranium oxide (UO2) enriched to about 5%, although the presence of the oxygen atoms makes the effective reactivity even lower.

    Finally, if you attempt to make critical assembly casually, it will just heat up as portions go critical before the full mass. Therefore a very rapid change of geometry is required, either compression or gun type in order to set off the final detonation.

    Reactors on the other hand are designed not to explode! Not only do they lack any means to effect the final geometry change, sufficient fuel enrichment, and also have far too many foreign nuclei in the way, they are carefully designed not to operate in dangerous reaction regimes. They do this by manipulating several types of criticality:

    In a sub-critical assembly the reaction is not self sustaining, and if the reaction was previously critical or supercritical the reaction power will be reducing.

    A critical assembly is one in which the number of neutrons released is precisely as many as is required for the reaction to be self-sustaining at its current power level.

    A supercritical assembly is one in which each reaction increases the neutron flux. The reaction thus grows exponentially.

    A power plant must operate in all these regimes. A plant that could not go supercritical could not start up.  By adjusting the number of neutrons absorbed in the reactor the power level is controlled.

    However there are two further types of criticality which are extremely important to the design of reactors, referring more to the response time than whether the power level is changing:

    In a prompt critical (supercritical) reaction, enough neutrons are immediately released in each fission reaction to sustain further reactions. The timescale of this process is on the order of the travel time of the neutron between reactions (milliseconds). This is the type of reaction required for a bomb, although for reasons discussed above it would still not cause a nuclear detonation in s power plant. The fuel gets (potentially extremely, damagingly) hot and the reaction slows/stops. The speed with which prompt criticality changes the power level of a reactor makes it impossible to control, and reactors are always* designed so that they cannot go prompt critical. Reactors will always absorb too many neutrons, even with all control methods withdrawn.

    The other type of criticality is delayed critical. A quirk of the fission reaction is that whilst each fission event creates neutrons, so too do the fission products a couple of seconds later as they decay. (It is for this reason a prompt critical reaction cannot be simply critical - if fission neutrons are enough to be self-sustaining, the delayed neutrons will later make it supercritical). If the reactor is operated such that on fissile neutrons alone the assembly is subcritical and delayed neutrons make up the difference to critical or supercritical as required, then the exponential coefficient is on the order of seconds and minutes rather than milliseconds. In conservative designs, reactors can take hours to build up to full power, leaving plenty of time for manual and automated control systems.

    And all that is why the worst that can happen given total control/coolant failure is a meltdown and not a mushroom cloud.**

    *The Soviet RMBK design can in certain situations, which is why Chernobyl had a prompt critical excursion when it was messed about with by people who didn't know what they were doing. The heat build up caused a steam explosion and graphite fire. Annihilation of the cooling systems caused the core to melt. 

    **Ok, conventional explosions can also cause mushroom clouds, but you know what I mean.

  2. Be aware that manufacturing solid rocket motors in the UK is illegal:


    "Under the conditions of the 1875 Explosives Act, the 1883 Amendment, and later Prevention of Terrorism acts, it is an offence to manufacture your own solid fuel rocket motors, since these are classed as an explosive."


    I don't know what the relevant US laws are. It's something to be aware of depending on where you're based.

  3. Wow, the Galactica's a beast!

    Following on from my previous attempt, I installed a docking port and an RCS system and attempted to hit some of the bonus achievements.

    Payload up, 1 xenon-electric probe sat:




    Docked to a space station:


    Ore probe for downlift (forgot to fill it - shh!)


    Ore probe docked:


    And landed at KSC for probe unloading:



    Full album:



    The hitches were that I stuck the xenon-electric thruster on backwards, and forgetting to actually put any ore in my ore probe.


  4. Much easier would be asteroid redirection. Eg Oort cloud objects don't orbit very fast - 3m/s ish. You could divert a very large object into the inner solar system and it would arrive at earth at approx 42km/s or even faster if it were slingshotted around Jupiter.

    Somebody check my maths on this, but a high performance ion thruster (exhaust velocity 5km/s) could halt the oort orbital velocity of a 10,000 ton object with 600kg of propellant. The sun would them do almost all the rest.

    If that could be brought in at 60km/s the kinetic yield would be 4 megatons, or 2 megatons without a slingshot.

    The drawback would be a painfully long deployment time, but if you brought in many objects periodically on a near miss trajectory they could be altered to strike the earth at much shorter notice.

    With the very small amounts of DV required and resource extraction, one spacecraft could potentially redirect many such objects.


    But considering the launch costs for getting out there in the first place, it would be far cheaper to just nuke it. Unless you were already in an oort civilisation.

  5. And that's just an argument for multiple colonies, not zero.

    Mars would actually be much less vulnerable to global catastrophe too. With power mainly from nuclear and enclosed habitats, there's not much an impact on the other side of the planet could do compared to a similar atmosphere-disturbing event on Earth.

  6. Earth could be rendered uninhabitable by any number of scenarios. In that situation humanity is saved by an offworld colony.

    Also, in a sufficiently large disaster, even with a 1-5% survival rate civilization on Earth would collapse. With all the easily extracted resources already extracted, there's no reason to assume advanced civilization could emerge again second time. An advanced society based on Mars could bootstrap us back into the space age.

    There are plenty of reasons why getting off this planet permanently is good for humanity's long term prospects.

  7. Centre of mass is the average position of all the masses in a body. It is always fixed unless you add/remove mass or allow those masses to move around with respect to one another. If the resultant force of all applied forces acts through the centre of mass the body will translate without rotation. Any resultant moment will cause the body to rotate about its CoM without translating unless there is also resultant force.

    Centre of Gravity is the average position of every mass weighted by the gravity field at the position of that mass. It is a concept that makes visualising resultant forces more easy by allowing a single bulk force to be applied at the CoG but it can be misleading. For instance, considering Gravity as applied to the CoG of an airliner will help you find the pitching moment, but it won't remind you that the weight of the wings still need to be supported back to the fuselage.

    In a uniform gravity field, CoM and CoG occupy identical positions.

    For shallow gravity fields (I.e. Earth's), the positional difference is usually negligible except for ridiculously precision applications or ridiculously large objects.

  8. Sure they will, compared to other Martians. It might not be rich by Earth standards, but that's a pretty irrelevant comparison when the nearest Earthling is an 80 day flight away.

    Who wouldn't want to have corporate/political/fiscal power over a significant fraction of a planet? When the colony is established it will have its own economy. Being top dog in that economy would be like being a big fish in a small pool, compared to being an irrelevant fish in an ocean.


    That's going to be an attractive investment to a certain type of person. It's trading Earth funds for Martian power.

  9. I would assume that the Spaceship is capable of blasting free of the booster if necessary. If there's a problem with the Spaceship itself there's probably nothing that can be done. 

    You can't just ejector-seat 100 people, and the Spaceship doesn't contain parachutes because it lands propulsively. Any cabin detatchment would have jettisoned its means of landing safely.

    But at least its launch abort scenarios are likely to be more benign than the shuttle's.

  10. Also, whoever first gets a foot in the door on Mars is likely to end up being the richest person/company on Mars. That's not nothing, even if the investment doesn't return to Earth or the RoI is small by Earth standards.

  11. New Shepherd attains an apogee of what, 100km?

     That's 0.981MJ/kg specific energy to the payload at apogee, conservatively.


    Falcon 9 first stage separates at about the same height, but can be doing 8000km/h +. That's an additional 2.46MJ/kg specific energy, or 3.35MJ/kg total.


    So before even involving the difficulties of piloting to a remote landing site, energetically what SpaceX does is nearly 3.5 times harder.


    Ok, Blue Origin did it first. But it's like the difference between landing Freedom 7 and landing Apollo 8.

  12. 4 hours ago, WestAir said:

    When you are training for a private pilots certificate the instructor will often have you close your eyes and put the aircraft into an unusual attitude, then have you open your eyes and try to return the aircraft to straight and level flight. This is often completed in less than 5 seconds. Do you remember where you read this study, I want to read it.

    I misremembered. I found the Google report, it's actually 17 seconds to respond to alerts and take back control. To actually react intelligently is another matter entirely.

    The situation you describe sounds like a pilot with hands on anticipating the situation that they need to rectify. In an emergency whilst operating on automatic the manual operator may have been reading, snoozing, facing the wrong direction. They aren't going to have the situational awareness of an alert pilot anticipating that they're going to have 

    Also 'often completed in less than five seconds' is not 'everyone completes in less than five seconds'.

  13. Studies have shown it takes at least 12 seconds for a passenger to become adequately aware and in control in an emergency situation. Vehicle accidents happen so quickly that a manual override is totally useless.


    Yes, a system failure will lead to deaths. So you design a system that cannot fail catastrophically or with redundant back ups as much as possible.


    Automated vehicles will be orders of magnitude safer than manually operated ones. They don't get tired. They don't get distracted. They can look in every direction at once and see through light obstructions. They can talk to each other in order to better manage traffic and reduce possibility of collision. They will never get reckless or exceed the safe operating envelope. They cannot be medically incapacitated at the controls. They can have back ups for failed components.

    Humans were not designed for piloting vehicles. To suggest we're somehow superior to a purpose designed system is human chauvinism.

  14. Four engines. In the event of failure, two have sufficient power to stay aloft at max throttle, with the third providing trim for offset load. 

    Obstacles and noise issues can be mostly avoided by a minimum height for non-vertical flight. Power lines in the UK aren't higher than 55m And anything taller than that would be both mapped (regulatory requirement) and detectable by the onboard collision avoidance system.

    Drones won't move as fast as aircraft, so the 200m mandatory minimum distance can probably be reduced. They'll also talk to each other to keep out of each other's way ideally.

  15. 3 hours ago, lajoswinkler said:

    Wasn't Pandora's atmosphere ridden with ammonia (NH3)? In any case, That much H2S is extremely dangerous and would lethally poison you in a few breaths (you'd die in minutes if you don't receive immediate help).

    Hydrogen sulfide is more poisonous than hydrogen cyanide and, being a small molecule, it diffuses through skin so at those concentrations it would poison you by entering your blood via skin.

    You could hold your breath but you wouldn't last for long even with the mask on. Kidney failure in case of ammonia and cell respiration failure (and suffocation) in case of hydrogen sulfide.

    Yet on screen evidence suggests it's safe enough to walk around in only a mask. That's either a goof or an indication that H2S is on the order of ppm.

    LCL0 is on the order of 600ppm (inhaled?) for 30 mins, or 800ppm for 5min. Max US regulatory exposure is 20ppm (inhaled?). 

    Lung surface area is approx 25 times greater than skin surface area (conservatively). It's also thinner and more readily diffused across. It therefore stands to reason that a lethal inhaled concentration of ~800ppm atmospheric equates to less than the regulatory equivalent skin dose.

    Put another way, the concentration required to kill you if you're wearing a mask by diffusing through skin would be of the order 20,000ppm or 0.02%.

    If Pandora's atmosphere is around 800ppm it should be safe to wear a mask and lethal in approximately the observed time frame without one. H2S is detectable at around 10ppb, so the smell of Pandora's atmosphere would be very, very strong.

  16. 9 hours ago, dharak1 said:

    While on the topic of why air pressure matters, does this mean the scene in Avatar where the Colonel opens the door on Pandora but holds his breath so he doesn't need a mask impossible? He holds his breath before the Pandoran air reaches him and air pressure on Pandora is supposedly very near earth pressure. From what I'm taking from this it would only matter if 1. He held his breath after breathing in the outside air or 2. The pressure difference was significant enough for the air to either be pushed in or pulled from his lungs even while holding his breath. 

    Here is the scene I'm talking about for those that haven't seen the movie:

    The part where he leaves is around 1:50

      Reveal hidden contents

    The part where he leaves is around 1:50


    The atmosphere of Pandora is apparently 0.9 Atm of pressure with a composition of:

    Nitrogen, Oxygen, Carbon Dioxide (over 18%), Xenon (5.5%), Methane and Hydrogen Sulfide >1%

    I don't know if the pressure is low enough to not be able to be held back by lungs or not.


    If you're in a plane or spaceship that depressurises it's a bad idea to hold your breath because the pressure in your lungs will dangerously over-inflate them without external pressure on your chest. And then once you've breathed out the zero/near zero partial pressure of oxygen in your lungs causes the oxygen in your blood to diffuse out down the pressure gradient and you've got 30s.

    It's actually a similar situation if you find yourself in an atmosphere at sea level composed of pure nitrogen, carbon dioxide, or other composition with zero oxygen concentration. Breathing normally, that's still a zero partial pressure of oxygen which will suck the oxygen out of your blood by diffusion. 30s.

    Unless you hold your breath. If you don't breath in the oxygen free atmosphere, then the air in your lungs maintains partial gas pressures not that different to that in your blood. It'll slowly deplete oxygen and accumulate CO2, but the driver is the gas concentrations in the blood. There's no sudden loss of partial pressure to suck the gasses out and you can last a lot longer.

    So then you have Pandora. That's not a lack of oxygen or a lack of atmosphere. That's toxic gasses. If you breath normally, there's suddenly a partial pressure of toxins in your lungs that force their way into your bloodstream. But if you hold your breath, it's pretty much equivalent to holding your breath normally. The colonel wasn't in immediate danger unless he took a breath. No toxins in the lungs means none in the bloodstream.

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