RCgothic

Because the mass of Krypton generated is negligible. On the order of 1.5% of the fission products, and only a few percent of the uranium fuel actually undergoes fission. For 1kg of uranium fuel you'd generate maybe half a gram of Krypton if what you start with was highly enriched. Even being generous, all the fission products are maybe 50g per kg U235. And they're heavy nuclei that don't accelerate well compared to H. Finally a kg of 5% U235 contains enough energy to raise the temperature of more than 100 tonnes of H2 by over 2000K. Fission products just aren't significant.

There are several reasons for going for a highly enriched fuel. The first is that U238 is heavy and you don't want to be bringing any if it's nut going to be doing you any good. The second is that a 'fast' reactor can function without a moderator if the U235 concentration is high enough. Moderators are heavy, so you don't want to bring one if you don't have to. I would also expect the fuel to be Uranium Dioxide, because Uranium metal undergoes a crystal phase change as it heats up that massively increases its volume. Ceramic UO2 is much more stable. UO2 has a density similar to lead.

Because Windscale wasn't a prototype for a water cooled reactor, it was a prototype for a gas cooled reactor. Why go gas cooled? Because water absorbs too many neutrons as it moderates, making it impossible to get a sustainable reaction out of un-enriched natural uranium and enriching uranium at the time was difficult and expensive and supply was basically dependent on the US, which was undesirable for political reasons. Also, enriched fuel is less good at generating plutonium than natural uranium, and we wanted Pu to make bombs with. Water-cooled graphite moderated is an option for a reactor operating on natural uranium, but as previously mentioned (Chernobyl RMBK design) that has stability issues. So that's why the UK went gas cooled. I think the logic for Windscale was that it would be cooled by an open cycle of air and thus they could skip the pressure vessel and heat exchangers and just exhaust up the chimney.

High five. One of my favourite incidents is Hunterstone B, Christmas 98. Total loss of power in a storm similar to Fukushima because an earlier loss of power had tripped the back ups, and with most everyone on holiday there wasn't enough manpower to reset them in case of a second loss of power. 4 hours before power was restored. Reactor was totally fine, and would have been fine for 20. AGR gas-cooled is just inherently safe comparatively. Magnox had even larger design margins. It's a shame the on-line refuelling doesn't work.

RTGs have specific power of around 5W/kg for comparison.

Graphite-moderated water-cooled was an extremely bad design choice from a stability point of view, but it wasn't the reason the reactor went prompt critical. It just made things worse once it did. The RMBK design was chosen because the graphite moderator absorbs fewer neutrons than water as it moderates, which allows for the use of unenriched natural uranium oxide fuel (cheap). The water coolant allows for higher power density than the other main graphite-moderated reactor type, which is gas-cooled, because of the higher heat transport capability. High power density and large size makes the RMBK design staggeringly powerful. But yes, in graphite-moderated gas-cooled designs you can't really get a loss of coolant from hotspot accident because it doesn't vaporize to form voids (it's already one big void), and the reactivity of the hotspot will reduce with negative temperature coefficient. In water-moderated designs, if you lose the water you lose the moderator, and unmoderated neutrons are less reactive, thus reducing power. Negative void coefficient. In the RMBK, if the water vaporizes, steam is less efficient at conducting heat away than water, but the graphite still moderates. Additionally, the lack of water means fewer neutrons absorbed and mute neutrons total. Positive Void coefficient. Steam explosion. But that that was just the endgame for Chernobyl. The prompt critical condition should not have been achievable in normal use, but it was being dicked about with, basically. As a fix they modified all the remaining RMBKs with neutron absorbers, and started using slightly enriched fuel. The graphite tips on the control rods were intentional, by the way, and sat in the middle of the reactor in the retracted position. They were there to boost the reactor power as they were being withdrawn. It wasn't realised that as the tips didn't fill the entire reactor they'd locally boost power at the bottom as they were being inserted... Hmmm. Well if I were designing a high power density reactor without shielding, water is heavy and doesn't get that hot, so I'd throw out all water-based designs. There have been a few designs that might inform design of a spacecraft engine. UHTREX is gas-cooled, graphite moderated, and operates without fuel cladding. Very efficient fuel burnup, and very high coolant outlet temperature (1300'C). The drawback is a very contaminated fuel circuit, but we don't care about that. Details are scarce, but based on an original diameter of 13ft with shielding I'd estimate 3m diameter spherical pressure vessel (~11t) filled with graphite (~29t) and .5t fuel with .5t for fuelling . 41t and 3MW output, burns through 6 fuel element per day for an endurance of ~200 days at full power. Roughly 73W/kg, but also required turbines and radiators. This is clearly dominated by the moderator, so let's throw that out too. I don't think molten salt will save any mass as a coolant when we consider the turbine loop volume, so Gas-cooled fast reactor it is. This type of reactor is very rare, with none built to my knowledge, but they're are a few later designs. Even so it's difficult to be specific. If I were to speculate, I'd say you could probably get the same power output out of a reactor half the diameter of UHTREX with no moderator. The trade off is enriched fuel, but money no object for the star federation, right? That would be about 1MWe/tonne, for the reactor itself. 3t for a 9MWth/3MWe reactor core. It would need to radiate about 2MWth per MWe, or ~70m2 to exhaust 6MWth for a 9MWth 3MWe reactor at 850'C reactor inlet temperature. I found a NSS article implying that would probably weigh around 0.7t. Best guess for a bespoke 3MWe turbine generator plant is approx 4t. Call that all 9t including refuelling and control. 333W/kg for the whole system. And if one of these let's go you still won't get a nuclear detonation. I expect the pressure vessel would go off like a fragmentation grenade, spraying glowing chunks of high velocity debris.

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.

Tidally locked to a gas giant is not necessarily a problem, it just means very long days.

Again, have to mention that RC rockets/ rockets with guidance computers are totally illegal in many jurisdictions because they can be used as missiles. You can just about get away with auto-stabilization, but seriously guys, don't get yourselves arrested. Contact your local NAR group for advice.

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.

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: Launched: 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: http://imgur.com/a/XyYWU The hitches were that I stuck the xenon-electric thruster on backwards, and forgetting to actually put any ore in my ore probe.

Yes, that looks like a normal F1 concept to me?

New entry for 1.2 before I reinstall FAR. Before Take Off: Full throttle: Take off Rocket mode engaged! Apoapsis established: Orbit acheived! Stayed up a few hours waiting for KSC Dawn: Beauty 1: Beauty 2: Reentry periapsis and fuel remaining: Coming in Hot: Coming in early: Final approach: Landed with 1000 units of fuel still onboard:

I followed up the above post with a calc on the half-period of an orbit with apoapsis at oort and periapsis at sol. 2 million years. So that idea's out. Although imparting an initial velocity would massively reduce that, but the maths is escaping me at the moment.

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.

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.

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.

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.

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.

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.

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.

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.

The F1-B engine is part of one proposal for advanced kerolox boosters to replace the shuttle derived ones as part of... Block 2? There are other options as well.

Except that when you hold your breath and that 14% residual volume expands 4x the pressure drops correspondingly to 1/4, which would then actively extract oxygen from your blood. Being a small volume, the partial pressures will equalise some, so it's not quite as bad as 3/4 of a total vacuum. But 60-75% as bad is still pretty bad.

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'.