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RCgothic

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Everything posted by RCgothic

  1. My mistake indeed. It's tricky to keep all these different payload figures straight! Guess that puts New Glenn squarely in SLS block 1 territory.
  2. Motokid600 already picked up two of my first thoughts on that vid: grid fins and moving landing platform. My other thought was that that is an excessive amount of cowling around those engines. As a two stage launcher it doesn't quite compare to falcon heavy to LEO or GTO, yet is much bigger. I guess the difference comes if they can stick a third stage on it'll have greater potential beyond earth orbit (Falcon is pretty much at structural limits). Certainly interested in seeing how they do though, and a second launcher in the 45t to LEO range makes it more likely the full payload of falcon heavy will be used, as sat operators like redundant options.
  3. How do they get a crew of four aboard if the space suit hogs the airlock? Other than that, the casualness of the whole thing is what struck me. The degree of autonomy of the ship, how easily things are constructed on orbit (and then we install the nuclear reactor!), the aggressive manoeuvres close to other vessels/structures, and the way a repair was made by just piloting a space suit into a jet of venting nitric acid at however many atmospheres and casually plug that right up. And our mars mission is under constriction even as we flyby the moon for the first time. No incorporating lessons learned for us! Oh, and the space station was at an altitude of 1000 miles, nicely within the inner Van Allen belt and squandering the oberth effect for moon missions.
  4. The big stuff is just a source for the little stuff, which is the real threat. The big stuff can be tracked and avoided. A cloud of little stuff is lethal. Big stuff spawns little stuff whenever it is struck by anything. Enforce an 'everything must be de-orbited at end of life' regulation. That takes away new debris sources. Lasers are the means to get rid of the smaller stuff, but it requires very good tracking and an exceptional power source.
  5. Latest I'm hearing is NET 12th March for Echostar 23.
  6. I think people would fall from a height much greater than 60m. Centripetal acceleration alone may put people that high, but remember that at the point gravity turns off everything on Earth's surface is pushing down with enough force to generate 1g. That force doesn't stop just because gravity does. Assuming that it takes 5cm for the average person to lose contact with the ground, they'd gain an additional m/s just through this spring force. That's an additional 60m height over a minute for anyone not inside, even at the poles. Snark is also right about the guidance issues for a shuttle due to the lack of inertics. It's not going to know it has an extra accelerating except by atmospheric wind speed. Possibly not fatal to a mission depending on what stage of the launch you're at, but it could make max Q very dicey. Over 60s the maximum possible dv gain is about 600m/s, which is not an enormous amount. And you'd actually gain less speed due to additional wind resistance and the fact that once you've turned you aren't directly fighting gravity.
  7. Actually I think there were two flights of Columbia with ejector seats and a crew of more than two. For ethical reasons the mission commander requested they be disabled so that the two on the flight deck would share the fate of the rest of the crew. They would have been of questionable use anyway. Pre SRB burnout there would be a high risk of passing through the fire trail. Post burnout you're going four times the speed of sound at sea level and ejection starts getting dicey.
  8. Getting a little off-topic, but I believe the OV-0 series were never intended for flight, and the OV-1 series were supposed to be the operational vehicles. Except it was more cost effective to rebuild STA-099 (Challenger) into a proper orbiter than OV-101 (Enterprise), which then never flew to orbit and wasn't retroactively renumbered, but Columbia was upgraded to OV-99.
  9. So CRS-8 first stage is due to be reflown as SES-10. How do we refer to reflown stages? CRS-8/SES-10 could get a bit unwieldy after a few flights.
  10. For those in the UK, it should be noted that manufacturing solid rocket motors is an offence under the 1875 Explosives Act, the 1883 Amendment, and later Prevention of Terrorism acts, since these are classed as an explosive.
  11. Log10(y) =x for the same values as y = 10^x Loge(y) = ln(y) = x for the same values as y = e^x Similarly there are log#(y) functions for arbitrary values of #^x, but they're rare. I think the most common non-10 non-e values are 2 (binary) and 16 (hexadecimal).
  12. Or, if you want to calculate the DV required for a particular manoeuvre between orbits, that's an orbital mechanics question, rather than a rocket equation question. Generally manoeuvres are either Hohmann manoeuvres, which go circular to elliptical to circular in two manoeuvres, plane change, which is negating velocity in one direction and adding it in another by pythagoras, or launch/landing. Don't try and calculate launch/landing unless you are going advanced. Just look it up. Plane change DV is easy. How much do you want to change your angular inclination by, and how fast are you going? By the rearranging the cosine rule: DV = SQRT(2 * v^2 [1-cos(angle)]) Thus for zero inclination change cos(0) is 1 and the DV required is 0. For a 180, the DV required is 2*v, because you have to negate all of v and add it again in the other direction. For a 90deg change you need 1.414*v. And of course any other angle can be calculated in the same way. Hohmann transfers are a little involved and I'm out of time. Maybe someone else can explain, or I'll be back later.
  13. DV = Isp * g0 * ln(m0/mf) I'll unpack that a bit. DV is change in velocity. It can be calculated for a specific manoeuvre or more generally as the total ability of a vehicle or rocket to change its velocity by. Isp is specific impulse. It is a measure of engine efficiency - basically how long in seconds an engine takes to exhaust a weight of fuel equal to its thrust. Longer is better because it means you get to burn longer for the same quantity of fuel. It's directly proportional to exhaust velocity, and you'll often find the rocket equation written in terms of exhaust velocity instead of Isp. g0 is the acceleration due to Earth's gravity, and is required for the conversion between exhaust velocity and Isp. It may seem weird that we use Earth's gravity no matter where the rocket is, but that's because we're implicitly converting from kilograms to Newtons and Newtons are defined in terms of Earth's gravity. m0 is the mass before the burn. mf is the mass of the rocket after the burn. m0/mf is therefore a measure of the mass ratio, or how much of the rocket you've used as fuel. The more fuel you've used, the greater this number, the faster you go. ln(m0/mf) is the natural logarithm of the mass ratio. It's a non-linear function, but any calculator or spreadsheet can work it out. The reason it's necessary is because the fuel you burn first also has to accelerate the fuel you burn later before that can be used, reducing its efficiency. Burning the 1st kg of a 1000kg spacecraft changes the velocity by less than 1/600th of the amount the 999th kg will get you. Where m0 is the total mass of a spacecraft topped full of fuel and mf is the dry mass after all fuel has been exhausted, the rocket equation gives you the total ability of the rocket to change its velocity. If you have the DV required for a manoeuvre, and the Isp of the engines, then the equation can be rearranged to tell you either how much fuel you need to bring along on top of dry mass in order to perform it, or if you have current mass, what your end mass will be after the manoeuvre (And therefore fuel required - but better hope end mass is higher than dry mass or the manoeuvre is impossible). The rearranged form is: m0/mf = e ^ (DV / [Isp *g0]) Where e is the exponential function (opposite of natural logarithm) and ^ means "to the power of" And that's basically all you need to know. Any questions?
  14. It's a good point that recovering the centre core of a Falcon Heavy is going to be harder than recovering a Falcon 9. The second stage isn't changing but the payload is getting bigger, meaning stage 2 has less DV so Stage 1 has to be going faster at separation, faster even than Falcon 9 on GTO. This problem gets worse with crossfeed, which I saw someone on Reddit mention is still being studied. (Pinch of salt duely taken). But perhaps on Falcon Heavy there can be more fuel saved to leave greater allowance for the entry burn or even return to launch site! It's difficult to guess what their plans are.
  15. 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.
  16. 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.
  17. 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.
  18. 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.
  19. 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.
  20. 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.
  21. Tidally locked to a gas giant is not necessarily a problem, it just means very long days.
  22. 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.
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