sevenperforce

Typically, 2:3 or 1:2 or 3:4 orbital resonance is necessary to make sure the moons remain stable. Remember that the strength of a tide decreases with the cube of the distance but increases linearly with the mass of the satellite, so if you had a moon 1/9 the mass of ours that was just 1/3 the distance, it would produce tides of the same magnitude. Next, keep in mind that the square of the period of any satellite is proportional to the cube of the radius. So if you want a 2:3 resonance, you need the nearer moon at an orbital radius that’s about 76% of the radius of the more distant moon (because ((2/3)^2)^(1/3) = 0.7631). If you want a 1:2 resonance then the nearer moon orbits at 63% the radius of the distant moon; if you want a 3:4 resonance then it’s 82.5%. So...because of the way the cubes and squares interact, the tidal effect of any two moons in orbital resonance is proportional to the square of that orbital resonance. With a 1:2 resonance, the farther body will have 1/4 the tidal impact of the nearer body. With a 2:3 resonance, the farther body will have 4/9 the tidal impact of the nearer body. With a 3:4 resonance, the farther body will have 9/16 the tidal impact of the nearer body. This gives you a good starting point. If you wanted a 3:4 resonance and you wanted both moons to produce tides of the same size, you’d want the farther body to be 16/9 the mass of the nearer body. With our single moon, the tidal cycle is forced by a combination of the moon’s orbital eccentricity and the [smaller but still significant] solar tide. But with multiple moons, the resonances are going to force near-circularity and wash out any eccentricity effects. Comparing only the two moons, you get the most aggressive tides when the two moons line up and the mildest tides when they are opposite.

That’s the dumb thing about the IAU definition: they say “planet” is ONLY for the big eight and “dwarf planet” is for anything smaller. But adding a diminutive modifier isn’t how the English language works. It would be like saying a “toy poodle” isn’t actually a type of poodle, or a “kiddie pool” isn’t actually a type of pool. If the IAU had said “They’re all planets but some are minor planets and some are major planets” then that would have made much more sense, linguistically.

If you are averse to admitting Pluto's status as a moon of Neptune, let it at least accept the role of a planetary satellite of Neptune. Y'all know my preferred definitions for natural objects in our solar system (or any solar system): Star. Any object with sufficient mass to fuse hydrogen-1 in its core. Stellar object. Any object which is or was once a star. Stellar binary. Any pair of stellar objects which orbit a local common barycentre. Planet. Any gravitationally-rounded object, other than a stellar object, which composes more than 50% of the mass of all objects crossing its orbit around a stellar object or stellar binary. Giant planet. Any planet too large to have a defined surface. Terrestrial planet. Any planet small enough to have a defined surface. Satellite. Any object, other than a planet, which is in a permanent orbital resonance with a larger object around a stellar object or stellar binary. Moon. Any satellite which remains permanently within the Hill Sphere of the larger object with which it is in orbital resonance. Planetary satellite. Any gravitationally-rounded satellite. Planetary moon. Any planetary satellite which is also a moon. Comet. Any object in orbit around a stellar object or stellar binary, other than a satellite, which crosses the orbit of the planet nearest the stellar object or stellar binary. Asteroid. Any object in orbit around a stellar object or stellar binary which is not a planet, a satellite, or a comet. Planetary asteroid. Any gravitationally-rounded asteroid. These definitions are simple. Every object in our solar system (or, indeed, in any conceivable solar system) has a place, and nothing fits into multiple categories other than categories which are a subset of other categories. A fifth grader (or earlier) can readily explain all of these categories. Ceres? It's a planetary asteroid. Jupiter Trojans? Those are just satellites of Jupiter, along with all of Jupiter's satellites, including a number of moons, some of which are planetary moons. Luna? It's a satellite, subset planetary satellite, and a moon, subset planetary moon. Pluto? It's a planetary satellite of Neptune. Charon? It's a planetary moon of Pluto and also a planetary satellite of Neptune. Phobos? It's a satellite of Mars, subset moon of Mars. Earth? It's a planet, subset terrestrial planet. Uranus? It's a planet, subset giant planet. Everything has a place. It's clear and impossible to confuse and matches the vernacular easily.

Looking back through some of the earlier update images and came across something interesting. Full-resolution image here. A couple of curious things. First of all, we don't know what this is part of; it isn't labeled. It may be part of SN20 or it may not be. The careful observer will note that the empty section over to the far right has the insulation cut away to match the tiles themselves and has bare metal exposed with no tile attachment points at all. It's a very curious shape, too. More interesting, to me, are the four rectangular sections which are all aligned together on the near side. The top two sections have the words "DO NOT INSTALL INSULATION OR TILES" scrawled on them in sharpie; the lower two sections have the words "REMOVE" with arrows pointing to all the tile attachment studs. What is this? If this is not part of SN20, could we be looking at external mounting points for landing legs? EDIT: Also noted that the curved heat shield tiles are attached directly to the aerocover with no insulative underlayment: I wonder if they will put insulation inside the aerocovers....

Pluto is one of Neptune's moons. I said what I said.

Alternately, if you want to be boring, there's a solution that uses the existing hex tiles and the existing truncated pentagon tiles along with just three other tile types: an elongated pentagon and two mirrored irregular hexagons. This one has a straight channel but it's extremely short -- just a few tiles long.

Okay, so I think I might be onto something here. We have only one decent image of studs on the nosecone, so I did a little bit of pixel counting and tried my best to map it onto a repeating pattern: There's a single inverted row in the middle, spaced to match the low rows, and a series of upper rows that are all spaced to match each other but are more widely spaced than the lower rows. I extrapolated that pattern out and filled in all the studs with regular hexagon tiles in the places where they would fit without colliding: (note: I'm not 100% sure that about having a 9-tile gap on the bottom and a 10-tile gap on the top. It may be as small as 7:8 or it may be as high as 12:13; I just have no real way of knowing for sure without better imagery) That's where I started thinking a little. We know these stud patterns are asymmetric; there is only one possible way to attach each tile to each stud pattern. You can't rotate a tile 120 degrees and just stick it on. Why do an inverted row? Is it just to remind the workers attaching the tiles that this is the "special" row? Maybe. But when I see asymmetric patterns laid out in a symmetric arrangement, I think about chirality and congruency. I played around with it a little bit, and here's what I came up with: I see two possibilities. On the left, you have just three unique tile types: the mirror pentagon, the right-handed pentagon, and the left-handed pentagon. The tiles on the upper layer are congruent with the tiles on the lower layer. There is no straight seam and the gaps are quite small. On the right, you have nine unique tile types: a mirror pentagon, four right-handed irregular hexagons, and four left-handed irregular hexagons. However, the right-handed and left-handed irregular hexagons are mirrored, and once again each tile on the upper layer is congruent with the tile directly below it, so you can simply rotate either matching tile 180 degrees to attach. This also has no straight seam, but it also has virtually no gaps at all. This seems like the most straightforward way of doing it. With this approach, you probably wouldn't even need any extra insulation under the seam regions. Almost the entire nosecone is tiled in the same ordinary hexagons as the body; you just have these mirrored, congruent tiles creating a seam every few rows. You can taper your ogive more and more by having fewer and fewer "regular" rows between seams, up until you are very close to the nosecone and you can do your final few layers in individually tapered, curved tiles. Just for fun, here's the final version showing both configurations with the stud marks removed and with several layers of tesselation up the ogive:

Probably a thermal curtain, in case of failure.

Hypersonic vehicles experience lift from wave drag rather than from airfoil drag, so their lift/drag ratio has an upper limit. The Küchemann relation says that the maximum lift/drag ratio cannot exceed (4M + 12)/M, where M is the Mach number. So Von Kàrmàn's line is based on a maximally streamlined hypothetical vehicle, since L/D has that upper limit.

Wait, no, I spoke too soon. If you look closely, there is a row of inverted tile studs. This row lines up cleanly with the mounting points below it, but it is misaligned with the row above, indicating that the row above has a different tile count. That middle, inverted row must be the transition between layers. We'll have to wait and see whether they go with a simple straight-edge cutoff or if they use some sort of adapter tiles...

I think for post V-2 rockets, he included the fins as a personal signature on the rocket. I think he called it his trademark or similar. I also suspect that between the simplicity and effectiveness on his earlier rockets, he wasn't about to give them up when moving to more complex rockets (when was the last time a fin failed on a rocket?). Fins were included on the human-rated Redstone, Saturn I(B), and Saturn V to help with launch abort. If things went south and the first stage shut down during boost, you’d want a way to keep the first stage passively pointing in the proper direction as long as possible to give the launch escape system time to act, rather than have the stage begin tumbling immediately. The concern was that without fins, a first-stage shutdown could lead to a tumble and a rapid conflagration with a fireball too large for the launch abort system to escape. Atlas-Mercury lacked fins because the bulky sideboard engines provided enough tail drag to maintain pointing in an abort. Titan-Gemini lacked fins because the Gemini capsule didn’t have a tractor abort system at all (and was also basically a death trap). Also IIRC Redstone may have had actuated fins........

No one in this thread has said this yet, so imma say it.... Please for the love of all that is good and holy, observe some basic caution when cooking rocket candy. Do NOT cook your rocket candy indoors. Do it outside, using a portable burner. Do NOT use a portable burner with an open flame; instead, use one that plugs in to an extension cord with an electric heating element. Make sure your area is well-ventilated. Follow instructions carefully. When cooking rocket candy, make sure you dissolve the ingredients in plenty of water. If possible, use an intermediate heat sink like a copper plate to maintain even heating and avoid the formation of hot spots. Hot spots tend to autoignite and then you WILL blow your face off. WEAR PROTECTIVE EYEWEAR AT ALL TIMES. Preferably something with full coverage. Protecting your vision is the single most important thing you can do. If you screw up the cook and your mixture ignites, protective eyewear WILL make the difference between sustaining serious burns and sustaining permanent blindness. I have made many sugar rockets. Grinding the ingredients (SEPARATELY) and then packing them together is the safest way to do it. Once you’re comfortable with that, you can start cooking. But please for the love of Thor be careful.

It looks like an ordinary pattern at this point in the curve, so perhaps they are simply accepting the gaps. If they are willing to accept the gaps then the simplest way of doing it is to introduce alternating smaller hexagons into the shield one layer at a time.

Looks like very large COPVs for the hot-gas thrusters are positioned in the interstage region. I wonder if this one is as armored up there as subsequent models will be.

As far as my understanding goes, it wouldn’t be significantly different than any other telescope mirror. The mirror of itself, of course, would be parabolic and circular, but you would merely need to cut it to fit. You can have an irregularly-shaped mirror or even a mirror with gaps and it will still provide light gathering capability just fine.

Something else to keep in mind is that an optical mirror need not be circular. You could just as easily have an optical mirror in an oval shape, mounted add an angle inside the starship payload bay. The width of the mirror would still be limited to 9 m, but the length of the mirror could be substantially greater, up to 15 m or so. As long as the chomper door can actuate open enough to expose the entire payload bay, you would have a dramatically greater effective light gathering area. Of course, such a design would be extremely heavy, so orienting would be propellant intensive in comparison to dedicated space telescope designs. You might want to mount several Starlink satellites around the payload bay to provide krypton-based RCS and thus lengthen duty lifetime. That approach would last much longer than trying to use hot gas for all the pointing. Every couple of years, you could simply fly it back down to earth to be serviced, then launch it again.

Minimum altitude orbit is a question of drag. 80-83 km is the point at which you could complete one full orbit without drag immediately taking you down (though probably not more than one). Maximum Earth orbit is a question of the Hill Sphere, the region where Earth’s gravity well dominates rather than the sun’s. Earth’s Hill Sphere ends at around 1.47 million km.

It doesn’t have to worry about cost-based competition until it has capability competition. SLS is enormously expensive for its payload to orbit, but it has the highest capability of any launch vehicle. Its supporters in Congress will continue to justify it as “expensive but necessary” until a vehicle comes along that can match or exceed its capability.

Better than being the guy who has to hold the matches under the business end of a Soyuz

I can see smartphones, computers, etc. in my dreams, but if I attempt to interact with them, they won’t respond properly to input. A page will scroll endlessly in unintelligible text or I will try to type and the letters will come out garbled.

This is a stupid question but I’m distracted by that SSTO thread, so.... The RVac is basically an SSME. It is optimized for a vacuum but it can fire at sea level. What gets more payload to LEO: a notional expendable SSTO based on the RVac or one based on the RS-25? I was counting the RVac lol, so......3.3 F-1s then.

That’s a whole lotta engine. How many F-1s is that? Looks like an Octaweb. Just imagine, if that was the entire thrust structure by itself, you’d already be looking at a rocket with almost as much liftoff thrust as Falcon Heavy and significantly more payload, in a single stick. And that’s less than a third of the total engines on this stage.

There are a number of things you could do to simplify the design somewhat. Small performance hit but you could probably get better thrust out of it. The RS-25 uses two fuel-rich preburners: one for the LOX pump turbine shaft and one for the fuel pump turbine shaft. Instead, you could just have a single preburner and split the exhaust out to two separate turbines, like the gas generator exhaust on the RS-68 and YF-77. Slight efficiency loss because the preburner exhaust would have a longer path to the turbines, but only using a single preburner would probably save weight anyway. Or, hear me out....... Expander cycle engines are simple and reliable, but they have poor thrust-to-weight and relatively low chamber pressure because there is only so much heat you can extract from the engine bell, and that gets worse and worse as your engine gets bigger. However, what if you did a combination? Keep the same fuel-rich preburner for the LOX turbopump, but use a closed expander cycle for the fuel turbopump. Use the additional heat from the preburner to increase the thermodynamic potential in the expander cycle loop. The LH2 would flow around the nozzle, chamber, AND preburner in order to absorb the maximum amount of heat. It would run through a turbine (which would turn the LH2 turbopump shaft), then exhaust into the preburner, which would burn with a small amount of LOX. The fuel-rich exhaust would run through another turbine (which would turn the LOX turbopump shaft) and then exhaust into the chamber. The highest pressure would be in the LH2 coolant loop, which is an extremely gentle thermodynamic cycle in comparison to a preburner cycle. You still need a seal between the preburner turbine shaft and the LOX pump but that’s not nearly as challenging as dealing with an oxidizer-rich preburner. And since the preburner is only running the LOX turbopump, it is smaller and uses less energy than if you had one preburner for both pumps or two separate preburners. Another approach, if you’re looking for a proper SSTO engine (for whatever reason), would be a variable-mixture tripropellant engine running LH2 *and* methane with LOX. The thermodynamic engine cycle for **that** would be insanity, though. Maybe you could do a hydrolox gas generator with exhaust to a LOX turbine and a fuel turbine, geared to both a LH2 pump and a methane pump. LH2 and methane play well enough together that the whole gearbox could be fuel-lubricated with no worries about leaks. The gas generator would run on LH2 the whole time due to its higher specific energy, but the chamber would ignite with methane alone at first and gradually shift the mix to hydrogen during the climb to orbit.

A closed expander cycle engine is somewhat less efficient for sea level purposes not so much because of low thrust, but because of low chamber pressure. Low chamber pressure means less of a pressure drop between chamber pressure and atmospheric pressure, which leads to wasted potential. Likewise, it is preferred for upper stages not so much because those stages don’t need as much thrust, but because operating in a vacuum means a larger nozzle can extract that wasted potential. Adding a larger vacuum nozzle to a gas generator engine will extract whatever wasted potential remains, but it cannot match the vacuum specific impulse of a closed expander cycle engine because it does not have as much remaining potential energy as a closed expander cycle engine. No matter how large of a nozzle you put on an RS-68, it will not be able to get anywhere close to what an RL-10 can produce with even a relatively small nozzle extension.

Ah, yes. The expander cycle. Famously inefficient. So inefficient, in fact, that the RL-10A-3 used on the Saturn I S-IV Second stage could only develop 444 seconds of specific impulse. Without a nozzle extension. Yes, you can achieve higher vacuum efficiency by using a longer nozzle, but that does not mean that any engine can achieve arbitrarily high specific impulse by using an arbitrarily long nozzle. The M1DVac is already nearly maxed out. If you doubled or even tripled the length of the nozzle, it still would not be anywhere near the 360 seconds that the RD-0124 gets. You could get 350, maybe 351 if you were really lucky, but that is it. The energy simply isn’t there. Similarly, adding an arbitrarily long nozzle extension to an RS-68 would not achieve arbitrarily high specific impulse. You might be able to get up to 437-439 seconds but you would never be able to achieve the 444 seconds that the RL-10 gets without any nozzle extension at all. You certainly couldn’t get the 460-470 seconds that the RL-10 achieves with a proper nozzle extension. You do need high thrust for sea level engines, but you are confused with respect to the way that gas generator engines and stage combustion engines achieve that. The specific impulse of any rocket engine depends on just two things: the amount of energy imparted to the exhaust and the average molecular weight of that exhaust. However, the amount of energy imparted to the exhaust, in turn, depends on the following sequence: The specific energy of the propellants The percentage of the propellant’s specific energy which is burned in the combustion chamber The percentage of the propellant which passes through the combustion chamber Total combustion efficiency The pressure in the combustion chamber The pressure at the nozzle exit All of these factors are important. You cannot ignore them. Adding a longer nozzle extension merely lowers the pressure at the nozzle exit, the very last step in the process. And while that is definitely very important, it cannot compensate for losses earlier in the process. Up until the last two variables, chamber pressure and exit pressure, a closed expander cycle engine is always the most efficient thermodynamic cycle because 100% of the specific energy is extracted in the combustion chamber and 100% of the propellant passes through the combustion chamber. A staged combustion engine is slightly less efficient because part of the specific energy is lost to the preburner(s), even though 100% of the propellant still passes through the chamber. A gas generator cycle is the least efficient of the three because part of the specific energy is lost in the gas generator and part of the propellant is lost in the gas generator exhaust. The hidden variable is the chamber pressure. The bigger the pressure drop, the more of the specific energy that gets converted into thrust. For a sea level engine, exit pressure at the nozzle is limited by atmospheric pressure, so you need your chamber pressure to be as high as possible so that most of the energy is converted into thrust before you reach 1 bar. Gas generator engines and staged combustion engines are good at this because they have a more energetic thermodynamic cycle, which gives them more available work potential to push the propellants into the chamber at the maximum possible pressure. That is why simply adding a larger nozzle to a sea-level gas generator engine cannot make it competitive with a full-sized nozzle on a closed expander cycle engine. The gas generator has already sacrificed efficiency by burning some of the propellant in the gas generator and dumping the exhaust overboard, and it has already extracted most of the remaining energy from its propellants before its main exhaust reaches one bar. There’s just not that much energy left.