Bunsen

I think the problem is the less-efficient trajectory needed for first stage recovery, where the first stage's horizontal velocity is kept lower. Those payloads are probably heavy enough that flying a recoverable ascent profile would leave an insufficient propellant safety margin. You never plan on burning the tanks dry for the primary mission, but you keep the option around in case something else goes wrong or to serve a secondary payload. SpaceX CRS-1 is a good example of this -- an engine failure made the first stage less efficient, and some of the propellant margin in both stages was consumed to compensate. That marginal capability had been sold, with the risks understood, to a secondary customer (Orbcomm wanted to test a prototype satellite). If everything had worked correctly, the second stage would have had plenty of leftover propellant after dropping off the Dragon, and would have carried that secondary payload to its intended orbit. When the primary mission ate into the safety margin, the secondary mission lost out and their satellite was left in an unusable state (so low that it deorbited in a few days).

The critical thing here is that our normal idea of how velocities add (e.g. if I'm running at 10mph and I throw a baseball at 30 mph, the baseball whizzes past you at 40 mph) is wrong (you see that baseball traveling a tiny, tiny, tiny bit slower than 40 mph), and the faster things go the more wrong it is. It's really close to right if the velocities are small enough (39.999999999999973 mph, in fact), but it breaks down horribly when the numbers approach c. That happens because distance and time depend on who's asking about them. I see you've already caught the distance part -- Lorentz contraction squishes things -- but there's also time dilation involved. Between the two of those, the apparent contradictions from the counterintuitive velocity addition disappear.

The Launch Escape System included a shroud that covered the entire CSM, for protection from the LES rocket blast and aerodynamic heating during ascent. The LES was jettisoned a little while after 2nd stage ignition, and it took the shroud with it.

Those grid fins are part of the launch escape system. They deploy to stabilize and slow the capsule after the escape rocket fires.

I think Shifty is right about the hydrogen serving as the moderator, at least in part. The reaction would be influenced by neutrons that leave the core and get reflected back, and hydrogen flow would have a large influence on their spectrum. Since thermal, or at least partially thermalized, neutrons are disproportionately effective compared to the intra-core fast neutrons, I expect that much of the reactivity control could be accomplished with external reflectors and controlled hydrogen flow.

When it's described as a "race," that suggests that it's won by whoever crosses a line first, not by who does it bigger or accomplishes how much science. Who could still run, or even walk, after the race is also separate question. Things are inherently ambiguous without specifying a finish line, but the Soviets crossed a hell of a lot of lines first (satellite in orbit, man in space, man in orbit, probes orbiting and/or landing on the Moon, Mars, and Venus). Defining manned landing on the Moon as the sole finish line seems pretty arbitrary -- If you had asked people in 1955 to describe the ultimate milestone in the space race, I don't think many would have said "land on the Moon, and robots don't count." It can be credibly argued that it was the most demanding line that anybody has yet crossed, but following your competitor until he collapses, passing him, and then declaring the finish line doesn't sound right. If you don't paint it as a race, though, and measure by points scored or yardage or what-have-you, then the USA is the clear leader today.

It would take a rather large amount of fuel to achieve a soft landing. The thing has about 180 m/s of delta-v left on board once it gets into lunar orbit; landing on the Moon requires decelerating from almost 1700 m/s to zero, plus extra to cover gravity losses during the burn. It would also require a larger engine, since its current engine would barely give it a 1:1 TWR on the Moon. With the additional structure, beefed up attitude control to slew the extra mass around, etc., it would probably more than double the spacecraft mass.

You ain't kidding. I'm not sure whether that's the move-it-or-lose-it ICBM heritage or just the fact that lots of stages means you can have a high initial TWR without excessive acceleration toward burnout.

I have seen instances, disturbingly many in fact, of the perverse unit called the "kilogram force" or kgf. Naturally, it is the weight experienced by a kilogram of mass at standard gravity, but I'm not sure whether it's the product of intentional trolling or if some deranged minds were simply jealous of the confusion and silliness enjoyed by users of the Imperial system. I think anyone who commits such a sin should be forced to spend a week doing thermal analysis in slugs, millifurlongs, BTUs, and microfortnights.

I think the sporadic little bits of flame at the tail end of the tanks really are normal, but it's not rocket fuel burning there. Those exhaust plumes kick out a huge amount of radiant heat, especially when there's more than one engine involved (each engine kinda shades the tank above it from its own plume, but the neighboring plumes have an unobstructed line of sight). Surfaces exposed to that are often covered with an ablative coating that keeps the underlying structure safely cool by boiling and burning away. The tail end of the Space Shuttle main tank did the same thing.

It's there, and whether or not it matters depends very much on how the radios are set up. Some systems are quite sensitive to frequency error, others not so much. Higher speed systems are usually less picky. For example, the cubesat I'm working on right now has one downlink transmitter on approximately 435 MHz with a 10 kHz channel width, and for low Earth orbit the ground station has to tune plus or minus one full channel to compensate for Doppler shift. It also has a 2.4 GHz radio with a 280 kHz channel width, which doesn't give a damn. Interplanetary missions see larger relative velocities, and often use rather low data rates to overcome the long distances and limited power availability. Doppler adjustment is designed into the system at about the same level as figuring out where to point the antennas, though, so it's not like it catches anybody by surprise.

Seriously? They don't. The only one of those that isn't faked is the rolling double cone, and it's actually rolling downhill. The first one has a pump hidden in the base (the tube doesn't go straight through the support under the vessel, watch how long it takes for the fluid to show up further along the tube), and all of the wheel variations have motors. I'm pretty sure those all have entries over in The Museum of Unworkable Devices, which is a good read if you want to understand why each of those ideas is stupid.

That's the most realistic aspect of it. There are a lot of complicating factors, and there are problems that come in at the bottom end where making engines really small also makes them inefficient, but the real-world tradeoffs between different fuels, different methods of driving turbopumps, whether you use pumps at all, what pressure your engine bell needs to operate in, etc. are a bit much for KSP's approach to engineering. So they distill it down to "efficiency is approximately proportional to inconvenience" and call it good.

That tends to happen when you're trying to express concepts for which human intuition provides little or no familiar reference. You explain them with silly, contrived little stories. Imagine several guys who've spent entirely too long at the bar, and now one of them is trying to explain intricate and wildly counterintuitive phenomena to the others, half of which he's making up and figuring out as he goes along, and doing it mostly with bar napkins and empty glasses for props. Now and then he needs a name for one of those phenomena, so he makes up something silly because he's drunk and on a roll. And now those few drunks are the only people on the planet who understand this new idea, and they tell other people about it before they've had time to think up a more respectable name, and it sticks. You now have a rather accurate picture of how a lot of the terminology came into existence, if not the ideas.

I just came up with the term "delta-L" then. Seemed natural enough to describe what attitude control systems do -- create changes (delta) in angular momentum (L). It's not delta-v, because wheels don't change the craft's velocity. The "unlimited" thing I was talking about was meant to refer to maneuvering, which involves changing the craft's angular momentum (from zero to something, to start rotating), then changing it back (to stop). RCS would have to burn propellant to start rotating, then burn more to stop, and you'll run out if you do it enough. Reaction wheels can keep on doing that almost forever, limited only by the availability of electricity and the lifetime of their bearings (which is finite, as Kepler has so recently demonstrated). For permanent changes in angular momentum, like counteracting external torques, you have to interact with something external to the spacecraft -- expelled RCS propellant mass and the Earth's magnetic field being the favorites, and RCS tends to be the practical option for manned craft. I think I thought of a good metaphor: Wheels let you borrow angular momentum for a while, over and over, but with a rather low credit limit. RCS makes you buy it every time. So if you only need it for a little while (say, to change which star your telescope is pointing at, which you're going to want to do a hell of lot of times), you borrow it from a wheel. But if you need to keep it (because your solar panels are asymmetric and you need to keep that antenna pointed at Earth), you won't be able to pay it back, so you buy it through RCS and pay with propellant. If you won't be operating for long, it can be simpler to just bring a fatter wallet full of propellant than to set up a credit account with the wheels.

They get emitted sooner. The big neutrino pulse is produced during the core's collapse, and those neutrinos zip right through the overlying layers of the star at light speed minus epsilon. Those layers are quite opaque to electromagnetic radiation, so we don't see a visual signature until a shock wave reaches the surface and heats it up a few hours later. Follow-up question, since K^2 showed up while I was checking that I had things right: is there a practically observable neutrino signal from type-1a supernovae? I'm sure all that fusion precipitates tons of beta decay, but the only observation I've heard of was SN1987A, which was a core collapse type where the temperatures get high enough for the weak force to poke its nose into thermal interactions.

Orbit is freefall. If you're in freefall while going fast enough sideways that you miss the ground, that's orbit. An accelerometer on board will read zero unless your engines are running, and KSP's gravioli detector is quite impossible in real life thanks to the equivalence principle.

Where did that image come from? I've seen it all over the internet today, and it's wrong in just about every possible respect. Ion engines use reaction mass and electric fields; some types use magnetic fields to guide or confine plasma, but all the work is thanks to the electric field. Secondly, that's a picture of a Hall effect thruster, not the grid-type thruster that's actually in the news. And as far as I can tell, the record it broke was for longevity, not efficiency.

I dunno exactly how things are arranged in there, but maybe look for a gear puller? Auto Zone and some similar shops often have free tool loan programs, so you might not even have to buy it. As for recommending a brass punch, I'd guess that's to insure that any deformation from an over-enthusiastic hammer strike happens to the punch, rather than mushrooming the shaft and making removal impossible.

Real spacecraft always need RCS, even if they have reaction wheels or CMGs, because there can be a small net torque on the craft (usually from drag or radiation pressure) that would force the wheels to keep spinning faster and faster. You have to dump angular momentum with RCS now and then. With only RCS, you have a limited delta-L (i.e. change in angular momentum) budget. That's fine for short missions, but long missions or lots of attitude changes require a lot of propellant. Wheels give you essentially unlimited delta-L (well, they do fail eventually) as long as you can afford the electricity, but they're heavy. So it only makes sense to add wheels to the design when they save more than their weight in RCS propellant, and that payoff generally takes a long time. They're also much better at fine pointing than chemical thrusters (which have a minimum thrust pulse size), so they're nearly indispensable for things like space telescopes.

0.236N (yeah, less than an ounce of force) and 4190s, according to the specs.

That's pretty similar to some other forms of electrothermal propulsion, a few of which have been flown. Arcjets heat a fluid with an electric discharge. Pulsed plasma thrusters operate similarly, but ablate a solid propellant block instead of zapping a liquid or gas. VASIMR does its heating with microwaves, but adds the magnetic confinement to allow higher propellant temperatures without cooking the chamber walls. They work fine, though they generally use a storable liquid rather than hydrogen. That's because high Isp necessarily implies a lot of kinetic energy in the exhaust, and that means a lot of electricity for a little thrust. It generally takes a long time to gather that electricity, so you need a working fluid that can hang around a while.

I managed to catch it about an hour ago from the back yard. Not the fanciest astrophotography ever (12x zoom on an old non-SLR Canon, craptastic tripod, light polluted back yard), but it's reasonably conspicuous. I'd call it a bit brighter than Eta Sagittae (mag 5.1), but much dimmer than Gamma (mag 3.5). Here's the widest-field view I caught (I was walking the camera toward the target by looking at these pictures, because I couldn't see anything dimmer than γ Sge in the viewfinder): And a closer-in view. It's the brightest thing in this poorly-focused frame: Edit: I should note that I could not see this well through my 8x42 binoculars. The nova was visible in those, but it took some effort to be sure of what I was looking at.

Thanks for the heads up! I certainly needed binoculars, thank you city lights, but I think that's the first nova I've knowingly looked at. Hope it keeps getting brighter for a while.

The first theoretical prediction and experimental observations of antimatter (positrons, specifically) happened around 1930ish. It took a couple more years for them to put two and two together and realize that those weird-looking solutions to Dirac's equation and those spirals going the wrong way in the cloud chamber were the same thing. I've met plenty of people who think antimatter is hypothetical just because it's such a standard feature of science fiction. Fun bit of trivia: the average banana, thanks to potassium-40 decay, produces about one positron per minute.