sevenperforce

One of the hidden advantages of the SRB is that it can be "throttled" by shaping the burn surface without essentially adding dead weight. In contrast, downthrottling a liquid-fueled engine renders those engines essentially dead weight; any thrust capacity you aren't using is useless dry mass and cuts directly into payload. An all-liquid launcher needs quite a bit of engine to hit that 2:1 T/W ratio at launch to minimize gravity drag, but instantly becomes overpowered as it loses fuel weight. Of course, the faster it launches, the faster drag builds up, so the extra thrust isn't completely wasted...but SRBs really have a big advantage here. Still hate them because they can't be restarted or refueled, but the usefulness is hard to deny. SpaceX seems to have sidestepped this issue by building ridiculously lightweight and powerful engines, so that the high T/W ratio mitigates engine downthrottle penalty. I like air augmentation for its easily-attainable positive effects on specific impulse, but it only meagerly augments static thrust, which means you still need either a supercharger or a larger engine cluster, both of which add to dry weight and thus reduce payload. Thrust-augmented nozzles and dry mass injection both help with vertical takeoff, while a rolling takeoff is also possible (not to build up aerodynamic lift but to build up airflow for air augmentation of thrust). Where does the 5:1 ratio come from?

Then again, for many many years it was considered standard practice for police to use a blackjack or nightstick to intentionally KO suspects. So while obviously the effects are far more than depicted, it was also expected. This. This. This. This movie rivals Armageddon for extreme errors. Let us not forget that after the ice SANK, one of the main characters got in a PLANE and chased down an ICBM. Twice. Two ICBMs. Launched from the North Pole, one headed toward Moscow and one headed toward Washington DC, traveled SO SLOWLY that a freaking jet aircraft was able to give them a headstart, fly to Moscow and shoot down the first one, then fly from Moscow to Washington DC and physically intercept the second IBCM. On one tank of fuel, I might add. Right.

Oh, good example. Of course, one difference there is that the second stage is already in motion, post-gravity-turn, and thus has less stringent thrust requirements than with a static launch.

Sounds awesome. There are a couple options I thought of. One is a trojan arrangement, where you have a large primary and a small secondary (hot brown dwarf, maybe) and the planet is at one of the trojan points of the secondary. Not sure where the third star could go to supply the other required illumination, though. Also possible to have a multiplanar arrangement, with the primary and secondary in a fairly tight binary and a third star in a highly elliptical orbit perpendicular to the ecliptic, but where the third star has forced an orbital resonance with the planet so that their orbits always coincide without disruption.

One of the ways that SpaceX keeps costs down is using the same fuel and engine on the second stage that it uses on the first stage. Only having a single engine design for the entire launch vehicle is a really good idea. I was wondering, though: how close to optimal is the 9:1 configuration? SpaceX started development of a 5:1 configuration so there is obviously some room for variance. I don't know what other TSTO launchers use matching engines so I'm not sure there is anything else to compare it to. The second stage engine has a higher specific impulse and thrust due to the extended nozzle, but that extended nozzle also weighs more, so that may or may not have an impact. I assume the primary driver here is the relative masses of the two stages: you want your launch vehicle to have a T/W ratio of nearly 2:1 at launch, but you want your second stage to be roughly one fifth the mass of your first stage and have closer to a 1:1 T/W ratio at separation, suggesting a nearly 10:1 thrust ratio...pretty close to Falcon 9. Any other considerations? What about altitude-compensating nozzles?

I'm always wracking my brain, trying to figure out how they imagine passwords and hacking and brute force attacks actually work. Does anyone think that you can just query the computer on the individual characters of the passcode? One particularly bad thing you don't see much anymore: trains stopping on a dime. Seriously, it does not happen. Headshots with a one-handed grip on a large-caliber handgun are a bit amusing.

I had assumed that this would be a thread about terrestrial planet formation as opposed to other types of formation from protoplanetary disks. I am also quite thrilled that my phone's voice text has the ability to understand the word protoplanetary. Back on topic: hypothetically, would it be possible for a trinary star system to form in such a way that one of the planets is continually bathed in sunlight, resulting in eternal day?

They'd save some in common development costs, but not very much at all in construction or maintenance or support. Any differences between the vehicles is going to make for a huge support structure difference, rendering common support for orbital and suborbital flight impossible. Only a limited amount of common support would be possible anyway, but use two separate vehicles and it's utterly impossible. The flight time reduction for supersonic travel is fairly meager compared to the flight time reduction for a hypersonic suborbital flight. So that might be a marked difference. What dV do you need for an antipodal suborbital hop? The ideal scenario would be a wingless blended delta/sears-haack lifting body with horizontal-attitude VTOL, either via biaxially-ducted fans, rotating engine nacelles, or something similar. Kerolox. Constructible in various configurations, but all with the exact same frame and engine config: Small tank, 20-30 business class seats, small payload cabin Small tank, large payload cabin Large tank, small cabin (5-10 passengers) Large tank, small payload bay The second two configurations are capable of reaching SSTO; the first two are not. However, all four configurations are equally capable of accepting a parallel first-stage booster, and all can return from orbital velocity. The booster allows the first two configurations to reach LEO and the second two configurations to have BLEO capability. This way you can offer daily antipodal flights with half-hour flight times for a large set of passengers, or scheduled hypersonic payloads, or ISS crew ferry launches. With the reusable strap-on parallel booster, the first two configurations can take a large group of passengers into orbit or a large payload into orbit.

No market as a separate vehicle, presumably. Doubling infrastructure and support costs is not really a great idea.

Four of each animal is also not nearly enough to create any sort of sustainable population, either. If you want a small temporary breeding population of tasty or useful animals, a few hundred embryos per desired species might be enough. But if you want a permanent sustainable population you will need 100,000 individuals minimum in order to have enough genetic diversity for the population to survive in the wild, and that is if you have already achieved 95%+ terraforming on the target world. If the trip only takes 45 years, why would most of the passengers have never seen Earth? Is artificial gravity based on linear acceleration, centrifugal acceleration, or handwaved? Handwaved selectable artificial gravity is technology on a level with Alcubierre FTL drive.

The SABRE engine cannot use kerosene, by definition, because it is an airbreather that requires you to dump LH2 through a precooler. But you wouldn't actually need LH2 to get Skylon to orbit. Strengthen the frame, replace the SABRE engines with a pair of steel-ducted Merlin 1D clusters, and fill it up with kerolox. It would carry triple the payload to orbit. Mass fraction wouldn't be as good, but that doesn't really matter because kerosene is cheap as heck and simple to tank. The proposed Skylon vehicle is REALLY big.

It is a battery, not a capacitor. That was my first question as well.

Perhaps a regular hypersonic transport between two nodes carrying a series of time-sensitive cargo packages + passengers?

The problem is that it actually has to be used in order to generate revenue. Intriguing. I wonder how large an average shipment is. Partial passenger/partial cargo is a possibility if there is some regular need...

You really would want to go cargo, simply because the LV requirements are thus rendered substantially less onerous and you have a lot more flexibility. Unfortunately, I can't think of any suitable cargo either. There's really nothing in the world that needs to be shipped in bulk to the other side of the world in a matter of hours...at least, not so desperately that people would pay for a suborbital spaceflight to accomplish it. I suppose there are certain particularly expensive consumables with short lifetimes that could be harvested and shipped to high-end restaurants, etc., but the demand would not be high enough to be a major driver. That leaves you with human cargo, with all its nasty "keep the cargo alive and air-conditioned" requirements. There's a glimmer of hope here, because while the difference between hypersonic and supersonic flight may be fairly low, supersonic options don't currently exist, and there is a big difference between hypersonic and subsonic flight. Subsonic flight means a trip to the other side of the world can take a day or longer; supersonic cuts this to several hours...suborbital hypersonic means no two points on the globe are more than an hour apart. There's a fair probability that the ability to commute around the world would prove attractive to enough people/businesses to service at least one or two routes from the start. I don't know what the maximum viable ticket price would be. Ideally, you could use existing airports by adding a dedicated spaceport terminal at lower cost than building an entirely new launchpad. The suborbital/orbital transition would probably be accomplished by having some portion of the passenger cabin be replaceable with an extended fuel tank. E.g., you can take 30 people to the other side of the world in an hour for $90,000 each, or you can take 10 people to orbit for $350,000 each. Well, this is a suborbital spaceflight, not a high-altitude sustained hypersonic flight. With a steep ascent and re-entry trajectory, only two sonic booms reach the ground. LH2 is a non-starter, I think. You need to be able to run on RP-1. Or LP/LNG at the very least. That's not a bad thing. A high propellant mass fraction is okay...you WANT to be able to carry a lot of fuel...and high density means better T/W ratios and a smaller overall vehicle, which drives down vehicle reuse costs. Yeah, but the horse is a lot closer to the cart than the rapid-reuse-orbital-flight version.

I just realized something. Chemistry is a compression algorithm for a subset of physics. There are certain physical particle interactions which happen the same way every time and thus can be represented in abbreviated form. Biology is a compression algorithm for a subset of chemistry. Psychology is a compression algorithm for a subset of biology. Sociology is a compression algorithm for a subset of psychology. A neat way of thinking about it, anyway. The supercomputer could "compress" by simulating chemistry rather than physics when particles were in the correct energy ranges, and so forth.

Lithium-7 has a vanishingly low probability of absorbing the high-energy neutrons in a nuclear reactor. It needs ultra-high-energy fusion neutrons for the absorption to have any reasonable probability.

SSTO RLV proponents often argue that once a vehicle with rapid reuse potential exists, it would create the market which would enable it to be cost-effective. Unfortunately, it's doubtful than anyone will bother to build or design such a craft unless the market for rapid small-payload LEO launch already exists. This creates a chicken and egg problem, where the market won't exist until the launch vehicle does, and the launch vehicle won't exist until the market does. It's possible, however, that a launch vehicle could be designed for a broader market, thus becoming available without requiring the rapid small-payload LEO launch market to exist first. From an economic/investment standpoint, it is much more attractive to build a vehicle which can use an existing market than it is to build a vehicle on the speculation that it will create a nonexistent market. Such a market could potentially be realized in a similar way to how we first got into space: using the same vehicles for suborbital and orbital flights. The US and the USSR both figured out that the gigantic missiles they were planning to use to lob nukes on suborbital trajectories could, when properly staged, be used to boost payloads into orbit. Many orbit-capable vehicles would make excellent hypersonic suborbital transports, suitable for transporting a large payload between any two points on Earth in an extremely short time. With a decent-sized fleet of such vehicles* servicing regular hypersonic transportation around the globe, you would have the support infrastructure necessary to service those same vehicles on orbital flights, either with reduced payloads or with a launch assist stage. The question, then: does such a market exist? Is there a need for large cargo (or, on the other hand, passenger transport) to be whipped around the world in a matter of hours, regularly? Who would pay, and what is the probable market saturation? *Skylon could potentially serve such a role, as it likely has excellent suborbital hypersonic flight capacities. It's not ideal, though, because it requires a great deal of LH2.

Also relevant...

Atom-by-atom is easy. But the universe is splitting atoms all the time. So it really needs to go quanta-by-quanta, which is not so easy. XKCD #505 is apt...

But wouldn't objects move back in during its million-year orbit? Nah. Planet 9's subtle adjustment to the sun's gravity well extends all the way across the solar system; it's not like its gravitational influence just drops to zero when you get far enough away. Otherwise the L3 Lagrange point wouldn't be a thing. Under my definition? No, it wouldn't be. Right now, the moon is tidally locked to Earth but Earth is rotating independently of the moon. When the dynamics of the system shift so that both Earth and the moon are tidally locked to each other, it will become a double-planet system. I mean, we would still probably call it "the moon" for the sake of history, but there's nothing wrong with recognizing such a change. After all, the Earth and the moon are already very close to being a double-planet system; the moon doesn't actually go around the Earth at all. They're just in very close solar orbits and the Earth's gravity perturbs the moon's solar orbit enough to tidally lock it and make it appear to orbit Earth.

Could have been a neutron star? I don't think so.

For anyone paying attention, they have rotated the F9 first stage and are lowering it onto the transport rings right now. http://www.portcanaveralwebcam.com/

Lithium-7 has a really low fast neutron cross-section and needs ultra-fast D-T fusion neutrons to actually fission into tritium and helium. Not enough neutrons to do it, even in an NTR.

'Twas pointed out to me elsewhere that Pluto is large enough to retain Rhea as a "real" moon -- that is, with the barycentre inside of Pluto. However, Pluto would still become tidally locked to Rhea in astronomically brief time. Thus, I propose an adjusted definition for "natural satellite". A natural satellite is a self-gravitationally-bound object orbiting a barycentre inside a larger body, too small to force mutual tidal locking with the larger body. This makes intuitive sense; if two objects are tidally locked then they are orbiting each other, even if the barycentre is within one body. A moon is a gravitationally-rounded natural satellite of a substellar object. A planet is a body large enough to have a moon. This cutoff is about 6.3e22 kg, comfortably greater than the mass of Eris but comfortably lower than the mass of Mercury. A dwarf planet is a gravitationally-rounded body too small to have a moon.