EDIT: I was wrong abuot turboprops and turbofans. If I understand correctly, turboprops actually use a turbojet-like engine to *power* a propeller, which is what provides the thrust. Turbofans fall somewhere in between a turboprop and a turbojet.
Today I Learned:
1) How jet engines work! Also, the difference between a turbojet, a ramjet, and a scramjet. First, what's a jet engine? A jet engine is, most simply put, a variant on a rocket. Rockets burn a fuel and an oxidizer, which makes a bunch of really hot gas, which is in turn funnelled out of the back to generate thrust. Jet engines also burn a fuel and an oxidizer, which makes a bunch of really hot gas, which in turn is funnelled out of the back to generate thrust. The difference is that jet engines don't carry their oxidizer. Instead they use the oxidizer that most of us use when we burn stuff -- air. That way, they don't have to carry giant tanks of oxidizer around, which would be heavy, expensive, and dangerous.
The catch is that the ratio of fuel to oxidizer (air) required to burn properly and generate thrust are fairly specific. In particular, you need oxygen at higher concentrations than is normally found in the atmosphere. Turbojets, ramjets, and scramjets attack the problem of compressing air in slightly different ways, all with the aim of burning fuel to produce thrust.
Turbojets are the most common form of jet engine. Any jet-looking engine with a propeller-looking spinning thing is a turbojet (actually, technically a turboprop or turbofan, but as far as I can tell they all operate under the same principle (see EDIT above)). Turbojets compress their air using, well, turbines. The propeller-looking thing in a jet engine isn't actually producing thrust -- it's actually jamming as much air as it can into the interior of the jet engine, where it can be burned with fuel to rocket the engine forward. Turbojets are very efficient in the regimes at which most commercial aircraft fly, and we're pretty darned good at building them.
Ramjets don't use a turbine to compress their air supply. In fact, ramjets have few or no moving parts -- they compress air by *moving really freaking fast*. Ramjets are basically double-sided funnels with complex fuel injectors in the middle. The funnel on the front collects air as the engine screams through it, causing the air to slow down, compress, and heat up significantly. The injector puts just enough fuel into it to keep it burning at the right rate, and the funnel on the back end directs the rocket flames backward to produce forward thrust.
This works great as long as you're already moving quite quickly, but is useless if you're standing still. Thus, ramjets are only useful on planes as supplementary engines for high operating speeds. Ramjets are also, ridiculously enough, used on some missiles and *artillery shells*. That's right. There are mortar shells with ramjets. Ramjets also don't work very well at extremely high speeds (several Machs). The reason is that the faster the ramjet moves, the hotter the incoming air gets as it's compressed. If the heat of the compressed air coming in becomes comparable to the heat of the burned fuel being used as propellant, the engine stops producing much in the way of thrust.
Enter the scramjet. The scramjet (for "supersonic combusting ramjet") is a variant of the ramjet that doesn't slow down incoming air to compress it. The scramjet features a more or less clear line of sight from the front to the back, with the aim of keeping the air flowing very quickly (faster than sound) while compressing it enough to burn well with injected fuel. This requires some absurdly fast (millisecond-level) injection control, which is technically quite challenging.
Scramjets also suffer the same speed limitations as ramjets, but much worse -- they only operate at all at multiple-Mach speeds, which limits their use to things that can get that fast using other methods. This and other design challenges have kept scramjets from seeing practical application... anywhere. Nevertheless, scramjets have technically existed since the 50s, and are currently being incorporated into several next-generation airplanes.
Thanks to JD for unknowingly pointing me to some good Wiki articles on jet tech.
2) There's a variant of Markov-Chain Monte Carlo called Parallel Tempering MCMC (PTMCMC) which lets you, among other things, relatively efficiently compute normalization constants when comparing alternate hypotheses, which lets you compute odds ratios exactly (and calculate exact probabilities of hypotheses).
3) Buffered solutions really are pretty robust -- I buffered some aqueous solution to pH 8.2 with TRIS today, then increased its volume by a good 25% by adding water, and it apparently didn't affect the pH at all.
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