January 10, 2016 at 03:44AM

Today I Learned: 1) Solid-state rockets (that is, rockets with solid fuel instead of liquid fuel) have the disadvantage that you can't stop them once you've turned them on -- the fuel is just a block, so once it starts burning, there's not a ton you can do to make it burn differently. In contrast, liquid-fuel rockets work by injecting fuel and oxidizer into an engine, so you can always stop injecting fuel. The advantage to solid-state fuel rockets is that they have fewer moving parts, for largely the same reasons. I also vaguely recall that they're more efficient, but I don't have anything to back that up. I have a nasty feeling I've already TILed this at some point -- anyone remember me posting this or similar information before? 2) Remember that M9 minimal media I mentioned a while back? In case you don't, M9 is a broth used for culturing bacteria. It's "minimal" in the sense that it's built from the ground up with known, quantified substances (typical broths like LB are mostly broken-down bacteria or yeast, so nobody *really* knows what's in them). Minimal medias are great because they're consistent and we know what's going on in them, but since they're pretty bare-bones in terms of nutrient diversity, bacteria tend not to grow as well in them. Today I learned about a possible problem with minimal medias. Most, it turns out, don't include trace metals like iron, selenium, nickel, and so forth, which are critical for growth. Yet bacteria somehow grow in those media(s?). How? It turns out that tap water contains plenty of trace metals (iron, for instance, is required at about 1 uM (micromolar, or about a 10^18 atoms in a liter)) for growth, and tap water usually contains several times this amount. Importantly, this means that if you make minimal media out of distilled water (or worse, 18 MOhm water!), you might find your bacteria don't grow as well. For a few minutes, I thought this might be the root of some problems I had a while ago that seemed to go away when I changed media batches... but then I remembered that I used a yeast-extract-based broth, not a minimal media. Oh well. This information, and many more fascinating facts, can be found in Cell Biology By The Numbers, by Rob Philips. It's about $50 on Amazon, or free here: http://ift.tt/1AvhcfE. If you've ever wanted to have a better quantitative idea of what living stuff *is*, I recommend checking it out. It's broken up into quite nicely-skimmable chunks, so I'd recommend it if you need some little chunks of reading to fill time. 3) SpaceX is quite operational, and has dozens of contracts lined up, which will keep it operational and building new rockets for many years. Also, SpaceX is currently valued somewhere in the neighborhood of $12 billion.