Blog-a-Day #2 -- Space Canoes

How do outrigger canoes relate to spacecraft? Well, I told you the relationships between the photo prompts and my blog entries might be a bit…tenuous. Actually, this one is a better analogy than most of the others will be. So, just how do outrigger canoes relate to our first topic in this series, namely suborbital boosters? Metaphorically, of course. As an aside, are similes special-case metaphors? Or are they two separate grammatical forms? Hmmm. And what about allegories? I’ve never been able to keep those three straight.

Anyway, outrigger canoes relate to supertankers or fast hydrofoils as suborbital boosters relate to most of the other spacecraft we’ll discuss in this series. Like outrigger canoes, boosters are short-range vehicles (yes, I know South Pacific islanders traveled across hundreds of miles to colonize other islands, but just go with me for a few minutes), driven by high-energy engines (paddlers) that quickly run out of juice (yes, yes I know they are incredibly fit and paddle for hours, but still, they eventually get tired, hungry, and thirsty).

Today’s state-of-the-art rocket boosters use either solid fuel or cryogenic liquid hydrogen and liquid oxygen. Solid fuel rockets date back to ancient China and the invention of gunpowder. Today’s most efficient (we’ll see why that’s important in a minute) boosters use a combination of powdered aluminum and ammonium perchlorate. The engine efficiency (otherwise known as specific impulse), measures the thrust output divided by the weight of the fuel, is not as high as its liquid hydrogen/liquid oxygen counterpart. But, the solid fuel burns so quickly and with so much thrust, that they are great for getting a payload off the ground and up to high velocities very quickly, at which point they can be jettisoned to lighten the load.

Solid boosters have other tradeoffs, though. They burn fast, but once lit, they can’t be turned off or even throttled. They are either fully on or fully off. Their specific impulse may be lower than cryogenic hydrogen and oxygen, but their engine mechanism, such as it is, is so much simpler than liquid fueled engines, especially the super-cold liquid hydrogen and oxygen. Basically, a solid rocket “engine” is a can strong enough to handle the high pressures and temperatures, and a nozzle to direct the exhaust to produce thrust. In engineering terms, simpler is always more reliable, assuming they are operated within the proper environmental conditions. This was learned to NASA’s peril because of the catastrophic failure of the Challenger Space Shuttle.

Whereas solid boosters generate a whole lot of thrust for a relatively short period of time, liquid fuel rockets generate lower thrust, but over a much longer time, and the total thrust they produce compared to the weight of the fuel, i.e. their specific impulse, makes them the kings of efficiency. To keep liquid hydrogen liquid, it has to be compressed and chilled to -423 degrees Fahrenheit. Liquid oxygen needs to be below -297 degrees. This means big, heavy, well-insulated tanks to hold the propellant (liquid hydrogen) and the oxidizer (liquid oxygen), and super high-speed pumps to move the cryogenic liquids from their tanks to the combustion chamber.

In contrast to the simplicity of solid rocket boosters, cryogenic liquid fuel rocket engines represent a most challenging engineering problem. One end of the “fuel chain”, the mechanism that stores, moves, mixes and burns the fuel, is hundreds of degrees below zero and the other end is burning at thousands of degrees above. The engineering challenge this presents is mind-boggling.

Still, whether you use solid boosters, liquid fuel, or a combination of both—a practice which has become quite common since the development of the Space Shuttle—you have to burn a whole lot of fuel to get a little payload high enough and fast enough for its second or third stage to deposit that small payload into an orbit. I’m reminded of the old saying about using a bazooka to kill a fly. There’s a lot of theory behind the next statement, which you can learn for yourself by searching for “The Rocket Equation.” The bottom line is that to get from the surface of the Earth into LEO (Low-Earth Orbit) takes five to ten times as much fuel by weight as the rest of the vehicle. And when you consider the actual payload tends to be only about ten percent of the tare weight—the weight of the vehicle without its propellant load--well, you get the picture. Of course, we can all do the arithmetic. Fully ninety-eight or ninety-nine percent of a spacecraft is, in fact, overhead.

Now do you see the connection? Six hunky cousins of The Rock jammed into an outrigger canoe, paddling their hearts out to get over the breakers outside their reef, versus two solid rocket boosters strapped to a couple of cryogenic rocket engines in order to get a single little satellite going high and fast. See? See? OK, it’s a stretch. But, in my defense, I didn’t pick the pictures, just the topic.

Anyway, next time we’ll look at a newer launch vehicle design that promises more efficiency and reusability, which should combine to lower the costs of getting a pound of payload into LEO from its current price of about $10,000 per pound.

Rob Johnson

May 17, 2020


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