atomic_fungus (atomic_fungus) wrote,

#1776: Aluminum and water

Apparently it makes rocket fuel. Well, that's not terribly surprising.

What I'm here to talk about, though, is this quote: "Current space shuttle flights release about 230 tons of hydrochloric acid in the exhaust left behind by their solid rocket boosters."

The space shuttle solid rocket booster (SRB) and the first stage of Ares both use the same kind of fuel: HTPB (a variety of rubber, as fuel), aluminum powder (as a burn-rate modifier), and ammonium perchlorate (as the oxidizer). Rocketry enthusiasts can obtain the same material--in much smaller sizes--to fill reloadable motor casings. It works well.

Problem is, it stinks. It is rather less than environmentally friendly and you certainly don't want to make a habit of breathing the stuff.

Since Ares is built--essentially--out of shuttle SRB sections, this isn't going to change with the new booster; in fact, it'll be worse, because the entire first stage will be solid fuel rather than liquid-fueled. And so they're trying to find eco-friendly ways to make fuel; hence the aluminum slurpee.

On the plus side, aluminum is cheap and readily available, as is water; and the exhaust shouldn't be corrosive or terribly bad for anything that gets a whiff of it.

On the minus side, solid fuel is stupid.

Until now no one built a heavy-lift booster with solid fuel; solid fuel causes too much vibration. A booster could have any number of strap-on boosters but the main thrust was provided by liquid-fueled engines.

Example: the shuttle boosts itself using liquid-fueled engines. The SRBs boost the tank of fuel required to operate those engines, at least until the tank has been emptied enough that the shuttle's main engines are sufficient to the task of boosting it further.

NASA is going for solid fuel in Ares because it's cheap: you get the casing back into round, put in a pre-cast fuel slug, add O-rings, and you're off to the races.

NASA's had all kinds of problems with the space shuttle main engines (SSME) for two reasons: first, they were designed from the top down, rather than the bottom up; second, they have to run the damn things beyond their rated capacity to get the shuttle into orbit.

Second point first: if you watch a shuttle launch and can hear the chatter from Launch Control--or if you read a typical launch profile--you find that the shuttle engines are run around 104% of their rated capacity during each launch.

Okay, now for the biggie.

When you design a complex system, you can approach the problem one of two ways.

You can build it from the bottom up: you start with components, designing and testing, then put the components together to form subsystems, which you--again--test, and correct design flaws. Eventually you have built a complete system, and you can have a reasonable amount of confidence that your system is as good as it can be. You will still find deficiencies with the design once it has entered service, but this process ensures that all of the obvious deficiencies have been dealt with and that all the subsystems work well together. This way is not fast and it is costlier than the other way.

This is not how the SSMEs were designed.

The other way is "top-down design": you start with the whole and drill down to the individual components. This lets you keep tabs on the big picture and prevent "mission creep" or "might-as-wells" from complicating the design. You design each component as it's needed, ensuring that the specifications agree with the requirements. Absent any serious design issues, it's quicker and cheaper than bottom-up.

...and whenever you find a problem you must fix the whole thing; it is likely that your fix for one problem will raise another one, because the whole thing must work together regardless of how you designed it.

Any engineering project will combine these two methods--you can't design a blade for a jet turbine without knowing how big the rotor will be--but generally speaking an all-new system will designed from the bottom up, rather than the top down.

That's not how NASA did it. NASA designed the SSME from the top down; and so every time the engineers found a problem with the design, it prompted a debate as to whether nor not the flaw really needed fixing.

The end result was an engine which must be completely disassembled after each flight and its components subjected to a battery of expensive tests before it can be reassembled and installed into an orbiter for its next flight.

NASA, wishing to avoid this, instead specified solid rocket motors for the Ares first stage.

Oh well. At least the Russians still make liquid-fueled boosters....

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