Saturday, September 15, 2007


So let's start examining the Emperor's architecture for returning to the moon. Prior to the release of the Vision for Space Exploration in Jan 2004, NASA had undertaken several programs, mostly managed by the Marshal Space Flight Center (MSFC), to take the next steps in developing a vehicle to replace the aging space shuttle. These concepts either violated basic physics (x-38), suffered from NASA interference (x-34 engine program), or did not improve on safety (Orbital Space Plane).

As the inevitable befell the OSP, a "soon, simple, safe" mantra started to be heard out of the astronaut office. Led by Doc Horowitz, the concept of flying a capsule on a space shuttle solid rocket booster (a.k.a. "the stick") started to take hold as an alternate means of lofting humans into orbit. The solid rocket booster, on the surface, looked like a simple way of accomplishing the goal. With only burning solid rocket propellant, and no high pressure turbo-machinery to go awry, how could this concept not be safer than using the existing, already taxpayer paid for, EELVs (Atlas and Delta) to get to orbit?

Today's liquid fueled rocket engines operate at high pressures, high temperatures, and the turbo-machinery running inside of them spin at high rpm. That enables the engines to develop high levels of thrust. With all of those "highs" in the preceding sentences, its easy to see that today's engines encapsulate high levels of energy. Properly channeled it lifts the rockets off the ground. But when something goes wrong, all of that energy can make for a big explosion, potentially putting crews riding along at risk.

Complexity, in and of itself, is not an argument for dismissing liquid fueled EELVs as potenitally man-rated rocket boosters. After all, your car likely has an internal combustion engine, composed of thousands of moving parts, that reliably starts every day under a variety of environmental conditions, and can run for well over 100,000 miles if maintained properly. The same goes for rocket engines. If assembled with care, they will work reliably, as designed. Indeed, hundreds of satellites have been succesfully placed in orbit by these same EELVs over the past 50 years. Heck, our first astronauts launched on EELVs and the liquid fueled Saturn 5 took us to the moon.

Nevertheless, in an attempt to improve upon the safety record of the space shuttle, NASA fell into the trap of doing a point design to remove one problem deemed likely to cause failures. Remove the initiating turbo-machinery and you remove the biggest threat to astronauts going uphill to low earth orbit. Right? And if you use a demonstrated space shuttle solid rocket booster (SRB), aren't you inheriting a man-rated system from the get-go?

Not so fast. When you do a point design, something unexpected usually falls out the other end. Tweak an existing design and its not an existing design anymore, its "new" in an engineering sense. The problem is exacerbated when you don't have competent managers with a respect for the history that came before them. In the case of ARES-1, you've got all of those things coming together in a future class case study of failure.

So what's wrong with the ARES-1 stick? For starters, imagine pushing a piece of cooked spaghetti along a table. Very hard to do. The segmented structure, bolted together at its field joints, is very much like the italian dinner staple. When attached to a space shuttle external tank , it is stiffened by being held at two points along its length. By itself, there are no attach points to grab onto and provide that stiffness. Consequently, when you light the rocket up, it will vibrate and bend in response to its inherent dynamics and environmental inputs, much like a rope being pushed from one end.

Why don't other rockets have this problem? In fact, they all do, but usually the structure is not segmented like the SRB and is made beefy enough to avoid this bending. So couldn't the SRB be made beefier to avoid the bending. The answer, of course, is yes, but only at a significant weight penalty which would take away from its ability to lift the already overweight CEV (more on that to come!). Still wanting to use the SRB for its safety advantages, NASA's manager must have hired Rube Goldberg to come up with an answer.

The bandaid answer is to ring the top of the SRB with small rocket motors around its circumference and to fire them as needed to keep the booster going straight along its trajectory when it starts to bend. Two sets of 32 motors do the job. Primary and back-up. Hmmmmm, that's starting to sound a little more complex than a simple SRB, doesn't it?

On top of that, let's add a 5th segment to a normal four segment SRB to get more performance out of the system so we can lift that overweight capsule. But now ,with this new segment adding its thrust to the exhaust system designed for four segments worth of propellant burning, we need to change the internal configuration of the propellant to slow down the burn so we don't overstress the system. While we're at it, let's change the grain we are using to optimize the performance. Hmmmmm, doesn't sound like the ARES-1 is exactly an off-the-shelf SRB anymore, does it?

It's not.

No comments: