Chair Force Engineer

Monday, February 12, 2007

Wider SRB's Are Better

As a consequence of adopting the Ares I design, NASA has to build a taller servicing tower. Ironically, a rocket with a smaller payload and smaller crew than the shuttle will require a taller servicing structure. The Ares I will exceed 300 feet tall, being dwarfed only by the Saturn V and N1 in terms of the tallest rockets built to date.

The long, narrow nature of Ares I makes its critics jittery about possible stability problems. They liken it to balancing a pencil on your palm The solution, in my view, is to make the rocket wider and shorter.

You may have heard the expression, "The shuttle SRB was designed by a horse's ass." The proponents of this phrase are referring to the railroad tunnels which dictated the SRB's maximum diameter, which is 16.7 feet at its widest, and the fact that the width of a horse, in an indirect sense, dictated the rail gauges. Because NASA is sticking with the casing size for the shuttle, the Ares I will fall under the same width constraint.

I maintain that the 5-segment SRB will be so different from the 4-segment SRB that it's essentially an all-new rocket. If NASA and ATK are going to spend the money on an all-new SRB, why not design it correctly this time? Why not cast the propellant as a single, monolithic grain instead of a series of segments? Why not barge the single-segment SRB's from south Florida to the Cape, and avoid the diameter constraint? The idea was tossed about during the late 60's, but this time there is no concern about barging the motors to Vandenberg.

Assuming that NASA were to order a 5.5 meter diameter SRB (to match the upper stage,) it could get away with an SRB that's just over 24m tall. The current Ares I first stage is 53m tall, and I'm modeling it as a 3.71 m diameter cylinder. It's a rough approximation, but the adoption of a 5.5 m SRB would shave almost 29m (95 feet) off the Ares I's height. That would bring Ares I from 309 feet tall to 214 feet tall, which isn't much taller than the shuttle (184 feet.)

Now let's assume that NASA ditched the 5.5 m upper stage (the S-IV redux, as I've pointed out) and went with a 6.6 m upper stage (a more literal redux of the S-IVB.) The SRB would be widened to match the upper stage diameter. The result would look a lot like the Saturn INT-05, which had similar capabilities to the current Ares I design and only stood 141 feet tall.

Adopting the shuttle SRB might have sounded like a good idea at the time, but when it's been thought through, it creates a lot of challenges that need not have been faced if another solution was picked. When it comes to modifying the Shuttle SRB, added girth is proving to be far better than added length.

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