...is nonsense like this.

The system consists of four pods, located around the aft skirt on the Ares I First Stage. Early graphics of a system - that are bound to mature if accepted as the way forward - show each pod will have a fuel tank, an oxidizer tank, a pressurant tank, and seven thrusters.

The downside of this concept - which is a completely separate system than the roll control system on the interstage - is the addition of failure modes, which would hit Ares I's LOC/M (Loss of Crew/Mission) numbers.

Also on the downside, the concept is a retro thrusting system (negative thrusting) - which would impact on Ares I's performance figures.

Gee, ya think?

Firing small thrusters to help dampen out some of the shocks is a clever engineering solution, but not all clever solutions to problems are good...especially when they entirely miss the point. This "solution" brings to mind homework problems and exam questions I sometimes saw in college -- it's a solution to a problem so abstracted from any relevance to the larger system that, while it seems reasonable at the microscopic level of the problem at hand, it immediately becomes impractical or even absurd when all elements of the system are taken into account. If you imagine a plot of the axial oscillation of the vehicle as a (grossly simplified) sine wave, it makes sense that small inputs of the opposite sign at the peaks would reduce the amplitude of those peaks. But translating such a concept into hardware is where the absurdity comes in -- here, the small opposing input isn't just reducing the amplitude of the oscillation, it's acting against the primary function of the larger system.

But the truly breathtaking information in the NSF post has to do with the payload impacts given for two of the proposed solutions (emphasis mine):

• Active Pulse Thrusters: "'Performance and aft skirt design challenge: (around) 500 lbm (pounds mass) payload impact."
  
• Interstage Isolation Mounts: "May reduce payload by 1000 lbm."
  

The wording is slightly ambiguous, and could just be an imprecise reference to the effects on the payload as seen from the first stage's perspective. But if by "payload" is meant here "everything above the upper stage", it's simply incredible that these "solutions" even passed the laugh test. Cutting into the payload capability by 500-1500lbs, when the payload itself is already overweight (in large part due to the choice of launch vehicle), misses the point even worse than having thrusters aimed the wrong way.

When is NASA going to recognize the obvious?

ADDENDUM: I'm also curious about what this means:

Residual mass - only recently added - is classed as 'Allocated 0 pounds (due to it being a new factor) - Estimated 2,687 pounds...'

Is this referring to residual propellants in the first and/or second stage? If so, does this really mean that the engineers failed to take into account something as fundamental as residual propellants in the design and performance analysis of Ares I? Wow.

Posted by T.L. James on May 2, 2008 11:10 PM | TrackBack