Friday, August 22, 2008

Ares Program's Shocking Development

Apparently the engineering team working on the ARES CEV are considering adding weights and shock absorbers between the first (SRB) and second stage in order to dampen hefty vibrations that might otherwise blur the vision of the crew or cause parts to fall off. The vibrational load is in the 5 to 6 g range. It's hard to honestly evaluate the impact of this new development, but one wonders if a liquid fueled first stage wouldn't have addressed this concern in the first place. An interesting blog to which I was recently introduced covered this story before I did, and it's owner wonders if this is a case of NASA carrying over lessons learned from the Apollo era. It may well be, although the solution to eliminate first stage pogo-ing in the Saturn didn't so much involve shock absorbers as it involved accumulators in the fuel and oxydizer lines.

POGO-ing is a longitudinal oscillation that involves pressure waves in the rocket fuel (not unrelated a water hammer) coupling into the engines, causing surges and reductions in engine thrust. A pressure wave in the fuel will occur just by suddenly applying thrust to the bottom of the rocket, which as one can guess occurs during every lift-off. Once the thrust spikes from the pressure wave, the fuel gets bounced back up the fuel tank. Then it comes slamming down on the bottom of the tank again, causing the next thrust spike -- repeat ad nauseum.

The Apollo solution to pogo-ing wasn't all that different from the common plumbing solution for water hammers, which involves adding a gas filled chamber -- closed at the top, attached to the fuel or water line at the bottom -- called an accumulator. When the surge travels down the line from the fuel tank, it encounters this detour into the gas-filled chamber. Since fluids don't compress, but gases readily do, the surge in the line is able to rush into the chamber, compressing the gas within as it goes, rather than surging into the engines. This way, the gas acts as a shock absorber for the fuel, eliminating or greatly reducing pogo-ing.

By the way, to the best of my knowledge there is no such solution for solid rocket motors, and besides which pogo-ing is a liquid fueled engine phenomena. Adding weight and shock absorbers to a rocket seems contrary to the basic direction of rocket improvement, so I am still left wondering, "what gives?"


Blogger Eyes on the Stars said...

You handled the pogo concept much better than I did. Excellent post

August 22, 2008 10:23 PM  

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