NASA said April 3 it was
leaning toward adding shock absorbers to
Ares 1 to address the vexing vibration problem that designers of the astronaut-crew-launching
rocket
have been wrestling with for the past six months.
Engineers at NASA’s Marshall Space Flight Center in Huntsville, Ala., became concerned last October after simulations suggested that
vibrations originating in the Ares 1’s solid-fueled main stage during liftoff could grow to dangerous levels as they travel up the length of the rocket toward the astronaut-carrying Orion capsule on its front end,
potentially shaking the launcher apart in flight. Subsequent analysis showed that the vibration issue, known as thrust oscillation, was not as severe as originally feared.
But NASA officials said April 3 that the predicted vibrations, if left unchecked,
still could be severe enough to harm the crew, or at
least shake the astronauts
hard enough to impair their ability
to perform routine functions like
reading Orion’s console and throwing switches.
“We have to get the crew vibrations down to a level that’s well below the health limit and would like to get it down to a level where the crew can actually perform functions during this very short period in the flight,” said Garry
Lyles, the veteran Marshall engineer who led the Thrust Oscillation Focus Team.
Thrust oscillation is common in
solid-rocket motors, the casings
of which tend to resonate like an organ pipe from the swirling gases produced as their rubber-like propellant burns from the inside out during flight. Trouble can result when the frequency of the vibrations originating in the motor match the natural resonance of the rest of the rocket, which can have an
amplifying effect
. Engineers
typically can address the problem early in a rocket’s design by stiffening joints or making the vehicle
heavier in order to “de-tune” it, preventing
the vibration frequencies from synching up in the first place.
Right now, NASA calculates that the Ares 1’s five-segment main booster will resonate at a frequency of 12 hertz, or 12 cycles per second, while the rest of the rocket stack is expected to resonate at 11.7 hertz – close enough to produce unacceptable levels of vibration. NASA would like to see at least 1
hertz difference between the two frequencies.
Lyles said it was possible that further analysis, aided by data NASA plans to collect on space shuttle flights
and from the Ares 1-X test flight slated for 2009, would show that the resonance of the main motor and the rest of
the stack have diverged enough that excessive vibration will not
be a problem. The Ares 1 core stage is based on the space shuttle’s solid-rocket boosters.
But NASA cannot afford to assume today that better analysis will make the problem go away.
“We can’t predict today that
it’s going to move far enough to get us out of any problems with the crew,” Lyles
said.
Lyles told reporters that de-tuning the rocket is one option NASA will spend the rest of the spring examining in greater detail. However, he noted that
Ares 1’s weight-conscious engineers are reluctant to add mass purely for the sake of de-tuning the rocket, and that stiffening the rocket’s joints does not look like an easy solution since the joints
already are “presumed to be pretty stiff.”
As such, the primary option under consideration, Lyles said, is to add shock absorbers –
tuned mass dampers in NASA parlance – to one of two spots on the rocket to dampen vibrations.
One approach would entail installing spring-mounted weights in the rocket’s aft
skirt. The other would be to spring-mount the parachutes inside the rocket’s first-stage recovery system. The seats aboard Orion
also could be outfitted with shock absorbers to take care of remaining vibrations.
Lyles said by concentrating its search for solutions on the Ares first stage, NASA expects to solve the problem without taking much of a performance hit.
Depending on the approach NASA takes, Lyles said, the mass hit would be on the order of 45 to 90 kilograms – a small fraction of the performance margin program managers are holding in reserve to deal with design problems that might crop up later.
“This is a very manageable design issue,” said Steve Cook, director of the Ares Project Office at Marshall. “It is not a show stopper.”
Lyles said his teams would spend the next 90 days converging on a solution. Cook said the thrust oscillation issue is not expected to hold up the Ares
and Orion
preliminary design reviews, currently slated for September.
Perhaps only coincidentally, NASA updated the media on the Ares thrust oscillation issue on the 40th anniversary of the launch of Apollo 6, the last unmanned test flight of the Saturn 5 rocket. Two minutes into that flight, the Saturn 5 experienced about 30 seconds severe vibrations that NASA traced to a type of thrust oscillation that can occur in liquid-fueled rocket engines.
While Lyles mentioned the Apollo 6 problem during the briefing, he did not note the anniversary.
Comments: bberger@space.com
