NASA Shakes, Bakes, and Rattles Lunar Spaceship

coondoggie writes to tell us that NASA has apparently successfully concluded putting the new Lunar Reconnaissance Orbiter through its paces. Using vibration and rotation tests NASA scientists were able to determine the center of gravity and were also able to observe the structural integrity during the vibration tests used to simulate launch aboard an Atlas rocket. "It is expected that the LRO will by the end of the year make its way to NASA's Kennedy Space Center in Florida for final launch preparations. The orbiter and the Lunar Crater Observation and Sensing Satellite, a mission to smack into the moon in search of water ice, are scheduled to launch atop an Atlas V rocket from Cape Canaveral Air Force Station in Florida sometime between Feb. 27, 2009 and the end of March 2009."

Comments

This is routine testing

[Animats]

That's normal stuff. I used to work for a company that built satellites. One of their test fixtures was a shake table connected to a water-cooled voice-coil actuator with a megawatt amplifier driving it. They had accelerometer data from actual launches, and they'd use that to drive the shake table.

Re:I hope not!

[Tmack]

. Using vibration and rotation tests NASA scientists were able to determine the center of gravity...

If they couldn't determine the center of gravity before they built the thing, they have some serious issues. /p>

THAT should be: If they couldn't engineer the thing with the center of gravity exactly where they want it... Center of Gravity and Center of Lift are usually two things aircraft are designed around. Hopefully a rocket based craft would be designed with the center of lift centered in the engine cluster, with the vector pointing generally up the middle and through the center of Gravity of the whole ship. Vectored or differential thrust shifting this lift vector around that CG point to change where its going. If you dont put the CG where you want it, it doesnt fly where you want it to. Granted, the CG of the capsule is high enough on the rocket and of low enough mass that its CG point can probably be off a good bit and still be compensated for, but the CG should still be an easy engineering calculation given the computer models, etc. They should even have the CG pinpointed for every load scenario of passengers and other cargo items, so they can load the craft without displacing the CG too greatly.

Tm

Re:The center of gravity is important.

[Anonymous Coward]

I happen to be an aerospace engineer (formerly at JPL), and the center of gravity is actually extremely important after launch as well. NASA is not pulling your leg.

You may replace the "center of gravity" with "mass distribution" or "center of mass" if it makes you feel better. The term is a hold-over from the early days, and is a term of art than any engineer would recognize and understand. You just need the contextual information that an engineering education provides.

The actual product of the test is both a location of the center of mass of the spacecraft and a mass distribution matrix, commonly called an inertia matrix or inertia tensor.

The attitude control system (my specialty) requires very accurate knowledge of where the center of gravity and the geometric centers are. These are used to compute both control and disturbance torques on the spacecraft. Without a C.G test it is almost impossible to get the pointing performance (sub-arc-second) many of the payloads require on modern spacecraft. The CAD/Solidworks models are rarely good enough for this, and although some things can be done to estimate the C.G. in-orbit, you still have to get through launch and initial checkout with whatever data you launched with. C.Gs are estimated in space to handle things like fuel loss.

Re:I hope not!

[vbraga]

Yes, it's only a confirmation of design phase estimates and calculations. If it's slightly off where it's required to be, small weights can be added to put it where it should. Some fine grained measures must also be taken for attitude control.

(English is not my first language, so, if there's something wrong with my post, I'll be glad to know and learn =))