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Weight and CG of Large Objects and Spacecraft

Product Details

Measuring the center of gravity (CG) location of large spacecraft presents many of the same challenges encountered when measuring small objects. Common requirements for the measuring device include: establishing known geometry, maintaining geometry over the full range of load conditions, selecting force transducers with appropriate sensitivity to support the desired measurement performance, preserving that sensitivity throughout the measurement, and devising a method to relate the spacecraft’s datum to the instrument’s datum.

The WCG High-Accuracy Series tests items up to 2,850 lbs, with ultra-high sensitivity models available. These instruments use a load cell system to measure the location of center of gravity using a weighted average. To measure an object, simply lower it onto the triangular table and position it relative to machine zero. The third CG coordinate can be measured after rolling an object 90 degrees. CG accuracy depends on test object weight, center of gravity height, fixture accuracy and leveling. Less than one minute is required to make a measurement, so these instruments are ideally suited to high volume production. This type of instrument is often used to measure “black boxes” used in aircraft and missiles. The five standard C series models are a lower-cost alternative and suitable for general purpose applications. We recommend them for laboratories that don’t require the highest precision.

Our hardware enabled the Boeing CST-100 Starliner team to take highly accurate mass properties measurements ahead of flight testing.

When you only have one chance to get it right

When dealing with smaller objects, the above challenges are often met with a combination of familiarity (“we’ve done this before, and we’ll do it the same”) and iteration (“it is easy to try several methods because the payload is easy to handle”).

However, when measuring large spacecraft that range in weight from 10,000 to over 35,000 pounds, these approaches do not work. First, given that the large spacecraft is unique with no legacy art, there is little applicable experience available; the effects of load cells and mechanical interfaces are not vetted; and the ability to mitigate problems on-the-fly is limited by experience, time, and budget constraints. Second, these efforts can be characterized as solutions unique to the spacecraft at hand, and considerable modification would be needed to accommodate different spacecraft. In other words, “we have not done this before”. Finally, given the elevated risk, difficulty, and time allowance (spanning days) for lifting, maneuvering, and handling large spacecraft, the iterative method is unworkable. Essentially you get one opportunity to measure correctly.

As many people working in the mass properties field well understand, mass properties verification occurs near the end of the integration cycle, where all previous delays compress the time allowed for measurement before the scheduled launch. Ad hoc measurement methods do not lend themselves to any schedule certainty the way that a dedicated instrument with well-rehearsed turnaround time can.

Getting accurate mass properties measurement is one of many required steps toward protecting assets, whether they be human lives, hardware, monetary investment, or corporate brand. How do you know you are getting the measurement right? Utilizing a Raptor Scientific instrument (and process) for independent verification and validation frees project engineers from the burden of interpreting results, deciding what data to throw away and what data to keep, and distances previous assumptions limiting confirmation bias. A proven, well characterized, Raptor Scientific instrument has the further advantage of being NIST traceable, featuring calibration stability across a wide dynamic range of payloads, and is an order of magnitude more accurate than ad hoc methods.

Raptor Scientific instruments enabled Boeing’s CST-100 Starliner team to take highly accurate mass properties measurements ahead of flight testing.  Watch this video compliments of Boeing to see the Starliner on their custom WCG system.

Weight & Center of Gravity Measurement Tools


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