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Product Details

0.1% of measured value, world’s most accurate instruments; 0.25 to 25,000 lb / 0.1 to 11,500 kg. Raptor Scientifics KSR series instruments continue to offer the highest accuracy available for CG and MOI measurement. These combined instruments are available in a variety of sizes and measure two coordinates of center of gravity and one moment of inertia in one setup.

Typical Applications

KSR series are used primarily by the defense and space industries for measurement and verification/validation of center of gravity (CG) and moment of inertia (MOI) in high end payloads such as:

  • Spacecraft: Satellites and Vehicles, Manned and Unmanned
  • Spacecraft components and cargo
  • Military payload: Bombs, missiles, torpedoes, and their components
  • Aircraft: UAV and Flight Hardware
  • Aircraft engines
  • Ground Vehicles: Automotive engines and transmission components

There are two basic mechanical configurations used in the new KSR instruments.

In Figure A the test object (and fixture) is supported by a spherical air bearing. A restraint is provided by a hollow tube which extends from the base of the spherical bearing. The lower end of this hollow tube is attached to a second (cylindrical) air bearing which is connected through a moment restoration transducer to the rigid instrument base structure.

A torsion rod extends downward from the upper surface of the bearing (the rotary table) to a clamping mechanism at the bottom of the rod. When this clamp is released, the spherical bearing is free to turn about the vertical axis for CG measurement and when clamped, the machine is converted to a torsion pendulum for MOI measurement.

These instruments are capable of withstanding a large overturning moment and are ideal for measuring tall payloads such as rockets. MOI tare is larger than the machine shown in figure B.

In Figure B, a flat gas bearing supports the test part and a cylindrical air bearing defines the rotational axis. The bearing and test table assembly is mounted on crossed web flexures which create a nearly frictionless pivot axis. A moment arm extends from the air bearing body and is connected through a linkage to the moment restoration transducer. The torsion rod is mounted similar to that in figure A. The measuring principles are the same for both configurations, but the flat bearing type has a lower MOI tare and can measure smaller objects.

Supreme Performance – Our KSR concept outperforms all other methods. These instruments offer advanced technology that maximizes measurement sensitivity, dynamic range, linearity, stiffness, and overload protection. Dynamic range and sensitivity are increased by a factor of 100 over conventional methods. Linearity and stiffness are improved by a factor of 5. Raptor Scientific KSR series instruments offer the Highest Accuracy available for CG measurement with the use of closed-loop force rebalance technology. For more than 30 years, our KSR systems remain the most accurate measuring systems of their type.

Test Small And Large Payloads On The Same Instrument – Our KSR instruments have a dynamic range that is greater than 30,000 to 1. A single instrument can cover a range of payloads previously requiring two or even three machines!

Center of Gravity – Measurement along two horizontal axes in a single setup with better than +/-0.001 inch (0.025mm) accuracy for payloads weighing as little as 4% of machine capacity!

Moment of Inertia – Measurement around a vertical axis with accuracy better than +/- 0.1%.

Fully Automated Operation – Select CG or MOI on the computer, and the entire measurement takes place automatically. The resulting test reports can be printed or exported in Excel or PDF format. All data is stored on a local hard drive.

Spherical air bearing – The extraordinary dynamic range and accuracy of the moment transducer would be wasted if conventional bearings were used as a pivot. This instrument uses a spherical air bearing to support the weight of the payload. The friction in this bearing is 1000 times less than conventional bearings. The runout is less than 0.03mm. Since the bearing rides on a film of air molecules, it has absolutely no wear. MTBF of our spherical air bearings is greater than thirty years.

Automatic elimination of leveling errors – Conventional center of gravity instruments will give erroneous answers if the mounting table is not perfectly leveled. This effect is particularly noticeable when measuring tall objects. On our KSR machine, the payload is mounted on a rotary table. The moment due to CG offset is measured at multiple positions of rotation. This data allows computation of CG coordinate locations along two horizontal axes. Averaging of CG offset moments displaced by 180 degrees eliminates leveling and zero shift errors.

Insensitivity to Payload Weight – Since the weight of the payload is totally supported by the air bearings and none of this force is applied to the torsion rod, the instrument is insensitive to payload weight and may be calibrated using a single test mass. The moment of inertia indication is then linear over the full range of payload weight specified for the instrument.

Computer controlled – A computer is supplied with each instrument. A series of menus allow the operator to select the desired procedure for calibration, measurement, or calculation. Prompts on the screen guide the operator through each procedure so that the operator rarely needs to refer to the instruction manual.

Software – The computer can be programmed to calculate the mass properties about axes other than the rotational axis of the machine. Custom report formats are available. Optional custom software calculates the correction weights required to balance a specific object. Optional interface and software permits communication with another computer.

Calibration hardware provided – Each KSR instrument comes with calibration hardware (beam and weights) certified to NIST.

Options & Accessories:

  • Custom interface plate
  • Explosion proofing
  • Integrated CMM arm
  • Dial indicator stand
  • Air supply
  • Integrated weight platform
  • Hydraulic load positioner
  • Product of inertia software: calculate POI from MOI measurement
  • Other accessories


  • Custom Fixtures
  • L Adapter fixture for measuring payloads in a horizontal orientation
  • Spacecraft Positioner for measuring all mass properties of a spacecraft

Electrical and pneumatic requirements These instruments require 115 VAC, 60 Hz, 15 Amp 1 Phase electrical power. The apherical air bearing requires clean dry air at a pressure of 80 to 120 PSI (550-825 kPa). Gas flow requirements vary according to the size of the machine and are listed on the accompanying data sheet. Generally shop air contains oil and water and is not suitable. Gas can be provided from a standard high pressure bottle, or you can order a compressor-dryer from Space Electronics.

Full Mass Properties


Point of Inertia Spin Balance
POI Spin Balance Series View Product
MP Weight/CG/MOI Series View Product

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