Impulse Recorder






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This is the same picture used in the brochure.

What it does

This machine, though looking crude, actually does a lot and does it accurately.  Here is what it does:

  • Ignites the engine remotely

  • Draws a thrust-time curve

  • Records the time delay for the ejection charge

  • Works with all sizes of engines

How it works

The engine is mounted on a rigid arm that pivots around an axis on one end.  A calibrated spring is attached to the arm.  When the engine fires-it moves the arm a distance proportional to the thrust developed.  At the end of the arm is a weighted ink pen that writes on a strip of paper wrapped around a drum.  The drum rotates at a specific speed so the thrust is recorded as the drum moves and the result is a thrust time curve.  The arm is long compared to the arc it moves in so the error due to moving in an arc rather than a straight line is minimal.  The mass of the arm is also very low compared to the engine force so that error is also minimal.  The motor that drives the recording drum is automatically started at the same time as the electrical ignition for the engine.

In addition, another pen scribes a line below the thrust time curve.  This pen is driven off a screw on the same axle as the drum and so moves a small amount across the drum as the drum rotates.  This way, it doesn't overlap itself as the drum makes multiple revolutions.  This is so that a delay can be recorded that lasts more than one revolution.  A small photo sensor is mounted on the forward end of the engine and detects the flash of the ejection charge.  This triggers a solenoid that causes this second pen to move and so records the time of the ejection charge relative to the main thrust time curve.  The crank is so that the coupling to the drum can be loosened and the screw can be backed up easily to the starting point for the next test.

Different size engines can be tested from the tiniest 0.25" ID to the largest 1.25" ID.  A number of different springs were used and each one was calibrated.  The stationary end of the spring was attached to an eye bolt which could be easily and quickly changed for different lengths of springs so all charts were made from zero force.


The construction can be seen fairly well from the picture.  All components were easily obtainable and were made with nothing more than standard hand tools, a table saw, drill press, and a gas welder (used to braze a few parts).  The drum was a piece of PVC pipe, probably about 3" sched. 40.  Wooden circles were cut and sanded to fit the two ends and a 1/8" steel shaft was run through it.  An old ErectorSet part was used to fix the shaft to the wooden end.  The drum motor I think was ordered from Graingers.  It was a 115V AC gear-reduced motor that only turned about 1.5 RPM.  I was thinking it was a BBQ rotisserie motor but from looking at the picture, apparently not.

The photo sensor, relays, amplifier for the photo sensor and transformer (115V to 12V for igniter) were ordered from Graingers. The solenoid was out of a 12V solenoid operated water valve.  The arm was aluminum angle and on the end was attached a piece of hack saw blade.  This acted as a spring and supplied the pressure to make the ink pen write.  All the other pieces were just wood, steel plate, steel rounds, and steel flats.

Hundreds of engines were tested on this machine.  Unfortunately, I didn't keep any of the charts but a few were reproduced for the manual and brochure.

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