CAUTION:  Appropriate safety proceedures should be followed when testing motors.  This test equipment will not prevent injury and can produce flying shards of PVC in the event that a motor fails.  For protection against container failures, an adequate shield and/or distance should exist between all personel and the test stand .  A shield should also be between the pressure gauge and personel.  All motors should be ignited electrically.  The circuit should be safed before approaching an armed motor.   Allow adequate safety period before approaching a motor which refused to ignite.

Theory Of Operation:  A rocket engine thrusts against a water filled syringe.  This produces pressure readings on a connected pressure gauge.  By videotaping the gauge during firing of the motor, one can record the time and pressure and use it to calculate instantaneous thrust and total impulse of the motor.

Choose a syringe size appropriate for your motor sizes.

The diameter of my 12 cc syringe produced a full scale (100 psi) reading corresponding to 30 lb thrust.   My 20 cc syringe produced a full scale corresponding to 50 lb thrust.  My 60 cc syringe produced a full scale at 82 lb thrust  and cost about $1.69.  It is appropriate for large motors only.

This entire test assembly (less mounting) totals only $15.00
even if nothing can be scrounged. 

Parts List:

• 12 cc syringe -  Tractor Supply    abt $ 0.59
• 100 psi pressure gauge - Home Depot   abt $ 6.00
• PVC - 1 1/4" x 3/4" bushing IP   abt $ 0.79
• PVC - 3/4" threaded plug   abt $ 0.79
• 1/4" O.D. polyethylene plastic tubing (about 4' long)  abt $ 0.28
• brass - 1/4" compression coupling (1/2" fpt)   abt $ 1.24
• 1 1/4" pipe (8 1/2" long)
• teflon tape for fitting (only need abt 2")
• 1/4" heat shrink tubing (only need 1" long) *

optional: 38 mm motors will require larger PVC pipe and fittings and a larger diameter syringe (i.e. 20 cc, 60 cc)

Assembly:

1.   Cut a 1" PVC pipe to 8 1/2" in length.
2.   Cement the PVC - 1 1/4" slip coupler x 3/4" threaded adapter on the pipe.
3.   Drill a 1/4" hole through the center of a 3/4" threaded end plug.
4.   Rough up the tip of your syringe and force a piece of heat shrink tubing over it. (Using a 1/4" long piece of heat shrink on the end of the syringe will make it a little larger assuring a snug fit by the plastic tubing.  The 1/4" hole in the end plug restricts expansion of the plastic tubing as the syringe end is inserted making the plastic pipe grip the syringe nicely.)
5.   Put 2 wraps of teflon tape on pressure gauge threads and put on brass fitting.
6.   Push about 10" of your 1/4" plastic tubing through the end plug.
7.   Fill the tubing with water.
8.   Connect pressure gauge to 1/4" plastic tubing.
9.   Push plastic tubing through pipe so that syringe may be inserted from other end.
   
   
   
10.   Fill syringe with water and insert into 1/4" tubing.
11.   Remove all air bubbles holding the tubing vertical and with the gauge on the bottom and thumping the tubing. (Occasionally pressing the syringe plunger will help.  Do not pull on the plunger even a little with the gauge attached.   Doing so may bend the needle on the gauge.)
12.   Disconnect the plunger and add or remove water so that finally syringe is only 1/4 full and contains no air.
      (You may find a bleed valve useful to remove any residual pressure from sliding on the tube.)
13.   Adjust tubing so that syringe tip extends with it into threaded end plug.
    
     
    
14.   Screw in end plug.

optional enhancements for large motors - Depending on the brand of syringe, high thrust levels may make reinforcement of the plunger desirable.  Also, a thin layer of Durahm's water putty may be used in the threaded plug to spread out the load on the end of the syringe near the polyethylene tubing attachment.

It is important to use lubricant on the plunger and make certain that it moves freely or the gauge will not fall off rapidly below 4 lbs thrust. (You should use KY jelly for this purpose.  Do not use teflon grease, vasoline, or soap on the plunger.  It actually prevents proper operation.)  Use of the KY jelly and complete removal of air in the plumbing and gauge makes the apparatus instantly responsive.  Surprisingly, with a free moving plunger, testing with a scale agreed exactly throughout the range of the pressure gauge.   Make certain that the motor also moves freely in the tube (lubricate if necessary).  NEVER test a motor containing a parachute deployment charge.  Be certain to seal the ejection charge hole or you'll be spending 59 cents for a new syringe.  The movie camera should be set to display the time with tenths of a second displayed while recording.  Alternately, a stopwatch can be placed near the pressure gauge if the videocamera does not display tenths of a second.  (Cameras record in about 1/30 sec frame rates.  So, it is possible to count frames.)  With attention to detail, you should be able to generate a good estimate of instantaneous thrust and total impulse.  The following chart was generated with a simple thrust curve spreadsheet.

TOTAL IMPULSE:  323 NS

Alternate Design:  140 lbs thrust.

This piston arrangement can be placed between the motor and a stop and with a 100 lb gauge will measure motor thrust to 140 lbs.  For lubricating rubber O-rings in PVC a different lubricant must be used than in the syringe design above.  Lube Jell with PTFE from Radio Shack works well in this PVC piston/plunger assembly.

Parts:

1    3/4" coupler
1    1"    coupler
1    3/4" end plug
1    1"    end plug
1    brass barbed hose fitting
1    O-ring

You should find that the 3/4" coupling fits into the 1" coupling's with very little extra room.  Glue the end plugs in place.  Drill for the brass fitting in side of the bottom half of the 1" coupler.  Thread the brass fitting in and out a couple of times.  Then, glue the fitting in place.  Cut an O-ring groove near the end of the 3/4" coupler.  Lubricate the O-ring and cut the O-ring groove deep enough that the greased O-ring slides easily.  Fill with water, attach a plastic tube and pressure gauge and you're in business for motors with peak thrusts up to 140 lbs.

A much higher power test stand makes use of a modified automotive brake cylinder as the hydrolic load cell.  Marlin Philyaw's test stand

note:  The noise of the engine may be lessened by pointing the exhaust into a tube into which a ring sprays a high volume of water.  This will reduce the sound generated by the shearing of the exhaust against the atmosphere.  However; it will also prevent viewing of the motor plume.  The tube must be suitable for high temperatures.  (Placing the motor nozzle too close to such a tube affects thrust.)