Mark-IIId PVC Hybrid Motor

Report of Hot Fire Test Sequence 1

October 21, 2007

Stephen Daniel

Last update, October 21, 2007

 

Test Report

On October 21, 2007 we conducted two static tests of the Mark-IIId PVC hybrid motor.  Photos of the test are available here.

Test Objectives

These tests had these objectives:

  1. Verify our infrastructure, including launch-control, nitrous supply plumbing, and the test stand.  All of these are common across both the Mark-IIId and the P1 systems.
  2. Test out a number of aspects of engine design common to all our motors, including our new fill-line disconnect, our pyro-valves, and our ability to predict engine performance
  3. Verify the readiness of the Mark-IIId PVC hybrid engines for launch, scheduled for 11/09/07.

Summary of Results:

We completely met our goals for objective 1.  Common design components (objective 2) performed flawlessly with the exception of the modeling software.

The first test firing went well; however there was a small burn-through in the PVC nozzle.  In our estimation a flight on this motor might have been successful.   The second test failed with a clogged injector resulting in a slow burn and loss of the motor.  A flight on this motor would not have left the ground.

Details

All infrastructure performed flawlessly. Lessons learned:

  • On a warm sunny day we went through 30 lbs of ice chilling the nitrous to about 50F and keeping it there for an hour or so.  We need to develop and deploy a battery-powered chill pump prior to the November launch.
  • The nitrous tank should be set up on the same side (left for White Nitro) as the nitrous fill port.  Otherwise when the fill tube disconnects it falls and lies near or in the blast zone.
  • The control-box LEDs are plenty bright enough to see in bright light, provided they are carefully aimed.
  • The bottom tie-downs on the test stand must be protected (with Al foil?) from exhaust gases.  There was a crack between the tower and the blast deflector on one side, and that side's tie-down burned through.
  • Water-soaked 1/2" CDX (cheap) plywood makes a very serviceable blast deflector.
  • We should use a leaf blower on the blast zone.  Clearing flammables is not sufficient; we need to clear dust.
  • Close all tool boxes and protect everything from dust prior to pushing the button!

Motor design components also worked flawlessly.  Our performance models needs work:

  • Fill line disconnect assembled easily, did not leak, held a full 600 PSI, and reliably disconnected on command.
  • Pyrovalves performed to spec, tightening into place easily and not leaking.
  • The current pyrovalves are 0.55" thick.  This is too thick.  (Note: quality control on the valve thickness is difficult on this design.  The 0.55" was supposed to by 0.625".)
  • Engine performance was off from nominal.  The burn appears to short (perhaps 2.4 seconds instead of 2.8) and the fuel consumed appears too high (perhaps 200g instead of 150g).  These facts suggest that the injector flow rates were higher than predicted.  Some (all?) of the excess fuel is consumed during the gas-phase burn-out.   Corrective action:
    • We urgently need combustion chamber pressure monitoring to verify and correct our performance models.  Second priority is tank pressure, third is thrust.

The PVC motors continue to struggle with reliability.

  • The nozzle failure on the first firing requires correction  This nozzle was made out of nested bushings holding a steel washer.  The washer failed and the burn through was under the failure on one side.  Corrective action:
    • The dead space under the steel washer will be plugged with plastic epoxy putty for further flights.
  • The failure of the second burn was caused by problems in the first.  During the gas-phase burn-out the motor appears to shut down and re-light twice.  Apparently during this oscillation hot exhaust gasses containing significant amounts of molten and vaporized plastic were sucked into and behind the injector.  We speculate that this was caused by too large an injector relative to the nozzle throat diameter.  Corrective actions:
    • Reduce the injector to the six-hole version.
    • Removed, inspect, and clean the injector and supporting nipple between firings.
  • The brass nozzle used on the second test survived without any damage.  It took some work to clean the gunk off it, but it suffered no visible damage or discoloration.