Tuesday, February 9, 2016

SteamFlash Rocket Motor 05 - First Test Run

First test run 7pm, Sunday, 07 Feb 2016 NZDT (8am, 07 Feb 2016 GMT).

Run at about 100W (peak 5.9 V across 0.36 Ohm).
This is the Flash Boiler only loaded with 6 ml of water for a short run.
The plan was to run for 20 sec then measure how much water had steamed away.  In theory this should be 1ml leaving 5ml.

The boiler ran strongly for 18 sec, giving a good proof of concept as a rocket motor.
At 18 sec the pressure opened the spring loaded o-ring joint at the bottom of the unit and we lost the water. The coil started to overheat and I cut off the power.

Starting setup - showing setup outdoors and remote from the control table.

This is me doing my introduction speech to my invited audience of neighbours.

Close-up of boiler running.  I held it together with spring loaded clamps so that excessive pressure would open the O-Ring seal. High pressure did happen and this "safety valve" did activate and do its job - see bottom of 2nd photo below.

End of run.

It was excellent value to have 2 x videos of this run. Between them we can see what happened and learn a lot about taking the next step. It is critical to keep the coil wet and the above photo shows what happens if it goes dry. Any flying model will need excellent methods for guarding against this.  This is the next big challenge - some early ideas are:

  • We may already have a safety method in that the coil is similar to a fuse. We need a test to destruction of the coil to see if it does a rapid burnout like a fuse.  Changing to a lower melting point wire may help.
  • Theory says that the resistance increases when the wire gets hot.  We can see on the videos that the voltage suddenly rises from 5.90V to 6.06V when this happens. Detecting that could be a safety method.
  • Thermistor = temperature sensor
  • Sensor to detect pressure drop
  • Limited run time timer with battery cutout
  • Change to heat resistant materials for the boiler - eg ceramics
Possibly a combination of most or all of the above.

The plastic and epoxy glue handled the 100 deg C temperature and steam pressure well.
Part of the syringe plastic body did melt a hole at the moment shown above with the glowing coil but that is to be expected in this not-by-design overheating moment.
Video currently editing - coming soon.

Lots of discussion happening here on next steps. A big question is the size of the nozzle hole and the nozzle shape. For this run we simply went with the hole that happens to come with the syringe body. 100W gives a surprisingly powerful result and looks like it may be near the upper limit we can use with these materials. We may be able to run with a smaller, lighter and cheaper LiPo battery than originally planned. 

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