December 6, 2004 notes

 

Wound engine failed

 

We got the fiberglass cord wrapping of the milled chamber done fairly well.  We put the chamber on the lathe, soaked 100’ of 1/8” thick fiberglass cord in a tub of epoxy, and set up a “human filament winder”.  One person held the tub with the epoxy and cord, one person rotated the lathe by hand, and one person guided the cord around the engine, maintaining tension with one hand, and squeezing off the excess epoxy with the other hand.

 

We found that we had to start at the throat, wind up towards the exit, then do a fast loop back down to the throat before going back up the converging cone and onto the chamber.  If we just started at the end and wound down towards the throat, it had a tendency to slide out of position.

 

http://media.armadilloaerospace.com/2004_12_06/winding.jpg

 

We left it spinning at low speed on the lathe overnight, which left a good finish on it.

 

To get the wax out of the channels, we first tried putting shop air in the inlet and blowing a heat gun through the engine, which did start extruding wax out of all the channels, but once about half of them opened up, there wasn’t enough pressure to push out the remainder, and all the air flow was cooling everything off.  We then tried filling the engine with water, and bringing it to a boil.  We then used a hose to carefully blow (manually) through the inlet to clear out the now very liquid wax.  This seemed to work fine, and we blew pressurized water through it to verify that all the channels were spraying properly.

 

Putting the wax in and getting it out was a pain.  If we try something like this again, we are considering just wrapping over the channels with pipe tape under the composite to serve as an epoxy barrier.

 

We were a little concerned that some waxy deposits left inside the chamber or channels might cause a hybrid-motor sort of burn through when we were running the hot oxygen preburner before starting fuel flow, but we went ahead with the test.

 

We put the engine on the test stand and dialed in the preburner temperature we wanted before starting the fuel flow.  On fuel throttle up it looked great for about half a second, then there was burning methanol spraying all over the place.

 

As soon as the chamber got hot, thermal expansion pulled the fiberglass cords apart in several places, including a 1/16” crack at the start of the converging section.

 

http://media.armadilloaerospace.com/2004_12_06/crack.jpg

 

It looks like some form of vertical reinforcement is really necessary to withstand the thermal cycling, even if the epoxy could handle the pressure load.  We could try thin vertical strips of fiberglass, or overlapping wraps of some narrow weave that conforms better than the pipe repair tape we tried, but we are probably going to go back to all metal engines.

 

If we want to try another one at this size, we will probably make a throat saddle like the XCOR and SPL engines, but I am going to get some quotes for making a gun-drilled motor, which may have some advantages, but will have to be somewhat larger.

 

In other work, to my continued amazement, our custom motor drive boards STILL AREN’T HERE.  We have gone ahead and built up half of the wiring harness and tested the computer, GPS, and IMU on the main board.