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.
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 wasnt 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
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
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.
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
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 ARENT HERE. We have
gone ahead and built up half of the wiring harness and tested the computer,
GPS, and IMU on the main board.