I finished the injector this weekend so all the parts for the regen engine are done. Photos are on the regen
page. I broke a drill bit in one of the 0.040" fuel holes of the injector but
luckily I was able to recover without any permanent damage. Well, we'll
see if all the holes flow straight when I do the water tests! I drilled
from the other side until I ran into the broken drill bit and then used
the shank of the broken drill bit to punch out the broken part. After that
I was much more careful!
I bought some 0.030" and 0.050" sanding cord from a woodworking shop to help
with deburring the intersecting coolant holes in the chamber. It seemed to work
Let's see, when I was working on the uncooled engine, right about now is when
I thought I was almost done when I really had another 18 months to go.
Hopefully, since the test stand got a good shakedown with the uncooled engine,
it will go smoother this time!
I should be able to start cold water tests to check the injector performance
along with the cooling jacket pressure drop here within a few weeks. Since
there is a significant pressure drop in the cooling jacket, I'll have to run
the propellant tanks at different pressures. This means I have to buy
another high-pressure regulator and do some plumbing work on the test stand.
I also have to design a different attachment for the engine to the test stand.
I was hoping to have it tested by the end of the year but it may slip a couple
of months into 2006.
I finished the regen chamber a few weeks ago and now I'm finalizing the
injector design. Several folks pointed out an irregularity in my thermal
analysis at the exit plane of the engine. I had been using the chamber stagnation
temperature for the gas temperature as a conservative value at the exit but I
went ahead and revised the analysis based on the assumed exit Mach number.
I took the opportunity to clean up my design spreadsheet and add sections
to help me analyze the as-built engine vs. the design parameters. The
updated spreadsheet in both OpenOffice and MS Excel formats can be found
in the Regen Chamber section.
The injector design is quite different from what I was
originally planning. The new design uses a set of unlike
split triplet injectors, with 6 total fuel and 6 total oxidizer
holes. Since the split triplet has the oxidizer holes pointing straight
down with an angle of 0 deg, it turns out to be a good solution to the
problem I had with the uncooled engine where the oxidizer holes pointed
out towards the chamber wall. I also have a good solution for injector
face cooling that uses face-milled slots as the manifold for each fuel
hole. There is an unfinished drawing on the regen drawings page.
Almost done with the regen chamber! Last night I drilled the chamber
coolant holes and the slots at the injector end. Photos
can be found here.
All that's left on the chamber itself is to drill
and tap the two holes for the inlet and outlet of the cooling "jacket".
Then I need to make the plate that covers the slots at the exit and the
plate that covers the angled holes on the side. But those are the easy
I'm leaning toward using SS-304 for the injector because I can't seem to get
the design to converge with 6061-T6. With aluminum, the analysis shows
either the injector face getting too hot and melting or
there is too much heat transfer to the fuel and the fuel
gets too hot and cokes up. From what I've read, SS-304 has
less ignition sensitivity to particle impacts with LOX anyway. If I go
with SS-304, I'll have to make sure the difference in thermal expansion
between the chamber and injector doesn't cause problems.
I finished drilling the coolant holes today for the nozzle converging and
diverging portions. The cross-drilled holes seem to match up nicely.
It really helped going slow and backing out the drill bit every
0.1" or so to clear the chips. I took a day last week and made a
test piece to see if I could really cross-drill holes and
make them line up. When I cut the test piece in half
to inspect it, all the holes were right where they should be. This is
one of those cases where you measure 1000 times and hopefully cut just once!
Now I just need to figure out how to debur the intersection
where the holes meet in the throat plane. Once I drill the chamber
coolant holes I can breathe a sigh of relief because everything
after that for the chamber should be pretty easy (bolt hole patterns, etc.)
I've been reading up on injector cooling and I'm surprised how little info there
is on the subject. The best source I've found is AGARD-AG-148-71 which
has a whole chapter on heat transfer to the injector. There just happens
to be some test data in the report on a 4 cm diameter thrust with
GOX/kerosene. For a chamber pressure of 10 bar, the total heat flux to the
cooled injector was 210 W/cm^2, which works out to 1.53 BTU/s-in^2.
That's pretty close to the calculated heat flux at the exit of my regen
engine of 1.49 BTU/s-in^2. So, assuming I'm in the ballpark with my
estimates on the rest of the chamber, I should be able to devise an injector
cooling scheme with the fuel. I have some more design work to do on the
Spent the weekend making an adapter
to mount my 6" 3-jaw chuck to the rotary table.
With a little more adjustment and alignment, I should
be able to start drilling the cooling holes in a few days.
Finished the first phase of the lathe work today on the regen chamber.
The inside contour is done and now I'm ready to start drilling the
holes for the cooling passages. There weren't any major screwups during
the machining but I had some chatter while doing the converging portion
of the nozzle - hopefully I can smooth it out with some sanding.
I've also updated the drawings to reflect the change back to 100 lbf thrust.
I went back and reprocessed the data from the uncooled chamber tests
last year to fix the thermocouple gain issue I had during the test. I
also compared the experimental results against the TMG analysis for the
same engine. The results are posted in the analysis section for the
uncooled engine. Since I had the issue at the start of the run with the
late LOX valve, it's hard to decide where to line up the "start" of the run
but it's interesting to note that the slope of the temperature curve for
the chamber thermocouple matches the prediction almost exactly. Hopefully
this is a good validation of my heat transfer equations that I used to
design the new regen engine.
Fabrication on the regen chamber has begun! Follow the machining
progress in the
Regen Chamber Photos section. I discovered the 3-jaw chuck on my
lathe had adjustment screws so I was able to get the runout to within 0.002
inches or so, much better than the 0.012 I had when I started! I'm only
able to spend a few hours a week on it so it will probably take a while
before it's finished.
My home shop is almost ready to start fabrication on the regen engine. I
picked up a used Bridgeport Series I mill and a used Nardini 14x40 lathe to
help out. My older and much smaller Grizzly mill/drill and lathe just
weren't going to make it for another engine. The Bridgeport should make
it easier to handle all the angled and cross drilled holes. I've spent
the past few weeks getting them set up and ready to use. Along the way
I had to get a rotary phase converter so I can run the 3 phase motors in
the mill and lathe with my 220 V single phase service. I'm almost finished with
cleaning all the old chips and gunk out of the lathe - what a mess! I was
originally going to buy new full-size Grizzly equipment but at the last
minute, I had a change of heart and decided to go with quality over new
stuff. My usual motto is to buy the best and cry only once. Time will
tell if I made the right decision or not.
I just couldn't wait! The project has been calling me for the past few
months, waiting to be finished. I've started design on a 100 lbf version
of the regen engine, similar to the uncooled one I previously tested. I
changed some of the equations I'm using and that combined with the belief
that I can get away with higher temps for kerosene for short duration made
it more practical to do a 100 lbf regen version. I've already run the
thermal analysis using Femap/TMG and it looks like it will work at least
as well as the 200 lbf regen version I was planning. Once I figure out a
good scheme for cooling the injector face, I'll start modifying the solid
models and generating shop prints. Again, the goal is to get an engine to
run for 30 seconds continuous, then I can start designing the flight
Well, not much has happened since the test. I took a new job at a startup
aircraft company and the free time is pretty much gone for a while. I
hope to get something going again this summer but we'll see. I guess I
can use the downtime to double-check the analysis on the regen engine.
The test frame and associated equipment survived the move so I still have
all the stuff to test again once I get the next engine built.
I figured out the problem with the thermocouples on the uncooled test
(gain problem with the amplifiers) and when I take that into account, the
temperatures come out as I expected. I ran an unsteady heat transfer
analysis using the same methodology as I'm using for the regen engine and
the time to reach the measured exterior temperature matches my test within
a second or so. This tells me that my assumptions for the insulating
effects of the LOX/Kerosene combustion products on the chamber walls are
pretty close. If I get time, I'll post the results of that analysis.