February 24, 2008 notes:
Film cooled engine work
We got set back up for full-throttle horizontal testing,
something we havent done in over a year.
We used to do tests at the shop in the loading dock bay, but we eventually
got people complaining about it, so we are now set up on the back of the crane
truck, and test out at the airfield where we do our flight testing.
The injector that we flew the vehicle on last month burned
through the oxidizer manifold when run at full throttle for an extended
period. The hover test was only at about
60% throttle, and had seen some initial erosion, so that wasnt too surprising. The chamber wasnt damaged.
We made another injector that was basically all we could do
to help the single ring of elements cooling we thinned down the deck to
increase heat transfer, reduced the area of the center lox manifold to decrease
the heat going into the cryogen, and decreased the element impingement angle to
move the impingement point away from the face.
This engine lasted longer, but the Isp was down. We were able to do several engine start
sequences with it, and collect a lot of 200 hz sampling data, but we did
eventually burn through the lox manifold on that as well. With the lower Isp, the chamber wasnt
getting very hot at all.
We gave up on the single ring injectors, and resigned
ourselves to using a fuel-ox-fuel manifold like the successful engines of the
past year and a half. There are three
construction options spokes in the element pattern and tricky manifold welds
like we have used in the regen engines of last year, external crossover pipes,
or buried welds in the manifold. I am
very concerned that spoke holes in the element patter will give hot streaks on
the film cooled wall, and external fuel bridges cause uneven manifold fill
rates and difficult fabrication, so we decided to just do the buried welds and
proof test very carefully.
Spacing the element exit points apart in the last engine
seemed to have killed our Isp, so I took extra steps to get them very close in
the next injector. Because of some
construction details on the new designs, I can drill the element holes from the
top side of the injector with a sacrificial plate on the chamber side,
resulting in clean exit holes that can be very close together without the need
for flat spot faces.
We used a new spun chamber from AMS for this engine, and
Phil polished it for best smoothness.
This chamber is more concentric, but it is still a 3.5 throat, and AMS
doesnt think they will be able to spin the tube all the way down to 3.0. We will see how the next couple go, but machining
out of billet may turn out to be a better option.
When we fired this engine, the Isp was great (we are only
doing relative comparisons by comparing chamber pressure with a constant
orifice are and feed pressure), but the combustion wasnt completely
stable. The plume jumps around a bit,
and there was a high pitched whistle.
The pressure transducer graph was absolutely level (until the transducer
died late in the test, another good sign of instability), so the instability
was well above the transducer bandwidth.
After about 20 seconds at full throttle, we saw stainless burning in the
plume and shut it down. This time, the
injector face was still perfect there was heat discoloration, but it was the
chamber throat that had started eroding.
We did another short firing with this engine, but we are making two
changes for next time:
We are cutting 3 off this chambers barrel section and
putting the same injector back on. I
doubt that will help the instability, but the film cooling wont have to absorb
as much heat, and it may reduce the combustion efficiency, which would also
reduce the cooling needs. If we get the
same chamber pressure with the shortened chamber, it means we had more L* than
necessary (at least for full throttle operation, deep throttle would be
different), which would be good information.
I am going to change the impingement angles on the elements
to hopefully fix the instability, and also hopefully keep the combustion
farther away from the walls. This may
reduce the Isp, but well see. Isp is of
secondary concern right now, so if we get an engine that lights 100% reliably
and can burn indefinitely, we will go back to flying vehicles.
One annoying aspect of firing the engine horizontally is
that we are having more issues with initial lox density. On our modules, the lox tank is all uphill
from the valve and engine, so we have full density lox right in front of the
valve. Using Pixel up on the crane truck
as our run tanks gives us a nice gas / low density lox trap. We cant just dump a whole lot of lox through
the engine, because it makes a big puddle in the horizontal engine and might
run back into the fuel manifold. We
found out that our manifold purges definitely do NOT sweep out liquid pooled in
the chamber very well. We added an
external lox line conditioning valve right at the engine to let us dump it
outside. We tried first with a big
solenoid, but it was clear that wasnt even close to good enough for the 20 of
feed line we have, and we eventually wound up using a spare main propellant
valve teed off the main one. When I let
this run for a while, the engine starts fast and smooth. If I dont let it go long enough, the engine
starts smooth, but then has a big drop in thrust for a second as it is
consuming the fluffy lox that had risen up and collected at the top of the
Rocket crane truck
We had to do it.
We pulled up the crane feet, took off the brakes, and let
Joseph and Russ ride in the truck cab with the engine firing. We built a little control box so they could
control the engine while I monitored the telemetry from our normal position.
A 1500 lbf engine doesnt move a 50,000 lb crane truck very
fast. If we put it on the back of James
We did a test assembly of our four module bodies to make
sure everything fits properly. After we
finalize the next generation engine, we will pull one of the modules off for a
set of single module tests, then go straight to four module tether tests to
prove out differential throttling.
Matt made some nice renderings of the six pack, our
current plan for commercial suborbital use.
Dont try to read too many technical details into these concept
renderings, there are lots of things not explicitly drawn.
Half the team will be at Space Access 08 next month, so you
can hit us up with detail questions there
On a final note, we did not get selected for the Air Force
Research Laboratory phase II project we were in contention for, but the NASA team
we are working with are very amiable, and that may lead to larger things in the
future. Our crop of potential commercial
customers continues to shift around, but it looks like some things will be finally
closing on that side soon.