I'm glad to hear the test went off fairly well.
Those mud daubers are certainly a pain. A good fifteen minutes of
every day's testing on a stand in Alabama was going through the
checklist uncovering and recovering various vent lines.
I used graphite plates for the flame redirectors on the tube rocket
launches at Armadillo, but they are kind of pricey, fragile, and
difficult to hold. For the 500lb engine testing at Masten we burned
through a lot of concrete pavers, which are cheap and replaceable but
throw off molten concrete. At Moonex I just used a big stainless pipe
elbow off McMaster, but it was peroxide monoprop so the thermal
environment was easier.
The farther the deflector can be from the nozzle the easier of a time
it has, but the wider the hot plume is.
The big boys mostly use refractory brick and special mixes of concrete
to deal with the conditions. If you do go for concrete, dry porous
concrete mixes survive longer than wet poured ones, aim more for
cinder block than driveway consistency.
Water sprays are less useful than you'd think, the exhaust just cuts
right through them. Adds a nice fog to the exhaust plume though.
On Mon, Sep 12, 2016 at 9:31 AM, Robert Watzlavick
<rocket@xxxxxxxxxxxxxx> wrote:
The dewar was at 20 psi in this case. Now I have to clean out the LOX tank
and all the plumbing since some crumbs may have made it downstream. I have
a LOX filter but it was upstream of this particular line. Lesson learned!
-Bob
On 09/11/2016 11:53 PM, Andrew Burns wrote:
Re. the mud daubers, from experience spiders can hold about 5 psi quite
comfortably...
Andrew
On Mon, Sep 12, 2016 at 2:49 PM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
wrote:
Oh, and I forgot the real excitement that happened. The one 3/8 inch tube
I forgot to cap off that was sitting in my garage the past two months got
clogged by mud daubers. I opened the dewar and the tank wouldn't fill at
all. It took a lot more effort than I expected to clean it out.
-Bob
On 09/11/2016 09:45 PM, Robert Watzlavick wrote:
I finished up the last of the water and LN2 cold tests I planned for my
250 lbf LOX/kerosene rocket. The goals for the tests were:
1. Test an alternate LOX loading scheme to reduce the tank ullage -
Ben's suggestion worked great. After filling, I closed the LOX vent valve
first, waited a few seconds, then closed the LOX fill valve. This improved
the pressurization time of that tank from 2 seconds to around 0.6 seconds.
Also, the helium pressure only dropped about 200 psi instead of 400 psi when
pressurizing the tanks.
2. Adjust the pressure regulator setpoints - The Aqua 1247 regulators
I'm using have quite a bit of droop under the flow rates I'm using so I was
able to tweak them statically to give the desired pressures under full flow.
During the run, the pressures drop a bit which will change the effective
mixture ratio from around 1.87 at the start of the run down to 1.81 at the
end. The LOX pressure falls off faster during the run even though it is a
lower pressure (340 psi) than the fuel (500 psi). We previously discussed
this and I believe it is due to the cooling of the helium as it enters the
LOX tank, probably not due to absorption in the liquid.
3. LOX insulation on tank and fill lines - I used 1/2 inch rubber pipe
insulation on the LOX tank (in 4 sections) and fill lines which reduced the
fill time from 8 minutes down to about 4 minutes. The Aerogel/Pyrogel
option looks like a better alternative for the tanks since it is thinner.
An alternative would be to use 1/4 or 3/8 inch thick pipe insulation.
4. Helium pressure vs. run time - In my previous tests using 2200 psi
helium, I wasn't even able to get 10 seconds of run time. By using a 3500
psi supply cylinder along with the tank ullage technique above, I was able
to load the onboard tank to about 2500 psi and that was enough for about 17
seconds of runtime, more than enough for a first flight. I'm planning on 10
seconds or less but I want enough helium in the tank to account for slow
leaks during pad operations.
5. Sticky LOX vent valve - I wrapped the LOX vent valve in a 4x6 inch
poly bag with appropriate cutouts and ran a slow stream of nitrogen through
it to displace all the ambient air. The idea was to prevent ice from
forming on the valve body and then dripping back into the bearing on a
subsequent fill operation. This worked as expected so I'll probably do the
same thing to the LOX fill valve and main valves. It also keeps
condensation from dripping on to the servos when the LOX tanks warms up.
Thanks everyone for your suggestions and advice. The next steps are do a
hot fire with the rocket strapped down so I'm looking for ideas on how to
hold down a lightweight stringer-based vehicle (no skin yet) for a vertical
test. I don't have a launch rail here so I was thinking of gripping two of
the four stringers and clamping them to my test stand. Another idea was to
tie into the engine/frame bracket somehow to handle the thrust loads and use
guy wires to keep the top of the vehicle pointing upwards. An unplanned
launch would definitely not be a good thing! I could also use some
suggestions on how to safely redirect the exhaust so I can still take video
and pictures during the firing. I've seen curved or angled plates used.
-Bob
