The chill down is mostly to let the LOX come to a thermal equilibrium
in its new home, the agitation from it boiling makes it likely to have
that last bit of headspace be filled with agitated boiling LOX which
makes it more likely for it to dump out the vent prematurely. One
thing with your thinwall tanks, is it looks like you have aluminum
brackets holding them, these act as pretty good heat-sinks to your
aluminum airframe. This can make it hard for the LOX to settle down.
The tell if this is an issue would be how frosty the rest of the
vehicle is getting if you let it sit. Or a thermal camera would be fun
to look at it with.
As to why it worked with your LN2 setup, you said you did have to vent
your LOX dewar for a half hour to get it down to 40psi. If the LOX
hadn't chilled down to equilibrium at its new pressure its possible
you were getting extra warm LOX when you did your fill. Which could
exacerbate your boiling problems. The boiling point of LOX at 200 psi
is -235F, at 40 psi its -278F, and -297F at ambient pressure. So your
bulk LOX needed to chill down over 60 degrees F in order to come to
equilibrium at ambient, which is a lot of LOX to boil off to cool it.
Since you did close the vent immediately after filling, its possible
your LOX is a lot warmer than you expected, its density will be much
lower. For example if your LOX temp was -250F when you pressurized to
600 psi, your LOX density would be 1008 kg/m^3, instead of 1148 kg/m^3
if it was at -297F. Which means even with a full tank you would have
13% less mass in your tank than expected even if full. This is another
reason for the settle at ambient, as it gives you a known density of
your LOX to start with, as you will know your LOX temp pretty
accurately, at least initially.
It is probably a combination of factors that lead to your short burn time.
If it were me I'd have your bronze filter travel with your LN2 setup
rather than have it be a permanent fixture in your flight vehicle.
Since I'd have that there shouldn't be any grease blobs in your LOX
dewars. If there were it could be rather exciting during the fill.
On Tue, Mar 13, 2018 at 9:40 AM, Robert Watzlavick
<rocket@xxxxxxxxxxxxxx> wrote:
Thanks for the suggestions. These were thin walled tanks, 0.09 or so there
shouldn't have been as much thermal mass to chill down. Of course now that I
think about it, there is less thermal mass to help keep it cool. Hmmm. But
I used the exact same setup (essentially) in LN2 tests and didn't see these
issues. I have a sintered bronze filter in the transfer line between the
vehicle and the transfer hose so I'm wondering if I didn't purge it
sufficiently and it clogged with ice. I added the filter because I found a
pea sized glob of grease in my test stand tank after an LN2 load a few years
back. The guys at the gas supplier couldn't explain where it came from.
I've done the let it sit a while and then refill technique in the past. I
think what I'll do for launch is set up a small load cell on a block for the
vehicle to sit on while on the rail. That way I'll know exactly how much Is
in the tank during filling (minus hose forces) and prior to launch.
-Bob
On Mar 13, 2018, at 12:01, Wyatt Rehder <wyatt.rehder@xxxxxxxxx> wrote:
Nice test Bob!
Small LOX tanks are annoying. You are definitely on the right track
with the lower fill pressure. Something that might also help would be
to after your first fill, close off the the dewar line and leave the
vent open on your tank and let it come to a thermal equilibrium in the
tank for about 10-20 minutes depending on how long it takes to stop
boiling. Then you can start filling again to which might get that last
bit of lox in there. Doing a couple of cycles of the wait/fill might
not be a terrible idea. If your tank doesn't stop boiling you might
want to add more insulation, especially putting something less
thermally conductive to mount your tanks to the frame.
On Tue, Mar 13, 2018 at 4:47 AM, Robert Watzlavick
<rocket@xxxxxxxxxxxxxx> wrote:
Bob,
There are other amateurs out there but they may not be as recently active.
Ones that come to mind are Paul Breed (Unreasonable Rocket), Rocket
Moonlighting, Graham Sortino, etc. There are others that email me on
occasion but for whatever reason, they decide not to post here.
To get my 100 and 250 lbf regen thermal designs to work using aluminum, I
reduced the mixture ratio down to 1.8 which lowered the chamber temperature
by 1000 degR at the expense of some performance. It wasn't just that
though, it required adjusting the chamber geometry, number of cooling holes,
and spacing to get it to converge. Finally it had to be producible on my
non-CNC shop equipment. I also took credit for some thermal resistance from
carbon layer that kerosene leaves on the chamber walls. Every time I think
of switching to LOX/alcohol to use a less messy fuel, I remind myself of the
benefit of that thermal resistance.
I haven't looked at lowering the temperature further since I have a design
that works good enough for my purposes. Even at a mixture ratio of 1.8, the
chamber temp is still around 5500 degR. However, it's not just the melting
point of the material which is 1100 degF for aluminum. The max allowable
for 6061-T6 in my design on the inside hot wall is 700 degF where it has 10%
of its strength left. I'm not well versed in the high temperature steel
alloys but I took a quick peek at MIL-HDBK-5 and the highest temperature
they show in any of the tensile strength tables is 2000 degF. ProPEP says
to get LOX/kerosene to burn under 2000 degF, it requires a mixture ratio of
0.18 which results in an Isp of 125 s. The longest I've run my engine is
20-25 sec at a time but that is enough for the wall temperatures to level
off and reach steady state. I'm sure if I ran it for continuously for
several minutes, I would run into other issues. There are engines out there
that have demonstrated longer burn times so it seems like giving up almost
half the Isp would make a hard problem (vehicle mass fraction) even harder.
-Bob
On 03/13/2018 03:19 AM, Robert Clark wrote:
Thanks for the update. I don’t know of another true amateur effort for a
bipropellant liquid-fueled rocket intended for launch. By true amateur I
meant not connected to a university which has many more resources, funding,
and expert advice available.
Quite impressed you were able to get regenerative cooling to work, which is
quite complicated for an amateur experimenter.
BTW, I’m investigating low temperature liquid-fueled rockets engines.
Suppose you ran your engine fuel rich so the combustion temperature was
lower than the metal’s melting point. How long do you think your engine
could run?
Bob Clark
On Monday, March 12, 2018, Robert Watzlavick <rocket@xxxxxxxxxxxxxx> wrote:
I ran a vertical static test of my flight vehicle this weekend using my
250 lbf LOX/kerosene engine.
Notes: http://www.watzlavick.com/robert/rocket/
Pictures: http://www.watzlavick.com/robert/rocket/rocket1/tests/index.html
I'll post videos later but it was a 12 second run. Everything generally
worked but I ran into some annoying test setup issues.
-Bob
