Structural steel is super cheap compared to any other part of a rocket
test. When an engine or motor explodes it's normal to destroy the loadcell,
because for them to function they can't have a large margin between
operating and ultimate strength, but the thrust structure should be
designed to take a large load. A reasonable worst case load is the cross
sectional area of the motor times perhaps 2x planned chamber pressure, that
is the instantaneous thrust of losing the aft bulkhead. It does mean that
squat boosters with a high contraction ratio between the ID and the nozzle
throat have a much worse failure multiple than a finer motor of the same
thrust.
Totally agreed on the concern about how casual some people appear on motor
testing videos. For liquids my conceptual danger zone is anything on the
plume side of a plane drawn through the injector face; it's rarer for a
liquid to throw things forward but lateral is quite common. For solids it's
probably better thought of 360x360 since components can keep thrusting
after disconnection from the stand.
On Wed, Jun 1, 2022 at 7:58 PM Rick Maschek <dmarc-noreply@xxxxxxxxxxxxx>
wrote:
Here's a question that may have been previously answered.
The easiest way to test a motor is verticle with nozzle up. You could
simply dig a hole, and drop in a load cell with the rocket on top.
The problem with that is you are also measuring the changing mass of the
propellant as the motor is thrusting at the same time.
Another problem with sugar motors is the accumulation of KCO3 slag on the
bulkhead that doesn't get blown out of the nozzle...
We had nearly 4" of hot molten slag on our bulkhead after our 12" motor
tests.
Testing horizontally does not measure the weight of the rocket, only
thrust but also has the issue of slag build up on the bottom side of the
motor. Not an issue provided your liner is able to take the heat. We have
in the past experienced burn-throughs when doing horizontal testing and
nearly always on the bottom side of the motor where slag accumulates. Also,
Tripoli does not like horizontal testing because the motor can fly cruise
missle style is something does wrong as shown in the video:
Eric Beckner 4 17 21 APCP CATO
<https://www.youtube.com/watch?v=8wousLLepx4>
Eric Beckner 4 17 21 APCP CATO
<https://www.youtube.com/watch?v=8wousLLepx4>
I try to do all my testing of motors (solid, liquid, and hybrids)
in-flight orientation vertically with the nozzle down. For sugar motors,
this also helps blow out the slag that would otherwise accumulate in the
motor case. In such an orientation, the motor would not move until enough
thrust overcomes the weight of the motor. We have thought about 'hanging'
the motor on a tension/compression load cell and thus seeing the amount of
thrust used to overcome the weight of the motor. After the burn the reading
would return to the weight of the motor minus the weight of the load cell.
We had not yet tried that method for recent testing and were simply using
a vertically mounted single BATES grain J-impuse sugar motor with a
65/20/15 KNO3/SB/SU formula nozzle down against a top mounted load cell.
After a dozen or so successful tests with various propellant formulas this
one CATOed. The load cell was mounted on a 1/2" thick angle iron support
bolted to an Ibeam. About two feet above that was a 1/4" angle iron support
botted to the I-beam used for testing hybrid motors. The nozzle blew out
shearing off the bolts to both supports.
The steel nozzle was found 7 feet away from the test stand after striking
the deflector shield and the concrete foundation. The 1/2" angle iron
support was found 40' away and the 1/4" angle iron support about 20 feet in
the opposite direction. The motor case took flight after taking out the
angle iron supports and was found about 200 feet away. The load cell has
still not been found.
I'm including this as a precaution when testing motors of how much power a
CATO contains. Any of the four large pieces striking a person could easily
have caused injury or...and I've seen some people in videos testing motors
in back yards or even in the front yard with traffic driving by. Please
exercise caution and in a safe place when doing motor testing.
