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Highlights of recent developments on the MiniSShot Project
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- Two telecon sessions were held on Saturday to discuss the findings
relating to the flight data analysis and motor dissection. In
particular, the discussion centered around possible causes for the
flight anomaly, based on the evidence presented. Prior to the telecon,
Richard prepared and distributed a report that presented a preliminary
analysis (
http://sugarshot.org/downloads/minisshot_prelim_flight_analysis.pdf ).
The report indicated a lack of evidence for nozzle blockage as cause of
the motor overpressurization. Analytical evidence was presented
indicating the event could be due to an increased burning area initiated
by an unknown event.
Following a general discussion of the flight anomaly, discussion focused
on two particular items. One related to the igniter that was used for
2nd phase ignition that differed from those used in the ProtoSShot
static firings. Specifically, the use of a Pyrodex pellet in addition to
the specified 10 grams of copper thermite (the pellet was added to
improve reliability of firing the hard-to-light thermite). The outcome
of that discussion was the agreement that further investigation and
possible testing may be need to ascertain whether or not this may have
led to grain damage upon ignition. The other key item related to
physical evidence. Description and photos of the recovered casting tubes
were examined for evidence of possible propellant disbonding. It was
noted that the casting tubes suffered little in the way of ablation in
the middle sections, which should have been protected by unburned
propellent until burnout for a nominal burn to have taken place. There
was no evidence of the burn pattern associated with disbonding.
Photo of cut & cleaned MiniSShot casting tubes:
http://sugarshot.org/downloads/dsc05529a.jpg
(photo courtesy Rick Maschek)
Photo of cut & cleaned ProtoSShot III static test casting tubes:
http://sugarshot.org/downloads/proto3_castingtubes.jpg
(photo courtesy Richard Nakka)
Video analysis of flight indicating key events before and after anomaly:
http://www.youtube.com/watch?v=BJqS81-tLn8
(video analysis courtesy Rick Maschek)
Participants of Saturday's sessions were Richard Nakka, Chris King, Rick
Maschek, Ed Mallory, Mark LaBarre and Robert Krech. A follow-up telecon
was held on Sunday for the benefit of invitees who were unable to attend
the Saturday sessions. Sundays participants were Richard Nakka, Rick
Maschek, Matt Campbell, Robert Krech and Nathan Asdourian.
- The day after the telecon, Hans performed a reverse calculation of
propellant surface regression. This was intended to provide a rough
estimate of the regression to better understand what was going on during
the second burn. Hans proposed the following theory:
Already at the beginning of the second burn, the segment(s) had multiple
fractures in the core surface. The fractures may have been several
millimeters deep - some more and some less - but were (mostly) not
penetrating the entire web thickness of the grain. The grain fractures
may or may not have been caused by the ignition charge. Some fractures
were ignited already from the beginning, while others may have ignited
later. The ignition and pressurization of the motor took roughly 500ms
and the fractures may have ignited gradually during that time. There
were many fractures, and during burn they expand and "collide". The
collision causes detachment of small chunks of propellant. This in term
causes a slight increase followed by a slight decrease of chamber
pressure, as can be seen many times on the pressure trace, already from
start of the burn. However, the grain is still basically in "one piece",
and chamber pressure increases gradually as regression continues to
carve the fractures wider.
If the motor had been able to absorb the increasing pressure, the
pressure would probably have continued its gradual increase until some
point where multiple fractures had collided, essentially eliminating
each other and the pressure would drop back to almost normal levels.
Instead, the rapid detachment of the aft part of the motor and the
subsequent corkscrew movement must have caused large chunks of
propellant to come loose and burn, as the burn surface increases
dramatically immediately after the casing failure. This stop the process
of expansion and collision of the fractures and probably mostly restores
the nominal grain surface.
http://sugarshot.org/downloads/regression1.gif
http://sugarshot.org/downloads/regression2.gif
- Nathan Asdourian has picked up the task of trying to recover on-board
video footage from the damaged file. Rick Maschek had put in a great
deal of effort toward that goal. Rick felt the task should be picked up
by a person with greater expertise, and Nathan kindly volunteered, and
has so far made some progress.
Capture of 1st frame of video footage:
http://sugarshot.org/downloads/videocapture.jpg
(photo courtesy Nathan Asdourian)
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