The DetMech mechanism was developed to automatically detach a rocket from its parachute line. It is designed to assist in recovering rockets snagged by their parachute in trees.
Total weight of prototype: 8 grams + 9 gram servo.
How It Works
The
mechanism
is
based
on a
simple
clamp
that
holds
down
a
parachute
cord.
At
the
appropriate
time,
the
clamp
is
released
by a
servo
motor
and
in
turn releases
the
cord.
You
may
loose
your
parachute
but
you'll
have
a
better
chance
at
recovering
your
expensive
rocket
and payload.
The
clamp
arrangement
shown
here
is
made
from
thin
aluminium
sheet,
but
could
well
be
made
of
plastic
or
wood.
The
grooves
in
the
top
and
bottom
plates
mesh
together
and
help
prevent
the
parachute
cord
from
sliding
within
the
clamp when
the
parachute
opens.
There
can
be a large
lateral
force
on
the
clamp
during
parachute
opening.
Aerodynamically
it
has
a
very
low
profile.
The
bottom
plate
is
attached
mechanically
to
the
collar
with
screws rather
than
glued
due
to
the
large
forces
involved.
The
main
parachute
line
should
also be
fitted
with
a
shock
cord
to
further
reduce
stress
on
the
clamp.
The
little
stops
either
side
of
the
bottom
plate
prevent
the
top
plate
from
moving
side
to
side
while
the
cord
is
pulling
on
it.
This
mechanism
is
controlled
by
the
second
servo
output
on
the
flight
computer.
An
activation
time
delay
of
around
20
minutes
after
takeoff
allows
the
rocket
to
be
normally recovered in
most
cases,
and
gives
the
recovery
crew
a
chance to
retrieve
the
rocket
from
an
easily
reached
a
tree.
Click on the
photos
below for a larger version.
Detail showing the different components that make up this prototype.
Clamp opened. The top clamp plate is also attached to the bottom plate mounting screw via a short length of Kevlar cord to prevent it from being lost. The Kevlar cord is entirely on the inside of the sleeve to prevent it getting tangled with the parachute cord.
The parachute cord is laid into the bottom groove. It is important that the cord has a strong knot on the end of it that will prevent the cord from sliding out of the clamp.
Closing the clamp pinches the cord in the groove. The string could have a section of heat-shrink tubing placed on it to help prevent it from being severed on the edge of the clamp.
The servo motor horn is then positioned to keep the clamp closed. The clamp can be made longer to provide more force to keep the parachute line secured.
A side view of the closed mechanism, showing the small drag profile.
A rocket with the DetMech mechanism located in the space between bottles. The DetMech servo is controlled by the flight computer in the nosecone. The flight computer also controls parachute deployment.
Videos
Prototype
Ground Tests:
Limitations
The
mechanism
will
not
always
ensure
that
a
rocket
is
freed
from
a
tree.
The
rocket
may
get
caught
by
its
fins
in a
branch,
but
it
should
at
least be
more
readily
shaken
out
of
the
tree.
The
rocket
may
be
substantially
high
above
the
ground,
and
releasing
the
rocket
may
cause
damage
when
it
hits
the
ground.
To
help
mitigate
this
issue,
the
controlling
circuit
can give
a
warning
prior
to
dropping
allowing
a
person
to
either
catch
the
rocket,
or
stand
clear.
Extensions
The
next
version
will
use
a
single
servo
motor
to
deploy
the
parachute
as
well
as
detach
it,
saving
on
weight
and
complexity.
The
dual
purpose
mode
of a
single
servo
would
be
based
on
Christian
Thomsen's
RC
dual
chute
release
mechanism.
The
servo
could
be
controlled
externally
by a
remote
control.
Allowing
the
rocketeer
to
release
the
rocket
when
appropriate.
A
variation
on
the
mechanism
could
make
it
completely
internal
to
the
rocket
body
reducing
drag.
Another
alternative
is
to
replace
the
servo
motor
with
a
water
soluble
paper
or
plastic
(such
as
Polyvinyl alcohol).
This
could
be
wound
around
the
clamp
to
hold
it
down,
and
when
it
rains
or
humidity
in
the
air
would
cause
it
to
dissolve
and
release
the
mechanism.
