THE TOMMY TIMER

There    are    numerous    approaches    for    parachute deployment   mechanisms.   In   this   tutorial,   we   show you   how   to   build   a   mechanical   system    based   on   a Tommy    Timer .    Tommy    Timers    can    be    found    in wind-up   toys   which   are   available   in   toy   stores   and on   the   internet.   After   removing   the   case   you   can cut   off   all   protruding   parts   of   the   timer   using   pliers. You   should   also   remove   the   plastic   handle   from the   main   axel   by   repeatedly   clamping   it   in   a   vice. After   that,   bend   the   axel   using   two   sets   of   pliers. The   modified   timer   will   later   be   the   centerpiece   of the   system.   You   may   wonder   why   we   use   these mechanical   timers   and   don’t   work   with   electronic systems.     Well,     the     answer     is     pretty     simple: Mechanical   systems   are   way   easier   to   build    and repair,    are    significantly    lighter     and    a    lot    more affordable     when    compared    to    electronic    ones    – this   is   especially   important   when   a   rocket   crashes and      you      have      to      build      a      new      parachute deployment   mechanism.   But   luckily,   this   shouldn’t be   the   case   all   too   often   since   the   two   systems   we want   to   show   you   in   this   video   have   already   proven their reliability  with numerous flights.

WHICH SYSTEM IS SUITABLE?

The    first    system    we    want    to    show    you    is    called Phoenix    6.    It’s    the    latest    version    of    our    Phoenix Parachute      Side      Deployment      System      and      is optimized   for   rockets   with   a   single   large   pressure vessel.   For   rockets   consisting   of   multiple   pressure vessel      segments      we      recommend      building      a Phoenix    Radial    Deployment    System.    This    system was    originally    developed    by    our    colleagues    from U.S.   Water   Rockets ,   but   we   adjusted   and   optimized it for the use with a lightweight mechanical timer.

PHOENIX 6

For     the     construction     of     the     Phoenix     6     Side Deployment      Mechanism      you      will      need      two bottles .   It   is   necessary   to   use   the   same   bottle   type you   have   already   used   for   your   pressure   vessel. Cut    off    the    bottom    of    one    of    the    bottles    using scissors   and   remove   the   thread   of   the   bottle   with   a saw.   Now   you   have   to   cut   a   big   opening    in   the cylindrical   section   of   the   bottle.   In   this   case,   the opening    is    about    10    cm    /    3.9”    wide    and    there should   be   about   3cm   /   1.2”   left   to   the   lower   edge. When    you    use    bottles    with    a    bigger    or    smaller diameter,    you    have    to    adjust    the    width    of    the opening.   The   bottle   with   the   opening   will   later   be the   case   of   the   parachute   system .   But   of   course, your   case   will   need   a   nosecone.   You   can   either   glue a   half   table   tennis   ball    on   top   with   epoxy   or   you can   use   the   nosecone   of   a   fireworks   rocket    for that.   Now   take   the   second   bottle   and   remove   both the   bottom   and   the   neck.   The   remaining   cylindrical section    will    be    separated    in    two    equally    large pieces.   One   of   these   pieces   will   be   the   parachute door . If   you   want   to   paint   your   parachute   system,   you can   do   this   now.   First,   sand   and   clean   both   the   case and   the   door.   We   recommend   using   plastic   primer before   applying   the   paint.   The   spray   paint   can   be applied   as   soon   as   the   primer   is   dry.   It‘s   maybe necessary to spray several coats. After   that,   you   can   cut   out   two   round   base   plates out   of   a   thin   plastic   plate.   You   should   be   able   to put   the   plates   into   the   system,   but   they   shouldn’t be   too   small   and   should   remain   in   place   without any   glue.   After   you   have   placed   the   plates   directly at   the   lower   and   upper   edge   of   the   opening,   you can   glue   them   in   place   using   a   hot   glue   gun.   Please be careful and don’t use too much adhesive. Now   it’s   time   to   mount   the   Tommy   Timer   you   have modified   at   the   beginning.   All   you   need   are   three small   holes   in   the   case   located   slightly   above   the lower   base   plate.   Put   the   bent   axel   of   the   timer through   the   hole   in   the   middle   and   use   a   small cable   tie   to   fasten   it.   The   parachute   door,   which   is placed   over   the   opening,   can   be   fastened   by   using a   rubber   band.   Just   mount   a   loop   slightly   under   the upper   base   plate   on   the   case.   Now   you   can   wrap the    rubber    band    multiple    times    around    the system .   Just   hook   the   rubber   band   over   the   bent timer axel. To   eject   the   parachute,   you   need   a   spring .   For   that, you   can   use   a   part   of   a   thin   walled   plastic   bottle. You   can   attach   the   bottle   piece   with   wire   on   the case   but   you   can   also   take   the   whole   cylindrical section   and   glue   it   in   place   with   a   hot   glue   gun. However,   if   you   use   this   method   you   have   to   watch out    that    the    parachute    will    pushed    out    of    the opening   and   not   to   the   side.   The   parachute   door can   now   be   attached   with   a   rubber   band   on   the parachute.  

PHOENIX RDS

But     before     we     finish     the     construction     of     the Phoenix   6   mechanism,   we   want   to   take   a   look   at the   Phoenix   RDS.   The   good   news   is,   that   it   consists of   even   less   components   than   the   Phoenix   6.   The most   important   component   is   a   thin   but   flexible plastic   cover .      You   can   use   a   document   cover   or   a thin    walled    bottle    section    for    that.    Mount    the modified   Tommy   Timer   at   one   edge   of   the   cover using   two   little   cable   ties.   On   the   opposite   edge you   can   attach   a   rubber   band   loop.   For   the   next steps,   you   will   need   a   rocket   with   multiple   pressure vessel     segments.     Wrap     the     cover     around     the connection   between   two   segments ,   then   wrap the   rubber   band   around   it   and   hook   it   on   the   timer axel.   Another   rubber   band   attached   to   the   cover and   the   nosecone   of   the   rocket   will   ensure   that   the cover   will   be   pulled   away    when   the   parachute gets   ejected.   The   Phoenix   RDS   is   now   already   on the    same    level    as    the    Phoenix    6    System,    whose construction   we   described   earlier   in   this   video.   The following steps are identical with both systems.

LAST STEPS

To   prevent   the   rubber   band   sliding   over   the   bend of   the   timer   axel,   glue   a   small   cable   tie   head    with epoxy   or   superglue   directly   at   the   bend.   Now   you can   hook   in   the   rubber   band   once   again   and   wind up   the   timer.   A   few   seconds   later,   the   rubber   band will   be   released   and   the   parachute   ejected.   Please be   very   careful   when   testing   the   system   because you   do   not   want   to   get   the   rubber   band   flick   into your    eyes.    Now,    the    only    thing    you    need    to complete   the   construction   is   a   small   trigger    which holds   the   timer   in   place   until   the   rocket   launches. Luckily   the   construction   is   fairly   simple.   You   just need   a   big   cable   tie!   Cut   the   long   part   off   so   that the   remaining   cable   tie   is   only   5cm   /   2”   long.   Now wind      up      the      timer      and      place      the      trigger underneath   the   small   cable   tie   you   used   to   attach the    timer    so    that    the    timer    can’t    start    running. Connect   the   trigger   with   wire   and   a   rope   to   your launch   pad   and   you   are   ready.   But   don’t   forget   to attach   the   parachute   and   the   system   to   your   rocket first.

Wind-up toy

Modified Tommy Timer

Painted system with nosecone

Base plates

System with spring and parachute

Rubber band wrapped around the system

1) System with trigger (before launch)

2) Timer is running (launch)

3) System opens

4) Parachute is ejected

FAQ

• What‘s the failure rate of your current parachute systems? Our   Phoenix   5   Side   Deployment   System   is very   reliable   with   a   failure   rate   of   only   3/70 (in   March   2017)   which   equals   4.2%.   Two   of the   three   failures   could   have   been   avoided if    our    current    checklist    had    been    applied correctly.               Our     version     of     the     Radial Deployment   System   had   a   few   issues   in   the beginning,   but   we   were   able   to   improve   the reliability   a   lot.      Our   latest   addition   to   our parachute   system   family,   which   is   used   in the     „Falcon“     rockets,     hasn‘t     completed enough     launches     yet     for     a     trustworthy valuation. • Why do you (still) use a mechanical tommy timer and no electrical system? Our    current    tommy    timer    based    systems like   Phoenix   5   and   Raketfued   RDS   are   more reliable   than   many   systems   which   are   based on    electrical    timers    or    sensors.    A    tommy timer   is   also   a   lot   more   affordable.   That‘s especially   relevant   because   most   failures   or crashes    also    affect    the    parachute    system. Last    but    not    least,    electrical    systems    are heavier   than   our   mechanical   systems.   Thus, we   see   no   reason   to   switch   to   an   electrical system. • Which altimeter do you use and where can I buy it? We   use   a   range   of   altimeters.   In   our   first water   rocket   which   included   an   altimeter   we used      the      “Altimax      Simply”      made      by Rocketronics    ,   but   this   altimeter   can‘t   output the   data   in   a   table   or   chart.   That‘s   why   we switched     to     a     prototype     made     by     the German   Aerospace   Center   DLR.   We   also   use the        Hobbyking    Altimeter     in    many    of    our rockets. • Which parachute size do I need for my rocket? That    depends    on    its    weight.    Usually    the recommended     payload     is     given     on     the packaging or on the website. • Is there any other method of building a recovery system? Yes, absolutely! You can 3D print the casing, for example. Learn more here.

Latest update: 6.2.1.2|2nd April 2020

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