Mission description

0064-EX-CN-2016 Text Documents

Cornell University

2016-09-20ELS_182190

Cislunar	
  Explorers	
  Mission	
  Description	
  
The	
   Cislunar	
   Explorers	
   are	
   competing	
   in	
   NASA’s	
   CubeQuest	
   challenge,	
   a	
   NASA	
  
Centennial	
   Challenges	
   program,	
   which	
   offers	
   a	
   total	
   of	
   $5.5	
   million	
   to	
   teams	
   that	
  
meet	
   the	
   challenge	
   objectives	
   of	
   designing,	
   building	
   and	
   delivering	
   flight-­‐qualified,	
  
small	
   satellites	
   capable	
   of	
   advanced	
   operations	
   near	
   and	
   beyond	
   the	
   moon.	
   The	
  
teams	
   will	
   compete	
   for	
   a	
   secondary	
   payload	
   spot	
   on	
   NASA’s	
   Exploration	
   Mission	
   1	
  
(SLS/Orion).	
  	
  
	
  
The	
   Cislunar	
   Explorers	
   team	
   is	
   led	
   by	
   Professor	
   Mason	
   Peck	
   of	
   Cornell	
   University,	
  
and	
   has	
   designed	
   their	
   spacecraft	
   to	
   demonstrate	
   a	
   game-­‐changing	
   technology:	
  
electrolysis	
  propulsion.	
  Each	
  spacecraft	
  carries	
  a	
  tank	
  of	
  inert,	
  liquid	
  water,	
  which	
  is	
  
then	
   electrolyzed	
   using	
   energy	
   collected	
   by	
   solar	
   cells,	
   splitting	
   H2O	
   into	
   hydrogen	
  
and	
   oxygen	
   gas.	
   This	
   is	
   a	
   readily	
   combustible	
   mixture,	
   widely	
   used	
   in	
   launch	
   vehicles	
  
and	
  spacecraft.	
  
           	
  
           Hydrogen	
   and	
   oxygen	
   are	
   cryogenic	
   liquids	
   and	
   must	
   be	
   stored	
   at	
   very	
   low	
  
temperatures	
   and	
   very	
   high	
   pressures.	
   For	
   CubeSats,	
   neither	
   are	
   acceptable.	
  
CubeSat	
   specifications	
   prohibit	
   extreme	
   pressures	
   and	
   cryogenic	
   storage	
   tanks	
   are	
  
too	
   bulky	
   to	
   be	
   usable	
   at	
   this	
   scale.	
   So	
   are	
   the	
   turbopumps	
   used	
   to	
   feed	
   LOX/LH2	
  
rocket	
   engines.	
   Electrolysis	
   propulsion	
   achieves	
   very	
   dense	
   propellant	
   storage	
   of	
  
1000	
   kg/m ,	
   several	
   times	
   denser	
   than	
   LOX/LH2	
   and	
   it	
   does	
   so	
   without	
   complex,	
  
                  3



expensive,	
   and	
   heavy	
   additional	
   apparatus.	
   This	
   is	
   a	
   way	
   to	
   bring	
   green	
   propellant	
  
into	
  the	
  CubeSat	
  framework,	
  producing	
  substantial	
  ΔV	
  in	
  a	
  compact,	
  simple	
  package.
	
  
           The	
   Cislunar	
   Explorers	
   have	
   been	
   carefully	
   designed	
   around	
   their	
   use	
   of	
  
water	
   propellant,	
   ensuring	
   passive	
   subsystem	
   symbiosis	
   to	
   reduce	
   the	
   cost	
   and	
  
complexity	
   of	
   the	
   spacecraft.	
   For	
   example,	
   the	
   presence	
   of	
   water	
   acts	
   as	
   a	
   partial	
  
radiation	
  shield	
  and	
  heat	
  sink	
  for	
  the	
  RF	
  Power	
  Amplifier.	
  
           The	
   most	
   significant	
   synergy	
   is	
   with	
   the	
   attitude	
   control	
   subsystem.	
   To	
  
separate	
  the	
  electrolyzed	
  gas	
  from	
  the	
  liquid	
  water,	
  the	
  spacecraft	
  each	
  spin	
  about	
  
their	
  major	
  axis,	
  centrifugally	
  forcing	
  the	
  water	
  away	
  from	
  the	
  spin	
  axis.	
  That	
  spin	
  is	
  
facilitated	
  by	
  a	
  spring-­‐loaded	
  mechanism	
  that	
  separates	
  the	
  two	
  Cislunar	
  Explorers	
  
from	
   each	
   other	
   after	
   deployment.	
   The	
   angular	
   momentum	
   of	
   the	
   spinning	
  
spacecraft	
  keeps	
  it	
  pointed	
  in	
  the	
  same	
  direction,	
  like	
  a	
  gyroscope. When	
  a	
  torque	
  is	
  
applied,	
  whether	
  deliberately	
  for	
  reorientation	
  or	
  as	
  a	
  side	
  effect	
  of	
  an	
  electrolysis	
  
thruster	
   firing,	
   the	
   spin	
   axis	
   will	
   begin	
   to	
   nutate.	
   However,	
   this	
   nutation	
   will	
   damp	
  
out	
  due	
  to	
  the	
  sloshing	
  of	
  water	
  in	
  the	
  propellant	
  tank	
  dissipating	
  energy,	
  and	
  the	
  
spacecraft	
   will	
   soon	
   return	
   to	
   a	
   spinning	
   steady-­‐state.	
   Therefore,	
   the	
   Cislunar	
  
Explorers	
  do	
  not	
  require	
  a	
  bulky,	
  power-­‐hungry	
  attitude	
  stabilization	
  system,	
  such	
  as	
  
reaction	
   wheels.	
   Instead,	
   each	
   needs	
   only	
   a	
   single	
   cold	
   gas	
   thruster,	
   used	
   to	
   exert	
  
torque	
  to	
  change	
  the	
  spin	
  axis	
  as	
  desired.


                                                                                                                                                 	
  
Figure	
  1:	
  Spacecraft	
  Separation	
  

the	
   Cislunar	
   Explorers	
   carry	
   several	
   inexpensive	
   onboard	
   cameras.	
   These	
   are	
  
repeatedly	
   used	
   to	
   capture	
   images	
   of	
   the	
   Sun,	
   the	
   Earth,	
   and	
   the	
   Moon.	
   The	
  
cameras	
   can	
   distinguish	
   between	
   the	
   three	
   bodies,	
   and	
   use	
   image	
   processing	
  
techniques	
   to	
   compute	
   their	
   apparent	
   size	
   and	
   angular	
   separation	
   in	
   each	
  
spacecraft’s	
   field	
   of	
   regard.	
   All	
   three	
   celestial	
   bodies	
   act	
   as	
   landmarks	
   in	
   space.	
   By	
  
comparing	
   the	
   apparent	
   locations	
   of	
   the	
   celestial	
   bodies	
   with	
   their	
   ephemerides,	
  
each	
  spacecraft	
  can	
  compute	
  its	
  own	
  and	
  attitude.	
  The	
  Cislunar	
  Explorers’	
  method	
  is	
  
uniquely	
   able	
   to	
   simultaneously	
   determine	
   position	
   and	
   attitude	
   onboard	
   a	
   spinning	
  
spacecraft.	
  This	
  technique	
  can	
  function	
  with	
  any	
  three	
  celestial	
  bodies	
  at	
  least	
  one	
  of	
  
which	
  is	
  resolvable	
  as	
  a	
  discrete	
  disc;	
  these	
  need	
  not	
  be	
  the	
  Sun,	
  Earth,	
  and	
  Moon.	
  
Brief	
   radio	
   transmissions	
   are	
   used	
   to	
   downlink	
   data	
   packets	
   of	
   computed	
  
coordinates	
   to	
   keep	
   the	
   ground	
   station	
   appraised	
   of	
   the	
   spacecraft’s	
   position	
   and	
  
attitude.	
   In	
   this	
   way,	
   the	
   Cislunar	
   Explorers	
   are	
   able	
   to	
   navigate	
   robustly	
   and	
  
autonomously,	
  in	
  a	
  way	
  that	
  is	
  well	
  suited	
  to	
  lunar	
  orbit.	
  
	
  
Apart	
  from	
  the	
  novel	
  technologies	
  to	
  be	
  demonstrated,	
  the	
  Cislunar	
  Explorers	
  make	
  
use	
   of	
   inexpensive	
   and	
   readily	
   available	
   components	
   wherever	
   possible.	
   The	
   flight	
  
computer	
   for	
   each	
   unit	
   is	
   a	
   Raspberry	
   Pi,	
   an	
   inexpensive	
   minicomputer	
   that	
   is	
  
popular	
   with	
   hobbyists	
   and	
   educators.	
   Both	
   the	
   attitude	
   and	
   propulsion	
   thrusters	
  
use	
   off-­‐the-­‐shelf	
   parts.	
   The	
   spacecraft	
   communicate	
   using	
   the	
   70	
   cm	
   UHF	
   radio	
  
band,	
   popular	
   with	
   amateur	
   radio	
   operators	
   across	
   the	
   world.	
   The	
   entire	
   design	
   is	
  
open-­‐source,	
   with	
   the	
   goal	
   of	
   enabling	
   further	
   innovations.	
   A	
   successful	
  
demonstration	
   of	
   their	
   optical	
   navigation	
   and	
   electrolysis	
   propulsion	
   technologies	
  
could	
   make	
   them	
   attractive	
   for	
   future	
   missions,	
   and	
   the	
   team	
   wants	
   the	
   space	
  
development	
  community	
  to	
  be	
  able	
  to	
  benefit	
  immediately.
	
  



Document Created: 0520-04-24 00:00:00
Document Modified: 0520-04-24 00:00:00

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