ESRIN Envisat coordination meeting

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LAUNCH WINDOWS FOR LIBRATION POINT
MISSIONSMartin Hechler, ESA/ESOCMission
Analysis Workshop, ESOC, 11 Dec. 2007
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Contents

• Orbit families at the co-linear Libration
  Points L1 and L2 in sun-Earth system
• ESA Missions to Libration Points (Herschel,
  Planck, GAIA, LISA Pathfinder)
• Transfers to L2 or L1 Stable manifold and
  fuzzy boundary
• Planck GAIA transfer optimisation
• Herschel/Planck ARIANE launch window
• GAIA on Soyuz/Fregat from French Guyana
• Summary and Conclusions
• Launch window for LISA Pathfinder on VEGA (L1)
  in other presentation

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Orbits Used for Missions at L1 and L2 (Sun-Earth)

• Lissajous orbits
• Earth aspect lt15o
• Survey Missions

• Quasi Halo orbits
• Free transfer
• Observatories

View from Earth
Herschel JWST LISA Pathfinder (L1) XRO Darwin
Planck GAIA

• Advantages of L2 for Astronomy missions
• Earth sun in same direction
• Easy shielding against infrared

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ESA Missions to L2 and L1 in Development Phase
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Fuzzy Boundary and Stable Manifold

• Fuzzy Boundary
• Perigee from launch conditions (i, O, ?)
• scan and bisection in perigee velocity (Vp)
• ? One non escape solution (free transfer)

• Stable Manifold
• Backward integration from orbit at L1/2
• Jump onto stable manifold
• ? Two local minima in ?V (fast/slow)

fast
slow
Example bisection
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Transfer Optimisation (Planck/GAIA)
Solved by forward/backward shooting
Herschel/Planck (i, ?, Vp) fixed
Cost functional (S?Vmin)
Departure variables launch (i, O, ?, Vp, Tp)
Arrival variables Lissajous
(Ay,Az,?z,Ty0) With prescribed properties (a lt
15o and no eclipse)
Tp
Day 2
Tm
Fast Transfer Ti Tp lt 50 days
Ti
Matching constraint (?X0)
Number of manoeuvres depends on case
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Planck Manoeuvre Model

• All manoeuvres are done in sun pointing mode
• Decomposition modelled in optimisation
• ?V of each thruster phase angle (5 variables)
• ?V sum of ?Vs of thrusters propellant

• Long manoeuvre durations by 10 cycling
• Gravity loss for day 2 manoeuvre
• Day 2 manoeuvre can also be delayed
• to avoid star mapper blinding by moon

• Optimisation in general converges to pure
  manoeuvres

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Launch Windows

• Definition
• Dates (seasonal) and hours (daily) for which a
  launch is possible
• Launcher target conditions ? number of flight
  programs
• For Herschel/Planck fixed launcher target
  conditions
• Constraints
• Propellant on spacecraft (e.g. propellant
  allocation for full year window)
• or mass in final orbit ? typical for
  interplanetary launch widows M0(V8)
• Geometric conditions
• eclipses
• sun aspect angles on launcher or in final orbit
• orbit stability ? typical for eccentric orbits
• Reduced case
• If lift-off time can be optimised for every day
  ? only seasonal launch window
• ? GAIA LISA Pathfinder

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Herschel/Planck Launch Window Selection of Vp

• Double launch on ARIANE 5 rp, i, ? for maximum
  mass
• Fixed launch conditions in Earth fixed frame at
  lift-off
• only one flight program on launcher (cost
  saving)
• Vp to be fixed
• Both spacecraft correct perigee velocity Vp (
  ra)
• Fast transfer vp about 3 m/s below vp of stable
  manifold

• Launch conditions of Ariane
• Vp Vesc - 30.32 m/s
• Ra 1 200 000 km

• Osculating at Planck S/C separation
• J2 must be on in integration

Remaining degree of freedom O TLaunch
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Herschel/Planck ?V Budget
Original budget (without decomposition losses)

• Perigee velocity variation correction for one
  Flight Program on ARIANE
• Orbit maintenance strategy for free non-escape
  orbit
• 50 saving in ?V
• predictability sufficient

Re-optimisation with decomposition and delay
(moon blinding) ? Increased budget
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Herschel/Planck Launch Window (WP 508)
?V limit is on effective ?V
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Remark System Transformations

• All launch conditions are defined in inertial
  system fixed at lift-off 3 seconds
• Launcher axis at Fairing Jettison and upper
  stage burnout for sun aspect angle
• calculation given in launch pad system (x
  vertical to Geoid)
• Orbital elements at S/C separation in Equator
  system with x at Kourou meridian
• Transformations

normal to Geoid to radius 248 arcsec Earth
fixed to mean system of date (pole) 4
arcsec Mean of date to J2000 444 arcsec Atomic
time to UTC 496 arcsec
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GAIA Launch Scenarios

• Launch Scenario 1
• Soyuz from French Guyana to circular parking
  orbit at 19o inclination
• Fregat burn at any time in circular orbit to
  reach L2 transfer (free ?)
• Two degrees of freedom (O, ?) ? optimum transfer
  near ecliptic plane
• Propellant originally budgeted for full year
  launch window with this scenario
• Problem uncontrolled re-entry of Soyuz third
  stage
• Launch Scenario 2
• Direct ascent (Fregat burn immediately after
  third stage cut-off)
• Controlled third stage entry in Atlantic
• ? fixed ? more propellant required or seasonal
  launch window
• Alternatives (investigated)
• Eccentric parking orbits with different
  ?-rotations (Pacific re-entry)

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GAIA Launch Scenario with Eccentric Orbits
Launcher Ascent Optimisation Study (WP 502)

• Cases 5-8 are with eccentric parking orbit
• Third stage re-entry in pacific ocean currently
  not accepted by Arianespace
• Perigee raising at apogee if required
• Fregat burn before perigee to turn line of
  apside
• Case 3 8 would open a winter launch window
• Circular orbit scenario could be re-instituted
  with Fregat tank increase
• Current baseline direct ascent only (case 3)

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GAIA Launch Window (Circular versus Case 3)
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GAIA Launch Window (Case 3 8)
Scenario not endorsed by Arianespace Walking
impact point of 3rd stage not allowed over Africa
!
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Summary and Conclusions

• Transfers to Libration point orbits
• Stable manifold free transfers to Pseudo-Halos
• Optimum transfers (with up to 3 manoeuvres) to
  Lissajous orbits
• Launch Windows
• Herschel/Planck (2008)
• Dates hours of launch fixed Ra at launch
• Detailed, sophisticated modelling
• Limits of daily slot by sun-aspect angle on
  launcher eclipses ?V
• At least 30 minutes daily slot for over 6
  months per year
• GAIA (2011)
• Launch scenario not yet frozen
• Short windows near equinoxes for direct
  injection of Soyuz/Fregat