Pre-Phase A Report - Lisa - Nasa

6.3 Injection into final orbits 137
the direction to the sun for these trajectories ranges from 10 ◦ to 60 ◦ , making it difficult
to provide power with a solar array fixed to the spacecraft body. The current spacecraft
design includes steered solar arrays to provide power for the ion-engines during the
transfer phase, so that the arrays can be pointed towards the sun regardless of the thrust
direction. These arrays would be attached to the propulsion module for each spacecraft
and jettisoned, along with the ion engine, after reaching the operational orbits.
6.3 Injection into final orbits
The spacecraft will have limited maneuvering capability once separated from the propulsion
module, due to the limited impulse of the micronewton thrusters. This requires that
the spacecraft be accurately delivered to the final orbits.
If chemical propellant is used in the propulsion modules, a fairly large (∆V ≈ 500 m/s)
insertion manuever would be needed upon reaching the desired operational orbit. With
typical manuever execution errors of 1 %, the error in this maneuver would be be about
10 m/s which would be much too large to be corrected by the micronewton thrusters.
Therefore, after the main insertion maneuver, the propulsion module would need to remain
attached to the spacecraft while sufficient tracking information is acquired to determine
the error in the orbit insertion maneuver.
The propulsion module would then perform a correction maneuver of order 10 m/s. The
expected error in a maneuver of this size would be about about 10 cm/s which would
still be too large for the micronewton thrusters to correct. Another period of spacecraft
tracking would be needed to design a final correction maneuver to be executed by the
propulsion module. After the execution of the final correction maneuver, the spacecraft
would separate from the propulsion module. The separation would be effected by a springseparation
system, with a separation velocity of order 10 cm/s. The error in the separation
maneuver is expected to be of order 1 cm/s which can be corrected using the micronewton
thrusters.
With the use of a solar-electric propuslion module, while detailed navigation studies have
not yet been performed, it is likely that a similar series of tracking periods, orbit error
estimation, and orbit correction would be needed before the propulsion modules could be
separated from the spacecraft.
6.4 Orbit configuration stability
Both the nature of the elliptical orbits and planetary perturbations will cause small
changes in the lengths of the sides of the triangle formed by the three spacecraft. These
orbital changes of distance between spacecraft will impose Doppler shifts on the interferometer
signals that will have to be removed using on-board oscillators (clocks). Noise
from the oscillators will then corrupt the distance measurements. The amount of noise
introduced depends on the size of the Doppler shift and the performance of the oscillator.
The spacecraft are designed to be drag-free so that the only significant forces affecting the
proof masses at the center of each spacecraft are gravitational. In the simplest case the
Corrected version 2.08 3-3-1999 9:33