Orbital Pegasus - Manual (Page 66)

Release 7.0 Apr 2010 55
Pegasus User’s Guide
autopilot. For the exo-atmospheric portions of
flight, the autopilot margins are similarly evaluated
at discrete points to account for the changing
mass properties of the vehicle. The control
system gains are chosen to provide adequate
stability margins at each operating point. Orbital
validates these gains through perturbed flight
simulations designed to stress the functionality of
the autopilot and excite any possible instabilities.
Due to the proprietary nature of Orbital’s control
algorithms, this analysis is not a deliverable to the
payload vendor.
8.3.3. Coupled Loads Analysis
Orbital performs a CLA (using finite element
structural models of the Pegasus and payload) to
determine maximum responses of the entire stack.
A single load cycle is run after a payload modal
survey has taken place and a test verified payload
model has been supplied. The CLA also contains
a “rattlespace analysis.” This analysis verifies that
the payload does not violate the payload fairing
dynamic envelope.
8.3.4. Payload Separation Analysis
Orbital uses the Pegasus STEP simulation to
ensure that the payload is in the desired
orientation for successful separation at the end of
boost. Orbital performs a separation tip-off
analysis to verify the three axis accelerations that
the payload will experience during the separation
event from the final stage. This analysis will only
be conducted on an Orbital-supplied separation
system.
8.3.5. RF Link and Compatibility Analyses
A RF link analyses is updated for each trajectory
to ensure that sufficient RF link margins exist
between range assets and the Pegasus vehicle for
both the telemetry and flight termination systems.
8.3.6. Mass Properties Analysis and Mass
Data Maintenance
Orbital tracks and maintains all mass properties,
including inertias, relating to the Pegasus vehicle.
Payload-specific mass properties provided to
Orbital by the customer are included. All flight
components are weighed prior to flight and actual
weights are employed in final GN&C analyses.
Orbital requires estimates of the payload mass to
facilitate preliminary mission planning and
analyses. Delivery of the payload mass properties
are defined in the mission ICD and tracked in the
Mission Planning Schedule (MPS).
8.3.7. Power System Analysis
Orbital develops and maintains a power budget for
each mission. A mission power budget verifies
that sufficient energy and peak load margin exist.
Battery usage is strictly controlled on the vehicle
and batteries are charged prior to vehicle closeout.
8.3.8. Fairing Analyses
Two payload-specific analyses performed by
Orbital relate to the payload fairing: a critical
clearance analysis (contained in the CLA) based
on the dimensions and payload characteristics
provided by the customer; and a separation point
analysis to select the timing for this event.
Payload fairing maximum deflection occurs at
approximately 5 seconds after drop of Pegasus
from the OCA during the pull-up maneuver.
The fairing separation point is nominally timed to
coincide with dynamic pressure falling below 0.01
psf that occurs during the Stage 2 burn. Payload
requirements specifying lower dynamic pressures
or aerodynamic heating environments at fairing
deployment may be accommodated by delaying
this separation event. In general, this separation
delay will lead to some degradation in Pegasus
payload performance, which will need to be
evaluated on a case-by-case basis.
8.3.9. Mission-Unique Software
Mission-unique flight software consists of the flight
MDL, which contains parameters and sequencing
necessary to guide Pegasus through the desired
trajectory.