Orbital Pegasus - Manual (Page 21)

Release 7.0 Apr 2010 10
Pegasus User’s Guide
trajectory design, payload mass, and the guidance
strategy requested by the payload. As a result,
the specific Pegasus orbit accuracy capabilities for
a particular mission are generally determined only
after these mission-specific details are defined and
detailed mission-specific analyses have been
performed. However, Figure 3-5 provides
estimates of 3-sigma orbit insertion errors for both
Pegasus XL and Pegasus XL with HAPS vehicle
configurations, which are representative of typical
Pegasus missions. For non-HAPS configurations,
these errors are generally dominated by the
impulse variability associated with Stage 3. This
variability is also responsible for the generally
larger magnitude errors for the non-insertion apse
relative to the insertion apse.
3.4.1. Actual Pegasus Insertion Accuracies
Figure 3-6 shows the actual Pegasus orbital
insertion accuracies achieved for all missions
since Flight 10. As this figure demonstrates, a large majority of these missions resulted in
perigee and apogee altitudes within 30 km of the
desired target values and inclination errors of less
than 0.05 degrees.
3.4.2. Error-Minimizing Guidance Strategies
Due to the large amount of actual flight experience
Pegasus has accumulated to date, the Pegasus
Program has been able to continually refine and
improve the fidelity and accuracy of the Pegasus
vehicle simulation. This process has allowed us to
develop a high degree of confidence in the
Pegasus simulation analysis results and to
accurately predict mission performance in flight.
To ensure that even a 3-sigma low-performing
Pegasus vehicle will achieve the required orbit,
Pegasus trajectories include a 67 m/sec
(220 ft/sec) guidance reserve. Pegasus flight
software provides the capability to manage this
reserve through the use of a variety of different
Figure 3-2. Pegasus XL with HAPS Mission Profile to 741 km (400nmi) Circular, Polar Orbit with a
227 kg (501 lbm) Payload