Orbital Pegasus - Manual (Page 46)

Release 7.0 Apr 2010 35
Pegasus User’s Guide
incorporated into launch vehicle telemetry using a
serial telemetry interface. This interface is either a
4-wire RS-422, or a 2-wire RS-485 serial
communication link between the Pegasus flight
computer and the spacecraft. Up to 250 bytes/sec
of payload data can be incorporated into Pegasus
telemetry. The payload data is available in the
launch control room during ground operations,
captive carry and powered flight. The serial
interface can also be used to send timed
commands from the Pegasus launch vehicle to the
payload during flight as required.
Pegasus avionics can be used to read and
process data from payload analog instrumentation
such as temperature sensors, strain gauges and
pressure transducers. Data from up to eight
separate sensors can be incorporated into launch
vehicle telemetry and displayed in the launch
control room during ground operations, captive
carry and powered flight. Pegasus can provide
10
V excitation for payload instrumentation if
required.
The Pegasus flight computer can generate
discrete commands on eight separate channels as
required by the payload. These opto-isolated
circuits can be configured as voltage switching or
switch closure. Each command circuit is capable
of producing multiple pulses of programmable
length. Minimum pulse duration is 40 ms.
Maximum current for voltage switching circuits is
500 mA. The commands may be manually
initiated during ground operations and captive
carry or automatically sequenced during powered
flight of the Pegasus vehicle.
All nonstandard services required by the payload
will be documented in a mission-specific EICD.
5.3.2.1. Range Safety Interfaces/Vehicle Flight
Termination
The Pegasus air-launched approach minimizes
interfaces with the test range. All ordnance on the
Pegasus vehicle is in the safe condition while in
captive carry mode under the carrier aircraft.
Ordnance is armed during a sequence that is
initiated on release from the OCA. Procedures for
arming ordnance on the spacecraft are determined
on a mission-specific basis. No arming of the
payload prior to drop of Pegasus from the carrier
aircraft is allowed.
Generally, the standard Pegasus FTS subsystem
satisfies all range safety requirements without
additional FTS support from the payload.
However, information on the payload, such as a
brief description, final orbit, spacecraft ordnance,
hazardous operations and materials summary, will
be required to support range documentation.
Additional range support for payload operations,
such as orbit determination and command and
control, can be arranged. Range-provided
services have long lead times due to Department
of Defense (DoD) and NASA support
requirements; therefore, test range support
requirements must be identified early in order for
Orbital to ensure their availability.
5.3.2.2. Electrical Isolation
All power carrying circuits are isolated from the
Pegasus XL and payload structures by at least
1 MΩ. The Launch Vehicle System (the Pegasus
XL, the integration site facilities, and the OCA) and
Space Vehicle System (the payload and all
ground-based systems required to process,
launch, and monitor the payload during all phases
of launch processing and flight operations) shall
each utilize independent power sources and
distribution systems.
5.3.2.3. Pre-Drop Electrical Safing
Prior to drop, all electrical power interfaces on the
umbilical interface circuits between the carrier
aircraft and the Pegasus vehicle shall be shut off
to the extent possible to minimize current flow
across the umbilical interface. Payload interface
circuits that cannot be turned off and will have a
current flow greater than 100 mA prior at drop
must be evaluated by Orbital on a mission-specific
basis.