Answers Your Questions
| From: Adam Krantz, of Seattle,
To: Derek Hassman, flight director
Question: Can you please explain the solar beta angle and how this affects the ISS? The status report on Nov. 20, 2002, describes a high solar beta angle from Dec. 21, 2002, for six days and describes using an orientation of YVV/"Barbecue" (y-axis in velocity vector, i.e., vertical body axis Y facing forward, in flight direction). Can you explain that orientation, and why it is necessary?
Answer: The beta angle is the angle between the orbital plane and a line drawn from the Sun to the Earth. As the beta angle increases, the ISS is exposed to more sunlight per orbit, and eventually it will be in constant sunlight -- in other words, there is no passing into the Earth's shadow for extended periods of time. This can create thermal problems, so special attitudes are chosen to counter these effects. The shuttle also has thermal constraints under these conditions. Since the attitude requirements of the ISS and shuttle would conflict when docked, shuttle/ISS flights are scheduled to avoid periods when the beta angle exceeds 60 degrees.
The ISS Y-axis is aligned along the truss (perpendicular to the modules). The plus Y direction is towards the starboard side. So a YVV orientation implies that the starboard truss will be pointing in the direction the ISS is traveling.
From the 11A Lead ADCO (Attitude Determination and Control Officer):
This is a GREAT question!
Normally, the ISS is oriented in an LVLH (Local Vertical, Local Horizon) attitude much like an airplane. As the beta angle increases, however, the solar arrays can no longer stay orthogonal to the solar vector and fail to produce adequate power. During these high beta ranges, the ISS typically orients to an XPOP (X-axis Perpendicular to the Orbital Plane) attitude where the aft end of the station points towards the Sun. This attitude is quasi-inertial, meaning it is fixed in space; an easy way of thinking about it is at "orbit noon" the bottom of the station faces the Earth, at "orbit midnight" the bottom faces away. XPOP allows the solar arrays to stay nearly orthogonal to the solar vector and maintain adequate power generation. While good for power, in XPOP the Russian Segment is exposed to the brunt of the Sun's radiation and as the beta angle increases, so do thermal concerns.
The YVV attitude still allows the arrays to remain nearly orthogonal to the Sun, while the ISS ôrolls" around the Earth permitting a more even heating of the ISS (hence the name barbecue). This is of particular concern in December when exceptionally high beta angles will be encountered.
While YVV is good for maintaining power generation and mitigating thermal concerns, at this time we do not have the capability to maintain this attitude with our non-propulsive Control Moment Gyroscopes. Instead, we must rely on Russian Segment thrusters. Due to the finite amount of propellant available onboard, we only fly YVV for six days in December when the beta angles are at their highest.
-ADCO/Aaron M. Butler