Solar Sail Attitude Control System for the NASA Near Earth Asteroid Scout Mission

Juan Orphee Ben Diedrich Brandon Stiltner Chris Becker Andy Heaton 1/19/2017

1 Introduction

• Near Earth Asteroid (NEA) Scout mission uses an 86 m^2 sail for primary propulsion

• The sail produces a significant solar disturbance torque

• NEA Scout is a class D NASA payload in the form of a 6U cubesat

• Limited volume and mass creates challenges

• NEA Scout uses reaction wheels for primary attitude control

• NEA Scout uses an Adjustable Mass Translator (AMT) to manage pitch and yaw momentum

• NEA Scout uses a cold gas Reaction Control System (RCS) for initial de-tumble, a Trajectory Control Maneuver (TCM), roll momentum management, and safe mode

• NEA Scout control software ensures all systems work smoothly together for successful mission 2 NEA Scout Spacecraft Configuration

Spacecraft body axes and relative scale of sail to spacecraft

1 – BCT Star Tracker/Drive Control Electronics 1 2 2 2 – BCT Coarse Sun Sensors x 3

3 – BCT 15 milli-Nm Reaction Wheel x 4 4 3 6 4 – Sensonor MEMS IMU

5 5 – VACCO cold gas Reaction Control System (RCS)

6 – NASA in house Adjustable Mass Translator (AMT) 2 3 NEA Scout Control System

Reaction wheel feedback control loop

Reaction wheels are primary attitude controller

Bode diagram for roll control

System bandwidth is 0.067 Hz

Stability margin 16 dB/67 deg

4 NEA Scout ACS Performance

Meets attitude pointing requirements

Meets science pointing requirement that is challenging for small sat

5 NEA Scout RCS Design

VACCO cold gas RCS

6 total control jets

- 4 for control - 2 dedicated to delta-V

Phase plane controller

6 NEA Scout RCS Performance

RCS required to de-tumble from 10 deg/sec per axis

RCS meets requirement

De-tumble is critical so sun pointing can occur in timely manner

7 Momentum Management Control System

The reaction wheels build up momentum from the solar disturbance torque

The momentum management control system uses a Proportional Integrator to manage X and Y momentum

The actuator for implementing momentum management in X and Y is the Adjustable Mass Translator

The AMT moves a large proportion of the spacecraft mass (~ 40%) to trim the solar torque for a given sun incidence angle

8 Momentum Management Performance

Momentum management for two 90 deg slews

Reaction wheels spin up and spin down twice

Reaction wheel torques to initiate wheel spin

Z momentum managed by RCS

9 Conclusions and Forward Work

Conclusions

• NEA Scout can control sailcraft despite volume and mass constraints

• RCS meets requirements and manages Z momentum

• X and Y Momentum is successfully managed by AMT and RCS

Future Work

• Verification of control software

• Off-nominal control case simulations

• Spacecraft integration of ACS hardware

10 Backup

11