Orbital Express: Testing On-Orbit Servicing
The Orbital Express advanced technology demonstration couples a prototype servicing satellite (ASTRO) and a surrogate next generation serviceable satellite (NextSat). Together, they are meant to test robotic, autonomous, on-orbit refueling and reconfiguration of satellites. If that were possible, it would mean faster, less risky missions to maintain and extend the lives of America’s critical military satellite fleet – and the technology would have more than a few civilian/NASA uses, as well.
The Orbital Express Program
USAF Lt. Col. Fred G. Kennedy, DARPA’s Orbital Express program manager, told Aviation Week that “it will take the technical excuse off the table for on-orbit servicing… In the 1990s, we talked about on-orbit servicing with Space Command – it was always seventh or eighth on the priority list. TThere were responses like ‘zero-g propellant transfer can’t be done.'”
There have been successful experiments by Japan and the US Air Force Research Lab that that demonstrated components of on-orbit servicing. Nonetheless, the task before Orbital Express is non-trivial. Even a small chance of collision will destroy the economic model for orbital servicing. In 2006, NASA’s Demonstration of Autonomous Rendezvous Technology (DART) satellite collided with its target, aborting the mission. The Mir Space Station also had a close call when a June 1997 robotic docking resulted in a collision that punctured the pressure vessel.
The Orbital Express program effort had a few near collisions of its own. Boeing makes no profit on Orbital Express, and its Other Transaction Agreement includes clauses on how Boeing and Darpa share expenses beyond the cost target. That almost proved the end of it.
In June 2006, Aviation Week reported that the program’s cost was $267.4 million from Fiscal 2001-07, which includes $25 million from NASA and “millions… a very significant fraction of the overall cost,” from Boeing’s own corporate research and development funds. This figure has grown from the $113 million March 2002 contract with Boeing to build and demonstrate Orbital Express, and changes in requirements have played a role. So has cost growth and underestimations of the complexity involved, however; the program was under threat of cancellation in 2005, before rescuing itself and getting back on track. The first launch would include a parallel drama, which also ended well.
Orbital Express team members include:
- Boeing (lead, ASTRO satellite, ARCSS autonomous rendezvous and capture sensor system optical sensor set)
- NASA (sensors and software developed for autonomous rendezvous and proximity operations, incl. the AVGS advanced video guidance sensor)
- Ball Aerospace (NextSat satellite)
- Northrop Grumman Space Technology (fluid transfer and propulsion system, including the thrusters and hydrazine tanks)
- MacDonald, Dettwiler and Associates Ltd. (Canadian firm, ASTRO manipulator arm)
- The Charles Stark Draper Laboratory Inc. (mission manager software/ operating system)
- Starsys Research (docking capture system, an enhanced version of a prior unit built under a government contract)
Astro will have an on-orbit fueled weight of about 2,400 lb.; its octagonal body is 69 in. across and 70 in. long, with a solar array span of 220 in. The array produces an initial 1,560 watts.
NextSat weighs about 550 lb. on orbit. Even though it carries hydrazine, it has no rocket engines; the fuel tank is just there for fluid transfer experiments.
Key Events & Contracts
June 16/07: Orbital Express performs a fully-autonomous “fly-around and capture” of its NextSat client spacecraft, marking another industry first. During the 5-hour test, ASTRO used its onboard cameras and advanced video guidance system to separate from, circle and re-mate with NextSat. Positioned in orbit 60 meters above NextSat, ASTRO followed an imaginary line called the “Rbar,” which extends from the Earth’s center to a satellite and beyond, to capture the spacecraft. The maneuver simulated the direction of approach needed to effectively service a satellite without interfering with its cameras or antennas. The test primarily used passive sensors with no active exchange of relative navigation information or involvement by ground controllers. Boeing release.
May 10/07: Autonomous Rendezvous. Boeing announces Orbital Express’ successful performance of a fully autonomous free-flight rendezvous and capture operation. Using its onboard cameras and advanced video guidance system, the Boeing Autonomous Space Transport Robotic Operations (ASTRO) servicing spacecraft separated from the NextSat client spacecraft, backed away to a distance of 10 meters (33 feet), maintained proximity flight with NextSat for a full orbit, and then approached and captured NextSat with its docking mechanism – with no active exchange of relative navigation information or any intervention or control from the ground.
During the next major unmated operation (Scenario 3-1), ASTRO will depart NextSat to a range of 30 meters (98 feet), then approach and perform a free flight capture of NextSat using its robotic arm.
April 16/07: In its first on-orbit demonstration 300 miles above the Earth, Boeing’s Orbital Express system autonomously transferred propellant fuel and a battery from one spacecraft to another. See compiled video link [MPG format].
In the pressurized demonstration, ASTRO transferred approximately 31.97 lbm (pounds mass) of hydrazine to NextSat, satisfying the objective of 32 lbm. In the transfer pump demonstration, ASTRO transferred 19.2 lbm, more than the 17 lbm target. The team then conducted a pump fluid transfer from NextSat back to ASTRO. The team also performed an autonomous transfer of hardware between the two spacecraft. Using its robotic manipulator arm, ASTRO placed a battery on NextSat. The battery was successfully integrated into NextSat’s power system following the transfer.
The demonstrations occurred at the lowest levels of spacecraft autonomy, which required several ground-based “approval to proceed” (ATP) confirmations. The team initiated ATPs to closely monitor and evaluate the operations. Future demonstrations will require fewer ATPs, allowing Orbital Express to conduct flight activities with increased autonomy. At the highest autonomy levels, no ATPs are required. Boeing release.
March 8-13/07: Following release from the Centaur upper stage, the Orbital Express ASTRO vehicle experienced a guidance anomaly during initialization that prevented the system from achieving a proper sun-safe attitude. To correct for this problem, control of the mated pair was shifted from ASTRO to NextSat, and the Orbital Express team used NextSat’s guidance system to successfully point the mated stack towards the sun. NextSat continued to control the stack while the Orbital Express team worked to determine the root cause. The team eventually uploaded a software fix to correct the anomaly and successfully tested ASTRO’s sun-safe mode on March 13, 2007. Orbital Express program release | NASAWatch partial problem report.
March 8/07: The Orbital Express duo compose 2 of the 6 on-board satellites successfully launched by a United Launch alliance Atlas V rocket. Launch video: high bandwidth – low bandwidth | Launch picture | DARPA release [PDF format] | NASA release.
April 04/06: Boeing announces that the Orbital Express system completed two major test milestones in March, 2006. The program completed its Baseline Integrated System Test (BIST) of the Autonomous Space Transport Robotic Operations (ASTRO) spacecraft, and also passed a series of electromagnetic interference and compatibility tests to verify component operation in the spacecraft’s actual electromagnetic environment. Boeing release.
Additional Readings & Sources
- Boeing – Orbital Express.