Endeavour to chart new ground in rendezvous demo
BY STEPHEN CLARK
Posted: April 21, 2011
It's a test pilot's dream. Use an existing vehicle to try out technology for its replacement. The U.S. military does it with fighter planes, and now NASA plans to demonstrate an important piece of space navigation equipment next month on the shuttle Endeavour's last mission.
The objective: Assess how the next-generation navigation sensors perform in the challenging environment of space.
The laser beam and camera are the crux of a next-generation optical space navigation system. NASA and contractor teams developed the instruments to help a new manned spacecraft safely dock with the space station and reach other destinations.
The practical goals of the exercise required some out-of-the-box thinking. Endeavour and its astronauts will fly a trajectory never before attempted in the shuttle's 35 previous visits to the research outpost.
"It's a whole unique array of burns," said Greg Johnson, Endeavour's pilot. "The trajectory is something very different than what we've ever flown before, but it's all the same sort of things that we've been doing with rendezvous. It just looks different."
Once the shuttle leaves the station at the end of the mission, Endeavour will mimic the approach of a future, still undefined, space capsule. Using its nimble flying qualities, Endeavour will undock and back away from the complex then approach the space station again.
Endeavour will complete a one-lap flyaround of the outpost before firing its thrusters for two separation burns, first to depart the immediate vicinity of the space station, then to put the ship on a looping trajectory back toward the million-pound complex.
"We're then going to do this series of burns where we're going to fall behind the space station a couple hundred thousand feet, and then we're going to come back in doing a profile that's actually quite similar to what Apollo used for a rendezvous," said Mark Kelly, Endeavour's commander.
Not much bigger than a kitchen microwave, miniature sensors and electronics inside Endeavour's cargo bay will be put to the test during the unique demonstration. It's called the Sensor Test for Orion Relative Navigation Risk Mitigation, but it's better known as simply STORRM.
The system was designed to help the Orion spacecraft navigate its way to the space station. Although the Constellation program, including Orion, was cancelled last year, work on the capsule continues. Orion could become the basis of a multipurpose crew vehicle, a new spacecraft designed to take astronauts to deep space on missions to asteroids.
It might also travel to the space station to serve as a lifeboat or ferry resident crews back and forth to Earth.
Congress directed NASA to develop the multipurpose crew vehicle in an authorization act passed last year. And the law said NASA should base the capsule on work already accomplished on the Orion spacecraft.
STORRM is an early example of Orion systems being tested in flight.
"All spaceflight hardware gets designed on the ground and tested to the best we can, but you can't really simulate the entire space environment here," said Gary Horlacher, lead flight director for Endeavour's mission. "Until you get into orbit, there's only so far we can take it."
Other crew members will support the re-rendezvous by operating other sensors and taking photos.
"The STORRM software is working itself," Feustel said. "I do some initialization of the code to make sure that it's operating properly. Everybody has been trained to some level on how to monitor the feedback on the computer screen to know if the system's failing. But it's essentially my job to really watch what's happening with the software and help the ground understand if they don't have the insight that they expect."
While Feustel makes sure the sensors are functioning, Kelly and Johnson will put Endeavour to work. With Kelly flying the shuttle from a flight deck station looking out the back window of Endeavour, Johnson will help oversee a series of computer-controlled jet firings to put the ship on a tightly-choreographed path back toward the space station.
"I will be flying the flyaround and I'll do the separation burns like we normally do, and then somewhere in there we'll swap roles and Mark will be watching out the back and I'll be up front, but typically the pilot does a lot of those little burns," Johnson said.
A series of thruster burns will push Endeavour away from the space station after the traditional one-lap flyaround, sending the shuttle miles above and behind the complex before it starts moving closer again.
"We will go ahead and do an orbit-lowering burn out there, which is going to bring us down below the space station and get us set up for the trajectory to mimic the Orion approach to the space station," Horlacher said.
On the way back toward the station, the orbiter will be catching back up to the outpost from a position below and behind it. STORRM's laser navigation sensor should be able to collect useful data within 5 kilometers, or about 3 miles, of the space station.
The astronauts will use the shuttle's existing Ku-band rendezvous radar for on-board navigation during the demo. STORRM will simply be along for the ride.
"The STORRM system is not integrated into the shuttle vehicle, so none of that information can get into the targeting capabilities of the shuttle," Horlacher said. "They will basically be operating separately from the orbiter."
Endeavour will halt its approach within 1,000 feet below and 300 feet behind the space station before the shuttle backs away to begin re-entry preparations. Officials say the entire operation should last about 4 hours from undocking through final separation from the station.
If Endeavour blasts off April 29 as planned, the shuttle's undocking and STORRM test is slated for May 11. But NASA could extend the mission by up to two days, pushing the demo to May 12 or May 13.
The shuttle crew will turn on the STORRM sensors at least twice earlier in Endeavour's mission to make sure they work. Feustel plans to check out the system on the second day of the flight, then STORRM will be taking data throughout the shuttle's initial rendezvous and docking with the space station on the third day of the mission.
"We're going to do a normal docking with the normal shuttle systems," said Mike Fincke, an Endeavour mission specialist. "STORRM will be there watching."
In the week-and-a-half between docking and the final STORRM demo, engineers on the ground will evaluate the system's performance during the rendezvous, potentially adjusting some parameters of the sensors before attempting the crowning test.
Both sensors are inside a protective enclosure. A separate box contains avionics and electronics.
None of the data will be used on Endeavour's demo because engineers aren't finished developing software to present the sensors' raw output to the shuttle crew.
Instead, engineers will analyze the data after the flight to help calibrate and tweak the navigation system.
"On this mission in particular, we'll be collecting raw data," said Jeanette Domber, manager of the STORRM project at Ball Aerospace & Technologies Corp. in Boulder, Colo. "There is some processing that has to happen to that data, especially with the VNS, to turn that into information for the astronauts in real-time."
Ball Aerospace developed the rendezvous sensors under a contract with Lockheed Martin Corp., builder of the Orion spacecraft. NASA's Langley Research Center in Virginia managed the integration of the sensors with an avionics suite.
Firing 30 laser pulses per second at five reflectors on the space station docking port, STORRM's primary sensor will compute a three-dimensional map of the lab's docking port.
The instrument will measure how long it takes for the light to be reflected back from the station and calculate the distance to the target.
"The Vision Navigation System is a flash lidar system, so it uses a laser to create a six-degree-of-freedom proximity map of the space station, or whatever we're trying to image," Domber said. "We send out a flash from the laser to return off the object that we're ranging to to get range and bearing information that can be used to compute trajectory solutions toward the object of interest."
The laser system can also tell an approaching spacecraft's alignment and orientation relative to the station docking system. Such information is critical for the space station's next-generation docking port, a planned common adapter designed to receive all of the partner nations' visiting vehicles. Engineers expect the adapter will require even more precise dockings than today.
A docking camera built with off-the-shelf components will take live imagery during the approach. The idea is for the camera to serve as a secondary visual cue to pilots flying the Orion capsule.
The STORRM laser has three times longer range than the primary space shuttle rendezvous sensor, and the docking camera has 16 times greater resolution than the orbiter's centerline camera, according to Horlacher.
"Many pulses over time allows you to determine range rate or relative velocity between the spacecraft," Horlacher said. "The system has the knowledge of where those detectors are and can do some computations on the relative orientation, which is critical when docking two spacecraft."
The shuttle crew visited Ball Aerospace in Boulder last year to visit with the engineers who designed the sensors. The astronauts also had the opportunity to train with the hardware that will fly on Endeavour, Domber said.
After the shuttle lands, the STORRM sensors will be shipped to Lockheed Martin's facility in Denver to run through more testing in a state-of-the-art docking simulation laboratory embedded in the bedrock of Colorado's Front Range.
The shuttle flight demonstration and additional ground testing will guide engineers writing software and developing algorithms to translate STORRM's raw data into comprehensible range, bearing and attitude information for automated docking computers and pilots, according to Domber.
"This is the best opportunity we have to test the sensors in the actual environment they will be working in, so in vacuum, at these distances, and with the right lighting conditions," Domber said.
The sensors were already thoroughly tested with a mock-up target inside a clean room at ranges between 6 feet and 200 feet. Ball also staged a unique outdoor test in which the laser system was installed on a building roof and aimed at reflectors positioned on other structures in the city of Boulder.
"We feel very ready," Domber said.
MISSION STATUS CENTER