NASA engineers at the Johnson Space Center in Houston, Texas are currently designing and flight-testing the prototype X-38, a spacecraft which could become the first new spacecraft to travel from orbit in the past two decades. Developed at a fraction of the cost of former space vehicles, the X-38’s most immediate application is a vital one: It will serve as a model for the emergency crew return vehicle (CRV), or lifeboat, for the International Space Station. The lifeboat is designed to hold seven astronauts, and with the touch of a button will bring the crew home within two hours of initiating the return-to-Earth sequence. The X-38 may also serve as the model for the crew transport vehicle (CTV), a space van meant to carry astronauts only, not cargo.
The mandate for the X-38 is to develop an economic spaceship. Because it will be a re-entry vehicle, the spacecraft requires precision assembly of thousands of tiles and parts. In the past, NASA engineers used typical manufacturing alignment techniques, such as levels, plump bobs and micrometers. It was labor intensive. Then NASA acquired a Leica LT500 Laser Tracker in January 1997.
"The first thing we noticed was the time savings", said Frank Jenson, an engineering technician with NASA’s Manufacturing, Material and Process Technology Division. "Man-hours spent for the inspection and analysis of large manufactured parts were reduced dramatically. I would estimate we save 70 percent of our time over conventional verification techniques. A typical ring frame, which used to require about two weeks for an inspection, now takes two-and-a-half to three days".
Steve Peterschmidt, Mechanical Engineer for Rothe Joint Venture on the X-38 project, is responsible for analyzing the 3D coordinate data collected by the LT500 and comparing it against their original CAD models. "We have to measure a bilateral profile tolerance of 0.010 inches over compound curvatures on parts up to 120-inches long. In standard dimensional inspection, we do not get the resolution we need; nor do we have the time to use a normal coordinate measuring machine. With the Laser Tracker, we have the inspection completed within a few hours", he said.
The X-38 engineers use the LT500 to measure the components manufactured within the enterprise and by outside suppliers. The X-38 project has the distinction of being the first in-house-manufactured project at NASA, rather than a contracted project. The Laser Tracker is also used to align thousands of parts on the large vehicles.
"There is a tolerance stack-up as you assemble the parts", Peterschmidt said. "If you assemble thirty parts together, you might multiply your deviations by thirty. The Laser Tracker allows us to place each part within the vehicle coordinate system, so it is attached to the assembly according to a single reference datum. We are trying to stay within an overall tolerance of 0.050 inches, and so far, the accuracy is around 0.020 inches. I believe we can stay within 0.020 inches to 0.030 inches for the entire project". The LT500 has an accuracy of 0.001 inches.
These are the tightest tolerances NASA has ever maintained for a spacecraft. Considering the X-38 is NASA’s first in-house design through manufacturing project, this is quite an impressive feat.
Another advantage of the Leica Laser Tracker is its portability. The LT500 sensor head weighs approximately 69 pounds, and can be easily moved around the part to be measured. The whole process takes about ten to fifteen minutes for repositioning and a few seconds for data collection. "We move it all over the building", said Brian Anderson, the X-38 Design and Production Manager, "and this is a large facility. We don’t have to bring the parts to the laser tracker, we move the laser tracker to the parts".
The Laser Tracker has such a reputation around the Johnson Space Center that engineers on many other projects want to utilize it. The X-38 team has been happy to oblige their fellow engineers. Nearby is Johnson Space Center’s main machining facility, with just about every type of manufacturing tool available. There has been no difficulty transporting the LT500 to this building and because the machine is simple to use and quickly does its job, there has been no loss of time on the X-38 project.
Jenson even packs the LT500 in its original shipping box, loads it on a government pick-up truck, and drives five miles to the Ellington Airfield. The Laser Tracker is then used to measure engine intake port on the astronauts’ T-38 trainer aircraft. Jenson can make the drive, take six detailed measurements with more than 6,000 data points each, and be back in time for lunch. Four other projects at Ellington have utilized the laser tracker in the first five months of 1998.
Lasers are notorious for calibration problems if repetitively moved. It is a common belief that the more precise the laser device becomes, the less it can be moved about. However, the LT500 is the sturdiest laser tracker in existence, built to be very portable. Jenson never needs more than fifteen minutes to check the laser tracker’s calibration after the appropriate warm-up. In eighteen months of heavy daily use, the Leica Laser Tracker has never been out of calibration, staying within the parameters of 0.0015 degrees of accuracy.
"That’s the whole idea of using a measurement device like this", Jenson said. "It has to be portable. We will even ship it to Dryden in California where they do the atmospheric testing on the X-38. We have to be able to put it through government shipping and still economize our time at each of the facilities".
The target date for the X-38 project is to have a working CRV attached to the International Space Station in 2003. Until then, the space station will use a Soyuz spacecraft similar to Mir’s emergency return vehicle. However, the Soyuz is cramped, and expensive. The X-38 will comfortably accommodate seven astronauts who may be injured or incapacitated and bring them automatically to safety without need of a pilot.
Use of the Leica LT500 on the X-38 project has resulted in a significant saving in time and effort, a benefit that has been collectively shared with many other engineering projects in progress at the Johnson Space Center and nearby Ellington Airfiled. The X-38 Project is on schedule to provide one of the most vital components of the International Space Station: a fast ticket home and perhaps an affordable ticket outbound. It is a project worthy of the planet’s sharpest engineering experts - minds that are much too busy to be slowed down by conventional measuring methods. The Leica LT500 delivers sharp, fast, accurate measurements at lightning speed.
(NASA’s policy is not to endorse any product, service or company. Leica Geosystems is very appreciative for the candid remarks by X-38 Project engineers, and their statements reflect solely on the technology presented by the LT500, in their individual experience).
