Research Horizons

Characterizing Missile Plumes for Tracking and Surveillance

Infrared surveillance satellite programs depend on accurate models of combustion chemistry and radiant emission in rocket exhaust plumes to model threats and interpret observed missiles. These models rely on physical and chemical mechanisms that are often unverified, incomplete, or unrelated to the real-world conditions being modeled. To address the need for accurate models, Paul Zittel together with Patti Sheaffer and William Dimpfl, scientists in the Aerospace Remote Sensing Department, have been conducting laboratory flame experiments and developing field instruments to generate and refine the underlying data. Their work should help to improve predictive models of plume phenomena as applied to surveillance, missile defense, and environmental studies.

Remote Sensing Department instruments on a Cine-Sextant tracking mount at Vandenberg Air Force Base.

The laboratory efforts recently culminated in detailed measurements of flame extinction at the low pressures characteristic of stratospheric altitudes. The experimental data were compared with simulations generated in PHOENICS, a commercial, state-of-the-art, computational fluid dynamic (CFD) computer model for simulating flames. One benefit of PHOENICS, Zittel said, is that it allows the user "to vary the flame chemistry mechanisms and rates used in the model to match laboratory observations, thereby defining the correct chemistry for use in other model applications." The results confirmed the importance of HO2 and suggested that the radical has not been correctly represented in many common combustion models. "We found that the rates typically used to predict the formation of the flame-carrying HO2 radical underestimated the formation rate and predicted flame extinction too early," Sheaffer said. "Adjustments of the rate led to better predictions." The team identified a new comprehensive reaction set that appears to model low-pressure hydrogen-oxygen combustion much more accurately, and the set will be tested in the next generation of high-fidelity rocket plume codes. Employing a technique developed to seed the flame with carbon particles, the team also plans to derive mechanisms and quantitative rates for soot combustion that could be used to further upgrade plume chemistry models. The fate of soot affects both the radiant properties of the plume as a function of altitude and the deposition of chemically active particulate into the stratosphere.

False-color, near-infrared image of Delta IV plume acquired by AMHI at 980 Hz, showing the main exhaust, a separate gas generator (GG) exhaust, Mach disks, and turbulent structures in the plumes. The scale of the image is 90 meters high, with a pixel resolution of 1 meter.

In conjunction with the laboratory experiments, the team has developed the Aerospace Multispectral Hypertemporal Imager (AMHI) suite to collect data during rocket launches at Vandenberg AFB. The imaging suite contains seven infrared instruments capable of both remote-tracking and unattended observation close to launch platforms, where ignition transient and launchpad contamination observations can be made. The suite includes a unique high-speed imaging radiometer that operates in several wavelength bands in the 1.1–5.5 micron range to collect spatially resolved data for research into the origins of turbulent mixing and plume fluctuations. It was used for the first time in November 2006 to produce kilohertz images of the Delta IV plume and data for the evaluation of new surveillance concepts. A new HgCdTe focal plane was also tested that will allow collection of high-frame-rate data into the long-wavelength infrared. Additional spectrographs and imagers, managed by Richard Rudy and George Rossano in the Aerospace Remote Sensing Department, are deployed with AMHI to Vandenberg to complete the acquisition of comprehensive, well-calibrated data sets spanning the short- to long-wavelength infrared spectrum. Thus far, highly successful collections have been made on five launches, and Zittel's team hopes to use the imaging suite to observe an Atlas V or similar launch next year.

In relating the fieldwork with the lab experiments, Zittel explained that "The real-world field data are compared with rocket plume model predictions that incorporate the fundamental physical and chemical parameters measured in the laboratory. The comparison may suggest the need for further improvements in the computational procedures, or chemistry, of the model, until an accurate predictive capability is achieved."

Reentry Breakup Recorder

When a spacecraft, launch stage, or other hardware reenters Earth's atmosphere, it experiences increasing aerodynamic heating and loads that cause the object to break apart. Aluminum and low-melting-point materials fail first, releasing other hardware that follows a similar process. Much of the hardware may be melted away, but as much as 10–40 percent of the original hardware may survive and collide with the ground, posing hazards to people and property.

Predictions of the hazards associated with the reentries of space hardware, however, have been limited by a lack of information on what hardware actually survives to hit the ground and on the response of the hardware to the reentry environment as reentry progresses, according to William Ailor of the Center for Orbital and Reentry Debris. "Hardware fragments can hit the ground anywhere along a footprint that is hundreds of miles long. Except for the Columbia accident, when the shuttle disintegrated over Texas during reentry into Earth's atmosphere, fewer than 250 fragments are known to have been recovered over the past 40 years, and only a very few of these fragments have been examined in detail."

"Excluding the Columbia accident, virtually no useful telemetry has ever been received," Ailor said, adding, "Since nearly 75 percent of Earth's surface is water, much of the reentered debris falls in water and is never seen. Most objects are much smaller than Columbia, so the number of fragments is much less. And the effort to recover the Columbia debris was unprecedented—there is no such effort for reentry of a rocket stage or satellite."

Ailor noted that observational data indicate that early breakup prediction models were inaccurate, and current models have been adjusted accordingly. Efforts are being made to understand the underlying physics better and to verify the models. One approach has been to retrieve and analyze reentry debris that has been recovered on the ground. Aerospace has been a leader in this work and has published the results of its investigations, Ailor explained. But this approach is limited by the rarity of finding such debris.

Ailor heads a team that has followed a second approach—to record the data during an actual breakup of a spacecraft or hardware during reentry. The team has been working since 2003 to develop the Reentry Breakup Recorder (REBR), a lightweight, autonomous instrument package that will attach to a host vehicle and record temperatures, accelerations, and other data as the host heats and breaks apart.

The REBR will be a small, lightweight, autonomous device containing a GPS unit, thermometers, pressure sensors, accelerometers, rate gyros, and other features.

As the host disintegrates, REBR, which has a heat shield to survive reentry, eventually separates and reaches subsonic free-fall conditions at about 60,000 feet, at which point it connects to the Iridium Satellite System and "phones home" the data to a computer at Aerospace. Use of the Iridium system allows the data to be recovered from a reentry event anywhere on the planet. REBR also uses the Global Positioning System to provide the impact location, although REBR is not designed to survive ground impact and does not need to be recovered.

Ailor said the current research and development continues a series of projects focused on the design of REBR. Progress so far includes securing a patent for the concept in 2005. The following year, the prototype design for the electronics package was completed and drop-tested at a site outside of Bozeman, Montana. The hardware to attach REBR to a launch stage has been designed, and Boeing completed a feasibility study for launching the recorder on a Delta second stage. NASA Ames Research Center, which is working with Aerospace to develop REBR, has made strong progress on the design of the aeroshell, a rigid heat-shielded shell that will protect the recorder's electronics and sensors during reentry.

The next goal, Ailor said, is to complete the basic design of the REBR prototype. This includes working with the Iridium modem provider to make a smaller modem, working with NASA to refine the aeroshell and thermal protection system, designing and testing the on-orbit thermal control system to allow REBR to remain viable after months in orbit, designing and testing a system that will activate REBR during a reentry, and testing an instrument suite that might be included in the attachment housing.

Knowledge and experience gained from the REBR project may lead to the design of space hardware that will react to the reentry environment in predictable ways, potentially allowing some space systems to avoid a directed reentry and saving propellant for extended mission life. It will provide detailed information that can be used to improve reentry hazard estimates and to improve the design of spacecraft and launch hardware to minimize reentry hazards. REBR could also be used as a black box for reentry vehicles and is a cost-effective platform for testing new thermal protection system materials and sensors.

REBR will provide data that is currently unavailable to anyone and will keep Aerospace in the forefront of reentry breakup technology. "It will position The Aerospace Corporation as the only source on Earth for actual reentry breakup data, since Aerospace is currently the only entity developing an instrument of the type described. The Boeing Company, NASA, the National Center for Space Research in France, and the European Space Agency have expressed an interest in using the device when it becomes available," Ailor said.

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