The frustrating communications blackout that can occur when a spacecraft reenters the atmosphere caused some tense moments in the earlier years of the space age—perhaps most memorably during the crippled Apollo 13 mission. But the phenomenon could also affect communications with new aircraft and weapons systems being contemplated now by the U.S. Air Force, which hopes to find ways to pierce the blackout.
The problem arises when a speeding vehicle heats the air in front of it, ionizing it into plasma that blocks radio transmissions. It resembles the shock waves created when an airplane hits Mach 1 and breaks the sound barrier. In the case of reentering spacecraft and hypersonic aircraft, the plasma-shock boundaries form at speeds of about Mach 10. The space shuttle avoids the blackout because the craft’s broad underside leaves an open area in the plasma plume trailing behind, enabling communications and telemetry data to be relayed to Earth through a network of satellites. But smaller craft are completely engulfed by the plasma.
That has concerned the air force, which plans to develop flight systems, perhaps including hypersonic missiles, surveillance craft and even manned craft that could top Mach 10. “Our standard paradigm [in the test and evaluation world] is we have a vehicle in the air and people on the ground and there’s a telemetry stream of data coming down so people can monitor the vehicle,” explains researcher Charles H. Jones of the Edwards Flight Test Center. Besides severing contact between the test craft and the ground, a plasma blackout also blocks the self-destruct signal that is sometimes necessary when a test vehicle strays off course.
Another crucial concern involves satellite navigation signals. “The military’s becoming ever more dependent on GPS systems,” notes Mark Lewis, a University of Maryland aerospace engineer and former air force chief scientist. Jones agrees, writing in the final report of a 2006 conference in Boston sponsored by the Air Force Office of Scientific Research (AFOSR) on the blackout problem: “Reception of GPS is the most critical capability for which we need to find a solution.... GPS is also the most difficult problem due to its innately weak signal.”
Jones has been something of a voice in the wilderness on the problem. “Mach 10 designs aren’t that common” just yet, he notes, and so “for a lot of people it’s not a pressing issue.” But as new higher-speed concepts in what he calls the “plasmasonic” Mach 10 realm begin to appear, the phenomenon is attracting more attention.
The 2006 conference revealed no lack of possible solutions. Ideas include designing the configuration of the craft so as to minimize the resulting plasmasonic sheath; building an “air spike” at the front of the leading edge that would protrude outside the plasma; finding frequency bands that might not be affected by the ionization; simply bullying through it with an enormously powerful transmitter; and using electrophilic injection, which means dispersing a deionizing substance, most likely water, into the plasma sheath to disrupt it. More exotic ideas involve employing high-powered lasers or ejecting a series of tiny relay devices, akin to messages in a bottle.
Although all the proposed solutions are theoretically feasible, Lewis notes that “the question then is, Are they practical from an engineering standpoint?” The water-injection idea, for example, was actually attempted back in the 1960s, during the Gemini program. But an operational version would require carrying far too much water to be practical.
A variation of electrophilic injection might be a strong contender, however, using a heat shield made of an ablative material that vaporizes parts of itself and deionizes the plasma. “In terms of an engineering solution, that seems like the simplest,” Jones says. Meanwhile Lewis considers himself “a stone agnostic. We’ve got a number of avenues to explore, and I think we’re still at the point where all those avenues should be on the table.” Jones has noted that a combination of techniques most likely will be required to meet all anticipated applications.