What we learned about stealth technology from the combat career of the F-117.
- By Bill Sweetman
- Air & Space magazine, January 2008
Tech. Sgt. Kevin J. Gruenwald/USAF
(Page 3 of 10)
Like the Have Blue, the F-117A was covered with radar-absorbent material—almost a ton of it. It was made in the form of flat sheets, cut to fit the skin panels, and glued in place. A putty-like material dubbed “butter” filled the gaps between the sheets. The engines were concealed from radar by Chrysler-like grills, and the hot exhaust gas from the engine was expelled through slits.
The jet had no radar. There were ideas for “low probability of intercept” radar that could work without giving away the jet’s position, but the technology could not be ready in time. (Such LPI radar uses tricks with modulation, frequency hopping, low power, and huge bandwidth to obscure its signal.) The aircraft’s use would be confined to attacks under the cloud base or in clear weather.
Even so, the jet was hard enough to build. The engineers were “inventing to schedule,” solving problems like keeping ice from forming on the inlet grills (a chemical-dispensing squeegee is hidden in the wings in front of the inlets) and on the air-data probes. They concealed the infrared sensor turrets from radar with a titanium wire mesh.
The haste paid off. The aircraft reached operational capability in October 1983, later than envisioned, but still only five years after the go-ahead on development. The angles of the wings were calculated to disrupt the radar waves as they scattered away from the aircraft and thus prevent them from returning to the source. Likewise, all the doors and opening panels featured saw-toothed forward and trailing edges to disrupt reflection of radar.
For all the hype, the aircraft was far from invisible. The RCS figures remain classified, but the airplane was more visible to radar from the sides than head-on. Planning the missions was difficult—because the jet could be seen by radar, the track had to be carefully adjusted to minimize the craft’s exposure.
The Model A of Stealth
Once Have Blue showed that attaining stealth was possible, the Air Force sought to replace it, and research began in earnest. Northrop, Boeing, and General Dynamics
launched programs plumbing shapes and technologies that promised greater stealth and better aerodynamics.
By the time the F-117A entered service, Northrop was flying the AP-1 Tacit Blue demonstrator, a stealth aircraft with curved surfaces, no inlet grills, and low-probability-of-intercept radar. The Air Force started studies of a stealthy supersonic fighter.