This is one of a series of reports on the Trailblazer II program. The particular aspect treated here involves the unmodified expansion-region plasma and its effect on an antenna located on the vehicle shoulder. This report describes some of the theoretical approaches used, discusses the levels of approximation involved, and shows the agreement between these various methods and the test data. The failure of a single set of assumptions to yield consistent agreement over a range of altitudes confirms the need to adopt flow models appropriate to the changing regimes encountered during reentry. One significant conclusion is that performance characteristics such as reflection and interantenna coupling which depend mostly on the level of peak electron density can be represented by simple plane wave, as well as by the more sophisticated slot antenna models. The latter approach, however, is necessary to describe propagation across the entire plasma sheath.

To develop alternative solutions to Air Force problems relating to signal transmission in the presence of ionization, AFCRL undertook an extensive investigation of techniques for modifying reentry plasmas. The program included laboratory tests and a series of reentry flight experiments. This report describes the flight test of one successful technique, Teflon ablation, a passive approach that requires no internal support systems. A reentry vehicle fitted with a Teflon-coated nosecap was instrumented to measure antenna impedance mismatch, interantenna coupling, signal attenuation, and charged-particle density. The probe data showed that the local boundary-layer electron density decreased by as much as a factor of 200. The Teflon coating effected a 25-dB decrease in S-band signal attenuation. High-power antenna breakdown was modified by the presence of the ablation products. Details of the vehicle design, flight dynamics, and ablation, are presented, and the results of the Teflon-ablation technique are contrasted with those of a successful liquid-injection technique that was tested on a previous flight.

The purpose of the third AFCRL Trailblazer II reentry vehicle flight was to study the effects of shock ionized air on the performance of a microwave antenna system located in the expansion region of the nose cone. The system consisted of a pair of cavity-backed circumferential slot antennas mounted one behind the other on the conical afterbody of the reentry vehicle. During reentry, measurements of antenna impedance mismatch, interantenna coupling, signal attenuation and antenna pattern distortion were made. The data have been compared with computed values and found to be in good agreement. This experiment thus establishes the validity of the various microwave and flow field models and calculations used to obtain the theoretical values in the shoulder region of the reentry body. These results will now serve as a standard against which to compare the properties of various chemical alleviants which are being studied in other flights. (Author).

The microstrip plasma probe, a new diagnostic tool for use in the investigation of ionized media, has been developed and successfully tested both in the laboratory and in an actual flight. It is flush mounted (therefore causes no aerodynamic disturbances); simple in geometric shape (therefore can be modified to fit curved surfaces and ruggedized to withstand shock and vibration); only a fraction of the operating wavelength in length (therefore saves on payload space); and operates with low power (therefore does not disturb the plasma which it is measuring). Experimental results, obtained from both the laboratory and from an actual flight, have been compared to theory and to experimental data from other probes. In both instances the results are close.