Start Date
4-1968 8:00 AM
Description
Laser communications has been proposed as one way to solve the "radio blackout" problem during the reentry of a manned vehicle. The AMICOM 8,000 kW Plasma Facility (PF)1 ' 2 was used in a set of experiments to simulate the conditions expected during the reentry of a high speed vehicle. This experimental study was designed to simulate a typical theoretical Apollo test vehicle reentry trajectory. No plasma effects on the transmitted laser beam were expected or observed since the cutoff frequency due to electron collision rates was calculated to be in the microwave region and not in the optical or infrared region. However, the high temperature gradients and anticipated gas density variations in the flow field were throught to be potential sources of local index of refraction fluctuations. These fluctuations may generate scintillation effects in the laser beam which would be expected to reduce the effectiveness of laser communications.
The limitation of maximum power in the PF flow field, approximately 4 megawatts, prevented simultaneous duplication of the desired Reynolds number (R ^ 7 X 104 per meter) and the stagnation point enthalpy (h/RT o « 100). However, by varying the PF parameters to lower the enthalpy, the Reynolds number, or both a satisfactory simulation of conditions just before the termination of "radio blackout" was obtained. The region of the "reentry corridor" simulated in this investigation is near 60 km altitude and 3.9 km/sec velocity. The Reynolds numbers were based on a characteristic length of 1 meter for the vehicle and 0.1 meter for the model. A flat disk model, normal to the PF flow stream, was used to generate the required local flow field for the laser experiments. The disk diameter, 0.09 m, was determined by the PF flow field blockage parameter; this also established the characteristic Reynolds number length (j£• Ki 0.1 m) for the experiments. The disk was followed by a smaller diameter cylindrical afterbody approximately 25 cm long containing instrumentation. The cylinder also had a pair of windows situated so the laser beam could pass through the flow field immediately behind the disk. An amplitude modulated He-Ne laser was used as a signal source with a photomultiplier tube as a detector. Model stagnation pressure and transmitted laser signals were recorded on an analog tape recorder. A frequency spectrum analyzer was used to analyze the data.
Laboratory Simulation of Laser Communications from a Reentry Vehicl
Laser communications has been proposed as one way to solve the "radio blackout" problem during the reentry of a manned vehicle. The AMICOM 8,000 kW Plasma Facility (PF)1 ' 2 was used in a set of experiments to simulate the conditions expected during the reentry of a high speed vehicle. This experimental study was designed to simulate a typical theoretical Apollo test vehicle reentry trajectory. No plasma effects on the transmitted laser beam were expected or observed since the cutoff frequency due to electron collision rates was calculated to be in the microwave region and not in the optical or infrared region. However, the high temperature gradients and anticipated gas density variations in the flow field were throught to be potential sources of local index of refraction fluctuations. These fluctuations may generate scintillation effects in the laser beam which would be expected to reduce the effectiveness of laser communications.
The limitation of maximum power in the PF flow field, approximately 4 megawatts, prevented simultaneous duplication of the desired Reynolds number (R ^ 7 X 104 per meter) and the stagnation point enthalpy (h/RT o « 100). However, by varying the PF parameters to lower the enthalpy, the Reynolds number, or both a satisfactory simulation of conditions just before the termination of "radio blackout" was obtained. The region of the "reentry corridor" simulated in this investigation is near 60 km altitude and 3.9 km/sec velocity. The Reynolds numbers were based on a characteristic length of 1 meter for the vehicle and 0.1 meter for the model. A flat disk model, normal to the PF flow stream, was used to generate the required local flow field for the laser experiments. The disk diameter, 0.09 m, was determined by the PF flow field blockage parameter; this also established the characteristic Reynolds number length (j£• Ki 0.1 m) for the experiments. The disk was followed by a smaller diameter cylindrical afterbody approximately 25 cm long containing instrumentation. The cylinder also had a pair of windows situated so the laser beam could pass through the flow field immediately behind the disk. An amplitude modulated He-Ne laser was used as a signal source with a photomultiplier tube as a detector. Model stagnation pressure and transmitted laser signals were recorded on an analog tape recorder. A frequency spectrum analyzer was used to analyze the data.
Comments
No other information or file available for this session.