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Plasma Resonance Could Overcome Radio Silence For Returning Spacecraft
Zothecula points out this article about a workaround for a long-standing problem with space-flight communications: some of the most cruicial time of a re-entry is also time when the craft cannot send data to or receive instructions from the ground controllers. From the article: Returning spacecraft hit the atmosphere at over five times the speed of sound, generating a sheath of superheated ionized plasma that blocks radio communications during the critical minutes of reentry. It's a problem that's vexed space agencies for decades, but researchers at China's Harbin Institute of Technology are developing a new method of piercing the plasma and maintaining communications.
This means coupling the craft's antenna to that plasma sheath, "[causing] the sheath to act as an inductor. Together, they create a resonant circuit."
I bet this includes some fancy use of ginseng root?
Mod me down, my New Earth Global Warmingist friends!
Antenna reciprocity applies (the level of the signal), but not noise. The plasma generates a very high thermal noise.
A directional ground based antenna with high output power will overcome the RX noise. The military use for this technology is obvious, you can re-target your balistic missiles (or abort) or use radar to see incoming interceptors and avoid them.
what can be done in that precise stage so the comunication channel becomes the difference between live and death?
We can send telemetry, making a difference between life and death for the *next* crew to go up.
Therefore, by the (faulty) logic you're using, you're just a cow with a keyboard - osu-neko (2604)
I thought this was no longer an issue? I think continuous communication had been in use for over a decade with the space shuttle before the end of the program. The solution was to use satellites, being on the other side of the plasma sheath, as relays to communicate between a reentering craft and the ground..
I am a Chinese
Although I do not enjoy the attitude displayed by those racists towards the Chinese, I do hope that there are even more of them --- the more of them look down on us, the more of them won't even notice what we have accomplished
We Chinese have a saying --- stay low but work diligently
In other words, the more we stay under the radar the more we can progress without Obama and his anti-Chinese gang looking over our shoulders
The effect is the same. Usually the capacitance is adjusted to series resonate with the inductance from length of feedline going to the drive point tap. Whether it's a physical series capacitor, or the effective capacitance of a coupling sleeve doesn't change the theory. Obviously the feedpoint impedance will vary with the length of the plasma plume. The transmission line length will be relatively short, so mismatched impedance isn't a problem there. The interfacing electronics needs to tolerate wide variations. While dynamic matching could be used, it would be tempting to have the operating frequency follow the resonance of the plasma plume. Unfortunately nearby planets might gripe about interference to other spectrum users. But if they set of an EMP first...
There is prior art for that... (load conditions, through feedback, setting the frequency) 7 inch 1945 Motorola television sets had a sleeve around the glass of the high voltage rectifier, essentially a feedback capacitor. It fed the grid (input) of the tube driving the high voltage transformer. Although it looked odd, the spring around that tube was very much a required part in the circuit. They could have used a manufactured capacitor, but it would have had to handle about 7,000 volts. The inductance of the transformer secondary resonating with the total of its own capacitance and that of the wiring and rectifier, set the oscillation frequency. Unlike larger screen sets that used magnetic deflection of the c.r.t. beam, the radar/scope-like c.r.t of the 7 inch sets had electrostatic deflection. That meant the high voltage transformer didn't have to run at a precise frequency to drive scan coils on the c.r.t. from a portion of the transformer winding.
https://web.archive.org/web/20...
So the plasma antenna should work, and it's probably less complicated than a 1945 television.
The resonant frequency is just going to dance around any way. Why not just modulate the voltage on the plasma directly? (Relative to the metal super structure of the craft.) If you do it with a square wave you can very easily recover the energy the same as if you were using resonance ... and these guys don't have to worry about the FCC.
The article specifies that spacecraft re-enters at about 5 times the speed of sound.
1) The spacecraft on low Earth orbit have orbital velocities of about 8km/sec, and the speed of sound is about 0.34 km/sec. That makes the spacecraft about 23 times faster than sound on re-entry. I remember reading bout the Columbia disaster, that the shuttle entered the atmosphere at about 26 times the speed of sound. That makes sense, as the potential energy of the above-atmosphere orbit is transformed into kinetic energy at the altitude of hitting the atmosphere.
For the Apollo spacecraft, they re-entered at even higher speed, close to the Earth escape velocity of 11.2km/sec. That makes them about 33 time faster than sound.
2) The plasma sheet forms a very narrow cone with the spacecraft at the tip of it, effectively enveloping the spacecraft. The angle is given by:
sin \theta = speed of sound / speed of spacecraft.
At mach 23 it is about 6 degrees. Plus the plasma is turbulent, so it is very difficult to aim a signal along this cone and hit a satellite.
At hypersonic velocities a trailing antenna is going to flail around so much that it'll probably snap.
I wouldn't be at all surprised to find that NASA tried something like this back in mercury days, as I've spoken with some of the scientists who worked on the unmanned and chimp craft. Many are now long-dead, but they had a lot to tell which isn't in any history file (such as desperately giving CPR to a chimp...)