Supersonic Rocket Communications

[user 31826]

Hi, we are a rocketry team from TU Yildiz, Turkey. This year we are planning to compete in a 6km altitude rocketry competition, our rocket will reach supersonic speeds. Last year we made a subsonic rocket for 3km altitude that used HopeRF RFM98W 433MHz RF module (It worked fine.). We are aware that this can't be used in 6km altitude due to the communication range not being enough.

We had a meeting where we were told that communication is not the same as supersonic speeds as subsonic (They said the due to relative speed of the rocket being higher, communication is not the same with subsonic). We are planning to use an RF Module called HUM-A-900-PRO-UFL. As of now, we aren't very aware of what to watch out for this time and we would really appreciate some support.

 

[gtg738w]

The speed of the rocket will cause a frequency shift (Doppler). Some modulations are very sensitive to frequency but LoRa uses a chirp and looks for the slope of the frequency. I think Semtech recommends 25% of the bandwidth for crystal tolerance. That is big enough you shouldn’t have an issue but you can calculate the expected shift and look up your crystal tolerance and see what your minimum bandwidth setting should be.

 

[thequick]

I’d be interested in hearing what you find out. The speed of rf (light) is about one million times faster than the speed of sound. I’d think any Doppler shift from going mach ( or a little faster) would be minuscule. But maybe you see issues because the frequency is so high?

 

[mikec]

The change in frequency is v/c*f0 so at 433 MHz the frequency shift at say 1000 m/s (about mach 3) is 1000/3e8*433e6 = 1443 Hz. That won't make any difference at all.

 

[cerving]

If your apogee is going to be 6 km, you're probably not going to be supersonic for more than a second, two at the most. You'll get your telemetry back when the rocket starts to slow down. Even then, your Doppler shift at Mach 2 is only about 2 KHz for a 915 MHz signal (see https://www.rfwireless-world.com/calculators/doppler-frequency-calculator.html), chances are that your receiver's bandwidth will cover it.

 

[thequick]

A Doppler shift of ~.003%, correct? (At max v. ). As noted, apogee velocity will be near zero.

 

[JoePfeiffer]

I'm curious who told you Doppler shift would be significant at those speeds?

 

[user 31826]

I'm curious who told you Doppler shift would be significant at those speeds?

They didn't say it would be significant. They just said that there's an effect. We just wanted to learn more about the topic.

 

[ProjectG]

This is a problem I'm trying to troubleshoot with my own tracking system that uses a Hoperf95W LORA module. I suspect rather than the speed and doppler shift, it's the acceleration affecting the transmitter that is the issue. For my set up, on a flight that reached a max acceleration of 8 g the tracking system worked great I had RF links all the way up and down. I did another flight with the exact same system that reached 17 g, and I lost the RF link the moment it launched, I didn't hear from it again until it was coming back down under parachute. From my research, and it would be great if anyone else here can confirm this, the crystals in these units shift their oscilating frequency when under acceleration, this can cause the transmistter frequency to drift enough that the reciever can no longer pick it up.

 

[UhClem]

Yes crystals are acceleration sensitive. Some more than others and orientation matters.
https://web.archive.org/web/20041110225836/https://mwrf.com/Articles/ArticleID/8990/8990.html
But I don't think that 17G's is enough to cause much trouble. There is probably something else going on. A RF link analysis (received signal strength) is probably in order. A minimum 15dB margin is the gold standard here.

 

[AllDigital]

I've been launching a telemetry package to about 6Km regularly and the test flights are in the Mach 1.5-2.0 range with g-force of around 20-24 gees. I'm using a two watt serial TTY radio on 433 Mhz and I never have dropped packets. In my testing, I've found that in addition to the radio, the quality/configuration/tuning of the antennas on both ends makes the biggest difference. I tested a dozen different types of antennas before finding the ones that work best for my package (different for each rocket).

 

[UhClem]

the quality/configuration/tuning of the antennas on both ends makes the biggest difference

The receive antenna should be circular polarized. With the typical linear polarized transmit antenna you get a constant 3dB polarization loss but this is much better than what happens if you end up with linear polarization that is 90 degrees off.

 

[CoyoteNumber2]

Quasi-related - an example of Doppler shift causing communications problems:

https://en.wikipedia.org/wiki/Huygens_(spacecraft)#Critical_design_flaw_partially_resolved

 

[OverTheTop]

Quasi-related - an example of Doppler shift causing communications problems:

Interesting article. OP please note that it does not relate to your question though .

I will check my M2.14 flight from last year and see if telemetry was solid all the way up. I don't recall seeing any dropouts during flight.

[EDIT] Confirmed. I had good packets at peak speed of M2.14 on my TeleMega telemetry system. I was on 435MHz for telemetry for that flight.