Falcon 9: Why does F9 first stage always lose Video Link during Entry Burn?

Question

Typically the F9 first stage and the drone ship each lose the video connection once during landing:

F9 first stage during entry burn
Drone ship during F9 first stage touch down (see here and here)

While item #2 has been discussed at length, my question is focusing on bullet #1.

In this video of the June 13 Starlink mission you can see the beginning of the entry burn.

Immediately on it the satellite connection regularly fails (signal loss, also visible in video). What is the reason for losing the live signal during entry burn?

The commentator says "We did lose the live signal of the first stage as expected" - why?

@uhoh: Those two questions target on the signal loss during touch down on the droneship. My question, however, targets on the signal loss during entry burn (which could also be reasoned by vibrations, but who knows?). — Dave, Jun 13 '20 at 23:12
I see that you'd linked one of them already. I made a small edit, how does it look? — uhoh, Jun 13 '20 at 23:21
Uninformed guess would be something akin to https://en.wikipedia.org/wiki/Communications_blackout#Spacecraft_reentry. Going up the ideal nozzle design leaves exhaust stream at low relative velocity to the atmosphere. For the entry burn the exhaust will be interacting with the atmosphere at much higher velocity/energy and more likely to do have exotic chemistry going on. Unsure what the relevant search terms would be for this though. — GremlinWranger, Jun 14 '20 at 01:10
OTV-5 mission has the complete reentry video, it looks like it was streamed live - https://m.youtube.com/watch?v=9M6Zvi-fFv4 — Joe Jobs, Nov 30 '20 at 19:18
In this video the link is lost after the entry burn - https://m.youtube.com/watch?v=W0MGgQZIYNk — Joe Jobs, Dec 18 '20 at 11:09
NROL 108 also has the complete video and, also like OTV 5, it is returrning to the lanunch site. Maybe they cut the link of sea landings in order to hide the location of the drone ship. — Joe Jobs, Dec 20 '20 at 15:40

score 4 · Accepted Answer · answered Jun 16 '20 at 20:39

4

Almost certainly this is a combination of two things.

The rocket when thrusting is more prone to vibrations.
The rocket is going through its own exhaust, which is a plasma soup.

My guess is the second is the primary cause, that going through the exhaust increases the RF noise, making it difficult to be understood. This was a well known problem, RF blackouts often happen during reentry, and only have been resolved by adding in satellites. But that plasma isn't caused by rocket exhaust, which is making things even more difficult.

answered Jun 16 '20 at 20:39

PearsonArtPhoto

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This is really interesting! So I've just asked Radio opacity of rocket exhaust? – uhoh Jun 18 '20 at 01:35
Then why they don't cut link at RTLS? (NROL 108 most recent). And why they cut link after entry burn at Starlink L12? – Joe Jobs Dec 20 '20 at 15:46
RTLS uses physical connections, which are robust to these kinds of problems. As for the last one, I have no idea, that sounds like a totally different question. – PearsonArtPhoto Dec 21 '20 at 13:49

score 0 · Answer 2 · answered Sep 01 '21 at 13:43

A big part of the problem is that the first stage is also very close to exiting the communications range from the Cape, and is losing altitude rapidly, tucking itself behind the horizon in the process.

Note that the second stage comms has by then already been handed over to Bermuda, as it is a good bit further downrange.

When the Falcon 9 is returning to landing site, it never exits line of sight with comms from the Cape, and should be able to retain link all the way down.

Falcon 9: Why does F9 first stage always lose Video Link during Entry Burn?

2 Answers2