[ghostfish]

>"Panels designed to relieve pressure within the engine bay were ejected to protect the stage and other engines."

Can anyone that knows something about the Falcon 9 design or rockets in general shed some light on this? That sentence makes it sound like the panels were purposefully jettisoned, which doesn't make sense to me. What do those panels do, what do they look like, and where are they?

[stephengillie]

Purposefully jettisonning panels is a way of preventing an explosion from damaging the rest of the ship. The rocket creates energy in a direction, but what if something prevents the energy from going in that direction? The energy, in the form of expanding gasses, has to go somewhere. If these panels didn't break away to give the energy somewhere to go, it could have forced into another rocket or into the ship's body, causing a much bigger problem.

[luke_s]

When they say "Jettisoned", it makes it sound like one of the flight computers decided to drop the panels. Wouldn't it more be the case that the panels are deliberately designed to be blown off? I have a hard time believing that any control system could react in time in case of an engine explosion...

[stephengillie]

The panels may have been designed to jettison once they reached a certain pressure differential, or they may be jettisonned by computer.

How quickly do engines explode? Is it faster than 2 cars colliding? Computers deploy air bags quickly because electricity travels faster along wires in your car chassis than the car travels into something else.

Visualize a bumper with sensors - as the bumper is deformed by a collision, a sensor shifts an electron in the copper wire, and the electron next to it shifts. There's a cascade of shifting electrons along the wire, and it races backwards through the car's body, chased by the destruction of the car as it collides with another object. The cascade of electrons hits the air bag computer, which begins another cascade of electrons to the air bag. The wave of destruction has covered most of the distance to your windshield by now. The air bag deploys as molecules of air rush from their high-pressure canister to fill it. As the bag hits its most pressurized point, your car is coming to a stop as its kinetic energy is combined with energy from the other object.

[schiffern]

Beautiful. A small correction – most airbags use pyrotechnic inflators, not compressed air.

Sidenote about airbags: they have to be folded to fit inside their module, so as it inflates it's also unfolding. In order to make sure it unfolds properly they coat it in a lubricant that can't evaporate – either talcum powder or cornstarch depending on the vehicle.

I learned this only after I scrambled out of the car my sister put in a ditch thinking it was on fire. The best part? It was a diesel car.

Who knows, maybe the engineers consider it a feature because after an accident it sure gets people out of the car quick!

[Swizec]

Most beautiful explanation of anything involving physics I have ever read.

[confluence]

Good god - you need to write books - or make movies :D.

Wonderful explanation.

[Pyrodogg]

I would venture to guess they're built to work both ways. If the computer has sufficient time and information to make the decision it releases the panels. Ex. Engine failed, shut it down, blow panel as precaution to drop pressure in that engine compartment.

They're also most likely built as the designated point of failure, like a safety valve on a boiler. If the pressure builds too high they are blown out by the pressure. Ex. engine goes critical and explodes before computer can react, the panels fail before other structural components do.

[schiffern]

They said "ejected", not "jettisoned". You can be 'ejected' from a vehicle if you're not wearing your seatbelt, so the word doesn't require active response.

[jlgreco]

So are these panels kind of a sort of relief valve?

[maaku]

They serve a similar function, yes.

[jccooper]

They probably refer to breakaway panel fairings above the engine isolation armor. Space X has an armor "cup"around the dangerous parts of the engine, and put fittings on top of that for some reason; aero probably.

Their statement suggests these came off when pressures radically changed during engine cut off, which is plausible. It was at max aerodynamic pressure, and removing all the pressure the motor generates is a big swing. I find it more likely that the nozzle (bell) shattered, as it is more exposed to both aero and combustion pressures. Maybe both happened.

A visual guide:

1) naked engine: http://farm3.static.flickr.com/2355/2230438397_487afd40a2.jp...

When a liquid engine fails energetically, it's usually going to be a failure in the chamber. Nozzle or throat burn-through or other failure may also happen. The turbo pump might also let go and shred some stuff nearby. Plumbing failures may also kill the engine, but not destroy much; there are plenty of valves to fix leakage before it gets bad.

2) un faired: http://www.spacenews.com/images/Falcon9engines_SpaceX02.jpg

Here you can see the armored tub around the chamber section of each engine. It is meant to contain any problems. Mostly these would be hot gas from holes burned in the chamber or throat, fuel or oxidizer from leaks in the plumbing, or shrapnel from the turbo pump coming apart. Note that the other major failure mode, excessive vibration, cannot be armored against, but that is more a design thing than a random failure.

Most to all of these failures are easily detected by various pressure and flow sensors, and usually before they become big problems. Turn off the propellant valves, and the engine rapidly becomes safe, though off. But the armor does keep the neighbors safe from any problems, presuming it holds. I would guess it is good enough for most failures.

3) faired vehicle: http://www.brendans-island.com/blogsource/20101015ff/falcon-...

Here you can see pretty white fairings hiding everything but the nozzles. Note that they appear to be a bunch of different pieces. Probably that is because they are meant to break away individually if something goes wrong.

Those are what I think the statement refers to. I doubt they are actively jettisoned, but may certainly be designed to pop off in an over-pressure situation. One could probably compare vs the video to see if the pieces look like that, the nozzle, or other engine parts. The corner fairing might also have failed under some circumstances.

Having seen the video in slow mo a few times, I think their assessment is plausible. Probably they will be able to tell what happened by telemetry. I don't know if they are still trying to recover stages, but if they are, they might get their hands in some physical evidence. Whether we will ever see any of this data, though, I don't know. They've been reasonably open in the past.