7 ms·

So I know very little about rockets, but here's my simplified understanding: The liquid oxygen tank provides the liquid oxygen which fuels the rocket. They use

by Osmium 11y ago

So I know very little about rockets, but here's my simplified understanding: The liquid oxygen tank provides the liquid oxygen which fuels the rocket. They use helium to keep the tank pressurised as the liquid oxygen gets used up (why helium? it's light and unreactive). The helium is stored in its own tanks, which are secured by struts like in this picture: http://i.szoter.com/741dc2bcf5762a48.jpg http://i.szoter.com/741dc2bcf5762a48.jpg (via reddit). The strut failed, causing a helium leak and a complicated series of events (including the helium tank bouncing around the liquid oxygen tank?), that basically resulted in the liquid oxygen tank being over-pressurised and exploding. Bonus (very cool!) gif of the liquid oxygen tank during a previous launch (unfortunately cameras apparently weren't included in this launch): https://i.imgur.com/WRp2ujX.gif https://i.imgur.com/WRp2ujX.gif (also via reddit) Some extra cool things: (1) They narrowed down the source of the failure using "acoustic triangulation", which, I think, is essentially using sound sensors (accelerometers?) located at various locations to pinpoint the location of the failure in 3D space. (2) The Dragon capsule could have been saved if it'd had the right software (which would deploy the parachutes). They'd already planned to do this, and will now have a software update for the next launch. Why hadn't they done this already? Because if the parachutes deploy accidentally, it could result in launch failure, so it's something they have to be careful about. But the capsule survived the explosion and they remained in contact with it until it was below the horizon.

deleted 11y ago
[deleted]
arcseco 11y ago
Not exactly relevant but there is a great video with narration(1) from a camera inside a kerosene tank from a Saturn 1 Rocket during flight. Gives appreciation to the amount of thought put into the design of something as simple as a pressurized fuel tank. (1) https://www.youtube.com/watch?v=fL-Oi9m2beA https://www.youtube.com/watch?v=fL-Oi9m2beA
ceejayoz 11y ago
> Bonus (very cool!) gif of the liquid oxygen tank during a previous launch (unfortunately cameras apparently weren't included in this launch) My understanding is that they have them on every launch, but bandwidth constraints with the downlink mean they have to pick between the many cameras to monitor in realtime.
MPSimmons 11y ago
Your simplified understanding matches my own.
ars 11y ago
> why helium? it's light and unreactive It's also one of the few gases that will stay at pressure even at the temperatures of liquid oxygen. Other choices are neon (but it's heavier) and hydrogen, which would react with the oxygen.
engi_nerd 11y ago
> (1) They narrowed down the source of the failure using "acoustic triangulation", which, I think, is essentially using sound sensors (accelerometers?) located at various locations to pinpoint the location of the failure in 3D space. An accelerometer is a poor device for doing acoustic measurement. At best, it can measure how the surface the accel is attached to responds to the acoustic environment (and that's only if the accel has a very high frequency response and is conditioned and sampled appropriately). For this kind of work they probably wanted to measure dynamic pressures within the tank, which they would most likely do with microphones. Other than that you have exactly the right idea.
- darkmighty 11y ago
  Well depends on their bandwidth. An accelerometer is essentially indistinguishable microphone also capable of detecting sounds all the way "DC" sound (where 'sound' is applied force on the accelerometer itself rather than a pressure plate that reacts to air pressure) -- if it's sampling rate is fast enough and it is coupled to the structure (i.e. the coupling system also has enough bandwidth) it is an acoustic sensor. Of course, a microphone was designed with high bandwidth in mind, while accelerometers probably value more precision around the DC input.
  - engi_nerd 11y ago
    Not all accelerometers have usable response at DC. The accels I have on my platform for dynamics measurement (high structural vibration and flutter) are meant to be AC coupled, for example. At rest they may give some garbage reading, but when they're vibrating, they accurately measure how the structure is responding. Your point remains though, if the accel has appropriate frequency response and is properly conditioned and sampled at a high enough rate, acoustic measurement is possible. What I was trying to say (maybe poorly) is that an accelerometer would not be my first choice for doing such work. But in the aftermath of an event like this, the data may be (and was, it sounds like) usable for such purposes.
baggers 11y ago
As final note the 'big explosion' was the air force blowing up the falcon9. This is standard procedure to stop the rocket doing damage in further locations and stopping a much of the fuel hitting the ground as possible (it's apparently nasty stuff).
- cloudwalking 11y ago
  For pedantry's sake: Falcon 9 blew itself up once an anomaly was detected. The Air Force kill command was not sent until dozens of minutes after the vehicle disintegrated.
  - MertsA 11y ago
    The rocket can initiate range safety itself like that?? What happens if a pressure sensor in one of the tanks fails a second after liftoff?
- phs2501 11y ago
  The F9's fuel is just rocket-grade kerosene, nothing particularly dangerous or exotic. The thrusters on Dragon on the other hand use monopropellent which is very toxic, but Dragon survived and impacted the sea.
  - hga 11y ago
    On the other hand, the oxidizer, LOX aka liquid oxygen, is a bit exotic and certainly dangerous in many ways. In general burning and dispersing as much of this and the rocket itself way up there is good, although of course the flight path is designed to be as safe as possible for failures.
- TeMPOraL 11y ago
  It's not really that nasty in terms of toxicity, it's mostly just that you don't want the huge fireball and resulting pressure waves to happen anywhere near people or infrastructure.

by Osmium 11y ago

deleted 11y ago

[deleted]

arcseco 11y ago

Not exactly relevant but there is a great video with narration(1) from a camera inside a kerosene tank from a Saturn 1 Rocket during flight. Gives appreciation to the amount of thought put into the design of something as simple as a pressurized fuel tank. (1) https://www.youtube.com/watch?v=fL-Oi9m2beA https://www.youtube.com/watch?v=fL-Oi9m2beA

ceejayoz 11y ago

> Bonus (very cool!) gif of the liquid oxygen tank during a previous launch (unfortunately cameras apparently weren't included in this launch) My understanding is that they have them on every launch, but bandwidth constraints with the downlink mean they have to pick between the many cameras to monitor in realtime.

MPSimmons 11y ago

Your simplified understanding matches my own.

ars 11y ago

> why helium? it's light and unreactive It's also one of the few gases that will stay at pressure even at the temperatures of liquid oxygen. Other choices are neon (but it's heavier) and hydrogen, which would react with the oxygen.

engi_nerd 11y ago

> (1) They narrowed down the source of the failure using "acoustic triangulation", which, I think, is essentially using sound sensors (accelerometers?) located at various locations to pinpoint the location of the failure in 3D space. An accelerometer is a poor device for doing acoustic measurement. At best, it can measure how the surface the accel is attached to responds to the acoustic environment (and that's only if the accel has a very high frequency response and is conditioned and sampled appropriately). For this kind of work they probably wanted to measure dynamic pressures within the tank, which they would most likely do with microphones. Other than that you have exactly the right idea.

darkmighty 11y ago
Well depends on their bandwidth. An accelerometer is essentially indistinguishable microphone also capable of detecting sounds all the way "DC" sound (where 'sound' is applied force on the accelerometer itself rather than a pressure plate that reacts to air pressure) -- if it's sampling rate is fast enough and it is coupled to the structure (i.e. the coupling system also has enough bandwidth) it is an acoustic sensor. Of course, a microphone was designed with high bandwidth in mind, while accelerometers probably value more precision around the DC input.
- engi_nerd 11y ago
  Not all accelerometers have usable response at DC. The accels I have on my platform for dynamics measurement (high structural vibration and flutter) are meant to be AC coupled, for example. At rest they may give some garbage reading, but when they're vibrating, they accurately measure how the structure is responding. Your point remains though, if the accel has appropriate frequency response and is properly conditioned and sampled at a high enough rate, acoustic measurement is possible. What I was trying to say (maybe poorly) is that an accelerometer would not be my first choice for doing such work. But in the aftermath of an event like this, the data may be (and was, it sounds like) usable for such purposes.

baggers 11y ago

As final note the 'big explosion' was the air force blowing up the falcon9. This is standard procedure to stop the rocket doing damage in further locations and stopping a much of the fuel hitting the ground as possible (it's apparently nasty stuff).

cloudwalking 11y ago
For pedantry's sake: Falcon 9 blew itself up once an anomaly was detected. The Air Force kill command was not sent until dozens of minutes after the vehicle disintegrated.
- MertsA 11y ago
  The rocket can initiate range safety itself like that?? What happens if a pressure sensor in one of the tanks fails a second after liftoff?
phs2501 11y ago
The F9's fuel is just rocket-grade kerosene, nothing particularly dangerous or exotic. The thrusters on Dragon on the other hand use monopropellent which is very toxic, but Dragon survived and impacted the sea.
- hga 11y ago
  On the other hand, the oxidizer, LOX aka liquid oxygen, is a bit exotic and certainly dangerous in many ways. In general burning and dispersing as much of this and the rocket itself way up there is good, although of course the flight path is designed to be as safe as possible for failures.
TeMPOraL 11y ago
It's not really that nasty in terms of toxicity, it's mostly just that you don't want the huge fireball and resulting pressure waves to happen anywhere near people or infrastructure.