Why are solid rockets considered unsafe?

[The Jedi Master]

Basically, it's an explosion that is kind-of-controlled-but-not-really and pushes the ship up. And it can't be turned off until it runs out of fuel. I don't want to be anywhere near one of those those things when it goes off.

[wasmic]

As others have pointed out, the danger of SRB's is that the oxidiser and fuel are pre-mixed. In a liquid or hybrid rocket, the fuel and the oxidiser is kept separately.

[Nuke]

this thread needs more explosions.

[nhnifong]

Yup. It's a fine line between a solid rocket and a leaky bomb, and in fact, as the performance of solid rockets increases, (higher chamber pressures, lighter casings) the solid rocket tends towards being a bomb.

I think a successful bomb actually needs a strong casing to contain the pressure until it builds to an optimal point before rupturing, therefore releasing the pressure during a shorter span of time, and producing a more destructive shock wave.

[Fuzzy Dunlop]

I think a successful bomb actually needs a strong casing to contain the pressure until it builds to an optimal point before rupturing, therefore releasing the pressure during a shorter span of time, and producing a more destructive shock wave.

For Low explosives like gunpowder and rocket fuel yes. For High explosives like TNT and C4 no.

[Dangerous_Beans]

Not all SRB's are outfitted with stopping capabilities, and the ones the shuttle used had pyrotechnics which BLEW UP the SRB's, they didn't just shut them down.

I think it's pretty cool that the emergency fail-safe is to just blow it up.

Though I don't think I will change career to work with them.

[KvickFlygarn87]

Can't shut 'em down? Couldn't you possibly make something to shut the nozzle., thus suffocating the fire in the engine?

[Dangerous_Beans]

Can't shut 'em down? Couldn't you possibly make something to shut the nozzle., thus suffocating the fire in the engine?

Stopping air from getting to a fire stops the fire by blocking the supply of oxygen. Rockets however supply their own oxygen, otherwise they wouldn't work in space where there is no air. so blocking the nozzle of a solid rocket engine doesn't stop it burning, but just makes it explode instead of going up.

Edit: which would be kind of cool, and at times may be preferable to having it go up, but probably not a good idea for a IRL space program.

[KvickFlygarn87]

Oh. But what if you cot off all sources to oxygen?

[drakesdoom]

Oh. But what if you cot off all sources to oxygen?

The problem is an SRB is a big pile of solid rocket fuel that is pre-mixed with oxygen and fuel and then lit on fire. Once lit it burns or blows up there is nothing else that can happen.

[Nibb31]

Oh. But what if you cot off all sources to oxygen?

You're not getting it. Solid rockets use a chemical compound mixture of fuel and oxydizer, like gunpowder. You can't cut off the oxydizer because it is mixed into the propellant.

The method used to terminate thrust on an SRB is to rip open the casing with an explosive cord that runs up the side of the booster. This effectively blows it up by rapidly burning all of the propellant at once outwards instead of letting it burn slowly through the nozzle.

Shutting the nozzle would terminate thrust, but wouldn't stop the combustion, so the SRB would explode uncontrollably under pressure.

[Nuke]

close off the nozzel you no longer have an srb, instead you have a giant pipe bomb.

[Seret]

For Low explosives like gunpowder and rocket fuel yes. For High explosives like TNT and C4 no.

Depends what kind of bomb it is. Most military bombs do have a tough case, and do indeed expend a huge amount of their energy fracturing it. That's because fragmentation is part of the show. Blast is really good at shoving things around at fairly short distances or in confined spaces, but frag tends to shred things instead of pushing them.

There are of course many exceptions to this, but in general heavy casings on HE weapons are very common. The ubiquitous Mk80 series of aircraft bombs, for example, are less than half explosives by weight.

[RRoan]

I think it's pretty cool that the emergency fail-safe is to just blow it up.

Not quite; there was a linear shaped charge running most of the length of the booster that effectively "unzipped" the side of it. This has two consequences: it decreases the thrust, since the gas can now escape out the side instead of the nozzle, and it releases the pressure in the chamber. Since the burn rate of solid fuel is very strongly dependent on the chamber pressure, this considerably slows the burn rate and, ideally, would prevent a detonation. The shuttle SRBs were originally going to have a thrust-termination system consisting of upwards-facing nozzles that would counter the main nozzle if opened (and relieve pressure as well), but this requirement was dropped very early on.

[Mr Shifty]

No they would have had to replace the O ring, after the cold weather, it would never have fit perfectly. It was a simple mistake, that cost more than the simple fix, but we learned from those mistakes.

There were noted O-ring deficiencies in at least 9 earlier Shuttle flights, including blow-by and erosion of the o-ring seal. The damage was correlated with lower temperatures, and Thiokol had previously expressed reservations about low-temp launches. It's true that by the time the STS was launched, there was no saving it, but the accident could have easily been prevented.

Note that the boosters worked fine. The failure was caused by the flame from the o-ring leakage impacting the external tank and weakening the joints holding the boosters to the Shuttle assembly. The flames burned a hole in the side of the ET, releasing a cloud of liquid oxygen that, as it expanded, provided a huge thrust to the assembly, then the H2 ignited, creating more thrust. The orbiter itself was mostly undamaged by flames; it was torn apart by the extreme aerodynamic forces caused by the excess thrust. There was never, for any Shuttle launch, any possibility of a survivable abort during the SRB burn.

[magnemoe]

close off the nozzel you no longer have an srb, instead you have a giant pipe bomb.

Opposite is also true, if you make the fuse hole on an pipe bomb to large it become an rocket.

Yes we was in an hole 7 meters from another hole with the bomb. The wannabee rocket bunched out of its hole and into ours where it was buzzing around. Later we tried to make actual rockets but worked as well as the pipe bomb.