Why are solid rockets considered unsafe?

[Stinkk]

Constantly I hear people saying that solid rockets should never be used for manned flight and how unsafe they are.

But looking at it objectively, to me it seems a little unfounded;

In terms of statistics, every shuttle launch consisted of two solid rocket motors.

Ok so one of the shuttle explosions was the result of a solid rocket failure, granted. But if you look at it there were numerous faults with the RS-25 engines thought out the shuttle history. statistically the solids did twice as many launches, and only 1 failure (albeit spectacularly).

You have to consider however the other shuttle explosion was a result of falling ice damaging the orbiter, a result of using cryogenic liquids required for both kerosene and hydrogen fuels.

Add to that Apollo 13 was a result of having cryogenic fuels, requiring stirring, electrical systems to keep them frozen etc..

And it's not just the fuel you have to worry about, with liquid oxygen, almost anything will ignite, don't forget Apollo 1.

Both hydrogen and oxygen slowly turn to gas even with the best insulation, this means you also have to worry about venting to avoid dangerous pressure build up.

I mean to use any liquid fuels you need lots of high pressure, high vibration tolerant machinery, and plumbing, high speed moving parts, electrics, just a whole series of points of failure.

To me it seems like mixing a high explosive liquids (hydrogen or kerosene) and a cryogenic highly reactive oxidiser under extremely high pressure is a lot more dangerous, and causes a lot more failures than a relatively inert fuel rubber mixture.

[Jaydee]

Because once it's going, you're going. You can't switch SRB's off

[K^2]

There are several parts to it. First, as Jaydee says, you can't shut a solid booster off. If you have a problem with it, you have to either ride it out or jettison it. There are no other options. Then there is the fact that a smallest crack in the fuel can lead to an explosion. It's rather difficult to inspect a solid engine for things like that. Finally, the biggest issue. When the SRB blows, the whole thing does. All of your fuel is sitting within microns of all of your oxidizer. It's a powder keg, almost literally. If your liquid engine explodes, it's bad, but all of your fuel won't go into a fireball all at once. You have a chance to fire up an emergency escape system. With boosters, the whole thing will be over before even automatic systems can react to it.

Liquid engines come with their own hazards, of course, but most of these are things where you'd have some sort of control over it. Apollo 13 came home safely. Columbia did not, but at least there was a window of opportunity for problem to be detected. With Challenger there was never even a chance. The shuttle was doomed while standing on the launch pad.

[Albert VDS]

Also the exhaust plume will eat right through your parachute if you had to abort and ended up in the plume.

[Fuzzy Dunlop]

Well the Challanger SRB is the only stage of any launch vehicle ever to directly kill its riders (though a Soyuz has come very close) so in that sense they are empirically more dangerous.

Also you can turn liquid engines off, which is especially useful on the pad. The Space Shuttle did this on several occasions. With solids you might not know about a fault until you launch it, at which point it's too late.

Another thing is that when solids fail they tend to explode spectacularly, which liquid engines generally don't do. Falcon 9 has survived a engine failure on it's first stage, if the same thing happened on a Delta 2 strap on the entire rocket would be toast.

Also solids tend to cause more vibrations than liquids, which can shake around vital equipment.

[Themohawkninja]

The only way to failsafe an SRB is to make it's casing so tough, that if all of the propellant combusted, it wouldn't explode as much as turn into a giant bullet, and seeing as the shuttle's SRB's, as tough as they are, still can explode via a self-destruct, and those things can withstand explosions themselves, the weight of a failsafe SRB would probably make the SRB really ineffective.

On the other hand, a liquid engine can simply shut it's pumps down (assuming the failure didn't destroy too much of the engine), and (this is just a thought, I doubt any real engines can do this) you can bleed off the liquid fuel so you don't build pressure, whereas you can't bleed off a solid.

[SargeRho]

In the majority of losses of vehicles with rockets that use SRBs, the SRBs caused the accident. When you light an SRB, it's not stopping. Hybrid rockets in turn, are like SRBs but oxidizer in a separate tank, and can thus be turned off and throttled.

[Stinkk]

But yeah I see your point about the difficulty of escaping a failing engine, I was just reading an article about the problems with Ares-1, that since the solid motor produced so much thrust so deep in the atmosphere it was extremely difficult to design a rocket motor for the escape system that could produce enough thrust on top of that during max Q faced with such wind blast, and even if it could it would probable cause excessive Gs.

I think a lot of the objection towards solids rockets is based on common misconceptions, namely;

Solid rocket engines can be shut off - by cracking them open, the space shuttle had pyrotechnics for this, - or if your in a vacuum by opening both ends of the rocket up.

Also the exhaust plume eating through parachutes is a problem with liquid fuels as well, just look up what happened to the Apollo 15 'cute.

Also a liquid rocket can explode fast enough to make little distinction between the two.

Just wanted to clear those two points up, they are almost everyone's go to in the discussion.

[Themohawkninja]

But yeah I see your point about the difficulty of escaping a failing engine, I was just reading an article about the problems with Ares-1, that since the solid motor produced so much thrust so deep in the atmosphere it was extremely difficult to design a rocket motor for the escape system that could produce enough thrust on top of that during max Q faced with such wind blast, and even if it could it would probable cause excessive Gs.

I think a lot of the objection towards solids rockets is based on common misconceptions, namely;

Solid rocket engines can be shut off - by cracking them open, the space shuttle had pyrotechnics for this, - or if your in a vacuum by opening both ends of the rocket up.

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.

Also the exhaust plume eating through parachutes is a problem with liquid fuels as well, just look up what happened to the Apollo 15 'cute.

No arguments here. Rocket exhaust on a parachute is equally bad either way.

Also a liquid rocket can explode fast enough to make little distinction between the two.

Keyword there is "can". As I have stated, there is a chance that you can keep the rest of the liquid from igniting.

[K^2]

Also a liquid rocket can explode fast enough to make little distinction between the two

It might look like equally fast to human eye, but the actual times aren't even in the ball park. The liquid fuel has to mix with oxidizer to ignite. That takes time. On large rockets, quite a bit of time. We are talking order of seconds here. Still almost instant to human eye, but long enough for electronic systems to respond and even do something about it. SRB is already a mix of fuel and oxidizer. Once the shockwave starts, it goes as fast as that shockwave propagates. We are talking small fraction of a second. Even if you have time to register it, you simply can't outrun this explosion.

And as Themohawkninja pointed out, an explosion of a liquid rocket's engine at least has a chance to go quietly. With SRB, there is no such chance. The only mode of failure is catastrophic explosion.

[DaRocketCat]

Space Shuttls SRB's never posed a problem, the infamous O rings did.

[sojourner]

Space Shuttls SRB's never posed a problem, the infamous O rings did.

Which was part of the SRB.

[DerekL1963]

Because once it's going, you're going. You can't switch SRB's off

That's the thing - it's a rare thing that you actually need to shut them off. Generally, just stopping them from providing net thrust is sufficient - and while that's non-trivial, it's old tech.

The real reason why solids are considered unsafe (by professionals) isn't that you can't neatly shut them off, it's that they pretty much have only one failure mode - BOOM! Worse yet, almost never do they give any indication that they're about to self disassemble.

[Stinkk]

Ahh I see, so while they are less prone to failure and fail less often, if they do it's catastrophic.

[DerekL1963]

That's pretty much it in a nutshell.

[Motokid600]

Correct me if I'm wrong, but the process of repacking the SRB's with solid fuel is a terrifying job.

[drakesdoom]

Correct me if I'm wrong, but the process of repacking the SRB's with solid fuel is a terrifying job.

Once it is established that rockets are slow controlled bombs I think it becomes clear any task involving rocket fuel of any type is terrifying.

[Apotheosist]

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.

[DerekL1963]

Correct me if I'm wrong, but the process of repacking the SRB's with solid fuel is a terrifying job.

Not particularly I suspect. It depends on what you're used to. But as I've said before, my opinions on such thing are suspect... I found it no problem to be (mumble) feet under the surface of the ocean accompanied by sixteen solid fuel rockets and a nuclear reactor - though I can neither confirm nor deny their were nuclear weapons present as well.

[asmi]

Another complication of using solids is that you can't transport the rocket empty to the launchpad and then fuel it there. Since vast majority of rocket mass is fuel, it makes a lot of difference when it comes to GSE costs, not to mention safety issue of handling fully-fueled SRBs vs empty liquid stages - latter can't explode since there is no fuel.

[Psycix]

SRB's are nothing but large scale pyrotechnic devices, which says enough about the required safety when handling them. Aside from the launch itself, things can also go wrong during production, assembly and transport.

There can be dozens of people working around the solid propellant, in some cases much more people than one crew of astronauts.

A few examples:

http://en.wikipedia.org/wiki/Brazilian_rocket_explosion

The rocket exploded on its launch pad at the Alcântara Launch Center killing 21 people.

http://en.wikipedia.org/wiki/PEPCON_disaster

The fire and subsequent explosions claimed two lives, injured 372 people, and caused an estimated US$100 million of damage.

http://www.waff.com/Global/story.asp?S=12429371

2 men die from injuries in Redstone Arsenal explosion

http://en.wikipedia.org/wiki/List_of_space_disasters

The third stage of a Delta rocket had just been joined to the Orbiting Solar Observatory satellite in the spin test facility building at Cape Kennedy. Eleven workers were in the room when the 205 kg of solid fuel in the third stage ignited. Sidney Dangle, 29; Lot D. Gabel, 51, and John Fassett, 30, were severely burned and later died of their injuries. Eight others were injured, but survived. The ignition was caused by a spark of static electricity.

A Titan 4 launch vehicle solid rocket booster was being hoisted by a crane into a rocket test stand at Edwards AFB, California. The bottom section of the booster broke free, hit the ground and ignited. One person, Alan M. Quimby, 27, a civilian employee of Wyle Laboratories, was killed and 9 others were injured in the accident.

Edited August 16, 2013 by Psycix

[Seret]

Another complication of using solids is that you can't transport the rocket empty to the launchpad and then fuel it there.

I'd imagine transporting one is a delicate job too. When I was in the military and handling small solid rocket motors in aerial rockets and ejection seats a fair bit of care had to be taken with them. If you crack the propellant it'll burn too rapidly, the result of which is splode instead of zoom when you press the button.

[Rocketscience101]

I'd like to point out that the solid rocket motor on Challenger performed well, in fact they had to be destroyed on their own, because they themselves did not blow up. It was a structural failure on the side of the booster, that can also happen to a liquid rocket if not carful. An O ring, around the nozzle had expanded and contracted in the extra cold day of the launch, and allowed flames to jet out of the side of the right booster. If you slow the video down, you can see what happened. It was no fault in the fuel, it was just a mistake made before launch.

[Nuke]

i think it was more of an operational error than anything. they launched at a temperature below which a part (an o-ring) was rated. that should have been caught and the launch scrubbed till it warmed up.

[Rocketscience101]

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.