Solid Rocket Motor Blowout Panels?

[Exosphere]

Since one of the main causes of SRB failure is overpressure (either due to design issues or production problems such as cracks or fissures in the grain), would it make sense to place "blow-out panels" on solid rocket motors used on manned vehicles such as the Atlas-V/SLS?

The idea I had was to intentionally create a small region on the booster that's slightly weaker than the rest of it (i.e. by etching the interior of the booster so that the wall is a bit thinner in that area). Obviously, even the blow-out panel would be able to handle the pressure produced by the SRB plus the standard factor of safety (I believe it's 1.25 in rocketry), but if it was a bit weaker than the rest of the booster, then a pressure failure would simply blow out the panel and extinguish combustion due to pressure loss, instead of destroying the whole rocket. Of course, doing so would force a mission abort, but it would seem that such a design would give the crew of the rocket a greater chance of survival, since the rocket as a whole would survive the initial rupture, allowing the LES to pull the capsule to safety, rather than the whole rocket exploding, potentially before even an auto-abort system could be activated.

As a disclaimer, I'm no expert on rocket design, so this may be a completely stupid idea. Even I can think of some potential issues -- intentionally weakening a section of an SRB could lead to production issues whereby the panel could accidentally be made too thin (i.e. due to someone setting the tool used to create the panel to an incorrect thickness), and the rapid rise in aerodynamic drag caused by blowing out a section of the SRB could cause the launch vehicle to tumble or break-up, complicating the design of an effective abort system.

However, since it appears that there are a considerable number of knowledgeable people on this forum who may be able to answer my question, would such a design in any way improve launch vehicle safety? Is it (even remotely) a good idea?

[Blue]

Having a blowoff panel probably would cause the booster to structurally fail if it was ever used while the booster was at speed due to the change in its aerodynamic profile. Unless the area surrounding the panel was extremely strong, which would cost a great deal of booster mass. I don't think such a thing would be justified, rather than having perhaps some kind of cutoff system mounted above or inside the nozzle, to stop solid fuel flow.

[Godot]

But wouldn´t the sidetrust that would be produced by the Jetstream exiting the blowout panel make the spacecraft going into an uncontrollable spin?

[Exosphere]

Having a blowoff panel probably would cause the booster to structurally fail if it was ever used while the booster was at speed due to the change in its aerodynamic profile. Unless the area surrounding the panel was extremely strong, which would cost a great deal of booster mass. I don't think such a thing would be justified, rather than having perhaps some kind of cutoff system mounted above or inside the nozzle, to stop solid fuel flow.

But wouldn´t the sidetrust that would be produced by the Jetstream exiting the blowout panel make the spacecraft going into an uncontrollable spin?

Yeah, I would assume that actually "blowing" the panel would cause the rocket to break up and/or tumble. The idea was that it might buy the abort system time, allowing the crew to be pulled away before a catastrophic failure occurred. The rocket probably wouldn't survive long after the blowout panel was activated.

[zekes]

Unless the blowout panel was at the top, then it might not be so severe.

[cpast]

Having a blowoff panel probably would cause the booster to structurally fail if it was ever used while the booster was at speed due to the change in its aerodynamic profile. Unless the area surrounding the panel was extremely strong, which would cost a great deal of booster mass. I don't think such a thing would be justified, rather than having perhaps some kind of cutoff system mounted above or inside the nozzle, to stop solid fuel flow.

Solid fuel doesn't flow. A solid-fueled rocket will keep burning until it's burned through all the fuel, or the rocket itself comes apart. The idea of blowout panels is to minimize thrust, but you can't stop the burn.

[K^2]

Should work. If it replaces the top plug, it should be able to fail without blowing the whole booster to bits. Of course, this only makes sense assuming that there is nothing above, or even close to the blowout panel. It will torch anything nearby. That will severely limit applications of such booster.

[Exosphere]

Solid fuel doesn't flow. A solid-fueled rocket will keep burning until it's burned through all the fuel, or the rocket itself comes apart. The idea of blowout panels is to minimize thrust, but you can't stop the burn.

Doesn't the rate of combustion change with the internal combustion chamber pressure? I thought that if the pressure inside a solid/hybrid rocket motor falls to or near that of the outside atmosphere, the motor would stop burning.

On another note, what's the general safety info on hybrid rocket motors that use non-monoprop oxidizers like liquid oxygen? It would seem like they'd be safer, since they're more tolerant of fuel grain errors than liquids, and are quite simple if a pressure-fed, pintle injector, valveless design is used. This, combined with the fact they use stable, nontoxic, cheap propellants like LOX and paraffin, and don't require special treatment while being handled, would seem to make them a good low-cost alternative.

The only problem with hybrids seems to be that:

1) They have a very low regression rate (although the paraffin-based fuel used by SPG seems to solve that issue).

2) From what I've heard from Copenhagen Suborbitals' use of hybrids, they seem to be prone to combustion instability (could a pintle injector solve this problem? I've heard that they're inherently stable).

3) Because of problem 1, a complex "wagon wheel" grain has to be used in large hybrids to develop acceptable thrust levels, which both reduces the mass fraction of the booster to ridiculously low levels (Copenhagen Suborbitals' page on their hybrid motors indicates that their motors have a fuel fraction of just 40%!), and causes the grain to weaken and fall apart during the burn, which causes chunks of fuel to be ejected from the nozzle. A high regression rate fuel like paraffin would probably solve this problem.

[cpast]

Doesn't the rate of combustion change with the internal combustion chamber pressure? I thought that if the pressure inside a solid/hybrid rocket motor falls to or near that of the outside atmosphere, the motor would stop burning.

Huh, did not know that. I always thought the idea was just to eliminate net thrust.

[Exosphere]

Huh, did not know that. I always thought the idea was just to eliminate net thrust.

Well, like I said, I'm not even remotely an expert on the subject, but I think that's what I've heard about how SRBs work.

I may be totally wrong, though, so don't take my word for it.

[cpast]

Well, like I said, I'm not even remotely an expert on the subject, but I think that's what I've heard about how SRBs work.

I may be totally wrong, though, so don't take my word for it.

I'm even less of an expert, I bet. Also, on searching with some new key terms, it looks like sudden pressure decrease can stop combustion (at the cost of ruining the SRB, but if you're aborting that was gonna happen anyways).

[K^2]

Doesn't the rate of combustion change with the internal combustion chamber pressure?

It does. In fact, the relationship is exponential, and the relevant exponent is one of the main characteristics of solid fuel.

That said, even at ambient pressure, it will be burning at a very high rate. So pressure drop won't exactly extinguish the booster. But it's way better to have a burning booster than an exploded one.

[magnemoe]

Unless the blowout panel was at the top, then it might not be so severe.

Know that ICBM uses blowout panels to control burn length with solid fuel stages so it look like it stop trust pretty cold without danger to upper stages.

They uses explosives who are triggered, the only problem as I see is that venting all the burning solid fuel might blow up the main stage, however of you only blow a panel on the part who points outwards this should work.

in this setting you lost the rocket, you only want to make sure no explosion damages the pod and that the SRB don't separate from main stage and come after you.

If you get catastrophic over pressure, or leaks in a SRB you activate the abort who separate pod, activate the escape tower, stop main engines and blow out the panels.