Author Topic: Reducing slag from solid rocket motors  (Read 3571 times)

Offline drbuzz0

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Reducing slag from solid rocket motors
« on: 03/02/2013 01:23 AM »
Hi -

Hope this is in the right section of the forum.

I'm wondering if anyone happens to know if it is possible to reduce the amount of slag ejected from solid rockets by modifying the design, fuel composition or operation of the motor.

Yes, I checked Google and could not find any info.

The reason for interest is the problem of orbital debris.  As solid rocket motors burn out, it is common for them to eject chunks of slag, which is generally composed of aluminum oxide.  It can range in size from dust particles all the way up to chunks a few centimeters across.

Solid rockets are becoming more common as apogee kick motors and upper stages.   So the problem of slag as a contributor to orbital debris is a concern.


Is there anything, short of not using solid-fueled rockets that can be done to reduce slag ejection?

Offline Jim

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Re: Reducing slag from solid rocket motors
« Reply #1 on: 03/02/2013 11:48 AM »

Is there anything, short of not using solid-fueled rockets that can be done to reduce slag ejection?

That is what is being done.  With the demise of Delta II and movement from SRM AKM's to liquid APS, the use of SRM's is the exception vs the rule. 

Offline edkyle99

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Re: Reducing slag from solid rocket motors
« Reply #2 on: 03/02/2013 09:29 PM »
I think there may have been studies looking at varying the size of aluminum particles as well as propellant binder composition to either reduce debris or to encourage it to deorbit more rapidly (in transfer orbit missions at least), but I don't think that any changes could have eliminated the problem entirely.  I suspect that liquid stages leave their own version of such "debris", though less extensive.

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Offline AJA

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Re: Reducing slag from solid rocket motors
« Reply #3 on: 03/05/2013 03:07 AM »
Caveat lector: I'm thinking out loud, and this probably comes under advanced, back-of-envelope concepts.

Would it be hard to engineer some kind of temperature triggered buckling of the inside walls of the combustion chamber to kind of 'seal the solid rocket motor' after burnout?

Here's how I picture it to work. Picture a cylinder of some kind of thermosetting material that sits between the grain and the outer surface of the SRM (more toward the nozzle, than the top). If you design the grain geometry appropriately, specifically - calculate your thermal conductivities right, you can probably pick a material that reaches some sort of setting temperature after x minutes of booster operation.

Coupled with the drop in chamber pressure, this can then buckle inward and seal off the nozzle from a lot of the slag.

Now, instead of a ring, you can have them longitudinally offset, like petals or something and flaps. Heck, it can even be a bimetallic strip that bends after a certain time, and once chamber pressure has dropped
                             /| 
  ________________/_|____/
/|  Slag    \ Slag /                 ---
\|________\___/__________ --  ----
                            \ |        \
                             \|

Of course, the stress/temperature endurance and transition points required might be unavailable with current materials, it might impair booster performance if there's premature deployment or mechanical failure, and it might just not be worth the effort.

Having typed it out, now it striked even me as cartoonish. lol


----

Alternatively, would electrostatic precipitators work?
« Last Edit: 03/05/2013 03:14 AM by AJA »

Offline mlindner

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Re: Reducing slag from solid rocket motors
« Reply #4 on: 03/11/2013 02:05 PM »
If you do that design wrong I can easily see you just created a bomb. Sealed off small container with oxidizer and fuel.

Offline tnphysics

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Re: Reducing slag from solid rocket motors
« Reply #5 on: 03/19/2013 09:07 PM »
Caveat lector: I'm thinking out loud, and this probably comes under advanced, back-of-envelope concepts.

Would it be hard to engineer some kind of temperature triggered buckling of the inside walls of the combustion chamber to kind of 'seal the solid rocket motor' after burnout?

Here's how I picture it to work. Picture a cylinder of some kind of thermosetting material that sits between the grain and the outer surface of the SRM (more toward the nozzle, than the top). If you design the grain geometry appropriately, specifically - calculate your thermal conductivities right, you can probably pick a material that reaches some sort of setting temperature after x minutes of booster operation.

Coupled with the drop in chamber pressure, this can then buckle inward and seal off the nozzle from a lot of the slag.

Now, instead of a ring, you can have them longitudinally offset, like petals or something and flaps. Heck, it can even be a bimetallic strip that bends after a certain time, and once chamber pressure has dropped
                             /| 
  ________________/_|____/
/|  Slag    \ Slag /                 ---
\|________\___/__________ --  ----
                            \ |        \
                             \|

Of course, the stress/temperature endurance and transition points required might be unavailable with current materials, it might impair booster performance if there's premature deployment or mechanical failure, and it might just not be worth the effort.

Having typed it out, now it striked even me as cartoonish. lol


----

Alternatively, would electrostatic precipitators work?

No.

The problem here is that the slag produced by an SRM is a substantial proportion of the exhaust mass flow, and its ejection is a substantial fraction of the thrust.

For the Space Shuttle SRB, the propellant is 16% aluminum (fuel) and 0.4% iron oxide catalyst. The other constituents all produce only gases. This translates to 30.6% slag in the exhaust by mass. Thus, nearly a third of the thrust comes from slag ejection.

It IS possible to design propellants that are slag-free. These propellants are also smokeless or reduced smoke and are used in military applications, among others. They have less performance, are detonable (a major safety hazard), or both.

I am not a propellant chemist, so I could be wrong on the last point, but I do not know of any practical counterexamples.

Offline Kabloona

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Re: Reducing slag from solid rocket motors
« Reply #6 on: 03/29/2013 04:55 PM »

Thus, nearly a third of the thrust comes from slag ejection.


Sorry, no. The vast majority of aluminum powder in the propellant is exhausted as microscopic aluminum oxide particles. All that white smoke from the Shuttle boosters? Aluminum oxide powder, more or less.

A tiny fraction of the aluminum does coagulate into slag, but it's nowhere near "a third of the thrust." One expert analysis says that for every 1000 lbs of solid propellant burned in space, there are about 4 lbs ejected slag particles. Yes, there will be a lot more aluminum oxide *dust* expelled, also, but that is not "slag." It's dust. Big difference. And the slag gets ejected during thrust tailoff at the end of the burn, so it's not really contributing anything to thrust.

And to answer the OP's question, there's no way to eliminate slag except to exclude aluminum from the propellant, but that results in a major performance hit in HTPB or PBAN solid propellants. You can get performance back by switching to a Class 1.1 propellant, but that's not going to happen for safety reasons.

Bottom line, as long as solid upper stages are used, they will eject large quantities of aluminum oxide dust and small quantities of slag.
« Last Edit: 03/29/2013 05:21 PM by Kabloona »

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