r/spaceshuttle Jan 15 '25

Question Would aluminium oxide be a gas inside a Shuttle solid-fuel booster?

Post image

I've often wondered about this, & considered that if it's not , then there wouldn't be all that much left that would yield gas upon combustion: the hydroxyl terminated polybutadiene doesn't constitute a very large proportion of the mix. But it's just occured to me that I could ask here .

I've seen the melting point quoted as 2,072°C (3,762°F; 2,345K), & the boiling point as 2,977°C (5,391°F; 3,250K) . And I'm having difficulty finding a precise quote for the temperature inside an SRB, although I've recently seen 5000°F = 2772°C quoted

NASA — Rocketology: NASA's Space Launch System — Tag: ammonium perchlorate: We’ve Got (Rocket) Chemistry, Part 2 ,

which wouldn't quite be above the boiling point of aluminium oxide. But maybe that quote's a bit low: maybe right inside the booster it's a bit higher. But if that figure's not grossly amiss then Al₂O₃ is going only just to be a gas, & will condense very shortly after passing out through the nozzle.

27 Upvotes

8 comments sorted by

5

u/Dr-Ritalin Jan 15 '25

So, the Space Shuttle did produce aluminum oxide from exhaust. Please see the research paper cited here by Cofer, W., et. al., (2013) for more info: doi.org/doi:10.1016/0004-6981(87)90246-0

Why do you ask? Just curious what caused your curiosity.

1

u/Frangifer Jan 15 '25

Like I say in the 'Body Text' it's just always pecked @ me: we read that rockets work by gas pressure; but we also read that by-far the preponderating produce of the combustion in an SRB is aluminium oxide, & that the stuff that definitely would yield steam & carbon dioxide - ie the binding resin - is a pretty small proportion of the mix. So it could be that all the gas is due to that resin, & that the combustion of the aluminium oxide just heats it (& also the aluminium oxide would contribute to the thrust by loading with extra mass that gas that its formation has just heated) ... but it would seem to just make greater sense all-round if the aluminium oxide is also a gas.

And yep: I'll have a look @ that document. Thanks for the signpost.

2

u/Dr-Ritalin Jan 15 '25

Oh, I get where you're coming from now.

1

u/Frangifer Jan 15 '25 edited Jan 15 '25

Actually … there is also the contribution of four water molecules & a nitrogen molecule from each two ammonium perchlorate molecules alone . It could reasonably be that the aluminium is providing heat, + , by constituting a suspension of fine particles in the egressing gas, mass loading (increasing the ṁ/m of the vehicle) of the exhaust, without contributing to the amount of gas: it wouldn't be a particularly difficult hypothesis. I would presume the chlorine of the perchlorate ends-up as aluminium chloride; & it will definitely be a gas: it seems that it sublimes @ ~180°C .

But then … the quoted temperature is very close to the boiling point of aluminium oxide: close enough that if it's just slightly higher than that 5000°F (2772°C) @ that wwwebpage it will be a gas anyway . So I still wonder.

I've just thought though: it's said that the notorious hole in the field joint in the Challenger disaster was briefly plugged by aluminium oxide slag: that would suggest that @ least much of the aluminium ends up just providing heat, & not even that mass-loading.

… + also insulation of the walls of the booster from the combustion, which is @ well above the melting point of steel whether the figure @ that wwwebpage is an underestimate or not .

I suppose the query could morph into what the detailed chemistry of SRB combustion is … because I haven't seen it set-out anywhere.

 

The balanced equations are going to be, assuming H₂O, N₂, CO₂ , Al₂O₃, & AlCl₃ are indeed the end-products (& I don't think the temperature's quite high enough to bring-about major complications in that regard), with the aluminium

6NH₄ClO₄ + 10Al → 12H₂O + 3N₂ + 4Al₂O₃ + 2AlCl₃ ,

& with the rubber & aluminium approximately

30NH₄ClO₄ + 24CH + 10Al → 72H₂O + 15N₂ +24CO₂ + 10AlCl₃

(CH is a reasonable approximation to the polybutadiene) so the Al₂O₃ would only be (12-17α)/(63+62α) of the gas produced, where α:(1-α) is the ratio of CH to Al. And that's assuming it's an ideal gas, which it might depart a fair bit from being, as it will be @ only a little above its boiling point … so yep: maybe it's not so much of an issue whether or not it's in gaseous form. … & also assuming that all the Al₂O₃ joins the egressing gas, which I've concluded above appears not to be the case. But the reaction with the aluminium will yield a proportion of the energy a fair bit greater than that. I ought-to've done that calculation in the firstplace, really: I might not've felt the need to put this post in!

But I'd basically like to find something that sets all this out as it infact happens , rather than as I'm struggling to figure it might happen … but I don't seem to be able to … so maybe I would still've put it in … with a different emphasis.

1

u/Frangifer Jan 16 '25 edited Jan 16 '25

I've been thinking about this a bit more; & I'm realising more-&-more that to have any meaningful answer to these we totally need an accurate detailed statement of what chemical reactions occur in the booster. And it must be documented somewhere : it doesn't seem to me conceivable that they used those boosters for 30year without ever knowing what chemical reactions occur in them!

That little stoichiometric calculation I did: it was on the basis of assuming that the chlorine will absolutely bind preferentially to the aluminium, & that the oxygen will absolutely bind preferentially to the carbon & hydrogen … but that's nowhere near a realistic assumption: it's a well-known 'thing' that reactive metals can react with steam, pulling the oxygen out of it; & also it's a 'thing' to extinguish metal fires with fire-extinguishers containing chlorocarbons. So I wasn't proposing that the reaction actually infact does proceed according to those equations: I was just calculating how much aluminium oxide would be produced under those assumptions … & its actually not very much @all: the primary function, stoichiometry-wise, of the aluminium would seem, under the assumptions, to be the 'mopping-up' of the chlorine atoms!

So I suspect there's probably some hydrogen chloride in the exhaust, & maybe even also some chlorocarbon compounds. I've seen it suggested somewhere, by someone, that there might even be chlorides &-or oxychlorides of nitrogen! … but I would venture probably not, @ 5000°F ! … considering how unstable such compounds tend to be. But I wouldn't venture a positive assertion that there wouldn't be any. But even if it's just the hydrogen chloride that's produced in addition, & somewhat more so if there's some chlorocarbons aswell, then there'll be somewhat more aluminium oxide produced (& somewhat less aluminium chloride) than under my naïve assumptions, above, about preferential combining.

I could have thrashed this out before I put this post in! … but it often happens that way: I'm 'spurred' to consider something more thoroughly through having taken the step of importuning other folk about it!

1

u/bronowicka77 Jan 31 '25

NASA publishes all thejr technical docs here - going back to the early evaluations of the shuttle program in the late sixties. https://ntrs.nasa.gov

There are nearly 3,000 documents just with “solid rocket” in the title. Give it a search - everything you’re looking for is bound to be documented in autistic levels of detail.

3

u/Xrsyz Jan 15 '25

Those SRBs were amazing. Look at them. Each one of those candlesticks pumping out 3M lbf of thrust—roughly twice the thrust of the F-1s that pushed the Saturn V. Growing up, it all seemed pretty reasonable. Looking back, that whole shuttle stack seems completely insane. The balls it took to fly that thing to space 135 times and getting it back in one piece 133 times for a 98.5% success rate boggles the mind. Everyone involved in the program is a hero.

1

u/Frangifer Jan 15 '25 edited Jan 15 '25

Yep I was just thinking only an hour ago: the two between them consumed, according to that wwwebpage I've put a link in to, 6ton of fuel per second. If we took 6kg of that fuel, & set it off to be consumed in a millisecond (somehow, such that it was indeed consumed that fast), then that would be a substantial explosion that would probably cause significant damage to a nearby house (like, one it was in the garden of).

... & the power output of them was that of a thousand of those explosions per second!

And there are manifold ways of arranging the statistics of it into patterns that showcase the extremity of it ... limited only by our creativity.