# How can phenolic (resin?) handle rocket engine nozzle temperatures?

I just saw @OrganicMarble's survey answer that bravely tackles the wide variety of technologies used to make different types of rocket nozzles.

One item caught my eye:

Nozzles may also be made of composite materials, such as the STS Solid Rocket Booster nozzles. They were built up of phenolic wraps and included a flexible bearing for thrust vector control.

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The phenolic resins I'm most familiar with were used in the past to make circuit boards, and to produce that stuff people of a certain age remember - bakelite. If you've ever noticed that unique "old radio smell", you've probably smelled phenolic resin in action. There are and have been (I am sure) many other applications.

Before flow techniques, soldering by hand with soldering irons could expose circuit boards to fairly high temperatures in certain contact points, and old electronics ran hot, so these organic compounds obviously offered some significant temperature resistance. This was important because I think the next step up at the time was probably ceramics.

But I would not have expected a mid-20th century organic compound to be used to make rocket engine nozzles for the space shuttle! Yes it's the booster rather than the main engine, but wow! Those plumes look really really hot!

Are these nozzles really made from phenolic compounds related to the components of old radios? What keeps them from melting or burning in the oxygen of Earth's atmosphere where they are used during the launch phase?

• Why do you think it needs to be centered anways? – Num Lock Jun 18 '17 at 9:32
• @NumLock when the battery on my laptop gets low it leans to the left. By moving the image to the right, I can keep it on my knee and type with one hand; balance is thereby restored in the universe. – uhoh Jun 18 '17 at 9:35
• @NumLock In other words, of course it doesn't need to be centered ;) I just wanted to center it in this case so I did. I suppose I could ask why you think that I should not center it? – uhoh Jun 18 '17 at 10:04
• Well, it doesn't add to the readability of the post. And on the mobile version of SE images are centered anyways. It just adds a blank line there. – Num Lock Jun 18 '17 at 14:40
• On mobile the viewport is already naturally narrowed down. It looks more natural that way. The maximum width is set to 90% here, so you get a small pleasing space around the image in any case. However, on bigger viewports you usually want your images (or actually any non-text content) left aligned. I can't quite remember the rule of thumb, so don't pin me down on this, but unless your content isn't at least two thirds of total width you don't want it centered. See Wikipedia for example: Small tables (or the ToC) are left aligned, bigger ones cenetered. Probably worth a question on UX.SE. – Num Lock Jun 18 '17 at 18:08

There is a great writeup on this in the wonderful book "Development of the Space Shuttle 1972-1981" by T. R. Heppenheimer. Highly recommended, as is his prior volume "The Space Shuttle Decision".

tl;dr: They did melt/burn. That was the whole idea.

Page 178-179:

Nozzle: The ﬂame within a solid motor burned at 5,700 degrees Fahrenheit, which was hot enough to boil iron. What was to prevent it from destroying the booster? A liquid-fuel engine relied on regenerative cooling, as it circulated hydrogen through numerous small tubes or channels, but this was out of the question with solid propellants. Insulation helped, it protected the casing as the ﬂame front approached the wall. The nozzles of large solid motors relied on a third approach, for they were lined with thick slabs of ablative material. Like a reentering nose cone, this ablative layer could slowly decompose, vaporize, and erode as the burning proceeded.

For the SRB nozzle, the basic ablative material was carbon cloth phenolic, a cloth woven of carbon ﬁber and strongly impregnated with phenolic resin. It cost thirty dollars per pound, and each SRB used it by the ton. Layers of this substance protected the throat as well as other regions that faced the full severity of the hot gas ﬂow. Silica cloth phenolic, woven from silica ﬁbers, protected parts where the thermal environment was less demanding. Glass cloth phenolic served as insulation.

These materials came from vendors in the form of tape, with widths from three-fourths of an inch to thirteen inches. Rolls of tape fed a wrapping machine that laid the tape in plies on a rotating mandrel. A blast of hot air, at temperatures up to 700 degrees Fahrenheit, softened the resin. A roller pressed the tape against the substrate, with a force of up to three hundred pounds for each inch of tape width. After rotating past the roller, the tape was exposed to a flow of carbon dioxide at -60 degrees. This prevented the resin from curing and produced a hard, solid surface as a substrate for the next ply.

Each nozzle used ﬁve tons of carbon cloth phenolic, two tons of glass phenolic, and one ton of silica phenolic, all tape-wrapped in this fashion. Finished carbon lay-ups were cured in a hydroclave, which used water to apply heat and pressure. Other lay-ups went into an autoclave, which used carbon dioxide. Cured components were machined using diamond cutting tools, achieving tolerances as close as 0.0025 inches.

To achieve thrust vector control, the nozzle was to swivel by up to 7.1 degrees in pitch and yaw. Designers avoided the use of sliding surfaces, which could prove difficult to seal against leaks of hot gas. Instead they used a ﬂexible support or bearing, built from ten steel plates interleaved with eleven layers of rubber. Similar ﬂexible bearings had ﬂown previously, but this was the largest ever built. Within the hot gas ﬂow path, the bearing lay in what amounted to a backwater, removed from the full force of this exhaust. Nevertheless, some gas would reach it, which meant that that this ﬂexible support needed ﬂexible thermal protection. It obtained this from a "boot", a barrier of laminated rubber that eroded or burned away at a calculated rate and that was thick enough to hold out until the motor expended all of its propellant.

• This serves both structural and ablative functions at the same time? It's not just a liner? – uhoh Jun 18 '17 at 3:14
• Yes. It was all composite except for those steel plates in the flex-bearing. – Organic Marble Jun 18 '17 at 3:15
• The nozzle extensions were explosively severed just before water impact to avoid damage to the TVC actuators. Check out this photo of a recovered SRB showing the severance plane: 2.bp.blogspot.com/-xP2QBTAF6dk/ThvC_SpzkTI/AAAAAAAAAMM/… – Organic Marble Jun 18 '17 at 3:26
• The photo is really striking - there's a lot of information there that you can't get from pre-launch photos. These are amazing machines, no wonder there was interest in using them as stand-alone launch vehicles. – uhoh Jun 18 '17 at 3:45
• Might be interesting to know how much of the material was ablated in a typical flight in both thickness (realizing it would be different in different places) and mass, in both absolute terms and as a proportion of the original thickness/mass. Also, does this mean that the entire nozzle was a "consumable", or were they in essence "re-lined" and reflown? – Anthony X Jun 18 '17 at 22:01