A B L A T I V E S
Ablatives can be a cost-effective solution to high performance engine requirements. Because its cooling capabilities are not compromised by drops in injection pressures, it is also very forgiving in throttled or blow-down thrust profiles. A high-performance rocket motor must endure intense mechanical stress in the presence of an ultimately unsustainable thermal load.
Ablatives are used to perform three core functions during a burn:
It must withstand the mechanical stress of the pressure in the chamber and the shearing action of high velocity gas.
It must insulate and contain the hot combustion products within the chamber.
It must shed away slowly, or ablate, when it is incapable of withstanding the heat any longer.
MATERIALS PROPERTIES
Ablatives are typically an advanced composite material, made up of structural fibers and a matrix. When stiffened by the matrix, the fibers provide the strength necessary to withstand the mechanical stresses. It may also provide a considerable portion of the insulative properties. Because it makes up layers of fibers or fabric, it can pull away from the rest of the composite when the matrix is burned away.
A typical composite material for ablative motors is Silica-Phenolic. A pure silica fabric is impregnated with a phenol-formaldehyde resin matrix. The silica fibers are not only strong, but the purity allows it to survive in and insulate from extremely high temperatures. Not high enough to survive chamber temperatures indefinitely, but long enough to prolong its survival and preserve its mechanical characteristics. The phenol-formaldehyde resin is rather brittle and not particularly strong, but it is an especially high-performance material due to its pyrolytic properties. Instead of merely burning away, the phenolic resin chars and puffs up into carbon rich layer. This layer acts as a semi-regenerative insulator. As this char burns away, it produces a gas film that further helps to insulate.
With Silica-Phenolic as a starting point, we can consider other common ablators, such as the many Garolite plastics. G10 is the most common ablator, made up of fiberglass E-glass and epoxy resin. E-glass is also a fiberglass like the pure silica described earlier, but it contains impurities that lower its working temperatures and strength. It has the benefit of being much cheaper and more readily available, and proves more than adequate for moderate combustion temperatures. The epoxy resin has the benefit of being strong and tough and food safe, but it does not char and pyrolyze in the same regenerative fashion that phenolic resins do. Because of this, G10 is nominally a lower performing ablative material, but its poorer performance is still often adequate for as many as 20 seconds of burn time in a moderately energetic motor. It can rely on its toughness and insulative qualities to contain combustion, and the quality and consistency of fabrication can often make it an attractive option over trickier silica-phenolic composites. There are legends of a Garolite silica phenolic material somewhere out there, waiting to be found.
*Moderate temperatures for rockets are a relative term, and holds little meaning except to put it in between low energy rockets like nitrous-alcohol motors with sub-200 second efficiency, and high intensity such as methane and hydrogen, approaching or exceeding 300 seconds. A moderate intensity is something like a LOX-alcohol motor.
On the other end of the Garolite compositions are the low strength phenolic garolites, like CE, LE, and XX. Each of these materials consist of a matrix of phenolic resin impregnating cotton, linen, or paper. These materials lack much of the mechanical strength possessed by fiberglass alternatives, but so long as it can withstand the mechanical stresses, it can readily ablate, producing insulating gasses upon pyrolyzing. Compared to G10, these Garolite plastics are easier to cut without wrecking cutting tools and lungs, and are almost an order of magnitude cheaper, making it a reasonable low-cost ablator for less energetic motors.
When selecting and utilizing ablative materials, there are other considerations to make. The opacity of the material is important for reducing the radiative heat transfer. In solid motors, a translucent material can pass radiation throughout the grain, heating up the bulk mass. Clear epoxies can allow more light through the layers, reducing its insulating effects.
MECHANICAL DESIGN
For a composite made up of layers of fabric, if a layer becomes compromised, it can choose to shear off entirely before it has the chance to fully work its magic protecting the chamber. This occurs most readily when hot gas impinges upon too large a proportion of the layer, burning away the resin that is holding the fabric to the outer layers. This can be seen in some motors in the form of chunks of ablator peeling off during the firing and burning up in the combustion stream. To mitigate this, high performance ablative composites seek to expose only a fraction of each layer at a time. Instead of exposing only a single inner layer at a time, the layers are stacked perpendicularly or diagonally, with the trailing edge making up the exposed surface area. If the leading edge were to be exposed, then the shearing action will want to peel it off like a scoop.
This kind of composite is difficult to wrap, but achievable by hand layup. Another less common method is to cut rings out of flat composite plate, and stack them to form the chamber profile.
Another way to wrap composite ablatives is by weaving it, such as with filament tow or with woven sleeves. The weaving helps to protect some proportion of the ablative from direct combustion gas exposure. To help produce a clean and consistent converging and diverging nozzle section, a test chamber was made using Phenol-Resorcinol resin and woven silica sleeves. The sleeves are able to expand or contract to fit the form, and the Phenol-Resorcinol resin is room temperature catalyst cured, making its fabrication easier to manage, and allowed for the use of a painted and waxed 3D printed mandrel. When working with phenolic resins, it is important to use ample ventilation and full protective gear and respirators, as the resin is especially nauseating.
Because Garolite tubes are made from rolls of fabric, it is inherently vulnerable to layers shearing off in chunks. This can be mitigated somewhat by protecting the edges of the tubes with overhangs and right angles. This characteristic makes it a poor material for anything other than straight combustion chamber liners, and so it should not be used for converging nozzles as the throat is quick to shear off. Some ablative motor designs take advantage of this by using a smaller nozzle insert in conjunction with a Garolite liner. This nozzle insert could be made of fine graphite or a phenolic composite designed to converge and diverge cleanly, and with better mechanical performance than the long wound tubes. Splitting the task between two components also reduces the machining requirements, and allows for tubes to be used at their nominal diameters.
Ablatives do not always make the strongest material on its own, but composites can be further reinforced by wrapping them in carbon fiber composites or containing them in a steel or aluminum tube. A carbon fiber overwrap allows for the chamber and nozzle to extend past any metal components, and can bond the outer wall of the chamber to the inner wall of a flange. The lightweight nature of composites help to produce a remarkably lightweight and high performance motor that is capable of crushing and absorbing the energy of landing.
THERMAL DESIGN
With different chamber materials and components, the heat absorbed still has to go somewhere. A graphite nozzle is an effective conductor, and waste heat absorbed through the nozzle will conduct towards the outside edge. If the outside edge of a graphite nozzle makes contact with metal retaining hardware, it will rapidly heat it up. This may not always lead to a failure, but heating up a tempered aluminum in this way can cause it to lose it’s tempered state, and it could fail when used again. To avoid this, it is important to retain graphite within an insulating material. In some heavy solid rocket boosters, a graphite composite is used as nozzle material, but it is also encased in a silica composite insulator before it makes contact with the metal structural components. A graphite nozzle may also be bonded with a phenolic nozzle component, as with the Newman 1 engine.
PRESSURE BEARING DESIGN
Heat transfer increases with local hot gas velocity, so a critical factor in improving material survivability is maintaining stagnant gas pockets in temperature sensitive regions such as uncooled metal parts, near a composite edge, or by an O-ring seal. Injectors are usually an exception because the flowing of fresh propellant into the injector helps keep it cool.
In the Newman 1 Engine, the ablative liner is slid into the aluminum motor case without any insulators aside from the ablator itself. There are no sealing surfaces preventing gas or liquid from passing from the combustion chamber to this gap, and this helps to keep installation simple. The reason this does not harm the motor tube is that there is no reason for the hot chamber gas to want to go there. O-ring seals towards the exit and the ends of the motor tube prevents a flow of gas from the high pressure interior to the low pressure exterior, except for out the nozzle itself. A series of right angles also helps to reduce any incident gas flow and radiation.
Motor components, like the nozzle and the injector, also cover the edges of the composite tubes, protecting them from direct gas exposure, and compensating for irregularities in the cut edges. This design manages to avoid relying on noxious single-use silicone RTV as sealing surfaces, and the phenolic nozzle extension, modified from COTS Aerotech nozzles, serves perfectly well in the supersonic expansion zone.
It is worth noting that silicone O-rings were used in the injector sealing surfaces, due in part to its high temperature performance and compatibility with alcohols. O-ring material compatibility is another important subject to cover separately.
INJECTION DESIGN
With the ablator actively burning away to perform a cooling or insulating function, the combustion mechanics must be managed to prevent an accelerated or imbalanced burn rate. Ablators are also a fuel, and should take care to avoid oxygen rich environments. By burning fuel rich in an ablative motor, not only is the fuel contributing to the insulating film cooling effect, but it also starves the chamber of free oxygen to feed the ablator. A fuel rich environment during steady state combustion is often preferred, but it should also be maintained following motor burn-out. Without a main valve shutoff, one propellant is likely to deplete first. LOX boil-off tends to reduce total oxygen supply, but should fuel deplete first, then the oxygen will find a steady supply of fuel in the ablator. For a single use motor, this is not inherently a problem, but it complicates post-firing analysis and static tests.
Study the liner of this Newman 1 engine from a previous test, and notice the distinct lines where there is more ablation. Impinging streams of propellant tend to create spray fans normal to their intersection. But these fans also interact with the fans created next to them. When these fans impinge with each other, the subsequent fan is directed halfway towards the chamber wall. This results in higher heat flux along the line of travel, and higher ablation rates, creating a distinct star pattern. This is partially mitigated by placing adequate film cooling orifices at this intersection, halfway between sets of orifices.
For the main propellant orifices, it also helps to direct the resultant angle of an orifice pair towards the center of the nozzle throat, instead of vertically down towards the converging nozzle. This helps to reduce the heat flux of the nozzle as well as the thermal shock.