O-Rings and Gland Design

The most common kind of seal used between components in standard liquid rockets is the O ring, so named for being a ring-shaped seal with a circular (O-shaped) cross-section. Typical O-rings are made of an elastomer material and form a seal by deforming inside a groove. The default standard for O-rings and O-ring gland design is the Parker O-Ring Handbook, but it comes with some caveats. It must be understood that the Parker guide is made for seals that are up to industry specifications, able to withstand potentially millions of cycles across wide temperature and pressure ranges. Designing to Parker specifications, however, can make assembly, servicing, and cost for a low-stakes amateur project far more burdensome than they need to be.

Half Cat Rocketry has demonstrated on many occasions that “improper” O-ring gland design can work just as well for the purposes of an amateur liquid rocket. It should also be noted that the focus of this subsection is exclusively on radial O-ring seals (a.k.a. piston seal), where a round part with on O-ring installed on the exterior is pushed into a round bore. Face seal O-rings, which seal between the flat faces of two parts, are not used in standard Half Cat liquid rockets.

Function

The main principle to understand about O-rings is that they require compression, or “squeeze,” to work properly, so that the elastomer flows into the microscopic surface imperfections under the pressure of installation. In practical terms, this means that the gland must be shallow enough for part of the O-ring to be protruding past the outer radius of the part it is installed on and make contact with the inner radius of the part it is being installed into. The other important parameters for an O-ring are stretch (how much its diameter increases when installed in the gland compared to the relaxed state) and gland fill (how much of the gland is filled once installed into the bore). As with squeeze, Half Cat Rocketry has demonstrated the acceptable range for these characteristics to be far wider than usually specified.

Material

There a wide variety of materials which O-rings can be made from, each with different characteristics. Most available materials are more exotic and niche than required for amateur liquid rockets, and costlier.

Buna-N (a.k.a. NBR or nitrile rubber), is among the most common and least expensive material. It is also compatible with most commonly used materials, but exhibits severe swelling when exposed to gasoline (even in very small concentrations, such as the 2% present in E98).

Silicone rubber is slightly costlier than Buna-N, but its softness makes silicone O-rings easier to install. It is also compatible with all commonly used materials, although it exhibits severe swelling after exposure to gasoline and minor swelling with prolonged exposure to isopropyl alcohol.

Buna-N and silicone O-rings should be replaced after use if in contact with chemicals that cause swelling, unless used in a semi-permanent static sealing application that does not otherwise require disassembly between tests or launches. In this case swelling will not prevent the O-ring from continuing to seal, but once removed from the bore the swelled O-ring cannot be re-installed.

Viton is a fluorinated silicone rubber, making it more compatible with oxidizers; however, compatibility with nitrous oxide in this specific context is not a concern, and it has never been the cause of failure for any Half Cat type rocket. Viton is more expensive and very similar in physical characteristics to Buna-N.

Note: Oxidizer compatibility on the rocket itself is not a requirement because personnel will never be near incompatible materials when oxidizer is present. See this page for detailed safety information regarding nitrous oxide bipropellant rockets.

Lubrication

It is necessary to apply grease to an O-ring to aid installation and prevent damage. Lubrication also helps the O-rings on the tank piston slide smoothly and prevent sticking during operation. A general rule of thumb is that any grease will work once, although it may cause the O-ring to swell or disintegrate over time. Silicone grease is a common option, although it has not been typically used by Half Cat Rocketry.

Red lithium grease (specifically Lucas Oil Red “N” Tacky #2) is the standard recommendation for its extremely low cost and higher viscosity, which make it easy to scoop and spread. As strange as it may seem to use literal axle grease in a bipropellant rocket motor, this lubricant has a long and successful heritage in Half Cat type rockets.

White lithium grease is similarly inexpensive and widely available. The white color comes from the inclusion of very small aluminum oxide particles, which do not meaningfully impact its lubrication properties.

Silicone grease should be avoided if using silicone O-rings.

Super Lube is a brand name synthetic grease that contains microscopic PTFE particles. It is slightly more expensive than lithium grease, but works well for all types of O-rings. It is commonly used in amateur rocketry due to its inclusion in Aerotech reloadable solid motor kits.

Krytox is an oxygen compatible grease which is somewhat more expensive – the same note about oxidizer compatibility (and why it is unnecessary) applies to lubricants. If oxidizer compatible lubricant is mandated by a third party, such as a university advisor,

Tribolube may be used in place of Krytox at lower cost. When applying lubrication to O-rings, use a liberal amount of grease to ensure that they install smoothly. It never hurts to add more grease, especially with cheap greases.

O-Ring Damage and Re-Use

In typical amateur liquid rocket applications, O rings rarely if ever need to be replaced. On parts which remain installed, O-ring life should be indefinite within the scope of this guide. The most common places where O-rings may need to be serviced are the forward tank bulkhead and tank piston, both exposed to fuel and un-installed regularly. If O-rings are not swelled, such that they still fit into their groove nicely, they may be re lubricated and re-installed. If an O-ring is damaged, it should be replaced or else may leak when pressurized. Minor damage such as small surface nicks may be acceptable, but tears, slices, rips, and other large damage are likely to cause failure and warrant replacement.

Note: An exception to the above applies to O-rings that are used to seal combustion chambers in locations other than adjacent to the injector fuel manifold, such as on a nozzle or nozzle carrier sealing to a case or ablative liner. Such locations are likely to exceed the melting temperature of any elastomeric material, requiring replacement of the O ring after each firing; an O-ring that melts during firing will usually maintain its sealing effectiveness for the duration of the burn, but should be replaced immediately thereafter. The Mojave Sphinx design does not include any O-rings that are expected to melt during normal operation.

O-Ring Glands

The source found to be most useful for amateur liquid rockets is the Marco Rubber Static O-Ring Design Chart. This reference has been used successfully in every Half Cat Rocketry project, and it has been found that the gland sizes can be safely increased even further than Marco Rubber specifies. To this end, provided below is a simplified chart for sizing O-ring glands.

It is recommended to leave at least .100” of length on either side of the O-ring groove whenever possible. This avoids a thin lip that is easily damaged, and in some applications provides axial distance margin before the O-ring protrudes past the end of the bore.

Note: The Marco Rubber O-Ring gland dimension chart will work as-is, but the table below offers more lenient dimensions, and the resulting gland will make installation easier without compromising seal integrity.