Port and Fitting Types

The topic of fittings is surprisingly heated in some engineering circles, as there are differing opinions on the use and proper occasions for various types of fittings. Discussion in this section will be limited to the relevant points required to understand and construct a simple amateur liquid rocket.

Pressure Ports

Simply put, a pressure port is a pressure-sealed passage into or out of a part – what that means in this context is that a pressure port is a hole with a fitting installed. For amateur liquid rockets, there are two main ways to seal a pressure port in a component: NPT and boss port. Both have their uses, advantages, and disadvantages.

NPT

National Pipe Taper (NPT) is a standard for tapered threads which seal by deformation. They form a conical profile as shown in the figure below.

NPT fittings have a reputation for being bad or difficult to use, which is often due to either a misapplication of NPT or a misunderstanding of the requirements in using it. There are a number of key advantages which make it ideal for amateur liquid rockets:

• Low cost and widely available with a large variety of form factors and a useful range of sizes

• Easy installation and low sensitivity to variation in installation torque and thread engagement, allowing controlled clocking when necessary

• Ports can be easily tapped into soft metals by hand using common and inexpensive tools that do not require high precision or rigid workholding (for 3/8NPT and smaller)

• Ability to withstand and seal at a wide range of temperatures

There are also distinct disadvantages which limit its usage:

• Limited number of removal and re-installation cycles (clocked fittings should not be removed and re-installed)

• Lack of deterministic indicators of leak tightness without conducting pressurized leak checks

• Less precise clocking control than swivel-type straight thread fittings with a jam nut and O ring

• Large sizes (1/2 NPT and above) can be challenging to hand-tap without sturdy workholding

The fact that NPT is so dominant in fitting catalogues makes it the default choice for pressure ports. NPT is also highly advantageous for building fitting trees because it goes together with Lego-like qualities: Tees, crosses, elbows, connectors, and reducers can be combined in many ways and branch out to line connections or adapt between two otherwise incompatible fittings. It has been the case at many (possibly a majority of) Half Cat Rocketry operations that the day has been saved by having a large collection of various NPT fittings to make, repair, or rework a connection.

In most applications, a few wraps of PTFE tape should be applied to the male threads of an NPT fitting prior to installation. This reduces friction against the threads of the female port and can act as a secondary sealant by helping to fill any remaining space between the threads.

Because NPT threads create a seal through deformation of the thread material rather than compression of an elastomeric or metallic seal, they are capable of sealing across a relatively wide range of torque and thread engagement, making them insensitive to variability in installation. This also allows NPT fittings to be installed in a specific rotational position (often called “clocking”), because the range of thread engagement across which an NPT fitting can seal encompasses more than one full rotation. Once the fitting reaches the minimum engagement required to seal, it can be tightened further by at least one full turn until clocked in the desired orientation, without detrimental effect. NPT fittings and threaded ports made from softer materials such as brass and aluminum are more forgiving in this regard than harder materials such as steel or stainless steel, which is also prone to galling. Stainless steel fittings are not recommended for amateur liquid rockets following the standard design discussed in this document, but can be used if no other options are available. When both the male and female threads are made from stainless steel, nickel bearing PTFE tape should be used to prevent galling.

In summary, NPT is a convenient pressure port connection that has the advantage of cost and ubiquity for the size of liquid rocket that most amateurs and students will build. There are challenges to using NPT, but it remains the most advantageous option for creating a pressure sealed connection in a machined component. A chart of NPT thread dimensions is provided at the end of this page for reference.

Straight Thread / Boss Ports

Boss ports are a broad class of sealing types which have straight threads and compress a sealing element between a fitting and the part it is threading into as shown in the figures below. The most common standards for boss ports found in the context of liquid rockets are BSPP, AS5202, and SAE J1926-1.

BSPP

The most suitable type of straight thread fitting for low-cost amateur rocketry applications is BSPP (British Standard Pipe Parallel). These fittings are somewhat less available in the United States than NPT. However, they are much less expensive than other types of straight thread fittings due to their use in plumbing applications, similarly to NPT. BSPP fittings have straight threads with either an O-ring or bonded sealing washer (elastomer gasket molded or adhered onto a metal washer) that is compressed between the underside of the fitting hex and the flat surface surrounding the port. In some cases, a copper washer may be used as a gasket; though capable of withstanding higher temperatures than elastomers, copper washers are less effective at creating a leak-tight seal, more sensitive to surface finish on the fitting and ported surface, and require higher installation torque.

No specialized seal gland geometry is used for BSPP; this greatly simplifies machining the ports, as they are simply straight thread tapped holes. A milled spot face around the port of at least the diameter of the sealing washer is recommended to provide a smoother sealing surface, but is often not required when using an elastomer gasket, especially if the ported surface is already machined rather than raw stock. If needed, the area surrounding the port can be polished by hand using fine sandpaper or abrasive finishing pads (such as ScotchBrite brand).

BSPT

BSPT (British Standard Pipe Taper) is – as the name implies – not a straight thread fitting type, however male BSPT fittings can be installed into BSPP female ports, using PTFE tape in the same manner as NPT rather than an O-ring to create a seal. The thread pitch of BSPP and BSPT fittings are the same, however BSPP male fittings are not compatible with BSPT female ports. While BSPT is not generally recommended for standard liquid rockets due to lower availability, it has virtually all of the same characteristics as NPT.

AS5202

In professional aerospace applications, AS5202 ports are the predominant method of connecting tube fittings to machined components. These ports use UNF straight threads and have a tightly controlled seal gland geometry compatible with with AS568 O-rings, as well as metallic K-seals for high-temperature and cryogenic applications. Machining AS5202 ports requires an expensive specialized porting tool, which cuts the seal gland geometry with the required surface finish, and in some cases also reams the hole to the precise tap drill diameter. AS5202-compatible fittings can adapt to most common types of metallic tube connections (i.e., flared and compression), and are typically made from 300-series stainless steel. Cost can range from tens to hundreds of dollars per fitting depending on the type and supplier. When operating on a limited budget, AS5202 ports and corresponding AS-specification fittings are not recommended for student- or amateur-built standard liquid rockets, unless supplied free or deeply discounted by a sponsor.

SAE J1926-1

Automotive and industrial applications often use SAE J1926-1 boss ports. These ports are very similar to AS5202 and also accept a range of UNF thread fittings, however the sealing feature is compatible only with elastomer O-rings. A specialized porting tool is also required for SAE J1926-1, although these are less expensive and more common than those for AS5202.

Straight thread boss ports have a few beneficial aspects for amateur liquid rockets:

• Effectively unlimited removal and re installation cycles

• Compression of O-ring provides a more deterministic indicator of leak-tightness upon installation

• Swivel-type fittings using a jam nut offer precise clocking control

• Some types of straight thread ports (BSPP) can be tapped into soft metals by hand using common and inexpensive tools that do not require high precision or rigid workholding

However, it comes with downsides that typically make it a non-starter in the design phase:

• Less common (in the United States) than NPT (typically more expensive and fewer varieties available)

• Often available only in steel or stainless steel, driving higher costs

• Some types (AS5202, SAE J1926-1) require specialized porting tools, and cannot be created from a drilled pilot hole by hand

The biggest reason that boss ports are not used more on Half Cat type rockets is the lack of vast, inexpensive catalogues of fittings in every form factor as is the case with NPT. The second biggest reason is that boss ports cannot be clocked as easily as NPT; boss ports have a very limited range of motion once they are tight enough to seal reliably.

One unconventional use of the boss port fitting concept which shows up occasionally is the bolt passthrough on a tank bulkhead. See the figure below and note how the two screws anchoring the recovery bulkhead pass through the forward bulkhead with an elastomer seal under the screw head. In the Mojave Sphinx design, for example, they are retained with nuts, but in some cases may be threaded into the bulkhead itself, in the same manner as a boss port fitting. This is an easy, reliable way to add fastener passthroughs to flat surfaces with only a tapped hole.

Tube Connections

By comparison, there are many options for connecting pressurized fluid lines. The types which are usually found in amateur liquid rockets broadly include compression fittings, flared fittings, and push-to-connect fittings.

It is important to understand that tube connections and pressure ports are two very different concepts! A single fitting may have different connection styles. In fact, fittings in this context are usually connecting between a tube and a pressure port and will have a different fitting standard on each end. Tube connections are not the same as pressure ports! For example: If someone says that their rocket uses NPT, they likely mean that the pressure ports in their machined components are NPT tapped holes. This says nothing about the tubes connections, which could be any one of several different types.

Compression

Compression fittings operate by compressing a ferrule onto the fitting with a nut. The ferrule deforms inwards, gripping the tube to form a seal and resist blowout under pressure. Compression fittings are quite common and therefore inexpensive. Their biggest advantage is that the tube does not need to be flared, and thus can be any length cut from standard tube stock (nylon, aluminum, copper, or stainless steel). However, one downside of compression fittings is a dubious torque spec, usually specified on a turn-basis (i.e., one and a quarter turns past hand-tight), which makes them less reliable for high-stress applications. This was most spectacularly the case in the first static fire of Full Cat, where a tube in the fuel feed system was ejected from a compression fitting due to insufficient retention by the ferrule.

The ferrule will be made of different materials depending on the tubing type; for nylon tubing, a PTFE or acetal ferrule should be used; for aluminum, copper, or brass tubing, a brass ferrule should be used; stainless steel ferrules should only be used with stainless steel tubing, which is not recommended for standard liquid rockets.

SwageLok, Yor-Lok, and other similar brands are a specialized type of compression fitting. They work in a very similar manner, but are often far more expensive and typically only used for metal tubing.

37° Flare

37-degree flare fittings are perhaps the most ubiquitous type of tube connections in professional aerospace applications, as well as the high performance automotive world and industrial hydraulic systems. They are generally available in two varieties: AN (Air Force-Navy Aeronautical Standard) and JIC (Joint Industry Council). The two are functionally identical for the purposes of amateur liquid rocketry, with the only differentiator being that AN uses more precise Class 3 “medium fit” threads, while JIC uses more loosely controlled Class 2 “free fit” threads, and are generally less expensive than AN.

These connections use a pair of male and female conical metal surfaces, which have a 37-degree half angle (measured from the centerline) and are pressed against one another by the preload from a threaded nut (commonly called a B-nut). The conical profile results in an annular contact line with sufficiently high contact stress to microscopically deform the surfaces. When the female side of the connection is a flared tube, a machined sleeve is placed between the flared end of the tube and the inside of the B-nut to reduce friction and distribute load, similarly to a washer in a bolted connection. Female flared fittings, such as those on flex hoses, typically integrate the flare and sleeve into a single machined part.

Both aluminum and stainless steel tubing can be used with 37-degree flare fittings, with the use of a flaring tool to deform the ends of the tube without cracking. Flaring tools compatible with stainless steel tubing are much more expensive than those suitable for aluminum. A tube bending tool is also required if using metal tubing in any other situation than a straight run.

Due to the difficulty of creating precise bends and the relatively high cost of flaring and bending tools, rigid metal tubing is not recommended for standard liquid rockets, except for straight runs for the portion of a fuel downcomer that is exposed to free stream flow during flight. In this context compression fittings are recommended, but flared connections may be used if desired.

In contrast to rigid metal tubing, flex hoses using 37-degree fittings are quite useful and accessible to the amateur liquid rocket builder, as discussed in Section 1.2.4. Such flex hoses comprise much of the fluid system in the Mojave Sphinx rocket and Ground Support Equipment design due to their reliability and ease of use. Flared fittings are indefinitely reusable, unless regularly over torqued to the point of yielding.

Flared fittings may be tightened according to a torque specification (many of which are available online) or based on the number of turns past the initial increase in resistance when all surfaces become clamped, however most sizes below ½” will seal at any reasonable value of “wrench tight.”

45-Degree Flare

45-degree flare fittings are not recommended for use in standard liquid rockets. They are primarily found in refrigeration and air conditioning contexts, and while their function is virtually identical to 37 degree flare fittings, they are less commonly available. 45-degree and 37-degree fittings are not considered to be compatible with one another, however a seal can be achieved between mismatched flare angles with sufficient torque, which will usually yield one or both of the fittings.

Push-to-Connect

Push-to-Connect (PTC) fittings seal by pushing the tube past an O-ring that is pre-installed in the fitting. The tube is retained by a ring of metal teeth that prevent reverse movement. Like compression fittings, the tube does not need to be flared; however, PTC fittings only work with flexible tubing made from nylon or similar polymers, as they rely on the softness of the material for the metal teeth inside the fitting to establish a grip on the tube. Another advantage of these fittings is that the tube can be rotated in place after installation, since the seal is not dependent on axial compression (which prevents movement).

To remove the tube from a PTC fitting, a release ring on the outer face is depressed to push the internal teeth away from the tube. The fittings are indefinitely reusable, although the plastic tubing may eventually wear out from repeated removal and reinsertion.

Fitting Material

Brass is the default fitting material for the standard liquid rocket architecture. Most NPT fittings are available in brass (bare or nickel plated), as well as mild steel, stainless steel, and sometimes aluminum. Brass possesses sufficient corrosion resistance to ensure that surface oxidation is not problematic, and its softness makes fittings more forgiving with regard to installation torque of NPT fittings, which is especially advantageous for controlling clocking. The relative softness also reduces wear on tapped ports in aluminum tank bulkheads and injectors when removed and reinstalled. Aluminum has similar characteristics with much lower density than brass, but is less durable due to its slightly lower yield strength and hardness (when comparing grades from which fittings are commonly made). Additionally, aluminum is not suitable for hot gas connections such as igniters and chamber pressure sense ports; brass is preferred for these applications, followed by stainless steel only when necessary.

Mild steel and stainless steel should be avoided when interfacing with aluminum or copper machined parts, as the harder fitting threads will increase wear on the tapped port when removed and reinstalled. Mild steel also suffers from poor corrosion resistance in humid environments.

Galvanic Corrosion

Galvanic corrosion is often cited as a major potential issue when fittings and the ports into which they are installed are made from dissimilar metals. The two materials can form a galvanic couple, which can result in corrosion when exposed to moisture. In practice, galvanic corrosion is rarely problematic for standard liquid rockets with a service life of less than two years. Even in humid environments such as Central Florida, the PTFE tape normally used on NPT threads is sufficient to mitigate galvanic corrosion between brass fittings and aluminum ports for a period of at least a year. In dry environments such as Southern California, galvanic corrosion can be considered a non-issue for standard liquid rockets.

Significant visible galvanic corrosion has been observed on a small number of occasions on Half Cat rockets when brass/aluminum interfaces have remained installed for greater than six months without any effort made to limit exposure to moisture. It has never resulted in the failure of a fitting or port. Even so, it is recommended to store standard liquid rocket systems in a climate controlled space whenever possible.