Fluid System

The fluid system consists of various plumbing components which direct and control propellant flow between the tank and chamber. In the Half Cat standard, this has been reduced as much as possible since it is very easy for a feed system to grow in complexity and rapidly become the most cumbersome part of a liquid rocket project.

Main Propellant VAlves

Both of the main propellant valves are typically servo-actuated ball valves. These are robust and uncomplicated mechanisms made entirely from off-the-shelf components with a custom 3D printed housing. Full port ball valves are used to minimize pressure loss, as they do not restrict flow. With a single PWM command from the ground system, both the fuel and oxidizer valves are opened fully, and remain open during and after the burn; the thrust terminates when propellant is depleted. Leaving the valves open after the burn allows the nitrous vapor remaining inside the tank to dissipate, and ensures no pressure or propellant remains in the system aside from a small residual volume of fuel in the downcomer.

As an alternative to servo-actuated ball valves, the main valves may be Half Cat style pneumatic poppet valves. These are not recommended in modern Half Cat style rockets due to the increase in complexity and number of machined parts, but they are suitable for high-performance vehicles that need to reduce mass. See this legacy page for information about Half Cat valves.

HCR-1100 standard for amateur liquid rocket main valves

The flow of oxidizer and fuel from the Propellant Tank to the Thrust Chamber Assembly shall be controlled by a main oxidizer valve and a main fuel valve. Each main propellant valve shall be either A) a servo actuated ball valve (recommended) or B) a Half Cat style pneumatic poppet valve.

A. The servo mechanism and ball valve may be directly connected by fastening the handles (recommended), by a coupling between the shafts, or by gears. Shaft couplings and gears shall not be 3D printed in plastic.

B. The body of a Half Cat style pneumatic valve shall be made of aluminum. The poppet shall be made of either PTFE (Teflon) or acetal (Delrin).

Feed Lines

Due to the imprecise nature of the rocket and its components, the tubes (also called feed lines) connecting each part must be flexible. The main two kinds of tubing typically used are high-pressure nylon and stainless steel-braided PTFE. High-pressure nylon tubing is quite cheap and easy to cut to length, which makes it ideal for longer and less critical fluid lines. Braided PTFE hose with a 37° flared connection is very easy to attach and remove, but these are more expensive and only available in pre-determined lengths. PTFE hoses generally have thinner walls and larger inside diameters than high-pressure nylon tubing for a given outer diameter, allowing higher flow rates and lower pressure loss. Hydraulic hoses, which may be cheaper, are acceptable for fuel lines, but these should be avoided for oxidizer since the rubber liner could potentially be impregnated with nitrous oxide.

It is often the case that some tubing and many of the fittings used on Half Cat type rockets will not be explicitly rated to the maximum expected operating pressures that are possible with saturated N2O. However, supplier ratings typically come with large safety factors built in and are acceptable to use for higher pressures so long as they are pressurized only when personnel are at a safe distance from the system. This is of critical importance – see this page on safety for more detail. PTFE lined stainless steel-braided hoses are commonly available with ratings of 1500 - 3000 psi, which makes them an ideal choice for the connection from ground supply bottle to remotely operated fill valve.

Below are some links to specific McMaster-Carr products which have proven suitable for feed lines.

• Braided PTFE hose

• High-pressure nylon tubing

HCR-1100 standard for amateur liquid rocket feed lines

Oxidizer and fuel shall flow from the Propellant Tank to the Thrust Chamber Assembly through either flexible stainless steel-braided PTFE hose or flexible nylon tubing rated to a minimum of 500 psi.

Quick Disconnect

Quick disconnects (QDs) are a specialized type of fitting that can be rapidly mechanically disengaged, without the need to unscrew threads. The use of a QD on the nitrous fill line allows it to cleanly separate from the rocket at liftoff, preventing damage to the rocket and ground system that can result from a still-connected line being pulled taut and breaking under thrust load.

The standard liquid rocket design uses a Flange Clip QD, developed by Half Cat Rocketry specifically for this application. The Flange-Clip QD is a completely passive device that uses the motion of the rocket itself to disengage the coupling, which then separates from pressure blowoff load. This eliminates the need for an active mechanism suchas a pneumatic piston or electromechanical actuator, with the added benefit of simplifying the countdown and launch procedure. The assembly consists of two custom flanged fittings (one male and one female) and a U-shaped clip that fits over the flanges. The fittings seal to one another via a small O-ring installed on the male half. Each fitting also has a 1/8 NPT port, allowing standard COTS fittings to connect the QD to the rocket’s tank and the fill line on the GSE.

As shown in the figures above, the two halves of the flange clip QD are coupled together before launch by the clip, which reacts against the axial force exerted by the pressure in the nitrous fill line during and after oxidizer loading. This force is relatively low (on the order of 50 lbf) due to the small area that the pressure acts on, and the clip - which is typically 3D printed plastic – is more than capable of withstanding it. The clip is tied down to the launchpad or otherwise secured to the ground using a strong, heat resistant tether such as Kevlar cord. When the rocket takes off, the fill line and QD fittings move upward while the clip remains stationary, pulling the flanged fittings out of the clip. At this point, the flanged fittings are axially unrestrained and separate nearly instantaneously from the internal pressure blowoff force.

Slack in the clip tether should be kept to a minimum so that the QD separates in the first 2-3 inches of the rocket’s motion, with at least two feet of slack remaining in the fill line. Too short a fill line (or too long a clip tether) can result in tension on the fill line damaging the male fitting or causing it to bind in the bore of the female fitting, preventing separation even after the clip is disengaged. It is critical that the strength of the tether is sufficient to resist the frictional force between the clip and fitting flanges, with a dynamic amplification factor of at least 2. Plastic zip ties should never be used to connect the clip to the tether, as they are prone to snapping well below their rated breaking force.

It should be noted that there are also a wide variety of COTS quick disconnect fittings available, most of which use some version of a ball lock mechanism and are intended for pneumatic or hydraulic systems. However, such devices do not lend themselves well to the type of passive actuation described above; they have a high risk of jamming under sudden shock load, especially when the force is not perfectly axial. The flange-clip QD was created in response to failures of an earlier design that used an off-the-shelf QD fitting with a tether attached to the sleeve that releases the ball lock. Despite apparently successful testing of the system both by yanking on the tether and manually sliding the rocket up the launch rail, the QD failed to disconnect during liftoff on multiple launches due to the much higher force and acceleration imparted by the rocket’s thrust, breaking the tether and fill line and leaving a still-connected portion of the GSE trailing behind the rocket.

HCR-1000 standard for amateur liquid rocket quick disconnects

Oxidizer shall flow from the Ground Support Equipment to the Propellant Tank through a Flange Clip Quick Disconnect, which shall consist of a male fitting, female fitting, and clip. Either the female (recommended) or male fitting shall be integrated to the vehicle and the other fitting shall be integrated to the ground-side oxidizer fill line.

The vehicle-side fitting shall be made from any alloy of aluminum or brass, and the ground-side fitting shall be made from any alloy of aluminum, brass, steel, or stainless steel. The male fitting shall seal to the female fitting with an elastomeric O-ring(s). Both the male and female fittings shall include flanges at the mating interface.

The clip shall be made of any metal or plastic material, slide freely over the flanges of the mated male and female fittings, and include a tie-down point.