The Problem

In Fall 2023, I discovered our rocket Phoenix had an inefficient fluid system design - two solenoid valves that could be replaced with one solenoid and two check valves. This would reduce cost, weight, and complexity. But this should have been caught a lot earlier. Our P&ID (Piping & Instrumentation Diagram) was so convoluted that even team members working directly with the components didn't fully understand how the system worked.

So, I decided redesign both the P&ID and document exactly how a liquid rocket's fluid system works, starting from basic principles. This guide walks through that process. By the end, you should be able to diagram this system yourself from first principles.

Liquid Rocketry Fundamentals

At the core of rocket propulsion lies the principle of Newton’s third law – for every action, there is an equal and opposite reaction. In the rocket’s nozzle, the rapid expulsion of hot gases generates a powerful downwards force, thus propelling the rocket upwards. The key to this thrust generation is the reaction that takes place within the rocket’s combustion chamber. Here, a fuel (such as liquid natural gas, or LNG) and an oxidizer (typically liquid oxygen, or LOX) are sprayed into the chamber through an injector and ignited by an external energy source. Ensuring the reliable delivery of the fuel and oxidizer to the injector is the role of the rocket’s fluid system. In pressure-fed systems like the Phoenix, an additional gas called a pressurant (often gaseous nitrogen (GN2)), is used to push down on the propellants so they reach the injector at a desired rate.

Base Configuration

The base configuration of our system starts with three vertically stacked tanks:

1. GN2 tank (top): Provides system pressure

2. LOX tank (middle): Stores liquid oxygen

3. LNG tank (bottom): Stores liquid natural gas

The tank arrangement ensures rocket stability through proper positioning of two key points:

1. Center of Mass (CM): The balance point where the rocket's mass is evenly distributed

2. Center of Pressure (CP): The average point where aerodynamic forces act

For stability, CM must be above CP - this creates a self-correcting effect that keeps the rocket oriented upright during flight, like an arrow. Here is a great visual intro to the topic:

In our rocket, heavier components go higher to maintain this relationship throughout the burn as propellants are consumed.

Critical Parameters:

• GN2 Tank: 5700 psi max pressure

• LOX Tank: 605 psi operating pressure

• LNG Tank: 516 psi operating pressure

The calculations behind the optimal tank pressures and propellant ratios are the work of our team’s propulsion experts. These are beyond the scope of this explainer, but if you’re curious to learn more, I encourage you to take a look into our technical paper available on Confluence or ask around.

Piping and Valves

Pipes and solenoids are installed from each propellant tank to the injector so they can flow downwards into it upon actuation. For the Phoenix rocket, all tubing is made of 3/8″ stainless steel due to its suitability in cryogenic conditions and ability to withstand high pressures. The base system requires three primary flow paths:

1. GN2 → LOX tank (pressurization)

2. GN2 → LNG tank (pressurization)

3. LNG tank → Injector (fuel delivery)

4. LOX tank → Injector (oxidizer delivery)

Solenoids work by controlling fluid flow via an electromagnetic coil. In a normally closed configuration, the flow is halted by a plunger when there’s no electrical power. Once powered, the electricity creates a magnetic field, retracting the plunger and allowing flow. Phoenix uses normally-closed solenoids, meaning they block flow when unpowered. This fail-safe configuration prevents uncontrolled propellant mixing during power loss.

Pressure Regulators

If the GN2 solenoid was opened in the current configuration, downstream components would immediately be exposed to the tank’s full pressure of 5700 psi. This could potentially overwhelm the structural integrity of the tanks and the lines, leading to bursts and potentially causing an explosion. Even if the tanks did survive, they would be overpressurized and release fuel far too fast, resulting in a highly rapid and uncontrolled combustion process.

Therefore,, we use pressure regulators downstream of the pressurant solenoid, which drop the pressure to our desired level. The first regulator, located immediately below the tank, steps down the GN2 tank’s 5700 psi to the LOX pressure of 605 psi. The second pressure regulator is located on the LNG line and further reduces the pressure down to 516 psi.

Regulators maintain output pressure through mechanical feedback:

1. Spring force sets desired pressure

2. Pressure-sensing bellows opposes spring

3. Force balance determines valve position

4. Higher downstream pressure compresses bellows, reducing flow

5. Lower pressure allows spring to open valve, increasing flow

This creates automatic compensation for input pressure fluctuations, changes in downstream demand, and flow rate variations.

Check Valves & Backflow

Check valves are components that permit flow in one direction while blocking flow in the opposite direction. Their primary function is propellant isolation - keeping fuel and oxidizer completely separated until they reach the combustion chamber. During normal operation, they are passive. During filling, they prevent cross-contamination between systems by acting as caps. Their most critical role comes during combustion, where they must prevent high-pressure exhaust gases from forcing propellants backward through the system, which would cause catastrophic mixing in the feed lines.

To prevent this and guarantee the placement of all necessary check valves in our rocket, I found that color-coding was a great visual aid. I first assigned unique colors to each tank and the injector as they all needed to be isolated from each other. I then highlighted every component touching each of them with their same color, stopping only at a check valve. A component marked with two different colors indicates a junction where a check valve is necessary. If there is a component that has no colors assigned to it, there is likely an extra check valve somewhere.

Consider the scenario where the CV-VN-VO check valve is installed before the branch instead of after it to check your understanding. How could the system break?

Rapid Emergency Depressurization System (REDS)

Working with cryogenic materials like liquid oxygen presents a challenge due to their high-pressure storage requirements and rapid expansion as they change state. For instance, liquid oxygen expands about 860 times when it turns into gas or “boils off.” To manage this, we add a Rapid Emergency Depressurization System (REDS) to all of our tanks.

Each REDS has two parallel paths: an active solenoid valve and a passive relief valve. In contrast to the flow control valves, the active solenoid on the REDS system is normally open to prevent overpressurization during power loss. 

The passive relief valve, on the other hand, does not need to be actuated. Instead, it is designed with a spring that is set to a desired pressure limit. If the tank’s pressure surpasses this limit, the high pressure overpowers the spring, triggering the valve to open and releasing the excess pressure. Once the pressure falls beneath this threshold, the spring automatically closes the valve, resealing the system.

And here is a video to help visualize the expansion. Hopefully this helps emphasize how dangerous working with cryogens can be. A poorly designed system can easily explode. Make sure you’re always taking the proper safety precautions to keep yourself and your teammates safe.

Instrumentation System

Lastly, we install a network of transmitters on our tanks to accurately reach the desired pressure levels in the tanks during fill and allow for ongoing monitoring of tank conditions when the rocket is on the launch pad.

Pressure transmitters operate by converting applied pressure into an electrical signal. The core of the sensor is a strain gauge, a conductive component that adjusts its electrical resistance when stretched or compressed.

This strain gauge is integrated into a Wheatstone bridge, a circuit designed to measure resistance variations. When pressure deforms the strain gauge, the resistance changes, disrupting the bridge’s balance and causing a voltage change across it. The transmitter processes this voltage, amplifying and converting it into a standardized output signal which is directly correlated to the pressure value.

Similarly, inside a temperature transmitter is an RTD (Resistance Temperature Detector). As you might have guessed from the name, they work by measuring the change in resistance of a metal element (typically platinum) as its temperature changes. They are also connected to a Wheatstone Bridge, where the small voltage imbalance is conditioned, converted by the transmitter into a standardized output signal, and correlated with a temperature.

Component Naming Convention

I created a standardized naming convention for our rocket designed to provide an easy reference and ensure consistency across all system components. The goal is for anyone to be able to understand what a component does and why it is there just by looking at the name. Here’s a breakdown of how it works: 

The naming scheme follows: COMPONENT-SOURCE-DESTINATION

Function Prefixes: V: Vehicle (flight system) F: Fill (ground system) C: Chill (thermal conditioning)

System Tags: N: GN2 system O: LOX system M: LNG system LN: LN2 system E: Engine/injector

Component Types: PRV: Passive Relief Valve SRV: Solenoid Relief Valve BV: Ball Valve SV: Solenoid Valve CV: Check Valve FV: Fill Valve QD: Quick Disconnect

Examples:

• SV-VO-VM: Solenoid valve from vehicle oxygen to vehicle methane

• CV-VN-VO: Check valve protecting vehicle nitrogen from vehicle oxygen

• PRV-VO: Passive relief valve on vehicle oxygen tank

• TT-VO: Temperature transmitter on vehicle oxygen tank

Fill System Integration

The GSE (Ground Support Equipment) system has three primary functions:

1. Tank pressurization

2. Propellant loading

3. Line conditioning

Our fill operations sequence is below:

1. LN2 Chill

   • Prevents thermal shock

   • Reduces boil-off losses

   • Conditions feed lines

2. Tank Pressurization

   • GN2 pressure verification

   • Relief system check

   • Regulator adjustment

3. Propellant Loading

   • LOX fill first (higher boil-off)

   • LNG fill second

   • Level verification

Each fill line requires:

1. Ground-side isolation valve

2. Quick disconnect fitting

3. Check valve protection

4. Vent capability

5. Pressure monitoring

Full Phoenix P&ID

The final Phoenix P&ID is below. Using what you’ve learned, take a look at the diagram below and see if you can figure out how the ground support equipment (GSE) we use to fill our rocket works.