Types of Threads

NPT

NPT stands for National Pipe Thread (and sometimes National Pipe Thread Taper). It is a common type of thread used on plumbing. If you see a threaded fitting or pipe around your house, it most likely uses an NPT thread. The most significant thing to know about NPT threads is that you need to use Polytetrafluoroethylene (PTFE) tape on the threads to ensure a leak-free seal! This tape is also commonly referred to as "Teflon tape". YOU MUST USE THIS TAPE! Generally one or two clean wraps around the threads is enough. To be safe, take off old Teflon tape and replace it if you think it's insufficient.

Some other points about NPT threads:

• NPTF is a variant of NPT with a slightly different major diameter. They are cross-compatible with NPT threads, but some care has to be taken and you should always use teflon tape! We generally don't use them, but be aware when buying components. NPTF does NOT mean NPT Female.

• NPT threads have a taper of 1 inch per 16 inches. Male threads are tapered in, and female threads are tapered out. This is to ensure a better seal.

Where we use NPT threads:

• Black flexible tubing to UFA (Universal Fill Adapter, see below) connection

• Most N2 (Nitrogen gas) fittings

• Non-Swagelok® 2-way ball valves

• Other large pipe threads (i.e. N2 cylinders, etc.)

Further Reading:

Swagelok® Tube Fittings

Swagelok® is a company which manufactures tube fittings and various other components. Swagelok® tube fittings are proprietary and only work with other Swagelok® tube fittings. These fittings offer several benefits over NPT, including:

• They do not require PTFE (teflon) tape

• The are good for high pressure

• They are vibration resistant

• They can be assembled easily

• They're easy to use on plain tubing

The first time a Swagelok® fitting is used on a pipe end, it must be properly prepared. This preparation process ensures that the ferrule (the conical frustum-shaped metal bit) is properly seated into the tube. The video linked below provides a good explanation of how to do this.

Video demonstration: https://www.youtube.com/watch?v=jB_Nyje_HNE

NPS

NPS stands for National Pipe Straight. It is the same as NPT but without the taper. It is NOT compatible with NPT. We do not use it at this time.

Common Components

Ball Valves (2-way and 3-way)

Ball valves control the flow of the working fluid. We currently have:

• 2-way ball valves with NPT threads

• 3-way ball valves with Swagelok® fittings

  • These valves have the inlet positioned in the middle of the T. The valve can either be closed (when the handle is upright), or opened to either side by turning the handle arrow to that side.

Ball valves are fairly straight-forward, but it is important to double check in procedures that the interior "ball" space in the valve is not pressurized after disassembly. For safety, don't put fingers around the openings when opening ball valves.

Check Valve

A check valve is a one-way valve that lets flow in one direction but not in the other. There is typically an arrow on the component indicating the direction of flow. We use both Swagelok® and NPT check valves.

It is critical that check valves be inspected to ensure that they are mounted in the right direction.

Regulator

A pressure regulator is a device that reduces the inlet pressure down to a specified output pressure. They are used for both gases and liquids. We have several regulators in our propulsion system:

• The regulator on the big N2 cylinders

• 1800 psi regulators on the N2 Composite Overwrap Pressure Vessels (COPV's)

• Adjustable Swagelok® regulators for use with either N2 feed pressure or liquid pressure regulation

  • These regulators have a high and low pressure port marked "HP" and "LP". They must be properly aligned!

  • These regulators do NOT work at cryogenic temperatures

  • It is normal to hear a loud buzzing when turning these regulators from closed to open under pressure. This is due to the internal diaphragm vibrating, and is perfectly normal.

Universal Fill Adapter

A Universal Fill Adapter (UFA) is the black knob assembly connected to the end of the black, flexible tubing. It attaches to paintball COPV N2 tanks and allows us to open or close the valve on the tank. It works by using the knob screw to push down on a spring-loaded ball, which allows N2 to flow out of the tank.

Some of these have an issue where the knob is very badly fixed with illegitimate Loctite. We prefer using the set screw-type knobs, particularly those manufactured by Ninja.

Remote Line

The remote line is the assembly of the UFA with the black flexible tubing.

Pressure Gauge

Pressure gauges measure gauge pressure of the fluid (as opposed to absolute pressure). They are typically used with a T-joint. Our pressure gauges are (theoretically) not cryo-rated.

Quick Disconnect

A quick disconnect is a type of fitting which allows a quick connection/disconnection (hence the name), without any threading. We use these on our mobile filler. They are less secure than other connections, so they should only be used where necessary.

To connect a quick disconnect:

1. Pull back on the outer ring on the female end

2. Insert the male end (AKA nipple) into the female end

3. Release the outer ring until it snaps back and you hear an audible click

4. The two halves should now be connected and you should not be able to pull them apart without pulling back the ring

5. If you can pull them apart, simply try again. You may have to wiggle the quick disconnect a little to get the outer ring to snap into place properly.

To disconnect a quick disconnect

1. Pull back on the outer ring on the female end

2. Separate the two halves

3. Release the outer ring

Relief Valve

We buy relief valves from Swagelok®. These valves work using a calibrated spring, which under a certain pressure, will allow fluid to flow through the valve. We use these as safety devices, to ensure a safe depressurization in the case of over-pressurization. It is important that the these vents be kept clear of personnel and high-pressure tanks at all times when the system is pressurized.

Safety Notes

• Never adjust a fitting on a high-pressure system. If you don't know whether a section of tubing or tank is pressurized, depressurize it first.

• Always check all fittings for tightness before pressurizing any system.

Basics & download instructions:

This YouTube video covers the basics of using the Rocket Propulsion Analysis(RPA) Software. This doesn't cover how to model the thrust chamber for the intro project, but it will help you become more familiar with the program: https://www.youtube.com/watch?v=F3W3zZj4zX4

NOTE: This YouTube video uses the RPA Lite version. In order to model the thrust chamber you must download the RPA Standard Edition Trial Version. (The C++ one is not needed)

Trial version of RPA Standard Edition v.2.3.2 has the following functional limitations:

• The user may run the analysis 3 times without restarting the software. To continue with evaluation, the application has to be restarted.

RPA downloads can be found here: http://propulsion-analysis.com/RPA/download.htm

Complete User Manual(v2.3): Rocket Propulsion Analysis - User Manual (rocket-propulsion.com)

Important Concepts:

• Engine Definition:

  • define the parameters for the combustion chamber and nozzle sizing

    • Nominal thrust, nominal mass flow rate, or throat diameter must be specified

  • switching on the flag for performing the thrust chamber thermal analysis

    • specify additional heat transfer and chamber cooling parameters on the screens Heat Transfer Parameters and Thrust Chamber Cooling.

  • Can define the type of engine feed system

    • parameters for cycle analysis can be specified on the screen Propellant Feed System

• Propellant Specification:

  • Define propellant type(s) and mixture ratio. The mixture ratio can be specified either as an O/F ratio (ratio of "oxidizer flow rate" to "fuel flow rate"), or as an oxidizer excess coefficient, given as ratio of desired O/F to stoichiometric O/F.

  • Click Add Oxidizer/Fuel. You can filter the list in the dialog window, using a regular expression(ex “oxygen”). The filter pattern is applied to both columns of the table.

• Nozzle Flow Model:

  • Nozzle Conditions: If you are solving the nozzle flow problem, you have to define at least nozzle exit conditions, specifying one of three parameters: nozzle exit pressure, nozzle expansion area ratio, or nozzle expansion pressure ratio.

WORK IN PROGRESS

Overview

Injectors are needed to spray the bipropellants (i.e. fuel and oxidizer) into the combustion chamber in a way that controls the atomization, combustion rate, and combustion efficiency of a liquid engine. Injectors are a vital component of a liquid rocket engine that will affect how efficiently the energy of fuel is converted into the needed thrust for a rocket. There are a variety of injectors to choose from. When designing an injector, some factors to consider are the bipropellants used, engine application, viability, etc.

Types of Injectors

Pintle

A pintle injector consists of two concentric tubes and a pintle. The cylindrical tubes are responsible for carrying the propellants to the combustion chamber. Generally fuel will go through the inner tube while oxidizer goes through the outer tube. The pintle is a protrusion at the end, which allows the fuel carried on the inner tube to deflect at a certain angle. The fuels will meet and mix at the impinging point and proceed to combust. By varying the size of the annular and center gaps that the fuel passes through, this allows for throttling of the engine and controlling of the flow into the combustion chamber.

Advantages

A properly implemented pintle injector can achieve combustion efficiency adequate for liquid engines (96-99%). The design is relatively simple and has proven dependability. Performance can be easily optimized by varying the gap sizes. It works in engines that have to be restarted. Overall, this injector is a simple, adjustable, and high performance option.

Disadvantages

Pintle injectors only work well for liquid and gelled propellants. Thermal stress is more concentrated in the certain parts of the combustion chamber which can lead to burn through. Another disadvantage is that there are no correlations for level of mixing and spray size.

Showerhead

Similar to an actual showerhead, the propellants are fed in a straight path into the combustion chamber where they will then atomize and combust. The propellants are sprayed through holes that would maximize atomization.

Advantages

This is the simplest option being relatively easy to make and implement such as by repurposing a commercial showerhead and integrating it into the engine plumbing.

Disadvantages

Mixing is dependent on the turbulence of the propellants entering the combustion chamber. Otherwise, the propellants will go straight in and have poor mixing. In general, the combustion efficiency of a showerhead injector will be low compared to other options making it viable for experimental, non flying engines.

Impinging

Propellants are fed into the combustion chamber at certain angles. To achieve this, many holes are drilled into the face of the combustion chamber. The fuel and oxidizer manifolds can be spaced in different orientations to vary where and how much mixing occurs. Some stream patterns include doublet, triplet, and self-impinging stream patterns.

Advantages

If done correctly, this can achieve strong combustion efficiency and is scalable depending on the size of the combustion chamber.

Disadvantages

This design can be quite complicated to drill sets of holes correctly accounting for entry angles, fluid velocity, and mass flow rate. Atomization efficiency decreases at high entry velocities because droplets will scatter in different directions. The degree of precision and equipment needed for this to be viable is most likely beyond the budget of the club unless a cheaper solution is found.

Coaxial Swirl

As suggested by the name, coaxial swirl injectors consist of coaxial tubes that will feed the bipropellant into a mixing chamber through tangential inlet ports. The oxidizer flows into a swirling chamber at an angle such that it will swirl and then spray out into the combustion chamber to thoroughly atomize. The fuel is fed directly into the combustion chamber where it will atomize with the oxidizer.

Advantages

In theory, can achieve the highest combustion efficiency and thus the highest performance. The spray pattern is similar to a pintle injector but without the need for a pintle to deflect the fuel due to its angular momentum.

Disadvantages

The variables involved in swirling such as the speed and the angle at which the swirling oxidizer is injected can be more difficult to optimize for this injector.

Injector Manufacturing and Assembly

More Resources

• Rocket Propulsion Elements

• Swirl Effects on Coaxial Injector Atomization

• NASA Technical Report on Liquid Rocket Injectors

• NSS on Pintle Injectors

• Pintle Injector used in the Falcon 9

• Scott Manley on Injectors

• Design and Analysis of a Fuel Injector in a Liquid Rocket Engine

Introduction

The feed system can be thought of as a combination of all of the valves, tanks, and pipes in the rocket. The main purpose of this system is to move propellants from tanks to the injector at a specified pressure and flow rate.

As the feed system is composed of pipes, valves, fittings, and tanks, these components greatly affect the operation of the feed system. The pressure and flow rate of the propellants that the feed system is able to deliver to the injector also influences injector and thrust chamber design, as the thrust chamber pressure and injector pressure drop combined must be less than or equal to the feed system pressure delivered before the injector. The feed system must also interface with some external structure to hold it in place during ground testing or flight while integrated into a rocket.

Viable Choices for Us

As is obvious from the above document(RPE Chapter 6), there are many choices for feed system types. For our purposes, a turbopump system is not viable because of its complexity. Under pressurized systems, a flexible bag within the tank and piston pressurization system are also too complex for a college rocketry team to do. Then, the remaining choices that are not too complex for a college rocketry team are Pressurized systems> Direct gas pressurization> By stored inert gas> As received> Regulated pressure & Blowdown. These are the two viable choices for our team that will be further examined below

Pressure Regulated

In this case, gas from a high pressure gas supply tank is flown through a regulator, resulting in a near constant pressure of gas to the propellant tanks. This results in a near constant pressure at the thrust chamber, in turn causing a near constant thrust. The propellants in the system can also be regulated instead by flowing the propellants through a regulator. This choice results in a simpler design for the thrust chamber than in a blowdown system, as it is easier to design a thrust chamber for propellant at a constant pressure than to design a thrust chamber that receives propellant at variable pressures.

Blowdown

A blowdown feed system works instead by storing the highly pressurized gas inside the fuel and oxidizer tanks instead of inside a different tank. As the gas and propellant is in the same tank, they both are at the same pressure. When the valve below the tank is opened, the propellant in each tank can then start flowing to the engine. This figure shows two engines, but the concept is the same with one engine as well. As more fuel is expelled out of the engine, more pressure is lost in the tanks, resulting in a lower pressure of the propellants over the firing time of the engine.

Comparison Between The Two

Useful Links

Rocket Propulsion Elements Chapter 6: Liquid Propellant Rocket Engine Fundamentals

Related Pages

Pipes and Fittings

Valve Types

Valve Actuation Methods

Fuel Choices

Oxidizer Choices

Getting started

Generally, there are two classes of ignition devices in high power rocketry (HPR): igniters/starters, and e-matches. The former are used to start Ammonium Perchorate Composite Propellant (APCP) motors from a 12-volt supply on the ground, while the latter can be used to start smaller black powder motors on the ground, parachute ejection charges in-flight, and second-stage motors in-flight.

Fully commerical-off-the-shelf (COTS) options

From the Apogee website:

• E-matches - While these are regulated by the government and require a Low Explosives User Permit (LEUP) to purchase, you can make your own from a kit without a permit. Order the Starter Chipboards and some special igniter dip designed for low-current igniters. We also have (as of November 2018) an ATF approved non-regulated e-match called the Firewire Initiator and the Firewire Mini that does not require a LEUP! This would be the primary choice if you didn't want to dip your own Starter Chipboards.

Chipboards

This part usually contains a nichrome wire that heats up when current is passed through them.

Pyrogen

This is the part that burns. Apogee sells the H-3 Compound E-match Dip for making e-matches (low-current, low-voltage, to ignite black powder usually) and QuickDip Pyrogen for longer-burning starters (starting APCP motors on the ground or in the air with high current).

Partially COTS options

DIY

Ematch substitute