A Quick Reference Guide

Some would say that welding is a last resort. In the aerospace industry, it's said that you should only resort to welding when all other options, including giving up, have been exhausted. You may think your welds are strong and deep until you find out they aren't. microfractures and hidden weak spots make it a reliability risk. Acknowledging this, you should assume the risks involved with welds, and never let your guard down when pressure is involved. Aluminum welds are especially tricky, and welding will remove the temper that gives it so much strength. Mitigate this and take advantage of the convenience of welds by using properly experienced welders and welding techniques.

Not all steels are equal. 304 Stainless steel is about as strong as 6061-T6 aluminum. steel comes in different strengths depending on its composition and how much money they alloy with it. Stainless steel also maintains its strength better at higher and lower temperatures than other steels.

6061-T6 is the standard aircraft aluminum, and is easy to obtain, but is T6 tempered. This means it can't be welded and left alone or it'll lose its temper. The resultant component will have to be re-tempered or designed for it’s less tempered state. It also makes it difficult to bend. Aluminum is more susceptible to fatigue and work hardening, and may someday fail suddenly. Unlike steel, aluminum eventually becomes brittle when heated.

When using longerons, the distance between lateral bracing is critical to its strength. Bracing helps prevent the longerons from bowing out and buckling well under its design strength. You can brace longerons using shear panels or a series of bulkheads. The width of the longeron is also important, as wider structural members are more efficient at resisting bending in its width than a solid core member of equal mass. Square tubes are fairly efficient and easy to work with.

A typical rocket will be designed for a 300 psi chamber pressure. The injector will have a 100 psi drop, and your propulsion system will have up to 20 more psi of major and minor losses. This coincidentally adds up to that magical number 420 psi.

You use tubes for flow, not pipe. Stainless steel 304 tubing is your bread and butter for any rocket, but you will need to use a proper tube bender and deburr the inside edges. Remember to clean the insides of grease and FOD.

Up to a 500lbf rocket can usually use 1/2" or -8 tubes as a minimum for flow. Any higher and you need to use 3/4"(-12) size or greater. Try to avoid 5/8"(-10) because it's an unpopular size that people don't like making parts for.

The best tube fittings to use for flight are double ferrule compression fittings, like swagelok or yorlok. They are less durable in removing and reattaching than flared fittings, but provide a solid and rigid metal seal for your launch vehicle.

With a regulated pressure system, a dome regulator is recommended for high flow pressure feed. The regulator opens up when the pressure in the dome is set, so it acts as a valve to hold pressure in the high pressure tank until the rocket is ready to fully pressurize.

The recommended main valve is a full port ball valve. It can be actuated a number of ways, including gearmotors and linear or rotary pneumatic or electric actuators. To save mass, rockets can use an external main valve linear actuator that stays on the ground and detaches when the rocket takes off and disconnects the actuator arm. Such systems may be available to borrow upon request, but are usually specific to each main valve system.

Propellants are filled and drained from the bottom. Placing a ball valve or quick disconnect stemming off from the main propellant lines lets you attach propellant fill lines.

Ethanol is a common and very forgiving propellant. It is similar in performance to Isopropyl Alcohol. Isopropyl Alcohol is readily available at pharmacies in high purities.

Kerosene is really messy. You may get better performance than Ethanol, but at what cost. It can be cleaned up with kitty litter, but if it should dump without igniting, it wont clean itself up like alcohol does. If you really need consistent performance and was considering RP-1 like what other big rockets use, just use Jet-A. It's all just grades of kerosene, so consider regular kerosene. The nastiest feature of kerosene is that it just gets everywhere, and kerosene can even creep up into oxygen systems can detonate. If a motor has a false start and needs to re-cycle, then kerosene makes this process more complicated.

Pressurant is filled from the top, but since this action is performed immediately before launch, the line that fills pressurant needs to be remotely disconnected. Such a system, a QD sep, is available for purchase from Honkawa Rocketry.

The ullage of both propellant tanks must be kept segregated so the fuel and oxygen vapors do not mix. When feeding pressurant to each tank, they must pass through a check valve first. Check valves can be damaged from normal use in high flow applications, so spares should be prepared, and backflow prevention tests a regular part of any hot fire procedure.

When developing your propulsion system, lots of factors create pressure drops that impact the pressure upstream of your injector. Major losses are generated from friction along the tube walls, and are proportional to the length, surface area, and velocity of the fluid travelling through it. Minor losses are generated from flow disruptions and bends and fittings. Having more fittings creates bumps and pockets, and can shift the flow path diameter. Tighter bends take more energy to divert flow. Dynamic pressure losses occur when draining propellants from a low velocity zone like a tank, to a high velocity like a tube path. Calculating all the losses together is an important initial step to designing your rocket with the correct size tubing, and can help identify what pressure to set your tanks to reach the ideal 100psi deltaP across the injector. It is only approximate however, and empirical test data is necessary to get a true value. There are 3 water flow tests to perform: A vehicle flow test, an injector flow test, and an injector impingement test.

1. The vehicle flow test is run at partial pressure, but the engine injector is replaced by a larger singular known orifice for each side equivalent in area to the actual orifice pattern. You can install premade orifices from McMaster Carr. You fill your tanks with the design amount of water and pressurize everything as if for the full run, but you subtract the engine chamber pressure from the equation so that you get the same deltaP across the injector into the atmosphere, ie. 100 psi. You cannot run this test with your actual injector since the high pressure drop will cavitate and damage the orifices.

2. The injector flow test is designed to determine the Cd of the actual injector orifices. Using a water hose, a pressure gage, and a discharge bucket, you run hose water through the injector and log the feed pressure, trying to get about 25 psi or so. during the test period, you capture the discharged water for a set amount of time, and then weigh the result to get a mass flow rate. You can then reverse calculate the Cd and make adjustments to your injector.

3. The injector impingement test needs to be performed periodically to make sure the orifices are not damaged or clogged. This test is as simple as running water at under 25 psi and observing that the injectors impinge properly and evenly. If any orifices are clogged, you may need to clear it out with needle or drill bit. It's important to thoroughly clean out your rocket so FOD doesn't clog the injector at any point.

Ports need to be capped and plugged to keep dirt and debris out of lines. Tanks and tubes and fittings also need to be cleaned and rinsed well with soap and then distilled water and then IPA.

Baffles help reduce swirling in larger tanks. It won't really be an issue until your tanks start getting wider, like 8" or so. Any less and it may not be worth worrying about. These do contribute to wasted propellant at the end of the burn though, so a good design will incorporate anti-swirl baffles.

When using LOX, you need to make sure your tank and lines are well insulated, and that you have a large enough vent line. LOX will constantly boil off to GOX, and filling the LOX tank requires that the generated GOX be vented out fast enough to be replaced by LOX. Too warm and you generate too much GOX to vent out. If you vent lines are too small then you can't vent out your GOX very well.

LOX will freeze up valves eventually. Ball valves running LOX get cold enough to form ice around it, and can lock up valves. This can be somewhat mitigated by keeping the system dry.

A LOX system will get really cold, and metals will shrink according to their CTE. Different metals shrink differently, which can loosen seals or seize valves.

Entrapped LOX is an explosion risk, since it will evaporate as it warms up. It cannot be contained, as LOX only remains liquid at room temperature at a pressure of 25,000 psi or so according to empirical testing by John Newman. If a tank bursts or fails with LOX inside, the sudden change to atmospheric pressure will cause the LOX to rapidly flash into gas. This is known as a BLEVE. This can occur when closing a ball valve with LOX inside of it, so valves that run liquid must be modified with holes drilled into the ball in the upstream side of a closed valve. The ball seals on the downstream side, but this modification makes the valve orientation critical. Needless to say, you must avoid a situation where LOX becomes trapped in a tank to warm up with no vent, or the whole tank will blow. If you do not understand this phenomena and the risks involved, you may not be allowed to operate the vehicle.

You'd don't need expensive LOX compatible PTs in your system. A neat trick to make any PT LOX compatible is to fill the PT port with a barrier of LOX compatible Krytox lubricant. The GOX will transfer pressure to the PT, without making direct contact with the transducer. A similar method of engine PTs is to use an oil filled port.

High pressure helium will leak proportionally to the pressure it's stored at and the amount of connections it has. The lower the pressure you store it at, the more time you have to use it. Use Buna N O-rings if possible since helium doesn't permeate it as much as other seals.

Pressurizing a tank, such as a COPV, also compresses the gas already inside it. SCUBA tanks are submerged in water to keep this compression heating from weakening the composite and causing a failure. In absence of this water, COPVs must be filled and pressurized slowly, or managed with temperature sensors along the way. With a negative Joule-Thomson coefficient, helium warms up when expanding from the fill bottle, which doesn’t contribute greatly to the warming, but doesn’t help either.

Helium is also generally incompatible with high pressure hydraulic hoses. It can be pumped through at first, but continued use will result in blistering and eventual failure. If used, inspect and replace regularly.

Long term helium shortages makes it difficult to obtain high pressure helium without existing contracts. Unless you already secured a source of helium for your rocket, you should count on using heavier and cheaper nitrogen.

Nitrogen is cheap and inert, which makes it an ideal pressurant for rockets, but it is also an asphyxiant. When venting nitrogen in an enclosed environment it can displace the oxygen in your lungs without you realizing it.

Nitrogen dissolves in LOX, but it does so in proportion to the exposed surface area and at a certain rate. At typical tank pressures, your LOX tank can remain exposed to a high nitrogen environment just fine for about 30 minutes. This doesn’t sound bad at all when you plan to only pressurize a tank in the final minute before firing.

Spark plugs aren't good igniters. Engine ignition of non-hypergolic propellants require they be heated enough to auto-ignite. The best and most reliable ignition source is a separate solid rocket motor, unless of course you have TEA-TEB. Many have tried to make plasma igniters or gas injection igniters over the years, but they tend to phase out.

Epoxy just burns, while phenolic chars and puffs up nicely. Epoxy will not work as well as an ablative, but is a strong and effective insulator. Phenolic is a higher performing, if weaker, material, working best with high purity silica cloth. G10 Tubing makes a reasonable short duration ablative for moderate intensity liquid motors. You can probably make do with 1/4” or 3/8" thick walls for 10 seconds as long as you avoid hot spots. If you can find phenolic-fiberglass, that will perform much better, while cheap phenolic-paper can be used for low intensity motors. A wrapped silica-phenolic is probably the best ablative you can make, without going into high performance carbon-carbon composites. See the ABLATIVES page for more details.

Graphite nozzles are adiabatic. They don't ablate, but rather they just take the heat like a champ. What they don't like is stress, and sonic effects will shatter graphite when it wants to. Graphite nozzles are honestly best left to specialist graphite machinists, and they seem to offer pretty good prices, especially in bulk. Try to avoid machining it yourself or at your local machine shop, since it's incredibly messy. Since it is adiabatic, all the heat that goes through the nozzle has to go somewhere. Make sure to insulate where the graphite is kept to avoid melting your motor casing. The latter half of the expansion section can be made of steel or some other less effective material since it is generally cooler and the boundary layer effects are pronounced. This also helps protect and house your nozzle.

Aluminum is a common and reliable injector material, but may not work well for the rest of the chamber. Exposed aluminum burns more readily in high oxygen, resulting in chamber rich combustion. The aluminum oxide layer is an important barrier protecting the rest of the material. Steel and stainless steel work well as nozzles for burns of 5 seconds or so. Aluminum can certainly be used to make combustion chambers, as demonstrated by Robert Watzlavick, and it's low cost and high conductivity is very appealing, but the risks of self-combustion is still ever-present. Half Cat Rocketry have also taken to using aluminum for their nitrous-alcohol regen motors. Though of a lower intensity, it still works and is very cost effective, especially as nitrous is generally less conducive towards combustion in favor of self detonation.

When 3D printing engines, stainless steel is reasonably cost effective and high performing. Inconel performs better at high temperatures but costs more. Copper cannot be reliably 3D printed as far as I’m aware and titanium will combust readily. It's usually better to use a separate conventionally machined injector since the injector elements will come out at a better and more consistent quality. Being able to access the engine from both ends enables better and easier surface post-processing, and the injector can be repaired or replaced easily. You absolutely do not need to resort to 3D printing to make a regenerative engine.

Regenerative engines derive different benefits based on surface roughness. The interior of the combustion chamber should be kept the smoothest to keep the surface area and friction to a minimum. Lower surface area decreases the heat transfer to the wall, which reduces the amount of heat that needs to be pulled out by the cooling channels.

The coolant velocity inside cooling channels work well with a minimum of 100 fps liquid velocity, based on what Mark and Kevin Baxter suggest. The size of the cooling channels need to be sized to account for this, making small regen engines all the more difficult to make.

Injectors often need to be iterated so the injector elements produce the required flow. The Cd is approximate when designing, like a fingerprint, and needs to be zeroed in through testing. Design your injector to be easy to replace and plan for multiple preliminary injectors. You can always drill in your orifices later.

Flat faced injectors are an efficient type of injector since it distributes flow fairly evenly and directs it downward through the throat. Pintle, coaxial, and swirl injectors produce conical patterns that are harder to regulate and impinge on the chamber walls, disrupting the protective boundary layer. You can have them machined at Honkawa Rocketry.

Injector orifice pairs make a fan when impinging, perpendicular to the plane of intersection. This fan continues to the side where it may impinge again with another fan. This produces a secondary fan that returns it to a radial pattern. This secondary radial fan may impinge on the chamber wall, creating a hot spot halfway between injector elements. This is a good place to put film cooling.

The radius of the impingement patterns should be balanced between the inside and outside to reduce radial flow and vortices. For a single impingement ring, the area on the inside must match the area of the outside.

Film cooling elements take a small percentage of fuel and injects them near the chamber wall to create a fuel rich boundary layer. They don't have to be angled if you don't want them to, but doing so helps to cling the fuel to the wall sooner. Directing them to impinge onto the nozzle converging section is recommended.

The following safety rules are a must-follow if you're going to be a liquid rocket engineer. Nobody benefits when you develop bad habits, least of all you. If you expect to be treated with respect by professionals, you should follow these rules.

Wear ANSI rated safety glasses when working with pressure! Lab goggles don't count either. Even 100 psi is enough to eject FOD or fittings at high velocity. Sharp objects have built-in active guidance directing it towards the closest eyeball.

K-bottles should never be left standing alone unattended. When working with the bottle or standing it up, it must always be strapped or chained to a heavy stationary object like a table. When transporting a bottle, you should use the appropriate hand truck and strap it onto it. Straps are only temporary, and they can decay and burn up, which is why chains are the standard. Please consider what may happen if your motor catches fire with K-bottles and LOX Dewars nearby.

Loosening or even tightening a fitting when pressurized can cause it to come loose. The moment you rotate a thread, static friction turns to dynamic friction, and the fitting may decide to loosen itself up on its own. It's a very rare occurrence, so just because it hasn't happened to you yet, doesn't mean it wont ever.

You must never approach an enclosed and unvented LOX tank with liquid. Don't put absolute trust in your PTs and relief valves. BLEVE can happen anytime.

Make sure surfaces near LOX are clean of grease and organic materials. Only use oxygen compatible lubricants like Krytox.

Oxidizer and Fuel vents should be kept apart and at different heights.

Remember to never fire your rocket without a FAR pyrotechnic operator or operator in training supervising. A liquid rocket requires an evacuation to bunkers before tanks are pressurized and any cryo tanks are closed.