Test Stand Engineering Pt 1
Test stand Engineering Part 1
On the long road to flying a liquid rocket, there is a long testing campaign that hopefully culminates in a working engine and that engine is what flies on the rocket and makes the entire program go forward. There are many components that need to be tested and most of them are to be tested in some form or fashion on the test stand. There’s been a bit of a learning curve to building my test stand - and for once I mean that I am physically fabricating it with my own hands. I’m going to go over all the pitfalls that I’ve made and all the tips and tricks Ken has taught me to solve my various problems.
Why is a test stand needed? I can’t just put everything together and turn it on, some things could fail catastrophically and probably will. However if they fail early in the program then they can just be remade instead of dooming an entire flight vehicle which is a massive amount of wasted time and money. A CATO is unbelievably destructive and often takes out some serious hardware with it. The main components that need to be tested are in order of testing, including the tanks, valves, injector and finally full chamber and captive test firing. The testing of the tanks includes hydro testing which serves to test the welds and pressure holding capacity. A failure in this case will necessitate the repair of the tanks and has occurred in my testing program. A failure in this step of the process is fairly benign, hurting nobody and harming only the faulty test article. The next step is valve cycling which involves running the solenoids at varying pressures to investigate opening and shut off behaviors under test conditions. It’s extremely important that the valves cycle as necessary because if they do not then there will probably be a CATO (big boom). This mostly just involves strapping them to the stand or really anything rigid, and then pressurizing them with the given fluid. This pressurization will probably happen using at least one of the tanks and the valves that need to be cycled are the propellant valve, pressurization valve and all purge valves. Check and fill valves ( which are either ball valves or check valves, do not need to be tested as they are passive and anyhow a failure in them is not really catastrophic.
Following the valve cycling is the cold flow of the entire system. This involves filling up the tanks, and opening all the valves in static test configuration to encapsulate the hydraulic characteristics of the system. This is what then allows for the necessary pressure-drop and temperature data to be collected for the static test to be conducted. Then the static test is where the most stresses are and where things are most likely to fail. This is what the entire testing campaign is leading up to. The engine is turned on and makes the full glorious 1,500 lbs of force and applies it to the stand for 15 seconds. This means everything has got to be bolted down tightly and this is the moment of truth for most of my engineering. The fail or fly moment. If this fails, the engine is probably scrap. And with it probably a good amount of my test apparatus. If it succeeds then the next step is to assemble the full rocket. Honestly, all things considered, that is a fairly easy task compared to the testing phase.
The specifics of the testing apparatus is what I’ve spent the last couple of weeks working on. I’m going to break this down into the following sections; structural, fluids and electronics.
Beginning with the structural portion, the first question to be answered is what kind of stand will be built, vertical, angled or horizontal. I chose horizontal as it was the most feasible option. This later morphed into angled, but it isn’t that big of a change. The distinctions are the direction in which the engine is pointing during firing. Horizontal points the engine horizontally away from the tanks and vertical points the engine down into a flame trench or diverter of sorts. This tends to be used for full vehicles and isn’t feasible for me because the stand would wind up being at least 12-14 feet tall and that is going to be really quite inconvenient in many different ways. An angled stand is like a horizontal stand but instead the engine is tipped towards the ground at an angle of 20 or more degrees to allow any fuel puddled in the engine to fall out of the nozzle as there is a 20-30 degree convergent angle in the nozzle. Any puddled fuel is extremely hazardous. The chamber was only designed for a certain amount of propellant energy in it and to detonate more energy than the capacity of the chamber leads to catastrophic failure. It also necessitates recleaning the injector to liquid oxygen standards which is a really tedious process because of likely contamination, depending on the amount of fuel puddled in the chamber.
Other structural concerns are to protect the tankage, so this is done typically with a metal, concrete or whipple blast shield. This prevents a bigger boom by protecting the propellant valve and tankage to prevent them from literally spraying more fuel on the fire. Oxygen rich fires also burn extremely hot and quickly spread, likely torching the test stand to the ground and are something to avoid at all costs. I picked a ¼” steel blast shield with holes cut into it for plumbing. The next point to consider is hard mounting the tanks. The tanks should be mounted using the same mounts as used in the flight vehicle, which in my case are placed at the bulkheads via axial bolt holes. I’ve been deliberating over this for a while now and came to the conclusion that making a small ring out of sheet metal and welding it to the stand would be optimal from a fabrication point of view. Last but not least in the structural considerations are mounting points and rigging configurations. Ken is suggesting that I drive stakes into the ground and need 6-8 stakes, maybe more depending on whether my trailer mounted configuration is final. These make sure that the stand does not roll, fly or otherwise tumble away. Unlike the rocket, the test stand MUST NOT MOVE. No exceptions. The reason wheels are included are for ease of use during fabrication, plumbing and assembly, which compared to the testing process is a fairly long process. I group them together because assembly leads directly into testing and is highly correlated. Incorrect assembly or plumbing will lead to failure and delays.