Pipes, Tubes and AN fitting history (10/25/20)

   The best stories are those told well, time to sit down and talk a little history. The kind not taught in school because that's most kinds. This is going to be the story of AN fittings. AN fittings are a standard of fittings developed by the Army and Navy hence the acronym ArmyNavy during the second world war to standardize what kinds of fitting they were using to send fire down on unsuspecting foes. These were fittings that worked on the basis of flaring, which was essentially you take a tube and stick a giant blunt metal cone into it and you turn it, making the tube have a kind of flare on the edge of it. There would be a nut on this tube to be put on prior to the flaring process, which is normally done with a readily available tool. This nut would bolt onto the male end of the other side of the fitting and would thread into the nut on top of the tube and would make a metal on metal seal which is just flat out amazing. You don’t have to worry about orientation or leaks or any of that. Amazing stuff. Veterans came home from the war and decided they needed a hobby and took up by and large the hobby of hot rodding and building race cars that would tear up the drag strip. These guys had seen the AN fittings and used them exclusively in their hot rods, which were clearly noted by their competitors to be much more reliable and leaked a whole lot less and so word spread and they became popular in the general hot rodding scene and then soon after in the general aftermarket scene and to this day they are dominating the aftermarket car scene. 

This leads into rockets very nicely because rocket leaks can be especially deadly. While many swear by compression or overpriced swagelok fittings the AN fittings are much cheaper and work just as reliably and more importantly without teflon tape. That stuff’s amazing by the way, but that’s a story for another time. Rocketeers, while few in number have used a variety of these fittings and really everybody just by and large says to stay away from NPT because they have all done it and have numerous leaks but our mentor with his 50 years of experience says AN fittings are the way to go and that is what we will be going with. 

Another thing that I read about which confused me a fair bit was the difference between pipes and tubes. While they are both various ways to describe a cylinder with a giant straight hole in the middle, they are differently made and more importantly differently measured. For pipes, you have two numbers that matter which are Nominal Pipe Size (NPS) and Schedule #. Which is normally in a multiple of 5 with common ones being schedule 10, 20, 40, 80 which denote in some way the thickness of the wall of the cylinder. The NPS tells us the diameter of the center of the wall of the pipe. So you have your outer diameter and your inner diameter and NPS lies smack in the middle of them. It’s confusing and it spits out weird diameters like 5.563” OD and 5.28” ID but I guess normal is what we know. Pipes in general are much less strictly made but they are made in larger quantities, often used in the oil industry and basically everywhere else too. Tubes on the other hand just have OD and ID outer diameter and inner diameter respectively and wall thickness is just calculated from that. Tubes tend to come in much thinner wall sizes and more sizes in general. We will be using a mixture of both in our rocket.

Thermodynamics, From a Cryogenic Rocket’s Point of View

LOx is probably the world’s best oxidizer. It will oxidize just about anything and everything. It’s nontoxic, not carcinogenic, cheap and widely available. The only caveat is that it is cryogenic, not LH2 temperatures but in the more manageable range of -295 degrees Celsius. LOx is for these reasons, our oxidizer of choice and it has been the rocket industry’s chosen oxidizer for decades. 

Working with cryogens tends to come with very careful design because if the liquid reaches ambient temperature, it will boil, sometimes causing the process of filling a tank to take a few hours for a small tank. Therefore, the process of filling a tank is typically preceded by chilling the cryo tanks with ice. Due to the relatively high thermal conductivity of aluminum, once the temperature of the tank is lowered it’s relatively easy to keep the temperature. Pressurized cryogenic tanks need to have relief valves or burst disks. In our rocket we have a fast-acting relief valve, which gets triggered if we hit a certain pressure and will vent the oxygen above that pressure. Other than needing to use PTFE seals and no other plastics allowed anywhere, and the metals being made brittle at lower temperatures, working with cryogens is pretty straightforward.

On the other end of the temperature spectrum, combustion temperatures in the engine tend to melt uncooled engines, necessitating active cooling, refraction or controlled ablation of the chamber walls, throat and injector face. We solve the injector cooling issue by making our injector out of a material with very high thermal conductivity, allowing the kerosene, which has a high specific heat, to absorb a lot of heat and cool all of the injector face. Note, cooling in this case means <500 degrees, so not something to be touched when in operation. The walls and throat in our case are going to be ablative. In our case, the material is Silica phenolic for the throat and phenolic canvas for the chamber liner. The phenol groups in the phenolic when burned separate the carbon into a different product, forming a carbonaceous foam that tends to burn very slowly, sacrificing itself for the rest of the engine and this happens throughout the chamber liner and throughout our burn. This, along with our film cooling, which due to the lowered mixture ratio around the boundary layer, preserves the engine liner for longer due to lower wall temperatures. At the nozzle essentially the same thing happens but with higher regression rates, higher heat fluxes and better ablation due to the different material.

Other thermal considerations we had were compromisation of the tank structure due to aerodynamic heating during flight but that is something we decided was not an issue after consulting some engineers. Other than that, this rocket is fairly straightforward from a thermodynamics standpoint. Turbopump-fed and other more complex systems are far more interesting from a thermal standpoint. I’m thinking of building a regen cooled self-pressurizing test bed rocket that I can launch locally and mess around with. I think the holy grail of small rockets are those that can launch with minimal infrastructure, minimal supervision, no destruction and no explosions most of the time.

Sources:

Physics (2005). Physics. For high school students. GIANCOLI