Amperka missile, part 2: engine housing, nozzle calculation
For those who entered the thread for the first time, please read the previous release.
Based on the obtained data on the gas pressure in the combustion chamber, it was necessary to select the material corresponding to these requirements for the housing. Our peak pressure reaches almost 25 bar. Without further ado, trying to get away from the use of complex materials, where possible, we decided to adopt the steel pipe DU-40 with a wall thickness of 3 mm. The corresponding pipe was successfully purchased in the first metal rolling on the market. Unfortunately, the warehouse for the production of iron ore was in the open, so the pipe was somewhat rusty.
Cleaning with the help of sandpaper and the “petal stick” of Lesha’s authorship (we say hello to Dr. Dew) did not give a normal effect, and it was too lazy to kill for this time. Why not try the chemical method for this. Of the chemicals within walking distance, there was only acetic essence, citric acid and salt, everything was purchased at the nearest grocery. As luck would have it, there was no suitable basin in which it was possible to pour the vigorous mixture and soak the pipe, it was necessary to construct it using the dendrofecal method from other boxes, using them as a support, and between them make a tray from the film left after airship, which was scraped with office clips. We put a pipe in this crisp sarcophagus and poured it with acidic acid, and added lemons and salt dissolved in water for a better effect. The reaction started instantly. Satisfied with ourselves, we left the pipe to pickle and left with a clean conscience for the weekend.
The smell that met us on Monday consumed our eyes and nose. Yes, they didn’t cover the bath in vain. The smell of vinegar seemed to be ingrained on the walls. Even the windows wide open did not save, then another two days I had to ventilate the studio, so do not repeat our mistakes: such things are best done either outdoors or in a tightly closed container. Nevertheless, the result of cleaning the pipe turned out to be quite satisfactory: the pipe was cleaned both outside and inside. Keep in mind that after applying chemical cleaning, it is necessary to rinse well with water and wipe the item cleaned dry, otherwise it will quickly become covered with a cloudy film in air. Even better is to protect the surface from contact with air with paint, varnish or spray polyurethane. But these are only our aesthetic considerations.
The nozzle is the main element of the rocket engine (your K.O.), because depending on the correctness of its calculation, you can get up to + 30% thrust on the same fuel with the same channel.
We approached the nozzle calculation thoroughly, in detail about the mathematics of its calculation, the principle of operation, the ongoing processes, and indeed, a lot of interesting things, you can read here and elib.osu.ru/bitstream/123456789/8572/1/1805_20110824.pdf. And also on website I found a very convenient Rocki-nozzle tool (scroll down on the page and look for the corresponding link).
Download the program, substitute in the corresponding fields the calculated values of the rocket obtained in Meteor (see article) and get the nozzle profile at the output. We process the data and in SolidWorks we draw a beautiful nozzle in compliance with all sizes.
There should have been a turner next, but it won’t get into this issue, because my friend turner refused to work with the CNC-cabinet and we couldn’t get to it. But by the next series everything will definitely be.
You can download the resulting model from the link at the end of the article.
Test bench, mechanical
Before launching the rocket, we wanted to take traction measurements on the bench in order to compare the real diagrams with those that Meteor calculated for us and check how much you can trust his calculations. In principle, the question of the stand was raised a long time ago and its solution was inevitable, and, as a rule, by trial and error.
The first version was the use of a 10 kg kitchen scale as a sensitive element with a potentiometer riveted to the axis of the arrow. Scales were successfully purchased in the online store and disassembled for ease of use. But by this moment, traction calculations appeared and the understanding that the measurement range of 10 kg would be small, and I did not want to make an error in the form of levers.
Then came option 2: use analog (with a rotating disk) floor scales for people. When disassembling the inside, it turned out to be a dead spring and a lever system, which is extremely unsuitable for use on a stand.
Option 3. For a long time I did not want to use it because of the low measurement speed, however, I had to. Load cells. Having scrubbed over the guts, I found several 50 kg load cells and a module on the HX711 chip at home.
The main problem is that the load cells were not bridge, but half-bridge. Well, you have to put 2 pcs. On the other hand, this is even a plus: we get a stand capable of measuring traction up to 100 kg, and the resolution of the ADC in the HX711 24bit will allow measurements to be made with fairly high accuracy. At least according to our calculations. As it will be, we will verify the most accurate method – the empirical one.
Meanwhile, Lesha assembled the frame of the stand from a 20×20 profile pipe, steel guides and linear bearings. At first they thought that it was possible to put the test engine in such a way that its thrust vector was directed downwards, that is, into the ground, but they abandoned this idea in favor of measurement accuracy, since at the start we will press on the sensor the weight of the engine itself, which will decrease as fuel burns out. Instead, it was decided to direct the thrust vector parallel to the ground, and we will protect the stand from shifting forward by fixing with the help of reinforcement or anchors driven into the ground. Well, or squeeze to a boulder – we will look at the place of testing.
In the next series we plan to completely assemble the stand, fasten the electronics to it, assemble the engine, equip it with fuel, put it on the stand and go to uninhabited places for conducting fire tests. Stay with us – there will be many interesting things.
You can watch the video in the article here:
The principle of operation of the Laval nozzle
Calculation and construction of the Laval nozzle profile
Model of our nozzle