Home made 3d printed rocket nozzle

One of the most important parts in a rocket engine is the nozzle. A well designed nozzle will provide the optimum expansion of the exhaust gases and maximize thrust. I am working on a home made rocket, and decided to do a few tests with a real nozzle just to get a real feel of how these devices work.

So for starters, how does a rocket nozzle work and what is it for? The nozzle is basically a tube that redirects the combustion gases into one direction so to push the rocket. While doing so, it also accelerates the gases and lowers their temperature. And all of that without a single moving part. Rocket nozzles achieve this because they belong to the category of Convergent-Divergent Nozzles, or de Laval Nozzles, which take advantage of the properties of gases at supersonic speed. I won't get into much detail about the thermodynamics of ConDi Nozzles, as there are many resources that explain them perfectly (https://en.wikipedia.org/wiki/De_Laval_nozzle).
The important point here is how to design the nozzle, and for that there are a few things we need to know:
- The properties of  the gas that will flow through the nozzle (i.e. air), or gamma-
- The amount of air we can provide per second (mass flow).
- Our working pressures.
About air, all we need to know is the heat capacity ratio, which happens to be just about 1.4.
The amount of air is a bit more tricky. In my case it is limited by the air compressor I use to power the system. It's not so important to know the exact amount of air that will flow as it is to be sure that our nozzle becomes the limiting factor. In order for the air to go supersonic (locally), it needs to choke the nozzle. That means that you must be able to supply air enough to saturate it, or conversely, that the nozzle must be small enough to saturate with the air you can provide. For this reason, the simplest thing you can do is to design the nozzle with a throat smaller than the smallest area of your air feed system. Just measure the ducts of your feed system and choose a smaller size for the throat. My throat is about 4 mm diameter because the smallest duct of my air compressor has a diameter of 6mm. It could actually be even a bit bigger than the duct and still choke (for a bigger area, a smaller pressure will be able to choke the same mass flow) due to the pressure losses, but this way we have some margin.
Finally, the working pressures (along with gamma) will define the expansion ration of the nozzle (the only thing we are left to know to fix its geometry). Expansion ratio, the ratio between the area at the end of the nozzle, and the area at the throat, follows the following formula:

Taking into account that Pe, the expanded pressure will be equal to ambient pressure (1.013 bars) and that my air compressor can give up to 8 bars, that results in an expansion ratio of about 4, so the expansion radius will be about twice that of the throat. I will build my nozzle with a contraction angle of about 30º and an expansion angle of 15º.

As you can see in the pictures, the nozzle is a simple revolution solid.

Notice I let a pretty big input hole. That is because I use 3/8" plumbing to feed the system. That is conventional plumbing that you can buy in any hardware store and is pretty easy to work with and to seal. I also printed a small cap to transform a PVC tube into an adapter to hold the nozzle in place. This way I can put it on top of a weight scale to measure thrust. The picture below shows my poor man's engine test bench, where I can measure pressure vs force.

I didn't expect much of this at first, but the results impressed me. The whole "engine" (the chamber plus nozzle) weight less than 10 grams, and it delivers more than 200 grams of thrust. A force to weight ratio of more than 20. Not bad for 10 grams of plastic.

Prometheus Arm: Latest changes.

In the last few weeks, we have made some improvements in the design of Prometheus arm.
In order to be able to use antagonistic actuation in a simple fashion, it is important that actuation of each degree of freedom remains symmetrical. This means that if you open a finger a little, both inner and outer tensors should be displaced the same length. Initially, a pulley system was our choice for granting this. This iteration was tested in the first version (A3 project) and showd some major inconvenients. Tensors used to degrade quickly and loosed the joints, leading to poor performance of the fingers. In order to solve this issue, we developed a system with adjustable pulleys that allowd tensors to be readjusted.

This moving pulleys are hard to print and require the printer to be very well calibrated. Besides, the teeth degrade rather quickly. Taking into account that, and the fact that fingers were starting to become complex to assemble, this solution didn't seem fitted for anyone to print and assemble at home. A deep redesign was needed.

So we wanted to do three things: Get rid of as many tensors as possible, increase robustness and simplify assembly. The solution is a simple bar actuated mechanism.

Two solid bars link the moving parts of the finger, and keep reduce the number of degrees of freedom to one. This way, the whole finger can be operated from the base and a symmetric mechanism is only needed there. The design is also robust if you choose the right orientation at print time, and the reduced complexity of the features and part count makes it easier to print and assemble. Here is a proof of concept of the finger. The next step is to print a complete finger with the servo adapter, and then test its actuation triggered by electrodes.

Helicopter Electronics: Test #1

Quadcopters are so popular these days, but I still prefer traditional helicopters. They are more efficient, can carry heavier payloads, give a faster response ... and are a bigger challenge. That's why I'm revamping an RC helicopter into an autonomous platform to test a few technologies. This is the first test of some of its electronics.


Sidenote: It is my little brother that you hear in the video, and actually operating the controls.

A perfect fit for A3

It's been a while since the last time I wrote about the A3 project. It has recently morphed into the Prometheus Arm project, in which I work with a friend of mine. What motivated the change? Take a look yourself:



I must admit it. I can't resist Iron Man. That's just it, and now I want to contribute. My friend Pablo and I are now working on a better design of the arm, balancing between cost, simplicity, repairability and functionality, and will try to contribute to Limbitless' project with it.

Talking specifics, we are making the first tests with a 3d printed, improved version of the antagonistic mechanism that allows for better, finer and simpler adjustment of the joints.

The main problem with antagonistic actuation is obviously the cost. Since you are basically doubling the number of actuators, the costs goes up very quickly. However, a good mechanical design can simplify things a lot. I still don't have it fully documented, but I will soon post a method that uses symmetric cinematic chains to factor out some actuators while retaining the basic functionality. Reducing the number of actuators necessarily reduces the number of degrees of freedom (which originally was two for each joint), but I think this is a good trade off if all you are loosing is strength degrees.
More specifically, the tension regulators are shared among similar joints, so they will all be adjusted at the same time, but individual elasticity and torque tolerance is fully kept. I don't think the use-case for trying to exert different amounts of force with each finger is much common, so it is definitely worth the cost drop.

Set up TortoiseGit to work with GitHub

Introduction:
TortoiseGit is the easiest and most comfortable Git interface I know for Windows. It integrates seamlessly with Windows explorer and simplifies common tasks as commit, checkout, pull, push, etc. I'm so used to it that I always recommend it to everyone.
If anything I enforce more than the use Tortoise, is the use Git itself. Version control systems are one of the most powerful tools a developer can have. Switching from traditional (manual) code back up systems to any form of version control system is a qualitative change that can easily increase your productivity by an order of magnitude. Not to mention the benefits of version control when you're working in a multi-person project. In such case, version control is simply a must. And of all VCS I've tried, Git is the by far. Under Windows, Git is supported by mSysGit.
Below, I will present a way to install both mSysGit and TortoiseGit and make the work together with GitHub, one of the most popular repositories over the internet, and the one I use most.

Using C++ in embedded environments

Although C++ is an incredible language, its use in constrained embedded environments is not much common. Somewhat surprisingly, even engineers with a good background of C++ in mid-size environments (e.g. Linux capable systems) often resort to plain old C when they work in small systems like an AVR. I believe there is more tradition than reason to this, and I will try to explain my reasons in this article.

SpaceX Falcon 9R Crash-landing explained.

By now we've all seen this amazing footage. This is Falcon 9's first stage trying to land on the "Autonomous Spaceport Drone Ship" as seen from an onboard camera (on board to the barge, not the rocket obviously). While this is an amazing showcase of great performance of the rocket, and it demonstrates SpaceX was really really near to nail it, it may not seem so at first sight. I mean, even if it had not exploded, it still hit the ship from the side and at about 45º from vertical, right? That doesn't look like the tests of the grasshoper at all, does it?