BT Blog

Reaction Flywheels

2022-01-28T03:12:24+00:00

Reaction Wheels

$I_s$ is the moment of inertia (MOI) of the spacecraft

$\omega_s$ is the desired angular velocity of the spacecraft

$\omega_m$ is the maximum angular velocity the motor can produce

$I_f$ is the necessary flywheel MOI

We start by calculating the inertial-frame flywheel angular velocity, which slightly differs from the motor’s as the stator is mounted to a spacecraft

\[\omega_f = \omega_m - \omega_s\]

We then apply this to the conservation of angular momentum

\[\omega_f I_f = \omega_s I_s\] \[(\omega_m - \omega_s) I_f = \omega_s I_s\] \[I_f = \frac{\omega_s I_s}{\omega_m - \omega_s}\]

To obtain the necessary flywheel moment of inertia to achieve the desired spacecraft angular velocity.

Note that this is independent of the maximum output torque of the motor, only the maximum angular velocity, as a lower torque will only reduce the angular acceleration and not the final angular velocity.

GPS, What The FAQ?

2022-01-22T04:40:00+00:00

1. How can I get my GPS happy even when he’s not?

Three easy solutions to get your GPS happy :

Power the GPS standalone before the rest of the stuff and wait for it to get a fix. Disconnect the standalone cable and reconnect the GPS to the rest.
Try powering runcams, video transmitters and telemetry radios ONLY ONCE GPS fix is acquired.
If none of above worked, the “guide of the good engineer 2022 (updated edition)” recommends using a hammer as a deterrent tool. GPS will quickly understand who’s the boss, and in case he doesn’t, he will quickly cease to be a GPS.

2. Sometimes my GPS is really happy and then a few hours after he’s not, what to do?

The integrated battery on your GPS keep the informations needed for a hot start (happy GPS) for about 3 hours, afterwards your GPS will have to wait to be happy on startup again

3. Can my GPS be happy even higher than so called “COCOM” limits?

YES, your GPS can perfectly be happy up to 50km or even 80km above sea level, you just have to put it in the “airborne <4G mode” if it’s a Ublox GPS. The SAM-M8Q can for example go up to 50km, and the NEO-M9N up to 80km. Oh and by the way “CoCom ceased to function on March 31, 1994, and the then-current control list of embargoed goods was retained by the member nations until the successor, the Wassenaar Arrangement, was established.” :)

4. Is it legal to put my GPS into the illegal dangerous bad (gone wrong!!) “airborne <4G mode”

NO. NO NOOOOOOOOOOOOO, it’s not illegal to put your GPS into the illegal dangerous bad (gone wrong!!) “airborne <4G mode”. It’s designed for. You have to send the right setup command to the GPS via serial. The command is not the same for different GPS models. Try looking for the GPS’s datasheet, and then the communication protocol for your specific GPS model.

5. I want my GPS to only send data when I want, how can I do this?

Most GPS sends data at a fixed rate that you can set using the settings. All the libraries proposing so called “polling” functions are just going to freeze your code until the next time the GPS decides to send a message because he’s happy.

7. Why does the military want to limit my GPS at 4G??!!!

They don’t give a shit about your GPS accelerating at more than 4G but physics does. This is not a limitation but a physical limit. After this limit the GPS position precision will drop.

8. I can’t get my GPS to speak the same language as my TEVECEEE flight computer, what can I do?

Check the baud rate used for the communication between your computer and the GPS (AKA the UART serial port). Use 9600 as a first value. Most GPS use this baud speed as a factory setting.
TX and RX are not gay, TX goes with RX and RX goes with TX.

9. I want to change the settings of my GPS how to do?

Download and install U-Center if it’s a Ublox GPS, otherwise you’re pretty much not happy to not say f.. yeah you got it no need to write it.
Get yourself a UART to USB converter (search for a FTDI dongle) to connect the GPS to your PC. Alternative solution: butcher an Arduino Nano to use it as and FTDI dongle.

10. My GPS can connect to U-Center but I can’t change the settings

Open the serial monitor right when you connect the GPS, and look at the first sentence sent by the GPS.
Open google and search for “Fake Ublox GPS, how to recognise them?” and learn why it was a bad idea to buy a GPS on Amazon for 7$ (you can replace “GPS” by any other hardware you want, it’s pretty much the same).

11. Damn.. so my GPS is a fake, what can I do now?

The SAM-M8Q GPS is a really good, small GPS. You can find a lot of differents breakouts easy to use. I’m sure you know what you have to do.

did you notice point 6 was missing?

Python vector framework

2021-12-16T00:12:18+00:00

A Python Vector Library

In preparation for simulation

In order to design a 6DOF simulation for vehicle testing, we will need a Vector3 object to store 3-dimensional values. Why 3? Think x, y, z; pitch, yaw, and roll. A 6DOF simulation has three axes, so vector values have the same. We begin with the class header:

class Vector3:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

But this setup is not very convenient, and also lacks any form of type hints or guarantees. Let’s spice it up a bit!

@dataclass
class Vector3:
    x : float = 0
    y : float = 0
    z : float = 0

    def __init__(self, x : float = 0, y : float = 0, z : float = 0) -> None:
        self.x = float(x)
        self.y = float(y)
        self.z = float(z)

This class definition and constructor will take advantage of Python’s type hinting, as well as the @dataclass decorator, which allows linters like pylance to report member variables of a class at dev time. The type hints also allow type linters like mypy to scan the file and uncover any potential mismatches, but in the absence of mypy, Python will still accept non-float’s for x, y, and z, so they must be explicitly cast.

For ease of use later on, we will also add a factory function to generate a zero-value Vector3:

@classmethod
def Zero(cls) -> Vector3:
    return cls(0, 0, 0)

However, at this point, we run into a fun problem with python’s type hinting. Types currently cannot be used for hinting until they are complete, which means you can’t say, specify that a method in Vector3 will return a Vector3 object. This will be changed in coming updates to Python, but it can be worked around by simply adding the line

from __future__ import annotations

to the imports of your project.

The @dataclass decorator will automatically generate __str__ and __repr__ functions for us, but the format is somewhat less than desirable, so we’ll override them with our own:

def __str__(self) -> str:
    return ''.join(["(", str(self.x), ",",
        str(self.y), ",", str(self.z), ")"])

def __repr__(self) -> str:
    return ''.join(["Vector3(", str(self.x), ",",
        str(self.y), ",", str(self.z), ")"])

And for comparisons, to allow for checking state variables and other generic checks on Vector3 objects, we implement __eq__ and __ne__:

def __eq__(self, other : object) -> bool:
    if not isinstance(other, Vector3):
        return NotImplemented
    return ((self.x == other.x) and (self.y == other.y) and (self.z == other.z))

def __ne__(self, other : object) -> bool:
    if not isinstance(other, Vector3):
        return NotImplemented
    return ((self.x != other.x) or (self.y != other.y) or (self.z != other.z))

Python requires equality & inequality functions to accept any objects, so the first line of comparison/inequality functions must be a type check to avoid accessing nonexistent member variables. Returning NotImplemented flags the interpreter to check the other object’s type for a compatible comparison function, and is a uniquely Pythonic concept.

More code for the vector math functions to come in pt.2 of this post!

WTF is the cross product

2021-12-15T17:27:04+00:00

Bro wtf

What is this thing like

\[\vec{A}\times\vec{B}\]

How does it

\[\vec{A}\times\vec{B}=|\vec{A}|\cdot|\vec{B}|\cdot\sin(\theta)\hat{n}\]

What in the

\[\vec{\tau}=\vec{r}\times\vec{F}\]

HOW

rej's corner

2021-12-15T17:00:00+00:00

you know what time it is

it’s time for another crusade

what is rej listening to

trash probably. i’m going through it

perry has just warned me

perry has just warned me that i took his repoginity

comedy section

call me a sidewinder cause she guide the missile

support

go give your local aerospace engineer some zip ties

Stock in the Chip Shortage

2021-12-15T04:41:18+00:00

The Chip Shortage Is Real.

There’s no doubt about it. From decades-long lead times to maximum-stock limits on popular chips, the effect of the global pandemic on supply chains is being felt worldwide.

In the midst of such n serious silicon scarcity, what is the humble hobbyist to do?

Finding Stock in a Stockless World

We’ve all felt it by now, the seemingly endless hours of scrolling, searching, trying new filters, only to see the last few items of that seemingly perfect component snatched by an unseen hand. Finding stock is hard enough without a global chip shortage, so here’s a few tips for getting those parts you still need:

Search every supplier you can
DigiKey, Mouser, Arrow, Newark, anyone you can get search terms in.
Alternatively, search octopart for many distributors at once.
Don’t restrict yourself to one package
Many components come in a range of package styles, such as a mixture of SOIC and USON/WSON for flash chips, or QFPs & QFNs for most microcontrollers. Apart from their footprints & dimensions, these part variants often have the same pin assignments as their package relatives, and can be directly swapped in (albeit, with a change of footprint) in lieu of other stock.
If all else fails, find pin-compatible replacements
Some components just don’t have alternate packages, or maybe they’re out of stock too. In that case, you might be able to find similar components using a supplier’s parametric search tool. Some suppliers, like DigiKey most recently, even offer suggestions for replacements in clear view when searched components have no stock. A mixture of compatible replacements and different packages may have luck too.

Sometimes, failure is inevitable.

No matter how far you search or how many alternatives you consider, there are a number of specialized or otherwise hard-to-find components that are just simply unavailable in the current state of the supply chain. Some designs can be reworked to accomodate different solutions, but some will just have to wait, and hope, for the shortage to end soon.

Welcome to the Blog!

2021-12-15T00:30:14+00:00

We’re sorry in advance

This blog is going to be full of a lot of random content, and a lot of stuff that we just think of randomly while working on projects.

This post will serve as a sort of format template so we don’t forget either lol

Format rules

Newlines are ignored. Double newlines act as section spacers.
To create a line break, end a line with two or more spaces
#, ##, and ### are three levels of header
Mathjax can be enabled by usemathjax: true in the header
*Asterisks* make italics
**More of them** make bold text

Mathjax can be used mid-line like $E=mc^2$ or put on its own line

\[E=mc^2\]

But both are formatted as $$math$$

Blockquotes can be made using > to mark the text

Nested blockquotes can be made >> by simply adding more

Formatting works in blockquotes

Unordered lists work by
simply starting each line with
a -

Ordered lists work by
putting a number instead,
like 4.

The rest of markdown should be hopefully self-explanatory.
A sample post front matter can also be found below. Enjoy!

---
layout: post
author: your-name
title: The post title!
description: Something descriptive
summary: Something short
tags: [any, tags]
usemathjax: true
---