Why Tolerances are Important

As rocket projects become more complicated and with the advent of cheap and readily available machinery, and machinery services (3D hubs for example) a lot of people are starting to push the limits with what is being made in the garage.

When it comes to rockets you are never going to get it right first time and you will soon find yourself in the iteration process as you improve on your designs. If you are having to remake parts from rough or non-existant drawings you may find yourself in a dilemma with parts not fitting and potentially botching a few.

From a mechanical standpoint, there are a few things you can do to make your life easier, a simple tolerance is one of them.

We must first understand what a tolerance is, in engineering, a tolerance is the limit or variation of a physical dimension. This can be set by yourself on how accurate you want your part or it is sometimes set by the machine used, a bad operator can also play a part but for this write up I will not consider this.
As a general rule the higher the tolerance you put on your part the more it will cost, if I wanted a shaft with a diameter of 20 mm ±0.1 mm (19.9 mm to 20.1 mm) this would be easily achievable on a lathe with no extra tooling. If I was to make this ±0.01 mm (19.99 mm to 20.01 mm) then things start getting harder, the shaft would now require a grinding process to achieve this, meaning more time and man-hours and thus a more expensive part.

Not only is cost a factor, but also the fit of the part, which is what we probably really care about matters. If my 20 mm diameter shaft had to fit inside a hole, a bushing for example, and if there were no tolerances involved then how would I know it would fit every time? It could be oversized, undersized or it could be ok.

Luckily for shafts and holes (or anything concentric like a rocket tube and bulkhead), there is a simple ISO tolerance letter/number designation system to make life easy, shown below.

Fits (Credit: Machinery Handbook 29th Edition)

To go along with this, there is a handy Limits, Fits and Tolerance calculator from Amesweb which makes this ISO system easy to understand.

Let’s look at out 20 mm diameter shaft and the bushing it must go into. From the above chart I’d like a sliding fit, with a basis on the hole (hole limits are maintained but shaft limits can vary), therefore I want a H7/g6 tolerance on the shaft and hole.
Plugging this into the above-mentioned calculator yields the following,

As can be seen, I have a nice tolerance dimension that will always enable a sliding fit, but what are these dimensions?

My bush dimension becomes 20 mm -0 mm on the lower end and 20 mm +0.021 mm on the upper end, while my shaft diameter becomes 20 mm -0.020 mm on the lower end and 20 mm -0.007 mm on the upper end.
This gives me a range that I can make each part too, and as long as each part is within that range I will always have a sliding fit, no matter who or where it is made.

This is a very basic introduction, more specifically relating to cylindrical components and fits. In a future post, I’ll go into a bit more detail into the next steps you can take to ensure your parts are concentric and cylindrical using the Geometric Dimensioning, and Tolerance (GD&T) language as well as covering the three basic types of tolerances you may see on a drawing.

BPS.space on TMRO

Video Caption: Joe Barnard of BPS.space joins us to talk about his work in making model rockets emulate larger, liquid fueled vehicles such as Falcon and Electron. He has created many amazing models and has been working on thrust vector control (TVC) steering of models as well as being able to stage and even land model rockets. This is his story.

If you would like to continue the conversation we have a few great ways to do that: – Comment right here on YouTube. We’ll comment back or even feature it in the show – Create a new post on our community forum at https://community.tmro.tv – Head over to our real-time Discord channel here: https://discord.gg/9NkkFWD

DARE: Parachute Research Group News

As well as designing the recovery systems for the upcoming Stratos IV flight, the team will get a chance to test their systems in the REXUS/BEXUS program, first flight in 2020.

From the REXUS/BEXUS website,

The REXUS/BEXUS programme allows students from universities and higher education colleges across Europe to carry out scientific and technological experiments on research rockets and balloons. Each year, two rockets and two balloons are launched, carrying up to 20 experiments designed and built by student teams.

DARE: Engine Testing and Recent Happenings

The team has also released a newsletter covering what they have been up to recently. Download it here to find out!!

Video Caption: Test 16 of the DHX-400 ‘Nimbus’ hybrid rocket motor for the Stratos IV student built sounding rocket.

Delft Aerospace Rocket Engineering is a student-team of Delft University of Technology and one of the largest and most advanced student rocketry teams in the world.

Follow the journey of Stratos IV to space on our social media!

FACEBOOK: www.facebook.com/daretudelft
INSTAGRAM: www.instagram.com/daretudelft
TWITTER: www.twitter.com/daretudelft

November 2018 at FAR

FAR is the Friends of Amateur Rocketry and is an organisation and test facility in the Mojave Desert where experimenters can test and launch their rocket projects.

Video Caption: A few of the larger projects at the FAR Saturday, November 17, 2018

Feed System Design for Pressure Fed Rocket Engines

Continuing on with USC LPL rocket lectures, this second one focuses on the feed system design.
This side of rocket engines is sometimes not heavily covered in the literature out there so this is well worth a watch.

Introducing the QUARK Rocket Project

Based out of the Federal University of ABC (UFABC) in Brazil the QUARK Rocket Project started in 2016. Focusing on the development of hybrid rockets, the team first entered the Spaceport America Cup in 2017, unfortunately, the team did not get to fly but walked away with maximum points on their technical project report.

In late 2017, teaming up with other universities and the Brazilian Airforce, the group launched a solid-fueled rocket to 12,795 ft, gaining a student altitude record in Brazil.

Fast forward and the 21 strong team, sponsored by PION Labs, a new space startup, continue their development of hybrid rockets, focusing on Nitrous Oxide and an in-house fuel called PWCB to deliver the 650 N of thrust from their Gluon engine. Which up to now has been fired 6 times this year.
The engine will power the teams Gluon 3k3 Mission later this year, which aims to test the remote launch control unit, engine control unit, avionics, and remote fill system, as well as reach an apogee of 1.5 km.

The team has their goals set high, after the Gluon mission the focus will shift to making an attempt at the South American apogee record for an amateur experimental rocket, planning to fly in 2019.

You can find out more about the QUARK Rocket Project by checking out their,