r/rocketry

So from what I've understood choked flow occurs if a fluid flows in a pipe with a velocity of mach 1 and no matter how much more we decrease the cross-sectional area of the pipe it won't go past mach 1. Why is that? I've heard explanations saying that mach 1 is the speed that information travels from the right to the left (assuming that the fluid flows from left to right) and so if we decrease the area from the right part of the pipe the information will not be able to travel back to say​ ​"​hey the pressure dropped here, I want​ you to push more". But I don't want that kind of explanations, I want to really understand what happens there. Even if we have to look at the macroscopic level. I want to understand the mechanism. Also I am asking this because I'm trying to understand how the De Laval Nozzle works so if anyone can help me there feel free to drop an answer to this post:​

https://www.reddit.com/r/AerospaceEngineering/comments/1vq87hu/how_does_a_de_lavar_nozzle_workand_what_is/

Hey everyone. I am very pleased to announce that I have founded the Madrid Rocketry club in Madrid, Spain. A free to enter and participate club for both beginners and experienced rocketry hobbyists! The idea is to get together to launch medium and small scale rockets, this is not an official registered club or company etc. The purpose is to launch rockets from safe and authorized locations in the Madrid area as getting authorization for these things in Europe is a pain in the a** (meaning i will take care of all of the bureaucratic stuff) all the clubs near Madrid I could find are closed to the public meaning they are from unis or for competitions. When I moved to this beautiful city I was hoping to enter a club like back in the U.S but there weren’t any! Anyways that’s it if you have any questions feel free to message me and che out the website!!! madridrocketry.com

Hey guys, I'm in high school and I started an aerospace and rocketry club at my community college about a year ago. I really want our club to compete in USLI, but I'm not sure where to get started at all; I know we have to submit the proposal by September 14th.

Sorry this is kind of vague, I'm completely new to this! If anyone would be able to help us that would be amazing. I would really appreciate any information as to how we can compete, what I should get started on first, and what kind of resources we should expect to need! Thanks!

Hi. I want to join my university's rocketry team. I have intermediate experience with Arduino and I am decent at C++ and I also know a bit of MATLAB. Interviews for the team are in 2 months. I want to join the electronics department and I was wondering what would be the best skills to focus on and learn until then. Also if you can recommend some project ideas to do in this 2 months( I learn best from practical work).

hi, so i was making a model rocket, but do not know how to write any code. can someone please help or suggest what i can do. i have components like arduino nano, lithium ion batteries, upconverters, downconverters, sensors and more.

Hey r/rocketry!

I'm planning my first High-Power Rocketry build. I'm aiming for about 450 meters (~1,500 ft) on an H-motor, and I want to set it up for dual-deployment right away to get comfortable with altimeters and pyrotechnics.

My main goals are to keep it light, affordable, and strong, while intentionally keeping the apogee modest so it's easy to track and recover.

Instead of a strict parts list, I'd love to hear how experienced HPR builders would approach this flight profile. What specific components would you pick for this?

Here are the flight profile parameters:

* Target Altitude: ~450 m (1,500 ft)

* Motor Class: 29mm or 38mm H-motor (single-use or reloadable)

* Recovery: Dual-deployment (Drogue at apogee, Main at ~120m / 400ft)

Here's where I'd love your recommendations:

* Airframe & Body Tube: What material and diameter would you use to hit ~450m on an H motor without overshooting? Would you go with a 3.0" Kraft paper/phenolic tube for drag/space, or a smaller 2.2" airframe? Is raw heavy-duty paper okay, or would you glass it?

* Motor Mount Size: Would you build a dedicated 29mm mount to keep it light and cheap, or build a 38mm mount with a 29mm adapter for future versatility?

* Motor Choice: What specific H-motor (AeroTech, CTI, etc.) is your go-to for a low-and-slow HPR flight?

* Altimeter & E-Bay: What's your preferred budget-friendly dual-deployment altimeter and switch setup for a first-time HPR electronics bay?

* Recovery Gear: What sizes/types of drogue and main chutes, shock cords, and retention (shear pins vs. friction fit) would you pair with this setup?

If you were building a 450m dual-deploy H-motor flyer from scratch on a budget, how would you put it together? Any hardware tips or wisdom would be awesome!

I'm the president of a newly established space club at my community college. I was initially planning on attempting to compete for the IREC, but I've seen somethings where its absolutely not for new teams. Is it possible to at the bare minimum qualify? what are some other rocketry competitions, or aerospace competitions that are beginner friendly?

I bought a Featherlight Swift tracker AND Jolly Logic Chute release- In the interest of KISS principles should I forego the chute release. Launching a 4" Loc EZI-65 on an H283 with a projected altitude of 1620 or so.

I'm in the UK and trying to hunt down a pro-29 aft closure. I got my last one from apogee which was fine but they are now sold out (typical). Does anyone know of anywhere I can get one?

Hi there,

I have an idea to build a model rocket with an electronic servo driven parachute ejection system. however, where I live I am not able to obtain any plugged rocket motors with no ejection charge.

my current idea is to have the ejection charge vent through several small, symmetric side ports instead of pressurising the recovery bay.

Has anyone done this before? Is there any reason this wouldn’t work ( regarding side forces, stability, hot gas damaging the airframe?)

I would greatly appreciate any advice :)

Thanks!

Many explanations from my perspective seem incomplete because they do never actually explain the why but only the what happens. Why does adding a diverging section on a throat which gases reach velocities of magnitude Mach 1, accelerate them even more reaching supersonic speeds instead of slowing them down? From what I've read this phenomenon occurs due to the gases being compressible instead of incompressible but still that does not explain the why. Why a compressible's fluid speed increases when the cross-sectional area gets bigger? Is it the opposite of what would happen with an incompressible fluid? It seems to. Also what if we keep just the converging and throat sections only and keep making the throat smaller and smaller? I've watched a video saying that Mach1 is the max speed the gases can have at the throat and if we keep making it smaller it doesn't make them faster. How is that? Can someone give me an in-depth analytical explanation?

I have at least 4 years of experience with robotics and have been wanting to try to create a self landing rocket like SpaceX's Falcon 9 for at least a year now. I have no idea where to start and what I need to know to do this. Thanks!

Hi everyone!

I’m excited to introduce Launch Lab Education, a new Australian business supplying model rocketry kits, components and accessories.

As someone involved in the Australian rocketry community, I know how difficult it can be to find rocketry products locally. Many items involve international suppliers, high shipping costs and long delivery times. Launch Lab Education has been created to help make these products more accessible within Australia and support the continued growth of our local rocketry community.

Our website is now open for pre-sale:

https://launchlabeducation.com.au

Use code LAUNCHLAB10 at checkout for 10% off your order.

The opening pre-sale runs until 31 August. Pre-sale orders will begin shipping after 31 August, with later orders processed each week in line with our Shipping and Delivery Policy.

For South Australian customers attending South Australian Rocketry Club launches, you can also select “Pickup at Blanchetown” at checkout instead of delivery. Your order will be available at the next scheduled club launch, subject to the order cutoff. If a launch is cancelled or postponed, pickup will move to the rescheduled or following launch.

This is only the beginning. We have more products, resources and projects in development, and I would genuinely welcome suggestions about what the Australian rocketry community would like to see stocked locally.

You can also follow Launch Lab Education here:

Instagram: https://www.instagram.com/launchlabeducation/
Facebook: https://www.facebook.com/launchlabeducation

Thank you for supporting a new Australian rocketry business. I’m excited to see where we can take this and how it can support clubs, university teams, schools, families and individual rocketeers across Australia.

Hi everyone,

I've been designing a ethanol nitrous unlike doublet injector, and have been struggling to understand how to deal with the dual phase stuff with sizing nitrous orifices. I posted earlier about some confusions I had about the NHNE model and using it to find area.

I was doing some more research and came across this page on the MIT Rocket Team Wiki that talked about how if you lower your Cd to around 0.4-0.5 you can get away with just using the SPI equation.

I was curious, so I looked up some papers and found 2, "A Two-Phase Mass Flow Rate Model for Nitrous Oxide Based on Void Fraction" and "Experimental Study on the Mass Flow Rate of the Self-Pressurizing Propellants in the Rocket Injector."

I'm kind of stupid, so the only thing I understood from the first paper is that by doing a bunch of formulas I don't understand you can find a correction factor Y to attach to the SPI equation that can make it more accurate, but it's only recommended for single-phase nitrous injectors, and not two phase.

SPC equation with Y formula

SPC with Y'

The paper also comes up with their own model that they call FML, but it has the same problems I have with the NHNE (Dyer edition, as they also label their equation NHNE) where the area is a variable in the SPC and in the HEM model. I also did not find any way of calculating the value of x2 in the paper, so even if I wanted to I couldn't find it.

FML equation

Downstream void fraction

The other paper was a little bit different and found that using the normal SPI formula and changing the Cd to 0.45, they could use the SPI formula to accurately model multiple different injector length/diameter ratios up until flow reached "critical flow," where flashing nitrous vapor chokes the flow and limits mass flow rate. In the figure you can see the critical flow happens at a certain pressure drop. My chamber pressure is going to be 350psi, and with a 20% stiffness my pressure drop is 70psi, or about 4.82 bar, which is well before the critical flow range.

Pressure drop to mass flow rate

Now to where I'm stuck. Both models, bot the Y correction factor and the Cd change to my understanding (which isn't a whole lot) model before the critical flow, and with my pressure drop being so low, I can use either one.

For the Y model, I'm confused whether I should use the Y or Y' version, as one is far more complicated than the other and I would much rather use the Y version.

For just changing the Cd, I'm hesitant about the fact that in their data, all of their mass flow rates were in grams per second, while mine are in kilograms per second. Would the difference in mass flow rate my the effect pressure drop has on critical flow be different? Would my pressure drop still be below the critical flow point? If the mass flow rate unit change doesn't matter, I would much prefer using the modified Cd SPI equation.

If I'm missing some obvious explanation in either paper or totally overthinking this, please let me know.

Is there a specific limit as to how high one can go?