Using Computational Fluid Dynamics for a Bottle Rocket

R

Russdawg1

Full Audioholic
Hey guys.

Anyone here have a background in fluid mechanics or CFD software?

I thought I’d try my hand at it and I gotta say it’s fun and really useful (even for stupid stuff!)

We’re making water bottle rockets in my physics class and while everyone thinks they have a good idea on what to do from researching the interweb for examples and think they know what they are doing, they are definitely no match for CFD software :p



Hard to make out from the picture (yeah I know I should have taken a better one but I already closed the software and turned off my computer) but it’s a 1 liter smart water water bottle with a 3.5” parabolic-ish(?) cone. Haven’t designed wings yet. I’ll be further improving the design as I get along with this project. Probably end up 3D printing my parts too instead of cardboard. I can get closer to a sharp edge for the wings than cardboard, along with a smoother surface that is just as light.

Anyways, just thought I’d share my plan to crush my classmates bottle rockets with an over engineered bottle rocket.

Feel free to share tips or stories of any time you’ve done something similar :)
 
R

Russdawg1

Full Audioholic
I believe the term for the nose is “ogive”

Interesting.
 
annunaki

annunaki

Moderator
Way to use tech to help aide your design! Do you have an adequate thrust model/airspeed applied in your program? I am not really sure how that works in the program since i have not worked with it. That will be really important as it pertains to control fins.
 
R

Russdawg1

Full Audioholic
Way to use tech to help aide your design! Do you have an adequate thrust model/airspeed applied in your program? I am not really sure how that works in the program since i have not worked with it. That will be really important as it pertains to control fins.
It’s set to 100mph. Not sure if it’s too high or too low. Part of the experiment is finding out all this stuff hahaha.
 
annunaki

annunaki

Moderator
Is the pressure for the bottles limited in anyway? Meaning do all rockets get the same PSI?
Also, what is the criteria for “winning”? Have to start at the end goal and work backwards ;)
 
R

Russdawg1

Full Audioholic
Is the pressure for the bottles limited in anyway? Meaning do all rockets get the same PSI?
Also, what is the criteria for “winning”? Have to start at the end goal and work backwards ;)
All rockets use the same bottle and 250L of water. Pressurized to 90psi or so.

There’s a calculation for how much force will be exerted based on the pressure and mouth area but I don’t have a bottle on me to measure mouth diameter. So I shall wait until tomorrow to get that. Then I can re run all the simulations using a mass estimate (also won’t know that until assembled due to weight variances between the eggs were using as payloads and my chosen way to try and protect the payload) and more reasonable speed based upon these calculations. Although 100 mph seems pretty reasonable.

Designed a 4” parabolic nose cone which modeled pretty well. Only 5N of drag on the rocket (without wings) compared to 48N without a nose cone.



Pictured is a 6” parabolic cone. 5.7N of drag.

Fun all around.
 
annunaki

annunaki

Moderator
Can you add anything to the outlet nozzle? Using Bernoulli’s principle, such as the shaped outlets on the space shuttle could you help better direct the thrust while also over coming the added weight penalty?

Also for your nose cone, have you looked at more of a “coke bottle” shape to the body? How does that model?
 
R

Russdawg1

Full Audioholic
Can you add anything to the outlet nozzle? Using Bernoulli’s principle, such as the shaped outlets on the space shuttle could you help better direct the thrust while also over coming the added weight penalty?

Also for your nose cone, have you looked at more of a “coke bottle” shape to the body? How does that model?
So I’m not allowed to do much to the Rocket other than pick and design the shape and style of the nose and wings. I am allowed to add weight at my discretion and use whatever materials I like, though there are plenty of common materials supplied.

Every bottle will be a 1 liter smart water bottle, so no coke shaped rockets.



I got my printed nose today. Excited for launch date of today or tomorrow.

Couldn’t draw up and print the wings fast enough, was only able to get the cone file out in time. It’s okay as I found some really thin cardboard that will work perfectly. Didn’t CFD model them either as there was no calculation for the proportion of wing effectiveness (stability) to drag.

I’ll let you guys know how my rocket fares compared to the rest.
 
M

Midwesthonky

Audioholic Chief
I’ll let you guys know how my rocket fares compared to the rest.
Please do. I'm interested to see how it turns out. The cone can help with the flow around the bottle, but the change in CG can make it more unstable. I used to play with model rockets as a kid and some of the homemade concepts were a tad more risky as moving the CG towards the nose made them a touch unstable with "surprising" results. No one got hurt and thankfully my mom was never around for those...
 
R

Russdawg1

Full Audioholic
Alright no one be mad.

...



This is where the nose ended up.

Lol. Not my rocket. Obviously.



I decided it would be near impossible to protect the egg with a stiff nose like that. So we used a paper cone nose, very precise, I measured it and cut it out perfectly, and this actually had the same amount of drag as the elliptical nose according to CFD modeling.

Anyways our rocket did well. Did not crush, but did well. My idea was limit drag, so I went with tiny wings, no more than a 2.5x1” half-elliptical shape. And I had multiple people tell me repeatedly that my wings were too small. Funny. Cause their wings were ugly as hell and waaaaay too big. Mine were as close to perfect as possible for a 1st trial. There was some slight wobble on the way up because of how tiny they were (come nose wasn’t centered perfectly), but the wings were effective enough and we got a really good acceleration due to the efficient design and lightweight build.

Good news. One team didnt launch so I will rebuild, with a new cone design and slightly larger, printed wings this time, as they will launch Monday giving me time to do so.

Hopefully I can crush them this time :)
 
annunaki

annunaki

Moderator
Glad it went alright and you get another shot.
Adjust your cone attach method. Too much weight there possibly throwing off your CoG. All the ripples from the tape is messing with your drag modeling. Also, if you sand and polish your nose cone it will help as well.
 
R

Russdawg1

Full Audioholic
Glad it went alright and you get another shot.
Adjust your cone attach method. Too much weight there possibly throwing off your CoG. All the ripples from the tape is messing with your drag modeling. Also, if you sand and polish your nose cone it will help as well.
So I believe our center of gravity was actually near perfect, but we attached our wings too high, pushing our center of pressure too far up and too close to the CoG.

So there’s a chance it could have been as simple as that at we had a helicopter seeking rocket ;)
 
R

Russdawg1

Full Audioholic
A little late to the party, but minor detail.

FWIW, there are rocket sim programs out there like openrocket to make things a little easier.

Looks like an elliptical nose cone, which is good for lower velocity rockets.
Yeah I thought I edited out the parabola part, but it is an elliptical cone.

The reason I was using CFD is eventually I want to be able to know how to model other stuff using it, not just rockets. Like perhaps an intake assembly for a Porsche Boxster haha.
 
M

Midwesthonky

Audioholic Chief
Also, if you sand and polish your nose cone it will help as well.
It would be interesting to model the differences between a smooth cone and one with the 3D printed surface irregularities. Sometimes the rougher surface can increase the thickness of the boundary layer and reduce drag much like a golf ball. We didn't have the CFD programs like this when I got the BSME. Had we had them, I would have preferred that over thermodynamics. I hated thermodynamics. You go along doing the homework thinking your doing alright, then get the test back that says "You idiot!"
 
Mikado463

Mikado463

Audioholic Field Marshall
I believe the term for the nose is “ogive”

Interesting.
well in the shooting world ogive describes the area on the bullet (projectile) where it makes contact with the rifling (lands) of the barrel.
 
Steve81

Steve81

Audioholics Five-0
well in the shooting world ogive describes the area on the bullet (projectile) where it makes contact with the rifling (lands) of the barrel.
No, it's still describing the nose of the bullet.
But what is an ogive profile? Simply defined, it's the curved portion of a bullet forward of the bearing surface.
 
Mikado463

Mikado463

Audioholic Field Marshall
No, it's still describing the nose of the bullet.
gotcha, understood, I was being rather vague in my description .........

 

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