Nozzle Geometry

Moderator: Mike Everman

Post Reply
lilrex
Posts: 22
Joined: Wed Aug 02, 2006 3:22 pm
Antipspambot question: 0
Location: Trinidad CO
Contact:

Nozzle Geometry

Post by lilrex » Tue Jan 08, 2008 1:43 am

I am in need of some help; I want to design a convergent divergent nozzle. I was wondering if someone can help me with the math to do so. I am specifically wanting the angles required to get a good expansion. Without the shocks. any help?

to start

chamber pressure = 150 psi

throat diameter = 1/4 inch

gas will be a mixture of 16.043 g/mol and 8 g/mol

and 188 J * mol * K, 29.378 J * mol * K Entropy.



discharge pressure 35 psi( to start with)

I actually need to iteratively find that base on the velocity of the jet, so I need to write a program to optimize the angles.

thanks!
Please excuse my malipropism when I say "my hypotenuse has been disproven" ~ Ray Rocha

heada
Posts: 118
Joined: Tue Nov 21, 2006 4:34 am
Antipspambot question: 0
Location: Indianapolis

Post by heada » Tue Jan 08, 2008 2:49 am

Will burnsim give you the numbers you're looking for?

http://burnsim.com/

-Aaron

Viv
Posts: 2158
Joined: Sat Oct 04, 2003 2:35 pm
Antipspambot question: 125
Location: Normandy, France, Wales, Europe
Contact:

Post by Viv » Tue Jan 08, 2008 4:02 am

Hi

This should help you out

http://www.aerorocket.com/

Viv
"Sometimes the lies you tell are less frightening than the loneliness you might feel if you stopped telling them" Brock Clarke

Viv's blog

Monsieur le commentaire

dgsharp
Posts: 12
Joined: Sat Nov 24, 2007 3:14 am
Antipspambot question: 0
Location: Audubon, NJ, USA
Contact:

Re: nozzle

Post by dgsharp » Thu Jan 10, 2008 11:58 pm

Keep going, it looks like you're doing good work! One nice thing is that the nozzle geometry really isn't going to kill you if you don't get it perfectly optimized though. You can usually get away with cutting almost any sort of nozzle (like using a stepped bit, a chamfering tool, or even just drilling a straight hole) and from what I've seen the losses compared to a nice convergent-divergent nozzle aren't much.

If it's about optimizing performance, you might want to crank up that chamber pressure. I'm pretty sure that would give you a lot bigger boost than using a more optimal nozzle. Lots of reloadable aluminum hardware out there is good up to about 1000 psi, and that'll give you a healthy boost in your Isp. Even lots of 1" Schedule 40 PVC pipe is good up to about 1000 psi, though it's obviously not rated that high. Jumping from 150 to, say, 500 would be well worth your while if your materials can take it.

Update: here's an example for a certain AN propellant formulation. The Isp at 150 psi is 182.7. Bumping the chamber pressure up to 500 psi brings your Isp to 214.8. A thousand psi brings it to 228.9 seconds. So if you could handle 1000 psi compared to 150, it'd be like carrying 25% more propellant, but the extra wouldn't weigh anything or take up any space (keeps drag down etc).

Here's a link to Wikipedia about the nozzle shape (I'll post the relevant text as well):

http://en.wikipedia.org/wiki/Rocket_eng ... imum_shape

"The simplest nozzle shape is a ~12 degree internal angle cone, which is about 97% efficient. Smaller angles give very slightly higher efficiency, larger angles give lower efficiency.

"More complex shapes of revolution are frequently used, such as Bell nozzles or parabolic shapes. This gives perhaps 1% higher efficiency than the cone nozzle, and is shorter and lighter. These shapes are widely used on launch vehicles and other rockets where weight is at a premium. They are of course, harder to fabricate, so are typically more costly.

"There is also a theoretical optimum nozzle shape for maximum exhaust speed, however, a shorter, suboptimal bell shape is typically used due to its much lower weight, shorter length, lower drag losses, and only very marginally lower exhaust speed."

-Dave

Mike Everman
Posts: 5007
Joined: Fri Oct 31, 2003 7:25 am
Antipspambot question: 0
Location: santa barbara, CA
Contact:

Post by Mike Everman » Sat Jan 12, 2008 5:26 am

Nice tidbit, that.
Mike Often wrong, never unsure.
__________________________

Irvine.J
Posts: 1063
Joined: Mon Jun 05, 2006 4:28 pm
Antipspambot question: 0
Location: Brisbane, Australia
Contact:

Post by Irvine.J » Sat Jan 12, 2008 6:15 am

Perhaps a 12 * expansion on an augmenter might be a good place to set for initial trials, they didnt specify it as a half angle though perhaps it was 12 all up. Interesting stuff sharp.
James- Image KEEPING IT REAL SINCE 1982
http://pulseairdefence.com
[url=callto://project42labs]Image[/url]

Post Reply