How do I handle this nozzle?

[JimJarvis50]

I'm attempting some RasAero simulations. The simulation asks for a nozzle diameter. For the nozzle on the 6" rocket below, is the answer 0 inches, 6 inches or 4-ish inches?

Thanks!

Jim

 

[claytonbirchenough]

I'd say it's 4 inches by the looks of it. I'm pretty sure RASAero is looking for the nozzle exit diameter to aid in calculating base-drag of the rocket when the motor is producing thrust and when it is not... I think the exhaust jet acts kind of like a boattail... All of this is what I think. I may be wrong or may have misinterpreted the question or just don't know what I'm talking about, so feel free to correct me!

 

[Reinhard]

I'm attempting some RasAero simulations. The simulation asks for a nozzle diameter. For the nozzle on the 6" rocket below, is the answer 0 inches, 6 inches or 4-ish inches?

Your answer is 4-ish inches. As ambient pressure decreases, the rocket thrust increases. Ae, the nozzle exit area, is the determining factor how big this effect is.

See the very first formula here:
https://www.braeunig.us/space/sup1.htm

Strictly speaking, pe is an unknown here, but RASAero probably makes the reasonable assumption that the rocket nozzle is expanded to see level ambient pressure.

Reinhard

 

[RocketHunter]

I'd say it's 4 inches by the looks of it. I'm pretty sure RASAero is looking for the nozzle exit diameter to aid in calculating base-drag of the rocket when the motor is producing thrust and when it is not... I think the exhaust jet acts kind of like a boattail... All of this is what I think. I may be wrong or may have misinterpreted the question or just don't know what I'm talking about, so feel free to correct me!

+1, that would be my best educated guess as well, though I don't know how accurate it would be - the thrust plume must vary a lot based upon nozzle design, angle, etc. Also I wonder if there is a way to account for the smoke grain burning - on larger motors I would bet it also has an effect in lowering drag.

 

[Reinhard]

Ok, looks like I was mistaken and Clayton Birchenough was correct. According to RASAero's user manual it uses it to correct the drag coefficient.
https://rasaero.com/dloads/RASAero Users Manual.pdf (page 14)

Reinhard

 

[claytonbirchenough]

Yeah so from the document user's manual Reinhard (thanks Reinhard!) posted above, it does say 0 can be entered if the nozzle exit diameter is not known and that would result in the power on and power off drag coefficient being the same... I messed around with it and it did seem a little finicky (at least for me) to get it to work/display right... I did a MATLAB project that involved ballistic missile trajectories and I simulated V2 missiles being intercepted by hypothetical Nike-Zeus ABMs. Was a lot of fun and actually really interesting... Anyway, I ended up not using the CD RASAero predicted because I couldn't put in an ogive boattail for the V2... Is there no way to do this in RASAero? I tried, but couldn't figure it out at the time... I'll also try and dig up an interesting jets-on and jets-off CD graph I found for the V2. I also found it interesting that they called the exhaust plume jets... As in when the rocket was under thrust, it would use jets-on CD and when there was no thrust, it would use the jets-off CD...

 

[ECayemberg]

Indeed. Rogers is looking for the exit dia, so 4" presumably. I'm watching, Jim!:wink:

 

[claytonbirchenough]

This is the V2 rocket's CD with jets and without jets courtesy of Principles of Guided Missile Design by Arthur Locke.

 

[JimJarvis50]

Indeed. Rogers is looking for the exit dia, so 4" presumably. I'm watching, Jim!:wink:

4-ish it is.

Thanks guys.

Jim

 

[Chuck Rogers]

In RASAero the nozzle exit diameter is used for two things. First, it is used to calculate the power-on drag coefficient. If a nozzle exit diameter of zero is entered, then the power-on CD will match the power-off CD.

The second thing the nozzle exit diameter is used for is to vary the thrust of the rocket motor with altitude. The nozzle exit diameter is used to calculate the nozzle exit area. RASAero assumes the motor data is for sea level (the motor was fired at sea level), and then varies the thrust of the rocket motor with altitude during the flight.

Thus it is important to enter an accurate nozzle exit diameter for two reasons; power-on CD, and thrust with altitude.


Chuck Rogers
Rogers Aeroscience

 

[JimJarvis50]

Thanks for weighing in here, Chuck. By the way, I just completed simulations of my 2013 flights at LDRS (74K) and Balls (118K). I've been meaning to get around to this for quite a while now, but I have finally done it in support of some current modeling. Both simulations were 10% above the actual altitude. Considering non-ideal conditions, wind, a little off vertical, etc., I think that is remarkable agreement.

Jim

 

[dixontj93060]

It would be really good to have nozzle exit diameter as part of the motor data set for the new Thrustcurve version.