How to switch the base-drag hack on and off

[MetricRocketeer]

Hi TRF colleagues,

Yesterday (Saturday, 18 Feb 2023) I started a short thread, which I entitled “Nosecone ballast”, in the High Power Rocketry forum. During the course of that thread, @JoePfeiffer very helpfully suggested that I use the base-drag hack. I did use it, and that enabled me to see the situation in a whole new light. Thank you, Joe.

Now, I know how to modify a rocket design to include that hack. What I would like to do is be able to turn the hack on and off. In other words I want to have the aft cone drawn on the OpenRocket design, but be able to use the cone or not use it. That is, if possible, I don’t want to have two separate design files — one with the aft cone and one without it — and then have to switch between the two files. I would like to have just one design file with the aft cone, which I would then deploy or not.

Can this be done?

Thank you.

Stanley

 

[mikec]

Not that I know of.

The right answer is to properly simulate base drag and not require this hack at all. See https://github.com/openrocket/openrocket/issues/1319

 

[thzero]

Can this be done?

Sure, learn the Java programming language and help the OR team with the above issue.

Otherwise two separate files works. I just copy it, delete it and then paste it back.

 

[rharshberger]

Sure, learn the Java programming language and help the OR team with the above issue.

Otherwise two separate files works. I just copy it, delete it and then paste it back.

Same here, create hack, delete, and undo delete as necessary to get information I need.

 

[kalsow]

Maybe just leave the conical transition in the model and set its length to zero when you don't want it??

I just did a quick test with a LOC Warlock, 7.67" diameter. Without the base drag transition: CP=36.3", apogee 1120' w/CTI H565. With the base drag transition (fore D=0, aft D=7.67', Length=24"), I get CP=43.3" and apogee at 911'. Leaving the transition in place, but changing its length to zero, I get CP=36.3, apogee 1120' -- just as if it wasn't there.

 

[lakeroadster]

On my 4 stage Saturn V, each stage has the hack.

What I did was create a pod for each stage, labeling it as "4th Stage Base Drag Hack", "3rd Stage Base Drag Hack", etc.

This allows you to enter the base drag transition, run the simulation, then delete the transition.

But on a single stage rocket, like the other fellas already stated, just add and delete. Easy Peazy.

 

[MetricRocketeer]

Maybe just leave the conical transition in the model and set its length to zero when you don't want it??

Hi @kalsow,

Thank you for your intriguing suggestion.

The problem is I cannot run the simulation no matter how I set the conical transition.

When I had the transition set as it is supposed to be, none of my simulations worked. They were all successful before applying the hack, and I got the message about a discontinuity in rocket-body diameter. I don't care about the warning, but every simulation failed.

I have now set the length to zero, as you suggested, and I get the same warning plus the warning that "zero-volume bodies may not simulate accurately".

Apparently you were not getting this warning, and you could fly your rocket.

Am I doing something wrong?

Stanley

 

[lakeroadster]

Hi @kalsow,

Thank you for your intriguing suggestion.

The problem is I cannot run the simulation no matter how I set the conical transition.

When I had the transition set as it is supposed to be, none of my simulations worked. They were all successful before applying the hack, and I got the message about a discontinuity in rocket-body diameter. I don't care about the warning, but every simulation failed.

I have now set the length to zero, as you suggested, and I get the same warning plus the warning that "zero-volume bodies may not simulate accurately".

Apparently you were not getting this warning, and you could fly your rocket.

Am I doing something wrong?

Stanley

Post up your .ork file... then we can analyze the issue. A .ork file is worth a thousand posts... typed words

 

[MetricRocketeer]

Post up your .ork file... then we can analyze the issue. A .ork file is worth a thousand posts... typed words

Hi @lakeroadster,

Here's the file. Thank you for asking.

Stanley

 

[lakeroadster]

Hi @lakeroadster,

Here's the file. Thank you for asking.

Stanley

I puled up the file and got this:

​

____________________________________________________________________________________________________________________________
Then deleted the base drag hack and got this

​

Looks like the base drag hack is working to me?

Your thoughts?

 

[MetricRocketeer]

Hi @lakeroadster,

I got the same thing.

Notice that you, like me, are getting the discontinuity warning. But the main problem is that none of my simulations were successful when I had the base-drag hack in place. But nearly all of the flights were successful without the hack in place.

Stanley

 

[lakeroadster]

Hi @lakeroadster,

I got the same thing.

Notice that you, like me, are getting the discontinuity warning. But the main problem is that none of my simulations were successful when I had the base-drag hack in place. But nearly all of the flights were successful without the hack in place.

Stanley

All simulations run with the hack in place

​

____________________________________________________________________________________________________________
And all simulations run with the CG moved to 1.22 as the hack shows the stability to be

​

 

[MetricRocketeer]

Hi @lakeroadster,

Then I am confused.

I am getting all the same results as you do. But the first set of values all show the red exclamation mark. Doesn't that indicate an unsuccessful flight?

Stanley

 

[lakeroadster]

Hi @lakeroadster,

Then I am confused.

I am getting all the same results as you do. But the first set of values all show the red exclamation mark. Doesn't that indicate an unsuccessful flight?

Stanley

No, not necessarilly an unsuccessful flight. To find out the issue double click the red mark, it will explain the issue.

But that doesn't matter. Override the cg of the rocket to get the stability that the base drag predicts... then the error messages go away.

 

[MetricRocketeer]

Over ide the cg of the rocket to get the stability that the base drag predicts... then the error messages go away.

Hi @lakeroadster,

First, I really appreciate your explaining all this to me.

Second, how do I override the CG of the rocket?

Stanley

 

[lakeroadster]

Hi @lakeroadster,

First, I really appreciate your explaining all this to me.

Second, how do I override the CG of the rocket?

Stanley

 

[MetricRocketeer]

This has been highly instructive, and I thank you for that. I didn't know about overriding the CG of the rocket.

Now I don't know if this is important. Before I applied the hack, all of my CG readings were quite low — approximately 0.23, depending upon the motor. But when I ran the simulations, I didn't get any errors. Therefore, changing the CG provides a reassuring number to me. But my simulations all work the same.

Am I making any sense?

 

[lakeroadster]

This has been highly instructive, and I thank you for that. I didn't know about overriding the CG of the rocket.

Now I don't know if this is important. Before I applied the hack, all of my CG readings were quite low — approximately 0.23, depending upon the motor. But when I ran the simulations, I didn't get any errors. Therefore, changing the CG provides a reassuring number to me. But my simulations all work the same.

Am I making any sense?

You can run simulations with very low stability numbers, sometimes the simulations show a successful flight, sometimes not, as is indicated by low apogee and / or ground hits before deployment.

It's on you to ensure the rocket is stable by using good data input, ensuring accuracy, and making sure the stability caliber is one you are comfortable with.

FWIW: Personally I leave the stability cone on, and set the CG and drag of the cone to zero and run the simulation. Then I'll delete the cone and move the CG of the rocket, then run the simulation again. I then compare the data. Why? I don't like moving the cg, it doesn't reflect that actual flight condition of the rocket... and the cg being in the incorrect location affects other calculations, such as pitch and yaw rates.

It's called the base drag hack for a reason....

I love Open Rocket... but in my opinion it would be better if we could move the rockets CP.... and leave the CG alone. And that's why I leave the cone on but zero out it's CG and Drag. That moves the CP... not the CG.

 

[MetricRocketeer]

Thank you so very much, @lakeroadster. I learned a tremendous amount, and I am grateful to you.

Stanley

 

[SolarYellow]

Anyone reading this thread should also read this other thread:

https://www.rocketryforum.com/threads/base-drag-hack-why-not-always-use-it.178095/post-2406767
We have stumbled across a bug in OR that's of critical importance to how the base drag hack is used. Should be fixed in the next release or beta, whenever that becomes available.