Let's put an end to the "Base Drag Hack"

[4regt4]

Gotcha !

So how about this ole-timey, tried -n- true method for determining the CP of a rocket that you KNOW is stable for those pesky RSO's:

Tools needed:

PENCIL
ERASER
RULER
FINGER

In addition, you will need:

A LOADED ROCKET, ready to present to the RSO and fly

Before approaching the RSO, say back at your Prep Area:

1. Measure the diameter of your LOADED ROCKET

That measurement is called One-Caliber.

2. If there is already a CP marker on the rocket, use the ERASER to remove it.

3. Balance the LOADED ROCKET on your extended FINGER.

That is the CG

4. Use the RULER to measure One-Caliber backwards from the CG, toward the Flamey End of the Rocket

5. Mark that point on your LOADED ROCKET with your PENCIL

That is the CP

6. Proceed to the RSO and if the RSO asks, show them the CP marker.

7. Fly your stable rocket



-- kjh

LOL!!

I have a few RSO stories - best left unsaid - that would REALLY leave you scratching your head. Some of these guys shouldn't be at the table.

Hans.

 

[drewnickel]

Imo the person doing the RSO work should be the person most experienced with whatever is coming to the table. You have a hybrid nut like me bring a rocket to the table saying it has 6 calibers of stability at lift off and with a visible pre-inserted igniter and odds are they're going to be like "what in the heck is going on." Two-stage rockets same deal, short stubby rockets, clusters, the list goes on. Not always practical especially at smaller launches, and not always necessary when half the launches are lpr/mpr flying on kits.

 

[lakeroadster]

Levison never explained the origin of using Pi*D for the length. He says many aerodynamic sources show that the flat disk CP is 2.2 calibers behind, yet doesn't bother to reference any of them. He also goes on to say that the CP measurement varies widely. There is not much engineering rigor in the article. People can write anything they like in the POF newsletter. TVM will publish it, but he may put a disclaimer on it, as he did for the Levison articles.

Its a hack:​

Hack: A creatively improvised solution to a computer hardware or programming problem or limitation​

​

Levison doesn't need to justify why he did what he did. He just needed to show how he did it, and define the parameters of its use so others can use the hack.​

​

Don't like it? Continue going about finding a "better" way... but stop trashing the work that Levinson has done.​

​

It works great for what I use it for... short fat odd rocs.​

 

[kjhambrick]

LOL!!

I have a few RSO stories - best left unsaid - that would REALLY leave you scratching your head. Some of these guys shouldn't be at the table.

Hans.

I had better say this:

I DO appreciate the sacrifices that the RSO, LCO and all the other Launch Site Volunteers make so that I can fly my rockets while they watch and work.

-- kjh

 

[4regt4]

I had better say this:

I DO appreciate the sacrifices that the RSO, LCO and all the other Launch Site Volunteers make so that I can fly my rockets while they watch and work.

-- kjh

Yup. And to be fair, most of them are very good. Especially entertaining if you get a funny LCO.

For my L1 cert flight, there were 2 guys at RSO. Both had a good sense of humor. One kept trying to trip me up by asking weird/dumb questions. Like "Did you attach the shock cord to both ends of the parachute?" He said that you'd be surprised by how many say "Yes", probably not knowing how else to respond. When he said "You DID secure the parachute with a rubber band, correct?" The other looked at him and said, "Actually, he did. He's using a JLCR." Both laughed.

OK, back to our regularly scheduled topic...

Hans.

 

[kjhambrick]

<<snip>>Continue going about finding a "better" way... <<snip>>​

​

@lakeroadster --

It is going to be hard to find a better way than your swing test -- that was awesome !

Thanks for the video !!

-- kjh

 

[lakeroadster]

Here's my Red Columbine rocket. It has a stability of 0.701 calibers with the Base Drag Hack, and (negative) -0.209 calibers without the hack.

It's another rocket I never would have built without the Base Drag Hack.

​

​

 

[kjhambrick]

John --

How did you pull the apex of the BDH Cone up inside the MMT on your Red Columbine OR Sim ?

EDIT: I ask because I've always got my MMT extended a tad past the Fin Can Body Tube ...

Thanks !

-- kjh

 

[lakeroadster]

John --

How did you pull the apex of the BDH Cone up inside the MMT on your Red Columbine OR Sim ?

EDIT: I ask because I've always got my MMT extended a tad past the Fin Can Body Tube ...

Thanks !

-- kjh

The MMT is an "inner tube". The Base Drag Hack cone is added to the end of the 2.6" BT-80 Body Tube.​

​

​

 

[lakeroadster]

My current project is a scratch build R2-D2. It's a 4inch body with a L/D of 2.5:1. It's stability with the BDH is 0.917 calibers. Without the BDH it's (negative) -1.41 caliber. I have yet to swing test it, it's not that far built yet. Again... I never would have started on the project had it not been for Levinson's Base Drag Hack.

​

 

[Buckeye]

Here's my Red Columbine rocket. It has a stability of 0.701 calibers with the Base Drag Hack, and (negative) -0.209 calibers without the hack.

It's another rocket I never would have built without the Base Drag Hack.

Now we are getting somewhere. This is an example I hoped somebody would bring up. Negative stability turned positive with the Hack, and it flies OK.

It has been mentioned a few times in this thread that there is probably some merit to the Hack for saucers, spools, and things that don't look like Barrowman rockets. Your thing looks like a shuttlecock and fits that category, I would say. Looks like you found a nice, specific use case for the method. Hack on.

 

[Spacedog49Krell]

I'm currently working on a project to measure base pressure gradients at 200+Hz to produce a 3-D image of base drag pressures from stubby rockets. I hope it enlightens us on this subject.

 

[4regt4]

Now we are getting somewhere. This is an example I hoped somebody would bring up. Negative stability turned positive with the Hack, and it flies OK.

It has been mentioned a few times in this thread that there is probably some merit to the Hack for saucers, spools, and things that don't look like Barrowman rockets. Your thing looks like a shuttlecock and fits that category, I would say. Looks like you found a nice, specific use case for the method. Hack on.

I'm wondering if the stability of this can be explained by the previously mentioned Munk Moment.

Hans.

 

[Buckeye]

I'm currently working on a project to measure base pressure gradients at 200+Hz to produce a 3-D image of base drag pressures from stubby rockets. I hope it enlightens us on this subject.

Like this?

 

[Spacedog49Krell]

Like this?

View attachment 633088

Yes, I'm currently testing a 30 pressure sensor configuration. Twelve sensors measuring the outer skin base pressure. Twelve sensors measuring the low pressure just inside the base with 6 sensors around the higher pressure motor area.

 

[Jeff Lassahn]

Here's my P-40 Warhawk. It's a 3" diameter rocket that is 10-3/16" long, which is a 3.4:1 length to dia. ratio.

I used the Base Drag Hack and stability is 1.04 caliber. Without the base drag hack stability is 0.002 caliber.

I never would have attempted the build without the Base Drag Hack.

Here's my Red Columbine rocket. It has a stability of 0.701 calibers with the Base Drag Hack, and (negative) -0.209 calibers without the hack.

It's another rocket I never would have built without the Base Drag Hack.

Would you be willing to provide the OpenRocket files for these? I'm trying to build up a collection of useful test cases for evaluating stability algorithms and these would be good to include.

Also, do you know how close the CG in the simulation is to the "as built" CG?

Thanks!

 

[bill_s]

My current project is a scratch build R2-D2. It's a 4inch body with a L/D of 2.5:1. It's stability with the BDH is 0.917 calibers. Without the BDH it's (negative) -1.41 caliber. I have yet to swing test it, it's not that far built yet. Again... I never would have started on the project had it not been for Levinson's Base Drag Hack.

View attachment 633068View attachment 633066​

Clearly the CP in front of the rocket is in error. It looks probably stable to me but very unlikely as stable as with your BDH version. That Red Columbine rocket is surely stable with that CG just from looking at it. In both cases the simulator is clearly in error or picking up on something weird about the unconventional data. Then it is being "fixed" in a way almost as questionable. If the simulator wasn't doing something odder than merely ignoring base drag, the hack would be even more overkill.

My previous post #168 got no comment. If part of the base drag effect is an asymmetrical drag, it has no conventional CP location. That portion has zero normal forces to place the CP, therefore its CP can only be merged into the conventional stability model by placing it wherever it needs to be to produce the equivalent result. This has the possibility of being behind the rocket if it is very short, but is more likely in front of the base of a merely stubby rocket.

 

[lakeroadster]

Would you be willing to provide the OpenRocket files for these?

After many hours of simulation and days of building and testing these one off custom designs, I must respectfully disincline to acquiesce your request.​

​

Or put another way.. Sorry, but no.​

 

[lakeroadster]

Clearly the CP in front of the rocket is in error. It looks probably stable to me but very unlikely as stable as with your BDH version. That Red Columbine rocket is surely stable with that CG just from looking at it. In both cases the simulator is clearly in error or picking up on something weird about the unconventional data. Then it is being "fixed" in a way almost as questionable. If the simulator wasn't doing something odder than merely ignoring base drag, the hack would be even more overkill.

My previous post #168 got no comment. If part of the base drag effect is an asymmetrical drag, it has no conventional CP location. That portion has zero normal forces to place the CP, therefore its CP can only be merged into the conventional stability model by placing it wherever it needs to be to produce the equivalent result. This has the possibility of being behind the rocket if it is very short, but is more likely in front of the base of a merely stubby rocket.

The CP in front of the CG is a reality for a given geometry. It's up to the designer to remedy the situation, or to abandon the design as unstable.​

​

Thus is the journey of an odd roc designer. A long and skinny 3/4FNC designer will never experience this.​

 

[RobertH3]

This went from argument to interesting quickly.

Cheers / Robert

 

[kjhambrick]

The CP in front of the CG is a reality for a given geometry. It's up to the designer to remedy the situation, or to abandon the design as unstable.​

​

Thus is the journey of an odd roc designer. A long and skinny 3/4FNC designer will never experience this.​

@lakeroadster --

I was wondering about the red CP dot above R2D2's head myself:

Clearly the CP in front of the rocket is in error ...<<snip>> ...

I believe @bill_s might have been referring to the right-hand R2D2 without the BDH Cone ?

-- kjh

 

[lakeroadster]

Sure, that's the way it works on a short fat rocket. Pull up OpenRocket and try it for yourself.

 

[kjhambrick]

Sure, that's the way it works on a short fat rocket. Pull up OpenRocket and try it for yourself.​

​

View attachment 633149​

That one looks the way I would expect -- The CP is partway down the Nose.

This is because at small angles of attack, the BEq CP of an Elliptic Nose is 'partway back' to the end of the nose (*) and the BEq CP of a cylindrical body is undef.

I was surprised that the CP for R2D2 without the BDH had a negative CP ( position about 1.75 inch before the nose ).

-- kjh

(*) - I would have to go look up the actual formula for an elliptic nose and I am too lazy

EDIT: See @Jeff Lassahn's wind tunnel rockets without the BDH Cone in Post #177 -- each of his CP's are in the Nose.

 

[kjhambrick]

p.s. This reminds me of a request for the OR materials database -- an imaginary material that is invisible when I 'paint' my OR Rocket but is still there.

That way the CP for my DBH Rockets could be printed in the proper spot but the BDH Cone would disappear in the painted view

EDIT: maybe that imaginary material is actually 'air'

 

[neil_w]

p.s. This reminds me of a request for the OR materials database -- an imaginary material that is invisible when I 'paint' my OR Rocket but is still there.

That way the CP for my DBH Rockets could be printed in the proper spot but the BDH Cone would disappear in the painted view

It is discussed a bit in the tutorial: https://openrocket.info/tutorials/base-drag.html
For more fun with transparency: https://www.rocketryforum.com/threads/stupid-openrocket-tricks-beveled-tubes.179618/

 

[kjhambrick]

View attachment 633150

It is discussed a bit in the tutorial: https://openrocket.info/tutorials/base-drag.html
For more fun with transparency: https://www.rocketryforum.com/threads/stupid-openrocket-tricks-beveled-tubes.179618/

Dang !

Thanks @neil_w !!

You guys, as always, are way ahead of me !!!

-- kjh

 

[bill_s]

Keep having to edit my reply to keep up...

The CP in front of the CG is a reality for a given geometry. It's up to the designer to remedy the situation, or to abandon the design as unstable.​

​

Thus is the journey of an odd roc designer. A long and skinny 3/4FNC designer will never experience this.​

Any type of rocket could end up unstable, however your rockets would indicate stable using the cardboard cutout method. Meanwhile the 3D view of the R2-D2, which I presume is the unhacked version, shows a red dot in front of the entire rocket. Surely this is just as odd as a spool being able to fly!. A mystery which would be better solved than by invoking some hack, in this case.

A reply (edited) was try it in OR. I had to add a tailcone to get the CP in front of the rocket. Clearly the R2-D2 shape is not a effective tailcone, remove it. However I then find the rocket still surprisingly hard to make stable. The large diameter nosecone has a very strong effect, some of which may be excessive, but the base here is also large and gets no credit. Once the tailcone issue is cleared up, this may be a good example for base drag being real, but the BDH as you used clearly overcompensates as the resulting margin both is unlikely and unnecessary.

 

[lakeroadster]

Clearly the R2-D2 shape is not a effective tailcone, remove it. However I then find the rocket still surprisingly hard to make stable. The large diameter nosecone has a very strong effect, some of which may be excessive, but the base here is also large and gets no credit. Once the tailcone issue is cleared up, this may be a good example for base drag being real, but the BDH as you used clearly overcompensates as the resulting margin both is unlikely and unnecessary.

There is a tailcone on the rocket... and the BDH is per Levinson's instructions, which have worked for me.​

​

 

[aerostadt]

Here's my Red Columbine rocket. It has a stability of 0.701 calibers with the Base Drag Hack, and (negative) -0.209 calibers without the hack.

It's another rocket I never would have built without the Base Drag Hack.

The proof is in the flying! Nice demonstration! I still have misgivings upon the fundamentals of the hack. It would be nice if there was some theoretical understanding showing that the hack is equivalent to some definite sort of tail-end flow phenomena. Perhaps, it is there, but so far I have not seen it. I have not examined all these posts in detail.

 

[kjhambrick]

Dang, John !

R2D2 is a beauty !!

Not to beat a dead horse but one more try ... @bill_s and I were referring to the right-hand OpenRocket PAINTED R2D2 in Your Post #220 where I believe you might have removed the BDH Cone for the painted image ?

Now that @neil_w told me I can make the BDH Cone invisible in my painted images, I'll not be removing it anymore just to look at my painted rockets

-- kjh