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

[Buckeye]

Saucers are stable due to the huge vortex drag off the sharp edges. That effectively moves the CP far rearward. I've never tried it, but I'd be willing to bet than any given saucer would fail a swing test miserably... not enough velocity to generate vortices. Has anyone out there done an OR sim on a saucer?

The Barrowman assumptions

 

[BigMacDaddy]

Nothing is missing from the short rocket simulations. Margin < 1 caliber does not mean unstable. Short fat rockets are stable because the CP is behind the CG. Barrowman simulations show that just fine.

The Hack is of dubious motivation, not validated, and violates the very theory in which it is trying to be applied. That "reassures" you?

So, 29 posts later and nobody has challenged the 3 points I made in the first post. "Don't bother me with the facts and data, Buckeye. The Hack just makes me feel better."

Is the CP behind CG on a saucer rocket? On a spool? On a pyramid?

 

[OzHybrid]

 

[sr205347d]

Use it, then build the rocket and swing test it.

Swing test! Wait . . . weren’t you the one who discovered that a good swing test didn’t necessarily mean that the rocket would be stable?
Thread 'Passes Swing Test, but Unstable in Flight'
https://www.rocketryforum.com/threads/passes-swing-test-but-unstable-in-flight.180232/

 

[kramer714]

Im still trying to figure out what a caliber is... hot dog or burger?

 

[rharshberger]

Swing test! Wait . . . weren’t you the one who discovered that a good swing test didn’t necessarily mean that the rocket would be stable?
Thread 'Passes Swing Test, but Unstable in Flight'
https://www.rocketryforum.com/threads/passes-swing-test-but-unstable-in-flight.180232/

Greater than Zero is supposedly stable, less than Zero is unstable...1 caliber is a suggested minimum because that allows more room for environmental factors to affect a rockets flight..like wind for example. Hence its a rule of thumb as we have discussed already.

 

[lakeroadster]

Swing test! Wait . . . weren’t you the one who discovered that a good swing test didn’t necessarily mean that the rocket would be stable?
Thread 'Passes Swing Test, but Unstable in Flight'
https://www.rocketryforum.com/threads/passes-swing-test-but-unstable-in-flight.180232/

If you are looking for a 100% success rate... find a new hobby.​

 

[Kelly]

So, 29 posts later and nobody has challenged the 3 points I made in the first post. "Don't bother me with the facts and data, Buckeye. The Hack just makes me feel better."

To point #2: As you said in a later post, Barrowman doesn't consider vortex flow. My understanding of stability in flow over short, wide objects is that vortices play a substantial role - that's why saucers and spools are stable. If it's true that vortical flow exists in short stubby rockets, then the Barrowman equations are wrong - according to Barrowman.

 

[JoePfeiffer]

Nothing is missing from the short rocket simulations. Margin < 1 caliber does not mean unstable. Short fat rockets are stable because the CP is behind the CG. Barrowman simulations show that just fine.

The Hack is of dubious motivation, not validated, and violates the very theory in which it is trying to be applied. That "reassures" you?

So, 29 posts later and nobody has challenged the 3 points I made in the first post. "Don't bother me with the facts and data, Buckeye. The Hack just makes me feel better."

I've refuted your first point enough times I didn't feel like doing it once again. Anybody reading the paper without an axe the size of Paul Bunyan's to grind will recognize the goal is to increase accuracy. As for your second point, I don't fully understand how the application of restoring forces from base drag is any less based on normal forces than Barrowman's analysis of pressure on nose cones.

You're right that it's never (to my knowledge) been validated, just like your CFD model -- though I do have more confidence in that than in Levison's article.

 

[JoePfeiffer]

I think I'm seeing Levison's mistake, and why he is showing base drag's effect on CP as much greater than reality. Does anyone know when pods were added to RockSim? In particular, were they present in RockSim 8?

 

[Alan15578]

Actually at least one of the articles starts off by explaining the stability of saucer rockets (or rather flying disks) and suggesting that the base drag hack simulates what we see there.

It is easy to say forget about the base drag hack if all you launch are 10:1 or longer aspect 3FNC or 4FNC rockets.

Many of the rockets I design violate many assumptions of the software and related formulas so need workarounds / "hacks" to try to confirm the likely CP position.

I would rather that base drag was properly modeled in the software but understand that is complicated and maybe only applicable in some cases.

I think we need a whole bank of techniques and approaches to try to confirm the best configurations to make stable rockets but no need to discount one approach that has been shown to be appropriate in a number of circumstances.

I'd be OK with a simulator that allows the user to override the estimated CP and input CP and other "real" aero data into the simulation, similar to mass overrides.

 

[Gus]

The United States just won the team world championship in FAI S1 altitude making extensive use of base drag hack and Bruce Levison's articles. Rocksim suggested our ultra high performance sustainers needed over 4 g of noseweight to fly on an Estes A3-6. Reality was they required less than a half a gram which was well predicted using Bruce's base drag hack.

As a team our cumulative score beat the Romanians by just 0.3 m. I personally finished second in the world helped in great measure by Bruce's techniques.

My daughter, who was on the team with me, and I first met Bruce and his son at Naram 49. We were terribly sorry to lose him to Covid. We considered Bruce was definitely on the medal stand with us for the contributions he made to help put us there.

I know that probably doesn't meet the level of "proof" you're requesting but I personally have no intention of abandoning the technique.

 

[Buckeye]

To point #2: As you said in a later post, Barrowman doesn't consider vortex flow. My understanding of stability in flow over short, wide objects is that vortices play a substantial role - that's why saucers and spools are stable. If it's true that vortical flow exists in short stubby rockets, then the Barrowman equations are wrong - according to Barrowman.

I agree that some different aero is going on with spools and saucers. Barrowman had "general" rockets in mind and probably never envisioned his methods being taken that far. The concept of CP starts getting pretty nebulous once you get past airfoils and rockets.

My beef is with adding the Hack to 3FNC designs, just because. I'll revise my previous statement. Short, stubby rockets (FatBoy) are stable because they have big-ass fins. If you make the fins smaller while relying on the base flow (which the Hack implies), you are probably asking for trouble.

 

[RobertH3]

This will be a truly interesting thread if the math starts to pop out with real world examples.
I flew a Mini-Magg with 16oz of nose weight for my L2. Successful and stable. IMHO it was overkill.
1lb of BB's and epoxy, with a dowel through the PET nose cone for a non-PET anchor. Textbook.

I "think" without having a mathematical argument that some Cg shift forward for weight is sensible in a stubby rocket.

I've heard two arguments - short fat rockets are stable, and the hack is there to make me feel better.
I've also heard length - width ratio, which is interesting.

I agree about ditching the hack, but I also agree that short fat flying vehicles appear to be stable.
One often sees them at launches and they all seem to perform well.
However, you also occasionally see a stubby tilt downwind instead of into it...

It would be interesting to see HOW MUCH stability is generated by base drag / vortexes created around the base of the model.
Safer too, probably.

It is also real world. I am scratch building a 2.6 Fat Boy with a 3x18 cluster. I weighed everything and added 70 grams to the nose cone.
I did not use the base drag hack in Rocksim. With 3xC6-5, the model is 0.88 cal. stable.
There is a concern here - with a slow launch (3xB4-4) it's slightly possible that the nose weight could pull her off trajectory when leaving the launch rod.

YES! Lets do math with this and share. An unstable stubby with a J or K is scary, and math is cool.

Cheers / Robert

 

[Rschub]

Short stubby rockets yeet because the moment of inertia about the CG is less. Balancing a baton on your finger is easier than balancing a spoon.

 

[Buster]

Madcow 4.0 Patriot took some nose weight but was 0.75 Caliber when launched. Started to cone but as the propellant burned it moved the C.G. forward and straightened right out. LOC/PML Stryker flew with a lot of nose weight and was stable. With the Base Drag Hack by adding a nose cone 12" x 4" moved the Cp back considerably. These were hand calculations using Barrowman.


Wouldn't "Bet the Ranch" on the BDH making an unstable rocket stable and launching it.

 

[user 30299]

For those of you interested in learning a little more about Base Drag, I have attached three publications
that will start you off in 1966, bring you a little further along to 1992, and then top you off in 2009.

Much of the work on Base Drag is tied to missiles and artillery shells. If you take a second and think
about it - an artillery shell is a fat stubby rocket; 6" to 8" dia., and 23" to 33" long. And the military
likes to figure ways to get that missile and shell to go that extra mile on existing weapons platforms.

The publications will take you through the subsonic to supersonic range. You will see how angle of
attack, fin location & dimension and boattails impacts base drag. You will read about Base Bleed
and Power On, and how they impact base drag.

If you can take the time to read these, they should provide an opportunity to see the Hack from
a missing perspective.

 

[ThirstyBarbarian]

Nothing is missing from the short rocket simulations. Margin < 1 caliber does not mean unstable. Short fat rockets are stable because the CP is behind the CG. Barrowman simulations show that just fine.

The Hack is of dubious motivation, not validated, and violates the very theory in which it is trying to be applied. That "reassures" you?

So, 29 posts later and nobody has challenged the 3 points I made in the first post. "Don't bother me with the facts and data, Buckeye. The Hack just makes me feel better."

You are mischaracterizing what I said, while I’m at least partially agreeing with you. Did you bring this topic up because you actually want to talk about it, or do you just want to have an argument?

Here, watch this and get back to me.



You said that a margin < 1 caliber does not mean the rocket is unstable, and short, fat rockets are stable because the CP is behind the CG. I agree with that. Less than 1 caliber does not automatically make a rocket unstable, but according to the common “rule of thumb”, you need to start taking a closer look at other factors once you get below that level. Those other factors include things like the length and diameter and the ratio between the two.

For example, I’ve flown short, fat rockets that have a stability margin of .2 or even less, according to the sim programs using Barrowman or similar methods. If I told you I wanted to fly a rocket with a stability margin of .2, and gave you no more information, would you automatically assume the rocket was stable because the CP was ahead of the CG by .2 calibers? Would it make a difference if I told you the length to diameter ratio were 5:1? What about 10:1? What if I said it were 60:1? For that matter, what if I told you I wanted to fly a rocket with solid 1.0 stability margin, right on the rule of thumb cutoff, but the length to diameter ratio were 100:1? Is that a stable rocket?

The hack is not just about the Barrowman equations. It’s also about the rule of thumb for stability margins and unusually proportioned rockets. Barrowman and the 1 caliber rule work great for 10:1 rockets. But they don’t work very well as you get further away from that ratio in either direction.

You have to use some common sense and experience with these kinds of things. For example, if your sims returned a negative stability number, but the hack moved stability into positive territory, that’s probably not something you’d want to rely on. But if the stability is in positive, but marginal, territory, I think the hack is a good way to check if base drag might be a factor. If you have a margin of .2 cal, and you apply the hack, and now the margin is .6 or .7, then I think it’s safe to say the rocket is probably going to fly better than .2 cal might suggest. If you apply the hack, and the margin is still only .3, well, maybe base drag is not going to help much, and maybe it needs some nose weight or bigger fins. That’s what I mean about reassuring yourself. It’s not feel-good voodoo. It’s about doing a sanity check on what your experience with drag is telling you. Is base drag a factor in this design? Do the hack and see.

And I have suggested the hack to people to literally reassure them. The most common scenario is someone flying a Warlock for their L2 test and they are freaking out, because it looks like they are going to need 5 pounds of lead to get to 1.0 calibers of stability. Hundreds of people have flown Warlocks without nose weight, even though the stability margin suggests they aren’t very stable. That information doesn’t necessarily reassure them. But if you explain a bit about short, fat rockets and base drag, and you tell them about the hack, it can alleviate some anxiety. And I’ve never had anyone use the hack and then come back and complain that their rocket flew poorly due to instability, so it seems the reassurance is warranted.

 

[ThirstyBarbarian]

Im still trying to figure out what a caliber is... hot dog or burger?

View attachment 630880

You need more than 1 Caliburger.

 

[Daddyisabar]

Where are you Gerald Bull? Where have you gone...Wernher Von Braun? They are looking down from Heaven (Or maybe up from Hades) and having a good laugh at us. Base drag Hacks, all of us!

 

[ThirstyBarbarian]

Where are you Gerald Bull? Where have you gone...Wernher Von Braun? They are looking down from Heaven (Or maybe up from Hades) and having a good laugh at us. Base drag Hacks, all of us!

Remember when Werner Von Braun got arrested in Florida for his Base Drag Story Hour?

 

[kjhambrick]

For those of you interested in learning a little more about Base Drag, I have attached three publications
that will start you off in 1966, bring you a little further along to 1992, and then top you off in 2009.

Much of the work on Base Drag is tied to missiles and artillery shells. If you take a second and think
about it - an artillery shell is a fat stubby rocket; 6" to 8" dia., and 23" to 33" long. And the military
likes to figure ways to get that missile and shell to go that extra mile on existing weapons platforms.

The publications will take you through the subsonic to supersonic range. You will see how angle of
attack, fin location & dimension and boattails impacts base drag. You will read about Base Bleed
and Power On, and how they impact base drag.

If you can take the time to read these, they should provide an opportunity to see the Hack from
a missing perspective.

Thank you VERY much for the papers @QFactor !

I've started reading the papers and they look very interesting and informative.

The subject of the papers is Base Drag which and not the same as the Base Drag Hack which is the topic of this thread.

Though off-topic for tthe Base Drag Hack discussion, these papers appear to be well worth the read !

-- kjh( if you go in for that kinda stuff, like I do )

 

[kjhambrick]

Remember when Werner Von Braun got arrested in Florida for his Base Drag Story Hour?

What was he wearing THAT day ?

-- kjh ( )

 

[user 30299]

Thank you VERY much for the papers @QFactor !

I've started reading the papers and they look very interesting and informative.

The subject of the papers is Base Drag which and not the same as the Base Drag Hack which is the topic of this thread.

Though off-topic for tthe Base Drag Hack discussion, these papers appear to be well worth the read !

-- kjh( if you go in for that kinda stuff, like I do )

Glad to hear you're taking a stab at the papers.

But they are not off-topic. Once you start understanding what is happening at the
bottom end of the rocket, you'll need to consider if the Hack's supposition squares
with known base drag conditions from the second the motor lights, to burn out,
and the coast.

You'll also need to consider how significant is the the change in CG as the motor
burns down. Remember, the stability margin is based on the distance between
the CP and CG - and that initial margin is "static". The CG quickly moves forward
on most motors and so your margin improves quickly.

Think about those rockets that for just a split second act unstable when coming
off the rail, and then a split second later are suddenly flying straight. Is that a
rocket that had an iffy margin until the CG shifted forward from loss of motor
weight at the bottom? I'm not saying that's the case for all those unstable
launches, but it is for some.

 

[OzHybrid]

The base drag Hack is simply a method of approximating what's going on. It reduces any amount of nose weight that would normally require adding to a stubby rocket and therefore reduces their overall weight which makes them fly higher.

If someone has another solution that approximates what's going on better, we'd all appreciate that contribution.

There is sufficient anecdotal evidence that it does work. So saying it doesn't, is not going to fly. Just like the original math analysis of bee aerodynamics that proved the bee could not fly. Nobody told the bee.....( subsequently updated theory that proves the bee was correct)
OpenRocket is not simulating fully what's going on without the hack for fat rockets.

Perhaps we should pay Joe and Sibo more......

 

[neil_w]

Perhaps we should pay Joe and Sibo more......

Good idea. Starting Monday we’ll double their salaries.

 

[Buckeye]

The hack is not just about the Barrowman equations.

It kinda is, because the hack is using a Barrowman formula for a Barrowman component (transition or nose cone) for something that is not applicable to Barrowman's method (base force). That is inconsistent. I would probably like the hack better if it was completely divorced from Barrowman.

For example, if your sims returned a negative stability number, but the hack moved stability into positive territory, that’s probably not something you’d want to rely on.

Yes, absolutely. That was part of my motivation for starting this thread. That's why I hope the OR developers don't bake the hack into the code, as they have been hinting at doing.

The most common scenario is someone flying a Warlock for their L2 test and they are freaking out, because it looks like they are going to need 5 pounds of lead to get to 1.0 calibers of stability. Hundreds of people have flown Warlocks without nose weight, even though the stability margin suggests they aren’t very stable. That information doesn’t necessarily reassure them. But if you explain a bit about short, fat rockets and base drag, and you tell them about the hack, it can alleviate some anxiety.

Correct. This is where static margin as percentage, rather than caliber, makes things more clear. If Levison was thinking in terms of percent, then the hack never would have been invented, and we wouldn't be discussing it today. His intentions were good, but his choice of metric was bad, imo.

If calibers are still preferred, the rules of thumb are easily modified:

10:1 = 1 caliber is good
stubby = less than 1 caliber is OK
skinny = more than 1 caliber probably needed

That seems easier/more logical than sticking a giant cone on the back of your rocket (and remembering to take it off when launching).

 

[NTP2]

Good idea. Starting Monday we’ll double their salaries.

0•2=0

I’m beginning to think you don’t think that through…

 

[user 30299]

0•2=0

I’m beginning to think you don’t think that through…

You forgot to add the royalties. Then is looks a whole lot better.

 

[JoePfeiffer]

0•2=0

I’m beginning to think you don’t think that through…

I am very confident he did think that through.

Years ago I had the lead in a community theatre play. One of the other actors was getting grumpy about sitting around while the director was working with me, and I told her there was a reason I was getting paid as much as the rest of the cast put together. Turned out she'd never been in a community theatre play before and thought that meant there were people getting paid...