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

[Buckeye]

The Base Drag Hack is one of those legends on TRF that gets repeated over and over and assumed to be true. Here are 3 reasons why this technique should be abandoned.

1. The origin of the technique is from Bruce Levison in the Apogee Reak of Flight newsletters

Simulating Short, Wide Rockets in Rocksim 8, Newsletter 154
Simulating Short, Wide Rockets, Part 2, Newsletter 158
Simulating Short, Wide Rockets in Rocksim 8.0, Newsletter 162

Read them closely, especially the first one. Levison wants to add a base vortex CP effect to short/wide rockets. Why? Did Barrowman forget? All rockets have base drag, so why is the technique limited to those less than 10:1? None of this is explained. Instead, Levison's motivation is to make the stable FatBoy (margin = 0.6) have a 1.0+ margin instead, so that it agrees with the 1-caliber rule of thumb. He artificially moves the CP rearward with no evidence that it is a more accurate location on a rocket.

Apogee warns users that they are at their own risk with the method and never implemented a base drag option in RockSim. Neither did Rogers in RASAero.

2. Barrowman's equations use normal forces, and adding an axial base force would be inconsistent. The man himself, Jim Barrowman at NARCON2021, notes that base drag not a normal force and has nothing to do with stability. Scroll to 49:00.

3. My CFD analysis predicts the Fat Boy CP location is around the fin leading edge, similar to Barrowman software and their variations. The additional Base Drag Hack puts the CP much further aft and stands as an outlier. The CFD may not be perfect, but it is an alternative prediction using proven industrial techniques that also disagrees with the hack.

Artificially moving the CP rearward without good reason is a potentially dangerous thing. Don't do it.

 

[lakeroadster]

The Base Drag Hack is one of those legends on TRF that gets repeated over and over and assumed to be true. Here are 3 reasons why this technique should be abandoned.

1. The origin of the technique is from Bruce Levison in the Apogee Reak of Flight newsletters

Simulating Short, Wide Rockets in Rocksim 8, Newsletter 154
Simulating Short, Wide Rockets, Part 2, Newsletter 158
Simulating Short, Wide Rockets in Rocksim 8.0, Newsletter 162

Read them closely, especially the first one. Levison wants to add a base vortex CP effect to short/wide rockets. Why? Did Barrowman forget? All rockets have base drag, so why is the technique limited to those less than 10:1? None of this is explained. Instead, Levison's motivation is to make the stable FatBoy (margin = 0.6) have a 1.0+ margin instead, so that it agrees with the 1-caliber rule of thumb. He artificially moves the CP rearward with no evidence that it is a more accurate location on a rocket.

Apogee warns users that they are at their own risk with the method and never implemented a base drag option in RockSim. Neither did Rogers in RASAero.

2. Barrowman's equations use normal forces, and adding an axial base force would be inconsistent. The man himself, Jim Barrowman at NARCON2021, notes that base drag not a normal force and has nothing to do with stability. Scroll to 49:00.

3. My CFD analysis predicts the Fat Boy CP location is around the fin leading edge, similar to Barrowman software and their variations. The additional Base Drag Hack puts the CP much further aft and stands as an outlier. The CFD may not be perfect, but it is an alternative prediction using proven industrial techniques that also disagrees with the hack.

Artificially moving the CP rearward without good reason is a potentially dangerous thing. Don't do it.

You are being overdramatic. It works as long as you follow the guidelines. Use it, then build the rocket and swing test it.​

​

Nothing "dangerous" about that at all.​

 

[user 30299]

You are being overdramatic. It works as long as you follow the guidelines. Use it, then build the rocket and swing test it.​

​

Nothing "dangerous" about that at all.​

Anyone have a video of a Warlock being swing tested? Or tried swing testing one?

 

[ThirstyBarbarian]

The reason people use it is that it provides an explanation for why short, stubby rockets that simulation software often predict to be unstable actually prove to fly in a stable way in the real world. I agree that the base drag thing is a “hack”, so it’s not perfect, but it’s trying to make up for something that the simulation software and stability “rules of thumb” are apparently missing regarding short rockets. The Barrowman equations, the software based on them, and the rule of thumb calling for 1 caliber of stability are also not perfect, so the hack is something you can use to basically reassure yourself. Until something that is more reliable comes along to explain why short, fat rockets fly stable, the hack has its uses.

 

[rharshberger]

Artificially moving the CP rearward without good reason is a potentially dangerous thing. Don't do it.

So is adding massive amounts of noseweight when its not needed, the base drag hack cuts the noseweight by significant amounts. I use it on my V2s and have yet to have one to prove unstable when flown, and my noseweights are much less than the ones I have inherited from other fliers.

 

[SolarYellow]

I would much rather we all stop using the caliber rule and change to a range of percent of airframe length. That would go a long way toward addressing short/fat rockets flying stably at less than 1 caliber and long, skinny, rockets needing several calibers to work properly. The 1-caliber rule is fine if you're making rockets that look kinda like an Alpha, but it becomes less useful as you move away from the middle of the data cloud.

Pretty much any dang curve you want to draw that goes near the middle of a cloud of data points can look like a pretty good fit to the points in the middle of the cloud.

 

[ThirstyBarbarian]

Things like saucers and spools fly well. Do the Barrowman calculations predict that they are stable? Can you get one caliber of stability in a saucer that itself is less than 1 caliber long? Do spools and saucers swing test well? If you shorten a rocket enough, does it become a saucer, or if you lengthen a saucer enough, does it become a rocket? The hack is about dealing with a messy middle ground where traditional equations and rules of thumb don’t make reliable predictions about rockets that fly very well in practice.

There are more things in heaven and earth, Rocksim,
Than are dreamt of in your philosophy.

 

[BigMacDaddy]

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.

 

[Rob Campbell]

Anyone have a video of a Warlock being swing tested? Or tried swing testing one?

As a Warlock owner, I was thinking the same thing. Or how about a Bruiser?

 

[ThirstyBarbarian]

I would much rather we all stop using the caliber rule and change to a percent of airframe length. That would go a long way toward addressing short/fat rockets flying stably at less than 1 caliber and long, skinny, rockets needing several calibers to work properly. The 1-caliber rule is fine if you're making rockets that look kinda like an Alpha, but it becomes less useful as you move away from the middle of the data cloud.

Pretty much any dang curve you want to draw that goes near the middle of a cloud of data points can look like a pretty good fit to the points in the middle of the cloud.

I do agree that percentage of airframe length is a better guide to stability than the 1-caliber rule of thumb, at least for short rockets. But any rule of thumb is going to be kind of arbitrary. How did the cutoff get to be exactly 1 caliber, not 1.27, or .78? It’s just a convenient round number. Above that 1 caliber, you can be very sure the rocket is stable, and below that, you should probably start taking a closer look at the design. That’s how you use a rule of thumb.

Sometimes there is a design that is right on the edge according to traditional methods and rules of thumb, but “hacks” or experience tell you it has a good chance of working, and the only way to find out is the away cell and a heads-up flight.

 

[ThirstyBarbarian]

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.

Agreed. And I like flying rockets that don’t always sim well, especially short, fat rockets. I like rockets with a lot of drag.

 

[Rschub]

Warlock swing test

 

[jsdemar]

Base drag aero dampening is susceptible to angle-on-attack, and it requires sufficient velocity. On the other hand, adding too much nose weight to a short fat rocket may have the same effect. Insufficient thrust-to-weight ratio, and tip off can happen either way. However, the <1 caliper 'hack' will tend to recover faster from a disturbance.

 

[NTP2]

Warlock swing test

 

[ThirstyBarbarian]

Warlock swing test

Send this one to the Away Pad.

 

[rharshberger]

Send this one to the Away Pad.

Every rocket has a pad...even if its the Away Pad...a long ways away....

 

[smstachwick]

Send this one to the Away Pad.

Away Pad for a swing test, that’s a new one.

 

[Wrightme43]

I just want to watch this thread.
This weekend I am launching a fury of fire 4" for the first time. I added nose weight to make it 1 caliber with a I180. It is a short squat rocket, it would of barely qualified for the base drag hack. I added bb's and epoxy then foam filled the nose cone. It weighs almost 5 pounds, and sims out great in open rocket. I will let you know.

 

[Handeman]

Away pad for testing is a good idea. Our away cell used to be a trailer launcher with a 20 ft. Unistrut rail. We add a 18ft 1515 rail that can be attached/removed if needed several years ago. With the growing number of team launches we've seen over the recent years, we've added a 10 ft. 1515 and 8 ft. 1010 pad to the away cell. Last launch, we had a G motor launched off the 1010 at the away cell. It was about 20x the recommended 30 ft. safe distance for a G motor, but hey, safety first!

 

[cerving]

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?

 

[KenECoyote]

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?

I would agree, but there's also a gravity component in that the saucer would normally be facing up and a swing test would be pretty bad at that if swinging sideways.

In my mind, base drag is almost like a low pressure area under wide rockets that help it to stabilize much like gds.

 

[lakeroadster]

... Has anyone out there done an OR sim on a saucer?

There's a Rocksim discussion of an Art Applewhite saucer in the Simulating Short, Wide Rockets - Pt. 3, Newsletter 162: Simulation of Flying Saucer Designs on Page 5 and 7.​

 

[Daddyisabar]

Just send the "Hacks" to the far far away pads. Way out there they can talk to the high level fliers who can talk some sense into them. The mindsimmer "Hack" with his unsimmable oddroc abomination is always ready for a good belly laugh from the bonified rocket scientists out at the away pads, hooking up their 3-4FNC beauties. They really really know what they are doing, well sometimes at least.

 

[lr64]

Hint: the older NASA and NACA papers are often easier to understand.

 

[mIcahel]

If OddRocs are outlawed only Outlaws will have OddRocs.

 

[cbrarick]

how would you explain the flight of a spool if base drag isn't a thing?

 

[Daddyisabar]

how would you explain the flight of a spool if base drag isn't a thing?

NECROMANCY! THE DEVIL'S WORK WORTHY OF A WITCH HUNT!

 

[JoePfeiffer]

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?

OR wouldn't have any chance of simulating a saucer correctly. It's 'way outside the assumptions in the program.

On the other hand, after rereading Levison's article, a saucer is exactly the shape used to justify the base drag hack, so I expect that would simulate quite well.

 

[user 30299]

If Base Drag is "pulling" the CP rearward, what happens if there is very minimal base drag on a stubby rocket.

 

[Buckeye]

The Barrowman equations, the software based on them, and the rule of thumb calling for 1 caliber of stability are also not perfect, so the hack is something you can use to basically reassure yourself. Until something that is more reliable comes along to explain why short, fat rockets fly stable, the hack has its uses.

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."