Base-drag hack: Why not always use it?

[SolarYellow]

Thanks Neil.

I just followed your procedure, outlined above, on numerous rockets I've already built. I don't see enough of an impact on the final simulation results to justify being concerned. Negligible impact.

With your builds trending toward the "oddroc" category, I'm speculating that they might be comparatively high drag to start with, in which case, the additional drag of the cone may not be significant. But it clearly makes a difference (~12-13% in apogee without busting out the calculator) with my 4FNC kitbash, even though it's a "low and slow" flyer by 3/4FNC standards. It's nowhere close to Big Daddy short/fat at 8.07:1.

 

[lakeroadster]

With your builds trending toward the "oddroc" category, I'm speculating that they might be comparatively high drag to start with, in which case, the additional drag of the cone may not be significant. But it clearly makes a difference (~12-13% in apogee without busting out the calculator) with my 4FNC kitbash, even though it's a "low and slow" flyer by 3/4FNC standards. It's nowhere close to Big Daddy short/fat at 8.07:1.

Yikes... that is pretty dramatic



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On my Level One rocket... Probably the biggest difference that affects the flight is motor delay. I'll need to be sure and review delay's before I buy motors.

I'm hoping to log some data with my recently purchased Altimeter Two to verify some of this data.

 

[QFactor]

2.60" O.D., 3" o.d. and 4" o.d.

Thanks for the info. Very helpful!

 

[lakeroadster]

Thanks for the info. Very helpful!

You're welcome.

But keep in mind, as @SolarYellow pointed out, the more aerodynamic the rocket is, the more this will effect the simulation.

 

[QFactor]

You're welcome.

But keep in mind, as @SolarYellow pointed out, the more aerodynamic the rocket is, the more this will effect the simulation.

At this time I don't use the "hack" on any of my stubby rockets. It's interesting to see what everyone comes up
with for "numbers" when running sims. But I don't think anyone has set out any flight results, other than what's
in the original POF articles on the "hack".

If someone has flight results, please point me in that direction.

For me the "jury is still out" as to whether the hack is applicable. From what I have read on Base Drag, it can be
a complicated condition, and it's a large portion of a rocket's overall drag.

 

[BigMacDaddy]

At this time I don't use the "hack" on any of my stubby rockets. It's interesting to see what everyone comes up
with for "numbers" when running sims. But I don't think anyone has set out any flight results, other than what's
in the original POF articles on the "hack".

If someone has flight results, please point me in that direction.

For me the "jury is still out" as to whether the hack is applicable. From what I have read on Base Drag, it can be
a complicated condition, and it's a large portion of a rocket's overall drag.

I think there is tons of flight evidence that short stubby rockets fly more stable than simulations would predict.

I have that experience with a number of rockets but none are as straightforward as a Big Daddy or similar (e.g., in my cases there is usually GDS or other draggy elements instead of simply a 3/4FNC sub 10:1 ratio short fat rocket).

 

[QFactor]

I think there is tons of flight evidence that short stubby rockets fly more stable than simulations would predict.

I have that experience with a number of rockets but none are as straightforward as a Big Daddy or similar (e.g., in my cases there is usually GDS or other draggy elements instead of simply a 3/4FNC sub 10:1 ratio short fat rocket).

I think there is incidental and inferred information, but not one where someone went out and
ran multiple sims and then ran multiple launches.

If you know of one, please post a link on TRF.

It would be very informative if we could look across at least three of the common large diameter rockets.

This would be a great project for high school or university students. Put all those L1 and L2 rockets
to work that the university students crank out before NASA SLI and Spaceport Cup competitions.

 

[neil_w]

I think there is incidental and inferred information, but not one where someone went out and
ran multiple sims and then ran multiple launches.

If you know of one, please post a link on TRF.

It would be very informative if we could look across at least three of the common large diameter rockets.

This would be a great project for high school or university students. Put all those L1 and L2 rockets
to work that the university students crank out before NASA SLI and Spaceport Cup competitions.

The problem is that it's somewhere between difficult and impossible to measure stability from doing test flights. I'm not sure what you could do to come up with any sort of accurate quantitative results, especially since the whole "1 caliber stability rule" is only an approximate rule of thumb in the first place.

I suspect a detailed CFD analysis would provide clearer conclusions, and that would be great university project as well.

If there's agreement that base drag increases stability (which seem hard to argue, unless you want to come up with another reason why saucers fly), then the debate boils down to the question of quantifying the effect. Until anyone can figure out how to do it, I'm satisfied with the rough approximation provided by the hack we all know and... well, not necessarily "love", but at least "use".

If you suspect that the existing hack is overly optimistic, then one possibility would be to simply use a smaller cone.

 

[Buckeye]

The problem is that it's somewhere between difficult and impossible to measure stability from doing test flights. I'm not sure what you could do to come up with any sort of accurate quantitative results, especially since the whole "1 caliber stability rule" is only an approximate rule of thumb in the first place.

Perhaps run a series of flights, moving the CG more rearward each time, until the flight is deemed "unstable." This would be the point where the CG crossed over the CP location.

 

[QFactor]

The problem is that it's somewhere between difficult and impossible to measure stability from doing test flights. I'm not sure what you could do to come up with any sort of accurate quantitative results, especially since the whole "1 caliber stability rule" is only an approximate rule of thumb in the first place.

I suspect a detailed CFD analysis would provide clearer conclusions, and that would be great university project as well.

If there's agreement that base drag increases stability (which seem hard to argue, unless you want to come up with another reason why saucers fly), then the debate boils down to the question of quantifying the effect. Until anyone can figure out how to do it, I'm satisfied with the rough approximation provided by the hack we all know and... well, not necessarily "love", but at least "use".

If you suspect that the existing hack is overly optimistic, then one possibility would be to simply use a smaller cone.

I prefer to look at it as a "challenge" somewhere between difficult and impossible. That's when we learn new things.

CFD would be part of the challenge's design process, but it's still only a mathematical tool.

Test results (flights) are always needed to see where the computational analysis falls. Accurate quantitative results are
difficult in any data collection, but if patterns can be discerned in those results, then we have a little more to go on.

 

[lakeroadster]

At this time I don't use the "hack" on any of my stubby rockets. It's interesting to see what everyone comes up
with for "numbers" when running sims. But I don't think anyone has set out any flight results, other than what's
in the original POF articles on the "hack".

If someone has flight results, please point me in that direction.

For me the "jury is still out" as to whether the hack is applicable. From what I have read on Base Drag, it can be
a complicated condition, and it's a large portion of a rocket's overall drag.

I have rockets that I've built that simulate as unstable without the hack.. yet they fly stable. If I didn't use the hack... I never would have built them.

Red Columbine: Stability 0.701 with the Base Drag Hack

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Red Columbine: Stability -0.209 without the Base Drag Hack

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[QFactor]

I have rockets that I've built that simulate as unstable without the hack.. yet they fly stable. If I didn't use the hack... I never would have built them.

Red Columbine: Stability 0.701 with the Base Drag Hack

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Red Columbine: Stability -0.209 without the Base Drag Hack

View attachment 569173

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Thanks for posting this!

 

[QFactor]

Perhaps run a series of flights, moving the CG more rearward each time, until the flight is deemed "unstable." This would be the point where the CG crossed over the CP location.

I would be able to do this on my Phoenix rocket; 4" dia., 29 or 38mm motors. Stubby rocket, significant diameter,
and know for stability challenges.



It has a nose-weight setup for a quick change-out.



I would need to add something like the Eggtimer Ion to collect basic flight data, or a more sophisticated
(don't mean to hurt your feelings Cris) data logger with more than a basic barometric sensor.

 

[lakeroadster]

I would be able to do this on my Phoenix rocket; 4" dia., 29 or 38mm motors. Stubby rocket, significant diameter,
and know for stability challenges.

View attachment 569209

It has a nose-weight setup for a quick change-out.

View attachment 569210 View attachment 569211

I would need to add something like the Eggtimer Ion to collect basic flight data, or a more sophisticated
(don't mean to hurt your feelings Cris) data logger with more than a basic barometric sensor.

If it's a scale build of a Phoenix, it's Length to Diameter ratio exceeds 10:1, so it's not a good candidate for base drag testing, IMO.

Ideally you want a rocket that simulates unstable, that the only reason it is stable, is due to base drag. And a 5:1 L/D would be a good choice, again IMO.

But in reality, as has been pointed out previously, what data are you looking for that would quantify the base drag hack, and how would that data be provided by onboard data gathering?

 

[SolarYellow]

"Stability" is not a bright line. The "1.0 caliber" rule is just a rule of thumb. I don't even think it's a good one. I've written elsewhere about how I think the proper denominator should logically involve the length of the rocket rather than the diameter. I've seen references to "real" scientific literature that supports that hypothesis, I just haven't read far enough to find that basis myself. On a forum I used to read, one person's sig was something like, "Never use a rule of thumb. It's prone to mildew and invariably not straight."

One of the problems with design of experiment for stability of an aerodynamically stabilized rocket is that it is about the response to disturbances and how the rocket recovers. The nature and magnitude of disturbance may differ enough from flight to flight (some variance systematic, predictable, measurable and some completely random) to generate a large scatter in the results. Ideally, you want a solution that's not marginal, but robust. On the other hand, an overstable rocket also leads to a variety of problems. So there's a "zone" of optimized compromise. The sweet spot of that zone may vary depending on conditions, and it will vary depending on motor characteristics. The "optimum" answer across the range of conditions one might fly in may involve changeable ballast.

All that is to say, I suspect a large amount of work could be done and money spent to generate data that is only directionally useful for a particular rocket configuration, and we kinda already have that level and type of information available.

 

[QFactor]

If all goes well, l will let you guys know what I find out, hopefully by the end of the year.

Pencil, paper, old calculator - good to go.

 

[Buckeye]

Love the HP11C calculator with RPN! I had a 15C until my workplace flooded, and it went swimming. I use an HP RPN lookalike app on my phone.

 

[QFactor]

Love the HP11C calculator with RPN! I had a 15C until my workplace flooded, and it went swimming. I use an HP RPN lookalike app on my phone.

One 11C I use at my desk. The other one goes to the job sites.

The 15C stays in the file cabinet and requires security clearance before you can pick it up.

All three work, are a little beyond 35 years in age, and I'm their original owner.

 

[SolarYellow]

That reminds me I need to buy a 48G one of these days.

 

[Handeman]

The 15C stays in the file cabinet and requires security clearance before you can pick it up.

Oh No! It's not properly labeled with the security level! VIOLATION!

Sorry, 35 years working in SCIFs.

 

[BigMacDaddy]

So obviously the base drag hack is most commonly applied (or maybe always applied) based on the body tube's base surface area. However, what other drag inducing elements are appropriate to estimate the drag for with the base drag hack?

For example, do the wedge-shaped X-15 fins generate more drag? Should they be modeled? What about the side pods?

View attachment 565557

More recently I am modeling the Moonliner. Should I be estimating the surface area of the rear side of the struts to estimate drag with base drag hack?

View attachment 565558

On the X-15 question apparently the wedge fins produce a ton more drag... What would be the correct way to model this if not with the base drag hack?

North American X-15 - Wikipedia

en.wikipedia.org

The X-15 had a thick wedge tail to enable it to fly in a steady manner at hypersonic speeds. This produced a significant amount of base drag at lower speeds; the blunt end at the rear of the X-15 could produce as much drag as an entire F-104 Starfighter.

 

[lakeroadster]

On the X-15 question apparently the wedge fins produce a ton more drag... What would be the correct way to model this if not with the base drag hack?

North American X-15 - Wikipedia

en.wikipedia.org

The X-15 had a thick wedge tail to enable it to fly in a steady manner at hypersonic speeds. This produced a significant amount of base drag at lower speeds; the blunt end at the rear of the X-15 could produce as much drag as an entire F-104 Starfighter.

But the question is how much drag is produced from those thick wings, and without some kind of wind tunnel testing, followed up by analytical data, just throwing base drag cones into Open Rocket makes no sense whatsoever.

If you built two identical X-15 models, one with thin fins and one with thick, then you could remove nose weight on the thin finned rocket until it became unstable in a wind tunnel. Now see if an Open Rocket Simulation supports that data.

Now use that same amount of nose weight on the thick finned rocket and see if it is stable. Reduce the nose weight until it is unstable. Now see if an Open Rocket Simulation supports that data.

Now you've got something to hang your hat on.

 

[neil_w]

If you go into the Component Analysis tool in OR, it'll tell you the base drag contribution of each component. That doesn't necessarily tell you what to do to properly adjust CP, but it should nonetheless be informative.

This build certainly opened my eyes as to what can be accomplished, stability-wise, with thick fins.

 

[BigMacDaddy]

Technical report about base drag in the X-15. Likely more interesting to the engineers out there...

https://ntrs.nasa.gov/api/citations/19660010056/downloads/19660010056.pdf

Ultimately, it would be great if we had better methods to account for base drag in CP calculations for all types of draggy elements not just the airframe (or ideally a method to account for drag of elements at various points along the length of a model).

 

[SolarYellow]

I don't remember reading discussion of the base drag hack with clusters.

I've been simming up a short-fat BT-60 with 7x13 cluster, completely filling the back end. With seven motors burning and filling the base drag area with pressurized gas, I'm thinking maybe it would not be appropriate to rely on base drag to stabilize the rocket during boost. Once the six A10-PTs are done burning and it's just the A3-6T finishing up and then smoking along toward ejection, I think it will be fine, but with OR showing 66g off the rod, I don't really want to take any chances with skywriting in the beginning.

Anyone have experience with such things?

 

[Buster]

One 11C I use at my desk. The other one goes to the job sites.

The 15C stays in the file cabinet and requires security clearance before you can pick it up.

All three work, are a little beyond 35 years in age, and I'm their original owner.

View attachment 569262

Wow I’ll have to get some of those things! Do they work well?

 

[pathtouch]

Slightly back on topic I just want to add a data point.

7.5” LOC Doorknob, fully glassed with with two layers of 6oz, L1090, no additional nose weight. Simmed with no base cone, stability is like 0.3 calibers. Simmed with base cone over one caliber of stability but only predicted 3900 feet of altitude. Removing cone sims to 4500 and that matched the observed altitude very well.

I now do an initial sim with the cone, then override COP to match that location and remove cone for accurate altitude predictions.
Editing after I re-opened openrocket and realized you can't override COP. I was leaving the cone in and editing Drag Cd for the entire rocket and all subcomponents until it matched my observed altitude while leaving the cone in to keep the "hacked" COP location. Until you have flight data, I would recommend overriding Cd with one from the component analysis of the model with no cone until you got flight data.

I also slightly adjust drag coefficient to match observed altitude since this is the biggest 54mm motor I can fit and now I have a sim I 100% trust.