It was pretty unlikely to go in anyway, but you’re free to take credit if you like.
Let's put an end to the "Base Drag Hack"
- Thread starter Buckeye
- Start date
Help Support The Rocketry Forum:
rharshberger
Well-Known Member
BDH is most useful IMO to reduce nose weight and nothing more and that's what I use it for, every rocket I have used it on has had the noseweight significantly reduced vs normal simulation and the rocket has flown stable at proper rail exit velocities for the conditions, the rockets include V2's, Patriot missles, Big Daddy's with stupid oversize motors, and a number of my own custom designs. I have built several dozen of these rockets using the "hack" and all fly fine with noseweights that under normal simulations would show as negative calibers ie...unstable. I really like to keep my Calibers of stability as low as possible for weather cocking reasons as long as the rocket is not going transonic, then I need to be more careful and make sure CP shift doesnt cause an issue. Its practical application in this case has proven to work and thats why I use it, even if I dont fully understand WHY its working.
ThirstyBarbarian
Well-Known Member
- Joined
- Feb 11, 2013
- Messages
- 12,264
- Reaction score
- 7,537
You are selectively quoting me again. I didn’t say the Barrowman equations don’t work. Here is what I said:
Those tools together — Barrowman and the 1-caliber rule — do not work for short, fat rockets. I agree that the 1-caliber rule of thumb isn’t a great guide to stability for stubby rockets, but I also think that the Barrowman equations miss something about stability for these kinds of rockets too.
Anyway, if you want to primarily fault the 1-caliber rule, then you have to come up with some other way to interpret the stability margin, right? It’s not just that the CG needs to be in front of the CP as calculated by Barrowman by some unspecified amount. You need a rule or guideline of some kind for determining if the CG is far enough ahead of the CP for the rocket to fly in a stable way. I agree 1-caliber won’t work for stubby rockets. So what is your rule? What guideline are you suggesting?
% of length is an improvement, which is why OR now shows it by default along with caliber. 10% is equivalent to the canonical 1 caliber.
It was certainly never going to go in as a conical transition grafted to the back of the tube. If we find a contribution of base drag to CP that we decide is appropriate to put in, it'll be as an appropriate normal force (and you can bet that, at least in the first release if any to include it, it'll be marked as experimental and users will be free to add it in to the calculations or not).
- Joined
- Aug 6, 2022
- Messages
- 3,625
- Reaction score
- 3,376
The only ordinary 3/4FNC rocket discussed by Levison was the Estes Fat Boy. A .ork file I downloaded at some point in the past and attempted to rehabilitate shows that a stability factor of 0.67 calibers is approximately 13.5% of overall length. So the 8-15% of airframe length rule is satisfied just fine.
Delving into this a little more.
Levison references, "WIND INSTABILITY - WHAT BARROWMAN LEFT OUT" by Robert Galejs. That work references two "edge case" rockets, the Fat Boy and the Rogue Aerospace Space Needle Jr. https://plans.rocketshoppe.com/rogue/rog10020/rog10020.htm As far as I can deduce, the latter is 11mm diameter tubing and is about 91 cm long, giving it an overall aspect ratio of 82.8:1. If that's correct, one caliber stability would be 1.2% of overall length. On the other hand, 8% of overall length would be 72.8 mm or 6.6 calibers, and 15% of overall length would be 137 mm or 12.4 calibers. Interestingly, per Galejs' analysis, "This rocket loses over 12 calibers of stability at only 5 degrees AOA!"
Galejs also references "Wind Caused Instability" by Bob Dahlquist. That mentions two rockets that Dahlquist had recently witnessed going unstable, a LOC/Precision Hi-Tech H45 and a Binder Design Thug.
The Hi-Tech H45 is 49.75 in (126.37 cm) long and 2.63 in diameter, for an aspect ratio of 18.9. One caliber stability would be 5.3% of overall length. On the other hand, 8% of overall length would be 4 in. or 1.5 calibers, and 15% of overall length would be 7.5 in. or 2.8 calibers.
The Binder Design Thug is 33 in long and 4 in diameter, for an aspect ratio of 8.25. One caliber stability would be 12% of overall length. On the other hand, 8% of overall length would be 2.64 in. or 0.66 calibers, and 15% of overall length would be 4.95 in. or 1.24 calibers.
I'm liking the percent rule more and more.
Last edited:
- Joined
- Jan 7, 2019
- Messages
- 1,392
- Reaction score
- 1,058
And Rocksim will say a rocket is stable right up to where the caliber = zero, then it goes unstable. That is not reality either. 1 caliber just provides a reasonable margin of safety based on experience. Between a caliber of 1 and zero, it gets generally less stable, but what the tipping point is is not exact, it depends on the rocket, the motor and the winds. In addition, a caliber over 3 says a rocket is "overstable", meaning it will weathercock like a mother in high winds. but will still fly like a champ in lower winds or on a punchy motor (Super Thunder, Warp-9) with gusty winds.
What plenty of people know from experience is that short/fat rockets are clearly stable at less than 1 caliber. A standard Estes Big Daddy has a caliber of .94 (marginal) without a motor and a caliber of .52 on an E12. It flies just fine.
I have a LOC Minie Magg that I built using the base drag hack to determine how much nose weight I needed to add to ensure stability. With the base drag hack, I only needed 50g. Without it, I would have needed 410g. I used 50g, it flies just fine. I got my L2 with it.
Finally, the other thing about short/fat rockets (and actually, even some long/fat rockets), if you put a BIG, LONG motor in it, Rocksim will have it being more stable than if you put a shorter, less heavy motor in it. The motor itself is so big it can move the CG forward.
So, in summary, the Barrowman equations are only reasonable approximations. They provide a guideline for a normal rocket. Change the baseline (short/fat, very long/skinny, whacky fins designs, draggy fin designs, spools, saucers) and the simulations become less accurate (or useless). There is no substitute for actual experience.
Please prove that assertion. (For stubby rockets, not saucers and plates.)
I'm glad people are taking some time to think carefully about the Levison paper. There are several things about the reasoning in the paper that I don't agree with.
But before talking about the base drag hack, or the Levison paper directly, I think we should all take a moment to understand why saucer and spool rockets are stable. This turns out to be pretty important for other kinds of rockets as well, and the biggest contribution to the stabilizing force on these kinds of rockets isn't the forces that people are talking about.
There's something that's sometimes called the Munk Moment, which is an aerodynamic torque that tries to turn objects so they're traveling flat on to the airstream. For a long thin object like a typical rocket the Munk Moment is destabilizing. For a short fat object like a saucer, the Munk Moment is stabilizing.
The Munk Moment is not a drag force, and it is not caused by vortexes or flow separation.
The simplest way to see the effect is to look at the airflow around an ellipsoid, ignoring viscous and compressibility efffects. People who know more math than I do can actually work this case out with pencil and paper, and the combination of ellipsoid shapes and being solvable by people without computers made this version of the problem very interesting a hundred years or so ago when folks were designing dirigibles.
I've tried to draw out the two cases below.
Air flowing around the object forms two stagnation points, one at the front of the object and one where the flow comes back together at the rear (in red on the drawings).
These two points are the points of highest aerodynamic pressure.
Since we're solving without viscous forces, the total drag is "negligible" and the pressure forces (green arrows) on the two stagnation points are opposite and cancel out.
The torques, on the other hand, do not cancel. The pressure forces act perpendicular to the surface of the object.
For long thin objects, this direction is mostly sideways, and the result is a torque that tries to make the object rotate farther away from the airflow so it is unstable.
For short fat objects the direction of the force is mostly front to back, and the result is a torque that tries to rotate the object back into alignment with the airflow.
The Barrowman equations make the assumption that all pressure forces act perpendicular to the rocket axis (so not just "mostly sideways" but "exactly sideways"). This is one of the ways that the approximation that rockets are long and thin is baked into the math. So Barrowman gets the unstable version of the Munk Moment pretty accurately, but the stabilizing version it gets very wrong.
But before talking about the base drag hack, or the Levison paper directly, I think we should all take a moment to understand why saucer and spool rockets are stable. This turns out to be pretty important for other kinds of rockets as well, and the biggest contribution to the stabilizing force on these kinds of rockets isn't the forces that people are talking about.
There's something that's sometimes called the Munk Moment, which is an aerodynamic torque that tries to turn objects so they're traveling flat on to the airstream. For a long thin object like a typical rocket the Munk Moment is destabilizing. For a short fat object like a saucer, the Munk Moment is stabilizing.
The Munk Moment is not a drag force, and it is not caused by vortexes or flow separation.
The simplest way to see the effect is to look at the airflow around an ellipsoid, ignoring viscous and compressibility efffects. People who know more math than I do can actually work this case out with pencil and paper, and the combination of ellipsoid shapes and being solvable by people without computers made this version of the problem very interesting a hundred years or so ago when folks were designing dirigibles.
I've tried to draw out the two cases below.
Air flowing around the object forms two stagnation points, one at the front of the object and one where the flow comes back together at the rear (in red on the drawings).
These two points are the points of highest aerodynamic pressure.
Since we're solving without viscous forces, the total drag is "negligible" and the pressure forces (green arrows) on the two stagnation points are opposite and cancel out.
The torques, on the other hand, do not cancel. The pressure forces act perpendicular to the surface of the object.
For long thin objects, this direction is mostly sideways, and the result is a torque that tries to make the object rotate farther away from the airflow so it is unstable.
For short fat objects the direction of the force is mostly front to back, and the result is a torque that tries to rotate the object back into alignment with the airflow.
The Barrowman equations make the assumption that all pressure forces act perpendicular to the rocket axis (so not just "mostly sideways" but "exactly sideways"). This is one of the ways that the approximation that rockets are long and thin is baked into the math. So Barrowman gets the unstable version of the Munk Moment pretty accurately, but the stabilizing version it gets very wrong.
Thanks for this. However ... The base drag hack provides us with a way of providing a solution within the existing framework of operation within Open Rocket or Rocksim.
When this gets to a point where it can be incorporated into OR, it would be useful. As it is, it's just interesting.
I agree @OzHybrid.
Borrowman is "baked in" to the stability calcs in OpenRocket and RockSim and countless 'home grown' stability calculators.
The title of Jim Barrowman's Masters Thesis was: "The Practical Calculation of the Aerodynamic Characteristics of Slender Finned Vehicles'" ( note that the Bold Font is mine )
And we're discussing the stability of rockets that don't fit Borrowman's "model rocket" and neither does the old "one caliber" rule of thumb for stability based on his initial assumptions.
It would be wonderful if all the popular sim programs could actually model stability of the 'fatties' without the hack ...
Maybe the Munk Moment is the way to go ... I dunno ... I remember reading about that long ago, now that @Jeff Lassahn has described it so well. I need to check out the math once again.
Until something better comes along, we have Barrowman -plus- the Base Drag Hack as long as it is applied wisely.
To throw another old mechanic's saying out there: "When all you have is a hammer, everything looks like a nail"
-- kjh
Borrowman is "baked in" to the stability calcs in OpenRocket and RockSim and countless 'home grown' stability calculators.
The title of Jim Barrowman's Masters Thesis was: "The Practical Calculation of the Aerodynamic Characteristics of Slender Finned Vehicles'" ( note that the Bold Font is mine )
And we're discussing the stability of rockets that don't fit Borrowman's "model rocket" and neither does the old "one caliber" rule of thumb for stability based on his initial assumptions.
It would be wonderful if all the popular sim programs could actually model stability of the 'fatties' without the hack ...
Maybe the Munk Moment is the way to go ... I dunno ... I remember reading about that long ago, now that @Jeff Lassahn has described it so well. I need to check out the math once again.
Until something better comes along, we have Barrowman -plus- the Base Drag Hack as long as it is applied wisely.
To throw another old mechanic's saying out there: "When all you have is a hammer, everything looks like a nail"
-- kjh
Per the old Redstone Arsenal paper (I bet Von Braun read that!! and Cool for Posting):
Only one other external stream parameter seems to exert a noticeable influence on power-on base drag. That parameter is body geometry. Experimental data illustrating the influence of boattail geometry are presented in Fig. 7a.(2) Although base diameter, boattail length, and boattail angle are varied in these data, the primary influence appears to be that of base diameter. This trend is apparent in other experimental data for both boattail and flare configurations. Analysis of the data shows that for a given base diameter the base pressure maintains a constant ratio to the base pressure of a cylindrical body. Values for this ratio, based on analysis of a large amount of experimental data (2,4,5,6), are presented as a function of the ratio of base area to body area in Fig. 7b. Although it is felt that a discontinuity exists between value for boattail configurations and flare configurations, a linear approximation allows convenient formulation of the following expression.
I'd assume a boat tail reduces drag somewhat by reducing the transition step.
Base drag is also speed dependent.
My big Q is: could this be simulated and taken into account along with the Barrowman / Rocksim / RASAero / Open Rocket stability equations or is it off the chart due to all of the different factors? Software taking this into account would need to do stability vs speed calculations and produce a graph.
It may also have "some" fit into the ROT that Mach-capable rockets need a higher caliber stability.
It would increase the use factor manyfold as well as safety.
I often "must" comply with rules of thumb in my job due to NFPA 70 (National Electrical Code). A sample would be sizing wires to a motor based on a table row instead of an interpolation of horsepower vs. current at a given supply voltage. Code does it for safety and #ss- cover. Engineers usually add a little more #ss- cover to an install just in case.
The situation is analogous. We all want an exact or near-exact stability calculation that takes body and fin area, Cg - Cp, geometry, and speed into account.
Now will a software mfr want to do this or add a little #ss- cover? Hmmm....
PS: I love old stuff from the Cold War - it goes into my history file as well as rockets.
Cheers / Robert
Only one other external stream parameter seems to exert a noticeable influence on power-on base drag. That parameter is body geometry. Experimental data illustrating the influence of boattail geometry are presented in Fig. 7a.(2) Although base diameter, boattail length, and boattail angle are varied in these data, the primary influence appears to be that of base diameter. This trend is apparent in other experimental data for both boattail and flare configurations. Analysis of the data shows that for a given base diameter the base pressure maintains a constant ratio to the base pressure of a cylindrical body. Values for this ratio, based on analysis of a large amount of experimental data (2,4,5,6), are presented as a function of the ratio of base area to body area in Fig. 7b. Although it is felt that a discontinuity exists between value for boattail configurations and flare configurations, a linear approximation allows convenient formulation of the following expression.
I'd assume a boat tail reduces drag somewhat by reducing the transition step.
Base drag is also speed dependent.
My big Q is: could this be simulated and taken into account along with the Barrowman / Rocksim / RASAero / Open Rocket stability equations or is it off the chart due to all of the different factors? Software taking this into account would need to do stability vs speed calculations and produce a graph.
It may also have "some" fit into the ROT that Mach-capable rockets need a higher caliber stability.
It would increase the use factor manyfold as well as safety.
I often "must" comply with rules of thumb in my job due to NFPA 70 (National Electrical Code). A sample would be sizing wires to a motor based on a table row instead of an interpolation of horsepower vs. current at a given supply voltage. Code does it for safety and #ss- cover. Engineers usually add a little more #ss- cover to an install just in case.
The situation is analogous. We all want an exact or near-exact stability calculation that takes body and fin area, Cg - Cp, geometry, and speed into account.
Now will a software mfr want to do this or add a little #ss- cover? Hmmm....
PS: I love old stuff from the Cold War - it goes into my history file as well as rockets.
Cheers / Robert
user 30299
L2 - NAR & TRA
- Joined
- Nov 6, 2019
- Messages
- 963
- Reaction score
- 907
I will need to keep this handy for the next university rocketry team with questions on harness length and BP calculations.
user 30299
L2 - NAR & TRA
- Joined
- Nov 6, 2019
- Messages
- 963
- Reaction score
- 907
Per the old Redstone Arsenal paper (I bet Von Braun read that!! and Cool for Posting):
Only one other external stream parameter seems to exert a noticeable influence on power-on base drag. That parameter is body geometry. Experimental data illustrating the influence of boattail geometry are presented in Fig. 7a.(2) Although base diameter, boattail length, and boattail angle are varied in these data, the primary influence appears to be that of base diameter. This trend is apparent in other experimental data for both boattail and flare configurations. Analysis of the data shows that for a given base diameter the base pressure maintains a constant ratio to the base pressure of a cylindrical body. Values for this ratio, based on analysis of a large amount of experimental data (2,4,5,6), are presented as a function of the ratio of base area to body area in Fig. 7b. Although it is felt that a discontinuity exists between value for boattail configurations and flare configurations, a linear approximation allows convenient formulation of the following expression.
I'd assume a boat tail reduces drag somewhat by reducing the transition step.
Base drag is also speed dependent.
My big Q is: could this be simulated and taken into account along with the Barrowman / Rocksim / RASAero / Open Rocket stability equations or is it off the chart due to all of the different factors? Software taking this into account would need to do stability vs speed calculations and produce a graph.
It may also have "some" fit into the ROT that Mach-capable rockets need a higher caliber stability.
It would increase the use factor manyfold as well as safety.
I often "must" comply with rules of thumb in my job due to NFPA 70 (National Electrical Code). A sample would be sizing wires to a motor based on a table row instead of an interpolation of horsepower vs. current at a given supply voltage. Code does it for safety and #ss- cover. Engineers usually add a little more #ss- cover to an install just in case.
The situation is analogous. We all want an exact or near-exact stability calculation that takes body and fin area, Cg - Cp, geometry, and speed into account.
Now will a software mfr want to do this or add a little #ss- cover? Hmmm....
PS: I love old stuff from the Cold War - it goes into my history file as well as rockets.
Cheers / Robert
Nicely put!
The take-away from your comments, and previous comments by others, is that you are all seeing that there
is more to the Base Drag condition than people realized. The part you quoted is from the Brazzel report,
from 1966. You learn that 50 to 70% of the rocket's drag is due to Base Drag. That is an enormous number.
And as Base Drag has moved forward in time, and research, some key points still hold up.
If you're a max altitude apostle, are you spending enough time on base drag reduction?
(Min. dia. rockets pose an interesting situation.)
I'm not questioning the use of the Hack. Knock yourself out. People continue to show "it works" with their
stubby rockets. I have my own collection of stubby rockets so I know their quirks firsthand. (And maybe this
summer I will join ThirstyBarbarian's army of Warlock aficionados.)
The desire is to have a "near-exact stability calculation", but that's currently beyond the means of RockSim
and Open Rocket. You still have to understand what's actually happening before moving to the next step.
- Joined
- Aug 6, 2022
- Messages
- 3,625
- Reaction score
- 3,376
It's beyond the means of any practical system, regardless of the calculation method.
Get into the wind/aoa instability papers referenced (not necessarily linked) above. The key takeaway from them is that the actual CP depends on AOA, which at rod/rail/tower exit depends on rocket speed and wind speed. Since it's fairly difficult to know moment to moment what the actual wind speed is, and often even more difficult to predict it more than a few seconds out, it's basically infeasible to plan to launch a rocket with an exactly-calculated CP in the real world, unless you just wait for a nearly perfectly still day.
We see this on the opposite end of things from the Fat Boy, too. One can reasonably assume that rockets like the Estes Ascender or Majestic, and the booster assembly accessory, were not introduced to the market without a good number of successful flights. Yet there are many reports of them going cruise-missile on F15 BP motors, especially when launched with the booster. What the wind/aoa instability articles teach us is that on long, skinny rockets like those, some cross wind can easily make a comfortably stable rocket uncomfortably unstable. (Looking at rail exit speed in a sim with E16/F15 motors and comparing those thrust curves to others will explain why "cruise missile" is more likely to happen with them.)
In general, inadequately stable rockets will turn downwind, while overstable rockets will turn upwind*. Either will fly straight up in still air. So it falls to the rocketeer to understand the effect of wind, including on long vs. short rockets, configure the rocket to be launched so that its stability factor is in an appropriate range for launch conditions, and make adjustments to it as and if appropriate, including motor selection. (For example, on a windier day, pick a motor with strong initial thrust and higher average thrust, but less total impulse. This will help get your rocket flying upward safely, and not send it so high up that it's going to be difficult to track/recover.) That's a lot more to know and understand than "one caliber." But it's what is really needed.
The wind/aoa instability issue is yet another reason that building rockets longer than they need to be is only for looks, not flight performance. In engineer-speak, the rate of change of CP location vs. aoa is greater on a long/skinny rocket, smaller on a short/fat rocket, so long/skinny rockets are more sensitive to wind. Cutting excess length out of a rocket is also a good way to reduce weight, so it will also improve launch speed (further improving the aoa issue), as well as reducing skin friction drag throughout the flight. It also reduces the tendency of the airframe to bend/buckle and shred due to flight loads, which is probably a reason why fairly lightly constructed short/fat rockets seem to be able to survive being launched on silly-big motors. This seems like as good a time as any to point out yet again that a rocket like the Star Orbiter would be an objectively better design by every flight performance metric I can think of if it was built with one body tube, not two.
*It occurs to me that I haven't seen it delved into how a long/skinny rocket might end up on one side or the other of this depending just on wind speed. I'll probably eventually end up running the numbers, but waving my hands in the air, it seems possible that at lower wind speeds, a long/skinny with several calibers of stability might be overstable and tend to turn into the wind, while at higher wind speeds, the aoa shift might make it unstable so it tends to turn downwind, all with the same c.g. and aerodynamic form.
-------
Side note, I noticed someone again talked about forward CP shift on supersonic rockets. To summarize, you can forget about that unless your rocket is going to exceed Mach 2. From the start of transonic to about Mach 2, it's actually going to gain stability vs. the subsonic/static condition, not lose it. So for the typical L1, skinny/MD HPR rocket that's going to get to Mach 1.3-1.7, it's a complete non-issue. You still have to design the rocket to get off the rail/out of the tower safely. Trying to shave that a little closer because the stability factor will increase at transonic and higher speeds would be dumb.
Last edited:
NTP2
Well-Known Member
I’ve been wondering if it would be possible for open rocket to interact/use open foam for CFD stuff, I’m imagining it could make a model of the rocket and have open foam do the math for it.
user 30299
L2 - NAR & TRA
- Joined
- Nov 6, 2019
- Messages
- 963
- Reaction score
- 907
With that kind of thinking we'll never land a man on the Moon . . . .
boatgeek
Well-Known Member
- Joined
- Dec 27, 2014
- Messages
- 7,643
- Reaction score
- 8,437
OK, two options. Pick whichever appeals to you.
1. Stubby models are that are not "supposed" to fly straight without accounting for base drag do fly straight. So something is making them fly straight. Since that impact appears to be larger as rockets transition from slender to stubby to saucers, base drag becoming a larger fraction of overall drag appears to be a significant factor.
2. Once more, with data. I'm currently working on the rocket below, 8cm diameter, 99cm long. It's actually well into 10:1 territory, so one wouldn't normally apply the BDH anyway. ~30% of the drag is at the base, 27.7cm aft of the CG. The CP (without accounting for effects of base drag) is 8.7cm aft of the CG. At M0.3 where the stability is measured, drag is around 14 N. So let's look at a 1-degree AoA. The restoring moment from drag (not counting lift from fins) is on the order of (0.7*14N)*(8.7cm)*(tan1) = 1.49 N-cm. The restoring moment from base drag only is (0.3*14N)*(27.7cm)*(tan1) = 2.03 N-cm, more than the impact of all other drag. I will grant that this is a somewhat simplistic analysis since it assumes that the drag forces operate through the CP (they are likely further forward because fin lift pulls the CP back) and it neglects the restoring moment from fin lift. However, I still think it shows that there is a significant impact, even once we're out of "stubby" territory. As the rocket gets shorter, the percentage of drag that is from based drag only goes up, and the effect will increase.
[minor edits made to clarify intent and fix math errors]
Last edited:
drewnickel
Resident Hybrid Nut
Not to mention Cp technically can't be calculated for zero angle of attack. By definition, Cp location is pitching moment about the nose divided by normal force (moment divided by force gives you the distance at which that force is applied). For an axisymmetric rocket, there should be no pitching moment at zero angle of attack and no normal force, so you just get 0/0. For the math minded people, there is a point discontinuity at zero alpha when plotting Cp vs angle of attack. It can be approximated by sweeping angle of attack near zero degrees (like maybe 1, 2, 3, 4 degrees etc) and extrapolating back to 0 degrees. In a perfect world without wind or alignment errors, a statically unstable rocket would still keep going straight if there was nothing to get it to pitch.
I do agree that the base drag hack is certainly not entirely accurate and seems more like a case of artificially getting static margin over 1 caliber when you really just have to have some non-zero positive margin to be stable and enough margin to remain stable at non-zero angle of attack (Cp moves as you vary angle of attack). Something working in simulations does not mean it is what is actually happening.
Last edited:
drewnickel
Resident Hybrid Nut
The margin you need depends on the rocket and how it's being flown. You need enough margin where a gust of wind or non-zero angle of attack does not move the cp ahead of the cg at any point in the flight. The longer the rocket, the more the center of pressure location will shift at non-zero angles of attack, so you need more margin for longer rockets and less margin for short rockets. The 1 caliber rule of thumb and 10% of length rule of thumb are reasonable starting points for people who for the most part don't need or want to go through and hyper-analyze the aerodynamics of a model rocket, but they are just rules of thumb. This is why I like using damping ratio to guide my decisions on stability rather than calibers of stability or % of length. You know if the rocket is going to weathercock because the margin is too high or if it's going to shimmy back and forth at high speeds and shake itself to pieces or destabilize.
- Joined
- Dec 31, 2009
- Messages
- 4,288
- Reaction score
- 3,776
The main limiting factor to a more accurate implementation of whichever model or method someone derives and verifies is... expecting it to be done for free and added to OpenRocket or RasAero. Or, expecting an investment into modifying a affordable commercial program for such a narrow market.
The next limiting factor is the compute power on the average user's computer to do anything like real-time CFD within the implementation. Even an iterative algorithm of higher order wouldn't be realistic. The challenge is to narrow the use cases to something that could be verified and programmed in closed form or using look-up tables. Anyone have the time and knowledge to take this on? I call "not it!".
The next limiting factor is the compute power on the average user's computer to do anything like real-time CFD within the implementation. Even an iterative algorithm of higher order wouldn't be realistic. The challenge is to narrow the use cases to something that could be verified and programmed in closed form or using look-up tables. Anyone have the time and knowledge to take this on? I call "not it!".
boatgeek
Well-Known Member
- Joined
- Dec 27, 2014
- Messages
- 7,643
- Reaction score
- 8,437
[touches nose]
After reading all this I am once again traumatized as to flying a high power Warlock or upscale Fat Boy. They may be stubby 3-4FNC beauties, but way too dangerous and unpredictable. I would have to seek out input from an experienced old dude, the grumpier the better. They have a lot of all thread rod glued everywhere for adjustable nose weight, and a crate full of different punchy motors. They do weird old school stuff like reefing chutes, licking their fingers and feeling the wind, launching off thick rods...
On second thought that would be too uncool, bad for my hipster image. Buckling down and using totally dialed in, hack free software like a true, forward thinking rocket scientist is the way to go. Knowing everything beforehand will lead to 100% perfection on the first flight. Safe and sound. Totally efficient with no stinking, performance robbing nose weight! Oodles of RSO praise and instant gratification. Come in with a skull full of mush and leave thinking like a rocket scientist! That's the ticket!
On second thought that would be too uncool, bad for my hipster image. Buckling down and using totally dialed in, hack free software like a true, forward thinking rocket scientist is the way to go. Knowing everything beforehand will lead to 100% perfection on the first flight. Safe and sound. Totally efficient with no stinking, performance robbing nose weight! Oodles of RSO praise and instant gratification. Come in with a skull full of mush and leave thinking like a rocket scientist! That's the ticket!
Would people not interested in this topic please simply ignore it instead of cluttering it with off-topic posts?
NTP2
Well-Known Member
This thread has really run its course, let them have their fun.
Very far from it.
ThirstyBarbarian
Well-Known Member
- Joined
- Feb 11, 2013
- Messages
- 12,264
- Reaction score
- 7,537
Personally, I’m just killing time waiting for an answer to my questions. We’ve been advised to do away with the base drag hack and do away with the 1-caliber rule of thumb, which I agree are not perfect, but my question is, if we do away with those, what do we use instead? What information does our sim software give us that tells us if our stubby rockets are going to fly straight or if they need nose weight and how much? If we are tossing out our current tools, what are we using now?
- Joined
- Dec 31, 2009
- Messages
- 4,288
- Reaction score
- 3,776
I bet you want a pony too.
NTP2
Well-Known Member
Really? I haven’t seen anything really useful for an entire page. It’s mostly the same arguments on repeat.
