MESOS Flight to 293K ft - RASAero II Prediction 290K ft

[Chuck Rogers]

I will make a mental note that these comparisons are after extensive back calculating from the measurements after the flight is complete. First time predictions are probably more like +/-10% agreement which is pretty typical of the softwares and good enough for design work.

Actually, as I (and others) have marched up that plot with flight after flight, the quote-unquote "preflight" predictions have gotten better and better. It's actually more like +/-5%, although there are outliers, we usually catch them all within +/-10%. You don't have all the answers before you start. Like Smooth Paint/Rough Camouflage Paint for a single stage rocket at Mach 3+ at low altitude, and a two stage or three stage rocket with delay staging reaching Mach 3+ at higher altitude.


Charles E. (Chuck) Rogers
Rogers Aeroscience

 

[OverTheTop]

Regarding the tumbling towards the end of the flight, it is caused by the rocket wanting to tumble around the axis with the largest angular momentum, hence the degradation to the end-on-end tumble. The stiffer the rocket is the longer it will take a rocket to degenerate into that tumble from a stabilised spin. If you look at many of the professional sounding rockets they have radax joints which have little or no hysteresis when flexed, contributing to staying nearer zero AoA for longer. Also, a rocket balanced around the long axis has less inclination to flex, so that also improves the attitude at the end of the flight, but is essentially the same mechanism as the aeroelastic flex mentioned above.

Search "NASA radax joint" for some useful information.

 

[Hobie1dog]

Regarding the tumbling towards the end of the flight, it is caused by the rocket wanting to tumble around the axis with the largest angular momentum, hence the degradation to the end-on-end tumble. The stiffer the rocket is the longer it will take a rocket to degenerate into that tumble from a stabilised spin. If you look at many of the professional sounding rockets they have radax joints which have little or no hysteresis when flexed, contributing to staying nearer zero AoA for longer. Also, a rocket balanced around the long axis has less inclination to flex, so that also improves the attitude at the end of the flight, but is essentially the same mechanism as the aeroelastic flex mentioned above.

Search "NASA radax joint" for some useful information.

you never cease to amaze me with your knowledge.

 

[Chuck Rogers]

I've re-done one of the plots using the last "preflight" RASAero II input file. Changes 1) through 5) noted previously have not been made to this file, this is the last "preflight" file. The file is attached below.

As Kip noted, all work on the creation of the .eng motor data files was completed prior to the flight.

The input file contains no adjusting of the Sustainer (Stage-2) Ignition Delay to match the flight data, the original planned ignition delay is used.

The input file has no adjusting of the launch angle to match the downrange distance at apogee. The flight is straight up.

There is No Wind.

The only thing that will be "switched " on the input file is the Surface Finish of Smooth Paint, or Rough Camouflage Paint. The attached file has Rough Camouflage Paint.

So to bracket the RASAero II flight prediction, both Smooth Paint and Rough Camouflage Paint were run.

I literally could have printed out this plot and handed it out at the launch site just prior to the launch.





And then after the flight, using the actual Sustainer (Stage-2) Ignition delay which differed from the preflight simulation (which used the preflight planned/expected delay), and the launch angle from vertical adjusted to match the GPS measured downrange distance at apogee, you get the following Postflight RASAero II Flight Simulations.





Note that using some of the data from the flight for the Postflight RASAero II Simulations, using the actual Sustainer (Stage-2) Ignition delay which differed from the preflight planned delay, and the launch angle from vertical adjusted to match the GPS measured downrange distance at apogee, the RASAero II accuracy actually went down, the preflight simulation was actually more accurate. Using data from the flight to try to get more accuracy isn't the point. The point is that by using the actual Sustainer ignition delay and the actual downrange distance at apogee allows a better assessment of the accuracy of the RASAero II aerodynamic and flight simulation models relative to the flight data.


As noted previously there was already a previous multi-stage rocket, with delay staging on the stages, where the Mach 3 to Mach 4 portion of the flight occurred at higher altitudes than previous Mach 3 to Mach 3+ rockets, where the data from this flight indicated a Surface Finish of Smooth Paint was more appropriate. Data from this flight was shown previously, it is repeated below.


So the predicted altitude for the flight was bracketed, and the flight provided a second data point that for multi-stage rockets with delay staging Smooth Paint is the more appropriate Surface Finish setting.







Charles E. (Chuck) Rogers
Rogers Aeroscience

 

[Chuck Rogers]

I will make a mental note that these comparisons are after extensive back calculating from the measurements after the flight is complete. First time predictions are probably more like +/-10% agreement which is pretty typical of the softwares and good enough for design work.

Buckeye:

This is not typical of the other software's.

RASAero II varies thrust with altitude. OpenRocket and RockSim do not vary thrust with altitude. (Someone correct me if I'm wrong.)

You can see what the altitude prediction would be with no variation of thrust with altitude by setting the Nozzle Exit Diameter for each stage to Zero in RASAero II. (Nozzle Exit Diameter for each stage is used to calculate the Nozzle Exit Area to vary thrust with altitude.)

Setting the Nozzle Exit Diameter to Zero for each stage is a little conservative, as the reduction in the Drag Coefficient (CD) for Power-On during the motor burn of each stage will not be included. The Power-On Delta-CD model also uses the Nozzle Exit Diameter input.


With the Nozzle Exit Diameter for each stage set to Zero, the altitude prediction is:

GPS Apogee Altitude = 293,488 ft
RASAero II Predicted Apogee Altitude (No Thrust with Altitude) = 168,979 ft (-42.42% Error)

An approximately 40% error in apogee altitude. Approaching predicting only one-half the altitude that the rocket achieved.


Clearly, for a rocket where there is an upper stage ignited at altitude, if the flight simulation does not include the variation of thrust with altitude, it is completely hopeless of getting any kind of accurate flight simulation.


Including the variation of thrust with altitude in RASAero II improves the accuracy of altitude predictions for lower altitude flights also.


Charles E. (Chuck) Rogers
Rogers Aeroscience

 

[Alan15578]

Cotronics 4525 was used on the fin leading edges. Then the entire rocket was painted in Rustoleum BBQ paint.

Before you laugh, Rustoleum BBQ paint behaves like very expensive intumescent paints. I apply 6 coats and do a final post cure to 225F. Stagnation temps at Mach 4.2 and 60,000ft are approximately 1,300F if I remember correctly. Spicy 🌶 but luckily air density is low.

This approach seems to work pretty well. I plan to look at different methods/coatings for the leading edges for the next flight. The rest of the airframe/nosecone was well protected via the paint.

MESOS is designed to be fully reusable except for the rocket motor liners and nozzles. In addition, the paint does need to be stripped and reapplied between flights.

Fin leading edge pic:

Thanks, I'm not laughing. Indeed I applaud you for finding something cheap, locally sourced, and effective at protecting the airframe. I had to google intumescent paint and I found more than I cared to read. Still, 6 coats sound like a lot. How much does that weigh per square foot? Is MESOS sensitive to the weight or are close to optimal mass? Still, stripping and reapplying 6 coats of paint between flights might not be the ideal solution.

One of my criticisms of HPR is that most of it is MR technology pulled up, rather than professional technology pulled down in an educational manner. MESOS to nearly 300K ft. is mind boggling. So I took a look at my personal library and scanned "Small Sounding Rockets". I found a few references to paints and coatings used on real sounding rockets: p 84 two coats of gloss white synalac lacquer enamel No. 7786, p 213 ablative material known as T-500 was specifically designed to ablate at 500 Deg. F, p 299 1 insulation of the interal payload, 2 use of "ThermoLag" T-230 a spray on ablative for 230 degree protection. There are probably additional references on that book, and there must be better references on thermal protection for aerospace vehicles. I doubt that any of these sounding rockets were reused. I am curious as to what sort of research you have done to address the paint and thermal protection issues.

 

[Alan15578]

Kip:

May I ask if you spin balance the payload (or whole vehicle)?

I find myself increasingly thinking that coning (both during boost and at high altitude) is in part due to non-asymmetric mass offsets. It is, of course, normal for an exoatmospheric logintudinally spin stabilized vehicle to transition to a flat spin due to momentum transfer, but any mass offest along the logintitudinal axis seems--to me--likely to abet that process.

I'm actively looking for a shop in N. Colorado that can do a dynamic balance of my current payload, if anyone has any suggestions....

Bill

I have been asking for decades for someone to design a DIY spin balancer for MR, it is always met by silence. HPR is a bit bigger, and I do not know of any shops that do that. However, every tire shop has some sort of spin balancer. If you find anything useful, please share.

 

[Alan15578]

Not quite the same thing. The base drag cone trick is to overcome limitations in the Barrowman static stability prediction. It is not used in flight simulation (I think) and besides, there is no flight data measurement for CP to compare to, anyway. Experimental CP would have to come from wind tunnel or CFD experiments, which are ground tests before flight.

They are both work arounds to try to get better results from existing software. You can adjust the surface roughness to change shear friction drag to better match measured performance, but shear friction drag could be modeled perfectly and the "error" could all be in the supersonic wave drag model used by the program.

Agree, this would be a nice feature in the software, like the old wRASP. The problem is that there are not many sources of Cd vs. Mach Number for the hobbyist. So, the hobbyist has to rely on the internal models. I have seen Cd backed out from accelerometer flight data and maybe some old wind tunnel measurements here and there. Do you have an available source?

I do not have a wind tunnel, or a good supersonic CFD program. If I had the need, I could find soemone who does, or do additional analysis myself.

 

[wclaybaugh2]

I have been asking for decades for someone to design a DIY spin balancer for MR, it is always met by silence. HPR is a bit bigger, and I do not know of any shops that do that. However, every tire shop has some sort of spin balancer. If you find anything useful, please share.

Allen:

Not to be discouraging but I too have found this an increasingly important issue, especially if flying above about 150k feet.

Because my main base is North Colorado I’ve talked to LM, Ball, and NTS about using their facilities, even for pay. Further, my former employment means I can get a return call from some of the very most senior folks. For a variety of good reasons, none are willing to take on occasional spin balancing of very small payloads.

I am now looking at *buying* a POI 50 from Raptor Scientific. This is a remarkably stupid purchase for the at most twice a year I would use it. But offering the capability to others would likely cost at least $1000 per balance if I just recover out of pocket costs…and I can number on the fingers of one hand everyone I know who both knows they need that service and could likely afford it. Dynamic balancing of a payload is more art than engineering: it can take most of a day with the very best current tools.

Bill

 

[Alan15578]

Alan:

RASAero II does not include any aerodynamic heating models. I have an excellent stagnation heating model in another program where I can get stagnation temperature with time and the total stagnation heat load. I plan to take a look at the various flights to do at least a stagnation temperature comparison and a stagnation heat load comparison between the flights.

Calculating stagnation temperature at the nose is just one simple equation and you could easily have RASAero II plot that temperature vs, flight time. calulating temerature at the wing or tail leading edge or other critical point is a little more complicated.

One issue is that this stagnation heating model is an equilibrium model, i.e., you'd use it for cruising at Mach 3 for 30-60 minutes, rather than a rocket going Mach 3 for tens of seconds, it will overpredict the heating. The analogy I give for our rockets is rather than soaking in an oven for 30-60 minutes, you put the rocket in the oven for tens of seconds and then quickly pull it out.

That might be a useful test procedure for sport rockets.

But this will answer the question of how much less is Mach 3 to Mach 4 aerodynamic heating at 10K ft to 20K ft, versus at 30K ft to 60K ft.


Charles E. (Chuck) Rogers
Rogers Aeroscience

My first professional job was determining loads on advanced missiles, mostly air-to-air missiles. As you can imagine, there are an enormous number of flight conditions to be evaluated. We used a coarse screen, or rule of thumb, to identify the more critical flight conditions. If we were below some temperature, we were good all day. If we went above that temperature we integrated temperature above that threshold over time, and if we stayed below another value we were still good. Critical cases were examined more accurately. Worst case, you required a design change or restricted the flight envelope. I can't tell you what these critical temperatures were, but the missiles were metal with a ceramic nose cone (radome).

When you wrote RASAero II you probably never expected that sport rocketeers could be flying above Mach 4 to nearly 300K feet. Adding one additional plot could be helpful.

 

[Kip_Daugirdas]

Thanks, I'm not laughing. Indeed I applaud you for finding something cheap, locally sourced, and effective at protecting the airframe. I had to google intumescent paint and I found more than I cared to read. Still, 6 coats sound like a lot. How much does that weigh per square foot? Is MESOS sensitive to the weight or are close to optimal mass? Still, stripping and reapplying 6 coats of paint between flights might not be the ideal solution.

One of my criticisms of HPR is that most of it is MR technology pulled up, rather than professional technology pulled down in an educational manner. MESOS to nearly 300K ft. is mind boggling. So I took a look at my personal library and scanned "Small Sounding Rockets". I found a few references to paints and coatings used on real sounding rockets: p 84 two coats of gloss white synalac lacquer enamel No. 7786, p 213 ablative material known as T-500 was specifically designed to ablate at 500 Deg. F, p 299 1 insulation of the interal payload, 2 use of "ThermoLag" T-230 a spray on ablative for 230 degree protection. There are probably additional references on that book, and there must be better references on thermal protection for aerospace vehicles. I doubt that any of these sounding rockets were reused. I am curious as to what sort of research you have done to address the paint and thermal protection issues.

The higher temperature paints like epoxies require a post cure to their max operating temp. I am limited by two things:
1) My oven
2) The lowest temp capable component.

In this case my fin cores with an advertised temp capability of 250F were my lowest temperature capable component. In addition, my oven is only capable of 250F. Therefore, none of my post cures went above 250F.

Lastly, cost and minimum order quantities need to be taken into account when purchasing the fancy stuff. I fund these projects myself so I’m very cost sensitive. I limit the fancy orders to only what is really needed. I think it’s great that hobby folk find unique off-the-shelf solutions. I’m certainly not the first to use either Rustoleum high-heat or Cotronics.

6 coats of paint is required because Rustoleum high-heat has terrible coverage. 8 rattle cans are used on the 3” diameter 5ft long second stage. Stripping the paint is not an issue. Acetone does quick work.

In regards to spin balancing, I just try to place my components balanced along the roll axis in CAD. The second stage is not designed to spin due to the off-center core of the moonburning propellant grain.

I have not been too concerned with spin balancing as coning/pitch-roll coupling has not been an issue for me. If I get into spin stabilization that will change.

 

[GalantVR41062]

I would look into automotive drive line shops for spin balancing long cylinders.

Another thought I have is setting up a horizontal mandrel or similar on a bench and spin the air frame then note what part of the airframe ends up down, kind of like balancing props.

Amazing work and will be looking forward to the next flight.

 

[wclaybaugh2]

In regards to spin balancing, I just try to place my components balanced along the roll axis in CAD. The second stage is not designed to spin due to the off-center core of the moonburning propellant grain.

I have not been too concerned with spin balancing as coning/pitch-roll coupling has not been an issue for me. If I get into spin stabilization that will change.

Kip:

I would expect that the 2ed stage—because it is axially unbalanced—would tend to gravity turn toward the heavier side if it was not spinning and, if spinning, would tend to cone.

Looking over the You Tube video, it appears to me that the second stage is coning in the ground view (as evidenced by the corkscrew exhaust trail) as well as plainly both quickly spinning up and then coning during 2ed stage burn as seen in the onboard video view.

It also appears—to me—that the video shows a transition to a wobble and later a flat spin as the vehicle nears peak.

Bill

 

[wclaybaugh2]

I would look into automotive drive line shops for spin balancing long cylinders.

That’s an interesting suggestion. I will look into it.

Bill

 

[Kip_Daugirdas]

I would expect that the 2ed stage—because it is axially unbalanced—would tend to gravity turn toward the heavier side if it was not spinning and, if spinning, would tend to cone.

I think issues of the slightly off-center mass of a moonburner are completely overblown. The moment caused by this is quite small. A high roll rate is more-so an issue. So far <2rps has not been a problem on my rockets.

The rocket did not cone. The smoke trails you see are from upper level winds which were not light that day 50kts+ above FL360. If the rocket had coned, the additional frontal area would have drastically reduced the apogee altitude. Rockets, particularly small ones like MESOS, are incredibly sensitive to this behavior.

Here is a video from the Kate data showing the 2nd stage in a 3D animation. I don’t see anything that resembles coning…some spin yes. This energy gets transferred into the flat spin at higher altitudes.

 

[wclaybaugh2]

I think issues of the slightly off-center mass of a moonburner are completely overblown. The moment caused by this is quite small. A high roll rate is more-so an issue. So far <2rps has not been a problem on my rockets.

The rocket did not cone. The smoke trails you see are from upper level winds which were not light that day 50kts+ above FL360. If the rocket had coned, the additional frontal area would have drastically reduced the apogee altitude. Rockets, particularly small ones like MESOS, are incredibly sensitive to this behavior.

Here is a video from the Kate data showing the 2nd stage in a 3D animation. I don’t see anything that resembles coning…some spin yes. This energy gets transferred into the flat spin at higher altitudes.

Kip:

It’s your call. If the coning that seems to me plainly visible in the ground video—and was clearly not present in the first stage burn—is not what it appears, then that’s that: a high altitude tornado….

Likewise, if the evident spin up and coning (as evidenced by the sinusoidal horizon line) in the onboard video is not what it appears, that too is your call and no business of mine.

I am given to looking at the actual data rather than at simulations of it. If the vehicle was simply spinning w/o coning during second stage burn than I would expect the pitch and yaw accelerations to be very similar. OTOH, if the stage was coning, I would expect those accelerations to show a sinusoidal pattern. That is where I would look to understand what was going on during this flight.

But that said, allow me to observe that the simulation plainly—in my viewing—shows that the vehicle was coning, indeed, in my viewing, it appears to be coning around two axises.

Bill

 

[Kip_Daugirdas]

Likewise, if the evident spin up and coning (as evidenced by the sinusoidal horizon line) in the onboard video is not what it appears, that too is your call and no business of mine.

Sinusoidal horizon line? You mean the rocket is not flying perfectly vertical and spinning - creates the same effect on video.

The rocket travels from 35,000 ft to 65,000 ft in 10 seconds under power. Winds can do quite a bit to the smoke trail of a small M motor over such a long distance. How one observes it from the ground isn’t necessarily indicative of how a rocket is flying.

I’m not saying the rocket didn’t eventually “cone”. But when it mattered, during 2nd stage boost and shortly there after, the rocket was flying as it should. If the rocket was coning, it would not have reached the altitude that it did. That is an actual FACT substantiated by flight data and simulation.

 

[wclaybaugh2]

Sinusoidal horizon line? You mean the rocket is not flying perfectly vertical and spinning - creates the same effect on video.

The rocket travels from 35,000 ft to 65,000 ft in 10 seconds under power. Winds can do quite a bit to the smoke trail of a small M motor over such a long distance. How one observes it from the ground isn’t necessarily indicative of how a rocket is flying.

I’m not saying the rocket didn’t eventually “cone”. But when it mattered, during 2nd stage boost and shortly there after, the rocket was flying as it should. If the rocket was coning, it would not have reached the altitude that it did. That is an actual FACT substantiated by flight data and simulation.

Kip:

I suspect that if you dispassionately look at the simulation from the flight data you will see that the nose tip of the vehicle is moving in a circle following the roll reversal at first stage burnout. (This is the data trying to tell us that the second stage was axisymmetricly unbalanced.) That coning continues through coast and second stage burn (23 to 33 seconds on the simulation timeline). None of this seems surprising given that the second stage was known to be unbalanced.

But this is neither my problem nor any business of mine; I apologize for pointing it out.

If you ever become interested in spin balancing your vehicles, I would be happy to help out.

Bill

 

[Kip_Daugirdas]

I suspect that if you dispassionately look at the simulation from the flight data you will see that the nose tip of the vehicle is moving in a circle following the roll reversal at first stage burnout.

Let me rephrase my question. Why would I spend my time/resources trying to eliminate a phenomenon that has no effect on the rocket’s performance?

I’m not blinded by passion - I’m realistic. Coning is only worth dealing with if it has a negative effect on performance or some other metric that actually matters. I guess that’s where I lost you.

 

[wclaybaugh2]

Let me rephrase my question. Why would I spend my time/resources trying to eliminate a phenomenon that has no effect on the rocket’s performance?

I’m not blinded by passion - I’m realistic. Coning is only worth dealing with if it has a negative effect on performance or some other metric that actually matters. I guess that’s where I lost you.

Kip:

The coning visible in the flight data and in the sky is effecting performance: it is increasing drag.

That it does not effect “modeled” performance implies an issue with the modeling:

Chuck Rogers has gone to a lot of effort to get RAS Aero II to accurately model the actual performance of amateur and hobby rockets. Since no such rockets have ever been intentionally balanced (so far as I know) I suspect Chuck’s empirical adjustments to the model have implicitly incorporated the drag effects of the coning that appears to be common to all hobby rockets. The test of this would be seeing if RAS Aero II underestimates the actual performance of carefully balanced vehicles.

Without exception, every single person to whom I have pointed out that a corkscrew exhaust indicates coning has insisted it was just wind…just sayin’: It ain’t the wind; it’s the rocket.

The sinusoidal horizon line displacement caused by the flight path angle is easily calculated given the camera focal length…since this vehicle was flying at a very small angle to vertical, I would expect that displacement to be very small. The displacement caused by coning is also calculable given knowledge of the distance between the lens centerline and the Cg; that displacement is going to be large for even small coning angles (measured at Cg) because of the lever arm…again, just sayin.

Bill

 

[jsdemar]

Kip and Bill,
I'll give a shot at resolving what you're both saying.

The sustainer had a sufficient stability margin to maintain its near-vertical trajectory while passing through turbulent layers of the atmosphere at 2->4 Mach. It's not surprising that the motor exhaust trail presented a helical signature while reacting to the significant forces. Keep in mind that the rocket is relatively small (3" dia), the moments of intertia (axial and radial) are much smaller that we're used to observing in overly stable amateur rockets and larger sounding rockets. Therefore the coning appears severe, but it's spread over a wide range of altitude.

In general, tightening the coning radius will result in less drag and higher altitude. Doing a spin-up past the resonant roll rate is the common method, albeit with some energy lost to the induced roll. Without the means of balancing the whole stack, I think the best approach is to make a reasonable effort to align the fins vertically and to remove major radial offsets in the enclosed masses. Then simulate and adjust the stability margin to above 1 over the range of velocity. And let it fly.

A critical mistake I've seen made is to let it spin without understanding the resonant roll rate. Rockets come apart trying to correct high, sudden, angles of attack. The resonant rate is relatively simple to calculate, but difficult to model for the as-built stack.

A lot more work could be put into the simulators to handle true 6DOF and dynamic changes to the rocket's distributed masses and inertias. I think, however, in the case of MESOS, there would be a minor difference in the ultimate apogee after all that effort. The helical path is tight and relatively spread out through thin atmosphere, from my observations of the flight (I was there!) and the data.

 

[FredA]

Bill - can you elaborate on how you get your spin-balancing done?
Beyond finding/owning a drive-line shop as step one ..... what's the process?

Can it be done in parts? Like booster & sustainer separately?
I would assume you need to have the propellant loaded, especially if using cores without axial symmetry.
Chutes too?

Once on the drive-line balancer - given the odd shape - how do you hold it and get it going? How fast does it spin?
Once spun - what data do you get from the machine and how do you translate that into know what mass to glue where?

Tell us more how this all works.
A little base knowledge would be very helpful for us that need to go beg random local shops to get this done.
Thx in advance.

 

[wclaybaugh2]

Kip and Bill,
I'll give a shot at resolving what you're both saying.

The sustainer had a sufficient stability margin to maintain its near-vertical trajectory while passing through turbulent layers of the atmosphere at 2->4 Mach. It's not surprising that the motor exhaust trail presented a helical signature while reacting to the significant forces. Keep in mind that the rocket is relatively small (3" dia), the moments of intertia (axial and radial) are much smaller that we're used to observing in overly stable amateur rockets and larger sounding rockets. Therefore the coning appears severe, but it's spread over a wide range of altitude.

In general, tightening the coning radius will result in less drag and higher altitude. Doing a spin-up past the resonant roll rate is the common method, albeit with some energy lost to the induced roll. Without the means of balancing the whole stack, I think the best approach is to make a reasonable effort to align the fins vertically and to remove major radial offsets in the enclosed masses. Then simulate and adjust the stability margin to above 1 over the range of velocity. And let it fly.

A critical mistake I've seen made is to let it spin without understanding the resonant roll rate. Rockets come apart trying to correct high, sudden, angles of attack. The resonant rate is relatively simple to calculate, but difficult to model for the as-built stack.

A lot more work could be put into the simulators to handle true 6DOF and dynamic changes to the rocket's distributed masses and inertias. I think, however, in the case of MESOS, there would be a minor difference in the ultimate apogee after all that effort. The helical path is tight and relatively spread out through thin atmosphere, from my observations of the flight (I was there!) and the data.

John:

That all sounds right to me.

Let me mention that I once modeled in Open Rocket (which is 6 DOF) a vehicle with an intentional offset in the payload area and Open Rocket did predict the induced coning. It was not clear to me that it correctly modeled the increased drag caused by that coning which might indicate an issue in the physics model of the antique 15.03 version, don’t know.

As you likely know, resonance in sounding rockets tends toward the 3-6 hertz range and so spinning at around 9 hertz tends to be optimal. For my six inch diameter sustainer that implies a fin displacement of about 0.020” front and rear.

While laser measurement tools can pretty accurately measure that displacement across the 10” fin length, I have found it simpler to design the drill jig for the fin mounting holes to be dead straight…the consequence of that is a very low induced roll rate which in turn leads to a tendency to gravity turn if there is any significant mass offset….

Bill

 

[wclaybaugh2]

Bill - can you elaborate on how you get your spin-balancing done?
Beyond finding/owning a drive-line shop as step one ..... what's the process?

Can it be done in parts? Like booster & sustainer separately?
I would assume you need to have the propellant loaded, especially if using cores with axial symmetry.
Chutes too?

Once on the drive-line balancer - given the odd shape - how do you hold it and get it going? How fast does it spin?
Once spun - what data do you get from the machine and how do you translate that into know what mass to glue where?

Tell us more how this all works.
A little base knowledge would be very helpful for us that need to go beg random local shops to get this done.
Thx in advance.

Fred:

To date, I have only done static balancing of the payload.

The stages are designed to be axisymmetric, I do make sure the fins are all within 0.1
gram but otherwise rely on the inherent symmetry of the design and propellant (Bates grains).

I have gone to a monolithic grain for the sustainer and while that grain is axisymmetric at all cross-sections it does vary some over the length. I do not expect that to be an issue but I’ve a flight test in a few months and we will see then.

With regard to payloads I start by measuring the Cg in two orthogonal planes (they are rarely identical although the difference is typically a few tens of thousandths). I then deduct mass by drilling adjustment holes at two 180 degree locations in the appropriate plane and either above or below the Cg as indicated by the measured offset. Repeat until the Cg’s coincide.

Once the Cg is located longitudinally I put the payload on a knife edge balance (typically supported near the base and near the top on disks that allow the payload to roll). Hand spinning with careful measurements will establish if there is imbalance in roll. Adding or deducting mass at the Cg will then bring the payload into static balance (indicated by no preferred orientation in roll).

I have not yet found any resource for dynamic balancing (which wants typically to be done with the payload mounted vertically due to all the antennae mounted at the top).

Bill

 

[jsdemar]

Fred:

To date, I have only done static balancing of the payload.

The stages are designed to be axisymmetric, I do make sure the fins are all within 0.01 gram but otherwise rely on the inherent symmetry of the design and propellant (Bates grains).

I have gone to a monolithic grain for the sustainer and while that grain is axisymmetric at all cross-sections it does vary some over the length. I do not expect that to be an issue but I’ve a flight test in a few months and we will see then.

With regard to payloads I start by measuring the Cg in two orthogonal planes (they are rarely identical although the difference is typically a few tens of thousandths). I then deduct mass by drilling adjustment holes at two 180 degree locations in the appropriate plane and either above or below the Cg as indicated by the measured offset. Repeat until the Cg’s coincide.

Once the Cg is located longitudinally I put the payload on a knife edge balance (typically supported near the base and near the top on disks that allow the payload to roll). Hand spinning with careful measurements will establish if there is imbalance in roll. Adding or deducting mass at the Cg will then bring the payload into static balance (indicated by no preferred orientation in roll).

I have not yet found any resource for dynamic balancing (which wants typically to be done with the payload mounted vertically due to all the antennae mounted at the top).

Bill

Bill,
Have you flight test anything using this alignment and balancing method?

 

[Kip_Daugirdas]

Bill - can you elaborate on how you get your spin-balancing done?
Beyond finding/owning a drive-line shop as step one ..... what's the process?

Can it be done in parts? Like booster & sustainer separately?
I would assume you need to have the propellant loaded, especially if using cores without axial symmetry.
Chutes too?

Once on the drive-line balancer - given the odd shape - how do you hold it and get it going? How fast does it spin?
Once spun - what data do you get from the machine and how do you translate that into know what mass to glue where?

Tell us more how this all works.
A little base knowledge would be very helpful for us that need to go beg random local shops to get this done.
Thx in advance.

Yeah I agree with Fred here. What I hate about TRF is people (like Bill) offering their two-cents like there’s some huge design oversight followed by some useless advice on how to “fix” it. In reality, perfectly spin balancing this rocket would be next to impossible as Fred points out. Even if it was accomplished, there are no guarantees it performs as expected or doesn’t drastically affect some other part of the design (payload, recovery, CP/CG relationship, etc).

Last I checked Bill has built exactly zero rockets of this caliber as a hobbyist. I am not NASA or some other professional company designing and building these rockets. I can’t launch a salvo of 10 rockets to get things perfectly dialed-in. I finance, source, design, and manufacture everything myself. These projects only fly once a year because of the work that goes into them.

Again these rockets are far from professional and expecting them to perform that way is ridiculous. It’s too bad Bill doesn’t understand that and goes on to sh*tpost, argue and derail threads…

 

[wclaybaugh2]

Bill,
Have you flight test anything using this alignment and balancing method?

John:

Yep; last flight.

Unhappily, two 3D printed battery boxes shattered at launch and one battery broke loose and swung on its leads through 90 degrees. This is turn lead to a gravity turn in the up range direction due in part to the lack of any roll.

Separately, the sep system activated at about one second—I have not been able to trace that failure but suspect the backup flight computer, which is why I have deleted it for the next payload.

Losing the payload seems to have been a good thing since it stopped the gravity turn that I expect would otherwise have quickly resulted in the vehicle hitting the deck under power. Instead, the rocket (w/o a nose) headed out at about a 75 degree flight path angle. Modeling based on the recovered rocket’s range suggested a peak velocity of Mach 1.3 (w/ a flat plate nose cone) and about 21k feet. Video suggests the rocket flew fine once it lost the payload.

Bill

 

[VernK]

A critical mistake I've seen made is to let it spin without understanding the resonant roll rate. Rockets come apart trying to correct high, sudden, angles of attack. The resonant rate is relatively simple to calculate, but difficult to model for the as-built stack.

John, can you elaborate a little more on how to calculate the resonate roll rate? Or point me to a reference. I would like to learn how to do it. I always assumed it is a fairly complex calculation.

 

[wclaybaugh2]

Yeah I agree with Fred here. What I hate about TRF is people (like Bill) offering their two-cents like there’s some huge design oversight followed by some useless advice on how to “fix” it. In reality, perfectly spin balancing this rocket would be next to impossible as Fred points out. Even if it was accomplished, there are no guarantees it performs as expected or doesn’t drastically affect some other part of the design (payload, recovery, CP/CG relationship, etc).

Last I checked Bill has built exactly zero rockets of this caliber as a hobbyist. I am not NASA or some other professional company designing and building these rockets. I can’t launch a salvo of 10 rockets to get things perfectly dialed-in. I finance, source, design, and manufacture everything myself. These projects only fly once a year because of the work that goes into them.

Again these rockets are far from professional and expecting them to perform that way is ridiculous. It’s too bad Bill doesn’t understand that and goes on to sh*tpost, argue and derail threads…

Kip:

Let me again apologize for your evident offense.

I am very sorry that this discussion has upset you.

I shall post here no more.

Bill

 

[Alan15578]

Kip:

Let me again apologize for your evident offense. I shall post here no more.

Bill

Bill,

I found your posts worthy of reading. It is a shame that the discussion became argumentative and somewhat uncivil. Please continue to post if you have anything to add.