Hi-Flier stability and weight; tuneup design for performance

[SolarYellow]

This may have been discussed at length elsewhere, but I haven't found those conversations with a reasonable amount of searching yet. Dipping my toe back into this after many decades of playing with much more expensive toys.

Did a little reading on the Estes Hi-Flier. I like the concept of small, light, low drag for maximum performance. Maximum altitude, or at least optimized altitude from cheaper engines. Specs caught my eye, and I've been trying to figure out why the skinnier rocket is listed at 31.2 g mass by Estes, when the (comparatively) big, fat Alpha is listed at only 22.7 g. Obviously, finishing practices like paint make a difference, but it's not easy to see where the extra mass goes. Then I encounter the discussion of stability. Common weights for the clay squares sold by Estes are in the .3-.4 oz range. 0.3 oz converts to 8.5 g, which is the total difference between the listed mass of the Alpha and Hi-Flier.

So the Hi-Flier, which is supposed to be a maximum performance rocket, ends up weighing 37 percent more than it ought to, just to ensure aerodynamic stability. As a habitual weight weenie, that's kinda lame. Thinking about it for a few seconds, it seems obvious that the fin design doesn't put the center of pressure as far back as traditional "swept back" fins. So if one wanted to run a Hi-Flier at an Alpha weight or lighter, redesigning the fins to be more traditionally swept back like the Alpha's fins and move the CP further back would probably accomplish that pretty easily without adding undue drag. Or even just use the standard fins, but cut new notches in them to hang them off the back of the body tube and a little taper on the inside to keep the overhanging portions out of the exhaust plume.

And since we're making it a different rocket, use the blunt nose cone from the "003161 - NC-20 Nose Cone (4 Pk)" kit to further reduce weight while probably actually enhancing aerodynamic efficiency at sub-0.8 mach. Actually, that would likely also help, as this paper https://apps.dtic.mil/sti/pdfs/ADA088130.pdf indicates a bluff nose cone improves static stability compared to a long, pointy one.

Any commentary on these tuneup ideas?

 

[tsmith1315]

So the Hi-Flier, which is supposed to be a maximum performance rocket, ends up weighing 37 percent more than it ought to, just to ensure aerodynamic stability.

Less mass isn't always better for altitude, there's usually an optimum for a given rocket/motor combination. Think of momentum and drag.

If you haven't already, download Open Rocket and try simulating them both with different masses.

Welcome back!

 

[smstachwick]

This may have been discussed at length elsewhere, but I haven't found those conversations with a reasonable amount of searching yet. Dipping my toe back into this after many decades of playing with much more expensive toys.

Did a little reading on the Estes Hi-Flier. I like the concept of small, light, low drag for maximum performance. Maximum altitude, or at least optimized altitude from cheaper engines. Specs caught my eye, and I've been trying to figure out why the skinnier rocket is listed at 31.2 g mass by Estes, when the (comparatively) big, fat Alpha is listed at only 22.7 g. Obviously, finishing practices like paint make a difference, but it's not easy to see where the extra mass goes. Then I encounter the discussion of stability. Common weights for the clay squares sold by Estes are in the .3-.4 oz range. 0.3 oz converts to 8.5 g, which is the total difference between the listed mass of the Alpha and Hi-Flier.

So the Hi-Flier, which is supposed to be a maximum performance rocket, ends up weighing 37 percent more than it ought to, just to ensure aerodynamic stability. As a habitual weight weenie, that's kinda lame. Thinking about it for a few seconds, it seems obvious that the fin design doesn't put the center of pressure as far back as traditional "swept back" fins. So if one wanted to run a Hi-Flier at an Alpha weight or lighter, redesigning the fins to be more traditionally swept back like the Alpha's fins and move the CP further back would probably accomplish that pretty easily without adding undue drag. Or even just use the standard fins, but cut new notches in them to hang them off the back of the body tube and a little taper on the inside to keep the overhanging portions out of the exhaust plume.

And since we're making it a different rocket, use the blunt nose cone from the "003161 - NC-20 Nose Cone (4 Pk)" kit to further reduce weight while probably actually enhancing aerodynamic efficiency at sub-0.8 mach. Actually, that would likely also help, as this paper https://apps.dtic.mil/sti/pdfs/ADA088130.pdf indicates a bluff nose cone improves static stability compared to a long, pointy one.

Any commentary on these tuneup ideas?

Remember that a rocket’s path to apogee isn’t done only under power. A good portion of it is coasting. A rocket that is too light will try to fly like a feather, with drag forces slowing it down excessively. More mass helps it push through.

On the other hand, too much weight will rob it of airspeed at burnout. The precise trade-off is unique to the design of each individual rocket.

 

[BABAR]

I'd be curious to see what people's experience has been, particularly people that fly with altimeters (@BEC , @Ronz Rocketz) with the Hi-Flyer. It is a cool LOOKING rocket, but seems like it has far more fin area than a rocket designed for maximum altitude would need or want. I would think the best altitude competitors would be minimum diameter rockets with the smallest fins sufficient to provide stability.

 

[BEC]

I don't think I've actually ever built one. They come to my club launches from time to time, but I don't think we've ever instrumented one (put in an altimeter) and gotten an idea of how high it really went.

You want minimum fin area for adequate stability with the motor you are using, and to make the fins as thin as possible (I use 1/64 plywood or thin carbon fibre sheet for small altitude models). The rest of it needs to be nice and smooth, though there is a trade there between smooth (via finishing) and weight. The smoothness becomes more and more important as you go higher in the impulse classes.

A long, slow burning motor is also better than a fast one as drag goes up with the square of velocity.

One typically doesn't gain by adding mass until you're up in the E/F impulse range. For an 18mm-motor-powered rocket, lighter is better.

To the OP: Alphas made from current kits typically come in at just over an ounce (so ~29-30g). To get one down to 21g one would have to bin that big honking thick "centering ring" used in the motor mount and go back to a pair of AR2050s, and go really light on the finish.

Also, if the current HiFlier has the clay weight in it (I haven't looked) that's there because on a C motor a HiFlier has been known to be unstable. If you're looking at a maximum altitude with an A or B, you probably don't need it. But to really maximize altitude on an A you'd want a 13mm minimum diameter model.

 

[SolarYellow]

Just to check my understanding, I'm reasoning that a C motor making a rocket unstable when it would be stable with an A or B is due to the greater mass (fuel load) of the C motor, which is all the way at the back and therefore shifts the center of mass rearward. Also, such a rocket might start out unstable, then become stable when enough fuel burns and head off in some random direction in a more or less straight line. (I've actually seen that one happen, but hadn't thought through why it happened that way.)

 

[Ronz Rocketz]

I'd be curious to see what people's experience has been, particularly people that fly with altimeters (@BEC , @Ronz Rocketz) with the Hi-Flyer. It is a cool LOOKING rocket, but seems like it has far more fin area than a rocket designed for maximum altitude would need or want. I would think the best altitude competitors would be minimum diameter rockets with the smallest fins sufficient to provide stability.

I built a slightly larger BT-50 scratch version. I got 820ft on a C6-5. This was the first tries with a Flightsketch before using a pouch to protect the altimeter from the sun. Ironically, I had the same issue as I had yesterday with the app and lost the data. Effneh...

 

[SolarYellow]

Well, this was interesting. I bought a Hi-Flier kit this afternoon. Busted out the gram scale and measured and weighed all the components. Then built it in OpenRocket. It's too light, as I didn't put anything in for paint or glue. With a C6-7, built according to Estes' design, the kit has a slightly negative stability factor. Adding paint to every surface would disproportionately increase the weight of the fins, moving the c.g. rearward and making it even more unstable.

By just trimming the stock fins and adjusting their position rearward to hang off the back, as well as shortening the main tube by 5cm, I got to a stability factor of 1.0 cal and added 7.5 percent to the sim's apogee height, so that should be decent as a baseline. Will probably just go with that as a "good enough" solution with zero additional cost. Might leave a little more length on it for a stability cushion after finishing, giving up a little apogee.

It turns out that long taper on the forward end of the fins is counterproductive. By playing around with fin shape, making them smaller and therefore lighter, I got to a stability factor of ~1.8 cal and added ~10 percent to the sim's kit-spec apogee height. Promising.

All of this is with no clay. And the clay in the kit weighed 7.16g.

I have some scraps of Monokote sitting around, so will probably use that to cover it. It's simply accepted in the model airplane world that covering is lighter than paint, even heavy covering like OG Monokote. I was pretty good with the iron and heat gun back in the day.

 

[ZoomieG]

Thinking about it for a few seconds, it seems obvious that the fin design doesn't put the center of pressure as far back as traditional "swept back" fins. So if one wanted to run a Hi-Flier at an Alpha weight or lighter, redesigning the fins to be more traditionally swept back like the Alpha's fins and move the CP further back

Estes Star Dart #2170

The Star Dart pre-dates the Hi-Flier by a couple years, but was only produced for five years. The Hi-Flier is identical other than the fins and decals/livery, and as far as I have been able to ascertain, are the only three small BT-20 sport models with a 9" body tube (Space Racer 2069 is the third). Most other are 7.75", 8.65", or 9.75".

Star Dart had smaller, swept delta fins that extended beyond the rear of the rocket. They can still be seen on ebay from time to time, and also makes an appearance in the Estes "High Altitude" set (Star Dart 2170 and Sizzler 2171).

I've attached an openrocket file I made last summer that you could use to replicate the fin pattern with 3/32" balsa. It is not pretty, as I haven't had the time to learn fancy graphics, but the materials are measured from an actual kit.

 

[SolarYellow]

Did a little engineering this morning. I drew up the stock fins in cad, then rotated them about the forward outer corner of the clearance notch until the leading edge took angles of 30, 37.5, and 45 degrees. The notch location became the new alignment point with the rear of the main tube, shifting the fin rearward. I reworked the rear shapes until I came up with something I liked the look of, and kept elements consistent across the different versions. I kept a flat on the rearmost part so it doesn't come down on a point and get beat up. Used cad to pick off the dimensions of the rotated and new points and enter them into OR. Lower left is the stock fin, the others are all just trimming of those same pieces of balsa.

One question I do have is whether that's likely to be enough clearance around the exhaust plume. I don't want to get these things all burnt. The first kick out is 1.5mm, and the second point is 8mm back, 8mm out. I suppose if I need to redo it, a new kit is only $6.99.



With more accurately measured dimensions on the fin, the stock kit's stability was even more negative, and the flight sim numbers show it being unsuccessful. Not sure whether it's attempting to calculate an unstable flight path or if the calculations just break down due to the instability.

I didn't see wadding in OR. I guess it could just be another mass element. But it's very light and slightly ahead of the c.g., so the difference it makes is likely to be insignificant, but in a favorable direction. I'm also looking at the motor holder clip and thinking I could shave the sides ahead of the retaining band to take some more weight out of it.

The good news is that the modeled stability gets really good for all revisions of the fin, and the performance looks really good. The stability factors were good enough that I looked at shortening the tube to reduce weight and drag. It started looking like I might be getting tight on room for wadding around 18 cm (down from 22.9 as the tube comes in the kit), so I called that good.





So far, not bad for just assembling the stuff that comes in the kit a little differently with zero extra cost. The suggestion is that those stock fins, although they may look zoomy, are actually kinda dumb.

So my plan is to build it with the 37.5 degree fins as seen in the OR screen cap above. It looks the best and OR shows slightly better performance than with either of the other options. I'll initially build it with the tube full length. I'll load the recovery pieces, gram scale it, find the actual c.g. location, and rerun it in OR to decide what length to trim the tube to. And work my way up to the big engines.

ETA: After making the comments above, I went back into OR and played with trimming the fins even more. Coming up with a better looking, more conventional, easier to make fin that is smaller and results in still better performance. OR shows it with a stability factor of ~1.2 with the short tube, >1.5 with the full-length tube. I think I'll build that with the full length tube and proceed as above.

 

[SolarYellow]

Estes Star Dart #2170

The Star Dart pre-dates the Hi-Flier by a couple years, but was only produced for five years. The Hi-Flier is identical other than the fins and decals/livery, and as far as I have been able to ascertain, are the only three small BT-20 sport models with a 9" body tube (Space Racer 2069 is the third). Most other are 7.75", 8.65", or 9.75".

Star Dart had smaller, swept delta fins that extended beyond the rear of the rocket. They can still be seen on ebay from time to time, and also makes an appearance in the Estes "High Altitude" set (Star Dart 2170 and Sizzler 2171).

I notice the instructions you linked to show the fins on the Star Dart being cut out so the grain was parallel to the body tube, rather than parallel to the leading edge. Rookie error. I wonder if it went out of production due to a reputation for the fins just snapping off?

Between that and the instability on the Hi-Flier, it makes me wonder how good the designers actually are. Is there not at least a best-practices checklist they follow?

 

[Spitfire222]

This may not be helpful to your endeavor, but have you looked at the Estes Xtreme? It's essentially a Hi-Flier with a different nose cone and cardstock fins, supposedly optimized for maximum altitude. That may be a better starting point?

 

[jmasterj]

This may not be helpful to your endeavor, but have you looked at the Estes Xtreme? It's essentially a Hi-Flier with a different nose cone and cardstock fins, supposedly optimized for maximum altitude. That may be a better starting point?

I think it's longer, too.

 

[SolarYellow]

Well, I already have the Hi-Flier kit. So I think I'll go ahead and build it.

From the work behind all the posts above, I decided the delta fin concept isn't the best, because it necessarily places the CP of the fins a lot farther forward than if they were hanging off the back, which means they have to be bigger, heavier and draggier to get the stability job done.

This adventure started out as just looking on the Hobby Lobby web site to make sure they had the Alpha before I drove over there. Then I saw the Hi-Flier and it was a bunch cheaper, so I started looking at it, and had to figure out why Estes was saying it was so much heavier. One thing leads to another, and I'm down this rabbit hole...

I have a solution I like all planned out. Basically, it's using the Hi-Flier kit to build a pretty different rocket. Time to build the dang thing and get back up to daylight.

 

[ZoomieG]

I notice the instructions you linked to show the fins on the Star Dart being cut out so the grain was parallel to the body tube, rather than parallel to the leading edge. Rookie error. I wonder if it went out of production due to a reputation for the fins just snapping off?

Between that and the instability on the Hi-Flier, it makes me wonder how good the designers actually are. Is there not at least a best-practices checklist they follow?

They aren't. The pic in the instructions is just a generic representation of parts. Actual fins have the grain along the leading edge.

 

[Daddyisabar]

Our old RSO hated high fliers, none to fly with a C motor. Look, here comes the scout group with a bunch of really poorly built hi fliers! Time for many tailgate rebuilds before even getting in line for the RSO table.

 

[lakeroadster]

What is this? Looks like an added mass of some sort?

The rocket is morphing into a clipped wing Alpha.

 

[SolarYellow]

What is this? Looks like an added mass of some sort?

Yeah, it's a mass component. I couldn't figure out any other way to get the engine hook in there. It's the length and mass of the engine hook and positioned abutted against the engine block. Not splitting hairs on the linear density difference of the different bends on the ends.

I can see the Alpha comment, but it's skinnier, like an Alpha on a diet. The fin chord is proportionally longer and they are shorter "span," with less taper. The clipped /flat rear ends of the fins make them a little beefier for streamer landings (not coming down on points), as well as helping shift the CP rearward. Speaking of not coming down on points, I might add a little belly to the trailing edge of the fins. When considering those differences, it's just a traditional rocket with more differences that matter than similarities to the Alpha, when you get into it.

Cad-calculated area of fins:
22.1 cm2 stock Hi-Flier
10.7 cm2 "clipped wing Alpha"

Weighing all the components in the kit, I'm at ~14.6g without glue or finish. Scaling the mass of the fins by the area reduction, I end up ~13.7g. OR model is saying 13.8g with no motor and a full-length tube, so small decimal differences on each component adding up, but not too much. I just rechecked the individual component masses as I worked them out in OR, and confirmed they are all pretty close to what I measured. With the tube cut to 18cm, OR shows the weight with no glue, finish, or motor as 12.9g.

Area of the full length tube and six fin sides is ~200 cm2. With the heavy OG Monokote I have sitting on the shelf being ~56g/sq.m, it should add around 1.2 g with overlaps. I'm pretty good at keeping glue mass down; doubt I'll have 2g of the stuff, unless I do big epoxy fillets or something. With possible shortening of the tube, it looks like 17g or less should be within reach. I would love it if I could hit 15.6g, half the Estes spec weight with clay. Wish I could find some Coverite Microlight, as it's only ~20g/sq.m. Debating whether to paint or Sharpie the nose cone. Haven't found any ways to dye polystyrene.

As noted, with the full-length tube, the OR stability factor is 1.54 cal with a C6-7. So I have a pretty good cushion starting out building it like that. OR says I should be able to get another 0.9g or so out of it just by cutting the tube to 18cm if I want to go that far (1.0g accounting for Monokote) and still have a ~1.2 cal stability factor. Will update the OR model so it matches the build once it's done, then make adjustments to tube length based on accurate data.

 

[BigMacDaddy]

Remember that a rocket’s path to apogee isn’t done only under power. A good portion of it is coasting. A rocket that is too light will try to fly like a feather, with drag forces slowing it down excessively. More mass helps it push through.

I never really fully appreciated this but just did a quick simulation with a minimum diameter rocket I made a while back with that gets close to Mach 1 (on paper). Anyway, confirmed that with added weight it goes higher (although slower)... This is also a C6-7...


Here is the same rocket with 13g of weight added to nose. Extra 52meters but quite a bit slower (more stable as a bonus)...

 

[BEC]

Interesting. I would have thought that's too small/light to benefit from added mass. I guess In need to spend some more time with OR myself....

Then again with seven seconds to coast...it would be interesting to see how much too early the ejection is for either model...especially since in the real world Estes delays tend to run short.

 

[SolarYellow]

Here is the same rocket with 13g of weight added to nose. Extra 52meters but quite a bit slower (more stable as a bonus)...
View attachment 531388

With that stability factor, it would be interesting to see how small you could get away with making the fins. I'd just start truncating them perpendicular to the body tube. Lighter, less drag, less leverage to generate damage on landing. What does the extra mass do to the descent speed during recovery?

 

[BigMacDaddy]

With that stability factor, it would be interesting to see how small you could get away with making the fins. I'd just start truncating them perpendicular to the body tube. Lighter, less drag, less leverage to generate damage on landing. What does the extra mass do to the descent speed during recovery?

Yeah, fins can get pretty darn small... I would probably rail launch so not too worried about the lug.

 

[SolarYellow]

There's an optimum with the nose weight. Too much, and it doesn't go as high. Not enough, and it doesn't go as high. Played with OR some more tonight. Super convenient: you can just click the up/down buttons on the mass component setup dialog to add or subtract a gram, and it will recalculate the apogee immediately. So just click the buttons up and down to quickly find the max value. Usually, there will be a range of 2-5g that gives the max simulated apogee. Within this range, more weight means greater stability factor. So set that to the upper end, then hack away at the fins to make them smaller, watching the stability factor go down and the apogee go up further. Iterate a few times between fin size and weight. Most of the benefit of adding the weight comes from the fin reduction reducing drag.

I looked at three models and tuned their configurations.

The Hi-Flier standard kit configuration with 7g added became stable with a factor of 1.37 cal and a calculated apogee of 430m. Played around and found the ~7g included in the kit was about optimum for the model. Shortening the body tube by 2cm increased the calculated apogee to 436m but dropped the stability factor to 1.07 cal.

The last small-fin version I posted in post #10 above has a stability factor of 1.18 cal with no nose weight and a calculated apogee of 491m. With anywhere from 2 to 6g of nose weight, the apogee increased to 493m, and went down with 7g. Stability factor correspondingly increased, ranging from 1.66 to 2.49. Looks like a win-win, although I'm not sure 2m is worth much for a sport model. Could be beneficial to have a slightly faster descent on the streamer, or maybe a slower descent would reduce risk of damage. This seems like a well-balanced model, as the optimum delay times come out a little under the nominal delay times for Estes A8-5, B6-6, and C6-7 engines. Still, no bulk pack engines with those delay times.

I then saved that model as a new .ork and hacked away at the fins, iterating between smaller fins and nose weight. Got the calculated apogee up to 541m with 4g of weight and a stability factor of 1.07 cal. With 6g nose weight, the apogee is still 540m with a stability factor of 1.46 cal. I played around with extending the body tube back toward the original length, but it was obvious that a shorter tube with more weight gave significantly better performance and stability factor. Proportionally, the fins look a lot smaller than we're used to on models, but not off the reservation when you look at high performance stuff like various military missiles.

The shorter the tube, the higher the apogee. Shortening the tube another 2cm to 16cm increased the apogee to 554m with 5g of weight with a 1.19 cal stability factor. (Image shown below is 18cm.) It may be a little short to pack in the wadding and recovery gear appropriately loosely. Will have to mess around with the parts on my bench and see how all that works; the best answer is the shortest tube that will fit everything properly.

One issue is that the heavier, low-drag configurations were coming out with C6 optimum delays in the 7.0-7.2 second range. The OR sim data table allows you to see the vertical velocity at the time of the ejection charge, and it may not be worth worrying about a few m/s velocity, depending on your recovery system. I'm thinking as much as 6-7 m/s with a small streamer shouldn't be a problem. B6 optimum delays overshot 6 seconds by even more then C6s overshot 7 seconds, but the velocity was about the same at deployment. A8-5s were OK, but what's the point? Depending on how long an Estes C6-7 delay is actually, one might sacrifice apogee for a shorter optimum delay to get a cleaner overall flight plan, at least for a sport model. Or maybe put a wrap of reinforcement around the top of the tube. Or don't worry about it.

Summary:
That's roughly a 118m or ~27 percent increase in performance vs. the standard kit just by playing around with the components. Depending on how accurately it all works out, that's around 1800 feet from a $7 rocket kit.

OR is a pretty cool tool.