Hydra-IRIS Two-Stage Sounding Rocket (3D Printed Parts + BT-80 / BT-60 + Plywood Fins)

[SolarYellow]

The sustainer fins also look larger in the pictures (there is obviously some perspective distortion going on here but they still look taller).

You obviously have some decent modeling software. Why don't you model up a 45-degree LE for the fin and then get a perspective view of the assembly that matches the viewing angle of this photo? If it looks like the picture, I think you'd be on track. As a preliminary step, you might try adding another red line to the non-vertical sustainer fin tip in the photo and see what that looks like.

Ah, also just noticed another photo from the 2nd page of the brochure above that shows the sustainer fin fairly square on. Unless some photo manipulation is going on this looks like a photo of the 3-fin sustainer. It also looks like the 45 degree sweep angle for the fin is more accurate proportion.

I don't have any confidence I know how that thing is clocked. But I agree 45 degrees looks better.

 

[SolarYellow]

Huh? The sustainer fins' height is right there on the drawing, 99.2 cm at the root and 52.7 cm at the tip.
View attachment 598493

I don't think you can do the analysis you're wanting to do. If you compare how close the booster fin roots are to each other in the side view, it's waaaaay closer than in the SECTION A-A view where they are necessarily drawn correctly positioned. I would not be confident basing any dimension off of having scaled this drawing.

 

[jqavins]

All of my analysis, save one thing, has been done from the dimensions given on the drawing, not the picture on it. That one thing is how the fin tips seem to be at the same distance from the center line.

 

[SolarYellow]

But the booster fin roots are obviously not positioned accurately, so I would not assume that the tips of the two respective stages are positioned accurately, which is what your analysis depends on. And which seems to contradict an available photograph of the actual assembly.

 

[jqavins]

I beg to differ. The section on the drawing shows the fins in line, since only three radial bits are visible, and shows those radial bits radiating from the farthest extents of the booster. That is the configuration I assumed, and that is corroborated by the photo.

As for the tip to centerline distance, the unknown clocking of the side view makes the distance itself unclear, but which is closer to the center line, or if they're the same distance, does not depend on the overall clocking, provided the two sections are clocked tne same.

 

[BigMacDaddy]

But the booster fin roots are obviously not positioned accurately, so I would not assume that the tips of the two respective stages are positioned accurately, which is what your analysis depends on. And which seems to contradict an available photograph of the actual assembly.

I tend to like to rely on actual photos if they disagree with scale documentation (especially something like this brochure that was more likely made by a marketing guy).

However, in this case, my biggest worry about relying on the photos from the brochure is that they may be edited. They are being used on the brochure so if the fins were not clear it is possible that someone doctored the photos to make the fins look more distinct. If you look at other photos (that seem to be the same as the one on the bottom of the IRIS brochure page) the booster fins are far less clear and distinct -- so maybe someone made the fins more distinct which could affect the dimensions that are apparent in these photos.

That being said -- the pictures seem to suggest 45 degree sweep angle.

 

[SolarYellow]

This looks to be an undoctored version of that photo: https://www.designation-systems.net/dusrm/app4/iris.html

 

[BigMacDaddy]

This looks to be an undoctored version of that photo: https://www.designation-systems.net/dusrm/app4/iris.html

Yes, I think so -- that is the picture I was talking about. It looks like someone filled in the top of the booster fins (or at least played with the contrast) in the brochure version - right?

 

[BigMacDaddy]

Looking at this from a different perspective. It is clear that the sustainer fin in that picture is not 35.6cm tall (or just 15% wider than the sustainer body tube). There is significant distortion here that makes measurements further back smaller than they should be so this suggests that the sustainer fin is more than 43cm tall. It would be 46.5cm tall if the 45 degree sweep of other IRIS fins is followed.



It may just be that there were different versions since this was an evolving prototype.

 

[BigMacDaddy]

I think I've got the answer, or at least part of it. The drawing clearly shows that the fins have the same span measured from the center line. But the booster has a greater body diameter, so the fin has less height (in your frame) from root to tip. And that changes a number of things. And I'm supposed to be working, so I'll have to get back to this.
View attachment 598655

To your point -- the fact that the fin span is measured in relation to the booster tube makes it even less likely that this measurement is supposed to also apply to the sustainer fins (in my non-engineering opinion). Why would you report a number that would require some relatively complex trigonometry (or 3D modeling) to calculate the actual height/span of the sustainer fin (relative to sustainer body tube). It seems just as likely that they forgot to include the measurement for the sustainer fin height/span on this marketing brochure.

 

[SolarYellow]

To your point -- the fact that the fin span is measured in relation to the booster tube makes it even less likely that this measurement is supposed to also apply to the sustainer fins (in my non-engineering opinion). Why would you report a number that would require some relatively complex trigonometry (or 3D modeling) to calculate the actual height/span of the sustainer fin (relative to sustainer body tube). It seems just as likely that they forgot to include the measurement for the sustainer fin height/span on this marketing brochure.

View attachment 598711

I'd say even more likely.

In that photo, it does look like they did something with the contrast on the booster fin and maybe applied a sharpening or some other kind of filter to the image overall.

Given that this is a situation of having to figure out what to go with in an environment of crappy information, I'd go with your 45 degree hypothesis.

 

[jqavins]

All this vaguery of getting measurements from photos is exactly why I avoid using photos whenever dimensioned drawings are available. None of these photos are shot square on, so there is perspective as you noted, and also foreshortening from the rotation angles to contend with. And picture graininess. I always avoid as much of that as possible by using numbers marked on drawings.

The drawing we have is a bit blurry and is incomplete, I'll grant. The equidistance of the tips from the center line is not certain. A section looking up at the sustainer fins is probably on the original engineering drawing but left off of the brochure. Still, the figures that are on the brochure certainly appear to be taken directly from engineering drawings, and doing that is the way for the marketing guy to make life easiest on himself.

The result of my method to determine the sustainer fin span yields that angle as 45.6°. That 7 mm discrepancy I mentioned in section AA might have something to do with that. Make the span 46.5 cm (instead of 45.5) and you'll have 45° exactly. Of this there's no doubt, since the root and tip cords are dimensioned.

 

[BigMacDaddy]

This shows that the motor tubes as having 20.3 cm diameter, but also seems to show that two side by side add up to 41.3 cm, so a 7 mm difference somewhere. That may be due to a 3.5 mm skin?
View attachment 598674
So, I'll go with the 41.3 cm, and calculate that the root of the booster fins is 22.25 cm from the center line, and thus the tip is 57.85 cm from the center line.
View attachment 598676
No, we know graphically that the tip of the booster fin and that of the sustainer fin are the same distance (22.25 cm) from the center line. The diameter of the sustainer is given as 30.8 cm. So the sustainer fin's span is 57.85 cm - (30.8 cm)/2 = 42.54 cm.

Thus, here is the revised sustainer fin.
View attachment 598681

Thanks very much for doing the calculations on this.

This is much closer to what the pictures look like and is a measurement I could live with -- and, as you state, is within a small margin of a 45 degree version. I guess I will do the slightly larger 45 degree version since it is easier for folks to cut the fin smaller than to extend it.

I also lose a couple of percentage points by converting the rear tubes to BT-60 tubes (BT-60 is 4.3% smaller than scale for a BT-80 sustainer tube) so everything is within the realm of sport-scale

Reassuring to have a few perspectives to triangulate a reasonable scale for the sustainer fins. Probably some engineering drawings on microfilm somewhere that a FOIA request would get copies of.

 

[BigMacDaddy]

I am away from the rocket till later in the week but plan to re-measure all the actual dimensions to compare to scale again to make sure I did not make any mistakes.

 

[SolarYellow]

I also lose a couple of percentage points by converting the rear tubes to BT-60 tubes (BT-60 is 4.3% smaller than scale for a BT-80 sustainer tube) so everything is within the realm of sport-scale

Still available, for now: https://estesrockets.com/products/pro-series-ii-2-5-od-body-tube

If you can print all the other stuff, you can print couplers.

 

[BigMacDaddy]

Still available, for now: https://estesrockets.com/products/pro-series-ii-2-5-od-body-tube

If you can print all the other stuff, you can print couplers.

I have a tube or two of that oddball size.

 

[ljwilley]

Is this going to be available to buy? If so, when and for how much?

 

[BigMacDaddy]

Been looking around for more pictures (and maybe that last piece of data). Came across this picture which is a good one that adds to our info. Pretty square shot on the fins and some nice details on the launch platform -- er... underwater launch tower -- if anyone ever wanted to make a prototypical launch setup (glad to know that these engines will ignite underwater).

 

[CTimm]

Been looking around for more pictures (and maybe that last piece of data).

info

 

[jqavins]

 

[BigMacDaddy]

info

Wow -- that is fantastic. Thank you so much for sharing that final piece of info!!!!!

Where were you able to find this?

 

[Leo]

With all the data now available, this rocket begs to be built with a BT-80 based size Iris and 3D printed parts where feasible.

Edit: HAHA... I just read the first post and BigMacDaddy is doing exactly that, building a BT-80 version. I should read everything next time

 

[ljwilley]

Wow -- that is fantastic. Thank you so much for sharing that final piece of info!!!!!

Where were you able to find this?

 

[SolarYellow]

info

Is there anything that calls out the maximum thickness of the sustainer fins at the root edge? I think everything else can be worked out from that, but I hate scaling poor resolution, old B&W photos to get small dimensions like that.

It's a common missing piece I've noticed in a lot of @PeterAlway's drawings.

 

[Jeff Lassahn]

Part of what I assume keeps the booster nosecone from blowing off when the 2x zero-delay motors go off would be the weight of the sustainer.

If I'm understanding what you are saying correctly, there may be a problem here. At the time of staging the rocket will be essentially in free-fall, so the weight of the sustainer doesn't put any force on the booster. The only forces will be aerodynamic plus any small residual thrust from the booster motors.

 

[CTimm]

Is there anything that calls out the maximum thickness of the sustainer fins at the root edge? I think everything else can be worked out from that, but I hate scaling poor resolution, old B&W photos to get small dimensions like that.

It's a common missing piece I've noticed in a lot of @PeterAlway's drawings.

There are 2 IRIS displays that I am aware of that "could" be measured.
The one at Goddard with tons of paint caked on over the years and the other is currently at Tuskegee.

I have not come across any data for the fin root thickness other than some info regarding the base of the fin middle bracket being beefed up from .3125" to .4475"

 

[CTimm]

Wow -- that is fantastic. Thank you so much for sharing that final piece of info!!!!!

Where were you able to find this?

On my hard drive.

 

[John Brohm]

Is there anything that calls out the maximum thickness of the sustainer fins at the root edge? I think everything else can be worked out from that, but I hate scaling poor resolution, old B&W photos to get small dimensions like that.

It's a common missing piece I've noticed in a lot of @PeterAlway's drawings.

I had the same issue some years ago when I was building my two-stage IRIS. Richard Morrow, the author of "Small Sounding Rockets", shared some IRIS info with me that helped clarify this. Richard's data indicates a sustainer fin root chord which, in planform, is a symmetrical circular segment (as Chris' earlier note also indicates). The maximum width of the root chord is 1.392", which includes the thickness of the stock sheeting that faces the fin sides. The maximum width of the fin tip chord is 0.522".

 

[jqavins]

Is there a radius or non-zero thickness at the leading and trailing edges, or is it a knife edge? Asking out of academic interest. It's the kind of detail I wouldn't be concerned with even if I were building a scale model; I wouldn't feel compelled to make it that scale. I'd radius those edges for durability.

 

[John Brohm]

Chris' fin photo (H-IRIS) a couple messages back highlights the curved airfoil shape of the fin skins. Those fin skins have a thickness of 0.071" (about 15 gauge sheet) and they meet tangentially at the edges. So that would make for an edge thickness of 0.142".

Chris' photo also reveals a leading-edge cuff that's riveted to the assembled fin. That cuff is 26 GA/0.019" stainless steel according to Richard Morrow's data, so that would expand the leading edge to a width of 0.18".