2.6-inch 29mm MPR Scratch Build

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Dane Ronnow

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It looks like Thursday is going to be nearly perfect for the first launch of my MPR build. Here's the weather report from Windfinder for 3/24 at Jean Dry Lake Bed, 25 miles south of Las Vegas:

windfinder.JPG

Mid 70s, sunny skies, and barely a breath of wind. From where I sit, it doesn't get much better than that.

I'll be applying a coat of Pledge Floor Gloss to the rocket tomorrow, then doing a final weight and CG check so I can run simulations in OpenRocket for the two motors I plan on using—G74W-9 and G80NBT-10.

Two days out and I'm already in the early stages of high anxiety. Then again, I've been nervous about launching this ever since I started spraying it with primer.

I just hope that wind report holds.
 

Dane Ronnow

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First Flights - MPR Scratch Build

Thursday, March 24th, was a near-perfect day for launching rockets. It could have been a little cooler—the Windfinder forecast called for a high of 79 degrees, but by 3 p.m., it was 101. But the wind was predicted to be 1 to 2 mph—and it was—and that was really all I cared about. That, and a clear, blue sky.

I was joined by my stepson Zac, who helped set everything up for the launches. He also shot the video from a smartphone. And I've got to say, he nailed the framing on the ascent every time, with the rocket never flying out the top of the video.

I had concerns about the first flight. First, would the parachute deploy fully—it's a big chute, with long shroud lines. And I made it that way so I could get soft landings, which was my second concern—would the fins be damaged hitting the hard surface of the lake bed. The answers were yes, the parachute deployed beautifully, and no, the fins weren't damaged, at least on the first two flights. They were absolutely perfect.

First flight, on a G74-9W:

View attachment MPR first flight 3-24-22 G74-9W.mov
















I want to say first that it was pretty awesome to see this rocket fly. I've never mentioned it in the thread, but this is my first build since I was a 15-year-old kid with an Estes Saturn 1B on a cluster of four B8-4s. That was in 1966. And it was the same old feeling of holding your breath until the parachute popped, then just enjoying the hell out of it—but this time on a G motor. Seeing this rocket streaking like a shot into that dark blue sky and disappearing out of sight, was really freaking cool.

The rocket landed approximately 300 feet away with no damage to the fins. Not even a chip. I attribute that to the descent speed of 10 fps, and the coat of Pledge Floor Gloss I applied the day before. That stuff is pretty amazing.

Data from FlightSketch Mini:

Apogee: 1559 feet
Max Speed 303 mph
Time to Burnout 1.1 secs
Time to Apogee: 10.2 secs
Time to Eject: 9.9 secs
Apogee to Eject: -0.3 secs
Total Flight Time: 165.4 secs
Average Descent: 10 fps

Compare that to the OpenRocket simulation I ran the night before with 2 mph of wind:

Apogee: 1636 feet
Max Speed 296 mph
Time to Apogee: 9.45 secs
Total Flight Time: 166 secs
Average Descent: 9.5 fps

That tells me that when you weigh every component that goes into the build, OpenRocket is pretty darn close to the actual flight. The only variable really is drag, and in the end that boils down to finish—regular, smooth or polished. And those can be pretty subjective.

Even though mine was coated with Pledge, and pretty close to having a polished finish, I simmed it as smooth because of the vinyl lettering. A lot of edges there. Sealed, but edges nonetheless. And looking at the OR-to-altimeter comparison, I'd say it's somewhere between smooth and regular.

Second flight, also on a G74-9W:

View attachment MPR second flight 3-24-22 G74-9W.mov
















The second flight was a carbon copy of the first, except it landed half the distance away from us—about 125 feet. Again, no damage whatsoever from landing on the hard lake bed.

I think the thing that struck me the most in both these flights was how straight the rocket hung beneath the parachute on descent. No circling at all. I think the spill hole is the key there.

The third flight, on a G80-10T, was a different story. At least from deployment, forward. The flight up was even more spectacular than the previous launches, of course, because of the bigger motor. But the parachute fouled, and the rocket came in hard, fins down. I can't be certain, but I believe it was the way I packed the lower shock cord cord in, S-folded next to the Nomex blanket bundle. Same way as I'd done it earlier, but also a little hastier, not being as meticulous. It was 3 p.m. and the temperature was 101, and Zac and I were frying. I just got in a hurry.

My rushing through the preflight prep and checklist for that flight also resulted in my failure to arm the altimeter for launch. So I have no data from that flight. But if the FlightSketch comparison to OR sims with the G74 held true with the G80, the apogee would have been around 2400 feet, with max velocity about 400 mph. Total flight time around four minutes.

Third flight, G80-10T:

View attachment MPR third flight 3-24-22 G80-10T.mov
















Incredibly, two hairline cracks along the fin fillets were the only damage.

Thankfully, this rocket will fly again. Lessons learned: Don't rush the process. Ever. Don't skip the pre-launch checklist just because you've done it once before. And finally, just because you have two perfect flights in a row, it doesn't mean you're going to have three. Prep the rocket for each flight as if it's the rocket's first time up.

In the end, there was nothing wrong with the rocket. It was built strong, and flew beautifully. It crashed because of my failures. On the brighter side, it's got its first dings, so I can quit worrying about marring the finish.

All in all, it was a great day. Zac and I had a blast launching this rocket together. He shot some excellent video. And we both got sunburned. We can't wait for the next time.

02.JPG
 
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Dane Ronnow

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Still shots from the first launch

It wouldn't be a post about a rocket launch without shots of the ascent. (And a couple of shots of a really cool parachute.)

29.JPG 30.JPG 31.JPG 32.JPG 33.JPG
34.JPG 37.JPG 39.JPG 41.JPG 42.JPG
 
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Dane Ronnow

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I post these pics mainly because there seems to be a lot of interest on Rocketry Forum in how to pack a parachute. This is how I do it.
(Shroud line S-folds are held by 3/8", light orthodontic elastics.)

IMG_0644.jpg IMG_0645.jpg IMG_0646.jpg IMG_0649.jpg IMG_0652.jpg IMG_0653.jpg IMG_0654.jpg IMG_0656.jpg IMG_0657.jpg IMG_0659.jpg
 

Dane Ronnow

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Really like what you have done with this baffle. I typically stay away from this style as I've had burning embers get past the baffle and up into the recovery gear and burn the shock cord and parachute.

In the past I've cut a third plate. Basically made a duplicate of the upper late with the holes around the outside and used it as the top and bottom plates and used the plate with the small holes in the middle of the baffle.

I think your design has even less chance of embers getting past and it's probably a little lighter as well.
I replied to your post back in February, saying I hadn't launched it yet, so I didn't know how well the baffle would work. Now I know. (See post #62 above.)

I inspected everything after each flight—airframe, shock cords, parachute and shroud lines, and Nomex blanket. No damage, and no burns. In the pics below, the blanket appears like new—no burns, no particle spots, nothing but dust from the lake bed.

From that, I'd say the baffle is working well. Of course, that's after three flights. Ask me again in 15 :)

IMG_0685.jpg IMG_0686.jpg
 

Dane Ronnow

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I have a question about data from the FlightSketch Mini. The screen shot below from the FlightSketch flight log shows a temperature of 68.5 degrees Fahrenheit. When I powered up the Mini for this flight, it said 90. I'm guessing the 68.5 would be at apogee, but would it be that much lower 1559 feet above the lake bed where it was 90? Or is the FS data wonky?

FlightSketch weather.JPG
 

Dane Ronnow

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Fin Damage

Closer inspection of the cracks along the two fin fillets told me the paint had cracked, not the fillet itself. Lateral flexing of the fin showed no sign of movement at the root, and I could feel that the internal joint between the tab root and the motor tube was solid.

IMG_0682.jpg IMG_0683.jpg

I repaired these by gently pulling the crack apart and carefully flowing a little CA into the opening. Then I pushed back on the fin, closing the crack as much as I could, and held it there while the CA dried.

After a thorough cleaning of the lower fin area—the gray section—I applied another coat of Pledge Floor Gloss to that area alone. I'll let that dry overnight, then apply one last coat to the entire rocket.

IMG_0691.jpg IMG_0692.jpg

While I was inspecting the rocket, I found a spot on the top of the body tube where the shock cord had stressed the edge—not a zipper, but getting close to one.

IMG_0684.jpg

I've got a roll of 1 inch white vinyl tape, and, prior to applying the next coat of Pledge, I'm going to wrap the top of the tube with two or three wraps of tape (with the nose cone in so I can pull against it). I'll lay a thin bead of CA along the bottom edge of the wrap to seal it to the body tube, and another bead around the top edge. When I apply the last coat of Pledge to the rocket, that tape wrap will be secure.

If I still end up with a zipper, I'll let you know.

In the end, I feel fortunate that the damage from that hard landing wasn't a lot worse. It's driven home the need for focus during the parachute packing process—the entire preflight, really, checklists included. Step by step. (And no talking, please. It's distracting!)

Later :)
 

Tractionengines

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I have a question about data from the FlightSketch Mini. The screen shot below from the FlightSketch flight log shows a temperature of 68.5 degrees Fahrenheit. When I powered up the Mini for this flight, it said 90. I'm guessing the 68.5 would be at apogee, but would it be that much lower 1559 feet above the lake bed where it was 90? Or is the FS data wonky?

View attachment 512396
The Flightsketch mini does not record flight weather itself. When you download the data it uses your devices location to get weather from somewhere on the internet. Lots of times my flight weather data was way off...
Having my phones location service on, and downloading the Flightsketch data right away gives "closer" data.
 

Dane Ronnow

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The fin repair turned out great. After closing the cracks in the paint with CA, I gave the entire fin section two more coats of floor gloss.

IMG_0698.jpg

I wrapped the top of the body tube with three turns of white vinyl tape, flush with the opening. Then I sealed the top edge of the BT and tape with CA, cleaned everything up with #600 sandpaper, and applied two coats of floor gloss to the top 2 inches of body tube (and tape), and to the nose cone. (Did I mention how amazing that stuff is?)

IMG_0694.jpg IMG_0700.jpg

And last of all, I replaced the existing 12 feet of shock cord with 15, for a little added insurance against zippering.

With that, this rocket is ready for another launch day. (I've got my eye on the weather :))

BTW, I want to thank everyone for reading and commenting on this build thread. I got a lot of good advice. I hope some of my build techniques helped others the way many of you have helped me. I could never have built this rocket without The Rocketry Forum. What a great place for ideas!
 
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Dane Ronnow

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Following the repairs described above, I made it back down to the lake bed yesterday for a single flight on the G80-10T motor. (I was planning for two additional flights, but the wind picked up after the first launch, and the blowing dust was pretty bad.)

Excellent flight—straight up, good parachute deployment (unlike the G80 flight on 3/24), then a very long descent—over 4 minutes. No damage whatsoever.

[Edit: Over 5 minutes]

Apogee was 2519 feet, with a max velocity of 342 mph. Data from the FlightSketch Mini was erratic, so I don't have any other numbers. (Regarding that, I posted a thread here: https://www.rocketryforum.com/threads/erratic-flightsketch-mini-data.172161/#post-2266636.)

My stepson Zac was out of town, so I don't have any pics or video of the flight. :(

But now I know two things—this rocket loves the G74 and G80 motors (9-second and 10-second delays, respectively), and careful packing of the parachute makes all the difference in the world. (Crazy, eh?)
 
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Following the repairs described above, I made it back down to the lake bed yesterday for a single flight on the G80-10T motor. (I was planning for two additional flights, but the wind picked up after the first launch, and the blowing dust was pretty bad.)

Excellent flight—straight up, good parachute deployment (unlike the G80 flight on 3/24), then a very long descent—over 4 minutes. No damage whatsoever.

[Edit: Over 5 minutes]

Apogee was 2519 feet, with a max velocity of 342 mph. Data from the FlightSketch Mini was erratic, so I don't have any other numbers. (Regarding that, I posted a thread here: https://www.rocketryforum.com/threads/erratic-flightsketch-mini-data.172161/#post-2266636.)

My stepson Zac was out of town, so I don't have any pics or video of the flight. :(

But now I know two things—this rocket loves the G74 and G80 motors (9-second and 10-second delays, respectively), and careful packing of the parachute makes all the difference in the world. (Crazy, eh?)
Don't forget the G64. My favourite motor. Pain to get lit unless you use the sliver of BP at the top of the grain to boost ignition of it. Great crackle..... Nice build thread.
 

Dane Ronnow

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Don't forget the G64. My favourite motor. Pain to get lit unless you use the sliver of BP at the top of the grain to boost ignition of it. Great crackle..... Nice build thread.
I'm flying single-use motors with this rocket, but the next two I want to try are the F25 and G38—longer burn, lower acceleration.
 

Dane Ronnow

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Fin Modification

I found new cracks after the April 13th launch. Like the others, these are hairline cracks in the paint along the fillet. And as much as I'd like to avoid rocket surgery, I don't want to keep filling these with CA. I need a permanent fix.

I thought about different ways of strengthening the fillet against ground impact, and settled on the idea of laminating the inner portion of the fin and the fillet with a 1" strip of .25mm carbon fiber veneer.

But before diving into that, I posted a thread seeking advice. And early on in that thread, @cls suggested it might be fin flex from velocity on ascent. From there, the discussion turned to fin flutter, and how to calculate the speed at which it occurs on highly-swept fins such as these. (Hint: You can't, using the standard method employed by flutter calculation spreadsheets.)

I won't rehash what was discussed in that thread, but I will say that John Cipolla devised a method of finding the flutter speed on these fins. Without his help, I'd still be spinning my wheels.

This is the thread for anyone who's interested:


So now I have a plan. And it involves removing all but 1/4" of the overhang—a reduction in fin area of nearly 40 percent.

Realizing the overhang was the cause of these cracks, I had already removed 1.25" (blue line in the photo below) before posting the thread referred to above.

After a lot of discussion, flutter calculations, and simulations in OpenRocket, I decided to cut the fins again (red line). And while this second cut would drop stability to less than 1 caliber, OpenRocket indicated the pitch rate would actually be much less—almost half as much.

Fin mod 00.jpg

So, following is the fin modification, beginning with the second (red line) cut.

First, I'm using a razor saw, so I need a way of holding the rocket stable and bracing the fin that's being cut. Otherwise, the saw blade is going to be jumping all over.

I have a box I've modified to hold the rocket firmly while I'm working on it. The bottom is a single panel of cardboard, so, unlike boxes with flaps on the bottom, this one lies flat, and doesn't rock at all. (I also use this to cradle the rocket when I'm down on the lake bed.)

With the rocket in the box, I found another box to support the fin, with a 1/8-inch scrap of balsa sheet as a shim. This is very stable, allowing me to make clean cuts.

Fin mod 01.jpg Fin mod 02.jpg Fin mod 03.jpg

The first cut is off the trailing edge, taking it up to 1/4" from the bottom of the body tube.

Fin mod 04.jpg Fin mod 05.jpg

The second cut removes 1.125" from the outer trailing edge at a 30-degree angle. This cut forms the tip of the fin.

Fin mod 08.jpg Fin mod 09.jpg Fin mod 10.jpg

Between these cuts and the cuts a few weeks ago, a little over .5 oz of weight was removed.

Fin mod 11.jpg
 

Dane Ronnow

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Next, I rounded the trailing edge and tip to help reduce drag.

Fin mod 12.jpg Fin mod 13.jpg

And finally, I sealed the exposed balsa with CWF.

Fin mod 14.jpg

I'll let this dry overnight, then sand the edges and brush on a little Testors matte enamel for primer.

I want to thank everyone who helped me work this through on the other thread, among them: @cls, @timbucktoo, @waltr, @ThreeJsDad, and especially @lakeroadster, who went the extra mile. And John Cipolla, who communicated through email, patiently educationg me in the realities of fin flutter. Thank you all.

More later.
 
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