New (and first) 2 stage rocket - Looking for advice as we move along.

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BryRocket

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I am somewhat new to HPR and building of rockets but I've had good guidance thus far. The first thing I was told was to build from scratch, so, that's where we are. I've built a couple scratch DD fiberglass rockets and then decided to tackle carbon fiber layup and tube rolling, active stabilization or multi stage rockets. I am super into the carbon thing, and it will happen, but I just couldn't commit to that now. So, multi stage it is.

My goal is to do this build in 4 stages and document it here as to obtain some feedback from others wiser and more experienced than myself. Jim Jarvis has been more than gracious and helpful since I got into this hobby so I hope I can give him a break and solicit some advice elsewhere, although since I am part of his local club, I will still get his opinion as we move along. I plan to make this rocket fly as a single stage first, and then build the booster for the 2 stage testing and subsequent flight.

1. Build the single stage booster (future sustainer).
2. Build the electronics bay (I plan to use a Raven for main/drogue/sustainer ignition, StratologgerCF for back up, some mobius camera and then the new featherweight tracker in the nose).
3. Finish the single stage rocket off.
4. Build the booster.

So, I already broke my initial plan and built the booster section and I will document that here. Hoping for some feedback going forward.

I started by using just an upscaled version of my 3" build that I made a few months back. It was built around a 38mm motor (I know, I know) and it's pretty much maxed at ~5400'. Here it is:

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From here, I just used OpenRocket to make it bigger and then cleaned up some of the dimensions. The new rocket was built around the 75mm CTI 4G case. I order all my fiberglass stuff from Tim at Wildman and he cuts and bevels my fins.

To start I worked on the layout of the first booster section that will ultimately turn into a sustainer. I started with laying out how I wanted my rail guides and centering rings. The aft centering was to be spaced 4" fwd from the tail end of the rocket to allow the use of an interstage coupler.

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I like to brace my rail buttons in some way so I have previously placed them to where I can stack a couple centering rings right under them. This has worked so far, but there may be a better way to do it. For this to work I had to place the aft rail button a bit more fwd than I would have liked. I'm thinking now I could have done some conformal glue-on ones (and will have to for MD rockets). For the conduit or tubing for the ignition wires, I used straight brake line ($6) and drilled holes in the CRs for that to pass.

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Once I had the brake line and fwd CR epoxied, I made sure it would slide inline with my rail button.

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Then nicked up the CR and epoxied it into the airframe and used the aft CR for support, unattached.

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Here is the aft end after.

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Next it was on to the fins:

Like I said, I get Tim at Wildman to make them for me from a .dxf file that I make with OpenLibre.

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I taped them off, scuffed them up and notched the bottom edges. I then used syringes to inject the epoxy into the fin slots and slid the fins in. After that I just kind of taped them to stay in place while I finished all 3 and then put my fin alignment cutout on them to hold in place over night.

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I apparently lost my photos of making the internal fin filets. What a pain! I basically used large 20cc syringes with a long tube hooked to them to get the epoxy down each side of the fin. I then set it at an appropriate angle so that the epoxy would settle at the fin where I wanted it and let it sit overnight. Repeat that process 5 more times. I can say with confidence that these fins are well attached. Perhaps heavier that I would have liked, but definitely attached.
 
Next was the attachment of the coupler:

I drilled a hole in the center for the 3/8" bolt to pass through for forward retention of the motor case. As you have probably noticed, I am using an aeropack retainer tailcone. This will be used on this rocket when flow as a single stage but will be moved to the booster once the full 2 stage configuration is complete and in that configuration, I will need some form of motor retention. I also drilled a small hole for the brake line to travel and added another small eye bolt to attach the ignition wire to for solid support when apogee event takes place and pulls the break wires apart. There is also a larger harness attachment.

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Here are some pics of the final product.

The new booster next to the previous smaller one. Its bigger brother I guess.

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Some others. You can see that the brake line looks like it will work great. It may not look like it from the picture, but there is plenty of room in between the motor retainer and ID of a coupler to pass the wires.

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So, here is my big "Oh heck" moment. I thought I had all this kind of worked out but now, after all the epoxy and aft CR going in, I now only have 3.75" of available real estate for the interstage coupler. Did I just completely screw myself there? I have always heard you want at least 1D. Is there any way to stiffen a coupler to get by with just 3.75" of insertion?
 
I have ordered the stuff for the electronics bay and we will start moving forward. This is where I'll def need some help. Drag separation vs charged? I suppose I'll have to have a computer for the booster since there is no motor deploy on 75mm motors. Do people ever just let the sustainer come up to pressure and blow the booster off?

I have lots of questions going forward. I'll update this once I get the electronics section done but in the interim, if there are any comments on the build so far, I'm all ears.
 
This looks great! I'm flowing with interest. I think someone needs to start a thread on TRF that contains links to two-stage build threads and data from two-stage flights.
 
You can use the regular aeropack screw on retainers in place of tail cone retainer. The bases are all the same. Im working on a similar setup and just ran into your same problem. Room for the interstage coupler to fit inside the sustainer...getting the CR back enough.
 
You can use the regular aeropack screw on retainers in place of tail cone retainer. The bases are all the same. Im working on a similar setup and just ran into your same problem. Room for the interstage coupler to fit inside the sustainer...getting the CR back enough.

I don’t have a regular retainer on hand but I think I remember doing the math and there would only be 1-2mm between the OD of the regular retainer and the ID of a 4” coupler. I was worried it might be a bit too tight. Do you know from experience that a 4” coupler will slide over the 3” aeropack regular retaining ring?


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...same problem. Room for the interstage coupler to fit inside the sustainer...getting the CR back enough.

We need to have someone with some experience comment on this I suppose. I literally have exactly 3.75”. I’d probably give myself 3.7” just so I know that the force is being applied at the airframe and not the CR.


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Well the rocket, as a single stage, is completed.

After building the initial booster, I then ordered the parts for the eBay, airframe upper and nose cone.

Here are the eBay build pictures. I am using a Raven for my main computer and a PerfectFliteCF for my backup. I'll use the Raven for both charged separation and sustainer ignition once we build the booster for that. I also have a RunCam 2 on board for perpendicular footage and then a Mobius Mini for parallel footage that will be removable. I wanted to incorporate that now because...I like using cameras but mainly because I want to be able to see the staging events in the future. I utilized a sled from Additive Aerospace (Andrew) and also bought my Mobius shroud from him (as well as various other switches).

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Here you can see how I mount a wooden coupler centering ring in the eBay. I then sink 4 T Nuts into that and epoxy over it all to strengthen it. This is how I suspend the eBay in the airframe. You can also see the little BP charge wells I make. I make these out of short pieces of copper pipe. They hold my 2.0mL centrifuge tubes perfect and help to direct the force.

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For the nose cone, I have a system that isn't exactly perfect but it works. I put some coupler centerings together in the coupler of the nose cone and put a 38mm tube in that. The aft CR actually has a wooden CR attached to the back and yes, TNuts again. Use 4 bots to hold the tracker sled into the nose cone. I have the same set up on my 3" rocket and this allows me just switch it back and forth and avoid the absolutely painstaking job of undoing 2 whole screws. I also suspend the Additive Aerospace sled with nylon all thread. It's not structural in any way really.


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Here are some photos of it laid out with the individual parts. There is a piston for the main. I get all my chutes made by Julie at Spherechutes. They are amazing, great price and value and quick turn around. The rocket was build around the CTI 75mm 4G case. It has some nice L loads in the 4G size (I'm not L3 cert) but I might convince one of my local rocket guys to "sponsor" an M flight. I will be using a new tracker in the nose that I'm not entirely familiar with so you might notice a little wad of shrink wrap around the drouge. That is one of my EggFinder Minis wrapped up with it's battery and logger. I'll be kind of a back up of sorts. I will fly it soon on a 2G load that sims out to roughly 6600. I have another 3G load that sims to 9700 so it'll be pushing my local waiver. Final is 90.75" and 14.4lbs sans motor.

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Now we are to the point of uncharted waters for me and where I will be needing the most advice. Questions to find answered to:

Drag vs charged separation, pros/cons? My thought on drag is that it seems less reliable maybe? Also, I like the solid feel I get w a coupler right now, do you have to make it a pretty loose fit to get the booster to separate?

With BP charged separation are there any real concerns? No worries with the charge going up in and igniting the sustainer motor or blowing the sustainer igniter out?

I’m sure there is some logic to choosing booster and sustainer thrust curves. I’ve read some but would like to get input from others. Does the booster thrust curve philosophy change if using charged separation vs drag?

Anyway, time to punch forward with getting this thing staged. In the interim, I plan to fire this thing off a few times as is.
 
Another quick note or “learning moment” was that by scaling my 3” version, that had favorable CG/CP design, is that I underestimated the significant weight difference in, not only propellant, but casing of the 75mm vs the 54mm/38mm cases. This rocket was fine for flying the 2G load in my 4G case w spacers but it was going to be close for if I flew the heavier 4G loads so I went ahead and added 20g epoxy and 150g lead shot all mixed up to the tip of the nose cone. All rosy now...unless that lead up there messes with my tracker or increases the chances of premature main deployment. [emoji848] I better go with 4 shear pins.
 
I rely on drag separation but use a 1/2 gram of BP for backup. I know a guy who uses drag separation with no BP & relies on 2nd stage ignition for backup separation. Most other 2 stage fliers I know also use 1/2 gram of BP as back-up. I use a piece of aluminum tape to keep igniter in place.
As far as booster motor, you want at least a 10:1 thrust to weight ratio for the whole stack on the pad.
 
I rely on drag separation but use a 1/2 gram of BP for backup. I know a guy who uses drag separation with no BP & relies on 2nd stage ignition for backup separation. Most other 2 stage fliers I know also use 1/2 gram of BP as back-up. I use a piece of aluminum tape to keep igniter in place.
As far as booster motor, you want at least a 10:1 thrust to weight ratio for the whole stack on the pad.

Thanks for the reply and advice.
 
It flew as a single stage this weekend. No problems. I'll fly it again as a single stage soon and then hopefully get a staged flight in this year. I'll update this with the booster build progress.

[video=youtube;3y66WYtYfao]https://www.youtube.com/watch?v=3y66WYtYfao[/video]
 
Quick update: The rocket is ready to go and targeting August 4th. This is my first build on here (4th total) and I did a horrendous job documenting it. I'll work on that.

First, here is the finished project.
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For my first flight, I am going to use the following configuration:

Booster: 75mm CTI L995 Red
Sustainer: 54mm CTI J244 White

We have a 10k waiver at this site and my goal is to get up around 8500 and see if I can get the sustainer to light and recover everything this go around.

I have the Raven set to fire the separation charge at 1 sec after burnout and then fire the sustainer at 2176' assuming that happens less than 7.3 seconds into flight. More on that: I am hoping for separation at 1 sec after burnout and sustainer ignition at 3 sec after burnout. I have ran simulations that show the rocket will be at ~2176' at ~5.3 seconds. I am just assuming it will take about 1.5s to come up to pressure and get rolling which would put ignition around 6.8s, ~3 seconds after burnout. I worked with Adrian and others on advice for setting the Raven up and I am using <7.3s as a qualifier. The rocket should (hopefully) be north of 3000' by then. If it tumbles or something goes seriously wrong, this will hopefully prevent unwanted sustatiner ignition.

IMG_Excel.jpg IMG_RAVEN.jpg

Here is the IC:
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I am going to use a StrattologgerCF to handle the cute deploy on the booster and I'll have a chute release on that. I also stuck a spare EggFinderTX in it as well. **Neat find: If you set the Strattologger's main deploy altitude to something greater than will be achieved, it will fire at 1 sec after apogee.** So, I have two charges, one set at apogee (Drouge) and the other set at 9999' (Main). This should fire each at apogee but one with a 1s delay. That is my only redundancy for the booster. 1 computer, 1 battery but 2 chances to fire the deploy.

There is some serious epoxy going on in the IC that you'll notice. That is to hold the bulkplate in place. I've blown coupler bulkplates out with charges on my pistons in the past so I went a little crazy here hoping to prevent that.

I was always targeting a static stability of 2.5ish for the whole stack but I had designed it around using 2 CTI 4 grain motors. An interesting oversight on my part was the stability once I used a heavy 75mm CTI motor in the booster and then a lighter 2 grain 54mm in the sustainer. It was back heavy and the stability fell to below 2 (I think it was ~1.8-1.9). So, we get creative. I am using a 2 grain 54mm motor in a 4 grain case with 2 spacers and an Aeropack MD Fwd Closure adapter. I'm pretty sure it accepts 5/16th threads. I was already planning to use this for providing the forward retention of the motor into the sustainer. I ended up make a few ~550g weights out of the CTI 54mm packaging, some lead shot and aeropoxy with a plastic center tube. I was able to bolt these onto the all thread supplying the forward retention to simulate a larger motor and manipulate the sustainer's (and ultimately whole stack's) weight and CG. This moved loaded stack to 2.26 and the sustainer to 3.19. It will work much easier when I get to using 2 larger motors of the same weight. In the picture you'll see two weights. I ultimately went with just one. Two improved the stability even further but hey, we do still have to go high right? If I had planned better I guess I could have added both and went with a 3 grain 54mm for the sustainer. Here are some photos of that:

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Photos of the booster filets:
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Photos of the 75mm motor. I put an eye bolt through a bulkplate. Mainly to prevent any of my laundry from getting snagged down in there but also to keep the inside of the spacer and the Fwd Closure clean.
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Photos of the sustainer with the wires ran for the ignition and separation duties. I was able to run an ematch wire through the conduit and a pair of 24awg solid core wires. I'll solder the igniter to those. I place my ematch in a 1mL centrifuge tube. It holds almost exactly 0.5g BP.
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So. That's about it for now. Hopefully we have a successful flight. I am pretty new to this and it certainly makes me nervous thinking about flying it. It ended up weighing 25lbs without motors. Looks like it could do about 24k' with 2 L motors (I'm level 2) but maybe closer to 30k' if I could talk some L3 into "sponsoring" it or whatever with a few Ms.

I'll report back in a few weeks.
 
Oops. Never got around to updating this. In short, first 2stager worked. Booster chute harness got pretty tangled up and tied the chute closed so it hit the ground hard and needed some repairs but we’re all ready to go again. Swivels added! Hope to find a waiver higher than 10k soon.

And, accidentally turned the down tube camera on then off again and only got a short video of me staring at it. Working on a solution there.

 
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Nice flight, thanks for sharing. It's interesting that the booster reduced spin to nearly nothing and that you can see separation by when the rocket starts spinning. Do you know if it drag separated before the separation charge went?
 
Nice flight, thanks for sharing. It's interesting that the booster reduced spin to nearly nothing and that you can see separation by when the rocket starts spinning. Do you know if it drag separated before the separation charge went?

Thanks!

I’m really glad you brought this up as I’ve been wondering about this myself. First, I’m quite confident that the booster separation was from the charge, not drag. This is speculation, but driven by how tight that IC coupler is, the weight of it and from what we witnessed from the ground.

About the spinning: if you go back to earlier in this thread you can see a video of the sustainer flying as a single stage. It spins in a similar manner. It’s the first rocket that I’ve ever made that does this. I think it is from one of two things: fins aren’t straight or the external camera shroud isn’t straight.

I had the fincan fin slots cut for me and have no reason to believe they aren’t straight. I also use a fin alignment jig when setting them up and they appear straight.

The camera shroud looks like it could perhaps be slightly skewed. Maybe. Am I reaching there? Could a shroud that is slightly misaligned cause the rocket to spin?

I am going to fly it as a single stage again in 2 weeks but I do not intend to attach the shroud. If it doesn’t spin, I’ll address attaching the shroud in a more aligned manner. If it does still spin, I’ll cus and pull my hair out and just rebuild the fincan because I just can’t handle the spinning.

To be continued...
 
Thanks!

I’m really glad you brought this up as I’ve been wondering about this myself. First, I’m quite confident that the booster separation was from the charge, not drag. This is speculation, but driven by how tight that IC coupler is, the weight of it and from what we witnessed from the ground.

About the spinning: if you go back to earlier in this thread you can see a video of the sustainer flying as a single stage. It spins in a similar manner. It’s the first rocket that I’ve ever made that does this. I think it is from one of two things: fins aren’t straight or the external camera shroud isn’t straight.

I had the fincan fin slots cut for me and have no reason to believe they aren’t straight. I also use a fin alignment jig when setting them up and they appear straight.

The camera shroud looks like it could perhaps be slightly skewed. Maybe. Am I reaching there? Could a shroud that is slightly misaligned cause the rocket to spin?

I am going to fly it as a single stage again in 2 weeks but I do not intend to attach the shroud. If it doesn’t spin, I’ll address attaching the shroud in a more aligned manner. If it does still spin, I’ll cus and pull my hair out and just rebuild the fincan because I just can’t handle the spinning.

To be continued...

I don't think the spinning is bad enough to need to rebuild the fin can. At least, I'm waaaaay too lazy to do that. :) It could be the camera shroud, but I wouldn't think the shroud would have enough "fin area" to make the sustainer spin at all. Could it be unevenly beveled fins?
 
Could it be unevenly beveled fins?

Perhaps. Eyeball test says it’s a possibility. I had them beveled when I had them cut. I’m not sure how to evaluate that or fix it. Any ideas?
 
I don't have any good ideas beyond trying to measure down from the leading/trailing edges to the edge of the bevel. A square would be helpful for that. It would probably be worth checking if the center of the bevel is on the center of the fin thickness as well. That's probably an eyeball job.
 
Second flight went a bit better. Booster recovery strategy was better and seems to work good. Probably going to shelve it until LDRS.

 
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