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Best configuration for combo alt/gps?

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Kelly

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So I thought it would be good to have a combination altimeter and GPS, and I got an eggfinder TRS as my first locator. My first rocket using it was single deploy, I built the avbay as an attachment to the nosecone, and the antenna (about 6") extended into the nosecone, out of the way, away from any metal, not using any valuable real estate. Great setup.

But I want to use it also for dual deploy, and I'm trying to figure out the configuration. Normally one would put the avbay into the middle of the rocket, with a drogue above and main below. Now, I can either make the bay 6" longer, and keep the antenna inside, or have the antenna extend through the bay endcap. Either way, it's 6" of wasted airframe. If it's inside the bay, I need to run igniter leads past the antenna for the ejection well on the endcap, which seems bad from an antenna standpoint - especially since one of those wires is probably grounded. If I put it outside the bay, it will be exposed to the ejection charge, so I need to protect it some way. And I'll still have ejection charge leads right at the base. Nothing here seems optimal.

Am I way overthinking this? Is there a good configuration for this setup? How do you guys do it?
Or, is it just not such a good idea to have a combo alt/GPS?
 

kjs

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... this was mine for 38 mm av bay with Raven4 and Featherweight Tracker... it used a charge for apogee and a cable cutter for main. we then used something similar for a 54 mm minimum diameter (> mach 2) and that worked as well. I can explain this pic more if there is interest.

1596762588307.png
 

Kelly

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Yeah, using a cable cutter occurred to me, it would make for a better configuration, but a bit more complexity (it seems - I haven't used a cable cutter before).
I am interested in hearing more about your setup - I assume your pieces stay linked together, not come down as a separate drogue and main?
 

kjs

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Correct on staying together - I had never used a cable cutter before either.

My learning on the 38mm is that if you don't have enough powder for apogee, the main is still stuck in the tube even if it separated - but it comes in fast (I could track the descent rate via the phone to the tracker - we knew on the ground that something wasn't right based on descent rate - we knew it wasn't ballistic, but it was too fast) .

It isn't obvious above, but those same electronics and nose cone survived a failed main.... i'll find that pic next (now below). it was a 'lawn dart' with the nose laying next to the body and the body tube cut the Kevlar in two.

By the time Ii made it back to the launch site after recovery, I had all the dirt out and it was fine to fly again except for severed kevlar...

Key learning - don't under estimate the apogee charge - you want the cable cutter and chute outside the tube for the cable cutter to work!!

What diameter is your rocket? My 3D prints av bays are for Featherweight stuff but I have it for 38 and 54. I'm still working on the 75 and 98. All are intended to be "in nose cone dual deploy with tracking"

For the 54mm version see: https://www.rocketryforum.com/threads/yes-featherweight-is-still-in-business.160770/

"Truth in advertising" - I write some software for the Featherweight Altimeters and Trackers so that is why my stuff is all Featherweight...
 
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kjs

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Here's the pic of "not enough powder for apogee" but all the electronics survived (same av bay and nose cone are in the current pic above now). It did separate at apogee but the chute was stuck in the body tube... from the tracker / phone, we knew it would speed up to ~150 ft/sec and then slow down to ~80 ft/sec. We didn't know what was going on at the time, but now I think it was starting to 'go ballistic' and it would flatten out, then nose down, then nose sideways. Luckily it landed nose sideways. Only real damage was the body tube cut the Kevlar in two on impact.

Again the failure here was "not enough apogee charge" - it had nothing to do with the electronics - they all worked flawlessly.

1596775479412.png
 
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n27sb

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Most of my configurations use a combination of GPS and Altimeter/DD in the nose.
This places all of the weight in the nose and reduces overall weight.
I usually use 2 different configurations.
1. Gps and Alt/DD mounted on a common board. Apogee charge pushes NC from BT and exposes drogue or just shock cord. Main is contained in a short piece of tube that is part of the NC.18858DAD-834E-4A3F-B3C2-3D42371DD4CA.jpeg

2. Gps and Alt/DD mounted on a common board in the nose. Apogee charge pushes fin can from BT and exposes drogue or just shock cord. The body tube has a bulkhead midway. Main is contained in the upper half of the body and is deployed by pushing the NC from the BT. In most cases the motor is the coupler for the lower half.

DC807760-F8C5-4312-B011-EA7F02D08894.jpegB5D6C3F7-8AE3-491B-8C31-037C0F8F3AA3.jpeg
 
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cerving

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I have a MD 38 with a TRS in the nose cone, what I did with it was to run the deployment wires down through the payload bay to the top of the AV bay, one set goes to an ematch in the Main and the other wraps around a pair of brass rods that run through the sealed AV bay. The Drogue ematch wraps around the brass rods on the bottom side of the AV bay. I'll get you a picture when I have a chance... it's easier to see than to describe, but the bottom line is that the TRS stays in the nose cone and nothing is in the AV bay.
 

n27sb

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I have a MD 38 with a TRS in the nose cone, what I did with it was to run the deployment wires down through the payload bay to the top of the AV bay, one set goes to an ematch in the Main and the other wraps around a pair of brass rods that run through the sealed AV bay. The Drogue ematch wraps around the brass rods on the bottom side of the AV bay. I'll get you a picture when I have a chance... it's easier to see than to describe, but the bottom line is that the TRS stays in the nose cone and nothing is in the AV bay.
I think I got it Cris. Similar to mine but you utilize the mid-rocket AVbay instead of a simple fixed bulkhead. This pushes the bottom half of the rocket apart.
 

Kelly

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I have a MD 38 with a TRS in the nose cone, what I did with it was to run the deployment wires down through the payload bay to the top of the AV bay, one set goes to an ematch in the Main and the other wraps around a pair of brass rods that run through the sealed AV bay. The Drogue ematch wraps around the brass rods on the bottom side of the AV bay. I'll get you a picture when I have a chance... it's easier to see than to describe, but the bottom line is that the TRS stays in the nose cone and nothing is in the AV bay.
OK, that makes sense. To paraphrase, you've got the traditional setup with AV bay in the middle, but you've 'faked' the AV bay electronics by running wires to the top and bottom from the electronics in the NC. That could work for me. What's the purpose of the brass rods?

But, if I understand, you have a lead that runs from the NC alt, through the payload bay (past the main), and to the bottom of the AV bay. Doesn't this lead effectively tie the NC/payload/AVbay together, and prevent separation forward of the AV bay and deployment of the main?
 

Kelly

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1. Gps and Alt/DD mounted on a common board. Apogee charge pushes NC from BT and exposes drogue or just shock cord. Main is contained in a short piece of tube that is part of the NC.
Sorry, Steve, I'm dense this morning, just not getting it. Are you using any motor ejection charges for drogue? I'm using electronics only. In this setup, how do you keep the main from deploying when the NC separates from BT?

2. Gps and Alt/DD mounted on a common board in the nose. Apogee charge pushes fin can from BT and exposes drogue or just shock cord. The body tube has a bulkhead midway. Main is contained in the upper half of the body and is deployed by pushing the NC from the BT. In most cases the motor is the coupler for the lower half.
I think I sorta get this; as you note it is like Cris's except with a bulkhead instead of a dummy bay. But same question, if you're not using motor eject, don't you need a wire running from NC past main to lower part of bulkhead, and doesn't this prevent NC separation from BT?

Thanks for the replies!
 

cerving

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OK, that makes sense. To paraphrase, you've got the traditional setup with AV bay in the middle, but you've 'faked' the AV bay electronics by running wires to the top and bottom from the electronics in the NC. That could work for me. What's the purpose of the brass rods?

But, if I understand, you have a lead that runs from the NC alt, through the payload bay (past the main), and to the bottom of the AV bay. Doesn't this lead effectively tie the NC/payload/AVbay together, and prevent separation forward of the AV bay and deployment of the main?
The brass rods just run the signal from the top of the AV bay to the bottom, where the drogue charge well is located. You wrap the ematch leads to them and tape it up. On the payload bay side, you simply wrap the wires a few times around the rods on the top side, when the NC blows it will pull them right off the rods.
 

Kelly

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BTW, the stuff I'm doing is in the 38/54mm, MD, size range right now.
 

n27sb

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Sorry, Steve, I'm dense this morning, just not getting it. Are you using any motor ejection charges for drogue? I'm using electronics only. In this setup, how do you keep the main from deploying when the NC separates from BT?
No problem,
There is a wire that runs through the tube that extends the base of the nose cone. The top of it is sealed from the electronics.
That wire extends into the middle of the rocket to a BP charge that is under the drogue this is the charge that expels the drogue and shock cord. Motor eject charge is removed.

There is a cap on the end of the nose cone tube. This is what contains the Main. It is a friction fit and the shock cord passes through a hole in it. The Main charge is behind the parachute and and pushes everything out at 500'.
This setup works well and was used in my 24mm flight to 5883'.
 

n27sb

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I think I sorta get this; as you note it is like Cris's except with a bulkhead instead of a dummy bay. But same question, if you're not using motor eject, don't you need a wire running from NC past main to lower part of bulkhead, and doesn't this prevent NC separation from BT?
Yes you do. I use a tiny little male and female pin like you would see in some small connectors. A piece of shrink tube covers the female side. This joint is placed above the bulkhead.
 

n27sb

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BTW, the stuff I'm doing is in the 38/54mm, MD, size range right now.
Most of my work is in 24 and 29mm so 38/54 would be like parking an MG Midget in a warehouse.

If you used method #1 your tube on the bottom of the nose cone would probably be 29mm or so. The nose cone has the typical shoulder that slip fits on the BT.
 

Kelly

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Yes you do. I use a tiny little male and female pin like you would see in some small connectors.
OK, also similar to what Cris does a pull-away connection.

There is a cap on the end of the nose cone tube. This is what contains the Main. It is a friction fit and the shock cord passes through a hole in it.
Ahh, shock cord bypassing the chute and attaching to the NC. That's what I was missing. Thanks!
 

Kelly

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Thanks for the replies, all! This will help me do the DD while keeping the electronics in the NC, where I want them.
 

n27sb

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Thanks for the replies, all! This will help me do the DD while keeping the electronics in the NC, where I want them.
You are welcome,
Don't be afraid to think outside the box. Be safe and Ground Test. Did I say Ground Test?
 

ksaves2

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Yup, if one has an open area, ground test and walk as far as you can to receive a signal. Put the tracker bay 6 feet above the ground and you’ll get the range to the last known position range down low.

One will eventually lose position information with the 100Mw, 900Mhz stuff within I’d say a mile unless there are no obstructions in the way. Correct me if I’m not right here.

Yeah, if the rocket is 3 miles away and one had a straight line of sight to the receiver one will get a last position 100 feet or lower in the air. Unless the wind is blasting the rocket across the field, that ought to be good enough to get within the ground footprint range of the tracker. I’ve always been able to get within this range and my 100mW, 900Mhz trackers always had a shot at the sky/GPS satellites so I can get a final position lie after the rocket was down.

Technically, if the tracker GPS receiver antenna is facing the ground, the position output should blank out. I’ve not had that happen as my recoveries always had positions coming in even with the rocket on the ground. I just had to get within the ground footprint range and although I never had to use a Yagi antenna with an attenuator to recover a GPS tracked rocket, (70cm, 2M or 900Mhz bands) the prospect is there as long as the tracker sends out an Rf signal.

Get close enough to the tracker and if there is a noisemaker on the harness, you will find your rocket. The ears are a great terminal tracker. Best regards

Kurt Savegnago
 

manixFan

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... this was mine for 38 mm av bay with Raven4 and Featherweight Tracker... it used a charge for apogee and a cable cutter for main. we then used something similar for a 54 mm minimum diameter (> mach 2) and that worked as well. I can explain this pic more if there is interest.
I'm working on something very similar - a 38mm CF rocket with Featherweight gear. I do have a couple of questions:
  • is the smaller hole the Baro vent hole? If not, how did you vent for barometric altitude?
  • was the chute packed in the red coupler?
  • can you discuss your battery setup
Thanks for your discussion in this and the other thread, very useful info.


Tony
 

kjs

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I'm working on something very similar - a 38mm CF rocket with Featherweight gear. I do have a couple of questions:
  • is the smaller hole the Baro vent hole? If not, how did you vent for barometric altitude?
Yes, I had three screws to attach the nose cone to the coupler tube and the small hole is the vent.

  • was the chute packed in the red coupler?
You over estimate my chute packing ability... ;) For me there was still room in the body tube and with the cable cutter, it was a tight fit.

  • can you discuss your battery setup
It's hard to see in the picture, but the Raven is sitting on the electronics end of the 24mm Featherweight Av Bay, so it's tiny lipo is inside the blue 'shroud' between the Raven and the Tracker. I removed the battery terminal from the Tracker and replaced it with the tiny white connector to get a flatter profile (or it wouldn't have fit). I then used one of the small tracker batteries and (cough..) shoved it in the space left available...

I attached the AV bay through the bulkhead with 2-56 brass screws and used wire crimp connectors on the bottom for the leads.

Picture here of the bottom of the av bay.

avBayBottom.jpg
 

manixFan

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Thanks for all that info Kevin, very useful. I've used the end of the A/V bay the same way, and I even found some connector cables that allow me to locate the battery elsewhere. I'm working on adding a switch to the cable, but for a couple of my rockets I'll just have to assemble them at the tower to be compliant with the new rules, there's just no easy way to retrofit a switch into the design. I've soldered JST connectors onto a bunch of ematches so I can still button up the A/V bay with the battery installed and then add the charges once it's out in the field.

FWIW, the Fruity Chute Iris Light series have the smallest packing volume of any chute I've found so far. It only uses 6 gores instead of 8 and that reduction in seam count really makes a difference, along with the thinner material.

1597157198634.png


(a 36" elliptical is needed to get roughly the same descent rate as the 30" Iris model)

Unfortunately, they don't give them away! But for small, altitude seeking rockets, they can really help keep recovery manageable. I've thought of using a reversed piston, (so I can stuff the chute into the body of the piston) but I haven't gotten around to testing it yet. Eliminating the need for a chute protector would help and if the piston is reversed the amount of linear space it would consume is negligible.


Tony
 
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rocketace

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Here is mine that is a work in progress for a 38mm minimum diameter. Using Featherweight Alt and GPS.
AVbay2.jpg
AvBay1.jpg
 

manixFan

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rocketace - I like the o-rings, I'll have to borrow that idea. Very nicely done overall. Thanks for the photos.


Tony
 

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Glad to help! Both O-rings are just used seal disk o-rings from 38mm AT reloads. I will use baby powder for the lower one that goes into the body tube to make sure it will slide out easy. The baro hole location and screw switch access was the most difficult.
 

rocketace

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I like that, very tidy. Where do you put your vent holes?

Thanks! The hole by the bottom O-ring is a set of 3 and those are the holes for the Baro. Those go directly out the body tube and are somewhat sealed from the ejection gases by the bottom O-ring. Even though I am using a Rave. Which is pretty immune to Mach, I still wanted the Baro holes as far from the NC as I could.

The hole by the top O-ring is a set of 4. 2 are to screw the Avbay to the nose come. The other two are for the shear pins.
 

Rob702Martinez

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Most of my configurations use a combination of GPS and Altimeter/DD in the nose.
This places all of the weight in the nose and reduces overall weight.
I usually use 2 different configurations.
1. Gps and Alt/DD mounted on a common board. Apogee charge pushes NC from BT and exposes drogue or just shock cord. Main is contained in a short piece of tube that is part of the NC.View attachment 427268

2. Gps and Alt/DD mounted on a common board in the nose. Apogee charge pushes fin can from BT and exposes drogue or just shock cord. The body tube has a bulkhead midway. Main is contained in the upper half of the body and is deployed by pushing the NC from the BT. In most cases the motor is the coupler for the lower half.

View attachment 427270View attachment 427269
What are the details of the antenna?
 

n27sb

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What are the details of the antenna?
In the first photo I am using the standard EF Mini antenna with the rubber removed.
The 2nd photo shows a antenna fabricated with curled copper wire.
 
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