The bay-less dual deploy rocket

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Not Quite Nominal

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The complaint: The traditional rod and lid bay made sense when electronics were large, as it uses internal volume efficiently. Now that electronics are smaller, it has some limitations.
  1. The altimeter and battery, which are checked every flight, are buried deep within the bay. Downloading from the altimeter or charging the battery requires taking apart the airframe and dealing with rigging. This is both time consuming and a source of errors.
  2. Since the altimeter is fastened deep inside the bay, moving altimeters from rocket to rocket on launch day is hard. This means that either you buy many altimeters (expensive) or have fewer launches
  3. Because the altimeter bay is part of the airframe and needs to be openable, it becomes less stiff. The upper bay, an open tube, is pinned to the avbay coupler, an open tube, which is stiffened only by a loose-fitting lid (compared to a fixed bulkhead). If you take a DD rocket and bend it lightly, you can see most of the bending happens at the avbay joints.
All these limitations result from components needing to perform multiple functions: The avbay lids are also the rigging anchor points, the nuts that open the bay also hold the airframe together, etc.

Proposed resolution:

Driving home from a launch it occurred to me that it's easier for me to change a motor than change my altimeter programming. Why not use the same system for both?

Changing a motor doesn't require taking apart the airframe or rigging. Why not use the same system? It's cheap, simple, and uses COTS hardware

Here's a little sled with a tiny LiPo, EasyMini, and MW3 screw switch.
IMG_3926.jpeg


It's now a cartridge
IMG_4138.jpeg


Cartridge with charges attached
IMG_3937.jpeg


Two motor mounts here. On the right, 24mm motor mount. On the left, 29mm "motor mount" the length of the coupler.
IMG_3922.jpeg


Here's the airframe and cartridge (Tried using acyrlic color shift paint. Doesn't look so great)
IMG_4142.jpeg


Load the altimeter in the mount like a motor, screw it in, done.
IMG_4143.jpeg

After launch, unscrew altimeter, plug into computer to download data, pop the cartridge into another airframe, wire up new charges, etc.
 

Jay Hart

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I have no experience at DD but I think your approach has value.
 

PatD

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How do you seal the "cartridge" to protect the altimeter from the deployment charges?
 

cbrarick

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you guys spoiled the party....good question, though
 

GaryT

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Plenty of ways to do that, get a piece of round G10 and a rubber washer the same size, put a tiny hole in it for the ematch. Place it in the ID of the screw down caps. Thinking outside the box nice.
 

PatD

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I was curious what Nominal's concept was. It is a good basic concept. Getting a good seal would require fairly close tolerances IMHO. The "motor mount" would need a bearing surface bulkhead with a large hole on the forward end for the charge to pass through and the cartridge to seal against with, as Gary noted, a fwd bulkhead that has a small hole for the charge wires to pass through. I believe that a soft silicone o-ring would produce a better seal than a rubber washer. Use the same system for the aft seal and mastic to seal the wire pass-throughs. I would use fiberglass components to survive the sealing forces and reduce tolerance creep. Just my thoughts.
 

Not Quite Nominal

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How do you seal the "cartridge" to protect the altimeter from the deployment charges?
The "cartridge" is a little longer than the motor mount and a little sticks out when fully inserted. When you thread on the retainer cap, the up end of the retainer presses against the thrust ring in the motor mount, and the back end is pressed against the retainer cap.

It's not a perfect seal, but the gas path isn't that much different that the gas path around an av bay lid.
 

BDB

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Interesting concept. But I’m confused by how you get the cartridge-altimeter to sample the air pressure outside the rocket.
 

Buckeye

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Years ago, there was a product called the "alti-tube" which is kinda the same concept? I agree with all your points in post #1, NQN. I like where you are going with this...
 

Not Quite Nominal

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I was curious what Nominal's concept was. It is a good basic concept. Getting a good seal would require fairly close tolerances IMHO. The "motor mount" would need a bearing surface bulkhead with a large hole on the forward end for the charge to pass through and the cartridge to seal against with, as Gary noted, a fwd bulkhead that has a small hole for the charge wires to pass through. I believe that a soft silicone o-ring would produce a better seal than a rubber washer. Use the same system for the aft seal and mastic to seal the wire pass-throughs. I would use fiberglass components to survive the sealing forces and reduce tolerance creep. Just my thoughts.
It does have such a surface actually: A plain old cardboard 29mm thrust ring on the forward end.

The screw-on retainer compresses the cartridge against itself on the aft end and the thrust ring on the forward end. Fiberglass may actually work less well, as it is less flexible and any misalignment would lead to a gap where air could pass through.

O-rings would certainly make a good seal, but they seem to be unnecessary traditional bays, and the gas paths here are pretty similar.

I just took a look at the pressure and temperature data from the EasyMini for the three times I flew this rocket so far and compared them with past flights with other rockets with traditional bays, and the moment of deployment looks pretty similar on all of them.

Interesting concept. But I’m confused by how you get the cartridge-altimeter to sample the air pressure outside the rocket.
I suppose I forgot to include those pictures.

The altimeter mount tube had a bunch of holes drilled into it before installation. The section of the upper airframe that is between the two centering rings is vented to the atmosphere as per Vern. That lets air into the "donut" bounded by the AMT and two CRs. The AMT has large holes drilled into it allowing air to flow from that donut to the cartridge.

Since the CRs have no holes and the cartridge is compressed between the retainer and the forward thrust ring, little to no air can enter from either parachute compartment, and the only air the cartridge sees is outside air.

Also this means I forgot to describe the most fiddly part of the project: Making sure all the holes align for the screw switch.
 

cerving

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I did something like this a few years back with a Quantum on a MPR rocket about that size, using the Estes 29mm retainers. There were small charge wells on each side of the "capsule", with a plywood disk at the end of the capsule that the retainer seated up against. It worked fine. This looks like a good opportunity for 3D printing. Using something like this for a variety of rockets would require standardizing the length of the AV bays or coming up with some kind of spacer system so a shorter capsule would work in a longer bay... the AV bay coupler for a 4" diameter rocket is going to be a lot longer than the one for a 38mm rocket.
 

Not Quite Nominal

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You basically replicated the Public Missiles CPR3K system, which I find very easy to use.

https://publicmissiles.com/product/cpr3000systems
It appears I did, minus the separating airframe. Without the use of pistons, there's no need to separate the airframe to get to the altimeters, and thus a cheap 29mm Estes retainer can be used.

This looks like a good opportunity for 3D printing. Using something like this for a variety of rockets would require standardizing the length of the AV bays or coming up with some kind of spacer system so a shorter capsule would work in a longer bay... the AV bay coupler for a 4" diameter rocket is going to be a lot longer than the one for a 38mm rocket.
This would absolutely be a great idea for 3d printing. We could simply print standardized cartridges. There are two ways to go:
  1. Standardize the altimeter cartridge length. In this case, the screw-on retainer is holding everything together tightly against the thrust ring, providing the air seal. However, different altimeters are different sizes.
  2. Use thrust rings. MPR and HPR motors come in a variety of lengths in each diameter, but they are all located simply by clamping the rear thrust ring between the retainer and the MMT.
The advantage of (1) is that it there is still a seal.

In the case of (2), since the cartridge isn't positively located against anything at the forward end, ejection residue can enter from there. In that case, the 3d printed cartridge should have two grooves, one forward and one aft, for an O-ring or similar seal. Good old Frost King foam weather sealing tape would probably work just fine at the pressures we are dealing with.

The other issue with (2) is standardizing the air vent holes. I vent air into the "donut" conventionally. However, air must also be vented from the "donut" to the cartridge. This is done with big holes in the MMT/AMT. If all cartridges are the same length, this is not an issue.

However, if one is significantly shorter than the other and leaves some of these holes exposed, ejection gasses could come from the upper bay, through the exposed holes, into the donut, and from there into the cartridge.

As such, it would be a good idea to create a best practice that cartridges must be at least a certain minimum length (50mm?) and vent holes should only be drilled in the aft-most 50mm of the MMT/AMT.

While we're at it, standardizing switch hole placement would be a good idea too.

Best of luck. Let us know how it goes.
Had three LPR launches so far, all with oversized ejection charges, which have all gone well. Pressure traces off the EasyMini don't show significant blowby using only the retainer pressure as the seal.
 

H_Rocket

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A really neat idea. I have some old CPR3K parts I might try something similar with. The idea of using the inexpensive retainers is very frugal. I wonder if I could go one step cheaper and just use some PVC plumbing parts.
 

alexzogh

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definitely a CPR derivative, but an area that could really use some innovation - you are definitely on to something. My last few "big guys" I've just put Marsa electronics in a 54mm tube in the nosecone with a threaded base for easy retrieval. For true dual deploy, I use a wireless MrfPyro in a dino chute pouch connected to the drogue harness. No AV bay, and super easy to move rockets. It takes about 30 seconds. Only issue I've had, which you may want to think about is if the electronics are not "locked down" well, it adds a lot of noise to the gyro sensors, and can overwhelm most Kalman filters. Not an issue for altimeter only electronics.

Keep iterating! would love to see this go further.
 
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