Ridiculous idea: DD without an av-bay

I don't like mass, and I don't like complexity. So here's something my brain came up with during a slow friday at work:


1) Create a small, strong casing for your avionics and battery of choice. 3D printing is a good candidate for this. For wires, use either pass-through pin headers or snugly fit direct routing.

2) Wrap your main chute in a tight burrito with a ziptie cable cutter.

3) Tightly secure the av-box to the recovery harness (probably best so it'll sit above the burrito in the BT).

4) Fill a soda straw or your preferred disposable container with apogee-charge BP. Wire this charge to the av-box, routed so it sits in the BT below the burrito.

5) Confirm main charge in cable cutter is wired, then slide the burrito into the body tube.


During boost, the av-box sits merrily atop the burrito, recording change in pressure through standard pitot holes in the airframe. At apogee the charge below the laundry fires, blowing off the NC --- now the burrito and av-box are flying together in free air. Add drogue/streamer to taste. At the preset Main altitude, the cable cutter releases the burrito, leaving the av-box hanging alone along the shock cord.

Does this make sense? Is this at all plausible, or have I inhaled too much bondo and CA fumes lately? Biggest issue I see is arming safety and care to avoid ripping wiring.

your casing with the altimeter will be in the same compartment as the motor ejection charge, the pressure at apogee in the compartment will trigger the main ( cable cutter )

Area66 said:

your casing with the altimeter will be in the same compartment as the motor ejection charge, the pressure at apogee in the compartment will trigger the main ( cable cutter )

Click to expand...

Hrm. That is a point. The pressure increase might be fast enough to be filtered out, but if it's not then you have a long walk...

Zeroignite said:

Hrm. That is a point. The pressure increase might be fast enough to be filtered out, but if it's not then you have a long walk...

Click to expand...

this is not only a point, I experience it with a Nike Smoke and Cable cutter deployment.

You could use motor deploy apogee and then just a timer for the main. A very long timer. At least it wouldn't be affect by pressure.


Mark

Area66 said:

this is not only a point, I experience it with a Nike Smoke and Cable cutter deployment.

Click to expand...

Evidence wins! Ok, the mass/complexity of one bulkhead in the NC shoulder isn't awful

Vaportrail said:

You could use motor deploy apogee and then just a timer for the main. A very long timer. At least it wouldn't be affect by pressure.


Mark

Click to expand...

That would be...of questionable safety. I wouldn't do that.

CarVac said:

That would be...of questionable safety. I wouldn't do that.

Click to expand...

Would you elaborate? If you started timer by break-wire, I see no reason it would be less safe than altimeters. As long as you maintain motor deploy back up at apogee, if it failed, it still wouldn't come in ballistic. Run two timers just in case. But I don't see a failure in a straight up timer being any worse than a failure with an altimeter.


Mark

Vaportrail said:

Would you elaborate? If you started timer by break-wire, I see no reason it would be less safe than altimeters. As long as you maintain motor deploy back up at apogee, if it failed, it still wouldn't come in ballistic. Run two timers just in case. But I don't see a failure in a straight up timer being any worse than a failure with an altimeter.


Mark

Click to expand...

A straight-up timer fails in the case of not reaching the desired altitude, even if the electronics work perfectly.

This could happen from powerful chuffs, catos, weathercocking, tangled drogues, and inaccurate simulations.

CarVac said:

A straight-up timer fails in the case of not reaching the desired altitude, even if the electronics work perfectly.

This could happen from powerful chuffs, catos, weathercocking, tangled drogues, and inaccurate simulations.

Click to expand...

These events can also mess up certain altimeters and MADs with timers for deploying the main. Nothing is totally perfect for what we do.

Sailorbill said:

These events can also mess up certain altimeters and MADs with timers for deploying the main. Nothing is totally perfect for what we do.

Click to expand...

You are correct, but with care, an altimeter controlling the apogee and main deployments is very reliable. More so then a timer where a human has to set the time. The timer will almost certainly work correctly, it's the time it is set for that is the issue. The altimeter will fire at apogee even if that is reached in 10, 15, or 20 seconds. The timing is apogee dependent, with an altimeter, not based on a time that might not be right, like a timer.

Zero, I get what you are saying. I don't like mass and I don't like complexity either. In fact, I seek the most simple solution every time because it provides a high level of repeatability and reliability. The traditional method of dual deploy avbays seems overly complex and wasteful to me. I tried it and did not like the prep time. I also did not like the double break points and odd weight distribution. Other things bothered me too like needing sheer pins to prevent drag separations that create airframe stress. Yet another problem was reliably sealing the bulkheads of the avbay to prevent ejection charges pressurizing the bay. I spent last winter kicking around and refining an idea that would simplify the dd process and reduce failure modes. I wanted an idea that was scalable as well. What I came up with is stupid simple and downright unsexy but an effective cure for all the above issues.

What I came up with is to use a payload section to house a cartridge containing the altimeter and tracker. The coupler has a glued in bulkhead that fixes the ejection pressure issue. I drilled two holes in the bulkhead to pass my charge leads but you could just as easily use terminal blocks as well. The leads are passed up through the bottom up to the top of the payload bay, attached to the altimeter mounted in the cartridge then the altimeter is slid inside the payload bay until it rest on the coupler. The nose cone holds the cartridge in place. Simple and light and very easy to construct. I flew this set up 4 times last season and only had one failure due to a user error with my cable cutter not releasing the chute. I found it way easier to set up. As far as the scalability goes, I could just as easily use a Tender Descender to control a chute bag in a much larger rocket.