Electromagnets and Landing Legs- Need Advice Please

[jmmome]

I'm building a 4" dia. HPR rocket lander with four landing legs- each 36" long 5/8" square poplar sticks to give a wide 40" stance when landing. Rocket will weigh around 7 pounds. One of the ways I could hold the legs in place during flight is small electromagnets- I found some that are powered by 12V and have 2.5KG lifting force.

I know just enough about electromagnets to screw this up. My biggest question is: how can I use the "MAIN" deployment circuit on a Perfectflight SL100 altimeter to cut the 12V power to the four electromagnets?

If there are other smaller electromagnets that you would recommend, I'm all ears. Any advice in general relative to the electromagnets would be wonderful. I'm completely out of my element here.

If you have questions as to how I plan to land this thing- first, I need to launch it on a relatively calm day. Two side drogues located in side motor pods will bring the rocket down at about 60fps to start the descent- tail first (the side pods each have "D" motor reloads that burn for 8 seconds but at only 0.5 pounds of thrust- just for show during descent) . Then at 500 feel, the main chute will deploy from the nosecone, and will be sized to reduce the descent speed to 15 fps. Lastly, at 100 feet, another main chute of the same size, restrained up to this point by a JL Chute Release, will deploy that chute, reducing the descent to about 7.5 fps.

 

[Bat-mite]

I know just enough about electromagnets to screw this up. My biggest question is: how can I use the "MAIN" deployment circuit on a Perfectflight SL100 altimeter to cut the 12V power to the four electromagnets?

So you are wanting the legs to "deploy" for the descent, but be tucked away during flight?

I can't help you with your goal; but you may want to watch this video to see an 800-pound rocket come down on four legs perfectly. It also uses side chutes and a top main, but not in your configuration.

 

[jmmome]

Yes- restrained during ascent, but deployed at maybe 500 feet during descent. Thanks for this video- someone else had suggested it for another project I'm working on.

The rocket will resemble my CINEMA X-1, which is an airstart rocket built years ago. 4" body all the way instead of the transition to 3", different fin shape, and will be closer to 5 1/2 feet tall instead of this 7 feet tall rocket.

 

[heada]

You can use normally closed relays. The event from your electronics would trip the relays which would open the circuit cutting power to your electromagnets. Look for solid state relays as mechanical relays have issues with g-forces.

I'd also recommend you look at the Eggtimer Proton. It has 6 programmable output channels, 4 of which can be used for things like servos which would work well for your project. 4 outputs for your electromagnets, as well as main and drogue.

 

[jmmome]

You can use normally closed relays. The event from your electronics would trip the relays which would open the circuit cutting power to your electromagnets. Look for solid state relays as mechanical relays have issues with g-forces.

I'd also recommend you look at the Eggtimer Proton. It has 6 programmable output channels, 4 of which can be used for things like servos which would work well for your project. 4 outputs for your electromagnets, as well as main and drogue.

Thank you for this advice!

 

[CoyoteNumber2]

Could you have some sort of tether on your main bridle that pulls release pins locking the legs to the airframe? Saves weight leaving magnets and batteries out of the equation.

 

[K'Tesh]

What about solenoids? It's locked in position when powered off, but releases when energized.

Also, I'd use carbon fiber instead of wood.

 

[Steve Shannon]

At 15 FPS, that second main might not have enough airflow to open within 100 feet or perhaps at all. Also, if it does 7.5 FPS is extremely slow and could even be slower than some updrafts.
Rather than getting tricky with electronics or solenoids, I would look for a way to have the ejection of the nosecone pull a pin that allows the legs to unfold or spring open.

 

[jmmome]

I may go back to my original and proven method- tying the legs tight to the body with fishing line, and burning through it with an igniter.

9 years ago, I successfully built and flew a clamshell main chute holder that rode inside the body tube, was pushed out by the motor's drogue charge as well as a drogue chute, and opened via the altimeter firing an igniter at a predetermined altitude, which melted the fishing line holding the clamshell together. I can adapt that method to hold the legs down, at the base of the rocket.

Thank you for all the guidance! I may have to up the descent speed and the opening altitude to get the second main chute to open.

 

[Bat-mite]

Nice pics. Thanks.