How to Retain Four Landing Legs?

[jmmome]

I'm building a Space-X inspired vertical landing rocket. 4" diameter with two side pods which will each contain a D2 reload, which burns for 8 seconds, simulating a pseudo re-entry burn (well above ground level). The pods will also each deploy a small drogue chute. Four landing legs will be retained against the main body tube through the powered flight, and most of the descent phase. When deployed, I expect to use elastic to spring them open, and use kevlar string attached to each through the 4" body tube as stops.

My thought is to use an altimeter to deploy the four landing legs at maybe 300 feet. My initial idea is to use fishing line threaded through the four legs to retain them in place against the 4" body tube, and use a small igniter to burn through the line- maybe two igniters at opposite locations on the surface of the rocket body for redundancy.

I did something similar to this years ago when I made a clamshell device which contained the main parachute and fit into the rocket body tube, and was attached to an altimeter bay above it. This allowed me to retrofit a single deploy rocket to become a dual deploy one without cutting up the original rocket. It worked, but the additional weight made it somewhat impractical.


I'd appreciate any thoughts as to retaining the four landing legs and deploying them via an altimeter. Thanks!

Mike Momenee
TRA #12430 L3

 

[JohnCoker]

I haven't implemented anything yet. Because of the small motors which the thrust vectoring system can support, I couldn't afford much weight.

I'd probably be tempted to use springs to hold them down and maybe a catch or an electromagnet to retain them against the body.

It's an interesting problem. I wonder if anything special would need to be done with the parachute to make the rocket come down slowly enough, and straight enough, for legs to work. I'm guessing you'd need triple deployment with the last parachute coming out very close to the ground and slowing the rocket down even more.

 

[JCRL]

What about making the legs spring loaded, using a rubberband to retain them during flight, and then nichrome wire to burn through the rubberband at the altitude you want?

 

[cerving]

Deploying the landing legs is easy, I think you've already figured that out. The hard part is the "retro" rocket... how do you ensure that it's not going to light unless you're on the main chute and at a decent altitude?

 

[BABAR]

My initial idea is to use fishing line threaded through the four legs to retain them in place against the 4" body tube, and use a small igniter to burn through the line- maybe two igniters at opposite locations on the surface of the rocket body for redundancy.

TRA #12430 L3

Ditch the fishing line and replace it with either #16 rubber bands or thin elastic thread. Using an elastic retention band allows you to keep the legs "tight" much better than fishing line or non-elastic thread. Non elastic thread always seems to get looser and looser between the time you tie it and the time you finally launch it.

Taut rubber bands and elastic thread also "burn through" very easily. #16 rubber bands are my standard go-to for rotor retention during the upward flight of my helicopter rockets. The bands criss-cross directly in front of the motor, so ejection charge burns right through them.

You could pass these bands through your pods just in front of your late firing motors (use standard hole punch size holes, may need to reinforce with an internal coupler). The small holes shouldn't vent too much of your ejection charges, so should still be able to burn the bands AND deploy the chutes, assuming relatively loose nose cone and loose chute and wadding packing.

Successful deployment with this technique for me is between 95 and 99 %.

Straight Trails!

 

[BABAR]

If you go with passing rubber bands or cords through the pods, an essential tool is a crochet hook.

Poke it through one hole, out the other, grab the band or thread, and pull back through.

The hooks are dirt cheap at Walmart or any hobby store. Get one a metallic color, NOT green. Makes it a LOT easier to find when you drop it at the launch site. Don’t ask me how I know this.

 

[BABAR]

I haven't implemented anything yet. Because of the small motors which the thrust vectoring system can support, I couldn't afford much weight.

I'd probably be tempted to use springs to hold them down and maybe a catch or an electromagnet to retain them against the body.

It's an interesting problem. I wonder if anything special would need to be done with the parachute to make the rocket come down slowly enough, and straight enough, for legs to work. I'm guessing you'd need triple deployment with the last parachute coming out very close to the ground and slowing the rocket down even more.

A trip rod out of the bottom of the airframe, just 2 inches longer than the deployed legs?

Comes down hopefully vertically under full chute. Trip rod activates 2 inches before legs touch down.
The trip rod when activated releases the shroud lines to 1/2 to 3/4 of the main chute. Turns the chute into a streamer less likely to topple the rocket over.

Just an idea......

 

[jmmome]

Thanks fellas! Some really great ideas that I hadn't thought about.

I was thinking about deploying my main at between 200 and 100 feet- 10fps descent or less. The two drogue chutes deployed from the pods would be sized to give me 60 fps descent or less, and I'd restrict the altitude to maybe 1,000 feet or less for the first flight.

I was trying to think of a way to jettison the main chute when the rocket touches the ground- I have a small Tender Descender, and maybe figuring out how to make the TD fire via the trip rod would be the method.

Mike Momenee
TRA #12430 L3