Increasing descent rate with a drogue

[Charles_McG]

I watch my 2-3” cardboard/plywood (with printed details) tumble during drogueless descent. The descent rate varies during the tumble, like this.



I’ve seen descent rates go up to 80-100fps if the string happens to straighten out vertically- it doesn’t usually hold that orientation long.

It seems to me that if I could size a drogue properly, it would hold the string orientation vertical and yield a higher descent rate than the tumble.

I’ve tried a streamer, but it didn’t stop the tumble. Does anyone know how I might go about calculating/ simming a better sized drogue? Or maybe my rockets are too light to pull this off?

 

[Kelly]

It seems to me that if I could size a drogue properly, it would hold the string orientation vertical and yield a higher descent rate than the tumble.

Hmm, just mind-simming it, I'm not sure it's possible...
The problem is, you want a drogue big/slow enough that it holds the rocket in an unstable position (descending rapidly, fins first). If your drogue is big enough to accomplish this, then you're not going to get the high descent speed you want (because of the drogue drag). If your drogue is smaller, then you're back to a tumble.

I think that if you had a flow simulator with which you could model the actual moment on the Cp of the booster when it is descending backwards, you could draw the FBD to see what force (from the drogue/harness) is necessary to counteract the aerodynamic moment and keep it fins-down. (And it might tell you that at the desired descent speed, the aerodynamic moment on the Cp is great enough that the restoring force required would not be consistent with the descent speed). I'm not sure what simulators are readily available that would give you an actual force/moment rather than just a Cp.

Hope some of that makes sense.

 

[Charles_McG]

Good points.

From what I’ve seen from the ground, and onboard video, I’d say there are three main descent modes: end over end tumble, flat spin, and backsliding. And a rocket may switch among them during descent, though it depends on the rocket.

I can see that keeping tail down might be hard to go much faster than the tumble without transitioning to it or backsliding.

But I’ve seen heavier dual deploy payload sections go nosedown and pull the fin section after it. That what I was trying with the streamer - get the nose weight portion pointed down and let it pull the rest after.

OpenRocket doesn’t [ do a good job at] simmming drogueless descent, alas.

 

[G_T]

I've had sustained low altitude descent rates up over 120fps using a drogue. YMMV.

If the drag of the drogue + the drag of the rocket with fins pointed down (which is fairly low drag) is less than the drag of the rocket descending sideways (quite draggy) then you can come down faster under drogue.

You'll have separated the nosecone, or broken it in the middle. In either case, the pointy end points down nicely once the drogue is big enough for that to work (needs to be a bit bigger for break in the middle rockets than for everything out the top rockets). The motor portion now has a CG that is rather towards the fins, and a harness attachment point well above its CG. It's not too hard to make that part stay vertical.

Gerald

 

[watheyak]

This is an interesting discussion.

All of my drogue-less rockets oscillate as described. The periods of low vertical velocity are also when it has the highest horizontal velocity, which I've attributed to the previously mentioned "backslide" behavior.

I was going to try a small ballute so see if it solves the problem. Mine are all small 54mm minimum diameter rockets, so I don't think much drag is needed. A 12" drogue seemed overkill.

 

[waltr]

Not sure if this helps:
I have a 54mm LOC IRIS and run a 6" drogue. Decent rate is about 35f/s. On the drogue the booster section stay fins down at a angle near horizontal. The AV/Main chute and Nose points more downward and the harness stays at an inverted 'V'. No oscillations of either part.

 

[Handeman]

Good points.

From what I’ve seen from the ground, and onboard video, I’d say there are three main descent modes: end over end tumble, flat spin, and backsliding. And a rocket may switch among them during descent, though it depends on the rocket.

I can see that keeping tail down might be hard to go much faster than the tumble without transitioning to it or backsliding.

But I’ve seen heavier dual deploy payload sections go nosedown and pull the fin section after it. That what I was trying with the streamer - get the nose weight portion pointed down and let it pull the rest after.

OpenRocket doesn’t [ do a good job at] simmming drogueless descent, alas.

You said your rocket tumbled during drogueless decent. What do you mean by tumble? payload passing fincan and vice versa? There is no control at all of what direction the main will be deployed to. Low but some chance it will be straight down and the rocket will foul the main and fall in.

The back sliding you describe is what I call "flying". Where the fin can takes off in one direction and pulls everything with it. It could be with the wind, against the wind or any other directions. You have zero control of how the rocket falls without a drogue and this flying can make the rocket land closer, or much further away from you depending on what random direction it ends up flying in.

The payload being nosedown and pulling everything with it is what I call a ballistic recovery. The only difference between a rocket together and hitting 300 mph and this kind of drop is there is more drag from the open fin can that keeps the speeds down some. Although the fastest, I consider this the worst type of drop between apogee and main deploys because I believe it has the highest potential for recovery failure when the main is deployed. I've seen the main charge shoot the nose cone and main straight down, the main open and the rocket fall through and foul the main. I've seen the main open, the payload section fall past and snap to the end of the shock cord almost colapsing the main and in a couple cases rip away. I've seen the payload section falling past the almost stopped chute and payload section and snap the shock cord/anchor points when it hits the end of shock cord and then hit the ground ballistic. Long shock cords makes the shock loads involved with these kind of main deploy's much worse because the sections fall much further and the chute has a chance to stop in mid air. Most of the time everything stays together and the flier is congradulated for a successful flight. I think they just happened to avoid disaster that flight because they overbuilt the rocket enough.

I've never found any type of simulator that can tell you what size drogue will work best or tell you how it will fall. What I do is adjust the size of the drogue up or down on each of the first several flights until I have it falling in a well controlled manner and go with that size drogue.

On most DD flights, the longest part of the flight is between apogee deployment and main deployment. It really should get most of your attention and be set up for the best controlled and safest recovery. IMHO.