Bellyflop Recovery

[sr205347d]

So it sounds like: minimize the overall mass, maximize the CG shift.

The only CG shift is due to propellant burn. There are no moving parts.

 

[Rktman]

Excellent research and development project, very well documented and detailed. Kudos!

 

[Rktman]

Since my last post, I have made multiple flights with variants of the BF 5 rocket. I tried small and large fins, tandem fins, longer body tubes, ogive vs elliptical nose cone, and of course, CG placement. Here is BF version 5.8.7, which worked pretty well yesterday after getting the CG placement right:

View attachment 554455

The body tube is 30" BT-60. The nose cone is a 3D printed (single perimeter) 4.75" ogive. This configuration seemed to resolve three issues that I have been having:

Boost: All boosts have been stable, but with varying amounts of corkscrewing. Small, low-drag fins minimized the corkscrewing, but were not stable in descent. I have decided that the corkscrewing cannot be eliminated short of using gyro stabilization. So, rather than considering it to be an "issue," I have decided to embrace it and call it a "feature!"

Transition from boost to descent: Versions 3 and 4 had the ejection charge vented out the side which caused an unnecessary and unwelcome tumbling. The rockets would naturally transition to flop mode, but the ejection charge venting would sometimes push the rocket back into a low angle of attack which would stabilize into a "lawn dart" descent. Using a larger diameter body tube and having the ejection charge vent out the back around the motor resolved this issue.

Descent: Center of gravity placement is critical. Too far aft, and the rocket glides tail first and lands hard. It is not as bad, though, as having the CG too far forward which would result in a lawn dart landing. The more challenging issue in descent is pitch oscillations which can result in hard landings on the nose or tail, or worse, pitching down far enough to reduce angle of attack and re-stabilize into forward flight and a lawn dart landing. To dampen the bobbing, minimizing the mass at either end of the rocket is good, as is having larger fins at the tail.

Back to my flights yesterday, the first two had the CG a bit too far aft which resulted in tail sliding. I packed a bit of some adhesive putty into the tip of the nose cone, and had three flights like this:

Impressive!

 

[dhbarr]

The only CG shift is due to propellant burn. There are no moving parts.

Yes -- short stubby motors will be less effective here than long skinnies.

 

[BABAR]

. I have decided that the corkscrewing cannot be eliminated short of using gyro stabilization. So, rather than considering it to be an "issue," I have decided to embrace it and call it a "feature!"
]

Amen to that!

Why shouldn’t the “Up” portion be as cool as the down.

What then drives you crazy is when you build an identical bird and it DOESN’T corkscrew, and you end up scratching your head trying to figure out what was different?

 

[sr205347d]

Saturday, I went to the HPR launch of the Wright Stuff Rocketeers (NAR #703) and for the first time flew in front of an audience. They went from "what the heck is that?" to "pretty cool!" The first two flights it still had a slight tendency to get into a back-slide, so I added a bit more weight to the nose. The last two flights were great, except for a bit of "nodding" still. Sorry, no pictures or video.

Today, I removed all 5.5 grams of weight from the nose as well as the forward fins. I had three flights with just barely noticeable "nodding."

I call this one Bellyflopper version 5.9:



Now I need to upscale this, and maybe add gyro stabilization to kill the corkscrewing.

As far as the forward fins, they would be useful if the CG were too far forward. Otherwise, they don't appear to be necessary.

 

[Dotini]

Since my last post, I have made multiple flights with variants of the BF 5 rocket. I tried small and large fins, tandem fins, longer body tubes, ogive vs elliptical nose cone, and of course, CG placement. Here is BF version 5.8.7, which worked pretty well yesterday after getting the CG placement right:

View attachment 554455

The body tube is 30" BT-60. The nose cone is a 3D printed (single perimeter) 4.75" ogive. This configuration seemed to resolve three issues that I have been having:

Boost: All boosts have been stable, but with varying amounts of corkscrewing. Small, low-drag fins minimized the corkscrewing, but were not stable in descent. I have decided that the corkscrewing cannot be eliminated short of using gyro stabilization. So, rather than considering it to be an "issue," I have decided to embrace it and call it a "feature!"

Transition from boost to descent: Versions 3 and 4 had the ejection charge vented out the side which caused an unnecessary and unwelcome tumbling. The rockets would naturally transition to flop mode, but the ejection charge venting would sometimes push the rocket back into a low angle of attack which would stabilize into a "lawn dart" descent. Using a larger diameter body tube and having the ejection charge vent out the back around the motor resolved this issue.

Descent: Center of gravity placement is critical. Too far aft, and the rocket glides tail first and lands hard. It is not as bad, though, as having the CG too far forward which would result in a lawn dart landing. The more challenging issue in descent is pitch oscillations which can result in hard landings on the nose or tail, or worse, pitching down far enough to reduce angle of attack and re-stabilize into forward flight and a lawn dart landing. To dampen the bobbing, minimizing the mass at either end of the rocket is good, as is having larger fins at the tail.

Back to my flights yesterday, the first two had the CG a bit too far aft which resulted in tail sliding. I packed a bit of some adhesive putty into the tip of the nose cone, and had three flights like this:

Really impressive inspiration, effort and results!

Could I ask for a detail pic of your ejection port system, please?

 

[sr205347d]

Really impressive inspiration, effort and results!

Could I ask for a detail pic of your ejection port system, please?

Thank you!

In post #25, there is a photo showing the motor mount. The ejection charge gas just vents around the motor to the rear and play no part. Plugged booster motors would work fine.

 

[Dotini]

Thank you!

I'm going to give that ejection system a try in a new horizontal spin recovery model!

 

[Dotini]

I'm thinking of terminating the fin can with a bulkhead at about the CG, then having a peripheral array of two to four holes of up to 1/4" diameter disperse the ejection charge laterally . Does that sound reasonable?

 

[sr205347d]

I'm thinking of terminating the fin can with a bulkhead at about the CG, then having a peripheral array of two to four holes of up to 1/4" diameter disperse the ejection charge laterally . Does that sound reasonable?

If the ejection charge is vented close to the CG, it shouldn't rotate the model.

 

[BABAR]

Thank you!

In post #25, there is a photo showing the motor mount. The ejection charge gas just vents around the motor to the rear and play no part. Plugged booster motors would work fine.

Interesting. This is sort of like the engineers saying the Bumblebee can’t fly (I think the story is a myth, but a good one.)

One of the “cool factors” of Horizontal Spin Recovery(HSR), Backslider Recovery (BSR), and now Belly Flop Recovery (BFR) is they transition from a stable low drag boost configuration to a stable HIGH DRAG recovery configuration WITHOUT any airframe configuration change.

The key at least for HSR and BSR has been to use the ejection charge to initiate a radical change in Angle of Attack. The Alway brothers? @Dotini , @Rktman I, and probable others have used an asymmetric side port usually at the nose, to initiate the shift. I am surprised that venting out the rear achieves it (not intuitive), but you have shown it can. Does it work consistently? For the record, at least for BSR, even forward ports in my experience do NOT work 100%, I theorize that there is always some possibility that since the final angle of attack post ejection “puff” is completely random, there is always a finite possibility the rocket will be oriented perfectly WRONG (momentary static nose nearly exactly DOWN, so starts to fall with near zero Angle of Attack….regaining and progressively increasing STABLE ballistic return.)


.
This probability decreases but I don’t think goes to zero with relatively high length to diameter ratios.

Your models seem to do pretty well with relatively “normal” ratios.

Great use, BTW, of longitudinal struts to replace centering rings, I have considered this for years for boosters but never in isolation for a single stage or sustainers, which typically need at least one “sealing” ring.

I will be interested to see @Dotini ‘s results with rear vent for HSR. The only downside I see is I don’t think it can be implemented on minimum diameter rockets. A plus side is that I suspect the side ports we use add a bit of drag.

One question on your rear vented bird(s): Is the ejection charge event BEFORE or AFTER apogee. Doubting Thomas that I am, I am thinking if significantly POST apogee, the “puff” would potentially augment rather than correct a stable ballistic recovery. But as @lakeroadster will tell you, I am not infrequently wrong

 

[sr205347d]

If you launch a rocket straight up, especially a long one, it will not arc over gracefully at apogee. It will enter a tail slide momentarily, then flop over. if If ascends at a bit of an angle, it may not tail slide and flop, but the AOA will increase to beyond the Barrowman assumptions, and then the body tube area will cause the CP to shift forward towards the center of lateral area (CLA). If the CG is aft of that, the rocket will turn broadside into the relative wind, at least momentarily. No “puff” is required. This has been 100% reliable in my experience. The ejection charge “puff” from the front at this point can spin it around to a low enough AOA to satisfy Barrowman and re-stabilize the rocket in forward flight.

WARNING: The key is to launch straight up. If you tilt the launch rod enough, the AOA may never increase sufficiently and it may gracefully arc over to a lawn dart descent. Here, a “puff” of ejection charge may save you.)

My recent flights used B6-4 motors, and the ejection charge went off most definitely well after apogee, with no apparent effect.

 

[BABAR]

If you launch a rocket straight up, especially a long one, it will not arc over gracefully at apogee. It will enter a tail slide momentarily, then flop over. if If ascends at a bit of an angle, it may not tail slide and flop, but the AOA will increase to beyond the Barrowman assumptions, and then the body tube area will cause the CP to shift forward towards the center of lateral area (CLA). If the CG is aft of that, the rocket will turn broadside into the relative wind, at least momentarily.

That has not been my experience for low power rockets, as there is almost always a slight wind that slightly turns the rocket. I have seen a number of on board rocket videos and I can’t remember seeing any that seemed to backslide. Truly completely windless days seem relatively in common.

I’d be interested in opinions of others that have far more experience than I, maybe @BEC , @kuririn , @Ronz Rocketz , @Rktman , @prfesser , or others might have an opinion.

i believe the Alway brothers, who patented the Back Slider recovery technique, also found they were not 100% successful, which suggest that their experience like mine was that getting a good backslide started is not as easy as you make it sound. It could also be that your Belly Flop is itself more reliable. I haven’t tried it yet. I have found Horizontal Spin easier to reliably achieve than Back Slide.

that said, the proof is in the pudding, and you’ve presented some outstanding examples that are hard to argue with!

 

[sr205347d]

I should qualify that by saying that I am talking about rockets designed for belly flop recovery that have the CG at burnout located aft of the CLA. You can see the lack of an arc at apogee in my previously posted videos. But, like I said, even with an arc at apogee, the AOA increase will destabilize the forward flight motion.

 

[BABAR]

center of lateral area (CLA).

I take it this is the equivalent of cardboard cutout, which for longer rockets can be significantly different from Center of Pressure (CP) calculated from Barrowman and multiple sim programs?

This is the difference that makes Back Slide Recovery work, relies on relatively long rockets .

 

[sr205347d]

Version 5.9 in action yesterday. Flying on a plugged C6-0.

 

[mbeels]

Really nice!

 

[Dotini]

Version 5.9 in action yesterday. Flying on a plugged C6-0.

Excellent!

Have you measured or calculated the rate of descent?

 

[sr205347d]

Have you measured or calculated the rate of descent?

No.

The rocket is fairly light weight as it is completely empty inside, so it descends slow enough that there is no damage landing on grass, hard dirt, or plowed field.

 

[Dotini]

No.

The rocket is fairly light weight as it is completely empty inside, so it descends slow enough that there is no damage landing on grass, hard dirt, or plowed field.

An altimeter is going to add a fair fraction of an ounce of weight, so its location will be critical. But your efforts are becoming impressive enough to warrant interest from curious rocket scientists to others who simply want to copy and reproduce your results on their fields. I know I do!

 

[sr205347d]

Have you measured or calculated the rate of descent?

Just curious, why is the rate of descent important?

 

[sr205347d]

An altimeter is going to add a fair fraction of an ounce of weight, so its location will be critical.

Absolutely true. Version 5.9 is 34.75 inches long and the CG is 11.5" from the aft end. So any avionics bay would have to be there. Or else, if the CG were further forward, a forward set of fins would be necessary.

 

[sr205347d]

But your efforts are becoming impressive enough to warrant interest from curious rocket scientists to others who simply want to copy and reproduce your results on their fields. I know I do!

If you want to copy version 5.9, here are some measurements: Fins are 4" semi span, 3" root, 2" tip, 120 degrees apart. Body tube is 30" BT-60. Nose cone is 4.75" ogive, 6.6 g. CP according to Rocksim is 4.7" from the aft end. CLA is 13" from the aft end. CG at launch with a C6 motor is 10" from the aft end. CG after burnout is 11.5". The weight, ready to fly except for the motor, is 2.8 oz.

 

[sr205347d]

If you want to copy version 5.9, here are some measurements: Fins are 4" semi span, 3" root, 2" tip, 120 degrees apart. Body tube is 30" BT-60. Nose cone is 4.75" ogive, 6.6 g. CP according to Rocksim is 4.7" from the aft end. CG at launch with a C6 motor is 10" from the aft end. CG after burnout is 11.5". The weight, ready to fly except for the motor, is 2.8 oz.

If you want to deviate from Version 5.9, I recommend doing a cutout (I use foamboard) to determine the center of lateral area (CLA). Your CG after burnout should be about a caliber aft of that. For the cutout, I printed a top view using Open Rocket, and pasted it on the foamboard.

 

[Dotini]

If you want to copy version 5.9, here are some measurements: Fins are 4" semi span, 3" root, 2" tip, 120 degrees apart. Body tube is 30" BT-60. Nose cone is 4.75" ogive, 6.6 g. CP according to Rocksim is 4.7" from the aft end. CG at launch with a C6 motor is 10" from the aft end. CG after burnout is 11.5". The weight, ready to fly except for the motor, is 2.8 oz.

Thanks for the data, I think I have enough to make one now. Maybe I will put an altimeter in a pod at the CG? I can also calculate the altitude when I have a launch partner armed with an Estes Alti-Track with a cellphone video camera attached. On my HSR models, I have been able to consistently achieve rates of descent comparable to a parachute.

 

[sr205347d]

On my HSR models, I have been able to consistently achieve rates of descent comparable to a parachute.

A higher rate of descent would mean a shorter walk! A bellyflopper is not landing on fragile fins, so a higher rate of descent should be survivable.

 

[BABAR]

Version 5.9 in action yesterday. Flying on a plugged C6-0.

Wow! Doesn’t get much better than that.

You have completely discombobulated my premise of the required destabilizing ejection side port “puff” for recovery of rockets with no configuration change at deployment.

Speaking of which, I am wondering if you, @Dotini , and I can name a new recovery class that included all known (and potentially to be discovered!) models where there is no change in ANY part of the rocket between launch and non-ballistic recovery. Essentially “no moving parts.” I will exclude the burning of propellant and delay and ejection charges and shift clay cap fragments.

Candidates I can think of thus far are

Belly Flop

Horizontal Spin

Back Slide

Monocopters and variants

I guess Saucers also fall into this category (pun intended). Maybe not, you could argue they have a very slow but ballistic Return.



Any others?

Featherweight is out, it ejects motor

Tumble is out, it SHIFTS the motor.

Rocket Boost Gliders to my knowledge always have SOME sort of surface movement.

Anyhoo, I was thinking of the terms

Fixed Configuration Recovery (FCR)

although that may provoke some problems with the RSOs, “Up next is a real cool FCR on pad 9.”

No Moving Parts Recovery (NMPR)

might be better.

 

[BABAR]

Okay, I have a theory as to why the physics of this cockamamie brilliant design works.

With two fins, the rocket is marginally stable.

We are frequently talking about minimum speed off the rod or rail for successful flight. Although the CALCULATION for rocket stability does not (@neil_w correct me if I am wrong) consider velocity in determining stability, experience and ThrustCurve tells us if you don’t reach a certain speed, the fins don’t catch enough moving air to be effective (otherwise we wouldn’t need a rod or rail guide rocket in desired direction until it achieves said sufficient velocity.)

Most symmetrical three and four finned rockets are stable enough that if you dropped them from a Hot Air Balloon in any orientation, starting at altitude they will naturally orient nose down and come in ballistic.

Your two ASYMMETRIC fin (Maybe ECCENTRIC is more accurate) rocket at low velocity (like a certain time after propellant burnout, but not immediately like gas stabilized rockets) becomes UNSTABLE, enters an inevitable high angle of attack, and settles into a horizontal descent.

Anyway, even if I am wrong your rockets are still really cool!

 

[cwbullet]

Interesting concept. I have seen a few do this unintentionally when they fly a fin off.