Bellyflop Recovery

[sr205347d]

Belly Flop

Horizontal Spin

Back Slide

What these have in common, and what differentiates them from all others, is that they take advantage of the shift of the CP towards the CLA at increased AOA.

Two fins are possibly not necessary for a belly flopper. (Well maybe just "flopper" as there would be no "belly.) I just don't want a fin to break on landing. I suppose it might be interesting to fly a "flopper" with three or four fins to see what the descent looks like.

 

[Dotini]

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.



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.

I've been toying with the term "monolithic", as if the rocket were carved or cast from a single block of material.

 

[BABAR]

What these have in common, and what differentiates them from all others, is that they take advantage of the shift of the CP towards the CLA at increased AOA.

Two fins are not necessary for a belly flopper. (Well maybe just "flopper" as there would be no "belly.) I just don't want a fin to break on landing. I suppose it might be interesting to fly a "flopper" with three or four fins to see what the descent looks like.

I think you do indeed need a "belly". I think any number of fins (within reason, after 10 probably diminishing returns!) would work, but I theeeeeenk they have to by eccentric. The "belly" is the side opposite the exaggerated opposite fin surface area.

My hypothesis is you put a third equal size fin on your current version on the opposite side equidistant from the other two and you will have a lawn dart.

What confounds me still on your rockets is that I think by "Cardboard Cutout" method of stability calculation (which is I think basically similar to your Center of Lateral Area) your two fin rockets are STILL stable, therefore by that method they should lawn dart. Your first statement of the shift of CP from Barrowman to CLA definitely works for relative SuperRocs, with large Length to Diameter Ratios, BECAUSE (if I understand it right) Barrowman essentially ignores the length of the body tube in the calculation of CP because it is negligible AT NEAR ZERO ANGLE OF ATTACK (again, @neil_w please correct me if it am wrong.) The Cardboard Cutout/CLA DOES ACCOUNT for the exaggerated long body tube surface area in long length to diameter rockets, and this surface are is enough to make the rocket unstable but only "kicks in" when the rocket is thrown off of near zero angle of attack. Your rocket flown with a plugged motor has no such "event", so "something else" must be happening between stable vertical flight and eventual horizontal recovery.

SOMETHING must happen to the rocket for it to transition from rapid pointy-nose first flight to slow sideways horizontal descent. For HSR and BSR, that even has classically been the puff port throwing the rocket into non-negligible angle of attack. You have proven by your plugged motor that Belly Flop Recovery doesn't need it. I don't buy (I may be wrong) that most rockets fly pointy nose up continuously until they run out of kinetic energy, and then start to fall tail first. The reason I don't buy it is that in every onboard video camera I have seen (and many of them are on high power rockets with electronic recovery timed for apogee) unless ejection is early the rocket always seems to be horizontal or nose down at ejection (@Steve Shannon have an opinion on this.) Here is Steve Eves' Saturn V (former world record largest rocket launch) video, it noses over before deployment, it doesn't start falling tail first. Jump to one minute.



So I still theeeeenk that with Belly Flop Recovery, the initiating even is the loss of forward velocity combined with the eccentric fin placement. I think any SYMMETRICAL placement of relatively normal thickness fins (@Flyfalcons got away with using really thick symmetrical fins in his award winning Guitar Rocket, hey if you ain't cheatin' you ain't trying!), any SYMMETRICAL placement of normal thickness fins that is stable on boost will REMAIN stable on descent without some kick (although we will see how @Dotini does with his next rendition, I think he is moving puff port to CG which I don't understand, and even if it works won't prove that "something" didn't happen. I don't have the guts to try HSR with a plugged motor.)

I still wonder if Belly Flop Recovery isn't essentially Back Slide with really poor forward velocity, and that MIGHT be BECAUSE your rockets aren't SuperRoc high length to diameter. I REEEEEEALLLLY miss having a nearby flying field, as I would love to play with this more myself.

 

[Steve Shannon]

SOMETHING must happen to the rocket for it to transition from rapid pointy-nose first flight to slow sideways horizontal descent. For HSR and BSR, that even has classically been the puff port throwing the rocket into non-negligible angle of attack. You have proven by your plugged motor that Belly Flop Recovery doesn't need it. I don't buy (I may be wrong) that most rockets fly pointy nose up continuously until they run out of kinetic energy, and then start to fall tail first. The reason I don't buy it is that in every onboard video camera I have seen (and many of them are on high power rockets with electronic recovery timed for apogee) unless ejection is early the rocket always seems to be horizontal or nose down at ejection (@Steve Shannon have an opinion on this.)

You’re absolutely correct. Most rockets have some horizontal air velocity that becomes more influential at turning the rocket as vertical air velocity decreases. Because the Cg is ahead of the Cp, they turn horizontal and then eventually downward.
Very long L/D rockets do sometimes backslide, but I’m not sure that I would take it for granted.
I have seen lots of rockets fall in a flat spin, but I don’t know how to cause that reliably.

 

[sr205347d]

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.

I am calling this class of rockets "CP Shift" rockets because they rely on the CP shift at higher AOA to a point forward of the (burnout) CG.

 

[sr205347d]

SOMETHING must happen to the rocket for it to transition from rapid pointy-nose first flight to slow sideways horizontal descent.

Well, SOMETHING must happen to a "normal" rocket to rotate it from nose up to nose down as it arcs over at apogee. That SOMETHING is an AOA increase which creates sideways aerodynamic force acting through the CP to cause rotation about the CG.

The rocket's rotational inertia will resist the rotational moment from the CP by some amount. So, the AOA must be high enough to create a large enough aerodynamic force. So all rockets experience some increase in AOA at apogee.

That increased AOA will shift the CP forward some amount. For "normal" rockets, the shifted CP is still aft of the burnout CG, and it remains stable in forward flight.

The same SOMETHING is happening to my belly flop rockets. When the rocket reaches apogee, the rotational inertia resists rotation, and the AOA increases. As the AOA increases, the CP shifts forward. But, unlike "normal" rockets, the CP moves forward of the CG and turns the rocket broadside to the relative wind.

I am sure the same thing would happen with your HSR rockets, making the ejection charge "puffs" un-necessary.

 

[dhbarr]

Well, SOMETHING must happen to a "normal" rocket to rotate it from nose up to nose down as it arcs over at apogee. That SOMETHING is an AOA increase which creates sideways aerodynamic force acting through the CP to cause rotation about the CG.

The rocket's rotational inertia will resist the rotational moment from the CP by some amount. So, the AOA must be high enough to create a large enough aerodynamic force. So all rockets experience some increase in AOA at apogee.

That increased AOA will shift the CP forward some amount. For "normal" rockets, the shifted CP is still aft of the burnout CG, and it remains stable in forward flight.

The same SOMETHING is happening to my belly flop rockets. When the rocket reaches apogee, the rotational inertia resists rotation, and the AOA increases. As the AOA increases, the CP shifts forward. But, unlike "normal" rockets, the CP moves forward of the CG and turns the rocket broadside to the relative wind.

I am sure the same thing would happen with your HSR rockets, making the ejection charge "puffs" un-necessary.

Right. CG shifts forward during the burn, CP shifts forward as we slow down. If the CG and CP are pretty close together in the end with a roll to the belly, we come in fairly flat.

EDIT: I had a dumb.

 

[BABAR]

Right. CG shifts aft during the burn, CP shifts forward as we slow down. If the CG and CP are pretty close together in the end with a roll to the belly, we come in fairly flat.

The same SOMETHING is happening to my belly flop rockets. When the rocket reaches apogee, the rotational inertia resists rotation, and the AOA increases. As the AOA increases, the CP shifts forward. But, unlike "normal" rockets, the CP moves forward of the CG and turns the rocket broadside to the relative wind.

I am sure the same thing would happen with your HSR rockets, making the ejection charge "puffs" un-necessary.

I think the CG shift from the burn is negligible for LPR (again I can beg @Steve Shannon ‘s indulgence, maybe for HPR with long burn motors it may be significant.)

More importantly is that with the exception of @Daddyisabar ‘s Jedi tractors, for tail end motored rockets which thus far include No Moving Parts Recovery (NMPR) which thus far includes Belly Flop, Horizontal Spin, Back Slide, and Saucer, even if the burned propellent mass change was significant, it would change CG, NOT CP, AND even more importantly, the change in CG would make the rocket MORE stable.

Also, as I understand it, technically CP does NOT change with changes in rocket speed nor angle of attack by Barrowman equation methods (@kuririn , @neil_w check me if I am wrong.) However, Barrrowman and Barrowman based simulations are based on an ASSUMPTION of a low angle of attack. When this no longer applies, the Barrowman solution is no longer applicable.

Your statement "I am sure the same thing would happen with your HSR rockets, making the ejection charge "puffs" un-necessary." for me is problematic, BECAUSE if have seen the same rocket HSR flying on the same motors fly multiple times successfully and then have a ballistic failure USING the puff technique. I think @Dotini has also experience. I've seen it more for Back Sliders, and I believe the Alway brothers discussed occasional failures as well. I am currently building an imitation of your Belly Flop (not sure when or where I will get to fly it, may have to mail it to someone along with my Screwballs!), and I may build a CADHSR (Cheap and Dirty Horizontal Spin Recovery) model with a rear ended vent (I don't want to buy any plugged motors, and while I respect those who fly with post market plugged motors, I prefer to keep my NAR insurance applicable, in case of fecal turbine interaction, which is EXACTLY what it think will occur with HSR without the puff.)

There is something very different (and obviously effective) with your Belly Flop design. Part of which is that, at least eyeballing it, even if you use Cardboard Cutout (or Center of Lateral Area) it looks like your models (at least the tail only finned ones) are STILL stable using any accepted standard. So I am still contending (hopefully in a collegial way if long winded way) that your eccentric fins are the key.

 

[StreuB1]

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

That is the coolest thing I have seen in a really long time.

Ok, now for a 4" upscale on an I motor aaaaaand go!

 

[StreuB1]

Very long L/D rockets do sometimes backslide, but I’m not sure that I would take it for granted.
I have seen lots of rockets fall in a flat spin, but I don’t know how to cause that reliably.

My Extreme Wildman fell from 11k at Mini WMP 2022 due to an anomaly where the booster ran into the upper section 0.5sec after ejection and separation and the impact destroyed all the electronics. It fell all the way down in a flat spin, zero damage other than the electronics.

 

[Steve Shannon]

Unless the Cg of the propellant mass is ahead of the Cg of the rocket at liftoff, a rocket’s Cg shifts forward, not rearward, when the motor burns. The amount of shift will depend on the mass of propellant and distance from Cg. It’s probably still a question on the L2 test because it’s important for prospective RSOs to understand.

 

[BABAR]

That is the coolest thing I have seen in a really long time.

Ok, now for a 4" upscale on an I motor aaaaaand go!

Might be pushing it. I wonder if it would pass muster for an L1 cert?

 

[dhbarr]

Unless the Cg of the propellant mass is ahead of the Cg of the rocket at liftoff, a rocket’s Cg shifts forward, not rearward, when the motor burns. The amount of shift will depend on the mass of propellant and distance from Cg. It’s probably still a question on the L2 test because it’s important for prospective RSOs to understand.

Yes, I had a dumb.

 

[sr205347d]

Upscaled BT-80 version, flying with a plugged D12-0.





My experiments on another model with gyro stabilization to kill the corkscrewing have not been successful yet.

 

[lakeroadster]

Might be pushing it. I wonder if it would pass muster for an L1 cert?

Since one of the HPR Level 1 Checklist checkboxes is: "Recovery System deployed: Yes / No" ... It would be prudent to get prior approval of this recovery type before attempting an L1 Certification flight.

 

[Raythain]

However, Barrrowman and Barrowman based simulations are based on an ASSUMPTION of a low angle of attack. When this no longer applies, the Barrowman solution is no longer applicable.

This is accurate. Barrowman presented this clearly in his presentation at this year’s NARCON. He also remarked that the CoLA is equivalent to the CoP at a 90 degree AoA.

If the AoA is off nominal, you’ll have to slap trigonometric functions all over the equations to separate the forces into horizontal and lateral components. The numerical analysis this would have required wouldn’t have likely been feasible in the 60s.

I’m gonna have to read up on flat spins, because I think @BABAR is right. The interesting question is why this design consistently enters a flat spin. It likely has to do with the unbalanced forces acting through the CoP, but why doesn’t it kick in until after apogee? Is it linked with low dynamic pressure?

Think I need to make a trip to the local hobby store for some BT-20 and balsa, I could fly 13mm tests in my backyard, and this question is interesting enough that I think I’m willing to do it in the snow. Might have to wait for a warm day to convince somebody to be my videographer.

 

[sr205347d]

The BT-80 version flies better with forward fins.

 

[sr205347d]

Estes Star Orbiter Kit Conversion to Belly Flopper



Pretty easy. I just left out the parachute and one of the fins.

Also didn't use the supplied 29mm motor mount. Instead, I put a thrust ring in the BT-60.



This way, I can slip in a motor mount for 24mm, 29mm, or a cluster of 18mm motors.



I flew it today with two C6-0 motors. All motors in this would need to be plugged.

The CP is 7.5" from the aft end. The CLA is at the break between the two 18" body tube sections. The burnout CG should be about one caliber aft of the CLA. Mine was a little aft of that, and so I had a tad bit of a back slide (which is better than having the CG too far forward).

It flew better than any of my previous belly floppers! The corkscrewing was minimal, and there was no bobbing.

Sorry for the lousy cell phone videography. It is hard to see what is in the frame with the sun shining on the screen.

 

[BABAR]

The BT-80 version flies better with forward fins.

View attachment 562177 View attachment 562178

Just beautiful!

 

[sr205347d]

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.

Well, after a day of flying in a stronger breeze than usual and getting some weathercocking that resulted in lawn dart landings on rockets without forward fins, I need to modify the above statement. I reviewed the flight records, and it seems that the forward fins provide a much more reliable transition to bellyflop mode. Going forward, I will include forward fins on all bellyfloppers.

 

[BABAR]

Well, after a day of flying in a stronger breeze than usual and getting some weathercocking that resulted in lawn dart landings on rockets without forward fins, I need to modify the above statement. I reviewed the flight records, and it seems that the forward fins provide a much more reliable transition to bellyflop mode. Going forward, I will include forward fins on all bellyfloppers.

NOW you tell me

 

[sr205347d]

It has been a while since I did this, but I thought I should report on it.

I tried to kill the corkscrewing with gyro stabilization, using an RC gyro connected to servos:



Still had corkscrewing. Tried different placements and larger tabs, but to no effect.

Running the whole problem through the old mind-sim, I realized that the corkscrew rotation was not due to imperfectly aligned fins. Here is what is happening: The asymmetrical fins add weight and drag to one side of the rocket, which causes the rocket to fly at an angle of attack to the relative wind. Now we have airflow across a cylinder, which results in a vortex forming over one side or the other of the cylinder. (There are lots of studies and reports on fluid flow across cylinders.) The static pressure in a vortex is lower than ambient, so it produces lift on that side of the body tube. Since most of the body tube is forward of the CG, the vortex lift causes the rocket to yaw toward that side, resulting in the corkscrewing. The vortex lift is much stronger than whatever torque my roll tabs could produce to counter the rotation.

Bottom line is that the corkscrewing cannot be stopped without resorting to some variable geometry arrangement. Gotta learn to love it.