Bellyflop Recovery

[sr205347d]

I recently re-entered the hobby, so it is quite possible that someone has already done this and I am unaware. That said, inspired by the SpaceX Starship, I thought that a model rocket descending in a bellyflop attitude could land with no damage, provided that there are no fins on the belly side to break off. An advantage could be a greater rate of descent and less wind drift.

Playing with Open Rocket and Rocksim, I came up with this:



Since I like to scratch build model airplanes quickly and cheaply with foamboard and packing tape, I thought I could do the fins that way. Here is what I built:


On the first flight, I had the nose cone come down separately on a streamer. The body turned out to be a back-slider. And that was before I knew that a back-slider was a thing! (See the Apogee Newsletter #447.) On subsequent flights that day, I suspended the nose cone from the rocket body with a bridle to adjust the CG during recovery, trying to get it to belly flop rather than backslide. I finally got a perfect bellyflop with the nose cone suspended as shown:



The problem with this is that on the next and last flight of the day, the shock cord wrapped around the forward fins causing another backslide.

To fix this, I removed the forward fins. I also tried to be more analytical about where the CG should be during recovery. I used Rocksim to compute the center of lateral area (CLA), or, "cardboard cutout" CP of the rocket without the nose cone. The computed CLA was about 5" forward of the aft end. I provided for attaching the nose cone cord so that I could adjust the location. Thus was born the Bellyflopper 2:



On the first flight of BF2, I let the nose cone come down separately on a streamer. I got a steep back slide, but no damage. With the nose attached so the recovery CG was at the computed CLA, there was still a back slide. Finally, with the nose cone attached so the recovery CG was about an inch forward of the computed CLA, I got a beautiful belly flop! Yay! And, no tangled shock cord.

A lesson learned from this is that the Rocksim calculated "cardboard cutout" CP is not accurate, at least if your fins are not axi-symmetric. I did an actual cutout (but using foamboard), and the actual CLA was right where the CG was on the successful belly flop, about six inches forward of the aft end.

By the way, the fin arrangement was stable. I will do another thread on the foamboard building technique.

BF3 will keep the nose cone in place with the recovery CG at the actual (not Rocksim) CLA. I will use the Alway method (see the Apogee newsletter) of inducing a high AOA by means of punching a hole in the body tube at the forward end. Stay tuned.

 

[Gus]

Really interesting thread! Thanks!

 

[dhbarr]

Neat!

 

[BABAR]

Excellent!

I have seen similar things with asymmetric fin rockets with small streamers that essentially “neutralize “ the nose cone. But first time I have seen it with just “nose blow “ recovery.

Can you give a forward cross section view or drawing of BellyFlopper 2? 2 fin rockets are NOT supposed to be stable, but with asymmetric placement easily do-able.

I think your design is unique enough to call it a “new” recovery technique, which is saying something in a hobby that has been around this long. @Dotini and @Rktman as well as many others may find this interesting.

Congrats, it’s also likely a good small field rocket that lands without damage. If you go to a club launch and bring 2 models, consider naming the second one “Tango Uniform” (Tips Up!). @cwbullet might appreciate that one,

 

[cwbullet]

Net design. I can't say that I have seen this before.

 

[sr205347d]

Can you give a forward cross section view or drawing of BellyFlopper 2? 2 fin rockets are NOT supposed to be stable, but with asymmetric placement easily do-able.

My second photo gives a pretty good cross section view. The BF2 is the same rocket but with the forward fins removed.

 

[BABAR]

Welcome to the Two-Fin Club. To my knowledge (which is far from exhaustive), there are at least two other two-fin derivatives.

The more spectacular is
https://www.rocketryforum.com/threads/summer-buildoff-roc-n-roll-flying-v-guitar.134436/ by @Flyfalcons

The more humble is the Lucky 7
https://www.rocketryforum.com/threads/stable-two-planar-fin-rocket-return-of-lucky-7.154443/
There is of course the One Fin Rocket, Bruce Levison's Corkscrew

http://modelrocketbuilding.blogspot.com/2011/03/corkscrew-2-build-backstory.html

Your model is interesting, it looks like it combines asymmetric fins (a trick I love to use), but also they look a bit thick, which was @Flyfalcons trick. What is the Fin thickness?

 

[sr205347d]

What is the Fin thickness?

3/16" I don't know how that would have anything to do with it.

 

[Rktman]

I recently re-entered the hobby, so it is quite possible that someone has already done this and I am unaware. That said, inspired by the SpaceX Starship, I thought that a model rocket descending in a bellyflop attitude could land with no damage, provided that there are no fins on the belly side to break off. An advantage could be a greater rate of descent and less wind drift.

Playing with Open Rocket and Rocksim, I came up with this:

View attachment 543251

Since I like to scratch build model airplanes quickly and cheaply with foamboard and packing tape, I thought I could do the fins that way. Here is what I built:
View attachment 543252 View attachment 543254

On the first flight, I had the nose cone come down separately on a streamer. The body turned out to be a back-slider. And that was before I knew that a back-slider was a thing! (See the Apogee Newsletter #447.) On subsequent flights that day, I suspended the nose cone from the rocket body with a bridle to adjust the CG during recovery, trying to get it to belly flop rather than backslide. I finally got a perfect bellyflop with the nose cone suspended as shown:

View attachment 543257

The problem with this is that on the next and last flight of the day, the shock cord wrapped around the forward fins causing another backslide.

To fix this, I removed the forward fins. I also tried to be more analytical about where the CG should be during recovery. I used Rocksim to compute the center of lateral area (CLA), or, "cardboard cutout" CP of the rocket without the nose cone. The computed CLA was about 5" forward of the aft end. I provided for attaching the nose cone cord so that I could adjust the location. Thus was born the Bellyflopper 2:

View attachment 543261

On the first flight of BF2, I let the nose cone come down separately on a streamer. I got a steep back slide, but no damage. With the nose attached so the recovery CG was at the computed CLA, there was still a back slide. Finally, with the nose cone attached so the recovery CG was about an inch forward of the computed CLA, I got a beautiful belly flop! Yay! And, no tangled shock cord.

A lesson learned from this is that the Rocksim calculated "cardboard cutout" CP is not accurate, at least if your fins are not axi-symmetric. I did an actual cutout (but using foamboard), and the actual CLA was right where the CG was on the successful belly flop, about six inches forward of the aft end.

By the way, the fin arrangement was stable. I will do another thread on the foamboard building technique.

BF3 will keep the nose cone in place with the recovery CG at the actual (not Rocksim) CLA. I will use the Alway method (see the Apogee newsletter) of inducing a high AOA by means of punching a hole in the body tube at the forward end. Stay tuned.

Really unique project and rocket, especially for a BAR!

 

[Dotini]

Congratulations on your project! I think the basic idea is very attractive.

Last year I briefly (one flight) experimented with a similar idea. It worked like a backslider, but came down too fast and sustained damage to the motor mount. Probably my fins were too small and not in the right place, as I certainly was out of my depth in the glider realm.


Fin can is a modified Estes generic.

 

[BABAR]

3/16" I don't know how that would have anything to do with it.

Not exactly sure either, but according to @Flyfalcons that’s how his worked. His were REEEAAALLLY thick. To my knowledge his is the only SYMMETRIC stable two linear fin rocket. By symmetric, I mean the fin span axis passes through the midline rocket longitudinal axis.

 

[lakeroadster]

I recently re-entered the hobby, so it is quite possible that someone has already done this and I am unaware. That said, inspired by the SpaceX Starship, I thought that a model rocket descending in a bellyflop attitude could land with no damage, provided that there are no fins on the belly side to break off. An advantage could be a greater rate of descent and less wind drift.

Playing with Open Rocket and Rocksim, I came up with this:

View attachment 543251

Since I like to scratch build model airplanes quickly and cheaply with foamboard and packing tape, I thought I could do the fins that way. Here is what I built:
View attachment 543252 View attachment 543254

On the first flight, I had the nose cone come down separately on a streamer. The body turned out to be a back-slider. And that was before I knew that a back-slider was a thing! (See the Apogee Newsletter #447.) On subsequent flights that day, I suspended the nose cone from the rocket body with a bridle to adjust the CG during recovery, trying to get it to belly flop rather than backslide. I finally got a perfect bellyflop with the nose cone suspended as shown:

View attachment 543257

The problem with this is that on the next and last flight of the day, the shock cord wrapped around the forward fins causing another backslide.

To fix this, I removed the forward fins. I also tried to be more analytical about where the CG should be during recovery. I used Rocksim to compute the center of lateral area (CLA), or, "cardboard cutout" CP of the rocket without the nose cone. The computed CLA was about 5" forward of the aft end. I provided for attaching the nose cone cord so that I could adjust the location. Thus was born the Bellyflopper 2:

View attachment 543261

On the first flight of BF2, I let the nose cone come down separately on a streamer. I got a steep back slide, but no damage. With the nose attached so the recovery CG was at the computed CLA, there was still a back slide. Finally, with the nose cone attached so the recovery CG was about an inch forward of the computed CLA, I got a beautiful belly flop! Yay! And, no tangled shock cord.

A lesson learned from this is that the Rocksim calculated "cardboard cutout" CP is not accurate, at least if your fins are not axi-symmetric. I did an actual cutout (but using foamboard), and the actual CLA was right where the CG was on the successful belly flop, about six inches forward of the aft end.

By the way, the fin arrangement was stable. I will do another thread on the foamboard building technique.

BF3 will keep the nose cone in place with the recovery CG at the actual (not Rocksim) CLA. I will use the Alway method (see the Apogee newsletter) of inducing a high AOA by means of punching a hole in the body tube at the forward end. Stay tuned.

Could you describe the post apogee flight profile? Does the rocket glide forward, does it fall straight down but in a horizontal orientation?

 

[BABAR]

Could you describe the post apogee flight profile? Does the rocket glide forward, does it fall straight down but in a horizontal orientation?

Tag on to that the boost profile, does it rotate/corkscrew or go straight with Little or no rotation?

 

[sr205347d]

Could you describe the post apogee flight profile? Does the rocket glide forward, does it fall straight down but in a horizontal orientation?

The desired descent is a horizontal attitude, belly down, and no gliding. It was gliding backwards until I moved the CG far enough forward.

 

[sr205347d]

Tag on to that the boost profile, does it rotate/corkscrew or go straight with Little or no rotation?

I got a little corkscrewing at the top, but I blame that on not getting the fins on exactly straight. I was a bit sloppy gluing the fins on with hot glue. I didn’t want to put lots of effort into a proof of concept that I thought might not survive launch.

I am thinking of going to Hobby Lobby and picking up some models and cutting fins off.

 

[sr205347d]

We left off with the Bellyflopper 2 showing a stable belly flop descent with the nose cone suspended below. However, I would rather leave the nose cone in place.

So, continuing my rapid prototyping, for BF3, I went to Hobby Lobby and got an Estes Dragonite, a beginner level kit. It was modified by leaving off one fin, leaving out the parachute, putting weight in the nose cone to have it balance at the CLA (center of lateral area as determined by foamboard cutout), securing the nose cone in place with electrical tape, and cutting a hole in the body tube below the nose cone to let the ejection charge escape. The rocket was expected to transition to an AOA higher than would be stable according to Barrowman with the aid of the ejection charge gas escape.

Here it is, along with my foamboard cutout. On the foamboard I marked the balance point (CLA) and the CPs calculated by Barrowman (BCP) and by the Rocksim equations (Rocksim).



I flew it four times with these observations:

First, there was some cork-screwing in boost on all flights, but nothing scary. I have some ideas how to resolve this.

Second, the rocket transitioned on its own to bellyflop mode at apogee before the ejection charge fired. And on one flight it appeared that the ejection charge actually pushed the nose back into a "stable" AOA, causing a lawn dart recovery! Fortunately, it was not severely damaged and I flew it twice more. To prevent going all lawndart again, I removed some of the nose weight which moved the CG about a half inch aft of the CLA.

The bellyflop attitude was marginally stable in pitch, with lots of pitching up and down. It did remain stable in roll, with the belly side down.

Lesson Learned:

Having the nose cone weight suspended from the belly of the BF2 rocket likely helped stabilize the descent.

The CG needs to be some amount aft of the CLA to prevent going to lawn-dart mode, but not too far else it will back-slide.

Future plans:

I will keep the nose cone in place, but use larger fins to move the CLA aft and reduce the requirement for added weight in the nose. The lowered moment of inertia and added damping should help reduce the pitching on descent. Larger fins may also reduce the cork-screwing in boost. Perhaps adding a dorsal fin will help with the cork-screwing also.

 

[dhbarr]

I think if you move your lugs around to the dragon's belly, inline with where the third fin would have been, you'll gain more stability and less corkscrew.

EDIT: Just realized the aft one is molded into the can, so scratch that.

 

[sr205347d]

Bellyflopper 4 made four flights today, with four awesome bellyflops! It is similar to BF2 as shown above, but the fins are bigger, the nose is taped in place, and there is a hole in the body tube just below the nose cone to let the ejection charge out.



Here is a cell phone video:



There is still some corkscrewing, but I think that a vertical fin should help with that.

This is just too easy, without any recovery system, no moving parts! (The best part is -- no part!)

I balanced it so the CG with a spent motor is about 3/4" aft of the center of lateral area (CLA) as determined by foamboard cutout.

If you try this, don't forget to check boost stability the usual way with Open Rocket or Rocksim.

 

[BABAR]

Doesn’t get much better than that!

 

[BABAR]

I will be interested to see if the vertical fin solves the corkscrewing. Although I have come to feel that corkscrewing on a rocket designed for unusual recovery techniques is more of an entertaining bonus rather than a fault or defect.

I find corkscrewing itself interesting, as while it occurs in most of my asymmetric fin rockets (not all) there are 2 things at least that I don’t understand:

1. For the ones that DON’T corkscrew, I don’t know what is different that makes them fly straight bs the others.

2. And more interesting, most of my asymmetric fin birds are asymmetrically loaded (most fins on one side) they are mirror image symmetrical, i.e. you could split it into two mirror image halves. So how does the rocket “decide” which way to START its turn?

 

[sr205347d]

The vertical fin cured the corkscrewing! It went up nice and straight.



Unfortunately, it came down nice and straight too, without transitioning to the bellyflop. The added fin made it too stable.



Cutting back the fin and/or moving the CG aft will fix it, I hope.

 

[BABAR]

The vertical fin cured the corkscrewing! It went up nice and straight.

View attachment 544749

Unfortunately, it came down nice and straight too, without transitioning to the bellyflop. The added fin made it too stable.

that darn law of unintended consequences.

 

[sr205347d]

After repairing the damage, I made four flights this morning, with the CG moved a bit aft from where it was. The fin configuration was unchanged.

It went back to corkscrewing on all flights. But, it did good bellyflops on the first three. On the last flight, it appeared to have transitioned to bellyflop mode at apogee, but the ejection charge pushed the nose down and it did the lawn dart thing again.

I don't know what the cure for the corkscrewing is. I thought I had it with the added fin area.

To prevent the ejection charge from messing with the bellyflop, I am thinking of venting it differently.

 

[Aeronerd]

Cool idea. You invented a new type of recovery system.

 

[sr205347d]

BF4 did nice flops, but with issues. It suffered corkscrewing on the way up, and, the ejection charge puffing out the hole below the nose cone can actually act to re-stabilize it by pushing the nose into a low angle of attack (AOA), leading to a lawn-dart landing and mangled death.

To resolve these two issues, BF5 was born. Since it seems to me that the corkscrewing is caused by a combination of rolling, along with asymmetric drag from the 3/16" thick foamboard fins, I tried to minimize fin drag by making them from 3/32" balsa and minimizing the size. I also printed an alignment jig to get them on straight.



To prevent the ejection charge from doing unpredictable things, I let that gas vent out the back around the motor. (Since the rockets have been naturally transitioning to bellyflop mode at apogee, no help from the ejection charge is necessary.)

This one only flew twice, behaving much like BF3. The corkscrewing was reduced to an acceptable level, and it transitioned to bellyflop mode at apogee just fine. But in descent it suffered from pitch oscillations, bobbing the nose up and down, to the extent that on the down oscillation, it would reduce the AOA and re-stabilize into a lawn-dart recovery.

BF5.1 is the same rocket but with a body tube extension. I hoped the longer rocket would have the bobbing oscillations damped a bit.



This one flew six times yesterday. Again, there was no noticeable corkscrewing and it reliably transitioned to flop mode at apogee. The bobbing was damped enough that it didn't go to lawn-dart mode, but still persisted. The bobbing can lead to awkward landings on one end or the other, but there was no damage. I would prefer it to land flat like BF4 did. So, I am thinking that BF6 will have larger fins to help dampen the pitch oscillations. I just hope that it doesn't go back to corkscrewing.

 

[BABAR]

Congrats. I take issue with the phrase “It suffered corkscrewing.”

I consider corkscrewing a novel side effect often seen with asymmetric fin placement. So long as the NET trajectory is straight and the delay appropriate to deploy at or just before apogee (late is bad) I think it is kind of fun.

That said, if you can prevent it, I’d be interested in how. I have had some asymmetric fin birds fly straight as an arrow with no corkscrewing, others leave a beautiful spiral smoke trail. I haven’t figured out what determines the difference . One corkscrewed with one motor but not a different size motor.

I suspect BELLY FLOP recovery, like Horizontal spin recovery HSR and backslide recovery BSR will also be more reliable with longer body tubes.

The only significant downside is I am not sure you’re going to like the abbreviation for your recovery method.

 

[sr205347d]

The only significant downside is I am not sure you’re going to like the abbreviation for your recovery method.

BFR -- Best by far!

 

[sr205347d]

That said, if you can prevent it, I’d be interested in how.

My guess is that corkscrewing is caused by rolling motion due to imperfect fin alignment, along with an angle of attack due to asymmetric fin weight and/or drag. Fix it by good fin alignment and minimum weight/drag fins.

 

[sr205347d]

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:



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:

 

[dhbarr]

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