Gas-dynamic Stabilization

I found this article quite interesting.

Peak of Flight 379

I have come up with a concept using these principles and was wondering if anyone here has explored gas-dynamic stabilization before, it would be nice to get some first hand info before I get started.

If you look at the end of the article, you will see the author retired from Rocketdyne and worked on the process since the 1960's so he knows what he's talking about.

Bob

I did see that, I just have a few questions that I think I have answers for, I just want to check and see if my answer is the same as someone who knows about such things.

I'll put my idea on paper this weekend and see if I can get someone in the know to chime in.

Screaminhelo said:

I did see that, I just have a few questions that I think I have answers for, I just want to check and see if my answer is the same as someone who knows about such things.

I'll put my idea on paper this weekend and see if I can get someone in the know to chime in.

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I don't actually have any answers. wfcook and I have both taken a shot at one of these. Our first tries did not work out as planned :surprised:

Since that article, Dean has updated his reference design. I'm going out on a limb and am posting it. I reworked mine and it performed much better. It is 2.125" in diameter and 18" long. I flew it on an E12 and it was stable at the start of the burn when the trust was highest. I haven't heard back from Dean regarding these results. I plan to try again on a motor with more thrust and a flatter curve.

The design that I have in mind is more along the pseudo-fin concept. It is based on the Painkiller and is intended to fly with D to F power. The MMT would be at or aft of CG (I may design in and adjustable nose weight to keep the CP aft of CG for larger motors) and the air intake would be similar to that used in the FWD MMT finless designs.

My initial thinking is that a larger than necessary intake would not have a negative impact on the GDS properties but would impose a drag penalty. I may have to adjust the position of the MMT and/or the length of the exhaust nozzle to make it all work.

I look forward to seeing a sketch or what you have in mind. wfcook and I discussed air intakes but I still just wanted to try the bare bones design first. I am surprised he hasn't found this thread (or responded to my PMs....knock...knock...hope all is well). Dean had been on TRF and is on FaceBook, however I can't seem to ping him either. Dean would obviously be an excellent sounding board and I think Bill has thought more about what is going on with these than I have. I mostly just believed and built something that matched the criteria in Dean's diagrams.

I read the article very interesting. If I was pursuing a design along these lines I'd iterate in on the finial solution. Build a couple of models with decreasing fin area until either I had successful finless flights or found the smallest fins I could safely fly with. Testing should be done in a careful manner, lots of field and few spectators for safety and such, also small test vehicles as well.

I am out sick today so I have been able to play in OR.

There are a few things that I don't know how to do in OR, so I made a couple of compromises in the design so that I could lay everything out.

1. Fin tabs added to account for the boat tail
   -Tabs will be trimmed to contour
2. Aft body tube is a single piece. Intakes are not depicted in the design file.
   -FWD edge starts at AFT MMT CR and extends AFT approximately 1/2", spanning the fin roots.

Items to be addressed:

1. Motor access
   -Hopefully, I can insert through the exhaust and seat using access through the intake.
2. Exhaust length
3. Stability estimation
4. Exhaust nozzle durability
   -Maybe epoxy/foil lined

The primary item that I need to address is the length of the exhaust tube. I believe that the exhaust nozzle is about 1.5" too long. Shortening the nozzle would necessitate moving the MMT AFT about the same distance. This will have a much smaller effect with the motors that I am likely to fly but the idea that got into my head was to use DGS to allow a wide range of motors while minimizing weight, which leads to the big question:

I have no reference for determining stability of this beast. With everything through the AT F72 single use, OR calculates the margin to be .5 or higher without consideration of DGS. This feels like cheating the concept. I may end up modifying the design to the point that I have slightly better than marginal stability at the low end and negative at the high end. As long as I have the ability to adjust nose weight, I should have the flexibility to explore the idea.

I am already modding the design as I post this so that I don't forget the ideas. I'll post an updated file later.

View attachment Andy1_DGS_rpt.pdf

View attachment Andy1_DGS_BT60_TRF.ork

Nice looking design. Based on my discussions with Dean, I think that any added stability from the effect he describes will have negligible effect on a rocket that big.

Oh, on my first try, I had a thin aluminum soda can (top and bottom removed, fit perfectly) in the base. It was held snugly with two very small screws. On the new one, I left it out and there was hardly any soot in the base tube.

Note that the red rocket shown in his article did not function properly when built to an early reference design/guidelines (may have been an earlier version than is shown there).

rstaff3 said:

I don't actually have any answers. wfcook and I have both taken a shot at one of these. Our first tries did not work out as planned :surprised:

Since that article, Dean has updated his reference design. I'm going out on a limb and am posting it. I reworked mine and it performed much better. It is 2.125" in diameter and 18" long. I flew it on an E12 and it was stable at the start of the burn when the trust was highest. I haven't heard back from Dean regarding these results. I plan to try again on a motor with more thrust and a flatter curve.

View attachment 265766View attachment 265767

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That's cool. Maybe drill more holes farther down - make it look like a recoiless rifle round. Maybe just a bit of a lip on top of the induction tube to help catch some more air flow at lower coasting speeds (or would that count as an unacceptable fin?) . This is just kind of a single, nasty ring fin. An F12 motor with its tiny nozzle, long even burn and lots of gunky exhaust blowing lower and cooler down the tube (yuck, who is going to clean that). Maybe some intake vents above and running through the motor mount, parallel to the motor tube. Get some of that Sergei Korolev open interstage action! Maybe a pointier nose cone. Anything to get more airflow by pass around the exhaust plume. I would be SOL on this one when the RSO asks "did you do the math?" All I could say on something like this is that it really "SUCKS" and hope the real rocket scientists at the club take it the right way.

The idea of air scoops is a good one and that would be my next step. I wanted to try a totally finless/scoopless one first.

You are right about the bottom beginning to look like a ring fin. The water rockets folks use a ring fin that is the same diameter as the rocket body but these have a lot longer gap. Make the gap even longer and leave off the tube and you get a bottle rocket of sorts.

I'm not exactly sure why Dean recommends a blunt nose but I do know he understands this stuff better than everyone else put together. Mine is blunt by accident. The nose cone is a spray paint cap and a ball from a ball pit. Glued together with the un necessary parts excised.

The oddroc built from the junk pile should go into a Museum of Fame! Hope you guys have lots of success.

rstaff3 said:

Nice looking design. Based on my discussions with Dean, I think that any added stability from the effect he describes will have negligible effect on a rocket that big.

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In the PoF article he refers to using this principle to augment the stability of small finned rockets and notes that nose weight is still required for these. With no nose ballast, all motor choices have positive stability margins up through F32. Through E30, only two motors simulate below .5 cal. The E9 sims at .283 cal and may be a good candidate to begin exploring the concept after a couple of shakedown flights on D12's.

I can not say yea or nay, but after seeing a few failures, I remain unconvinced that this will significantly effect the stability. I would love to see it work.

rstaff3 said:

I can not say yea or nay, but after seeing a few failures, I remain unconvinced that this will significantly effect the stability. I would love to see it work.

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Thanks for the lack of confidence:wink:

Seriously, it was naysayers motivated me to follow through with the Fatter-By and tumble recovery. I definitely respect your experience here, there is a reason that you are doubtful. That makes you a very good devil's advocate for me. Keep the thoughts coming, good, bad or indifferent, they all contribute.

By adding an adjustable nose weight, I can easily change CG if necessary. As designed, no provision is made for this so even adding a nut to the NC will bring the CG forward and increase the stability margin.

If there is any evidence that GDS has a positive effect in such an arrangement, a later version may be built a bit smaller. The size chosen here was partially for ease of experimentation during construction. An 18mm version will be a bit cheaper to experiment with.

We'll be ordering parts shortly and the cutting will begin!

Certainly don't let any negativity on my part hinder your efforts. Like I said, I look forward to seeing your progress!

rstaff3 said:

Certainly don't let any negativity on my part hinder your efforts. Like I said, I look forward to seeing your progress!

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On the contrary, doubt spurs me on! I welcome your perspective. The thing is, you have some experience with this concept and I am a self made expert after reading one article:surprised: The image that you posted a couple of days ago prompted a design change to compensate for the length of the exhaust nozzle. As I said before, I welcome all comments, good, bad or indifferent. I have a tendency to fly by the seat of my pants and skepticism helps me to maintain perspective.

BTW. I had enough BT-70 for the airframe so that is what I am using. I'll post data later so you can see where this is going.

Please be aware that the terminology used in the article is misleading. Aerodynamic stability requires that a body attempt to return to a trimmed angle of attack (zero for a rocket) after encountering a disturbance. Mathematically, that is:

Cm = 0 when alpha = 0
dCm/dalpha < 0

Mr. Black's equations show that for a RAM-D "stabilized" rocket, the following is true:

Cm = 0 when alpha_dot = 0 (rotation rate is 0)
dCm/d(alpha_dot) < 0

What this basically means is that a RAM-D rocket is not aerodynamically stable, just that any rotation rates are slowed enough that it doesn't over-rotate before the engine cuts out. Keep that in mind when building/flying a rocket of this type. (I have further misgivings about the physics involved, but these conclusions are based on the equations that Mr. Black provides.)

TheAviator said:

What this basically means is that a RAM-D rocket is not aerodynamically stable, just that any rotation rates are slowed enough that it doesn't over-rotate before the engine cuts out. Keep that in mind when building/flying a rocket of this type. (I have further misgivings about the physics involved, but these conclusions are based on the equations that Mr. Black provides.)

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Although I am just a bit lost in the equations, I believe I get the concept of what you are saying and I my own gut feeling tells me to not to rely on the concept initially. The first flight will be an airframe of proven stability (swing test). I intend to use an adjustable nose weight to maintain a predictable margin of stability and proceed carefully when the margin starts to get tight. I do not intend on relying on GDS to stabilize an otherwise unstable airframe unless the concept proves effective.

I appreciate your input Brian. My job here is to keep my head in the clouds, so I am documenting this on the forum so y'all can keep my feet on the ground.

No real progress to note at this point. I cut body tubes and a MMT that I had on hand last night. Today, I picked up some 1/32 sheet and a balsa block to begin playing around with the boattail. I'll be ordering the rest of the parts this weekend.

As promised, the BT-70 file is attached.

View attachment Andy1_DGS_BT70_TRF.ork

I got parts in the mail this weekend!

I can't seem to upload files at the moment, so I'll post the play by play and add pics when I get the problem figured out.

I had originally planned on building this as a BT-70 airframe but decided to scale down to BT-50 at the last minute due to economies of scale. We have a couple of other projects waiting in the wings that need 24mm tube, so this got the most bang for the buck when we placed the parts order. The boattail also entered into the decision. I had been playing with the idea of scratch building it, just in case, and decided to scale it down to make that process simpler if it came to that.

My first efforts were focused on the boattail. I have never made one before and I learned a lesson or two along the way.

1. Start with a shorter nose cone. I had to do quite a bit of shaping after I cut it to length.
2. Use plastic if possible. Hollowing a BNC-50Y took a little while.
3. When working with balsa, cut the shoulder to length before thinning the walls.

The tail is 1.5" long with a 1/8" shoulder for gluing. I had planned on fitting it with a larger exhaust tube in order to protect it from heat but scaling this down to a 24mm body tube made that a bit of a challenge. In the end, I fitted a BT-20 exhaust and coated the ID of the tube with epoxy. I am relying on cool air from the intake to protect the tube. This will make motor installation quite a challenge but I have a couple of ideas in mind to deal with this.

The MMT is of standard construction and the AFT end is recessed 1" in the BT. The AFT centering ring is 1/4" above the AFT end of the MMT and I am planning on 1/8" of overhang. This will allow for 1/2" cut outs in the body for fresh air intake and allow for a couple of different options for motor retention.

Next on the list is cutting fins and intakes. The intakes will span the fin roots, so my plan is to mark the length for the intake, glue the fins and then cut the tube. I will be using epoxy fillets since the fins will be providing a structural element to the airframe. A layer of glass over the fin can may be in order as well. The only thing keeping the exhaust section on will be the fin and a thin strip of airframe under the fin.

As always, your thoughts on this are appreciated.

I'll add some pictures when I get it sorted out.

Still can't upload images so here is a link to the album.

LINKY

Yeah, hollowing balsa is tedious. The smaller the part, the more chance clumsy hand will ruin the part. Your linky presentation is just fine with me.

rstaff3 said:

Yeah, hollowing balsa is tedious. The smaller the part, the more chance clumsy hand will ruin the part. Your linky presentation is just fine with me.

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I rather enjoyed the process and ended up with a passable piece in the end. I think that it was MUCH easier than a stick built part.

I may just keep the album updated and just repost the link as I make progress. That may have the added benefit of having previous photos close at hand.

I have finally been able to get some more work done on this project.

Since the last update, the MMT and boattail have been installed, the aft end has been glassed, fresh air inlets have been cut and fins have been installed.

Link

The aft 3 inches were glassed to reinforce the inlet area. I used two layers of glass with the second layer being a sacrificial layer to protect the first during final shaping and finishing.

The inlets are about 1/2"x3/4" and should allow adequate fresh air to the exhaust stream.

The fin fillets were made using some left over 9309 adhesive filler at work yesterday. This is a strong, light weight material that just happened to be available after a repair.

Next on the list is to make the nose cone with an adjustable weight system. I am planning on hollowing the BNC-50 cone and epoxying a nylon nut in the end and using an allthread for the adjustable weight. I am hoping to use nylon for the allthread so that I can extend it to the shoulder area for easy access. What is you experience in doing this?

The next two weeks or so will be a washout. I'll be gone for work and then heading out on vacation, so I won't be doing much on this during that time other than mind sims.

Thanks for the update, it's coming along nicely. I have only done an adjustable nose weight system once. It did use nylon all thread, but was on a 38mm 5.5" dia. rocket. I mainly did it in the event I wanted to use that expensive cone on a project that didn't need any weight. The exact design wouldn't be appropriate for a small rocket but the general idea is fine. That is, there is a lead filled compartment that is screwed in down the all thread, which is attached to the tip of the cone.

This one lends it's self better to a replaceable design i believe. I was thinking of making separate rods of specific weight. If I am able to use steel or aluminum nuts for weights, I could also shift the CG if needed for a specific setup.

For a rocket that size, that sounds like it would work. Lots of ways to skin that cat.

Rap...rap...rap...anyone home? Anxious to see your progress...

Sorry for the delay. I am a bit of a slow builder and on vacation to boot.

I am currently working on the nose cone. I have cut off the tip and made a well for an 8x32 nylon nut. The aft portion has been hollowed some for a long screw, and I have a bag of plain he nuts for weights. I'll add to the pictures soon but the building is a bit slow right now.

My son and I are also working on a couple of kits along the way. He's building an Estes Hornet and I am finally building the Bullpup that I have wanted for so long.

It's nice to know I have a follower for this!