Gas-dynamic Stabilization

With over a thousand views I bet you have quite a bit more than just one follower. Inquiring minds want to know: is GDS for real?

I will give my opinion if a fellow MDRA'er got a good video of the flight of the Inductor! tap, tap, tap....just waitin'

Here is the latest update on my progress. Link

I cut about 3/4" off of the BNC-50Y nose cone and hollowed a well in the tip for a nylon nut. I then hollowed the bottom portion of the cone for access to the adjustable ballast. As you can see from the photos, the nut is off of the center line. This required me to enlarge the bore in the nose cone so that the screw/allthread does not contact the side.

In the next to last photo, you can see the short version of the rocket. The final length will be determined with some swing testing. The original design calls for an overall body length of 16" but I plan on doing some initial base stability testing in both the short and long configurations.

The last photo is the Bullpup that has been distracting me this week. My son and I always build a pair of rockets on vacation each year. We do it in bits and pieces throughout the week and shoot paint when we get home. I have wanted a Bullpup since I came back to the hobby and decided to do it this year. My son is building an Estes Hornet with a 24mm mount.

Well, I couldn't get the 8x32 nylon allthread at a price that I was willing to pay so I grabbed a steel one at HD yesterday. I'll play with it today and see what I come up with.

Well, I have something to call an adjustable nose weight. The allthread weighs in at 9g and each nut is about 1g. I am still trying to get my hands on a nylon piece. I'll be checking the local Granger Friday for a 3/16 rod that I can thread. I'll be playing with the OR file to see just what I'm going to need with the final configuration and doing some basic finishing until then.

PhotoBucket link

You can see that the allthread is a bit crooked, but it shouldn't present a problem.

Whoops! I forgot to upload the OR file that most closely matches the current status of the build.

View attachment Andy1_DGS_18mm_Post.ork

Nothing to see here right now. I ran into a snag on the nose cone. I wanted to see how well the aerosol primer that we use at work would do on balsa, so I sprayed the cone after doing a little touch up job. Well, it turned out to be a bad can and the primer never cured. My touch up job had to be completely cleaned and re-done and my NC was still sticky after 2 hrs in the oven. I managed to clean most of the primer off and baked it for another 2 hrs and it doesn't stick to me any more. To seal it, I used some left over 956 from work and that is curing right now.

Sorry for the long absence. Work, Scouts and baseball have all conspired against me for the last few weeks. I have been doing a little here and there, mostly priming and painting. I hope to do some ground checks this weekend to check stability with lighter motors.

I'll post some pics this weekend when I get back in town.

Well, glad you're back in the fold.

Thanks!

One issue that I am going to have to deal with is the nozzle diameter. Without thinking things all the way through, I used a BT50 for the nozzle as well as the MMT. My plan was for friction fit and tape thrust rings. It looks like I may be hogging out the nozzle a bit so that I can make everything work as planned.

Check out the diagram I posted earlier, which give rough guidelines on the L/D of the induction tube and specified additional venting for longer tubes. I didn't try to understand the 'why' on some of the constraints, but can say when I build mine to an earlier reference diagram, it was very marginal. When I modified it to comply with the new version, it worked.

Check out the diagram I posted earlier (post #4), which gives guidelines on the L/D of the induction tube and specifies additional venting for longer tubes. I didn't try to understand the 'why' on some the constraints, but can say when I build mine to an earlier reference design, it was very marginal. When I modified it to comply with the new version, it worked.

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So, it looks like I need to scale it down next time!

I did calculate the necessary intake area based on length and diameter of the nozzle. The problem that I have to deal with is that I am planning on 18mm motors with friction fit and a tape thrust ring on the motor, and the tube that I used for the nozzle is a BT-20, just like the MMT! I'll have to enlarge the ID of the nozzle so that I can slip the prepped motor in the MMT. I may be able to fish tape in through the intake for the thrust ring but I'll have to allow for clearance of the strip for the friction fit at least.

Oh, I see what you were saying. The rocket looks stable to me under any amount of nose weight. Looking forward to the flight report!

According to OR, it will be marginal with larger 18mm motors. I don't have the numbers on hand right now but I designed it so that the low end would have an acceptable margin and the high end would be iffy. I was inspired by the Painkiller for the sleek look and fin shape that would have a nice, balanced look as it scaled up and down to dial in the numbers that I was looking for. I'm not sure how much of the flight envelope we will get to explore but I am looking to get a couple of flights this weekend just to check basic stability. Once we can confirm that we have a safe base line, we can play with the effects of the gas stabilization concept.

I also have a Mini HoJo that I threw together that flirts with the concept. It is lengthened by about 1/2" overall with the motor recessed about 3/4". It is an otherwise stock build just to see where I can go with this idea. I figured that it would be a good one to play with since it is somewhat marginal to begin with. I have mind simmed it quite a few times and the longer airframe, coupled with the weight shift looks promising. I plan to send it up at the same time.

Swing testing was kind of interesting. When swing testing the rocket, i was having fits trying to stabilize it but if i added any nose weight at all, centripetal force kept the nose to the outside of the circle regardless of the string location. It may be a heads up flight on a B4 but I am seeing signs that it is trying to behave and I am inclined to give it a try. I'll chime in tomorrow with results one way or another.

Well, if it passes a swing test, it don't need no steenkin' GDS. Its flight will either be cool or really cool

Well, the idea was to start off with a basically stable vehicle with lower power, lighter motors (B) and use heavier (C and D composite) to test the theory.

Well, the maiden flight was a heads up launch since the swing test was questionable. The rocket came off the rod O.K. and arced over into a slight breeze. At about 100 ft, it corkscrewed 4 times as it continued its lazy arc and apogee was no more than 150 ft and landed under a partial chute about 70 yards down range. You can see in the attached photos that the nozzle got torched pretty good and I suspect the Dr. Krushnic payed a visit which limited power which resulted in the low altitude.

I did the calculations and had an intake that was more than adequate for the nozzle I was using according to my understanding of the concept. My thought is that using an 18mm motor and a BT-20 nozzle does not allow for enough air flow with the motor recessed as deeply as I have it here. I think that Orange Gush 1.0 needs to be retired and Orange Gush 1.1 needs to be a BT-60 airframe for a couple of reasons:

1. Inadequate space to prep the igniter
2. Tape thrust ring had to be done with the motor installed
3. BT-20 nozzle is too narrow

I like the general layout and I don't think that a complete re-design isn't necessary but I have learned a few things. This won't be the last one that I do, it is just too much fun to try something new.

I also attached a couple of shots of the Mini HoJo. Other than the stretch and recessed motor, it is a stock build. Four launches in a moderate breeze and all were arrow straight under thrust. It tended to weather cock slightly during coast but apogee was consistently about 200 ft on an A3-4T. I have a second one in the build pile that may get the 18mm treatment but I may have to lay up an over sized FG aft ring to make launch prep easier.

One more note on the day. I was finally able to recover the top section of the Fatter-By today after two months in the weather. Fortunately the tube was the only piece that seems the worse for wear and I have some on my bench ready to go!

Nice report! The initial straight flight is consistent with the test flights of my Inductor. My guess is that these need a fair amount of motor relative to what you would think. As the thrust drops they tend to get squirrelly and when it is gone, they are unstable (yours may do better slightly better if is was just marginally stable).

FAIW, Dean Black thought my Inductor was likely too heavy to work. Initially I added nose weight to get the CG where I wanted it. On an E is was totally unstable. On an F is started straight, then wobbled and then went unstable. I adjusted the Induction tube, removed nose weight and added tail weight...and it flew well on an F (still was unstable during coast). Another observation is that the L/D ratio of yours is far different than the baseline GDS rockets that Dean presented.

On the HoJo, how did it's static stability stack up? ie was it stable without any potential GDS?

rstaff3 said:

On the HoJo, how did it's static stability stack up? ie was it stable without any potential GDS?

Click to expand...

I didn't do any ground testing on the HoJo since the stability of the stock model is a known and the modifications that I made moved things in directions that would only improve the stability margin. By observation I can say that it flew very well on the smaller motor, even tolerating a moderate breeze quite well, which is not my experience with a stock version. In the end, the principles of GDS afforded me a way to lengthen the rocket and shift the CG forward without radically modifying the rocket. I am thinking that the next one may just get a longer induction tube with the motor shoved up even further.

Orange Gush v1.2 will have to be a BT-60 with an AT D21 I believe. I am thinking that high exhaust velocities relative to the tube diameter will make a difference here since that is what is going to be drawing the air into the tube. This conclusion based on extensive mind sim work and extensive consultation with the voices in my head.

I agree the bigger body and the D21 will help. I asked about the HoJo because I wondered if GDS was actually going on. It sounds like the benefit was more that the rocket was longer and the main rear mass, ie the motor, was further forward. I assume you are following the other GDS thread with aerostadt's analysis. That looks like the 'pseudo fin' deal was more of a factor in the HoJo than 'induction stabilization'. Both could be considered GDS.

Yes, I have been keeping up with the other thread and aerostadt's analysis is quite good but that thread is running a slightly different direction than what I am doing. I am, indeed exploring the pseudo fin effect. That is my reasoning for trying to start with an airframe that is stable with lower power motors but marginal with higher power. What I think that I saw Sunday is that, under power, GDS will help but once you begin coasting, the model needs to be basically stable.

My Inductor is basically retired for the reason you noted. It was interesting to see that it worked but it is kind of a waste of an F motor. I guess the altitude is probably as good or better than a saucer. Although I love my saucers and saucer-like objects, the wild spinning during coast annoys me. I think you hit the nail on the head as GDS, in my book, is better suited to marginally stable models where you can get stability without what daddyisabar calls 'performance robbing nose weight'

Yeah, I think that I am going to play around with the HoJo's some more and maybe build OGv1.2 but I don't think that relying on GDS for hobby rockets is an Avenue that is worthwhile for me to explore. The pseudo-fin effect may have some merit though.