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# Thread: New GDS document from Dean

1. So I hadn't fully worked out what upscaling by motor size would look like.
I was out doing laundry, started plugging motor sizes into my scaling spreadsheet, and came up with the following representation.

It'll be interesting to see the effects of composite motors at smaller scales. Make that Necessary if I ever want to try this out in a Hi-power setting. Although I dislike the name, maybe a trip to BALLs will be required (Research motors? I do research rockets!)

Nyt riding a Dino included for scale. (The dinosaur was a bit taller than me, I'm ~2m)

On a previous gas induction thread a few years ago I wrote some equations for a Control Volume analysis. The formulation is in the attached file.

Attachment 323170
I think I've seen your sketch before when i was originally researching induction.
It's a good way to start the problem, but it looks as if you're forces are dependent on the velocity of the rocket driving air entering the control volume while neglecting the exhaust's role in the dynamic stability.

I'd be down to collaborate on fleshing this out if you're interested.

3. Originally, I put three C.V. analysis diagrams in a thread by BigDaddy found here:

http://www.rocketryforum.com/showthr...amic+induction

I found that several good GDS (Gas Dynamic Stabilization) threads on TRF can be found using "gas dynamic induction" as a filter.

Nyt, I would be interested in exchanging ideas here, but I have too many project going to devote too much time to this subject. Conceivably, this project on a theoretical basis can be as big as the report that was given on a different thread discussing equations for optimum piston launch volumes. (I was thinking of printing out that report and studying, but don't have the time now.)

It's a good way to start the problem, but it looks as if you're forces are dependent on the velocity of the rocket driving air entering the control volume while neglecting the exhaust's role in the dynamic stability.
You are correct that the simplified C.V. analysis on post #30 does not include the exhaust. As a minimum it looks like the restoring force (perpendicular force) should have something like the equation shown in post #30. Note that in this analysis we don't know exactly what the mass flowrate into the rocket model is. One could always use the external velocity multiplied by a typical flow area and use a fudge factor. Perhaps, the fudge factor could be determined by a CFD analysis, assuming that the fudge factor is constant and does not change much with angle of attack. Probably, after motor burn-out the restoring force will have this simple relationship. I tend to think that partial cut-out vents, which your designs seem to have, may offer offer more surface area for the development of side force than struts that have hardly any surface area at all.

While the motor is burning, one can bring up the issue of the exhaust. As I recall (I could be wrong) I think Dean had all the incoming flow acquiring the exhaust velocity. It this was so, it was extremely optimistic. If the incoming flow is accelerated to some fraction of the exhaust velocity, but ends up exerting a force exactly in line with the model axis and goes through the c.g., it will have no moment-arm and will not exert a restoring force at all.
Last edited by aerostadt; 4th July 2017 at 11:12 PM.

4. Ah yes, I remember finding that thread! I'm 85% positive I found all the induction threads on the forum during my initial research phase (before the rocket got built )

I really should return to setting up simulations for the 18mm version. If I can set an inlet at the motor nozzle with roughly suitable values for the exhaust of blackpowder/composite motors, it could give me an idea of what kind of pressure drop is created (reducing some of fudge content of your fudge factor. Diet fudge factor anyone?).
Something about the inlet air acting on the inner walls of the tube being solely responsible for the stabilization effect just doesn't sit right with me. Especially looking at that low pressure region playing tricks with the incoming air.

This is way more fun than filling tube spirals and waiting for paint to dry!

Edit: Yeah, I do have inlet cutouts. Those are based off of Dean's suggestions for longer induction tubes. The strait gaps with the support struts are equal to double the tube cross section area, but the cutouts are there to avoid thrust loss in the tube (see his 2nd paper I think?). I'm breaking about 3/5 of his guidelines in order to keep this model's geometry, so I know I'm asking for more trouble than this design already has. Totally worth it to weird out the need-fin purists!

5. 6 months, time for an update I guess!

I haven't had much time for building lately, and less opportunities to fly, but I was able to get a flight of the GBI mk18 on Saturday.
I used my last AT D10 to see what would happen. Took a second for the sonic igniter to light the old W propellant, but.....

Turns out, it makes awesome liftoff photos! (Rocketeer's Corner)

Unfortunately, about 1 second after that photo, the rocket lost it's flight line and started to skywrite. I'm examining the thrust curve of the D10 and running some flow numbers to see if I can glean anything from the results. I also REAlly need to change that induction tube, because the interior is crying out for help.

Last edited by Nytrunner; 6th December 2017 at 04:59 AM.

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