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  1. #1
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    New GDS document from Dean

    Dean sent me a new document on induction stabilized rockets, which I have reposted with his permission. It doesn't look like there is any new stuff that will help us design them but it does present a real world pr'fessional rocket that functions via GDS. He also has specs on the thin walled metal cans that he uses. No commentary on the NAR safety code is needed, that horse has been well beaten when he first presented the concept.

    Enjoy!

    Induction Stabilized Rockets Revisited, 2016(1).pdf

    Dick Stafford
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  2. #2
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    Pretty cool.


    John S. ---- NAR #96911 ---- TRA #15253 ---- MDRA #067 ---- BARC #028
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  3. #3
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    This is a nice read and I think is better than some of the previous versions. The distinction between the so-called induction principle (vents are near the aft-end) and the GSD (vents are near the c.g.) is clearer and quicker to the point. I noticed that on Hall's model there is a "Not recommended" in the caption. I am not exactly sure what this means. Does it mean that the rocket is not as stable as a GSD or it does not fall within the full capability as a GSD. I noticed that Cook's model has "induction" in the caption, but it probably falls within Dean's definition of a GSD.

    I am now beginning to understand the gist of Dean's survey of metal cans. Since the motor is so far forward in the GSD, some fire-resistant material is needed. I am wondering if some other solution might be possible. For example using sodium silicate solution painted on cardboard or paper tubes. However, Dean has been looking at this subject matter for a long time and he should be aware of what works good and what does not. I keep thinking I should try this, but I don't know if I will get around to it.

  4. #4
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    Having tried four variations, I am surprised how little burning there is on untreated paper tubes. Some surface treatment would be good if you are going to fly them often.
    Dick Stafford
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  5. #5
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    I played with the idea on a BT-60 tube and was surprised that there was very little charing after I realized that the MMT ended up 1/2" further forward than I had planned. The very end of the boattail was a bit toasty but I was able to hog it out and fly it again without further damage.
    Bill

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  6. #6
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    Please tell me more about how your experiment worked out!
    Dick Stafford
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  7. #7
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    I built a marginally stable rocket modeled after the Painkiller. The motor ended up being recessed about two inches and I used a BT-50 liner in a balsa boattail that started out as a nose cone. It actually launched better after the boattail was hogged out, probably had a visit from Mr. Krushnik due to the length/diameter ratio of the induction tube on the first launch.

    My conclusions? I would not rely on GDS for stability at this point. For me, it is a means to open up other design possibilities by shifting weight forward. I used it in the Der V-Max build because it helped me to achieve a look that I was going for in the design. I was able to recess the motor forward about 4" and keep the CG in an acceptable range without sacrificing thrust. As far as the contribution to stability, it may have helped some as the tube fins on this rocket have a height of just uber 2" but I did add a small ring fin and the design is flirting with the aerodynamics of short-fat rockets, so base drag may have played a part. One thing to note though, I never got a good swing test with this one. I decided to launch because it looked like it would fly and my mindsim agreed. Both flights were reasonably straight and definitely stable. Did GDS help? Possibly, but I think that more research is needed to see how far I can go with it. Right now, I am simply glad that I can make use of the principles and have a little more freedom in design.
    Bill

    I won't be wronged, I won't be insulted, and I won't be laid a hand on. I don't do these things to other people and I expect the same from them.
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  8. #8
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    Interesting, thanks for sharing. In his latest, Dean makes a distinction between two types of induction: 1) Finless Re-direction of Air Flow—Air Inlet at or Near the Exhaust and 2) Induction Tube Stabilization—Air Inlet at or Near the Center of Gravity. Mine have all been the latter. And, I agree this shouldn't normally be relied on. But, it was fun trying it out.
    Dick Stafford
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  9. #9
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    I followed your thread and was encouraged by what I saw. I am nowhere near trying pure GDS but there is enough info out there now to make it a nice addition to the bag of tricks.
    Bill

    I won't be wronged, I won't be insulted, and I won't be laid a hand on. I don't do these things to other people and I expect the same from them.
    John Wayne, The Shootist

    In the Hangar:
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  10. #10
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    I had no intention of creating a profile or posting on TRF (been reading and mining technique info for ~1 year), but the topic of gas dynamic stabilization and finless rockets is irresistible.

    My goal was to create a semi scale Ground Based Interceptor (GBI) (which clearly has no fins), so I began researching options that wouldn't just turn it into a giant SRB looking thing. Starting with Dean Black's papers (google searches led me back to TRF, surprise surprise) and getting advice from my uncle and a friend of his (professional CFD men), I've begun trying to figure out some relations between the gasflows and any stabilizing effects.
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    However, as you guys have already figured out, theory and application are sometimes quite far apart. So I think I found an adult rocketry project of my own.
    I basically want to take this concept and (likely over the course of years) scale it up as safely and reliably as it will go. Introducing, the GBI Mk 18. I don't have Rocksim yet, but the barrowman calculation from OpenRocket gives it a .35 stability margin (since this is not a traditional geometry, it may have some trouble modeiling it accurately) and a swing test actually works, so I'm optimistic for its post-burnout flight behavior.
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    Now I just need the burn-ban in AL to lift so I can try it out.
    "I'm at least 70% confident about whatever I say (90% of the time)"- college me

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  11. #11
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    Quote Originally Posted by Nytrunner View Post
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    What flow simulator / CFD is that?

  12. #12
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    Quote Originally Posted by Nytrunner View Post
    I had no intention of creating a profile or posting on TRF (been reading and mining technique info for ~1 year), but the topic of gas dynamic stabilization and finless rockets is irresistible.

    My goal was to create a semi scale Ground Based Interceptor (GBI) (which clearly has no fins), so I began researching options that wouldn't just turn it into a giant SRB looking thing. Starting with Dean Black's papers (google searches led me back to TRF, surprise surprise) and getting advice from my uncle and a friend of his (professional CFD men), I've begun trying to figure out some relations between the gasflows and any stabilizing effects.
    Click image for larger version. 

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    However, as you guys have already figured out, theory and application are sometimes quite far apart. So I think I found an adult rocketry project of my own.
    I basically want to take this concept and (likely over the course of years) scale it up as safely and reliably as it will go. Introducing, the GBI Mk 18. I don't have Rocksim yet, but the barrowman calculation from OpenRocket gives it a .35 stability margin (since this is not a traditional geometry, it may have some trouble modeiling it accurately) and a swing test actually works, so I'm optimistic for its post-burnout flight behavior.
    Click image for larger version. 

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    Now I just need the burn-ban in AL to lift so I can try it out.
    Welcome on-board. I'll be doing a rain dance to help with that burn ban.
    Dick Stafford
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    The Original Rocket Dungeon
    Volunteer compiler of product news for ROCKETS Magazine

  13. #13
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    Part of what I liked about GDS is that you don't need nose weight and a statically stable model. Of course, without some natural.stability, there isn't much of a coast phase.
    Dick Stafford
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  14. #14
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    Quote Originally Posted by dhbarr View Post
    What flow simulator / CFD is that?
    This is the flow simulation package from Solidworks '15. For external flows, its actually pretty simple to get some roughly real results. Of course, the rough results include several neglected conditions and some assumptions which take alot more knowledge and experience (and a better computer) than I have. I show runs to my uncle and modify according to his verdicts: "You know, that looks reasonably accurate", "Mmmmmm......I'm not sure about what it did there...."
    "I'm at least 70% confident about whatever I say (90% of the time)"- college me

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  15. #15
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    Quote Originally Posted by Nytrunner View Post
    This is the flow simulation package from Solidworks '15. For external flows, its actually pretty simple to get some roughly real results. Of course, the rough results include several neglected conditions and some assumptions which take alot more knowledge and experience (and a better computer) than I have. I show runs to my uncle and modify according to his verdicts: "You know, that looks reasonably accurate", "Mmmmmm......I'm not sure about what it did there...."
    I do not know what Solidworks is. It looks like you have something like fluid path lines and the motor exhaust modeled. In a sense you have external and internal flows. The exterior flow from the front and around the model is external flow, but the motor inside the airframe is like internal flow (although, you do not have the internal burning inside the motor itself.) If you do have a true CFD simulation, you could give the model a slight angle of attack, the calculate the pressure distribution all around the vehicle, and upon integrating the pressure distribution find the correcting force or moment.

  16. #16
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    Quote Originally Posted by rstaff3 View Post
    Part of what I liked about GDS is that you don't need nose weight and a statically stable model. Of course, without some natural.stability, there isn't much of a coast phase.
    No sorcery needed hopefully . I know my nose ended up weighty because I used paper forms (what you guys would call shrouds probably) with inner coatings of epoxy and cardboard bulkheads to make the nose profile for the GBI. No regrets: I could probably drop it nosefirst from my apartment balcony without damage.
    "I'm at least 70% confident about whatever I say (90% of the time)"- college me

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  17. #17
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    Quote Originally Posted by aerostadt View Post
    I do not know what Solidworks is. It looks like you have something like fluid path lines and the motor exhaust modeled. In a sense you have external and internal flows. The exterior flow from the front and around the model is external flow, but the motor inside the airframe is like internal flow (although, you do not have the internal burning inside the motor itself.) If you do have a true CFD simulation, you could give the model a slight angle of attack, the calculate the pressure distribution all around the vehicle, and upon integrating the pressure distribution find the correcting force or moment.
    Solidworks is a CAD and design software with numerous useful addons like FEA, motion studies, Flow Simulation, etc...
    I tried the opensource SimFlow (based on OpenFoam software) but the "free" version only allows 100,000 computational nodes which is woefully inadequate for even a half-rocket cutaway like in my case above.

    You read the flow pretty close. The tubes are fluid streamlines colored by pressure (the predominant yellow corresponds to low altitude atmospheric pressure), and you can see the flow being drawn in by the motor exhaust. Speaking of exhaust, the red/yellow/blue jet corresponds to decreasing velocity of the plume as it expands (I was really happy when I got that modeled successfully). Near the vents, you see teal and green contours that represent pressure below the free stream reference (atmosphere) and those contours appear again as pressure rises down the length of the induction tube. The lo pressure region and streamline deflection match wonderfully (you can almost see a nozzling effect as the outside air is introduced to the plume)

    As far as Internal vs External, I use an External case because the conditions Inside the induction tube are influenced heavily by the conditions outside. Internal flow is defined as 'bounded by a surface' for which I'd have to create Lids and Boundary Conditions for. Since neither my job or life depend on this, I'm content to let the External conditions and motor plume work out those conditions for me .

    As for angle of attack, I think we're on the same page. It took awhile to get the plume and freestream behavior working right (Solidworks FlowSimulation is notorious for crashing ), so that is the next case I'd like to run provided I get the time to work on it. Harvesting the quantified pressure distribution may be something else entirely. I haven't tried that yet and I'm predicting another chat with my uncle when it comes time for it.
    "I'm at least 70% confident about whatever I say (90% of the time)"- college me

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  18. #18
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    Quote Originally Posted by Nytrunner View Post
    Solidworks is a CAD and design software with numerous useful addons like FEA, motion studies, Flow Simulation, etc...
    I tried the opensource SimFlow (based on OpenFoam software) but the "free" version only allows 100,000 computational nodes which is woefully inadequate for even a half-rocket cutaway like in my case above.

    You read the flow pretty close. The tubes are fluid streamlines colored by pressure (the predominant yellow corresponds to low altitude atmospheric pressure), and you can see the flow being drawn in by the motor exhaust. Speaking of exhaust, the red/yellow/blue jet corresponds to decreasing velocity of the plume as it expands (I was really happy when I got that modeled successfully). Near the vents, you see teal and green contours that represent pressure below the free stream reference (atmosphere) and those contours appear again as pressure rises down the length of the induction tube. The lo pressure region and streamline deflection match wonderfully (you can almost see a nozzling effect as the outside air is introduced to the plume)

    As far as Internal vs External, I use an External case because the conditions Inside the induction tube are influenced heavily by the conditions outside. Internal flow is defined as 'bounded by a surface' for which I'd have to create Lids and Boundary Conditions for. Since neither my job or life depend on this, I'm content to let the External conditions and motor plume work out those conditions for me .

    As for angle of attack, I think we're on the same page. It took awhile to get the plume and freestream behavior working right (Solidworks FlowSimulation is notorious for crashing ), so that is the next case I'd like to run provided I get the time to work on it. Harvesting the quantified pressure distribution may be something else entirely. I haven't tried that yet and I'm predicting another chat with my uncle when it comes time for it.
    Cool analysis...far beyond my abilities.
    Dick Stafford
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  19. #19
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    Long time, no update on the GBI mk18 (Great Big Inductor, 18mm).
    I flew it with the slight base flange a couple times. Mixed results that were kind of encouraging, but I wasn't entirely pleased with them.

    First flight on a B6-2 traced a Z in the sky. No exaggeration. For whatever reason it made two rather sharp turns at about ~100 and ~150 ft up.
    It also ripped the shockcord from my thin cardboard nose bulkhead, so the epoxy and paper shroud nose separated and landed on its own (that thing's rock solid).
    LCO made the most of it: "And we have smooth capsule deployment. Booster away!"

    I reattached the shockcord with epoxy this time. Second flight on a C6-3 traced a kind of logarithmic path to deployment. Slight wiggle at burnout, but nothing crazy.

    Worth noting is that both these flights had only barely noticeable black spots in the induction tube from exhaust. Looks like the additional airflow did a great job of protecting the tube interior.
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    For the third flight, I cut off the flange, and put a coupler in the end to friction fit either the flange, or a new ring piece with the same height and diameter. Both of them were made to match what looks like a loading ring on the real ground-based interceptor. This ring is on no way sufficient to stabilize a conventional rocket of this dimension, but I wanted to see how it affected the GBI mk18's behavior.

    It was the best flight of my inductor yet on a C6-5. Almost perfectly straight.
    Unfortunately, my Estimeter jammed the parachute and it nose dived onto its open body tube. The chute is kinda charred.
    Further inspection shows that the rougher coupler caught more of the exhaust and started to char, which then caught and dislodged the ring piece, burning it too.
    Plus the motor hook pushed forward and crimped the motor tube a bit.
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    Autopsy results:
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    I'm going to save the nosecone, extract the shockcord from the old motor mount and make a new body/induction section.
    It won't have the coupler in the back, I'll build it with the ring from the start, make the portholes cleaner, and actually paint it!
    After the ring testflight, I'm encouraged for the chances of upscaling. If this rebuild works out, GBI mk24 will be greenlit!
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  20. #20
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    Great report. The ring seemed to help that little bit but, as you noted, it hardly counts.
    Dick Stafford
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  21. #21
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    My one real regret is not getting a video of that final flight.
    Because of the unorthodox design, I launched it myself and didn't think to hand off my phone.

  22. #22
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    Success! The damaged 18mm GBI has been sitting in my apartment mocking me, so I finally took some time to fix it instead of rebuild.

    I flattened out the tube damage, trimmed the motor mount so I could get motors in again, added a blocking ring (the motor hook was traveling forwards....), and fixed the little cradle ring onto the aft end.

    Took it out to test on my way to buy filler primer:


    My favorite shot is just when its a few feet off the ground and outlined by the trees. You can see it flying straight up, and the flame is clearly visible through the ducts.
    The parachute worked this time also, and I've got it back.
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    Induction tube is showing some heat damage, and part of the inside has ablated away. I'm going to attempt to reinforce it with wood hardener, or just coat it with aluminum foil.

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    "I'm at least 70% confident about whatever I say (90% of the time)"- college me

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  23. #23
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    Decided I'd stick this back here and not drive the RAIS thread off course.

    The GBI mk. 18 behaved quite well on a C6-5 at Southern Thunder this weekend (it got squirrely the last time it had a chance to perform for a crowd).

    A fellow HARA member took this amazing photo just after liftoff and posted it on his blog (Rocketeer's Corner)
    I include it below with his permission.

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    The whole rack was tilted out away from the trees/flightline, so I loosened the rod chuck and clamped it between the teeth so it stuck up more neutrally vertical. The GBI launched from in between the 2 and 3 signs, and you can see the direction of the wind by the plume drift. The issue I'm running into is that the C6 has such a short burn that any dynamics action cuts out at the end and it starts to wiggle. Now I really want to put my last D10 in there and see what happens. Slightly shorter burn time, but ~2.5x the propellant massflow when comparing blackpowder and composite characteristics.
    I think its interesting to compare the difference in vectors between the flight of the rocket and the wind. For those that like to do "Mindsims" *shudder*, which way would a finned rocket be flying in that wind?

    Its had about 8 flights on it now, and the induction tube interior is definitely looking ashy. I coated it with Minwax ~3 flights ago, and that's helped, but I'm considering removing the current tube and replacing it with one that's removable. (2-56 nylon screws may turn out to be useful for more than just shearing!)

  24. #24
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    I saw the photo on the blog, thanks for the report. I agree a D10 would be a good choice. The more mass flow the better. The removable tube would be useful too. You have as many flights on that one than I have on all my GDS rockets.
    Dick Stafford
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  25. #25
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    Quote Originally Posted by rstaff3 View Post
    I saw the photo on the blog, thanks for the report. I agree a D10 would be a good choice. The more mass flow the better. The removable tube would be useful too. You have as many flights on that one than I have on all my GDS rockets.

    I really think that has more to do with my own laziness than any robustness of the rocket lol. I've been optimistically tweaking the inlet holes and making display wraps instead of buying parts for the mk. 24.

    Upon successful D10 flight, I will have to start the upscale. I promise!
    I've even got another club member offering to let me use his wood lathe for the cone.

    I'm wondering how an E9's longburn will perform....... (provided I can find a safe pack)

    ***Scratch massflow, that's wrong*** I meant exit velocity!

  26. #26
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    I think mass flow is accurate, but momentum more accurate .
    Last edited by rstaff3; 28th June 2017 at 10:15 PM.
    Dick Stafford
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  27. #27
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    Quote Originally Posted by rstaff3 View Post
    I think mass flow is accurate, but momentum more accurate .
    I can get on board with momentum. The D10 has less propellant mass to start with.

    If I'm remembering his theorem correctly, its actually the entrained air massflow that is the prime contributor. To draw more air into the mixing tube, I'll need a bigger pressure drop near the nozzle exit which should result from increased exhaust velocity. Thus, I'll lean towards momentum (velocity) increase contributing more than massflow.

  28. #28
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    Now that you mention it, the air flow momentum is the mechanism involved so you are correct. Looking back, my jump from BP to composites mostly involved larger class motors with higher average impulse. Propellant mass was also higher but not by much.
    Dick Stafford
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  29. #29
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    Quote Originally Posted by rstaff3 View Post
    Now that you mention it, the air flow momentum is the mechanism involved so you are correct. Looking back, my jump from BP to composites mostly involved larger class motors with higher average impulse. Propellant mass was also higher but not by much.
    Unfortunately, I'm still hanging out in the land of conjecture because I've been building rockets more than analyzing Dean's equations (I'm still a little suspicious about their mechanics). And its been a lot more fun to experiment......I'm a bad engineer when I'm not being paid for it lol.

    Edit: Just saw your Cheat Sheet page. I really need to steal that safe/un/marginal chart the next time they let me RSO low/mid power at a launch (and have a scale!). I correctly marked 4/5 suspicious rockets as Heads-up, based on feel, but if I can have data at the desk it should cut down the amount of arguments that tend to happen.....

  30. #30
    Join Date
    27th October 2009
    Location
    Brigham City, UT
    Posts
    2,150
    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.

    GDS Inductor.pdf


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