noobie question

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heada
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noobie question

Post by heada » Tue Nov 21, 2006 5:19 am

Sorry for the noobie type of question but will the Maggie Muggs type ramnjet work at transonic or supersonic speeds?

I have the ability to launch a model rocket that weights about 25 pounds to just over supersonic speeds (mach 1+) (APCP motor that produces about 400lbs of thrust for 0.65 seconds, J2135 if you know common motor designations) I hold a NAR Level 2 certification so I can do this legally. If a design similar to the Maggie Muggs will work efficiently over mach 1, I should be able to scale up the design slightly and maybe have positive thrust for a second stage rocket. It would have to be tested somewhere like Blackrock (unless the army wants to let me test at WhiteSands) Also, as for using a spark plug as an igniter, why not use a rocket motor igniter? I have some that I use to start larger rocket motors that produce a ball of flame at about 2000 degrees for 1 second. Its 32 gauge wire and would burn off after motor ignition leaving nothing inside the motor to foul the air flow.

Thanks for your input and bearing with another noob.

-Aaron

tufty
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re: noobie question

Post by tufty » Tue Nov 21, 2006 11:17 am

Larry will probably chime in here, but here's my take.

The Maggie Muggs design was, I believe, intended more as a easy-to-build demonstration of the concept of a ramjet using easy-to-find parts than as a real, workable propulsion unit. As such, its thrust is likely to be suboptimal, although if you run it fast enough I think it would probably produce net positive thrust, even if "only just".

You're probably better off designing a ramjet to run at the speeds you're expecting to see. The math isn't too hard hard, and the construction needn't be, either.

Your igniter idea might work, but the timing has to be bang on, remember there will be air running through the motor at the point when you trigger it.

Simon

larry cottrill
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re: noobie question

Post by larry cottrill » Tue Nov 21, 2006 1:44 pm

heada -

Simon is entirely correct. Maggie was designed as a LOW SPEED ramjet - its geometry is unsuitable for really fast operation. What you need for a "classic" high speed ramjet is a LONG conical diffuser and much more conventional (i.e. small, low drag) flameholders. The exit nozzle also needs to be carefully designed for optimum efficiency.

The idea with Maggie was that a fast propeller plane could get it up to a speed where it could start operating to boost net thrust, pushing the whole thing up to a somewhat higher speed for decent operation. That's the reason for its awkward-looking diffuser and flameholder design and the extremely lightweight materials. The use of epoxy bonding for construction was just a totally experimental idea, not to be taken too seriously - even if Maggie turned out to be wildly successful at low speeds (very doubtful), I would never have used that construction method for a production version.

L Cottrill

jthompso
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re: noobie question

Post by jthompso » Tue Nov 21, 2006 2:07 pm

Drag losses, fuel weight, fuel delivery, static weight... There are so many problems associated with a ramjet stage in a model rocket--though they are not insurmountable. I'm in the research stage of building just such a rocket, but due to limited time and funds it'll be a long time before I ever actually start working on it. I think the only way that a ramjet can find use in a model rocket is if the rocket motor makes the transition into ramjet: liquid fuel motor to liquid fuel ramjet, or solid fuel motor to ducted rocket. The ducted rocket shows a lot of promise, essentially you would need to construct a motor that had some grains with full oxidizer and others with the oxidizer ratio reduced, you also need a way to allow air to enter the motor when it gets up to speed. The ducted rocket is the ultimate balance of weight to additional power. If you're still interested in a traditional ramjet to add to your rocket you should talk to Multispool--he built a model afterburner with a (I believe) low profile "v" flameholder which is what you need to reduce drag.

heada
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re: noobie question

Post by heada » Wed Nov 22, 2006 7:49 pm

Thank you all for your responses. I was not intending to build and use Maggie directly for production but was more curious if it could be used in high speed as well as low speed. I think I will build one (welded) and play with it so that I have something to compare to when I do build a high speed ramjet. Speaking of which, has anyone in the ramjet/pulsejet hobby built a working motor in the sizes/thrust classes as those made by professionals?

As for my igniter idea, how often do ramjets need to be restarted? If it takes 3 or 4 attempts to get the initial air/fuel mixture correct then my idea wont work but if the motors normally start on the first go (either rich or lean) then I think it might help the performance of the motor to not have the spark plug in the air stream.

Again, thanks for the answers.

-Aaron

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re: noobie question

Post by jthompso » Wed Nov 22, 2006 8:03 pm

You may have read it already but in case you haven't there is a very good paper on ramjet design in the main page of this site. If you start with the MaggieMuggs as the basis of your design you will be limiting yourself, unlike most of the people on the forum building ramjets you have the ability to bring the motor to the speed of sound where the ramjet is king. If you haven't found this paper yet you should read it as soon as possible, it will help you greatly in the design of your motor.

heada
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re: noobie question

Post by heada » Wed Nov 22, 2006 10:12 pm

I have read it and it does contain lots of useful information. When it comes time to build the first motor, I plan on studying the information with more detail. I am finishing up a few rocketry related projects before I start this ramjet project.

I did more research on what it would take for someone like me to launch a rocket boosted ramjet and the legal part looks quite daunting. The FAA has a classification for what I want to do called Unguided Suborbital Launch Vehicle (USLV) The rules and regulations surrounding getting a license to launch a USLV are more than likely out of my reach. I may be able to launch a small vehicle to less than 100,000 feet at the launch site at BlackRock, NV but nothing above that without a USLV license from the FAA.

-Aaron

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re: noobie question

Post by jthompso » Thu Nov 23, 2006 3:28 pm

Have you given any thought on how to test the motor once you've built it?

heada
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Re: re: noobie question

Post by heada » Thu Dec 07, 2006 4:35 am

jthompso wrote:Have you given any thought on how to test the motor once you've built it?
Good question...and I think that I've actually been thinking more about this lately than I have about actually building the ramjet.

I'm going to build an open ended wind tunnel with a large industrial sized fan suppling the in feed air. I have found one that I think will fit into my budget that will provide more airflow than I will need (7900 cfm) The Fan is 30" diameter and if I build the wind tunnel the way I'm thinking it would work, that 30" would go down to about 8" diameter and then open it to about 12" I hope it would produce a good test airflow that will not only channel air into the ramjet about also around the ramjet to help with cooling which should better simulate what would happen during flight. Naturally, I would have to test not only the air speed but also the air pressure as it exits the wind tunnel.

As for a test stand, I've been tossing around two versions of almost the same idea. Unfortunately, I've tried to type up a description about 3 times now and haven't found a way to describe it. My artistic skills are very lacking so I think I may just have to go build one.

As for building the ramjet, NASA has an interesting simulator called enginesim. I don't think it will quite do what I have in mind as it is mostly geared for horizontal flight, but it is still very interesting.

http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html

-Aaron

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