Entrance area

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rafa
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Entrance area

Post by rafa » Thu Jun 16, 2005 5:53 pm

Good afternoon.

There is so so so far away the last time that i can write here , and my english is so ¿oxidizer?. I really am sorry and i will write the best way that i can.
My question is very easy , somebody knows what is the minimum entrance area for a ramjet???
I have a knew project in mind , build a RC model helicopter with ramjet tips. What do you think about it?

Thanks a lot.

rafa
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re: Entrance area

Post by rafa » Fri Jun 17, 2005 11:47 am

Thanks , Ben , i understand wat you say.

Al Belli
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re: Entrance area

Post by Al Belli » Fri Jun 17, 2005 12:02 pm

Hi rafa,

This may be of interest to You.

Al Belli
Attachments
PRACTICAL SUBSONIC RAM JET DESIGN.doc
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rafa
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re: Entrance area

Post by rafa » Fri Jun 17, 2005 3:23 pm

Thanks Al Belli , but i have these document many years ago and i don´t remember if it says something about the minimun entrance area.In the document that you post is impossible to see the diagram about the entrance area and thrust.

Thanks.

Glenn Olson
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re: Entrance area

Post by Glenn Olson » Mon Jun 20, 2005 7:48 pm

Rafa posted:

>Thanks Al Belli , but i have these document many years ago and i
>don´t remember if it says something about the minimun entrance
>area.In the document that you post is impossible to see the diagram
>about the entrance area and thrust.

Try http://www.alt-accel.com/arla/decker/ramjet.htm

I've found nothing in the literature about a minimum size for ramjets but I'm sure that there is a practical limit. The smallest I've heard of is approximately bullet size used in ram accelerators. The smallest I've seen for open air ramjets is the one pictured by Windspire (http://www.windspire.com/jetpower/) which is about the size of two large fists.

For open air useage the limit would depend on maintaining a flame and keeping heat losses through the sides to an acceptable percentage. For something really small you may need to go to a special fuel such as high percentage peroxide with a catalyst screen inside the ramjet. There are other fuels that are very combustible, such as ethylene (C2H4) which NACA used extensively. Or you could go with a bi-propellant such as acetylene and oxygen. With one of these methods you might get down to a thumb-sized ramjet but plan on some experimentation.

Glenn

rafa
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re: Entrance area

Post by rafa » Wed Jun 22, 2005 5:39 am

Good Morning.

I want a 10 to 15 cm of lenght ramjet , no more . I think that i can use simply gasoline , with the maximum purity , adding a bit amount of Nitromethanol ( very rapidly combustion near an explosion )
Can i make the body of the ramjet larger than one ramjet bigger , in compartion with the entrance area???

Thanks a lot.

Glenn Olson
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re: Entrance area

Post by Glenn Olson » Wed Jun 22, 2005 5:52 pm

Rafa said:

>I want a 10 to 15 cm of lenght ramjet , no more . I think that i can use
>simply gasoline , with the maximum purity , adding a bit amount of
>Nitromethanol ( very rapidly combustion near an explosion )
>Can i make the body of the ramjet larger than one ramjet bigger ,
>in compartion with the entrance area???

If Nitromethanol is as you say then you should have no trouble holding the flame. Keeping the flame length short may require a grid-type flame holder.

If you've seen the ramjets on the proposed Triebflugel and the Windspire then you know that they can be made short relative to their lengths. One of the important aspects is the flight speed you are planning for. Low speed inlets typically have fat lips and short diffusers. High subsonic inlets typically have thinner, or sharper, lips with longer diffuser sections. I suggest you do a web search of early jet aircraft and match your inlet shape to a jet with similar cruising speeds. With that shape in mind you can size the rest of your ramjet to fit within your length.

As an alternative you can get a larger diameter, and hence more thrust, by using inlet and exhaust cones, but this gets more complex.

Glenn

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