ideas

[Zippiot]

has anyone tried a solid fuel ramjet that gets initial thrust and ignition from a model rocket engine? ill draw a pic of wut im thinkin of to help, but basically its gonna be a rocket engine blowing thru a ramjet with solid fuel stuck to its walls. i have already made a few ramjets but i always ground test them with a blower to feed it air, how fast does the avg model rocket (c engine) go and will it be enough to power a small solid fuel ramjet?

[MJD]

A C engine carying any kind of load or drag will not even be remotely close to enough power to get you going. You need about 200mph minimum airspeed for a ramjet. A C engine can push a skinny rocket weighing an ounce or so to this speed, but that's about it. How small is your ramjet going to be, and how much do you think it will weigh?

Plus, if you fire the rocket engine down through the ramjet you are going to lose a lot of efficiency so you'll also need to factor that into the picture.

Even with a very small air-breathing motor, you will likely need at minimum an E or F class motor to get sufficient airspeed.

[Zippiot]

i was thinkin tiny, like 1 inch diameter of the combustion chamber, might even be able to get it down to 3/4 inch. i have ramjets that work at 230 mph, with some good intakes and math calcs i can probably get it down to 180-200...
it would be short, the fuel can be pretty much anything from pvc pipe to candle wax to cement epoxy mix

at the moment the tiny ramjets i have are just made of pipe steel, i have access to a machine shop to make a thin stainless version, it wont weigh much but it would have to be placed below the rocket engine, this might cause stability issues

[larry cottrill]

Here's a liquid-fueled one I thought of, but of course, never tried. It is probably not a very good design - there is no diffuser, it just relies on the air entrainment and crude flameholder to achieve combustion stability.

With an "ejector" like the front end of this jet, the air mass taken in greatly slows down the flow, to a fraction of the driving velocity (the rocket exhaust stream, in this case). I am quite sure the basic idea would work, but I'm also sure it will take a lot of experimentation to get it right. It would almost certainly have to be a lot better than what I show here.

The reason this is called a "rocket started ramjet" is that the idea was to use it on a model airplane where the rocket would get it moving so that the ramjet could take over after the rocket burn ends. For any model I could build, this would take a heck of a long-burning rocket engine!

L Cottrill

[Zippiot]

the flameholder i have is a washer with some tiny holes drilled in it, the big hole in the middle lets the rocket exhaust flow by without interupption. in theory...
this should help it accelerate to a higher speed...but getting it to hit 200+ mph going vertical might be a challenge, and even when it does the air starts to thin so it would have to go even faster as it climbs...

but less air lss drag...oh god how many variables are there?