Compressed air/Ramjet = Cramjet

[Tony~]

I've been trying to find a way to build a small, semi-efficent ramjet so that it can work on a rocket. But the problem is to get pressure to the ramjet, a motorjet/thermojet was my first thought but soon realised they're too bulky and not very efficent. Then I figured, what if you put a pressurized-air tank to provide the engine with high velocity air? It should work, as long as you have the right pressure and exact air inlet. But the engine would only work as long as there is pressure inside the tank, but then you put a one-way valve on top of the tank so that when pressure is inside the tank it is held shut by it and when the pressure drops the valve opens and air from the outside can rush in with force from the airspeed. What do you think? Workable?

[larry cottrill]

Tony -

It's hard to know what would happen from just a description. I think the problem would be that there is a big difference between usable tank pressure and ram air pressure. For a tank of air to be usable for sufficient time to accelerate the vehicle to reasonable speed, you would want fairly high pressure that exits through a fairly narrow high-speed nozzle. This air jet could drive the front end of your engine as an "ejector" - a large air volume driven by a much smaller, higher speed flow up the middle. That is not an unreasonable design. The problem will be in the transition to ram air, which will be slower and will be equivalent to a much lower inlet pressure, even if fairly high speed is attained.

Because the outlet of the tank must be small, I don't think your idea of a valve in the leading end of the tank is workable. Instead, just let the outside air flow around the tank into the engine inlet. This would be similar to the rocket-fed ramjet I proposed on the old forums [see below], but you would have a larger air cylinder in place of the solid rocket. The rocket would work, theoretically, because the amount of air pulled in would far exceed the rocket gases. Similarly, what you want to do could even possibly be done with a CO2 cylinder rather than air!

Good luck!

L Cottrill

[Harabec]

Tony
you could try it with compressed air before launch. But then you would need a recovery system and a way to get a stable air intake and fuel pressure. What type of fuel do you plan on using?

[Tony~]

I've now managed to construct a engine/combustor out of a pipe of about 5 cm in diameter and 30 cm in lenght. I've squashed the pipe in one end to seal it and ther's 2 holes for compressed air and butane as fuel. As the flameholder I simply used a round steel plate with alot of small holes in. I haven't mesured any thrust yet but I guess it's not big, maybe I should squeeze down the end of the pipe so that it works like a nozzle?

[Greg O'Bryant]

OMG! Larry;
Once again another idea that somebody else has had on this forum that is similar to one of mine. About 7 years ago I thought of making a ram jet rocket hybrid too. Unlike your engine mine was a rocket engine that was a core burner type to get it going, and then have it transition to end burner. While it would be in the end burning stage the propellant core would be encased in a larger grain of parrafin wax. The wax would be vaporized and come out of the rocket engine with great force to be burned up in the ram jet section of the engine. I am getting reluctant to say things on this forum becuase it is hard to have anything to say that somebody else hasn't said already........LOL

[steve]

It's certianly not a new idea, take a look at the bottom/middle of this page:
http://www.islandone.org/LEOBiblio/SPBI104.HTM

[Greg O'Bryant]

I know that is the problem you think you have got a great new idea and find out that it was thought of years ago. Oh well it was new to me at least for a little while.:)