New Design (Unfinished) - Ram Jet

[Feyd]

I had the crazy idea of making a model rocket that (kind of) looks like a ramjet engine. I've got it started but it's not stable yet. I think it needs a TON of noseweight at this point but I'll keep playing with it.

I inset the rear tube with the engine hoping the smoke and flame coming out the back would look more like engine exhaust. I was also thinking of using three canted engines to make it even more spectacular. But at that point I think I needed something like 12 oz of nose weight. :eek:

I've been playing with body tube sizes and nosecone shapes but nothing seems to help. I think the internal fins would help keep the whole thing stable going up, but I'm not sure whether I believe RockSim as far as stability goes. Also, I'm not quite sure how to shorten the fins in the ring tail - I don't want them going all the way up the nosecone. That would make it difficult for the parachute to deploy.

Comments are welcome.

[CPMcGraw]

I had the crazy idea...

Congratulations! Now you're sounding like one of us... :D

...I'm not quite sure how to shorten the fins in the ring tail - I don't want them going all the way up the nosecone...


On the Ring Tail dialog, General tab, look over to the right at the 'Pylon Count' field. Set this to '0'.
Set the 'Pylon Thickness' to 0.0001 (essentially '0', but you cannot use '0' here - it has to be a non-zero value. This would effectively become zero...
Now, draw some Custom Fins to the shape you want them, adjusting the height of the fin to meet up with the inside surface of the ringfin tube. You will need to zoom in with the side view to 'make it meet'.


I'm not sure you can do this without having some external fins. If you had a gimbaled engine, then a computer would be in charge, but we don't have that. We have to hang it out in the breeze. There may be a compromise 'minimal fin size' that will work, but I think you will still have something visible on the outer surface of the tube...

[Feyd]

Update!

I've changed things around significantly, and added a "skinny" portion near the aft end but I'm not sure how to cover it (I guess a custom transition is in order).

The first try with a single 18mm engine only made it to 115 ft and had a horrible DV. So I put in three engines (I'm thinking canted since three won't fit in the body tube I'm using) and after adding a LOT of noseweight it makes it to 330 ft and has an excellent DV.

Take a look. I'm going to modify the engine to 24mm next to see what happens.

[CPMcGraw]

Feyd,

Look this revision over and see if it is closer to what you wanted. I'm getting between 460' and 483' on three C6-3 engines, and the Dv numbers are from 16 FPS to 26 FPS.

Length: 14.50"
Diameter: 3.70" (RT-99)
Fin Span: 10.04"
Weight: 3.61 oz

Model reaches safe flight V on a 36" x 1/8" standard rod.

I'm not sure this can be done without having some fin area, and it took quite a bit of tweaking to get this combination to work right. I'm still trying to eliminate the ballast (I hate adding dead mass to a design!).

Your idea is a good challenge, Feyd! Thanks for presenting it!

[Feyd]

Thanks, I'll have to play with your model a bit. I was trying to make it look as much as possible like an actual engine. I was thinking the airflow over the internal fins would keep it stable. So I'm not sure RockSim is taking that into account.

[CPMcGraw]

... I was thinking the airflow over the internal fins would keep it stable. So I'm not sure RockSim is taking that into account...

I think it does, but once RockSim is told it has a 'ringfin' component, it gives that more significance than the pylons (or any custom-shape fins) inside.

Sometimes, RockSim gives totally bogus numbers. Take a look at the stability margin when you switch from 'Barrowman' to 'RockSim' calculations on the first tab. Barrowman says it's stable...