Lakeroadster's Finless Rocket Utilizing Gas-Dynamic-Stabilization

[lakeroadster]

I wanted to take a stab at building a finless rocket.

I've created a CAD model and created drawings of a rocket roughly based on the IKAROS design that was posted HERE. Kudos to @BigMacDaddy for the inspirational thread.....

The variation here, away from the IKAROS design, is I stretched it a bit to utilize a streamer and a nose cone.

Plan is to make a liner out of a piece of a pop can to slide into the body tube below the air vents to protect the body tube from the motor exhaust.

 

[BigMacDaddy]

Very cool -- love your CAD / planning skills... Glad my design inspired further exploration of this and looking forward to seeing the results.

For GDS, I was never clear on how much space was needed inside the tube for the outside air to mix with exhaust. When I made my attempt I had an 18mm mount inside a BT60 tube. Here you are planning a 18mm engine inside a BT20 tube.

I did get the impression from reading a couple of articles that the optimum length for the tail portion was 2.5x the diameter of the tube. Do you think you might need a slightly longer ring tail portion (i.e., body tube portion behind the engine)?

 

[lakeroadster]

Very cool -- love your CAD / planning skills... Glad my design inspired further exploration of this and looking forward to seeing the results.

For GDS, I was never clear on how much space was needed inside the tube for the outside air to mix with exhaust. When I made my attempt I had an 18mm mount inside a BT60 tube. Here you are planning a 18mm engine inside a BT20 tube.

I did get the impression from reading a couple of articles that the optimum length for the tail portion was 2.5x the diameter of the tube. Do you think you might need a slightly longer ring tail portion (i.e., body tube portion behind the engine)?

Yes, It's an 18mm design.

As for the dimensional data... I pulled the sketch of the IKAROS up into CAD, scaled it to fit a 18mm mount, then made the opening and lengths exactly the same as the photo. But the actual aluminum housing shown in the photo below does appear to have a longer skirt?

If it doesn't fly well... I can always make the pop can liner longer and try it again...

I'll review the Apogee Newsletter below and compare it to the design though. It'll be a while before I build this rocket.

Thanks!

 

[Cape Byron]

Subscribed...

 

[lakeroadster]

@BigMacDaddy The IKAROS design seems to fly in the face of almost everything M. Dean Black specifies in the Apogee Newsletter.

If you are not familiar with the Ikaros, it's a parachute rocket flare

Here's a video... and a couple of screen grabs....

Disclaimer: I'm not contemplating building a flare.. I simply want to build a finless rocket, that mimics this design from a flight perspective.

 

[GlenP]

Do you think the tube of that flare acts like a piston launcher to help it get up to speed quickly with out a launch rod?

 

[lakeroadster]

Do you think the tube of that flare acts like a piston launcher to help it get up to speed quickly with out a launch rod?

Good question, no piston though, just the pressure generated from the motor, pushing it out of the tube.

 

[DES]

I've played around with the gas stabilized, finless designs a little bit. Typically the finless section is on the order of 4x the diameter of the motor

Something to be aware of - unless you provide some degree of ballistic stability, the rocket is stable only while the motor is burning. The moment it burns out, the rocket will become unstable, and typically spin around in a tumbling ball a few hundred feet up in the air until the ejection charge goes off. Doesn't seem to hurt anything, but you don't gain any altitude once you lose stability.

In the case of the rocket launched flare, the center of gravity is well forward because of the mass of the flare composition. That keeps it going in the same direction. You are also getting a pretty good kick of thrust as the rocket "pops" out of the launcher / chamber.

 

[Daddyisabar]

GDS is not the end all of model rocket stability issues one would wish for. Spend the money on a fancy gimbal and a long burn motor. The last resort is to turn to the Dark Side...

 

[lakeroadster]

In the case of the rocket launched flare, the center of gravity is well forward because of the mass of the flare composition. That keeps it going in the same direction. You are also getting a pretty good kick of thrust as the rocket "pops" out of the launcher / chamber.

Add Nose Weight, check
Launch Lug Delete: Update Design To Utilize A Closed Bottom 3/4" PVC Launching Tube ...

That's doable.... thanks for the input!

 

[BigMacDaddy]

Add Nose Weight, check

Ah but how much? I added nose weight till the rocket was stable without GDS (i.e., swing test). But if GDS was really working I should have been able to do this without that much nose weight.

 

[lakeroadster]

Ah but how much? I added nose weight till the rocket was stable without GDS (i.e., swing test). But if GDS was really working I should have been able to do this without that much nose weight.

Without the motor firing (during the static swing test), GDS isn't "a thing"... that's why during flight as soon as the motor thrust stops, the stability provided by GDS is no longer present.

 

[lakeroadster]

How about some Open Rocket simulations, to estimate apogee and motor delays?

I threw on some "faux fins" that are 0.000" thick just to make Open Rocket think it is it stable.

The first simulation is a standard Ikaros parachute flare. The second is my version of a GDS rocket.

 

[GlenP]

Good question, no piston though, just the pressure generated from the motor, pushing it out of the tube.

Essentially the rocket is the piston, when launched from the tube like that.

 

[lakeroadster]

Essentially the rocket is the piston, when launched from the tube like that.

 

[BigMacDaddy]

Without the motor firing (during the static swing test), GDS isn't "a thing"... that's why during flight as soon as the motor thrust stops, the stability provided by GDS is no longer present.

Sorry, yes - I know and meant to say: "But if GDS was really working [when I launched] I should have been able to do this [i.e., launch successfully] without that much nose weight."

In other words, if I added enough weight to be stable on swing test (which I did) then I probably added too much weight given that GDS should have been a stabilizing factor when I actually launched. So question still remains -- how much weight is enough to stabilize in addition to GDS?

 

[lakeroadster]

Sorry, yes - I know and meant to say: "But if GDS was really working [when I launched] I should have been able to do this [launch successfully] without that much nose weight."

In other words, if I added enough weight to be stable on swing test (which I did) then I probably added too much weight given that GDS should have been a stabilizing factor when I actually launched. So question still remains -- how much weight is enough to stabilize in addition to GDS?

As for me, I don't really care. If it launches, flies straight, and it's stable, it's a success. Unless we're talking "Competition Rocketry" maximizing the apogee has little meaning, especially on a one off scratch build, that isn't a paint by number kit.

But to your point, fly the rocket, remove some nose weight, fly it again. Rinse and repeat until the rocket becomes unstable, * during the coast phase, after thrust..

* edited .. thanks @GlenP

 

[GlenP]

… until the rocket becomes unstable…

…during the coast phase, after thrust.

 

[lakeroadster]

…during the coast phase, after thrust.

Exactly... (updated for clarity)

 

[BigMacDaddy]

Mine got a bit less stable in later phases of the launch (less thrust?) but once the engine stopped burning it just slowed down and started falling backwards (no tumbling or spinning or anything) but I think that mine was a bit heavy...

 

[lakeroadster]

Mine got a bit less stable in later phases of the launch (less thrust?) but once the engine stopped burning it just slowed down and started falling backwards (no tumbling or spinning or anything) but I think that mine was a bit heavy...

What's the definition of "finless"?

On your rocket, I'm seeing a boat tail design, that has an exposed motor, (3 or 4) regular small fins and (1) ring fin....

 

[BigMacDaddy]

In my naïve view, probably any planar object that creates aerodynamic forces is a fin (or works like one). For example, the rear tube section of your rocket likely works as a ring tail / ring fin, particularly given that GDS pull air into that space and over those surfaces.

From my perspective "finless" was no fins sticking beyond the body tube dimension. However, I am not defending the finless term, not really trying to make a finless rocket and break the laws of physics or anything. I just wanted something streamlined and different looking and for the purpose of this thread I think both models likely use GDS which was why I brought of the flight characteristics.

 

[GlenP]

In my naïve view, probably any planar object that creates aerodynamic forces is a fin (or works like one). For example, the rear tube section of your rocket likely works as a ring tail / ring fin, particularly given that GDS pull air into that space and over those surfaces.

From my perspective "finless" was no fins sticking beyond the body tube dimension. However, I am not defending the finless term, not really trying to make a finless rocket and break the laws of physics or anything. I just wanted something streamlined and different looking and for the purpose of this thread I think both models likely use GDS which was why I brought of the flight characteristics.

I agree, I think those struts that hold the lower tube in place are not really acting as fins, they are attached to the boattail and would basically be in the wake of the main body tube, not that effective aerodynamically. You could probably use round dowels in their place and not notice much difference, but they might break on a hard landing.

 

[Y3kankerous]

The reason why @BigMacDaddy 's streamlined finless rocket is GDS is that the flow of the gas (exhaust-air mixture) thru the duct created by the pseudo-ring fin is generated by thrust not the rocket movement and so mostly independent of the rocket velocity, in fact the duct gas velocity will actually drop once the rocket is moving. It is the lateral components of the intense fluid flow field in this region that provides stabilisation because any perturbing lateral force will be strongly counteracted (imagine the lateral forces as pieces of elastic going out radially from the ducts). Hence why it starts to be unstable once the thrust drops because the lateral components are now similar or lower than the normal perturbing forces and can't counteract them. Thinking about it the highest gas flow rate is most likely actually at launch before the rocket starts to move. During thrust phase, flow thru the duct is faster than the air velocity (MUCH faster), in the delay phase and after the flow is less than the air velocity due to drag. I'm no sort of expert on this stuff so please put me right if I've missed the mark on any of that.

 

[Y3kankerous]

Really interested in this build and the sim methods as I have some ideas of my own for a couple of GDS designs that I'm hoping to get to work on soon (doubtless they have been done at some time or other in the past lol!) so will b keeping an eye!
Also the designs are fantastic!
I read PoF 379 recently and it lit an E motor inside my brain, scrambled everything I thought I knew in my (def rudimentary) understanding of stability.
So my question related to this build (and a possible way to estimating the reqd nose cone weight)... Is there any reliable way to estimate the velocity of the gas going thru the vents and hence the magnitude of stabilising force? My guess is this is moderately advanced CFD but maybe there is a way to estimate? It looks like the existing CAD designs would be plenty detailed enough if anyone with CFD skills+package was interested to try it.
If so, there might be some seriously cool possibilities, because the stabilisation is a function of the gas velocity NOT the rocket velocity (exactly opposite to a normal roc)... Like tuning the roc mass Vs a specific thrust profile (esp a long burn) to get only a low altitude lift off, then going into GDS stabilised semi hover (or even descent!!! oh no I've gone too far, forget that...). Not sure if this is realistic in practice (yep v long launch cables) ie how strong the stabilisation wd have to be to counteract lateral disturbance forces (effectively acting as a gyro) to avoid total instability, but as above if the rocket velocity falls while under thrust, that should favour stability because the duct velocity rises. So what about a staging arrangement that activated different ducts at different phases of flight!?!? (Think this needs a new thread...)
Would love to make this work for a saucer probs with some ultra bright down facing LEDs to light the massive smoke plume and warn any earthlings nearby to take cover ahahahaha

 

[Daddyisabar]

Save us GDS! You are our ONLY HOPE!

 

[BigMacDaddy]

deleted...

 

[Y3kankerous]

Moved also...

 

[lakeroadster]

@Y3kankerous and @BigMacDaddy ... how about moving your Gas Dynamic Stabilization theorizing over to @BigMacDaddy 's thread, here is a link:
Engine-Driven Gas-Dynamic or Induction Stabilization Streamlined Finless Rockets

This is a build thread...

Thanks!

 

[DES]

Apogee Newsletter 379 provides a good discussion of why and how GDS works. Complete with math.

See attached.