"Finless" Rocket Design - Ram Air Intake Stabilization?

[KenECoyote]

(Note that updates are at the bottom of this first post.)

GOAL: To create a "finless" rocket similar to that defined by M. Dean Black (author of the Apogee Peak of Flight article on GDS) "Finless rocket design has long been a subject of debate among rocketeers wishing to build and fly true scale models of space launch vehicles and large military rockets which do not normally possess prominent fins typical of tail heavy model rockets."

Hi,

There's been a lot of talk recently on the subject of finless rockets and I had thought up an idea for one a while back and I kind of hijacked a thread (AFAIK I hadn't heard of similar before and if there is, it's still cool to test this design myself and see what I can do with it).

Since I've started building a test bed for this and someone has mentioned "I might want to watch its maiden flight from an armored bunker", I decided I should give it a separate thread so all of you can share my journey as well as cheer, laugh and cry with me. :grin:

Here's the link to the other thread as well as the post and sketch I did of the idea I had...I'm calling it RAIS/"Ram Air Intake Stabilization" for now (better than Forward Air Rearward Throughput) :

https://www.rocketryforum.com/showt...less-Rocket-Flies-Again&p=1539666#post1539666

I've pondered finless design as well. I love cup rockets and figured a finless rocket would be a cool variation of it; however if you make the tube straight, you likely need some other way to make it more stable with less wobble (which is what fins normally help with). I figured maybe air inlets in the nose/sides with internal ducting leading out at the aft section (possibly thin vertical slots) at an angle downwards and sideways...invisible air fins. Way beyond what I'm willing to start right now, but maybe one day.

...

I spoke too soon! I've been losing some sleep thinking about this one. I'll likely make a few test samples and right now I'm thinking of using an Estes Converter or Converter XL (luckily they've been on clearance recently and I picked some up...the smaller one for just $1.65!). I've included a drawing below and I figure I'll first use the converter with the test shroud added only to the bottom of the three sections since this one would be very difficult to sim and I don't want it unstable right off the bat. Also with the shroud only on the bottom of 3 sections, it looks and should behave more like a conventional rocket and would be less of a concern for RSOs. If that works, I may then try two of 3 sections and then after that remove the top 1/3 and replace with a nose cone. I'm guessing there is also some magic ratio of width to length since if it is too narrow, the air intake wouldn't be significant enough to send enough out the slots for stabilization. Additionally, this is complicated by having the right size of outlet hole/slot. Maybe short & squat when slow and narrower if faster is better since you need significant air flow to make a difference. Design is such that if the rocket arcs off vertical (assuming no wind), the outer side of the arc gets more wind, which then pushes the bottom back to vertical from what I'd guess.

Really rough sketch:

If the prototypes work, there is possible future versions with perhaps an inlet cover around the shroud (using a large conical nose cone with most of the top cut off), so it looks like the intakes of a Bomarc or the front of a Talon missile. Also depending upon how big the inlets need to be, it may be possible to use a large nose cone and drill inlet holes vertically down into it so the inlets are in the nose cone itself.

Thoughts?

The design I have isn't a tube fin since it has a blocked bottom and includes splitters going down the length of the area between the shroud and the main bt (represented by the dotted line down the side). This allows the air to funnel by individual section into each of the air outlets (simulating "air fins"). The sectioning also allows for some stabilization in that if let's say the rocket arcs over to the right of vertical, the left side of the rocket will get more air as it arcs, which sends more air to that side of the outlet, thus pushing it a bit more back to a straight path.

Additionally, at higher speed, the back pressure should make the upper shroud behave a bit like a larger nosecone, allowing for some base drag. Also at higher speeds, there may be a system loop formed when the higher volume of air flowing over the outlets helps to draw more air out as well to facilitate keeping the pressure building steadily/smoothly in the shroud (mindsim things so).

I already have a plan set out to test first in a B/C sized Estes Converter with BT70 shroud and if that works, I'll test a larger version using the Eliminator XL and BT-80 shrouds. The reason I'm using these rockets is that they are modular and have separate sections which allow me to test different configurations...starting with the RAIS as a small part and gradually increasing the portion of the rocket with shroud until only the nose cone is visible above the shroud (assuming successful steps along the way).

Estes Converter mock-up (1 shrouded out of 4 and 2 of 2 config):

*1/28/15 Edit: After the initial small scale tests, the shroud on this mock up appears too small. More small scale testing will be done before moving to this model.

If that 2 of 2 config works in both Converter and Eliminator XL, I'd like to try a Mid-Power version like this:

Thanks to all the contributors so far that have provided a LOT of great feedback, thoughts, ideas and critiques (in response order): CrazyOB; rstaff3; neil_w and Daddyisabar.

I'll be posting more about the testing and builds as I go along, but in the meantime please feel free to provide feedback and thoughts (plus good rocket mojo).

1/22/16 - Some updates:

Changed name to RAIS - I've added "Ram" since it seems to be a big part of this.

Here's a sketch of how I think airflow on this design will behave.

Here's a mini prototype rocket I've made which shows the design from a bottom 3/4 view:

This was able to fly up reasonably straight; however coast phase was difficult to track due to its small size.

2/2/16 UPDATE:

My second design "Well De-Finned" had two nearly perfect launches:

"Well De-Finned" Launches #2 and #3 (1/2 A3-2T):

Launch #2 Video Link: https://www.youtube.com/watch?v=Cp0HLCHuFGs&feature=youtu.be

Launch #3 Video Links (note the wind sent it over to the right; however the rocket didn't noticeably weathervane and instead drifted to the right):
https://www.youtube.com/watch?v=qycwPCPfZPw&feature=youtu.be
https://www.youtube.com/watch?v=xrgaIUVnC8w&feature=youtu.be

I would consider "Well De-Finned" to be a successful design! More testing will be required; however I think this one is good enough to get the ball rolling on variations.

2/3/16 UPDATE:

Build instructions for "Well De-Finned" are now in post #156.

2/5/16 UPDATE:

Decided to take down build instructions for "Well De-Finned" until I do more testing.

2/6/16 UPDATE:

Changing thread title to "Finless" (with quotes) and "Induction" to "Intake".

2/7/16 UPDATE:

Added Goal to the top.

5/30/16 UPDATE:

My best test results came from heavily weighted nose cones (think full of bb's and epoxy). Prior to that, I had a lot of unstable versions and the heavier weight seemed to give the design a very nice straight flight. RStaff3 suggested to me to try the design with the vents closed to see if the rocket was stable with just the weighted nose. I didn't think it would be considering my experience with the design; however I tried it and it was stable! It's a tricky situation and I would've preferred to give it less nose weight, but doing so means less stable/predictable flights, so right now this design doesn't look so good since it is also aerodynamically inefficient. I'll put this on the backburner since I have a few more ideas going on and want to focus on other things right now. To be continued...

 

[neil_w]

op:

 

[KenECoyote]

Here are my thoughts on the subject of why rear stabilization would be good on a finless rocket...

A rocket without fins (like a cup rocket) can fly as long as the CG is ahead of the CP by enough margin. However there is more of a tendency for it to have less stability at the rear since there isn't a stabilizing effect from fins*. Fins help a rocket travel in a straight path...much like ice skates would allow you to go straight on ice, but running with flat shoes on that ice will allow for a lot of sideways sliding.

*Saucers are an exception and are more stable due to the flat plate effect causing base drag and there is almost no "rear"; however we're aiming for a "long tube"-type rocket without fins here rather than a saucer.

My cool "spirally" cup rocket:

So that is the reason most standard rockets have fins. You don't necessarily need them, but without them you give up stability. Additionally, fins help to bring the CP back, so if you have a long rocket and no fins, you usually need to add a lot of nose weight to bring the CG forward enough of the CP and that can result in a lot of nose weight.

 

[neil_w]

By the way, how do you know where the CG needs to be on this thing? Does it matter? And will there be a speed where the stabilization starts to become effective?

 

[GlenP]

I have been thinking along similar lines as this, but instead of vertical fin type air ducts, using a set of closely spaced vent holes around the entire bottom circumference of the rocket to make an air-disc type of fin. You could precede that with an internal transition to force the air into the air slots from the through tube to the outer body tube. you want to have sufficient air flow through the fin holes so that you don't have air blockage, this would create an air dam at the front of the rocket as the air is spilled out the front because it can't get through, shifting the effective Cp to the front. Consider the extreme case where there are no fin holes, what would the air do? So, how big should they be? Use some basic area ruling as a start. What is the frontal area of the inlet? Make the slot fins total area equal or bigger.

 

[KenECoyote]

LET THE BUILD COMMENCE!!! :w:

Here's the first LP testbed...Estes #2029 Converter. I picked up two of them on clearance for under $2 each from Hobbylinc about a month or so back (just gotta keep mentioning that since it was one helluva deal). I love the convertible nature and the connectors are like designer's gold to me. As noted earlier, I can change the length/config as I want. I plan on making a shroud sized for the bottom section (which will always be used for testing) and one additional section with a shroud attached as well. With that done, I can fly in multiple configs.

First step is to open and build. Unfortunately my kit came with the wrong directions! It included the ones for a Mean Machine (too bad it didn't also include the decals...I wanted those).

Okay, I can do this without directions, right? The plan is to first remove the plastic fins and build the fin can. Hmmm...where is that tube of plastic cement (fingers crossed it isn't dried out or the consistency of Silly Putty).

Cutting off fins:

X-acto tip of the week: When using knives, always make sure your fingers and other parts aren't under the blade or blade path (unless you're experienced and have a calloused fingerpad to stop the blade when doing small whittling).

Finless!!!

Funny side note is that the included plastic chute isn't printed and had a note about that (hmmm...2009...that kit has been sitting around for quite a while at HobbyLinc!):

 

[KenECoyote]

By the way, how do you know where the CG needs to be on this thing? Does it matter? And will there be a speed where the stabilization starts to become effective?

Unknown at this time. I'm pretty sure it's stable with all the sections and shroud only on the bottom (first test config) given the length of the rocket and since I'm using what was a commercially available tested kit. As I progress, I'll likely have to do swing tests to check for the CP; however this design would be hard to calculate due to the front intake area likely creating base drag and since it varies with outlet size, it makes it very hard to determine (except in Mindsim). Later on I can easily add nose weight...and likely will need to.

 

[neil_w]

I have been thinking along similar lines as this, but instead of vertical fin type air ducts, using a set of closely spaced vent holes around the entire bottom circumference of the rocket to make an air-disc type of fin.

In that case, why have holes at all? Why not just leave a gap between the shroud and the rear disc? To lessen drag a bit (?) you could have the rear disc be more of a cone than a flat disc (hard to describe, hope you catch my gist). In either case, if the theory is that jetting the air out the sides creates the stability, then might as well send it in all directions equally.

Maybe.

(mind-sim seg-fault)

 

[neil_w]

In that case, why have holes at all? Why not just leave a gap between the shroud and the rear disc? To lessen drag a bit (?) you could have the rear disc be more of a cone than a flat disc (hard to describe, hope you catch my gist).

...which perhaps ultimately behaves like a rocket with a tail-cone, except the tail-cone is mostly hidden below the shroud. Hmm.

(mind-sim jumps off the roof, screaming)

 

[GlenP]

In that case, why have holes at all? Why not just leave a gap between the shroud and the rear disc?

Yes, that is another option. Take a look at this finless rocket, it has base cone fin. Now, put a larger tube duct around this rocket, but leave a gap at the base where the air gets pushed out laterally through the gap between the inner transition fin/cone and the outer body tube. you might need to add a slightly large dia base transition disc to the bottom of the fin cone.

https://modelrocketbuilding.blogspot.com/2011/10/centuri-finless-rocket-plans-from-jim.html

 

[KenECoyote]

I have been thinking along similar lines as this, but instead of vertical fin type air ducts, using a set of closely spaced vent holes around the entire bottom circumference of the rocket to make an air-disc type of fin.

I initially thought of using holes and also considered a lot of holes; however as you've noted that creates more of an air-disc rather than air-fin and I'm trying to imitate a finned rocket. While a rocket with a cone/disc at the base does work (almost like a saucer-rocket hybrid or saucer with a very long motor jutting through), it may not have as much of a straight-line stability compared to fins.

You could precede that with an internal transition to force the air into the air slots from the through tube to the outer body tube. you want to have sufficient air flow through the fin holes so that you don't have air blockage, this would create an air dam at the front of the rocket as the air is spilled out the front because it can't get through, shifting the effective Cp to the front. Consider the extreme case where there are no fin holes, what would the air do?

I'm not clear on the internal transition you're referring to. Regarding sufficient air flow, that is yet to be deterimined and there should always be some higher pressure in the shroud since i is trapping air and as long as anything gets in the way of the air on the way through (including any thing covering the bottom), there will be higher pressure. However some high pressure may be good since it can help the front of the shroud act like a larger nose cone, possibly adding some base drag.

So, how big should they be? Use some basic area ruling as a start. What is the frontal area of the inlet? Make the slot fins total area equal or bigger.

In this case, my mindsimming says that the air outlet doesn't necessarily have to be equal to the air inlet for the reason mentioned earlier and because higher pressure within the tube would allow the air exiting the slot to exit with higher pressure and behave more like a fin in the air stream (I think). If the hole had no obstruction straight through, it would be basically like a tube fin.

Great questions!

 

[KenECoyote]

(mind-sim jumps off the roof, screaming)

:rofl:

 

[neil_w]

Take a look at this finless rocket, it has base cone fin. Now, put a larger tube duct around this rocket, but leave a gap at the base where the air gets pushed out laterally through the gap between the inner transition fin/cone and the outer body tube. you might need to add a slightly large dia base transition disc to the bottom of the fin cone.

https://modelrocketbuilding.blogspot.com/2011/10/centuri-finless-rocket-plans-from-jim.html

That's exactly what I was thinking. Dunno if the shroud would help or hurt. Maybe it subverts what Ken was trying to do in the first place.

Gotta stop thinking about this.

 

[KenECoyote]

Yes, that is another option. Take a look at this finless rocket, it has base cone fin. Now, put a larger tube duct around this rocket, but leave a gap at the base where the air gets pushed out laterally through the gap between the inner transition fin/cone and the outer body tube. you might need to add a slightly large dia base transition disc to the bottom of the fin cone.

https://modelrocketbuilding.blogspot.com/2011/10/centuri-finless-rocket-plans-from-jim.html

For rockets with cone bases, there isn't any need for a shroud since the cone base is already in the air stream and adding a shroud would likely only add more weight and drag. The design I've sketched has a shroud since the air flow has to change from oncoming to sideways and narrowed/directed. Will it work? We'll see. Should be fun and hopefully not too painful.

Boy, you smart guys are melting my brain!

 

[GlenP]

The mass flow rate is density*velocity*area, if you are subsonic then density is constant. So area*velocity at the inlet = area*velocity at the outlet (forgetting about pipe friction for a first order estimate) if the areas are equal, then the air flow should exit at close to the same speed as the inlet. If you make the exit slots 50% of the inlet slots, then the speed at the exit should be twice a much as the inlet speed. Maybe rather than making the exit area larger, it should be a little bit smaller, so the fin slot is a higher speed than the rocket, but even if equal speed, it is in a different direction, so I would try to keep it close to a 1:1 ratio, but maybe smaller. this is one parameter which could be studied, if you had a simple way to make that variable, like changing how the internal transition is mounted relative to the body tube to make the gap bigger or smaller, or have an internal or external coupler that can be taped over the slots to change their length or width, kind of like a Kraft Parmesan Cheese dispenser top, you know?

 

[KenECoyote]

Okay, crisis averted...plastic cement was good!

I've glued the now finless fincan (or should it just be "can"?) together and did some test fitting on a cluttered table.

Next I wanted a hard stop above the motor retainer so I can slip on the shroud and possibly change shrouds later...this way I can experiment with different size outlets as well.

I traced the "can" onto a piece of fin and cut it out like a ring and glued it to the bottom.

 

[KenECoyote]

The mass flow rate is density*velocity*area, if you are subsonic then density is constant. So area*velocity at the inlet = area*velocity at the outlet (forgetting about pipe friction for a first order estimate) if the areas are equal, then the air flow should exit at close to the same speed as the inlet. If you make the exit slots 50% of the inlet slots, then the speed at the exit should be twice a much as the inlet speed. Maybe rather than making the exit area larger, it should be a little bit smaller, so the fin slot is a higher speed than the rocket, but even if equal speed, it is in a different direction, so I would try to keep it close to a 1:1 ratio, but maybe smaller. this is one parameter which could be studied, if you had a simple way to make that variable, like changing how the internal transition is mounted relative to the body tube to make the gap bigger or smaller, or have an internal or external coupler that can be taped over the slots to change their length or width, kind of like a Kraft Parmesan Cheese dispenser top, you know?

Thanks for all that info! I studied aerodynamics in High School, but took a different career path, so that is very helpful to know.

Yes, I had considered different ways to test the outlet size. For the first/LP version, I'll just swap shrouds or add tape. For the next larger one (if I get that far), I'm thinking of using a coupler inside of the BT80 shroud and both would have slots and I can rotate and tape into position to vary the size of the outlet. Thinking about this stuff is one reason I don't get enough sleep.

 

[KenECoyote]

I needed something to block the bottom of the shroud and also hold it in place, so I figured something like a centering ring makes sense. Unfortunately I couldn't locate the ones I had (don't even know if I had the right size), so I looked for heavy cardstock and found it in a small box of chocolates. I figured I'll first try it for the larger BT-80 since I did have a centering ring for that first, then once that worked I cut the same for the current rocket.

Next I plan to build the internal shroud supports. I was initially thinking maybe heavy card stock, but now I'm thinking balsa. Done right I can easily swap the shrouds (which would be just plan cut BT70 (?) body tubes with slots on the bottom).

Here's the mock-up again:

Time for dinner! To be continued...

 

[KenECoyote]

Brainstorm: if it works...make it whistle! :grin:

 

[rstaff3]

Wow, I'm away from the 'office' for a while and look what happens. I haven't digested all the posts but that won't let me spewing more G into the GIGO model. More power to the mindsims, Scotty!

I'll state again that if this is unstable it will be very unstable. Even if it is stable, it will be very unstable shortly after burnout. Launch far from people and it will never reach 'em. GDS, which I have demonstrated personally, is quite sensitive to things like airspeed, position of the motor, length of the rocket, LD ratio etc etc etc. I don't want to pessimistic but my sense is there are a lot a variables and it will likely take, without some actual maths, some trial and error and luck. That being said, this is one of the more fascinating ideas I've heard about. GO FOR IT! If it fails, iterate and try again. If it works, do the same

Since I'm not an aerospace guy nor do I play one on TV, I find it hard to believe that the airflow in will equal the airflow out. Due to turbulence, flow resistance, the mythical clogging of tube fins, or whatever. (Actual aerospace people are welcome to tell me I'm ignorant.). Maybe you need four sets of downscaled exhaust headers to install in da' gap. LOL

 

[rstaff3]

I needed something to block the bottom of the shroud and also hold it in place, so I figured something like a centering ring makes sense. Unfortunately I couldn't locate the ones I had (don't even know if I had the right size), so I looked for heavy cardstock and found it in a small box of chocolates. I figured I'll first try it for the larger BT-80 since I did have a centering ring for that first, then once that worked I cut the same for the current rocket.

Next I plan to build the internal shroud supports. I was initially thinking maybe heavy card stock, but now I'm thinking balsa. Done right I can easily swap the shrouds (which would be just plan cut BT70 (?) body tubes with slots on the bottom).

Here's the mock-up again:

Time for dinner! To be continued...

Looking good!

 

[KenECoyote]

Wow, I'm away from the 'office' for a while and look what happens. I haven't digested all the posts but that won't let me spewing more G into the GIGO model. More power to the mindsims, Scotty!

I'll state again that if this is unstable it will be very unstable. Even if it is stable, it will be very unstable shortly after burnout. Launch far from people and it will never reach 'em. GDS, which I have demonstrated personally, is quite sensitive to things like airspeed, position of the motor, length of the rocket, LD ratio etc etc etc. I don't want to pessimistic but my sense is there are a lot a variables and it will likely take, without some actual maths, some trial and error and luck. That being said, this is one of the more fascinating ideas I've heard about. GO FOR IT! If it fails, iterate and try again. If it works, do the same

I did recall your note about instability after burnout; however I think this design is different than GDS (Gas Dynamic Stabilization). GDS uses the motor exhaust gases combined with the airflow to create a cone at the base of the rocket for stability; while it can work, the inherent problem appears to be that once boost stops, there isn't any more stabilization, hence you get instability after burnout. However the RAIS design should in theory continue to stabilize the rear of the rocket as long as there is airflow into the front of the rocket (which means it should continue to stabilize after boost to apogee). This is verified with MSSS! (Mind Sim Says So!) :grin:

Since I'm not an aerospace guy nor do I play one on TV, I find it hard to believe that the airflow in will equal the airflow out. Due to turbulence, flow resistance, the mythical clogging of tube fins, or whatever. (Actual aerospace people are welcome to tell me I'm ignorant.). Maybe you need four sets of downscaled exhaust headers to install in da' gap. LOL

I'm not an aerospace guy either; however I pretend to be here. The airflow in won't equal the airflow out...this is actually a positive in my view since that means higher pressure at the front of the shroud caused by the deflection of extra air around the high pressure, so it then starts to behave more like a large nosecone and possibly creating base drag, which is good for stability.

Looking good!

Thanks!

 

[rstaff3]

I did recall your note about instability after burnout; however I think this design is different than GDS (Gas Dynamic Stabilization). GDS uses the motor exhaust gases combined with the airflow to create a cone at the base of the rocket for stability; while it can work, the inherent problem appears to be that once boost stops, there isn't any more stabilization, hence you get instability after burnout. However the AIS design should in theory continue to stabilize the rear of the rocket as long as there is airflow into the front of the rocket (which means it should continue to stabilize after boost to apogee). This is verified with MSSS! (Mind Sim Says So!) :grin:




I'm not an aerospace guy either; however I pretend to be here. The airflow in won't equal the airflow out...this is actually a positive in my view since that means higher pressure at the front of the shroud caused by the deflection of extra air around the high pressure, so it then starts to behave more like a large nosecone and possibly creating base drag, which is good for stability.



Thanks!

I'm thinking it will need a lot of speed to get the stabilization active and this will drop as it slows. Maybe I'm also incorrectly assuming this will slow faster than a 3FNC.

My mindsim doesn't indicate significant base drag or any pseudo base drag. Wouldn't a high pressure area at the tip be pseudo draggy? I may change my sim randomly in the middle of the night or while contemplating this week's nor'easter.

Where are the real rokit scientists when you need 'em?

 

[KenECoyote]

Here's a sketch I came up theorizing what I think will happen and what I'm trying to achieve (all reference is to this half shown):

A: Airflow represented by 5 arrows goes towards the front of the rocket
B: Much of the airflow goes into the shroud; however some of it spills away/gets deflected since there is no clear airflow straight through, causing a high pressure zone in front of the shroud
C: The slot at the bottom allows the air which went into the shroud to escape. Since the slot is narrower than the intake, the pressure is higher coming out as well and also directed in a narrow and long pattern which when it meets with the outside air stream, forms almost the same pattern/drag/effect as a wide fin
D: The mixed air continues out away from the base...the movement away from the base of the slot may actually help to draw the air out as well (airflow dynamics across a linear slot with air coming out isn't known to me at this time, so I can only theorize what may happen. To me, it should have some sort of Bernoulli or Venturi effect to help draw the air through the slots; however I haven't figured it out yet.

 

[KenECoyote]

I'm thinking it will need a lot of speed to get the stabilization active and this will drop as it slows. Maybe I'm also incorrectly assuming this will slow faster than a 3FNC.

My mindsim doesn't indicate significant base drag or any pseudo base drag. Wouldn't a high pressure area at the tip be pseudo draggy? I may change my sim randomly in the middle of the night or while contemplating this week's nor'easter.

Where are the real rokit scientists when you need 'em?

I'm giving my mindsim all she's got Capt'n!

I'm not quite sure if you need a lot of speed to get the stabilization...perhaps for lower speed you just need wider slots & slightly wider shroud. However I do think it will slow faster than a 3FNC for the simple fact that the system in place is draggier than a rocket of the same diameter with a full nosecone and tail fins. Also the shroud itself isn't as aerodynamic as a large nosecone and sending the air out sideways creates a lot more drag and slipping through it. If the basic model works, I can possibly move on towards other models that are more efficient (part of it is determining how much intake of air as well as exhaust out to the sides is needed) including a version with a large nosecone which has 3-4 holes in it for the intake or maybe a nose cone with just a single hole in the tip.

 

[rstaff3]

Here's a sketch I came up theorizing what I think will happen and what I'm trying to achieve (all reference is to this half shown):

A: Airflow represented by 5 arrows goes towards the front of the rocket
B: Much of the airflow goes into the shroud; however some of it spills away/gets deflected since there is no clear airflow straight through, causing a high pressure zone in front of the shroud
C: The slot at the bottom allows the air which went into the shroud to escape. Since the slot is narrower than the intake, the pressure is higher coming out as well and also directed in a narrow and long pattern which when it meets with the outside air stream, forms almost the same pattern/drag/effect as a wide fin
D: The mixed air continues out away from the base...the movement away from the base of the slot may actually help to draw the air out as well (airflow dynamics across a linear slot with air coming out isn't known to me at this time, so I can only theorize what may happen. To me, it should have some sort of Bernoulli or Venturi effect to help draw the air through the slots; however I haven't figured it out yet.

The diagram needs some chaotic curly queues.

 

[GlenP]

You have the right idea, and this is a good sketch. for the three arrows that go into the tube, this represents a particular mass flow rate. The same mass must flow out of the tube, this is a fundamental conservation of mass law. Mass going out must equal mass going in, this is what leads you to the product of area and velocity being constant for constant density incompressible subsonic flow, or a simple area rule. If you have a 1:1 area ratio, the the velocity of the exit is equal to the velocity at the inlet. Now the net airspeed at the exit slot is the sum of the outside air along the rocket and the fin slot exit perpendicular to the air stream, if those also have the same 1:1 ratio, then the effective air flow angle at the fin slot would be 45 degrees. Again, this is a very rough idea based on very simple aero, no friction effects considered in or out of the tube. The air stream coming out the fin slots at 45 deg is sort of like a body that produces a streamline of the same shape, ie a swept back fin with the leading edge at 45 deg. the inlet to exit area ratio will determine the velocity ratio and the net angle of the air flow at the fin slots. there will be some friction and blockage effects such that you won't have perfect flow through the tube, but this gives you a starting point to evaluate the size of your fin slots and experiment with their effectiveness.

 

[KenECoyote]

The diagram needs some chaotic curly queues.

LOL! My mindsim doesn't adjust for that. Actually, I'm guessing you're referring to turbulence...there likely would be some in different areas; however that is more complex than I'm capable of figuring out. If you have any ideas, please feel free to adjust or add to the pic on what you think may happen...would be interesting to see other views of what may go on.

Also regarding your earlier note about base drag...I originally thought of using a BT-80 for the first design and later decided to go slimmer. So my original thought was for a "squatter" ("squatty-er"?) where base drag may be more present and help to reduce the amount of nose weight. In the current testbed case, the length to width ratio is just under 10:1, but not if I include the nose cone, so I think you're right about base drag not being present in that first design.

 

[KenECoyote]

You have the right idea, and this is a good sketch. for the three arrows that go into the tube, this represents a particular mass flow rate. The same mass must flow out of the tube, this is a fundamental conservation of mass law. Mass going out must equal mass going in, this is what leads you to the product of area and velocity being constant for constant density incompressible subsonic flow, or a simple area rule. If you have a 1:1 area ratio, the the velocity of the exit is equal to the velocity at the inlet. Now the net airspeed at the exit slot is the sum of the outside air along the rocket and the fin slot exit perpendicular to the air stream, if those also have the same 1:1 ratio, then the effective air flow angle at the fin slot would be 45 degrees. Again, this is a very rough idea based on very simple aero, no friction effects considered in or out of the tube. The air stream coming out the fin slots at 45 deg is sort of like a body that produces a streamline of the same shape, ie a swept back fin with the leading edge at 45 deg. the inlet to exit area ratio will determine the velocity ratio and the net angle of the air flow at the fin slots. there will be some friction and blockage effects such that you won't have perfect flow through the tube, but this gives you a starting point to evaluate the size of your fin slots and experiment with their effectiveness.

Thanks for your feedback Glen! Thinking back to what you and CrazyOB said (in the earlier thread), I'm starting to think that perhaps area rule may be a good idea in this case considering there likely won't be base drag in my first test iteration and that for this first design it seems better to make it faster and less draggy. Perhaps along the way I should try a squatty version which is short, wide and slow to see what happens there...the extra drag may result in instability shortly after boost; however as rstaff3 said, that also means slow/tumble recovery.

For more brain-twisting, I'm also thinking that another version can use a cone base at the bottom of the shroud to help direct the incoming air to the sides (much like what you had mentioned earlier) and maybe even extend the shroud out further to add Gas Dynamic Stabilization for an extra measure of safety with faster rockets. I think with the Converter base, I can probably try those out as well with swaps as long as I go with installing the internal vanes to the shrouds rather than the main bt (still deciding...tricky to figure out and commit since this is hard to reverse once built).

Well, back to work week for me! Possible good news is that I just discovered there's a "bonus launch" this Saturday at RR (weather permitting), so there is a possibility I may do a test launch as soon as this weekend. :grin:

 

[KenECoyote]

Just a quick note that I got around to cutting the first shroud to the correct length and so here is the mock up of what I'll be flying first. Should be pretty close to what I'll end up with:

(I've also updated the pic in the OP since this is a better representation of what I want to first test.)

neil_w - Any armored bunker thoughts for this one? :wink: