Triple Threat build thread

[Zebedee]

I'm thinking of making a scratch built 3 D/E engine cluster rocket which would consist of 3 body tubes, each glued to the other two along their long axis in a triangular arrangement as you look from the end. Each tube would have it's own nosecone and one fin.

I'm wondering about the aerodynamics of the hole down the middle of the rocket (which would fit a 1/4 rod for launching) and what fins would be required for stability since I don't think I can simulate this in OR.

A single tube on a D12 would hit ~1000 feet and 250ft/s - I'm not sure if that's slow enough to allow airflow down the central cavity or it would create sufficient turbulence at the front of the rocket to make it unstable. I read in another thread that launch lugs are draggy partly because air does not flow nicely through them at higher speeds.

My other alternative is to have two tubes on each side of a longer central tube (like a Falcon Heavy plus fins).

Thoughts?

 

[CarVac]

The hole down the middle will be completely irrelevant to the aerodynamics. Feel free to put the launch rod there.

 

[bill_s]

EDIT: Ack, I was thinking the "hole" was a separate tube, possibly longer.

Still I believe the hole down the middle would act like a tube fin, except only the leading edge effective, in other words like putting a small fin or extra blunt nosecone on the front end, so you'd need a bit more fin area. Might could sim as a slight oversize nosecone-transition. Testing would be preferred.

It might look cool to put some space between the tubes.

[...]I built a rocket once with a single, coaxial tube fin, starting about 60% back. It did have some small fins inside the tube fin, to support it (this was actually a Big Daddy with the fins cut to stubs, no centering rings or nose cone, and another BT50 rocket grafted onto the motor mount).

My latest rocket is finless, and believe it or not, Open Rocket said it needs far less nose weight than it actually does (overly optimistic CP). However, it predicts the needed CG, or matches the "cardboard cutout" method calculation, by using angle of attack = 90 degrees (see Analyze>Component Analysis).

 

[luke strawwalker]

My suggestion would be to make it look something like a Saturn IB, with the three tubes going into a single tube near the front and a suitable sized nosecone. You can still run the launch rod down the center of the three tubes, but the drag will be a lot less. Drill a hole in the tip of the nosecone if needed to allow the launch rod to pass.

You CAN build it with regular cones, but the airflow will 'stall' in the area where the air flowing off of them converges in the central area, and cause a lot of drag. With a long central area between the tubes, the airflow down the central channel will be minimal.

As for stability, obviously three fins would be easiest. Putting them sticking out directly from the tubes themselves would put them in the 'cleanest airflow', but putting them in the joint between two tubes would probably give the strongest joint. Since the fin span (distance from root to tip edge) contributes more to stability than fin chord (length from front to rear) I'd probably make the fins a little longer and put them in the saddle between tubes... However, both work. The Quest "Future Launch Vehicle" has three fins, out on the side "booster" tubes.

If you want to minimize drag on the rocket to an absolute minimum without resorting to a "Saturn IB" style common tube enclosing all three tubes at the front (which would also make a handy recovery compartment and need only be long enough to contain the parachute and wadding or heat shield material, and the nose cone shoulder (or use baffles in the BT-50 motor tubes and eliminate the need for that stuff) I'd suggest making the cones "off-center" so that "lean in" towards each other at the tips. Basically it's the "slanted nosecones" like you see on the first stage side tanks of the Russian Proton Rocket, or the booster nosecones on the SRB's of the Ariane V... or similar to the "conformal" nosecones on the Atlas V SRB's that basically curve away from the side of the Atlas centerbody. You could probably cut a "sliver" out of regular nosecones and glue them back together to get the appropriate degree of "tilt" to the cone to bring the tips of them very close together while leaving clearance for the central launch rod.

While regular "straight axis" (on-center) nose cones next to adjoining structures works (like the SRB nosecones beside the much larger external tank on shuttle) there IS a lot of "interference drag" as the airflow tends to stagnate as it flows directly down the side of the nosecone toward the wall of the other tube... this sets up a shock wave as the airflow has to turn "sideways" to flow out of the way as it meets up with that wall-- this is very visible in the scorch marks on the first two shuttle ET's which were painted white (air friction heat topped 400 degrees in flight and was particularly pronounced where the shock waves intersected the tank walls, leaving scorch marks in the paint).
If the idea is to reduce drag, minimizing the "stagnant" area where the three cones are facing each other, all "dumping" air off them into this "hollow" between the three cones, which will then have to "spill out" through the narrowing space between cones, will maximize performance by minimizing drag.

If you want the simplest construction, though, the straight cones will work...

Later! OL JR

 

[BABAR]

Not exactly what your plan is but 3 side by side body tube and nose cones. No stability issues encounter ed
See post 8
https://www.rocketryforum.com/showt...Glider-and-New-Gyskelion&highlight=Triple+sec

 

[Zebedee]

Thanks for all the feedback and info guys. This was actually inspired by the Semroc trident someone posted in another thread recently. Just remove the center pieces from it push the three outriggers together and stick motors in them

OL - what you're describing was my concern - I'm not so worried about performance more about the stuff happening upfront pulling the CP close to or even forward of the CG and making the whole thing unstable.

I like your idea of turning the nosecones inward - the shoulder on the semroc balsa cones I bought is way too wide for the tubing I have (which is a thick walled cardboard inner tube from a roll of fabric) so I actually have room right there to shape the shoulder totilt the NC's inward.

I think I'll also attach the initial fins pretty lightly so if it is unstable I can whip them off and put larger ones on to help. I'll post some parts pic's in the next few days.

Zeb

 

[luke strawwalker]

Ok tried to upload this earlier and for some reason my modem wigged out, and then the forum wouldn't load for awhile...

Here's a drawing I whipped up on MS Paint showing what I'm talking about. They say a pic is worth a thousand words... Guess we'll see... LOL

Later! OL JR

 

[mjennings]

again not exactly the same but similar the Sunward Gravity Rider has a 3 tube set up, two joined in the aft and one projecting forward.
https://sunward1.com/gravity-rider-model-rocket.htm
https://www.rocketryforum.com/showt...Sunward-Gravity-Rider&highlight=gravity+rider

Off set cones aren't hard to make out of paper either, it could be a lot easier than carving them out of balsa. Fliskits uses them in the ACME spitfire and thunderbird
https://www.fliskits.com/products/rocketkits/kit_detail/acme.htm
https://www.fliskits.com/products/rocketkits/kit_detail/thunderbird.htm

I've seen directions for calculating them in a few places but can't think of them right now

 

[luke strawwalker]

again not exactly the same but similar the Sunward Gravity Rider has a 3 tube set up, two joined in the aft and one projecting forward.
https://sunward1.com/gravity-rider-model-rocket.htm
https://www.rocketryforum.com/showt...Sunward-Gravity-Rider&highlight=gravity+rider

Off set cones aren't hard to make out of paper either, it could be a lot easier than carving them out of balsa. Fliskits uses them in the ACME spitfire and thunderbird
https://www.fliskits.com/products/rocketkits/kit_detail/acme.htm
https://www.fliskits.com/products/rocketkits/kit_detail/thunderbird.htm

I've seen directions for calculating them in a few places but can't think of them right now

Isn't there a pattern generator for paper offset cones on payloadbay.com??

I know I've seen them online at one point or another... been toying with making a Proton rocket scaled to fit with the other Zoochies like the Saturn IB and Soyuz...

Later! OL JR

 

[Zebedee]

Awesome diagram! We're on the same page.

As I mentioned I have the three cones already and no more of that tubing to use them in after this project so they shall be adapted. It should be relatively straight forward to remove the shoulder completely, sand down the base at a slight angle and re-attach the shoulder. I can take advantage of the increased base area to smooth/round the angle of the transitions into the tube on the inner and outer "faces" too.

My takeaway from the links Michael shared (thanks also) was that those models seem to have large draggy fins at the rear which I assume is compensating for the drag induced by the airflow at the front so I'll plan for somewhat larger fins to start and make it easy to change them (no fillets for example) for ever larger ones if the first flight doesn't go well.

As I said - performance is not the goal for this - just an interesting looking rocket than flies well - this should be fun

 

[luke strawwalker]

Awesome diagram! We're on the same page.

As I mentioned I have the three cones already and no more of that tubing to use them in after this project so they shall be adapted. It should be relatively straight forward to remove the shoulder completely, sand down the base at a slight angle and re-attach the shoulder. I can take advantage of the increased base area to smooth/round the angle of the transitions into the tube on the inner and outer "faces" too.

My takeaway from the links Michael shared (thanks also) was that those models seem to have large draggy fins at the rear which I assume is compensating for the drag induced by the airflow at the front so I'll plan for somewhat larger fins to start and make it easy to change them (no fillets for example) for ever larger ones if the first flight doesn't go well.

As I said - performance is not the goal for this - just an interesting looking rocket than flies well - this should be fun

Sounds good... neat project...

Be aware that changing the cone angle does some weird stuff geometrically... of course it sounds like you're prepared for it and will resand the cones to fit the tubes better at the base... The back ends of the tilted cones will no longer be circular, but an ellipse.

Hope that stuff helps..

Good luck with your project! OL JR

 

[mjennings]

not sure about the online calculators but I'm pretty sure it's in here
https://www.apogeerockets.com/Rocket_Books_Videos/Books/Model_Rocket_Design_And_Construction

Be aware that changing the cone angle does some weird stuff geometrically... of course it sounds like you're prepared for it and will resand the cones to fit the tubes better at the base... The back ends of the tilted cones will no longer be circular, but an ellipse.

yup that will make for a lot of balsa dust!

 

[dave carver]

I have had thoughts of a rocket along the same lines. It's been my thought that due to aerodynamics the three triangular setup nosecones will act like one big blunt nosecone. Because of compressability there will be very little air going down the center.

Your demo plan makes the rocket look like it's being built from 3 Baby Berthas

 

[Zebedee]

I got my build pile cleared out this week (finished an Estes Crossfire, Estes CC Express and Semroc Thunderchief) so got started on this guy last night.

Pics are the gently used fabric roll inner tube, marked and taped for the three 18" long body tubes, three beautiful Semroc nose cones (gonna miss those guys), 1/8" Basswood fins marked out (large-ish and with grain lined up on the leading edge and a bonus pic of the first rocket I made from that same fabric tube (but 12" long) and same nose cone as for this project (but smaller fins).

I'm also thinking about recovery - I think it might be fun to have a small-ish chute in each tube but not sure how badly they will interfere with each other. Also thinking about installing baffles...

 

[Micromeister]

Zebedee:
As JR mentioned the drag form from three cone-3 tube clusters really robs performance lowering altitude substantially.
Been flying NAR Cluster Altitude competitions for a bunch of years now. Experimenting with configurations for 3x1/2A Cluster Altitude competition for Naram-35 in 1993 brought to light many of the issues JR showed in his example.

Even 3 BT-5 body tubes with nosecones caused almost 1/3 reduction in achieved altitude then running 3 inline tubes with a tapered shroud. Ultimately I came up with a flat Tapered side tube cap that smoothed out most of the induced drag allowing some very impressive cluster Altitude flights. Below are some of the original 3x1/2A cluster Designs we tried. perhaps they will give you a few ideas for you fun flying Cluster(s).

Ps: One of the most important things to keep in mind designing Cluster models is that Frontal AREA is absolutely the most important factor followed closely by Optimum Mass (Coasting mass or Throw Weight).

 

[Zebedee]

Thanks for the photo's - lots of cool ideas there.

For this build I'm pretty set on something that looks like your 34e model but with longer nosecones canted towards the middle (leaving enough room for the launch rod.

My primary concern is making sure it's stable which I think I have a handle on thanks to the info posted so.

 

[Zebedee]

I had to go on a business trip last week so not much progress on this yet. I also got distracted by my Wildman Vindicator mini and Semroc ThunderStrike in the build pile. I did manage to get the tubes cut and the nose cone shoulders sanded down for the first fit.

First impressions are that the central hole is pretty small - it will (barely) fit a 3/16 rod but probably not a 1/4 rod. That may mean I choose to put 1/4" lugs on the exterior nestled in the gap between two tubes.

Second thoughts are that when canting the NC's inward they immediately impinge on each other which means more shaping that previously thought. I kinda like the look as is - I might decide to leave them straight and see how she flies.

Lastly - still thinking about the chutes. 3x12" would support the weight but I'm concerned that if one pops significantly before the others the weight of the whole rocket plus motor cases might rip the shroud lines on the standard plastic chutes, so probably one larger or three smaller nylon chutes from top flight will do the trick.